Research for the heat leakage caused by gaps on barrel insulation structure of reactor pressure vessel

International Journal of Advanced Nuclear Reactor Design and Technology xxx (xxxx) xxx

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Research for the heat leakage caused by gaps on barrel insulation structure of reactor pressure vessel Qiu Tian, Luo Ying, Qiu Yang*, Li Yu-guang, Xie Guo-fu, Yang Li-cai Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu, Sichuan Province, 610213, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 April 2019 Received in revised form 3 November 2019 Accepted 4 November 2019 Available online xxx

Due to the function requirements of the Cavity Injection and Cooling System, the barrel insulation structure of HPR1000 reactor pressure vessel has been greatly improved compared with the previous projects. In the past, the empirical formula method was used to evaluate the thermal properties of the insulation, but the influences of the support structure and the excessive gap on insulation are difficult to evaluate. In this paper, the fitting formula of the equivalent thermal conductivity and the qualitative temperature of the insulation have been obtained firstly through the heat transfer test. Then, ANSYS Fluent was used to simulate the barrel insulation structure with gaps of HPR1000. A circumferential 15
model was established to obtain evaluation method for flow leakage with gaps in hot state, and then a whole model of the barrel insulation structure with gaps was established to obtain heat loss of insulation based on calculation of flow leakage. The sensitivity of gap size and ventilation parameters to heat leakage was further analyzed. Finally, it was indicated that, by minimizing joint gap length, setting of overlaps, controlling cold gap, optimizing structure of insulation support, the effect of heat leakage caused by gaps can be reduced. © 2019 Xi'an Jiaotong University. Published by Elsevier B.V. This is an open access article under the CC BYNC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Barrel insulation structure Simulation analysis Gap Flow leakage Heat leakage

1. Introduction HPR1000 is the 3rd generation nuclear power technology of million kilowatts independently developed by China National Nuclear Corporation. It has advanced design features such as 177 reactor core, CF3 advanced fuel assembly, active & passive safety systems, comprehensive prevention and mitigation measures for severe accident and so on. Metal reflective insulation covers the whole reactor pressure vessel (RPV) of HPR1000. As shown in Fig. 1, barrel insulation, bottom head insulation, insulation support and flow liner comprise the barrel insulation structure. HPR1000 adopted the concept of active & passive safety, which meets the safety requirements for the 3rd generation nuclear power plant. Meanwhile, Cavity Injection and Cooling System (CIS) is designed to control the risk of core melting. Moreover, barrel insulation

* Corresponding author. E-mail address: [email protected] (Q. Yang).

Production and Hosting by Elsevier on behalf of KeAi

structure and RPV outer surface form a specified annular passage. In the case of severe accident, the cavity cooling water would be injected into the annular passage through the injection pipe at the bottom of the insulation, thus to cool the RPV and maintain its integrity. Correspondingly, some new requirements are put forward to the design of barrel insulation structure: - CIS injection pipe and water-steam vent set on the insulation; - A sufficient annular passage between insulation and RPV; - To keep the integrity of annular passage, flow liner is added between insulation and RPV, and fixed to reactor pit by insulation support. Improvements on the structure design take great influence on thermal property of insulation. Besides the restraint of heat loss, concrete temperature limit shall also be considered in the design of insulation. Otherwise, improvements on the structure design and adjustment on the installation procedure will bring great difficulty to on-site installation and increase the risk of dimensions unconformity and cold gap unconformity problems. For previous projects without CIS, only empirical formula method combined with heat transfer test were used to evaluate the thermal properties of the insulation. Based on the single model of

https://doi.org/10.1016/j.jandt.2019.11.001 2468-6050/© 2019 Xi'an Jiaotong University. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).

Please cite this article as: Q. Tian et al., Research for the heat leakage caused by gaps on barrel insulation structure of reactor pressure vessel, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.11.001

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Fig. 2. Sketch of metal insulation structure.

- Total average heat flux of RPV insulation outer surface 235 W/ m2; - Service temperature limit for concrete: maximum average temperature65
C, maximum local temperature95
C. Fig. 1. Sketch of RPV insulation.

empirical formula method, substantive simplified and conservative analyses were performed for the insulation supports and their gaps through insulation. During on-site installation and debugging, it is difficult to use the empirical formula method to evaluate the thermal properties effectively when there are problems such as non-uniform dimensions and flow leakage. Little literature [1] focuses on heat leakage caused by gaps on insulation due to previous insulation without CIS. In this paper, thermal conductivity of insulation has been obtained through the heat transfer test. ANSYS Fluent was used to simulate the barrel insulation structure of HPR1000 and to analyze the flow leakage/ heat leakage caused by gaps in hot state. By this way, thermal property of the barrel insulation structure has been validated. The simulation results can provide a reference for subsequent design and engineering problems. 2. Heat transfer mechanism and heat transfer test for insulation 2.1. Heat transfer mechanism Some of the heat generated by the reactor is carried away by reactor coolant, while the rest escapes through the RPV wall and insulation. Combined with heat conduction, convection and radiation, the process of heat losing to environment is a complex process. The RPV of HPR1000 uses metal reflective thermal insulation, which has several layers of stainless steel foil placed in stainless shell, as shown in Fig. 2. The surface of stainless steel foil is bright, clean and with high reflectivity. These foils can reflect most thermal radiation, and thereby greatly weaken the radiation heat transfer. In addition, the heat convection between foils is also restrained by disposition of foils in the shell. The thermal performance requirements of RPV insulation of HPR1000 are as follows: - Equivalent thermal conductivity 0.10 W/(m$K); - Nominal average heat flux of RPV insulation outer surface 175 W/m2;

The outer diameter of RPV core shell and that of the lower head of the corresponding insulation structure are all greater than 1000 mm. So the heat transfer model of insulation structure can be simplified as flat slab thermal conductivity [2]. Forced ventilation is existed on the outside of insulation. For barrel insulation structure in ventilation zone with high wind speed, the heat transfer boundary conditions of the cold surface can be regarded as the hemispherical surface with natural convection. For lower head insulation structure in ventilation zone with low wind speed, the heat transfer boundary conditions of cold surface can be treated as half spherical with natural convection. Base on ASTM C680 [3], equation (1) has been used to calculate the conduction heat transfer of flat slab:

Qi ¼ qi ,Ai ¼

Ti  To ,Ai d=ls

(1)

Where, Qi is the heat loss of each insulation component, qi is the heat flow density of each insulation component, Ai is the heat transfer area of cold surface of each insulation component, Ti is the hot surface temperature of insulation, To is the cold surface temperature of insulation, d is the thickness of insulation, ls is the average thermal conductivity coefficient of insulation. ls is the key parameter for both empirical formula method and software simulation analysis, and generally got from heat transfer test [4]. The cold surface heat transfer of insulation includes convection heat transfer and radiation heat transfer. The heat transfer effect of cold surface is usually characterized by the total equivalent heat transfer coefficient, which is comprised of convection heat transfer coefficient and radiation heat transfer coefficient. The heat flow density of the cold surface is calculated by the following formula [3]:

qi ¼ hðTo  Ta Þ

(2)

h ¼ hc þ hr

(3)

hc ¼ Nu  lf

. L

(4)

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hr ¼

sε T 4o  T 4a

 (5)

To  Ta

Where, Ta is the environment temperature, h is total equivalent heat transfer coefficient of cold surface, hc is the convection heat transfer coefficient, hr is the radiation heat transfer coefficient, Nu is total Nusselt number, lf is the thermal conductivity coefficient of air, L is characteristic dimension for insulation heat transfer, s is Stefan-Boltzmann constant, ε is cold surface emittance between outside surface and the ambient surroundings. 2.2. Heat transfer test 2.2.1. Test samples and equipments The heat transfer test was conducted in accordance with ASTM C1061 [5], and the thermal conductivity of insulation specimen at different temperatures was determined. The type of test specimen was T-joint, each of which was composed of 2 pieces with dimension of 900mm900 mm and 1 piece with dimension of 1800mm900 mm spliced together to form a specimen with a total dimension of 1800 mm (length)1800 mm (width). The thickness of specimens is 85 mm, 100 mm, 115 mm, 155 mm, 180 mm, 200 mm, 215 mm and 240 mm respectively. Guarding heat-box method was used for the test. Test apparatus is shown in Fig. 3. The metering box was put in the guard box, then temperature of the protection box and metering box was adjusted to test temperature, meanwhile, the edge heat flow loss (f2) and heat flow through the metering box (f3) were reduced to a minimum, so the heat flow through insulation specimen was approximately equal to the power of heater (f). After entering the steady state, metering box could be deemed as a one-dimensional heat transfer, depending on the Fourier law [7]:

f ¼  ls А

dt dx

(6)

In the steady-state process, the heat conductivity of any section along the wall thickness was equal. Integrating x from 0 to the thickness (d) in the above equation could be obtained:

ðd

Tðc

f dx ¼  ls A 0

In equations (6)e(8), f is heating power for metering area (W), ls is thermal conductivity of specimen (W/m$K), d is specimen

thickness (m), A is metering area of specimen (m2), Th and Tc, are average hot and cold temperature of specimen, respectively (
C), Tm is qualitative temperature (
C), Tm ¼ ðTh  Tc Þ=2.

2.2.2. Test results Under normal operation condition, the outer wall temperature of HPR1000 RPV is about 292.2
C to 327.8
C. During heat transfer test, 200
C, 250
C, 300
C, 350
C and 400
C were chosen as hot surface temperature, while wind speed was approximately 0. The surface temperature of insulation was measured and the thermal conductivities were calculated. Thermal conductivities are shown in Fig. 4(Total test error is about±4%). There were three different thickness specifications of HPR1000 RPV insulation: 86 mm, 102 mm and 115 mm, which were corresponded to T-joint specimens with thicknesses of 85 mm, 100 mm and 115 mm in heat transfer test. Measuring results of thermal conductivities for specimens with three different thicknesses are shown in Table 1. The thermal conductivity of specimen 115 mm thick was slightly greater than that of specimen 85 mm and 100 mm thick. Therefore, specimen 115 mm thick was used for subsequent thermal conductivity analysis. According to linear fitting of measured data, the functional relation of equivalent thermal conductivity (ls) and qualitative temperature (Tm) is:

ls ¼ 0:02582 þ 1:93472  104 ,Tm

(9)

When using the formula method to analyze the thermal performance of insulation, heat loss of insulation joints with high possibility of thermal bridge formation is usually calculated by using the measuring results of the thermal conductivity of the Tjoint specimen from above tests. However, when the joint gap exceeds expectations, the thermal leakage cannot be effectively evaluated.

3. Analysis for flow leakage 3.1. Model

dt dx dx

(7)

Th

Thus, the thermal conductivity ls of insulation specimen was derived:

ls ¼

3

f,d 2f,d ¼ ðTh  Tc Þ,A Tm ,A

Fig. 3. Sketch of heat transfer test device.

(8)

In previous projects, there were some problems such as airflow reduction at outlet of reaction pit and ventilation temperature alarm due to the excessive insulation gap. In order to analyze the influence of heat leakage on HPR1000 barrel insulation structure, flow leakage caused by gap was firstly modeled and analyzed to determine the calculation method. Considering the symmetry of barrel insulation structure, the circumferential 15
model was selected for modeling analysis, as shown in Fig. 5. In axial direction, barrel insulation structure consisted of 9 layers of insulation panels. Each layer of insulation panels was connected to embed parts through 16 or 24 insulation supports distributed in the circumferential direction. The insulation supports were numbered layer 1 to 9 from top to bottom. The model was established by tetrahedral grids, as shown in Fig. 6, with grid number of about 84.29 million. Gaps of barrel insulation structure included gaps between panels and panels, gaps between penetrations and panels, and gaps between supports and panels. In accordance with the principle of equal area, gap area of each layer was equivalent to the contact zone between insulation supports and panels at the same height. Arrangement of flow liners was the same as that of insulation panels, and gaps between flow liners were treated in the same way. For example, the equivalent area for 2 mm gap in hot state is shown in Table 2.

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Fig. 4. Testing results for thermal conductivity.

Table 1 Thermal conductivity of insulation samples. Hot face temperature (
C)

ls (W/m$K)

85 mm 100 mm 115 mm

200

250

300

350

400

0.0463 0.0465 0.0474

0.0490 0.0521 0.0534

0.0550 0.0563 0.0583

0.0599 0.0624 0.0635

0.0629 0.0660 0.0682

Fig. 5. Circumferential 15
model.

3.2. Boundary conditions The defined boundary conditions mainly include: - Inlet temperature is 25
C and inlet volume is 500 m3/h; - Outlet pressure is 1 atm;

Fig. 6. Gridding for circumferential 15
model.

- Temperature of RPV outside wall is 292.2
C below the nozzle center line plane, and is 327.8
C above the centerline plane; - The boundary condition of concrete exterior wall is taken as air cooling condition, and the temperature is set as ambient temperature of containment vessel (40
C).

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Table 2 Gap area of insulation with different heights (unit: mm2). Insulation support

Gap between insulation panels þ gap between penetration and insulation panel

Gap between support and insulation panel

Equivalent area

Layer Layer Layer Layer Layer Layer Layer Layer Layer

64145 74465 74465 74465 74465 70648 83134 71146 90015

11520 11520 11520 11520 17280 17280 17280 17280 17280

4729 5374 5374 5374 3823 3664 4184 3684 4471

1 2 3 4 5 6 7 8 9

Materials in the model include air, stainless, low-carbon alloy steel and concrete, etc. The physical parameters of materials were determined according to the literature [6,7], and thermal conductivity of insulation was calculated by the fitting formula (9) obtained in section 2.2. The standard k-ε model has been adopted for turbulence model, and the Surface-to-Surface model was selected for the radiation heat transfer model. In order to ensure the convergence of solution process, the flow and heat transfer process (except radiation heat transfer) were simulated firstly. After calculation results converged, radiation heat transfer module was added for calculation.

3.3. Calculation results and analysis Fig. 8. Cloud chart of temperature distribution in fluid domain (unit: K).

Gaps of 2 mm, 2.5 mm and 3 mm were calculated, respectively. For instance, calculation result of 2 mm gap is introduced. Fig. 7 shows the velocity vector of whole flow domain, while Fig. 8 shows the cloud chart of temperature distribution in fluid domain. It can be seen that air flows into the space between insulation and RPV through the gaps of lower layers, and the flow rate in the supporting gap section of the bottom layer 7 to 9 is large. Between the outer wall of RPV and the inner wall of insulation, the air closing to the outer wall of RPV is heated and flows upward due to its decreasing density. In order to maintain the balance of the system, when the pressure difference between both sides of the

lower gaps exceeds the pressure difference resisting the air pass through gap, the ventilation air has been sucked into the insulation. The heated air flows upward and escapes from the upper gaps in the same way. The heated air mainly escapes from the upper three layers of supports gaps, among which both the velocity and temperature of the outflow air through the first support gaps are the highest. Fig. 9 shows the average temperature, average velocity and mass flow of the gap section from layer 1 to 9. Positive mass flow indicates that air flowed from inner side to outer side of insulation, while negative value indicates the opposite direction. When the gap is 2 mm, the calculated flow leakage from the ventilation passage to the space between RPV and insulation is 0.05171 kg/s, namely 151.96 m3/h, accounting for about 30.4% of inlet volume. When the gap is 2.5 mm and 3 mm, the flow leakage calculated by the model is 183.04 m3/h and 211.08 m3/h, accounting for 36.6% and 42.2% of inlet volume, respectively. By comparison, it can be seen that the smaller the gap size is, the smaller the flow leakage is, and the smaller the velocity of the leaked air is. The leaked air can be fully heated by RPV outer wall and the temperature is higher. Flow leakage of a certain gap size under different inlet conditions can also be calculated by the same method, and flow leakage of the entire barrel insulation structure can be obtained by multiplying the value of calculation results by 24. 4. Simulation analysis for heat leakage 4.1. Simulation model

Fig. 7. Velocity vector of fluid domain (unit: m/s).

In order to analyze the influence of heat leakage, simulation analysis was carried out on barrel insulation structure. The study area was the area between RPV outside surface and exterior surface of reactor pit shielding wall. Due to the asymmetry of ventilation

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solid homogeneous structure, and the concrete part only considered the part within radius of 5.1 m, and the RPV support was also simplified. ANSYS Meshing integrated grid generation strategy was adopted to establish the model. The mesh was generated to ensure that the mesh nodes of fluid-fluid, fluid-solid and solid-solid interfaces correspond with each other one by one. The total number of meshes was about 32.44 million, as shown in Fig. 11. 4.2. Boundary conditions According to the calculation method in chapter 3, the leakage mass, leakage velocity and leakage temperature of air moving from ventilation passage to the inside of insulation, which was through the gaps at different heights, could be obtained. In the model of section 4.1, the real geometry of insulation gap was not established. Instead, user-defined function (UDF) was used to introduce the heat leakage through the gap to the space between insulation and RPV outer wall. The heat leakage was introduced in the form of mass, momentum and energy source phase. The inlet boundary conditions are the ventilation parameters of HPR1000, i.e. inlet temperature is 17
C and inlet air volume is 13600 m3/h. The other boundary conditions are consistent with section 3.2. In work condition I, a 3 mm gap is adopted. According to the existing inlet conditions, the total flow leakage is 5429 m3/h. In order to ensure the inlet volume consistent with the given work condition, the total inlet volume was subtracted by the mass source phase loaded into the fluid domain through the UDF, then the result was taken as the final calculation of the inlet boundary. At the same time, condition II of 0 mm gap was calculated and compared. 4.3. Calculation results and analysis

Fig. 9. Leakage data of insulation support locations of each layer.

passage, an overall modeling of related structure was required. The 3D model included RPV outer wall, insulation, RPV support, reactor coolant piping and its insulation (part located in the inner side of primary shielding wall), concrete, and the fluid domain formed by solid structure of each part and RPV outer wall. The study area and the model are given in Fig. 10. Considering the influence of the complexity of equipment and reactor pit structure on the grid quantity, the model was built with partial simplification. For example, the insulation was simplified to

Fig. 12 shows the distribution of streamline velocity in the pit. Air flows into the pit from the air inlet. Since the air inlet direction almost coincides with the normal direction of concrete, the velocity direction changes after air enters the lower chamber and impacts the concrete inner wall. After rotating in the lower chamber, the air flows upward and cools the insulation. The shrinkage zone of concrete leads to the decrease of air passage area and the appearance of the maximum air velocity. Due to the flow leakage in work condition I, the maximum flow velocity of 5.61 m/s in condition I is less than that of 6.56 m/s in condition II. It can be clearly seen that the average velocity of ventilation passage in working condition I is less than that of working conditionII. The streamline temperature distribution in the pit is shown in Fig. 13. The air continuously cools the insulation along the height direction, and the air temperature keeps rising and reaches the maximum at the upper annular chamber around RPV nozzles. Although the maximum streamline temperature of work condition I is 112.35
C, which is only slightly higher than the work condition II of 111.61
C, the average streamline temperature of work condition I at the upper annular chamber is obviously higher than that of work condition II. Fig. 14 shows the temperature distribution of insulation supports. From the overall trend, the support temperature of the two work conditions gradually increases from bottom to top, which is due to the gradual increase of the air temperature in the ventilation passage from bottom to top and the gradual decrease of cooling capacity. The highest support temperature in work condition II is 265.26
C, which occurs at the point where insulation support penetrates insulation panel and contacts with high temperature air, far higher than the work condition I of 68.08
C. This is because there is no leakage in the calculation of work condition II. Firstly, temperature distribution of inner wall of insulation was calculated using the circumferential 15
model, and then the UDF was used to

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Fig. 10. Study area and integral model.

Fig. 11. Meshing of each part of the model.

load the temperature distribution curve into the model as boundaries. The temperature of inner end face of insulation support is the same as that of inner wall of insulation at the same height, which is closer to the true situation of insulation support. In work condition I, in order to increase the leakage source term and avoid unreasonable calculation results, the inner end face of insulation support was set as adiabatic boundary. This practice did not affect the evaluation of overall thermal performance, but the temperature distribution of insulation support would deviate from the real situation. Fig. 15 shows the temperature distribution of fluid domain section. It can be seen that the temperature of fluid domain rises along the height direction, and air with high temperature enters the ventilation passage from gaps. The maximum temperature of 95.15
C appears at the contact position between upper insulation support and insulation panel. The air temperature in the ventilation passage of work condition II does not change significantly, but only

gets the maximum value of 162.40
C in the downstream area closing to insulation support. The maximum value is occurred due to the formation of a detention zone downstream when airflow is hindered through the support. The air velocity in the detention zone is low, and the heat transfer coefficient is small, which contributes to make high temperature. In the upper annular chamber, it can be seen that the temperature of condition I is obviously higher than that of condition II. Fig. 16 shows the temperature distribution of concrete. It can be seen that the temperature of concrete inner wall is obviously stratified under working condition I, which is due to the impact on the concrete of high temperature air from the upper gap. The stratification phenomenon of work condition II is not obvious. The maximum temperature in these two conditions is close to each other. The highest temperature 74.62
C of work condition I appears in the concrete surrounding the upper annular chamber. Due to the effect of thermal conductivity of insulation support, local hot spots

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Fig. 12. Distributed drawing for flow line velocity in reactor pit.

Fig. 13. Distributed drawing for flow line temperature in reactor pit.

Fig. 14. Temperature distributing drawing of insulation support.

has been appeared on concrete surface contacting with the insulation support. The highest temperature of 74.02
C in work condition II also appears in the hot spot. In condition I, there is no obvious local hot spot, which should be related to the setting of

inner boundary of insulation support mentioned above. Table 3 shows the comparison between calculated values and design requirements. It can be seen all data of work condition II meet the design requirements, and there is still a large margin of

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Fig. 15. Temperature Distributing map of Fluid Domain Section.

heat loss value. The total heat loss of condition I is 15.22 times that of condition II because of the heat leakage. The total heat loss and nominal average heat flux are far beyond design requirements. The increase of heat loss of insulation structure will lead to the increase of air temperature in the pit, thus affecting the concrete temperature. From the calculation results of concrete temperature under work condition I, the average temperature increases 2.92
C compared with that under work condition II, and the highest temperature I increases slightly but still within the design requirements. It shows that the ventilation volume of HPR1000 is sufficient to withstand the additional heating load caused by heat leakage. 4.4. Sensitivity analysis for heat leakage On the basis of the above calculation method, sensitivity analysis is further carried out on the factors affecting heat leakage, including gap size, inlet volume and inlet temperature. 4.4.1. Gap size Work condition III, Ⅳ and Ⅴ have the same ventilation parameters but different gap sizes. Flow leakage of three conditions and thermal performance of insulation under the flow leakage have been calculated. Fig. 17 shows the temperature distribution of

concrete inner wall with different gaps, and Table 4 shows the detailed calculation results. It can be seen that the increase of gap size directly leads to the increase of leakage. The leakage under work condition Ⅴ increases about 38.9% compared with condition III, which has accounted for 42.2% of the total inlet volume. With the increase of gap size, the heat loss of insulation also increases significantly. The total heat loss under work condition Ⅴ increases by about 38.1% compared with that under work condition III. The change of gap size has little influence on the average temperature of concrete. The temperature value under work condition III increases by about 0.45
C compared with work condition Ⅴ. Local maximum temperature of concrete increases slightly. The temperature value under work condition III increases by about 5.55
C compared with work condition Ⅴ. 4.4.2. Inlet volume The inlet volume directly affects the air velocity on the surface of insulation, and then affects the thermal performance of insulation. Under work condition III, Ⅵ and Ⅶ, the thermal performance of insulation was calculated with the inlet volume of 12000 m3/h, 13600 m3/h and 15000 m3/h, respectively. Fig. 18 shows the temperature distribution of concrete inner wall in different inlet volume. Detailed calculation results are given in Table 5. It can be seen that with the increase of inlet volume and the increase of air

Fig. 16. Temperature distribution map of concrete section.

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Table 3 Thermal performance evaluation. Parameters

Work Condition I

Work Condition II

Design requirements

Inlet temperature (
C) Inlet volume(m3/h) Gap (mm) Average temperature of concrete (
C) Highest local temperature of concrete (
C) Nominal average heat flux of insulation (W/m2) Total heat loss of insulation (kW)

17 13600 3 37.19 74.62 1321.91 304.04

17 13600 0 34.27 74.02 86.84 19.97

65 95 235 90

Fig. 17. Temperature distribution map of concrete inner wall in different gaps.

Table 4 Thermal performance in different gaps. Parameters

Work Condition III

Work Condition Ⅳ

Work Condition Ⅴ

Gap (mm) Inlet temperature (
C) Inlet volume(m3/h) Flow leakage (m3/h) Average temperature of concrete (
C) Highest local temperature of concrete (
C) Nominal average heat flux of insulation (W/m2) Total heat loss of insulation (kW)

2 25 12000 3647 38.90 71.64 843.52 194.01

2.5 25 12000 4393 39.12 75.06 1011.65 232.68

3 25 12000 5066 39.35 77.19 1165.22 268.00

velocity in the ventilation passage, the heat transfer effect of concrete-air and insulation-air is enhanced, so the concrete temperature, especially the local maximum temperature, decreases significantly. The temperature value under work condition Ⅶ decreases 7.22
C compared with work condition III. Total heat loss of insulation increases slightly. The heat loss value under work condition Ⅶ increases by about 8.9% compared with work condition III. 4.4.3. Inlet temperature The thermal performance of insulation at the inlet temperature of 25
C, 20
C and 17
C were calculated under work condition III, Ⅷ and Ⅸ, respectively. Fig. 19 shows the temperature distribution of concrete inner wall in different inlet temperature. Table 6 gives the detailed calculation results. It can be seen that with the decease of inlet temperature, the heat loss of insulation increases slightly. The total heat loss of work condition Ⅸ increases about 14.3% compared with work condition III, which is due to the increase of heat transfer coefficient of insulation caused by the increase of temperature difference between cold and hot surface of insulation.

With the decreasing inlet temperature, the average and highest temperature of concrete work condition Ⅸ decreases 1.99
C and 3.56
C compared with work condition III, respectively. Fig. 20 shows the proportion of heat leakage in the total heat loss under all work conditions. It can be seen that the proportion of heat leakage decreases from 94.6% to 78.6% with the increase of gap size. As analyzed in section 3.3, when the gap increases, the overall air velocity decreases and the leakage air temperature decreases in the path from ventilation passage into gap and back to ventilation passage, so the proportion of heat leakage decreases with the increase of heat leakage and total heat loss. Besides the convection heat transfer directly caused by gap, factors including heat leakage which makes the support temperature rise will also indirectly increase the total heat loss. From work condition Ⅵ to Ⅸ, although the change of ventilation parameters has a certain influence on the total heat loss of insulation, the heat leakage is more than 89.4% of the total heat loss of insulation. In each of the above work conditions, heat leakage plays a dominant role in heat loss of insulation.

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Fig. 18. Temperature distribution map of concrete inner wall in different inlet volume.

Table 5 Thermal performance with different inlet volume. Parameters

Work Condition III

Work Condition Ⅵ

Work Condition Ⅶ

Inlet volume(m3/h) Inlet temperature (
C) Gap (mm) Average temperature of concrete (
C) Highest local temperature of concrete (
C) Nominal average heat flux of insulation (W/m2) Total heat loss of insulation (kW)

12000 25 2 38.90 71.64 843.52 194.01

13600 25 2 38.31 67.21 883.09 203.11

15000 25 2 37.95 64.42 918.30 211.21

Fig. 19. Temperature distribution map of concrete inner wall in different inlet temperature.

Table 6 Thermal performance with different inlet temperature. Parameters

Work Condition III

Work Condition Ⅷ

Work Condition Ⅸ

Inlet temperature (
C) Inlet volume(m3/h) Gap (mm) Average temperature of concrete (
C) Highest local temperature of concrete (
C) Nominal average heat flux of insulation (W/m2) Total heat loss of insulation (kW)

25 12000 2 38.90 71.64 843.52 194.01

20 12000 2 37.27 68.76 921.62 211.97

17 12000 2 36.91 68.08 964.43 221.82

Please cite this article as: Q. Tian et al., Research for the heat leakage caused by gaps on barrel insulation structure of reactor pressure vessel, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.11.001

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Q. Tian et al. / International Journal of Advanced Nuclear Reactor Design and Technology xxx (xxxx) xxx

Fig. 20. Proportion of heat leakage in the total heat loss.

4.5. Discussion of heat leakage It can be seen from the above analysis that heat loss of insulation will not meet design requirements due to heat leakage caused by gaps, and the gap in hot state should be strictly limited. Design of HPR1000 barrel insulation structure is fully considered the experience feedback from previous projects. Following considerations are made in control of gap in hot state: 4.5.1. Minimize joint gap length Barrel insulation structure of HPR1000 is composed of more than 280 insulation panels and the same number of flow liners. For example, the joint gap length for a single layer insulation panel is estimated in Table 7. Arrangement of flow liner is the same as insulation panel, and its joint gap length is also the same as insulation panel. Obviously, under the same gap size, gap area can be reduced by shorting joint gap length, which is beneficial to reduce the heat leakage. 4.5.2. Setting of overlap and control of cold gap Considering the thermal expansion of insulation panel and flow liner from cold to hot, there is a certain cold gap between insulation panels and between flow liners, to offset the effects of thermal expansion during installation. At the same time, it is expected that the gap in hot state could be small enough to avoid heat leakage during the reactor operation. In order to settle the problem, on the one hand, reasonable cold gap should be stipulated according to the thermal expansion of insulation panel and flow liner, on the other hand, overlaps are needed to block the convection once there are hot gaps. Taking insulation panel of HPR1000 in Table 7 as an example, both the insulation panel and flow liner are manufactured by stainless steel. The thermal expansion is calculated according to the material properties, working temperature and insulation panel sizes. The thermal expansion and corresponding requirements of cold gap are shown in Table 8. Meanwhile, according to the

different installation positions, the longitudinal and circumferential joints of adjacent insulation panels are combined by hot and cold surface overlaps and insulation panels lapping; the longitudinal and circumferential joints of the adjacent flow liners are covered with overlaps, as shown in Table 8 and Fig. 21. Except the position of support connecting with insulation panels, the other joints are all covered with overlap. For the insulation panel, when the installation status meets the requirements, the gap value shall approach to 0 under hot state. Even if some large cold gaps occur due to the manufacturing tolerance of insulation, position deviation of the embedded parts, welding deformation of insulation support and other factors, the gap can be covered by the 45 mm overlap. In order to ensure the overlap on cold and hot surface better fitting the insulation panel surface, some precise adjustment shall be performed between layers during panel installation, so as to avoid excessive staggered edge. Bolts and fillet welds are used to connect the overlap on longitudinal and circumferential joint, and the leakage can be negligible. Excessive staggered edge, however, shall also be avoided for the installation of flow liner. 4.5.3. Optimization of insulation supports Rectangular tube has been applied for insulation support. The penetration gap between support and insulation panel is hardly affected by thermal expansion, so non-metal insulation material is used to fill the gap. In addition, it is required to reserve a margin of 20 mm length for insulation support when the manufacturing finished. According to the size adjustment of on-site measurement, the staggered edge problem of flow liner mentioned above can be avoided to a certain extent. In conclusion, by reducing the length of joints, setting overlaps to control the cold gap and optimizing structure design of insulation support, the hot state penetration gaps of insulation panel and flow liner can be reduced to extremely low. The simulation analysis of section 4.3 has validated the thermal performance of 0 mm gap of HPR1000.

Table 7 Comparison for joint length of insulation panel. Parameters

Previous project

HPR1000

Outer diameter of barrel insulation (mm) Height of a single layer insulation panel (mm) Number of a single layer insulation panel Total length of circumferential joint (in cold surface, mm) Total length of longitudinal joint (in cold surface, mm)

5041.9 957.3 85 ~19440 ~36672

5358 849 24 ~16824 ~20376

Please cite this article as: Q. Tian et al., Research for the heat leakage caused by gaps on barrel insulation structure of reactor pressure vessel, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.11.001

Q. Tian et al. / International Journal of Advanced Nuclear Reactor Design and Technology xxx (xxxx) xxx

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Table 8 Joints of insulation panels and flow liners of HPR1000. Average thermal expansion (mm) Requirement of cold gap (mm) Cover method of joint Insulation panel Circumferential joint 1.9 Longitudinal joint 2.3

2

Flow liner

4e8

Circumferential joint 3.2 Longitudinal joint 4.0

cold surface and hot surface: overlap of about 45 mm panel lapping of about 40 mm cold surface and hot surface: overlap of about 45 mm overlap of about 45 mm, connecting with bolt and fillet weld overlap of about 45 mm, connecting with bolt and fillet weld

Fig. 21. Schematic of connection between insulation panels and between flow liners.

5. Conclusions (1) Fitting formula of equivalent thermal conductivity of insulation (ls) and qualitative temperature (Tm) was obtained through the one-dimension heat transfer test, and applied to thermal performance analysis of HPR1000 barrel insulation structure. (2) ANSYS Fluent was used to carry out simulation analysis. By establishing a circumferential 15
model of barrel insulation structure, evaluation method for flow leakage with gaps in hot state was obtained. Given ventilation parameters, when gap in hot state is 2 mm, 2.5 mm and 3 mm, the proportion of flow leakage in inlet volume is 30.4%, 36.6% and 42.2%, respectively. (3) Based on calculation of flow leakage, a full-size model of barrel insulation structure was established, and models with 3 mm and 0 mm gap were analyzed to obtain heat loss of insulation, temperature field distribution of surrounding concrete, flow field distribution of ventilation, and so on. The model with 0 mm gap could meet the design requirements of thermal performance. For model with 3 mm gap, both the total and average heat loss exceeded design requirements, but concrete temperature was still within design requirements. (4) Sensitivity analysis for heat leakage was carried out on gap size, inlet volume and inlet temperature. Proportion of

convection heat transfer directly caused by gap in total heat loss of insulation was compared under each work condition. The proportion of heat leakage caused by gaps was more than 78.6%, which was dominant in heat loss of insulation. (5) By minimizing joint gap length, setting of overlaps, controlling cold gap, optimizing structure of insulation support in the design of HPR1000 RPV insulation, gap in hot state can be reduced to extremely low.

References [1] Dong-jie Liu, Xiao-bing Ran, Dai Chang-nian, et al., The modified design of RPV metallic reflective insulation.Excellent papers of China nuclear industry survey and design institute in 2014 87 (4) (2014) 46e51. [2] GB/T 8175, Guide for Design of Thermal Insulation of Equipments and Pipes, 2008. [3] ASTM C680, Standard Practice for Estimate of the Heat Gain or Loss and the Surface Temperatures of Insulated Flat, Cylindrical, and Spherical Systems by Use of Computer Programs, 2010. [4] NB/T 20343, Design and Fabrication Specification for the Thermal Insulation of Reactor Pressure Vessel and Reactor Coolant Pipings and Equipments of PWR Nuclear Power Plants, 2015. [5] ASTM C1061, Standard Test Method for Thermal Transmission Properties of Nonhomogeneous Insulation Panels Installed Vertically, 1986. [6] Yang Shi-ming, Tao Wen-quan, Heat Transfer, Higher Education Press., Beijing, 2006, pp. 46e438. [7] RCC-M, Design and Construction Rules for Mechanical Components of Nuclear Islands, 2007.

Please cite this article as: Q. Tian et al., Research for the heat leakage caused by gaps on barrel insulation structure of reactor pressure vessel, International Journal of Advanced Nuclear Reactor Design and Technology, https://doi.org/10.1016/j.jandt.2019.11.001