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Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies
Annals of Nuclear Energy xxx (xxxx) xxx
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Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies Da Wang, Fenglei Niu ⇑, Ruixian Liang, Weiqian Zhuo, Shengfei Wang Beijing Key Laboratory of Passive Safety Technology for Nuclear Energy, North China Electric Power University, Beijing 102206, China
a r t i c l e
i n f o
Article history: Received 16 February 2019 Received in revised form 15 July 2019 Accepted 7 September 2019 Available online xxxx Keywords: LOCA Fuel assembly Debris Pressure drop
a b s t r a c t A loss of coolant accident (LOCA) would generate debris from thermal insulation and other materials within the containment of a pressurized-water reactor (PWR). Some of the debris may pass through the strainer and accumulated on the fuel assembly (FA), causing resistance to flow for core cooling and could inhibit the LTCC capability of the plant. To address this safety issue, a quantitative assessment of the head loss across debris bed accumulated on a full-scale FA under limiting cases has been experimentally studied. Additionally, the formation of debris bed was also observed during tests to analyze its characteristics in different conditions. All tests results indicated that almost all debris was captured on the bottom nozzle (BN) and P-grid. Fiber acted as a critical role to form a filter bed. Fine particles include SiC particulate and chemical precipitate could make a ‘‘Compact effect” on debris bed, exerting a significant impact on pressure drop. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction A pipe break in the nuclear plant’s primary system leads to a rapid depressurization of the primary reactor system, thermal piping insulation and other materials (e.g. paint and concrete) in the vicinity of break would be dislodge by dynamic propagation of pressure wave followed by the ensuing vapor/water-jet impingement. The cracked materials with other latent impurity such as painting chips, coatings and concrete dirt would be transported by the flow in containment and eventually into the containment sump equipped specialized strainers. Most of debris could be captured by the strainers (Johns et al., 2003), however a portion of debris would pass through sump strainers and enter into the core, finally accumulated on the bottom nozzle and grids of FA, even on the surface of fuel rods, which could impeded the performance of ECCS and reduce the ability of the coolant to remove decay heat from the core. Debris transported to the FA has a tendency to form a uniform bed, could cause a significant head loss across the FA, the resulting additional resistance may reduce recirculation flow margin. If the sump pump cannot provide enough cooling water to a reactor core because of the debris blockage, this can lead to a serious consequence like core damage. To address this safety issue, the PWR owners group (PWROG) prepared WCAP-17057-NP Revision 1, ‘‘GSI-191 Fuel Assembly Test ⇑ Corresponding author.
Report for PWROG (Baier et al., 2011)” and submitted it to the United States Nuclear Regulatory Commission (U.S.NRC). This report describes the acceptance criteria for the mass of debris that can reach the RCS and not impede LTCC flows to the core. The NRC’s position is that plants must be able to demonstrate that debris transported to the sump screen after a LOCA will not lead to unacceptable head loss for the recirculation pumps, will not impede flow through the ECCS and CSS, and will not adversely affect the long-term operation of either the ECCS or the CSS (Sisk et al., 2011). The PWROG performed tests to evaluate in-vessel effects of debris and chemical precipitates that pass through the sump strainer following a LOCA. These tests were conducted by Westinghouse and AREVA on their respective fuel designs using various fiber, particulate, and chemical precipitate debris loads to examine the pressure drop across a single fuel assembly. The ultimate goal of the tests is to determine the maximum sump strainer debris bypass loads that result in acceptable pressure drops across the mock-up fuel assembly (Geiger et al., 2009). The NRC staff observed a Generic Safety Issue-191-related downstream effects fuel blockage test that simulated the down-flow conditions that occur during a cold-leg break in a Westinghouse 17 17 fuel assembly, the length of the test assembly was about 48 in., approximately one-third the length of an actual fuel assembly. Comparison of pressure drop measurements between the top nozzle/1st grid support and the full assembly indicated that the initial collection of debris occurred at the top of the fuel assembly but quickly distributed over additional spacer grids. At no time during the test
E-mail address: [email protected] (F. Niu). https://doi.org/10.1016/j.anucene.2019.107037 0306-4549/Ó 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx
did the pressure drop exceed the acceptance limit of 13 psi across the fuel assembly (Ruth et al., 2010). Also the NRC staff observed fuel blockage testing conducted on AREVA fuel assemblies by Continuum Dynamics Inc. The observed mock-up fuel assembly test with scaled flow rate, scaled debris load, scaled 1/3 height mockup fuel assembly. As expected, NRC staff observed that the pressure drop across the fuel assembly did not approach the acceptance limit of 13 psi. Also, chemical precipitate addition did not result in a large increase in pressure drop across the mock-up fuel assembly (Geiger, 2008). Furthermore, in-vessel downstream effect tests with a mock-up PLUS7 FA for the advanced power reactor (APR) 1400 were performed by KHNP Central Research Institute to confirm that the head losses caused by debris meet the available driving head following a LOCA. All the test results showed lower pressure drops than the available head limits and a sufficient driving force is therefore available to maintain an adequate flow rate, and the LTCC capability is adequately maintained in the APR1400 (Suh et al., 2015). However, few studies have been done on debris impact on full length fuel assembly and little attention has been devoted to characteristics of debris bed accumulated on the BN. In this study, a downstream effect program was performed to quantify the head loss across the FA under the debris loading conditions of a postulated LOCA and determine the maximum pressure drop in a limiting case. Additionally, the test loop was intended to test the debris capture characteristics on a full-area FA. The mockup FA was full scale in all dimensions include height and structures. There are total 8 spacer grids along the longitudinal direction with a bottom/top nozzle component and 15*15 fuel rods. Moreover, the fuel rod diameter is 10.0 mm and the pitch is 13.3 mm. The BN which is a box-type structure contains a square shape of plate with orifices. Over the plate, there is a filter screen used to prevent debris from passing through it. 2. Experimental method The experimental method is to simulate appropriate PWR containment debris accumulation and head-loss conditions, with flexibility for controlling local flow conditions, debris quantity and other important parameters and with the capability of taking applicable measurements and visual observations of the phenomena under examination. 2.1. Experimental facility
The experimental facility is illustrated schematically is shown in Fig. 1, and a photo of the facility with a 3D layout is shown in Fig. 2. The experimental facility is mainly composed of five parts: a test column, mixing tank system, collection system, circulation system, and control and monitoring system. The test column contains a full-scale FA in a 3.5 m height and steel vessel with a lower/upper plenum. Adequate transparent acrylic-windows were installed in some region of vessel for viewing debris accumulation on FA during the test. The lower plenum was designed as a pyramid structure to preclude debris settling. There was a perforated baffle plate between FA and lower plenum for distributary. As debris caught on the FA, the differential pressure is measured constantly across the lower/upper half fuel assembly and the entire fuel assembly. There are extra ports available on the sides of the test column if a measure of the differential pressure across a specific portion of the fuel assembly as required. The mixing tank is mainly manufactured as stainless steel with cylindrical shape and several transparent acrylic window with scaleplate for observing water level and turbidity. A debris stirring tool is installed downward vertical at the top of tank to mix debris well and preclude the settling on the bottom of the tank. The collection system is composed of a collecting tank manufactured as transparent acrylic material with a cubic shape and relevant pipes and strainer with different size filter screen. The collection tank is placed under the mixing tank, in that way, the collect tank could collect debris solution from mixing tank and filtrate debris by gravity head. Thus the quantitative study of debris without being captured on FA could be analyzed. The circulation system pumps water from mixing tank, through the circulation piping and test loop, finally back into the mixing tank. The butterfly flow control valve downstream of the recirculation pump was set to a precalibrated position to generate an average flow rate in the loop that produces the desired approach velocity at the fuel assembly. An electromagnetic flow meter measures the flow rate and provides feedback to the control system to maintain a constant flow rate. A bypass loop is used to prevent pump damage from serious debris blockage. The control system regulates the flow rate and water temperature. The monitoring system records the transient data of pressure drop (dP), flow rate, and water temperature in test loop. The monitoring system is also used to check the slope of dP versus time graph in order to evaluate whether the dP meets a steady state condition. As debris caught on the FA, the differential pressure was measured constantly across various locations including the
A closed-loop hydraulic test apparatus was designed and used to measure the pressure drop and flow rate across a mock-up FA. Stirrer Fuel Assembly
T
T
Mixing Tank
Pump
dP3 Heater
dP4 T
Bypass Strainer Collecting Tank
dP2
dP1 T
Pump Flow Meter
Fig. 1. Schematic diagram of experimental facility.
Fig. 2. Three-dimensional diagram of experimental facility with photo of components.
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx Table 1 Debris types and amounts per fuel assembly.
Table 2 The bypass fiber length distribution ratio.
Debris types
Transported to strainer (kg)
Assumed bypass debris (kg)
Per FA (g)
Fiber Latent fiber Coating debris Latent particle Chemical precipitates
168 13.7 23.4 77.4 84 (maximum in 30 days)
54.51
450.5/ 100 833.06
23.4 77.4 84
Length
proportion
<0.5 mm 0.5 mm–1.0 mm >1.0 mm
67.1% 8.9% 24.0%
694.7
show a particulate debris surrogate of SiC power and an SEM image.
lower half fuel assembly (dP2), upper half fuel assembly (dP3) and the entire fuel assembly (dP4). The differential gauge of dP1 measured the pressure drop across the BN and P-grid. 2.2. Debris preparation During the LOCA event, debris transported to containment sump mainly include fiber insulation material, paint coating, chemical impurity and latent debris (Yu et al., 2008). The debris types and amounts are summarized in Table 1. All the debris except fiber transported to the sump was assumed to bypass the strainer. In all tests, the circulating coolant may entrain debris that can be categorized as particulate, fiber, and chemical precipitates4. In a homogeneous debris bed, the densities and size characteristics of the individual constituents are necessary to determine the porosity of the debris bed. In general, the lower bound values for the characteristic sizes of the debris were adopted. This is conservative for head loss calculations because the specific surface is inversely proportional to the characteristic size of the debris particle. The smaller the characteristic size of the debris bed constituents the higher the pressure drop across the debris bed (Mattei et al., 2004). 2.2.1. Particulate debris There are several sources of particulate debris. Paints and coatings might be broken up into flakes, or be reduced to their basic constituents such as micron-sized powders. Solid insulation, such as CalSil, may contain particles, again with a potential for reduction to micron-sized elements if they are broken up to their smallest basic units. The NRC Safety Evaluation (SE) for NEI 04-07 (NEI, 2004) identified particle size as a key parameter for the selection of representative debris. Specifically, the SE states that major contributors to head loss are the increasing smaller particles. All testing was conducted with silicon carbide particles approximately 10 lm in size, allowing the debris to act as a fine particulate debris that collects within a fiber bed and results in a maximum head loss. Silicon carbide is representative of all particulate debris. Fig. 3
2.2.2. Fiber debris The steam jet in high temperature and pressure could cause damage to piping thermal insulation, which would be split into fiberglass pieces, mostly individual fibers with a high mobility act as a crucial component to build up a filter bed, causing ‘‘Thin bed effect ” (Suh et al., 2015). Once a filter bed is created, there is potential to progressively trap smaller and smaller particles until the flow is effectively impeded. NUREG/CR-6877 indicates that the latent fiber can be substituted by fiberglass during the test (Ding et al., 2005). A sump strainer bypass test about fiber debris was performed primarily, and the bypass fiber length distribution ratio is shown in the Table 2. Fig. 4 shows the SEM image of fiber. The fibers do not appear circular but somewhat ribbon-like, with the narrower dimension significantly smaller than 15 mm. Smaller fibers also can be seen. 2.2.3. Chemical precipitates Chemical reactions occur when hot borated water sprays over the containment. More important reactions occur when many constituents reside together over days in the containment sump, which is usually buffered to increase the pH. One important product is sodium aluminum silicate (SAS, NaAlSi3O8), which can precipitate into micron-sized elements and lead to complete clogging of a filter bed if it accumulates in or on it. Therefore only NaAlSi3O8 was conservatively used in the test. The properties, particularly the size, of the precipitates depend on the chemical and temperature history. Thus, NaAlSi3O8 precipitate was prepared in strict accordance with procedure of WCAP-16530-NP (Lane et al., 2008). The concentration of NaAlSi3O8 did not exceed 11 g/L and the settling tests were also performed to ensure it met the settling criteria defined by WCAP-16530-NP-A. Fig. 5 shows the chemical precipitates for the test and an image of SEM. 2.3. Experimental conditions In the event of a hot-leg (HL) break of double ended guillotine, the ECCS liquid must pass through the core to exit the break and no spilling of ECCS liquid occurs. Therefore, an additional driving head
Fig. 3. Particulate debris surrogate of SiC powder.
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx
Fig. 4. Morphology image of fiber.
Fig. 5. NaAlSi3O8 precipitate and SEM image.
from the build-up of liquid level in the downcomer and in the steam generator tubes to the spillover elevation is present. Thus the maximum flow rate to per fuel assembly is expected to be 9.917 m3/h to obtain the maximum pressure drop at the test column. However, the higher flow rate can also result in faster debris build-up and there is more debris available to accumulate. So, the HL break condition represents the conservative case in terms of debris loading. For postulated cold-leg (CL) breaks, most of the ECCS liquid spills directly out of the break location, which decreases the driving head of core flow to a minimum. The flow to the core is selected as 2.5 m3/h which is required to make up for core boiling to remove the decay heat. With different debris mobility, the fiber was easier to trapped by sump strainers in low flow rates, thus less fiber debris would be transported to the core in lower flow velocity than HL break. Furthermore, to create the worst possible scenario, the limiting break case for modeling purposes will be a combined condition by HL break and CL break. Namely, a limiting flow at the core inlet, combined with more debris loads that occurs for a high flow HL break. This ensured the development of debris beds with the maximum resistance and highest pressure loss. In prior testing programs the manner in which the debris is added was observed to make a difference in the overall head-loss through the FA. Therefore, two debris additions methods were used in this study: For sequential debris addition tests, the all SiC particulates was added in one batch; the fiber was divided into several batches; surrogates for chemical reaction products were added to the test loop after all of the fiber and particulate had been added. For concurrent debris addition tests, the three types of debris in the amount prescribed in the test procedure were well mixed in water until completely suspended and fine enough that it dispersed across the pipe cross section, thereby approaching the BN
relatively uniformly and subsequently producing a uniform debris bed. The debris mixture was introduced into the test loop by slowly pouring the bucket contents into the top of the test section. Tap water was used to simulate the post-accident coolant, because this condition was expected to be conservative relative to actual reactor coolant condition (Baier et al., 2011). Additionally, the previous tests demonstrated that surrogates made with highpurity water were not as effective or as stable as those made with tap water (Technical Letter Report on Evaluation of Chemical Effects, 2008). Thus, the tap water was used in this study. 3. Results and analysis The recirculation pump was turned on and allowed to run for 10 min before test, allowing air to be flushed out of recirculation loop piping. Benchmark tests as comparison with debris tests were performed firstly before adding debris. Fig. 6 shows the typical pressure drop behavior of fuel assembly without debris. These benchmark-test results presented a repeatable character. Flow variation tests were performed once the debris bed was established. The test matrix is described in Table 3. Three different cases in HL break condition and two different cases in CL break condition were performed. Also the same case had been repeated and the results presented good repeatability within an acceptable range. The HL break condition in concurrent adding method presented a highest pressure drop than other tests. 3.1. Hot-leg break condition During the HL break condition tests, the flow rate was constantly sustained around 9.917 m3/h. The typical pressure drop
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx
dP
Allometric1 y = a*x^b 1.65459 0.99827 Value Standard E a 0.038 0.00384 b 1.465 0.05094
0.4
50
Flow rate Total FA Lower FA Upper FA BN/P-grid Particulate Fiber Chemical
8
3
Model Equation Reduced ChiAdj. R-Squar
10
Flow rate(m /h)
Pressure drop (kPa)
0.8
0.6
60
HL-1 HL-3 HL-5 CL-1
6
40
30
4 20 2
Pressure drop(kPa)
1.0
10
0
0
0.2
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Time (h)
Fig. 7. Pressure drop under the HL break condition in concurrent adding method with 100 g fiber (Test HL-1).
0.0 2
3
4
5
6
7
8
Flow rate (m3/h) Fig. 6. Results of benchmark tests.
60
Flow rate Total FA Lower FA Upper FA BN/P-grid
8
Flow rate(m3/h)
behavior in concurrent adding method with 100 g of fiber is shown in Fig. 7. These debris was equally divided into 12 batches. The particulate, fiber and chemical precipitant were represented by black dot, dark yellow triangle and navy square respectively in the following figures respectively. The pressure drop increased notably when the first batch and the second batch were added into water tank. However, the each addition of subsequent debris did not remarkably affect the pressure drop as the first or second batch did. The pressure drops among upper FA, lower FA, BN/P-grid increased gradually with the debris addition, especially in lower FA, which took up more than 70% of head loss compared with total FA. This trend is also shown in Fig. 8, which indicated that the debris distributed in a relatively decentralized manner among FA in concurrent adding method with high flow rate. Fig. 9 shows the typical pressure drop behavior in concurrent adding method with 450.5 g of fiber. This test conditions were all same with test HL-1 except the mass of fiber. Similarly, the pressure drop increased sharply when the first two batches were added into tank and the pressure drop caused by each addition of subsequent debris was much lower than the first two batches. Moreover, the pressure drop caused by first two batches of debris in this test was 50.75 kPa, almost equal to the final pressure drop in test HL-1, which demonstrated that the fiber debris had a significant impact on head loss. This trend is also illustrated clearly in Fig. 10. The maximum pressure drop recorded under the HL break condition was 88.37 kPa, which was much lower than the available head limit. The lines of lower FA and upper FA break as shown in Fig. 9, because the pressure drop of these two location exceeded the full-scale value of the gauge. Thus these data was abandoned.
40
6
4
20
Pressure drop(kPa)
10
2
0 0
2
4
6
8
0 12
10
Batch Fig. 8. Pressure drop versus debris adding batch (Test HL-1).
The typical pressure drop behavior in HL break condition with sequential adding method is shown in Fig. 11, which provides the limiting head loss curves. When the test started, all SiC particulate was added into the mixing tank, and the system was allowed to equilibrate for a while. The particulate debris had negligible effects on pressure drop, as shown in Fig. 11. Fiber was then add added in amounts of 10 g form first batch to 10th and 40 g/batch from 11th to 18th, the last batch was 30.5 g. The pressure drop increased gradually during the addition of fiber and the rising rate of pressure drop depended on the amount of debris. After the first addition of chemical precipitate, the pressure drops increased considerably, and subsequence addition of chemical did not affect the pressure drop. A likely reason for this trend is that there was a
Table 3 Test matrix. Test-ID
Flow rate (m3/h)
SiC (g)
Fiber (g)
NaAlSi3O8 (g)
P/F ratio
Adding method
DPmax (kPa)
Benchmark HL-1 HL-2 HL-3 HL-4 HL-5 HL-6 CL-1 CL-2 CL-3 CL-4
2–8 9.917 2–9 9.917 2–9 9.917 2–9 2.5 2–9 2.5 2–9
– 833.06 833.06 833.06 833.06 833.06 833.06 833.06 833.06 833.06 833.06
– 100 100 450.5 450.5 450.5 450.5 100 100 450.5 450.5
– 694.21 694.21 694.21 694.21 694.21 694.21 694.21 694.21 694.21 694.21
– 8.33 8.33 1.85 1.85 1.85 1.85 8.33 8.33 1.85 1.85
– Concurrent – Concurrent – Sequential – Sequential – Concurrent –
0.8 50.53 – 88.37 – 82.37 – 32.76 – 64.46 –
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx 100
100 10
10
2
0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
6
80
60
40
4
20
2
0
0
20
0
4.5
1.0
1.5
2.0
Fig. 9. Pressure drop under the HL break condition with concurrent adding method (Test HL-3).
3.0
Fig. 11. Pressure drop under the HL break condition with sequential adding method (Test HL-5).
HL-1(Fiber 100g) HL-3(Fiber 450.5g)
80
Pressure drop (kPa)
Pressure drop (kPa)
2.5
Time (h)
Time (h)
80
Pressure drop (kPa)
40
4
3
60
Flow rate (m /h)
8
Pressure drop (kPa)
3
Flow rate (m /h)
6
Flow rate Total FA Lower FA Upper FA BN/P-grid Particulate Fiber Chemical
80
Flow rate Total FA Lower FA Upper FA BN/P-grid Particulate Fiber Chemical
8
60
40
dP of total FA
NaAlSi3O8
60
Fiber 40
SiC 20
20
0
0
0
2
4
6
8
10
12
0
300
600
Batch Fig. 10. Pressure drop versus debris adding batches in test HL-1 and HL-3.
uniform debris bed could be compacted by chemical precipitates. This phenomenon is called ‘‘Compaction effect”, which may cause a higher head loss. When the vast mass of smaller particulates passing through the uniform debris bed, the bed porosity decreased substantially because of bed compression and reduce of interfiber spacing, with a corresponding increase in the particulate filtration rate and the entrapment of the finer particulate. Once the debris bed was already saturated with fine particles, the extra chemical precipitate addition had no effect on pressure drop, further compression was limited by the density of the particulates. After the first addition of chemical precipitate, the pressure drop of BN/P-grid decreased sharply, a probable explanation is that the debris bed at BN/P-grid could not support that pressure and portion of debris bed was destroyed and separated. Thus the pressure drop decreased subsequently. In Fig. 11, the pressure drops in full height (Total FA) are very similar to those in lower FA on which most of debris deposited, especially in the BN/P-grid. After injecting all types of debris, the pressure drop across upper FA was negligible, which was different from the test results with concurrent adding methods. Fig. 12 displays the relationship between debris loads and pressure drop in test HL-5. As the figure shows, even though the mass of SiC reached at 833.06 g, there was negligible effect on pressure drop. However, once the fiber debris was added into loop, the pressure drop increased apparently and with the next addition of fiber, the pressure drop increased continuously. Only the first batch of
900
1200
1500
1800
Debris loads (g) Fig. 12. Pressure drop versus debris loads in test HL-5.
chemical precipitates made an obvious increase on pressure drop. This trend pointed out that the formation of a fiber bed at the FA serves as a collector for particulates which in turn result in an increase in pressure drop across the FA. Without a fiber bed, particulates that are transported into the FA are sufficiently small that they will not collect at BN, the debris flows freely throughout the FA. Without a fiber bed to capture particulates, there is no impact on the head loss at FA. Also, the chemical precipitates act an crucial role for core blockage that can cause a significant pressure drop across a uniform fibrous debris bed if debris bed is not already saturated with particulate. Fine particles could fill internal pores within the fibrous bed. Fig. 13 presents the comparison of pressure drop behavior between concurrent adding method and sequential adding method. Although their rising trends were different, the final total pressure drop was almost close. 3.2. Cold-leg break condition During the CL break condition tests, the flow rate was constantly sustained around 2.5 m3/h. The typical pressure drop behavior in CL break condition with sequential adding method is shown in Fig. 14, which was similar to those observed during the test HL-5. All the SiC particulate was added to test loop at one time and had negligible effect on pressure drop. The 100 g of fiber was equally divided into 10 batches. Pressure drop gradually increased
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx 100
35
HL-3 Concurrent HL-5 Sequential
dP of total FA
30
Pressure drop (kPa)
Pressure drop (kPa)
80
60
40
25
NaAlSi3O8
20
Fiber 15
SiC 10
20
5 0 5
10
Batch
15
20
0
25
0
1000
1200
1400
1600
60
Flow rate Total FA Lower FA Upper FA Particulate Fiber Chemical
3
Flow rate (m /h)
2
1
40
20 0
0 2
4
6
Time (h)
8
10
Fig. 16. Pressure drop under the CL break condition in concurrent adding method with HL debris loading (Test CL-3).
100
HL-3 Q=2.502m3/h CL-3 Q=9.917m3/h 80
60
40
30
20
Pressure drop (kPa)
20
Flow rate Upper FA Total FA BN/P-grid Lower FA Particulate Fiber Chemical
3
800
80
Pressure drop (kPa)
40
Flow rate (m /h)
600
Fig. 15. Pressure drop versus debris loads in test CL-1.
with the addition of each batch of fiber. After adding the first batch of chemical precipitate, pressure drop had a steep elevation immediately, with a corresponding sharp decrease in the flow rate. However, after that the pressure drop decreased obviously, a likely reason for this trend is that with increasing of pressure and particulates load in recirculation loop, debris bed could not sustain its structure. Some fine particles would penetrate the debris bed, with a corresponding decrease in the particle filtration efficiency, thus causing a decrease in flow resistance. Before adding the second batch of chemical debris, there was a significant increase on pressure drop, with a corresponding notable drop on flow rate. The erratic behavior associated with this test is probably due to transient debris bed behavior as the lumps of debris disassociate or disintegrate with flow turbulence. Disintegrated debris subsequently deposited in the bed again, whereby the bed tended to become more uniform, thus incurring increased head loss. The second batch addition of chemical precipitate caused a slight increase on pressure drop. Pressure drop curves of total FA, lower FA and BN/ P-grid are nearly overlap during the whole test, which indicated that almost all debris was trapped in the BN/P-grid. Fig. 15 presents the relationship between debris loads and pressure drop in test CL-1. Same situation with test HL-5, the addition of SiC made barely effect on pressure drop. The pressure drop increased linearly with addition of fiber debris. However, the first batch of chemical precipitates clearly resulted in a substantial increase on pressure drop, as shown in Fig. 15, which indicated that ‘‘Compaction effect” was more prominent in CL condition than that in HL condition. The reason for this is probably that the fibrous
1
400
Debris loads (g)
Fig. 13. Pressure drop versus debris adding batches in test HL-3 and HL-5.
2
200
Pressure drop (kPa)
0
10
0 0
2
4
6
8
10
12
Batch Fig. 17. Pressure drop versus debris adding batches in test HL-3 and CL-3.
0
1
2
3
4
5
6
0 7
Time (h)
Fig. 14. Pressure drop under the CL break condition with sequential adding method (Test CL-1).
bed formed in low velocity was more uniform and the interfiber space was more broader. Fig. 16 shows the typical pressure drop behavior in CL break condition with hot-leg break debris loading in concurrent adding method. Similar with test HL-1 and HL-3, each batch addition of
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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60
30 4 20
60 3
Flow rate (m /h)
40 6
80
Flow rate Total FA Upper FA Lower FA
6
Pressure drop(kPa)
3
Flow rate(m /h)
50
4 40
2
Pressure drop (kPa)
Flow rate Total FA Upper FA Lower FA
8
20
2
10
0 5.2
5.3
5.4
5.5
5.6
5.7
5.8
0 6.0
5.9
0
0 7.6
Time (h)
7.8
8.0
8.2
8.4
Time (h)
Fig. 18. Transient Head Loss Curve for a Formed Debris Bed (Test HL-2/CL-2).
100
80
y = Intercept + B1*x^1 + B2*x^2
Equation
40
No Weighting Weight 10.84332 Residual Sum of Squares 0.99892 Adj. R-Square Value
30 Pressure drop
Intercept B1 B2
0 2.27029 0.30891
Pressure drop (kPa)
Pressure drop (kPa)
Decreasing Velocities Increasing Velocities Polynomial Fit of test data
Decreasing Velocities Increasing Velocities Polynomial Fit of test data
50
Standard Error -0.16913 0.02261
20
y = Intercept + B1*x^1 + B2*x^ 2 Weighting No Weight 27.16239 Residual Sum of Squares 0.99918 Adj. R-Squar Equation
60
Value Pressure drop
Intercept B1 B2
0 3.2599 0.5543
Standard Erro -0.21256 0.02709
40
20
10
0
0 0
1
2
3
4
5
6
7
8
9
0
10
2
4
6
8
10
Flow rate (m3/h)
Flow rate (m3/h) Decreasing Velocities Increasing Velocities Polynomial Fit of test data
80
y = Intercept + B 1*x^1 + B2*x^2 No Weighting Weight 22.53191 Residual Sum of Squares 0.99931 Adj. R-Square
Pressure drop (kPa)
Equation
60
Value Intercept Pressure drop B1 B2
0 5.63579 0.23877
Standard Error -0.19547 0.02494
40
20
0 0
2
4
6
8
10
Flow rate (m3/h) Fig. 19. Head loss versus flow rate (Test HL-2/4/6).
debris caused a obvious increase on pressure drop, especial the first and second batch. However, the pressure drop increased slowly during the test, with a corresponding decreasing in flow rate. The trend could be explained by that with low flow velocity, the debris was transported much slower than hot-leg break condition in test loop. Furthermore, the pressure drop in lower FA is very similar with that in total FA, which is different from test HL-1 and HL-3. It indicated that the debris was easier to trapped by lower FA in cold-leg break condition no matter what methods the debris was added into loop. Fig. 17 provides the comparison of pressure drop behavior between test HL-3 and CL-3. These two pressure drop increasing trend were similar. However, the final total pressure drop of test CL-3 with 2.5 m3/h flow rate was 66.46 kPa, which accounted for 72.9% of final total pressure drop of test HL-3 with
9.917 m3/h flow rate. Thus, the flow resistance under CL break condition was much higher than that under HL break condition. A likely reason for this is that debris was scattered among FA in a high flow rate and a single debris bed in a low flow rate with the same amount of fiber gives rise to increased flow resistance by higher packing effects. Likewise, the maximum pressure drop recorded under the CL break condition was lower than the available head limit. 3.3. Flow variation tests Once the debris bed was established, the flow velocity was gradually altered so that the pressure drop could be measured for various flow velocities for the same debris bed. The general idea
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx 70
50
Increasing Velocities Decreasing Velocities Polynomial Fit of test data
80
y = Intercept + B 1*x^1 + B2*x^2 No Weighting Weight Residual Sum 81.22011 of Squares 0.99513 Adj. R-Square Equation
Value Intercept Pressure drop B1 B2
0 9.43965 0.00209
Pressure drop (kPa)
Pressure drop (kPa)
60
Increasing Velocities Decreasing Velocities Polynomial Fit of test data
Standard Error -0.87141 0.15709
40
30
60
40
Equation
y = Intercept + B1*x^1 + B2*x^2
No Weighting Weight 1679.12032 Residual Sum of Squares 0.96813 Adj. R-Square
20
Value Standard Error 0 -20.05927 3.56427 -1.14489 0.69892
Intercept Pressure drop B1 B2
20 10 1
2
3
4
Flow rate (m3/h)
5
6
7
2
3
4
Flow rate (m3/h)
5
6
Fig. 20. Head loss versus flow rate (Test CL-2/4).
3
2
Stop Adding
4 Upper FA Lower FA Total FA Flow rate Water Level
Stop
Drain Water Only
1
2
Flow rate (m3/h)
Pressure drop (kPa)
6
Add and Drain Simultaneously Add Water Only
0
0 0
500
1000
1500
2000
2500
3000
3500
Time (s) Fig. 21. The impact of water purity on pressure drop.
was to decrease/increase the flow, then wait until the head loss became relatively stable, record the head loss and flow, and then move on to the next velocity. Once the flow rate reached at 2 m3/h or 9 m3/h, the flow was sequentially decreased/increased with the data recorded on the descent/ascent, as well. These processes are shown schematically in Fig. 18, respectively. The head losses were somewhat less when the flow velocity was being decreased than when the velocity was increasing. A similar trend was also observed for the CL break condition tests. The reasons for this behavior are not clear, but the effect is not compression hysteresis. It may be possible that the debris bed was disturbed and subsequently shifted into a less uniform configuration so that its resistance to flow was lessened. This trend is also
illustrated in Figs. 19 and 20, which plots pressure drop as a function of the flow rate. In these figures, the velocity direction is illustrated with an arrow. As these figures shown, this trend was slight in HL break conditions, however it was relatively obvious in CL break conditions. Thus, the stability of the debris bed is an important consideration when analyzing these tests data. It will be apparent that debris beds reoriented following changes in the pump flow if the debris bed was not steady enough. Debris bed formed in low velocity is relatively more susceptible to movement if the bed is perturbed. An increase in turbulence was shown to enhance the disintegration of the debris bed, so that the debris bed changed either slowly or suddenly when the pump flow was altered in low velocity. After test, the water in the loop turn to be cloudy because of slight disintegration of debris bed caused by turbulence of pump flow. It indicated that plenty of particulates were suspended or transported with flow in the loop and could not be trapped by debris bed accumulated on the BN. The filtration rate of debris bed already attained to a stable value in the specific flow rate, although there were still plentiful particulates in the loop. A tentative test was performed after test, the water volume and purity in water tank were changed sequentially but the flow rate was sustained as a steady value. The pressure drop of FA was also maintained a stable value, as shown in Fig. 21. 3.4. Test observation Debris was observed to accumulate on the bottom of BN in a relatively uniform manner in the condition of that the flow passing through the BN was smooth and stable. The continuous hydraulic forces gradually caused late-arriving fibers to blanket over the pile of debris that accumulated earlier. However, the flow pattern pass-
Fig. 22. Surface morphology of debris bed during test Test HL-5.
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx
Fig. 23. Surface morphology of debris bed during test Test CL-1.
Fig. 24. Photographs of BN bottom and debris bed.
Fig. 25. Debris collection after test.
ing through the BN was various with different flow velocities. In the case of high flow velocity, the eddy phenomenon was obvious and could cause debris bed uneven. The formation process of debris bed was recorded during test HL-5 and CL-1, as shown in Fig. 22 and Fig. 23 respectively. The debris bed presented different morphology in different flow rates (flow patterns) on the BN. In HL break condition, the debris accumulated nonuniformly on the BN. The surface of the debris bed, which initially was somewhat wavy in appearance, took on a rugged look with clearly defined lumps and depressions. After adding the chemical precipitate, the debris bed was compressed notably. In CL break condition, debris accumulated on the BN in a fairly
uniform way after a few batches of fiber had been added into the loop, and there was only a single debris bed. The debris bed remained at this location and continued to develop as fiber was added. The surface of debris bed was flat and smooth, took on a fluffy look. The addition of chemical precipitate did not influence the number of debris bed. However, it resulted in a compaction effect on the debris bed. During these two tests, when the SiC particulate was added into tank, the water appeared to be cloudy immediately. The test loop gradually turned to be clearer after adding fiber, because the particulate continuously embedded into debris bed. Post-test observation of FA showed that fiber and particulate were captured primarily on the bottom of inlet nozzle in CL break condition and on the P-grid in HL break condition. Also, a respectable fiber was observed to be captured on the spacer grids and among fuel robs in HL break condition. The reason for this phenomenon might be that the high flow rate could drive debris further into the core. Fig. 24 shows the photographs of BN bottom and debris bed disengaged from BN after test. The flow paths of BN seemingly congested by debris, but there was enough flow could be improved and passed through debris bed into core during tests. A relatively uniform mixed debris bed could be collected after CL break condition test. The photo also revealed that particulate debris dispersed through the base of fiber bed. This was partially because the method by which debris was introduced to test loop allowed the debris constituents to commingle in water before the BN and gradually settle together. SiC particulate was visible as ‘‘inclusions” in a web of tightly woven fiber. Additionally, debris on different locations was collected upon separating FA from test vessel. Fig. 25 illustrated the accumulation characteristic of debris across the FA. In both break conditions, debris primarily deposited on the lower FA, minor debris could
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx
be collected from upper FA. However, much more debris was intercepted by BN in CL break condition than that in HL break condition. 4. Conclusion In-vessel downstream effect tests with a full-scale fuel assembly were performed to confirm that the head losses caused by debris in limiting break conditions meet the available driving head following a LOCA. Following conclusions could be summarized. Debris would not form a impenetrable debris bed on FA, and a sufficient driving force was available to maintain an adequate flow rate, and the LTCC capability was adequately maintained after LOCA. The hot-leg break condition in a concurrent addition method represented the most conservative results and should be used for testing designed to define debris limits. The parameters most affecting the magnitude of the head loss across the debris beds were the bed thickness and the bed porosity. The thickness of the bed depended on the quantity of debris in the bed and flow velocity passing through it. The debris was likely to transported into higher location in high flow rate. A nonuniform bed would not cause a head loss as high as an ideally uniform bed. The porosity depended primarily on the compression of the bed. The chemical precipitates acted an crucial role for core blockage that could cause a substantial pressure drop across a uniform fibrous debris bed if debris bed was not already saturated with particulate. When the debris bed became sufficiently compressed, further compression effect was limited by porosity of the debris bed. Acknowledgment This research is supported by the National Natural Science Foundation of China (Grant Nos. 11635005 and 11805068).
11
References Baier, S.L., et al., 2011. GSI-191 Fuel Assembly Test Report for PWROG. Westinghouse Electric Company LLC, WCAP-17057-NP Revision 1; accessible through U.S. NRCADAMS, Accession No. ML11293A103. Ding, M., Abdel-Fattah, A., et al., 2005. Characterization and Head-Loss Testing of Latent Debris from Pressurized-Water-Reactor Containment Buildings. U.S. Nuclear Regulatory Commission Office of Nuclear Regulatory Research, Washington DC, pp. 8–11. Geiger, Ervin, Klein, Paul, Smith, Stephen. Trip Report on Staff Observations of Generic Safety Issue (GSI)-191-Related Areva fuel blockage testing at continuum dynamics INC[R]. U.S. 2009. Geiger, Ervin. Trip Report Flow Blockage Testing of AREVA 17 x 17 Fuel Assembly at Continuum Dynamics Inc, ML090720039. Ewing New Jersey. December 17 and 18, 2008. Johns, R.C., Letellier, B.C., Howe, K.J., 2003. Small-scale Experiments: Effects of Chemical Reactions on Debris-bed Head Loss. U.S. Los Alamos National Laboratory, pp. 38–40. Lane, Ann E., Andreychek, Timothy S., et al., 2008. WCAP-16530-NP-A, Evaluation of Post-Accident Chemical Effects in Containment Sump Fluids to Support GSI191. U.S. NRC, pp. 66–70. Mattei, J.M., Hannon, J.N., Armand, Y., et al., 2004. Debris Impact on Emergency Coolant Recirculation. U.S. Nuclear Safety. NEI, 2004. Pressurized Water Reactor Sump Performance Evaluation Methodology, vol. 2, NEI 04-07 Revision 0; accessible through U.S.NRC ADAMS, Accession No. ML0505550156. Ruth, K.L., Baier, S.L., et al. Evaluation of Debris-Loading Head-Loss Tests for AP1000 Fuel Assemblies during Loss of Coolant Accidents, WCAP-17028-NP APP-MY03T2C-004, Revision 5.U.S. NRC, 2010. Sisk, Robert B., et al., 2011. Evaluation of Long-Term Cooling Considering Particulate, Fibrous and Chemical Debris in the Recirculating Fluid. Westinghouse Electric Company LLC, WCAP-16793-NP Revision 2; performed under PWROG Project Number PA-SEE-0312. Suh, Jeong Kwan, Kim, Jae Won, et al., 2015. Experimental study of pressure drops through LOCA-generated debris deposited on a fuel assembly. Nucl. Eng. Design 289, 49–59. Technical Letter Report on Evaluation of Chemical Effects; Studies on Precipitates Used in Strainer Head Loss Testing, January 30, 2008. NRC Contract #JCN 3216. [ADAMS Accession 0 Number: ML080600223]. Yu, L.I.U., Chun, L.I., Qinghua, L.I., 2008. Corrective actions study on issues of containment sump strainer clogging of NPP. Nuclear Safety 4, 42–45.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.anucene.2019.107037.
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Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies Da Wang, Fenglei Niu ⇑, Ruixian Liang, Weiqian Zhuo, Shengfei Wang Beijing Key Laboratory of Passive Safety Technology for Nuclear Energy, North China Electric Power University, Beijing 102206, China
a r t i c l e
i n f o
Article history: Received 16 February 2019 Received in revised form 15 July 2019 Accepted 7 September 2019 Available online xxxx Keywords: LOCA Fuel assembly Debris Pressure drop
a b s t r a c t A loss of coolant accident (LOCA) would generate debris from thermal insulation and other materials within the containment of a pressurized-water reactor (PWR). Some of the debris may pass through the strainer and accumulated on the fuel assembly (FA), causing resistance to flow for core cooling and could inhibit the LTCC capability of the plant. To address this safety issue, a quantitative assessment of the head loss across debris bed accumulated on a full-scale FA under limiting cases has been experimentally studied. Additionally, the formation of debris bed was also observed during tests to analyze its characteristics in different conditions. All tests results indicated that almost all debris was captured on the bottom nozzle (BN) and P-grid. Fiber acted as a critical role to form a filter bed. Fine particles include SiC particulate and chemical precipitate could make a ‘‘Compact effect” on debris bed, exerting a significant impact on pressure drop. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction A pipe break in the nuclear plant’s primary system leads to a rapid depressurization of the primary reactor system, thermal piping insulation and other materials (e.g. paint and concrete) in the vicinity of break would be dislodge by dynamic propagation of pressure wave followed by the ensuing vapor/water-jet impingement. The cracked materials with other latent impurity such as painting chips, coatings and concrete dirt would be transported by the flow in containment and eventually into the containment sump equipped specialized strainers. Most of debris could be captured by the strainers (Johns et al., 2003), however a portion of debris would pass through sump strainers and enter into the core, finally accumulated on the bottom nozzle and grids of FA, even on the surface of fuel rods, which could impeded the performance of ECCS and reduce the ability of the coolant to remove decay heat from the core. Debris transported to the FA has a tendency to form a uniform bed, could cause a significant head loss across the FA, the resulting additional resistance may reduce recirculation flow margin. If the sump pump cannot provide enough cooling water to a reactor core because of the debris blockage, this can lead to a serious consequence like core damage. To address this safety issue, the PWR owners group (PWROG) prepared WCAP-17057-NP Revision 1, ‘‘GSI-191 Fuel Assembly Test ⇑ Corresponding author.
Report for PWROG (Baier et al., 2011)” and submitted it to the United States Nuclear Regulatory Commission (U.S.NRC). This report describes the acceptance criteria for the mass of debris that can reach the RCS and not impede LTCC flows to the core. The NRC’s position is that plants must be able to demonstrate that debris transported to the sump screen after a LOCA will not lead to unacceptable head loss for the recirculation pumps, will not impede flow through the ECCS and CSS, and will not adversely affect the long-term operation of either the ECCS or the CSS (Sisk et al., 2011). The PWROG performed tests to evaluate in-vessel effects of debris and chemical precipitates that pass through the sump strainer following a LOCA. These tests were conducted by Westinghouse and AREVA on their respective fuel designs using various fiber, particulate, and chemical precipitate debris loads to examine the pressure drop across a single fuel assembly. The ultimate goal of the tests is to determine the maximum sump strainer debris bypass loads that result in acceptable pressure drops across the mock-up fuel assembly (Geiger et al., 2009). The NRC staff observed a Generic Safety Issue-191-related downstream effects fuel blockage test that simulated the down-flow conditions that occur during a cold-leg break in a Westinghouse 17 17 fuel assembly, the length of the test assembly was about 48 in., approximately one-third the length of an actual fuel assembly. Comparison of pressure drop measurements between the top nozzle/1st grid support and the full assembly indicated that the initial collection of debris occurred at the top of the fuel assembly but quickly distributed over additional spacer grids. At no time during the test
E-mail address: [email protected] (F. Niu). https://doi.org/10.1016/j.anucene.2019.107037 0306-4549/Ó 2019 Elsevier Ltd. All rights reserved.
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did the pressure drop exceed the acceptance limit of 13 psi across the fuel assembly (Ruth et al., 2010). Also the NRC staff observed fuel blockage testing conducted on AREVA fuel assemblies by Continuum Dynamics Inc. The observed mock-up fuel assembly test with scaled flow rate, scaled debris load, scaled 1/3 height mockup fuel assembly. As expected, NRC staff observed that the pressure drop across the fuel assembly did not approach the acceptance limit of 13 psi. Also, chemical precipitate addition did not result in a large increase in pressure drop across the mock-up fuel assembly (Geiger, 2008). Furthermore, in-vessel downstream effect tests with a mock-up PLUS7 FA for the advanced power reactor (APR) 1400 were performed by KHNP Central Research Institute to confirm that the head losses caused by debris meet the available driving head following a LOCA. All the test results showed lower pressure drops than the available head limits and a sufficient driving force is therefore available to maintain an adequate flow rate, and the LTCC capability is adequately maintained in the APR1400 (Suh et al., 2015). However, few studies have been done on debris impact on full length fuel assembly and little attention has been devoted to characteristics of debris bed accumulated on the BN. In this study, a downstream effect program was performed to quantify the head loss across the FA under the debris loading conditions of a postulated LOCA and determine the maximum pressure drop in a limiting case. Additionally, the test loop was intended to test the debris capture characteristics on a full-area FA. The mockup FA was full scale in all dimensions include height and structures. There are total 8 spacer grids along the longitudinal direction with a bottom/top nozzle component and 15*15 fuel rods. Moreover, the fuel rod diameter is 10.0 mm and the pitch is 13.3 mm. The BN which is a box-type structure contains a square shape of plate with orifices. Over the plate, there is a filter screen used to prevent debris from passing through it. 2. Experimental method The experimental method is to simulate appropriate PWR containment debris accumulation and head-loss conditions, with flexibility for controlling local flow conditions, debris quantity and other important parameters and with the capability of taking applicable measurements and visual observations of the phenomena under examination. 2.1. Experimental facility
The experimental facility is illustrated schematically is shown in Fig. 1, and a photo of the facility with a 3D layout is shown in Fig. 2. The experimental facility is mainly composed of five parts: a test column, mixing tank system, collection system, circulation system, and control and monitoring system. The test column contains a full-scale FA in a 3.5 m height and steel vessel with a lower/upper plenum. Adequate transparent acrylic-windows were installed in some region of vessel for viewing debris accumulation on FA during the test. The lower plenum was designed as a pyramid structure to preclude debris settling. There was a perforated baffle plate between FA and lower plenum for distributary. As debris caught on the FA, the differential pressure is measured constantly across the lower/upper half fuel assembly and the entire fuel assembly. There are extra ports available on the sides of the test column if a measure of the differential pressure across a specific portion of the fuel assembly as required. The mixing tank is mainly manufactured as stainless steel with cylindrical shape and several transparent acrylic window with scaleplate for observing water level and turbidity. A debris stirring tool is installed downward vertical at the top of tank to mix debris well and preclude the settling on the bottom of the tank. The collection system is composed of a collecting tank manufactured as transparent acrylic material with a cubic shape and relevant pipes and strainer with different size filter screen. The collection tank is placed under the mixing tank, in that way, the collect tank could collect debris solution from mixing tank and filtrate debris by gravity head. Thus the quantitative study of debris without being captured on FA could be analyzed. The circulation system pumps water from mixing tank, through the circulation piping and test loop, finally back into the mixing tank. The butterfly flow control valve downstream of the recirculation pump was set to a precalibrated position to generate an average flow rate in the loop that produces the desired approach velocity at the fuel assembly. An electromagnetic flow meter measures the flow rate and provides feedback to the control system to maintain a constant flow rate. A bypass loop is used to prevent pump damage from serious debris blockage. The control system regulates the flow rate and water temperature. The monitoring system records the transient data of pressure drop (dP), flow rate, and water temperature in test loop. The monitoring system is also used to check the slope of dP versus time graph in order to evaluate whether the dP meets a steady state condition. As debris caught on the FA, the differential pressure was measured constantly across various locations including the
A closed-loop hydraulic test apparatus was designed and used to measure the pressure drop and flow rate across a mock-up FA. Stirrer Fuel Assembly
T
T
Mixing Tank
Pump
dP3 Heater
dP4 T
Bypass Strainer Collecting Tank
dP2
dP1 T
Pump Flow Meter
Fig. 1. Schematic diagram of experimental facility.
Fig. 2. Three-dimensional diagram of experimental facility with photo of components.
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx Table 1 Debris types and amounts per fuel assembly.
Table 2 The bypass fiber length distribution ratio.
Debris types
Transported to strainer (kg)
Assumed bypass debris (kg)
Per FA (g)
Fiber Latent fiber Coating debris Latent particle Chemical precipitates
168 13.7 23.4 77.4 84 (maximum in 30 days)
54.51
450.5/ 100 833.06
23.4 77.4 84
Length
proportion
<0.5 mm 0.5 mm–1.0 mm >1.0 mm
67.1% 8.9% 24.0%
694.7
show a particulate debris surrogate of SiC power and an SEM image.
lower half fuel assembly (dP2), upper half fuel assembly (dP3) and the entire fuel assembly (dP4). The differential gauge of dP1 measured the pressure drop across the BN and P-grid. 2.2. Debris preparation During the LOCA event, debris transported to containment sump mainly include fiber insulation material, paint coating, chemical impurity and latent debris (Yu et al., 2008). The debris types and amounts are summarized in Table 1. All the debris except fiber transported to the sump was assumed to bypass the strainer. In all tests, the circulating coolant may entrain debris that can be categorized as particulate, fiber, and chemical precipitates4. In a homogeneous debris bed, the densities and size characteristics of the individual constituents are necessary to determine the porosity of the debris bed. In general, the lower bound values for the characteristic sizes of the debris were adopted. This is conservative for head loss calculations because the specific surface is inversely proportional to the characteristic size of the debris particle. The smaller the characteristic size of the debris bed constituents the higher the pressure drop across the debris bed (Mattei et al., 2004). 2.2.1. Particulate debris There are several sources of particulate debris. Paints and coatings might be broken up into flakes, or be reduced to their basic constituents such as micron-sized powders. Solid insulation, such as CalSil, may contain particles, again with a potential for reduction to micron-sized elements if they are broken up to their smallest basic units. The NRC Safety Evaluation (SE) for NEI 04-07 (NEI, 2004) identified particle size as a key parameter for the selection of representative debris. Specifically, the SE states that major contributors to head loss are the increasing smaller particles. All testing was conducted with silicon carbide particles approximately 10 lm in size, allowing the debris to act as a fine particulate debris that collects within a fiber bed and results in a maximum head loss. Silicon carbide is representative of all particulate debris. Fig. 3
2.2.2. Fiber debris The steam jet in high temperature and pressure could cause damage to piping thermal insulation, which would be split into fiberglass pieces, mostly individual fibers with a high mobility act as a crucial component to build up a filter bed, causing ‘‘Thin bed effect ” (Suh et al., 2015). Once a filter bed is created, there is potential to progressively trap smaller and smaller particles until the flow is effectively impeded. NUREG/CR-6877 indicates that the latent fiber can be substituted by fiberglass during the test (Ding et al., 2005). A sump strainer bypass test about fiber debris was performed primarily, and the bypass fiber length distribution ratio is shown in the Table 2. Fig. 4 shows the SEM image of fiber. The fibers do not appear circular but somewhat ribbon-like, with the narrower dimension significantly smaller than 15 mm. Smaller fibers also can be seen. 2.2.3. Chemical precipitates Chemical reactions occur when hot borated water sprays over the containment. More important reactions occur when many constituents reside together over days in the containment sump, which is usually buffered to increase the pH. One important product is sodium aluminum silicate (SAS, NaAlSi3O8), which can precipitate into micron-sized elements and lead to complete clogging of a filter bed if it accumulates in or on it. Therefore only NaAlSi3O8 was conservatively used in the test. The properties, particularly the size, of the precipitates depend on the chemical and temperature history. Thus, NaAlSi3O8 precipitate was prepared in strict accordance with procedure of WCAP-16530-NP (Lane et al., 2008). The concentration of NaAlSi3O8 did not exceed 11 g/L and the settling tests were also performed to ensure it met the settling criteria defined by WCAP-16530-NP-A. Fig. 5 shows the chemical precipitates for the test and an image of SEM. 2.3. Experimental conditions In the event of a hot-leg (HL) break of double ended guillotine, the ECCS liquid must pass through the core to exit the break and no spilling of ECCS liquid occurs. Therefore, an additional driving head
Fig. 3. Particulate debris surrogate of SiC powder.
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx
Fig. 4. Morphology image of fiber.
Fig. 5. NaAlSi3O8 precipitate and SEM image.
from the build-up of liquid level in the downcomer and in the steam generator tubes to the spillover elevation is present. Thus the maximum flow rate to per fuel assembly is expected to be 9.917 m3/h to obtain the maximum pressure drop at the test column. However, the higher flow rate can also result in faster debris build-up and there is more debris available to accumulate. So, the HL break condition represents the conservative case in terms of debris loading. For postulated cold-leg (CL) breaks, most of the ECCS liquid spills directly out of the break location, which decreases the driving head of core flow to a minimum. The flow to the core is selected as 2.5 m3/h which is required to make up for core boiling to remove the decay heat. With different debris mobility, the fiber was easier to trapped by sump strainers in low flow rates, thus less fiber debris would be transported to the core in lower flow velocity than HL break. Furthermore, to create the worst possible scenario, the limiting break case for modeling purposes will be a combined condition by HL break and CL break. Namely, a limiting flow at the core inlet, combined with more debris loads that occurs for a high flow HL break. This ensured the development of debris beds with the maximum resistance and highest pressure loss. In prior testing programs the manner in which the debris is added was observed to make a difference in the overall head-loss through the FA. Therefore, two debris additions methods were used in this study: For sequential debris addition tests, the all SiC particulates was added in one batch; the fiber was divided into several batches; surrogates for chemical reaction products were added to the test loop after all of the fiber and particulate had been added. For concurrent debris addition tests, the three types of debris in the amount prescribed in the test procedure were well mixed in water until completely suspended and fine enough that it dispersed across the pipe cross section, thereby approaching the BN
relatively uniformly and subsequently producing a uniform debris bed. The debris mixture was introduced into the test loop by slowly pouring the bucket contents into the top of the test section. Tap water was used to simulate the post-accident coolant, because this condition was expected to be conservative relative to actual reactor coolant condition (Baier et al., 2011). Additionally, the previous tests demonstrated that surrogates made with highpurity water were not as effective or as stable as those made with tap water (Technical Letter Report on Evaluation of Chemical Effects, 2008). Thus, the tap water was used in this study. 3. Results and analysis The recirculation pump was turned on and allowed to run for 10 min before test, allowing air to be flushed out of recirculation loop piping. Benchmark tests as comparison with debris tests were performed firstly before adding debris. Fig. 6 shows the typical pressure drop behavior of fuel assembly without debris. These benchmark-test results presented a repeatable character. Flow variation tests were performed once the debris bed was established. The test matrix is described in Table 3. Three different cases in HL break condition and two different cases in CL break condition were performed. Also the same case had been repeated and the results presented good repeatability within an acceptable range. The HL break condition in concurrent adding method presented a highest pressure drop than other tests. 3.1. Hot-leg break condition During the HL break condition tests, the flow rate was constantly sustained around 9.917 m3/h. The typical pressure drop
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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dP
Allometric1 y = a*x^b 1.65459 0.99827 Value Standard E a 0.038 0.00384 b 1.465 0.05094
0.4
50
Flow rate Total FA Lower FA Upper FA BN/P-grid Particulate Fiber Chemical
8
3
Model Equation Reduced ChiAdj. R-Squar
10
Flow rate(m /h)
Pressure drop (kPa)
0.8
0.6
60
HL-1 HL-3 HL-5 CL-1
6
40
30
4 20 2
Pressure drop(kPa)
1.0
10
0
0
0.2
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Time (h)
Fig. 7. Pressure drop under the HL break condition in concurrent adding method with 100 g fiber (Test HL-1).
0.0 2
3
4
5
6
7
8
Flow rate (m3/h) Fig. 6. Results of benchmark tests.
60
Flow rate Total FA Lower FA Upper FA BN/P-grid
8
Flow rate(m3/h)
behavior in concurrent adding method with 100 g of fiber is shown in Fig. 7. These debris was equally divided into 12 batches. The particulate, fiber and chemical precipitant were represented by black dot, dark yellow triangle and navy square respectively in the following figures respectively. The pressure drop increased notably when the first batch and the second batch were added into water tank. However, the each addition of subsequent debris did not remarkably affect the pressure drop as the first or second batch did. The pressure drops among upper FA, lower FA, BN/P-grid increased gradually with the debris addition, especially in lower FA, which took up more than 70% of head loss compared with total FA. This trend is also shown in Fig. 8, which indicated that the debris distributed in a relatively decentralized manner among FA in concurrent adding method with high flow rate. Fig. 9 shows the typical pressure drop behavior in concurrent adding method with 450.5 g of fiber. This test conditions were all same with test HL-1 except the mass of fiber. Similarly, the pressure drop increased sharply when the first two batches were added into tank and the pressure drop caused by each addition of subsequent debris was much lower than the first two batches. Moreover, the pressure drop caused by first two batches of debris in this test was 50.75 kPa, almost equal to the final pressure drop in test HL-1, which demonstrated that the fiber debris had a significant impact on head loss. This trend is also illustrated clearly in Fig. 10. The maximum pressure drop recorded under the HL break condition was 88.37 kPa, which was much lower than the available head limit. The lines of lower FA and upper FA break as shown in Fig. 9, because the pressure drop of these two location exceeded the full-scale value of the gauge. Thus these data was abandoned.
40
6
4
20
Pressure drop(kPa)
10
2
0 0
2
4
6
8
0 12
10
Batch Fig. 8. Pressure drop versus debris adding batch (Test HL-1).
The typical pressure drop behavior in HL break condition with sequential adding method is shown in Fig. 11, which provides the limiting head loss curves. When the test started, all SiC particulate was added into the mixing tank, and the system was allowed to equilibrate for a while. The particulate debris had negligible effects on pressure drop, as shown in Fig. 11. Fiber was then add added in amounts of 10 g form first batch to 10th and 40 g/batch from 11th to 18th, the last batch was 30.5 g. The pressure drop increased gradually during the addition of fiber and the rising rate of pressure drop depended on the amount of debris. After the first addition of chemical precipitate, the pressure drops increased considerably, and subsequence addition of chemical did not affect the pressure drop. A likely reason for this trend is that there was a
Table 3 Test matrix. Test-ID
Flow rate (m3/h)
SiC (g)
Fiber (g)
NaAlSi3O8 (g)
P/F ratio
Adding method
DPmax (kPa)
Benchmark HL-1 HL-2 HL-3 HL-4 HL-5 HL-6 CL-1 CL-2 CL-3 CL-4
2–8 9.917 2–9 9.917 2–9 9.917 2–9 2.5 2–9 2.5 2–9
– 833.06 833.06 833.06 833.06 833.06 833.06 833.06 833.06 833.06 833.06
– 100 100 450.5 450.5 450.5 450.5 100 100 450.5 450.5
– 694.21 694.21 694.21 694.21 694.21 694.21 694.21 694.21 694.21 694.21
– 8.33 8.33 1.85 1.85 1.85 1.85 8.33 8.33 1.85 1.85
– Concurrent – Concurrent – Sequential – Sequential – Concurrent –
0.8 50.53 – 88.37 – 82.37 – 32.76 – 64.46 –
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx 100
100 10
10
2
0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
6
80
60
40
4
20
2
0
0
20
0
4.5
1.0
1.5
2.0
Fig. 9. Pressure drop under the HL break condition with concurrent adding method (Test HL-3).
3.0
Fig. 11. Pressure drop under the HL break condition with sequential adding method (Test HL-5).
HL-1(Fiber 100g) HL-3(Fiber 450.5g)
80
Pressure drop (kPa)
Pressure drop (kPa)
2.5
Time (h)
Time (h)
80
Pressure drop (kPa)
40
4
3
60
Flow rate (m /h)
8
Pressure drop (kPa)
3
Flow rate (m /h)
6
Flow rate Total FA Lower FA Upper FA BN/P-grid Particulate Fiber Chemical
80
Flow rate Total FA Lower FA Upper FA BN/P-grid Particulate Fiber Chemical
8
60
40
dP of total FA
NaAlSi3O8
60
Fiber 40
SiC 20
20
0
0
0
2
4
6
8
10
12
0
300
600
Batch Fig. 10. Pressure drop versus debris adding batches in test HL-1 and HL-3.
uniform debris bed could be compacted by chemical precipitates. This phenomenon is called ‘‘Compaction effect”, which may cause a higher head loss. When the vast mass of smaller particulates passing through the uniform debris bed, the bed porosity decreased substantially because of bed compression and reduce of interfiber spacing, with a corresponding increase in the particulate filtration rate and the entrapment of the finer particulate. Once the debris bed was already saturated with fine particles, the extra chemical precipitate addition had no effect on pressure drop, further compression was limited by the density of the particulates. After the first addition of chemical precipitate, the pressure drop of BN/P-grid decreased sharply, a probable explanation is that the debris bed at BN/P-grid could not support that pressure and portion of debris bed was destroyed and separated. Thus the pressure drop decreased subsequently. In Fig. 11, the pressure drops in full height (Total FA) are very similar to those in lower FA on which most of debris deposited, especially in the BN/P-grid. After injecting all types of debris, the pressure drop across upper FA was negligible, which was different from the test results with concurrent adding methods. Fig. 12 displays the relationship between debris loads and pressure drop in test HL-5. As the figure shows, even though the mass of SiC reached at 833.06 g, there was negligible effect on pressure drop. However, once the fiber debris was added into loop, the pressure drop increased apparently and with the next addition of fiber, the pressure drop increased continuously. Only the first batch of
900
1200
1500
1800
Debris loads (g) Fig. 12. Pressure drop versus debris loads in test HL-5.
chemical precipitates made an obvious increase on pressure drop. This trend pointed out that the formation of a fiber bed at the FA serves as a collector for particulates which in turn result in an increase in pressure drop across the FA. Without a fiber bed, particulates that are transported into the FA are sufficiently small that they will not collect at BN, the debris flows freely throughout the FA. Without a fiber bed to capture particulates, there is no impact on the head loss at FA. Also, the chemical precipitates act an crucial role for core blockage that can cause a significant pressure drop across a uniform fibrous debris bed if debris bed is not already saturated with particulate. Fine particles could fill internal pores within the fibrous bed. Fig. 13 presents the comparison of pressure drop behavior between concurrent adding method and sequential adding method. Although their rising trends were different, the final total pressure drop was almost close. 3.2. Cold-leg break condition During the CL break condition tests, the flow rate was constantly sustained around 2.5 m3/h. The typical pressure drop behavior in CL break condition with sequential adding method is shown in Fig. 14, which was similar to those observed during the test HL-5. All the SiC particulate was added to test loop at one time and had negligible effect on pressure drop. The 100 g of fiber was equally divided into 10 batches. Pressure drop gradually increased
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx 100
35
HL-3 Concurrent HL-5 Sequential
dP of total FA
30
Pressure drop (kPa)
Pressure drop (kPa)
80
60
40
25
NaAlSi3O8
20
Fiber 15
SiC 10
20
5 0 5
10
Batch
15
20
0
25
0
1000
1200
1400
1600
60
Flow rate Total FA Lower FA Upper FA Particulate Fiber Chemical
3
Flow rate (m /h)
2
1
40
20 0
0 2
4
6
Time (h)
8
10
Fig. 16. Pressure drop under the CL break condition in concurrent adding method with HL debris loading (Test CL-3).
100
HL-3 Q=2.502m3/h CL-3 Q=9.917m3/h 80
60
40
30
20
Pressure drop (kPa)
20
Flow rate Upper FA Total FA BN/P-grid Lower FA Particulate Fiber Chemical
3
800
80
Pressure drop (kPa)
40
Flow rate (m /h)
600
Fig. 15. Pressure drop versus debris loads in test CL-1.
with the addition of each batch of fiber. After adding the first batch of chemical precipitate, pressure drop had a steep elevation immediately, with a corresponding sharp decrease in the flow rate. However, after that the pressure drop decreased obviously, a likely reason for this trend is that with increasing of pressure and particulates load in recirculation loop, debris bed could not sustain its structure. Some fine particles would penetrate the debris bed, with a corresponding decrease in the particle filtration efficiency, thus causing a decrease in flow resistance. Before adding the second batch of chemical debris, there was a significant increase on pressure drop, with a corresponding notable drop on flow rate. The erratic behavior associated with this test is probably due to transient debris bed behavior as the lumps of debris disassociate or disintegrate with flow turbulence. Disintegrated debris subsequently deposited in the bed again, whereby the bed tended to become more uniform, thus incurring increased head loss. The second batch addition of chemical precipitate caused a slight increase on pressure drop. Pressure drop curves of total FA, lower FA and BN/ P-grid are nearly overlap during the whole test, which indicated that almost all debris was trapped in the BN/P-grid. Fig. 15 presents the relationship between debris loads and pressure drop in test CL-1. Same situation with test HL-5, the addition of SiC made barely effect on pressure drop. The pressure drop increased linearly with addition of fiber debris. However, the first batch of chemical precipitates clearly resulted in a substantial increase on pressure drop, as shown in Fig. 15, which indicated that ‘‘Compaction effect” was more prominent in CL condition than that in HL condition. The reason for this is probably that the fibrous
1
400
Debris loads (g)
Fig. 13. Pressure drop versus debris adding batches in test HL-3 and HL-5.
2
200
Pressure drop (kPa)
0
10
0 0
2
4
6
8
10
12
Batch Fig. 17. Pressure drop versus debris adding batches in test HL-3 and CL-3.
0
1
2
3
4
5
6
0 7
Time (h)
Fig. 14. Pressure drop under the CL break condition with sequential adding method (Test CL-1).
bed formed in low velocity was more uniform and the interfiber space was more broader. Fig. 16 shows the typical pressure drop behavior in CL break condition with hot-leg break debris loading in concurrent adding method. Similar with test HL-1 and HL-3, each batch addition of
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60
30 4 20
60 3
Flow rate (m /h)
40 6
80
Flow rate Total FA Upper FA Lower FA
6
Pressure drop(kPa)
3
Flow rate(m /h)
50
4 40
2
Pressure drop (kPa)
Flow rate Total FA Upper FA Lower FA
8
20
2
10
0 5.2
5.3
5.4
5.5
5.6
5.7
5.8
0 6.0
5.9
0
0 7.6
Time (h)
7.8
8.0
8.2
8.4
Time (h)
Fig. 18. Transient Head Loss Curve for a Formed Debris Bed (Test HL-2/CL-2).
100
80
y = Intercept + B1*x^1 + B2*x^2
Equation
40
No Weighting Weight 10.84332 Residual Sum of Squares 0.99892 Adj. R-Square Value
30 Pressure drop
Intercept B1 B2
0 2.27029 0.30891
Pressure drop (kPa)
Pressure drop (kPa)
Decreasing Velocities Increasing Velocities Polynomial Fit of test data
Decreasing Velocities Increasing Velocities Polynomial Fit of test data
50
Standard Error -0.16913 0.02261
20
y = Intercept + B1*x^1 + B2*x^ 2 Weighting No Weight 27.16239 Residual Sum of Squares 0.99918 Adj. R-Squar Equation
60
Value Pressure drop
Intercept B1 B2
0 3.2599 0.5543
Standard Erro -0.21256 0.02709
40
20
10
0
0 0
1
2
3
4
5
6
7
8
9
0
10
2
4
6
8
10
Flow rate (m3/h)
Flow rate (m3/h) Decreasing Velocities Increasing Velocities Polynomial Fit of test data
80
y = Intercept + B 1*x^1 + B2*x^2 No Weighting Weight 22.53191 Residual Sum of Squares 0.99931 Adj. R-Square
Pressure drop (kPa)
Equation
60
Value Intercept Pressure drop B1 B2
0 5.63579 0.23877
Standard Error -0.19547 0.02494
40
20
0 0
2
4
6
8
10
Flow rate (m3/h) Fig. 19. Head loss versus flow rate (Test HL-2/4/6).
debris caused a obvious increase on pressure drop, especial the first and second batch. However, the pressure drop increased slowly during the test, with a corresponding decreasing in flow rate. The trend could be explained by that with low flow velocity, the debris was transported much slower than hot-leg break condition in test loop. Furthermore, the pressure drop in lower FA is very similar with that in total FA, which is different from test HL-1 and HL-3. It indicated that the debris was easier to trapped by lower FA in cold-leg break condition no matter what methods the debris was added into loop. Fig. 17 provides the comparison of pressure drop behavior between test HL-3 and CL-3. These two pressure drop increasing trend were similar. However, the final total pressure drop of test CL-3 with 2.5 m3/h flow rate was 66.46 kPa, which accounted for 72.9% of final total pressure drop of test HL-3 with
9.917 m3/h flow rate. Thus, the flow resistance under CL break condition was much higher than that under HL break condition. A likely reason for this is that debris was scattered among FA in a high flow rate and a single debris bed in a low flow rate with the same amount of fiber gives rise to increased flow resistance by higher packing effects. Likewise, the maximum pressure drop recorded under the CL break condition was lower than the available head limit. 3.3. Flow variation tests Once the debris bed was established, the flow velocity was gradually altered so that the pressure drop could be measured for various flow velocities for the same debris bed. The general idea
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037
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50
Increasing Velocities Decreasing Velocities Polynomial Fit of test data
80
y = Intercept + B 1*x^1 + B2*x^2 No Weighting Weight Residual Sum 81.22011 of Squares 0.99513 Adj. R-Square Equation
Value Intercept Pressure drop B1 B2
0 9.43965 0.00209
Pressure drop (kPa)
Pressure drop (kPa)
60
Increasing Velocities Decreasing Velocities Polynomial Fit of test data
Standard Error -0.87141 0.15709
40
30
60
40
Equation
y = Intercept + B1*x^1 + B2*x^2
No Weighting Weight 1679.12032 Residual Sum of Squares 0.96813 Adj. R-Square
20
Value Standard Error 0 -20.05927 3.56427 -1.14489 0.69892
Intercept Pressure drop B1 B2
20 10 1
2
3
4
Flow rate (m3/h)
5
6
7
2
3
4
Flow rate (m3/h)
5
6
Fig. 20. Head loss versus flow rate (Test CL-2/4).
3
2
Stop Adding
4 Upper FA Lower FA Total FA Flow rate Water Level
Stop
Drain Water Only
1
2
Flow rate (m3/h)
Pressure drop (kPa)
6
Add and Drain Simultaneously Add Water Only
0
0 0
500
1000
1500
2000
2500
3000
3500
Time (s) Fig. 21. The impact of water purity on pressure drop.
was to decrease/increase the flow, then wait until the head loss became relatively stable, record the head loss and flow, and then move on to the next velocity. Once the flow rate reached at 2 m3/h or 9 m3/h, the flow was sequentially decreased/increased with the data recorded on the descent/ascent, as well. These processes are shown schematically in Fig. 18, respectively. The head losses were somewhat less when the flow velocity was being decreased than when the velocity was increasing. A similar trend was also observed for the CL break condition tests. The reasons for this behavior are not clear, but the effect is not compression hysteresis. It may be possible that the debris bed was disturbed and subsequently shifted into a less uniform configuration so that its resistance to flow was lessened. This trend is also
illustrated in Figs. 19 and 20, which plots pressure drop as a function of the flow rate. In these figures, the velocity direction is illustrated with an arrow. As these figures shown, this trend was slight in HL break conditions, however it was relatively obvious in CL break conditions. Thus, the stability of the debris bed is an important consideration when analyzing these tests data. It will be apparent that debris beds reoriented following changes in the pump flow if the debris bed was not steady enough. Debris bed formed in low velocity is relatively more susceptible to movement if the bed is perturbed. An increase in turbulence was shown to enhance the disintegration of the debris bed, so that the debris bed changed either slowly or suddenly when the pump flow was altered in low velocity. After test, the water in the loop turn to be cloudy because of slight disintegration of debris bed caused by turbulence of pump flow. It indicated that plenty of particulates were suspended or transported with flow in the loop and could not be trapped by debris bed accumulated on the BN. The filtration rate of debris bed already attained to a stable value in the specific flow rate, although there were still plentiful particulates in the loop. A tentative test was performed after test, the water volume and purity in water tank were changed sequentially but the flow rate was sustained as a steady value. The pressure drop of FA was also maintained a stable value, as shown in Fig. 21. 3.4. Test observation Debris was observed to accumulate on the bottom of BN in a relatively uniform manner in the condition of that the flow passing through the BN was smooth and stable. The continuous hydraulic forces gradually caused late-arriving fibers to blanket over the pile of debris that accumulated earlier. However, the flow pattern pass-
Fig. 22. Surface morphology of debris bed during test Test HL-5.
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx
Fig. 23. Surface morphology of debris bed during test Test CL-1.
Fig. 24. Photographs of BN bottom and debris bed.
Fig. 25. Debris collection after test.
ing through the BN was various with different flow velocities. In the case of high flow velocity, the eddy phenomenon was obvious and could cause debris bed uneven. The formation process of debris bed was recorded during test HL-5 and CL-1, as shown in Fig. 22 and Fig. 23 respectively. The debris bed presented different morphology in different flow rates (flow patterns) on the BN. In HL break condition, the debris accumulated nonuniformly on the BN. The surface of the debris bed, which initially was somewhat wavy in appearance, took on a rugged look with clearly defined lumps and depressions. After adding the chemical precipitate, the debris bed was compressed notably. In CL break condition, debris accumulated on the BN in a fairly
uniform way after a few batches of fiber had been added into the loop, and there was only a single debris bed. The debris bed remained at this location and continued to develop as fiber was added. The surface of debris bed was flat and smooth, took on a fluffy look. The addition of chemical precipitate did not influence the number of debris bed. However, it resulted in a compaction effect on the debris bed. During these two tests, when the SiC particulate was added into tank, the water appeared to be cloudy immediately. The test loop gradually turned to be clearer after adding fiber, because the particulate continuously embedded into debris bed. Post-test observation of FA showed that fiber and particulate were captured primarily on the bottom of inlet nozzle in CL break condition and on the P-grid in HL break condition. Also, a respectable fiber was observed to be captured on the spacer grids and among fuel robs in HL break condition. The reason for this phenomenon might be that the high flow rate could drive debris further into the core. Fig. 24 shows the photographs of BN bottom and debris bed disengaged from BN after test. The flow paths of BN seemingly congested by debris, but there was enough flow could be improved and passed through debris bed into core during tests. A relatively uniform mixed debris bed could be collected after CL break condition test. The photo also revealed that particulate debris dispersed through the base of fiber bed. This was partially because the method by which debris was introduced to test loop allowed the debris constituents to commingle in water before the BN and gradually settle together. SiC particulate was visible as ‘‘inclusions” in a web of tightly woven fiber. Additionally, debris on different locations was collected upon separating FA from test vessel. Fig. 25 illustrated the accumulation characteristic of debris across the FA. In both break conditions, debris primarily deposited on the lower FA, minor debris could
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D. Wang et al. / Annals of Nuclear Energy xxx (xxxx) xxx
be collected from upper FA. However, much more debris was intercepted by BN in CL break condition than that in HL break condition. 4. Conclusion In-vessel downstream effect tests with a full-scale fuel assembly were performed to confirm that the head losses caused by debris in limiting break conditions meet the available driving head following a LOCA. Following conclusions could be summarized. Debris would not form a impenetrable debris bed on FA, and a sufficient driving force was available to maintain an adequate flow rate, and the LTCC capability was adequately maintained after LOCA. The hot-leg break condition in a concurrent addition method represented the most conservative results and should be used for testing designed to define debris limits. The parameters most affecting the magnitude of the head loss across the debris beds were the bed thickness and the bed porosity. The thickness of the bed depended on the quantity of debris in the bed and flow velocity passing through it. The debris was likely to transported into higher location in high flow rate. A nonuniform bed would not cause a head loss as high as an ideally uniform bed. The porosity depended primarily on the compression of the bed. The chemical precipitates acted an crucial role for core blockage that could cause a substantial pressure drop across a uniform fibrous debris bed if debris bed was not already saturated with particulate. When the debris bed became sufficiently compressed, further compression effect was limited by porosity of the debris bed. Acknowledgment This research is supported by the National Natural Science Foundation of China (Grant Nos. 11635005 and 11805068).
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.anucene.2019.107037.
Please cite this article as: D. Wang, F. Niu, R. Liang et al., Assessment of head loss through LOCA-generated debris deposited in PWR fuel assemblies, Annals of Nuclear Energy, https://doi.org/10.1016/j.anucene.2019.107037