Numerical analysis of the passive heat removal system for molten salt reactor at steady state

Applied Thermal Engineering 102 (2016) 1337–1344

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Research Paper

Numerical analysis of the passive heat removal system for molten salt reactor at steady state Xiangcheng Wu, Changqi Yan ⇑, Zhaoming Meng, Kailun Chen, Shaochuang Song, Zonghao Yang, Jie Yu Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, Harbin Engineering University, Harbin 150001, Heilongjiang, China

h i g h l i g h t s  We analyzed the interplay between the boiling loop and the condensing loop.  The time-averaged pressures of calculation agree well with the experimental data.  The system pressure is very sensitive to heat flux and HTCC.  A transition from bubbly flow to slug flow occurs in the annulus.

a r t i c l e

i n f o

Article history: Received 7 September 2015 Accepted 30 March 2016 Available online 19 April 2016 Keywords: Natural circulation Passive heat removal system Molten salt reactor Numerical calculation

a b s t r a c t The passive residual heat removal system of molten salt reactor consists of a boiling and a condensing loop, which are connected by a steam drum. A numerical model based on two-fluid model in RELAP5 is developed to demonstrate the use of the passive system and investigate the interaction between the boiling loop and the condensing loop at steady state. The numerical work is partially validated by experimental results obtained in a single-tube natural circulation loop. Heat flux, initial water inventory, the height of the condensing loop and the heat transfer capacity of the condenser are changed separately to study how the system responds. The results show that the mass flow rate in the boiling loop indeed has a maximum. Both the maximum value and the system pressure are mainly affected by heat flux and heat transfer capacity of the condenser. The mass flow rate in the condensing loop is controlled by heat flux instead of other parameters, thus it seems that the condensing loop is a follower of the boiling loop. The initial water inventory and the height of the condensing loop have little influence on the system performance. Additionally, Bubbly flow, slug flow, vertical stratified flow, horizontal stratified flow and annular-mist flow appear in the natural circulation system. Ó 2016 Published by Elsevier Ltd.

1. Introduction In molten salt reactor, nuclear fuel is uniformly dissolved in the molten salt coolant that is as high as 650 °C. The reactor is operated at high temperature and lower steam pressure compared with pressurized water reactor. The high temperature can provide high thermal efficiency and the lower steam pressure can decrease the mechanical stresses. The study of molten salt reactor dates back to 1950s when the Oak Ridge National Laboratory (ORNL) designed and constructed a 10 MW Molten Salt Reactor Experiment (MSRE), which reached critical state and operated for four years with great success, even though radiation damage effect occurred on the surface of nickel-based alloy [1,2]. Molten salt reactor was also inves⇑ Corresponding author. E-mail addresses: [email protected] (X. Wu), [email protected] (C. Yan). http://dx.doi.org/10.1016/j.applthermaleng.2016.03.176 1359-4311/Ó 2016 Published by Elsevier Ltd.

tigated in Russia and Japan [3,4]. Now molten salt reactor has been chosen as one of Generation IV reactors that need more attention and investigation in the future. Removing the residual heat effectively should take precedence in the safety of nuclear reactors. In the residual heat removal system of MSRE, pumps were used to send cooling water through the condenser, and the cooling water was provided by a cooling tower. Additionally, fans in the cooling tower were used to control water temperature. The residual heat removal process would be interrupted in case of pump or fan failure. If a passive concept is introduced in the removal process, the possibility of pump failure can be eliminated. In a passive residual heat removal system (PRHRS), pump is no longer needed and a natural circulation loop driven by gravity is adopted. Park et al. [5] performed experiments on the PRHRS of the SMART-P using the high temperature/high pressure thermal– hydraulic test facility. The results showed their Emergency

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Cooldown Tank (ECT) was large enough to remove the core decay heat. However, because of this, all the steam was condensed in the upper part of the ECT and an uneven distribution of fluid temperature was recorded along the vertical direction. Xinian et al. [6] analyzed the transient flow in a three-fold natural circulation loop that was the candidate of the safeguard systems. Capability analysis of emergency passive residual heat removal system (EPRHRS) of CPR1000 was performed with RELAP5 code by Zhang et al. [7]. An unstable process during the transition period from single-phase circulation to two-phase circulation was mentioned in the brief explanation to the accident condition. But they claimed that the decay heat could be taken away effectively. In recent years, some novel designs of PRHRS were proposed for the next generation reactors [8–12]. Natural circulation has been widely utilized in nuclear reactors. Wu et al. [13] experimentally investigated the circulation characteristics of secondary PRHRS for pressurized water reactor. They claimed that the PRHRS was not only sufficient to remove the decay heat, but also had strong natural circulation stability when encountering a power drop disturbance. It should be noted that due to low driving pressure, instability usually appears in natural circulation loops, especially in two-phase loops. These instabilities include density wave oscillation, natural circulation oscillation, instability induced by flashing and so on. The features, frequencies and amplitudes of these instabilities have been widely investigated [14–16]. Durga Prasad et al. [17] and Boure et al. [18] both gave detail reviews of instabilities in two-phase circulation loops. A PRHRS should take away the residual heat and avoid the severe impact induced by instabilities in natural circulation. At the same time, it must be capable to operate for a long time. So some important parameters should be studied so as to know their effects on system operation and find an optimal design. Now, China initiates a project to investigate the Thorium Molten Salt Reactor (TMSR) technology. The goal in the first stage is to design and construct a 2 MW TMSR experiment. Hence, a system model based on RELAP5 was developed to study the PRHRS for Chinese 2 MW test reactor at steady sate. A partial experimental validation was performed. The main goal of this research is to analyze steady-state performance of the two-phase natural circulation system. Since the cooling medium circulates in the condensing loop and the boiling loop, the coupling and interaction between them would be discussed. The analysis would focus on the effect of some design parameters and gives hints for qualitative or quantitative design improvements to avoid undesired effects.

2. Description of PRHRS In Fig. 1, the PRHRS is coupled by a boiling loop and a condensing loop that are connected by a steam drum. The condensing loop (CL) refers to the pipes above the steam drum, and the boiling loop (BL) is below the steam drum. Heat source comes from decay of fission fuel dissolved in molten salt and atmosphere is served as heat sink. Coupled natural circulation loops and the BL arranged in a thimble tube are two important features in the PRHRS. The steam drum is served as a separator dividing water–steam mixture into two parts: an upper part being steam and a lower part being water. Decay heat is discharged to the atmosphere via a natural draft aircooled condenser. The feed water tank acts as a buffer pool by providing or collecting water. The heat transfer tube in the drain tank is composed of a center tube, a bayonet and a thimble (see Fig. 2). Water in the bayonet absorbs decay heat released from molten salt and turns into steam. The gas gap between the thimble and the bayonet is to keep the bayonet from boiling crisis because at the initial state, molten salt temperature is as high as 933 K. Water in the steam drum flows

Fig. 1. Passive residual heat removal system [19].

down along the center tube, reverses and boils in the annulus between the center tube and the bayonet. Accordingly, the flow is driven by buoyancy resulted from density difference between the annulus and the center tube. Steam–water mixture flows into the steam drum, and then steam goes up again into the condenser. As the latent heat is released, condensed water flows back into the steam drum due to gravity, thereby closes the big circulation. 3. Experimental apparatus Fig. 2 shows the apparatus for a single tube experiment. The dimensions of the center tube, the bayonet and the thimble are consistent with the original designs, which are listed in Table 1. Distilled water is used as working fluid. The thimble is heated by a high-temperature tube furnace that could reach 700 °C and remain constant for over 10 h. The heat section that is consistent with the thimble in length is divided into three segments. Due to PID technology, each of the segments can adjust furnace temperature automatically. At steady state, temperature difference among the three segments is lower than 2 K if measurement error of ceramic thermocouples arranged in the furnace is ignored. The furnace is wrapped by adiabatic ceramic fiber of 250 mm in thickness to reduce heat dissipation. Likewise, water tank and the part of the bayonet exposed to the ambient are wrapped by aluminum silicate. Four heating elements are arranged in the water tank to raise water temperature quickly. Flow meter cannot be installed in the BL due to the special nested structure. So the time-averaged flow rate is measured by weighting. Water–steam mixture flows through the right side of the three-way valve to Separator where water flows downward to Receiver 1 and steam goes to Receiver 2. After weighting, water would be pumped back into the water tank. The maximum deviations of water flow rate and steam flow rate are 0.6% and 1% respectively. A pressure transducer is installed on the outer surface of the bayonet 10 cm away from the heat section. The uncertainty of the pressure sensor is 0.04%. The temperatures at the inlet of the

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Outlet

Three-way valve Water tank

Condenser Seperator

Collector 2

Measurement loop in the dashed frame

5

Bayonet

4

Pressure sensor

Pump

Collector 1 3 1 2

3 5

1

Inlet of the heat section Thimble

4

2

Center tube

Outlet of the heat section

Inlet of the center tube

Fig. 2. Experimental loop.

Table 1 Dimensions of the components. Component

Size

Center tube Bayonet Thimble Water tank

12.7 mm  1.0 mm, 2.0 m in height 25.4 mm  1.5 mm, 2.2 m in height 38 mm  1.5 mm, 1.5 m in height 500 mm  500 mm  500 mm

center tube, the bottom of the bayonet and the outlet of the bayonet are measured by K-type thermocouples with an uncertainty of 0.1 °C. When the experiment is started, water is gradually heated to saturation. Then a steady circulation is established and experimental data is recorded in the computer.

4. Numerical model RELAP5 code was developed at the Idaho National Engineering Laboratory and has been widely used for system analysis [20,21]. The hydraulic component and the heat structure contacted with fluid can be simulated by its corresponding module, e.g., volume, junction and heat structure. It also has modules to simulate some special component, e.g., pump and accumulator. Fig. 3 shows RELAP5 nodalization of the PRHRS. The BL is composed of 101A, 100P, 102SV, 103SV and 104SV. 100P simulates the center tube, and the bayonet is modeled by 101A. 100P and 101A

have 20 and 22 volumes, respectively. The steam drum is composed of 203B, 202P and 201B. The condenser is modeled by 303P that has 10 control volumes. 301P, 302P, 304P and 305P are associated pipes between the steam drum and the condenser. 307P simulates the chimney. It should be noted that Chen’s formula is used to calculate boiling heat transfer coefficient before CHF [22]. The default option of filmwise condensation is the maximum of the Nusselt (laminar) and Shah (turbulent). In the gas gap, radiation and conduction cannot be calculated simultaneously due to lack of specified model. So heat flux boundary of the system would be given on the surface of the bayonet. 306TDV and 308TDV simulate the atmosphere. So a constant pressure boundary is given in 306TDV and 308TDV. The heat power is input on the outside of the bayonet and discharged into the ambient. The volumes without heat boundary are assumed to be adiabatic. The calculated results in the BL are validated because the most complicated heat transfer modes including subcooled boiling, saturated boiling, convection, and condensation appear in the BL. Fig. 4 shows that pressure fluctuations of the experimental data are more random and of larger amplitudes. But the timeaveraged values are very close to that of experiment. Fig. 5 shows the validation of velocity in the center tube. Excluding the biggest heat flux point, the calculated results are about 20% higher than the experimental data in the range of 8.2–17.2 kW/m2, which is attributed to the simplification of the model and the empirical correlations. To get more reasonable results, most of our analyses are under 15 kW/m2.

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308TDV

1

2

3

4

302P

5

3 0 7 P

4 3

3 0 3 P

301P

2 1

1 203B

306TDV

2

202P5 202P4 104SV

202P3

304P

3

202P2 103SV

4

202P1

201B

5

305P

1 1 0 1 A

1 0 0 P

SV

2

3

Time dependent Volume

Juncon Heat boundary

TDV

Single Volume

B

Branch

A

Annulus

Pipe

P

4

102SV

Fig. 3. Thermal hydraulic nodalization of PRHRS.

5

Experimental value

Experiment Calculation

1.62

4

Velocity (m/s)

Pressure at the measuring point (KPa)

Calculated value

3

2

1.08

0.54

0.00

1

5000

10000

15000

20000

2 Heat flux (W/m )

0

0

100

200

0

100

200

Fig. 5. Velocity in the center tube.

Time (s) Fig. 4. Comparison of pressure.

5. Results and discussion Pressure in the steam drum, mass flow rate in the BL and the CL and some related parameters of the closed system without a pressurizer would be discussed and analyzed at steady state. The calculation results are considered to be stable when time-averaged values are unchanged.

5.1. Effects of heat flux Obviously, steam generation speeds up in the annulus as heat flux make a linear increase. So more steam is discharged into the steam drum and the condenser. If the heat-transfer capacity of the condenser is infinite, steam whose temperature is higher than the ambient would be condensed and the system would operate under high vacuum. But in reality, the heat-transfer capacity of the condenser is finite and unchanged when heat transfer coefficient and temperature difference between the condenser’s interior and exterior are constant. Extra steam that cannot be condensed

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state, steam generation in the annulus and consumption in the condenser reaches equilibrium. Figs. 6–9 describe the variation of the parameters as heat flux increases. In Fig. 6, the system pressure that is close to saturation pressure increases faster than heat flux. From 112 KPa to 925 KPa, the system pressure is over 8 times as large, while heat flux only doubles from 7200 W/m2 to 14,800 W/m2. It implies that the strength of steel structure need to be high enough if the system is closed and no further steps are taken to lower the pressure. In Fig. 7, mass flow rate in the BL reaches a maximum value of 1.02 kg/s at about 10,000 W/m2. As heat flux increases further, driving force and friction force both ascend, but the increase of two-phase friction force is larger than that of buoyancy. The mass flow rate has to decrease so as to achieve a balanced state. In theory, the driving force has a maximum value when liquid fills the center tube and the annulus is full of steam. The smallest flow rate is 0.87 kg/s at about 7200 W/m2. It indicates the range of mass flow rate is small. Mass flow rate in the CL increases linearly with heat flux in Fig. 7. Since liquid temperature in the system is close to its local saturated temperature. Heat flux for temperature rising is much smaller compared with latent heat of vaporization. So most of power is used for steam generation. On the other hand, it can be

780

520

260

0 8

10

12

14

16

Heat flux (×10-3 W/m2) Fig. 6. Variation of system pressure with increasing heat flux.

increases the system pressure. So saturated temperature also increases, which in turn raises temperature difference and improves the heat transfer capacity of the condenser. At steady

Mass flow rate in the boiling loop (kg/s)

1.2 0.009

0.008

1.0

0.007 0.8

0.006

0.005 0.6 0.004

0.003

0.4 6

10

8

12

14

Mass flow rate in the condensing loop (kg/s)

6

16

Heat flux (×10-3 W/m2) Fig. 7. Variation of mass flow rate.

0.006

0.9

0.8

0.005

0.7

Static quality

0.004 0.6 0.003 0.5 0.002 0.4 0.001

0.3

0.000

0.2 7

8

9

10

11

12

13

14

15

Heat flux (×10-3 W/m2) Fig. 8. Static quality and void fraction at the outlet of the heat section.

Void fraction

System pressure (kPa)

1040

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Heat flux (×10-3 W/m2) 10

12

14

System pressure (Pa)

Distance from the steam drum (m)

8 1.0 1.2 1.4 1.6 1.8

130000

117000

104000

91000 280

300

320

340

360

380

400

420

440

Initial inventory ratio (kg/m3)

2.0

Fig. 11. Variation of system pressure with increasing initial inventory ratio. Fig. 9. Initial position of slug flow occurrence.

5.2. Effects of initial water inventory The steady state should be the same if initial water mass is fixed regardless of the initial water temperature. Nevertheless, it may not be true when initial water volume is given because water volume is also affected by pressure and temperature. To draw more

0.8 0.7 0.6

Void fraction

seen that steam inventory in the system depends on heat flux. Fig. 8 illustrates static quality and void fraction at the outlet of the heat section in the bayonet. Since the subcooling is very low at the bottom of the bayonet, the static quality increases almost linearly with heat flux. While the void fraction shows a downward trend after the first rise due to the effect of slip ratio and flow pattern change. The width of the annulus between the center tube and the bayonet is only 4.85 mm, which may have negative effects on the occurrence of annular mist flow. Consequently, according to the vertical flow regime map in the calculated results, there are only bubbly flow and slug flow, and annular mist flow does not appear. The initial positions of the occurrence of slug flow are shown in Fig. 9. Due to fluctuation, some positions where bubbly flow and slug flow may appear alternately are excluded. Only the positions entirely occupied by slug flow are selected. As can be seen in Fig. 9, slug flow appears at the bottom of the bayonet when heat flux is higher than 13,800 W/m2.

0.5 0.4 0.3

outlet of the heat section outlet of the bayonet

0.2 0.1 280

300

320

0.004

0.9 0.003 0.8

0.002 0.7

0.001 320

340

360

380

400

Initial inventory ratio (kg/m3) Fig. 10. Variation of the mass flow rate.

420

440

Mass flow rate in the condensing loop (kg/s)

0.005

300

380

400

420

440

general conclusions and make the calculated data universally applicable, a new independent variable, initial inventory ratio, is defined and used on the horizontal axis. It is the ratio of initial water mass to the volume of the system and has the same unit as density. In each calculation, only initial water mass is reentered, other parameters remain unchanged.

1.0

0.6 280

360

Fig. 12. Void fraction in the BL.

1.1

Mass flow rate in the boiling loop (kg/s)

340

Initial inventory ratio (kg/m3)

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1.0

Void fraction

0.9 0.8 0.7

node 1 in 305P node 2 in 305P

0.6

Slug to Annular-Mist (%)

60

40 30 20 10 0

0.5 0.2

50

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.2

2.2

0.4

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Height of the condensing loop (m)

Height of the condensing loop (m)

Fig. 15. Flow pattern in the downward channel (node 5 in 305P).

Fig. 13. Void fraction in the downward channel (305P).

10

720

System pressure (kPa)

8

Velocity (m/s)

0.6

6

liquid steam

4 2

540

360

180

0 100

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

120

140

160

180

Capacity of the condenser (%)

Height of the condensing loop (m) Fig. 16. Variation of system pressure with increasing HTCC.

The variations of the parameters are shown in Figs. 10–12. The mass flow rate in the BL increases with initial inventory ratio, which is easy to understand. Bigger inventory ratio means higher water level in the water tank and smaller height difference between the outlet of the bayonet and the water level, which leads to an increase of the driving force. In Fig. 10, the mass flow rate in the BL increases by 8.6%, from 0.81 kg/s to 0.88 kg/s. The mass flow rate in the CL and the void fraction in the BL remain nearly unchanged (Figs. 10 and 12) because steam generation depends on heat flux instead of the water inventory. The system pressure is also insensitive to the water inventory (Fig. 11), which means an arbitrary amount of water can be filled at the beginning of the experiment. The cross-section area of the annulus expands at the top of the center tube. The cross section becomes circular due to absence of the center tube (see Fig. 2), which makes water slow down and occupy more space. That is why the void fraction at the outlet of the heat section is higher than that at the outlet of the bayonet in Fig. 12.

5.3. Effects of HCL The height of the condensing loop (HCL) refers to the height difference between the condenser and the steam drum. According to the calculation results, the parameters, including mass flow rate, void fraction and velocity of liquid and steam in the BL, are nearly unchanged with the increase of the HCL. It seems that the HCL does not have a strong influence on the BL. In Fig. 13, the flow rate in the CL is almost unchanged. Node 1 in 305P is full of steam and the void fraction remains 0.98. But the

Mass flow rate in the boiling loop (kg/s)

Fig. 14. Velocity in the downward channel (305P).

1.00

0.95

0.90

0.85

0.80 100

120

140

160

180

Capacity of the condenser (%) Fig. 17. Mass flow rate in the BL.

void fraction of node 5 in 305P has a large increase from 0.51 to 0.83, which offsets the effect of increasing the HCL. From Fig. 14, it can be seen that steam is almost stagnant but the liquid velocity sharply increases to bigger values due to gravity. The transition of flow pattern from slug flow to annular-mist flow is depicted in Fig. 15. It is noted that the transition only happens in node 5 of 305P while other nodes remain annular-mist all the time. Percentage on the vertical axis refers to the degree of the transition. When the HCL falls below 0.6 m, annular-mist flow does not exist anymore.

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2.0 1.8

Velocity (m/s)

occurs in the annulus. Vertical stratified flow is in the water tank. The steam upward channel of the CL is full of annular-mist flow. The flow pattern in the downward channel shows a transition from annular flow to slug flow.

liquid steam

1.6

Acknowledgements

1.4 1.2 1.0

This work is supported by the National Natural Science Foundation of China under grant no. 11475048.

0.8

References

0.6 100

120

140

160

180

Capacity of the condenser (%) Fig. 18. Variation of the velocity above the heat section.

5.4. Effects of HTCC The Heat Transfer Capacity of the Condenser (HTCC) is changed by altering the number of the condensing tubes. Higher HTCC means more steam can be condensed under the same condition, which results in a decrease of the system pressure until a new balance state is achieved. Fig. 16 shows the corresponding results as expected. It can be seen that the system pressure is very sensitive to the HTCC. When the HTCC increases from 100% to 180%, the system pressure decreases from 726 kPa to 129 kPa. In Fig. 17, the mass flow rate in the BL increases to 0.91 kg/s and then declines, corresponding with the liquid velocity in Fig. 18, which may be attributed to the transition from bubbly flow and slug flow. Higher HTCC also causes an almost linear increase in the steam velocity. 6. Conclusions A theoretical method is developed in this work to study steadystate flow and heat transfer in the PRHRS of MSR. Numerical calculations are based on two-fluid model in RELAP5. Results have been presented to demonstrate the use of the PRHRS and investigate the effects of the relevant parameters including heat flux, initial water inventory, HCL and HTCC. This numerical work is partially validated by experimental results obtained in a single-tube natural circulation loop. The time-averaged pressures of calculation agree well with the experimental data. The calculated velocities are about 20% higher than the experimental data in the range of 8.2– 17.2 kW/m2. The driving force and the friction force achieve a balance at steady state. At the same time, energy input from the bayonet also equals energy output from the condenser. Even though some parameter variations that need more information from the transport equation cannot be obtained directly, all of them are under the two limitations. Heat flux and HTCC that control the rate of steam generation and consumption respectively play an important role in system performance. The system pressure is very sensitive to the two parameters. When heat flux and HTCC increase, the mass flow rate in the BL shows a downward trend after the first rise due to the transition of the flow patterns. Higher heat flux and larger HTCC both increase the mass flow rate in the CL. The HCL and the initial water inventory don’t have a significant effect on the mass flow rate and the system pressure. Single phase flow occupies the center tube. A transition from bubbly flow to slug flow

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