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Accident analysis of tungsten target coupled with ADS core
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Accident analysis of tungsten target coupled with ADS core Tianji Peng a, Minggang Lang a, Zhiwei Zhou a,*, Long Gu b a
Institute of Nuclear and New Energy Technology, Collaborative Innovation Center of Advanced Nuclear Energy Technology, Key Laboratory of Advanced Reactor Engineering and Safety of Ministry of Education, Tsinghua University, Beijing, 100084, China b Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 730000, China
article info
abstract
Article history:
The Accelerator Driven Subcritical Reactor System (ADS) is a kind of nuclear reactor which
Received 22 October 2015
can burn minor actinide waste produced from conventional reactors with inherent safety
Received in revised form
features. Because of the characteristics of a sub-critical reaction process, the fission chain
26 January 2016
reaction is maintained by additional neutrons generated by protons in spallation target.
Accepted 26 January 2016
This paper presents a concept design and the accident analysis of helium gas-cooled
Available online xxx
tungsten target for 10 MW helium-cooled experimental ADS.
Keywords:
solid domain was simulated with three-dimensional heat conduction model and the fluid
Helium-cooled tungsten target
domain was simulated with one-dimensional quasi-static model. In order to analyze the
ADS
transient characteristics of the target with cooling system, a RELAP5-TRCAP coupling
Accident analysis
model for cooling system was established, in which the decay heat of the target and core
RELAP5-TRCAP
was considered. The simulated results indicate that the peak temperature in the target is
The thermal-hydraulic model of the target coupling with core was built, in which the
lower than the limiting value under the rated operation state and during four typical transient with actuation of the safety system. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the large-scale application and development of nuclear energy, post processing of spent nuclear fuel became a subject of much attention, because of the radioactivity of fission products (FPs) and minor actinides (MAs), as well as the low utilization of fuel. Recently the Accelerator Driven System [1,2] (ADS) is introduced for transmutation of spent nuclear fuel. A concept design of 10 MW helium-cooled experimental ADS with prismatic reactor core and solid target is proposed. And a target-core coupling thermal-hydraulic code TRCAP was developed. In order to perform a preliminary safety assessment
on the target, a RELAP5 model coupling with TRCAP for the cooling system was built. The simulated results indicate that the peak temperature in the target is lower than the limiting value during typical transient scenarios with protected system.
Concept design of ADS Subcritical reactor The annular subcritical reactor is shown in Fig. 1. The subcritical reactor can be divided into the fuel zone,
* Corresponding author. E-mail address: [email protected] (Z. Zhou). http://dx.doi.org/10.1016/j.ijhydene.2016.01.164 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Peng T, et al., Accident analysis of tungsten target coupled with ADS core, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.164
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Fig. 2 e Fuel arrangement and the top view of core. The configuration of the spallation target for the gascooled ADS is schematically illustrated in Figs. 3 and 4, and the proton beam is injected in z-direction through the target window. The target is made into an integral cuboid with flow channels to bear the high pressure of helium coolant. As shown in Figs. 3 and 4, compact round flow channels inside the target increase the convective heat transfer area. To flatten the temperature distribution in the target, the cross flow mode of helium in two nearby layers of flow channels is selected. That is to say, helium flows in x-direction in the channels of odd-numbered layers, and in y-direction in the channels of even-numbered layers. Table 2 shows the parameters of the target. Fig. 1 e Configuration of reactor.
Thermal-hydraulic model and simulation permanent reflector and replaceable reflector. As shown in Fig. 2, there are four kinds of hexagonal prism fuel pin-in block type of graphite fuel assembly in the fuel zone. Since the large surface area and the low power density of the core, after reactor shutdown, the decay heat will be transferred radially through the fuel assemblies, side reflector and reactor vessel to the cooling panel by heat conduction and radiation without any active cooling system. The central part of core is occupied by the spallation target which produces neutrons to drive the subcritical core. The main parameters of the core are listed in Table 1.
Spallation target In ADS, fission chain reactions in the subcritical core are driven by the spallation target. Neutron yield and heat dissipation are the concerns in the design of target. Tungsten is an excellent spallation material for the target due to its high neutron production rate, low neutron absorption, high melting temperature, good heat conductivity and mechanical strength.
Heat transfer mechanisms Fig. 5 shows the mechanisms of heat transfer in ADS. The blue lines (in the web version) demonstrate the flow path of helium, and the red lines demonstrate the heat transfer between solid regions. Under the operation condition, the vast majority
Table 1 e Parameters of core. Geometry Layer number of fuel assembly Assembly number at each layer Height of assembly Assembly spacing Gap between adjacent assemblies Diameter of coolant hole Coolant hole spacing Operating condition Thermal power Pressure Inlet temperature Mass flow
5 30 400 mm 360 mm 2 mm 41 mm 51.5 mm 10 MW 4 MPa 480 K 5 kg s1
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Fig. 3 e Configuration of spallation target.
Math model and simulation program
Fig. 4 e Geometry of target.
Table 2 e Parameters of target. Geometry Length Width Height Number of channel Channel pitch Channel diameter Operating condition Proton energy Current intensity Beam diameter Pressure Inlet temperature Inlet velocity
216 mm 216 mm 104 mm 27 13 8 mm 5 mm
In the heat transfer problem of ADS, the temperature distribution is the main concern. Thus, the solid zone with a threedimensional heat conduction model is analyzed. Because the length of flow channel is much larger than the hydraulic diameter, the three-dimensional effect in helium flow channels can be ignored, and so the one-dimensional model is accurate enough to simulate the flow and convection heat transfer. While the inertia of helium can be negligible, so the quasi-static model is used [3]. The solid domain of the target is divided into 75,816 cubic grids with the size of 4 mm 4 mm 4 mm, shown in Fig. 6 a; helium channels are divided to 18,954 cylinders with 4 mm in length to match the solid grids, shown in Fig. 6 b. For the subcritical reactor, the one sixth of the reactor can represent the whole reactor with the periodic boundary condition, because of the rotational symmetry. The 1/6th of the reactor core is employed as the computational domain, as shown in Fig. 7, and the Reactor Pressure Vessel (RPV) is the outer
250 MeV 4 mA 200 mm 4 MPa 500 K 30 m s1
of deposition heat in target is removed by high pressure helium flow in the channels, while the rest is transferred to the reactor via heat conduction and radiation. The most of heat in core is taken away by helium flow in the fuel regions, and a very small part is removed to cavity cooling panels surrounding the reactor vessel. If the primary coolant system fails, the cavity cooling panels are the main heat sink of the reactor in the residual heat removal phase.
Fig. 5 e Heat transfer mechanisms of ADS.
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Fig. 6 e Target grids (partial).
boundary. In the vertical direction, the computational domain is divided into 12 layers, as shown in Fig. 7; in the horizontal direction, the solid computational domain is divided into 26 nodes, as shown in Fig. 8. The formula [4,5] used to calculate the frictional coefficient in the helium flow channels are shown below. 8 < 64Re1 f ¼ 0:3164Re0:25 : 0:376Re0:25
Re < 2300 Re < 2300; circular Re 2300; annular
(1)
The correlations [4,5] used to calculate the Nusselt number are depicted below.
Nu ¼
8 > > > 1 > > d 3 > > > 3:663 þ 1:663 RePr > > l > > < > 0:0214ðRe0:8 100ÞPr0:4 1 þ > > > > > > > > > > 0:0215Re0:8 Pr0:4 > :
Re < 2300 23 ! d l
Re 2300;
circular
Re 2300;
annular (2)
Based on mathematical model above, the in-house code TRCAP (Target-Reactor Coupling Analysis Program) is developed using FORTRAN language. A RELAP5-TRCAP coupling model is established to simulate the system, as shown in Fig. 9. The RELAP5 code is modified to couple with TRCAP, and the “Equivalent component method” [6] is used to simulate the target and core. Take the case of the target, the target model in RELAP5 is replaced by an equivalent componentda heating pipe with variable loss, which is modeled by “single-volume component”, “valve junction component”, and “heat structure”. The data transmission between RELAP5 and TRCAP is shown in Fig. 10.
Fig. 7 e Reactor nodes & vertical number.
The operation duration of ADS is assumed as one year. The decay heat of tungsten target is calculated according to the data in the Ref. [7], the decay heat is about 3.2 kW when shutdown. The formula in Ref. [8] is used in the work to calculate the decay heat of core. Fig. 13 shows the time variations of the decay heat of target and core.
Results of steady condition With the inlet parameters of helium and the boundary conditions for the cavity cooling panels (70 C), the results of
Heat source of ADS MCNPX code is used to calculate the heat deposition distributed in both target and core. MCNPX (Monte Carlo N-Particle eXtended) is a software package for simulating nuclear processes developed at Los Alamos National Laboratory. The heat distributions of the target and core are given in Fig. 11 and Fig. 12 respectively. The total thermal powers of the target and core are 0.8632 MW and 9.988 MW respectively.
Fig. 8 e Reactor node horizontal number.
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Fig. 9 e RELAP5-TRCAP coupling model.
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Fig. 12 e Power distribution of core.
Fig. 10 e RELAP5-TRCAP coupling calculation method.
Fig. 13 e Decay heat of target and core.
Fig. 11 e Power density distribution of target (W cm¡3).
rated condition are obtained. Fig. 14 shows the threedimensional temperature distribution. The temperature distribution profile is similar to the heat source profile. Because of the convection in the channels, the helium temperature of the downstream is higher than that of the upstream. The hot spot diverges 8 mm from the center along the direction of flow. The peak target temperature is located at (114 mm, 114 mm, 70 mm), its temperature is 2223 K, which is substantially below the melting point of tungsten, namely 3673 K.
Fig. 14 e Target temperature distribution of solid (K).
Safety protection system Without any protection, the target will melt in 9 s during loss of flow accident [9]. To ensure the safety of target, a protection system which can be triggered by low pressure, high pressure
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Table 3 e Trigger signals of protection system. Signal & condition
No. ①
②
③
④
⑤
⑥
variation rate absolute value, low flow, high temperature and other signals is adopted. When the system is triggered, the proton beam, reactor and two circulators of helium loop will be shut down. The triggering signals and triggering conditions are listed in Table 3. In order to simulate the delay of detector, all signals are the average values during last Dt (for the thermal-hydraulic parameters of gas, Dt ¼ 0.5 s; for the temperature of solid, Dt ¼ 0.2 s; for the proton beam current, Dt ¼ 0.01 s); and the execution delay time from trigging is 1.0 s. With conservative considering, assuming that the first signal fails, then the protection system is triggered by the second signal.
Accident analysis Loss of flow accident Circulator is the key equipment of the cooling system, whose reliability concerns the safety of target. Once the circulator is stopped, coolant flow rate will decrease and mismatch the thermal power. In the case of protected LOFA, the circulator 201 loses power at 0.0 s, the accident sequence is shown as Table 4. The process of LOFA can be divided into two stages: short term cooling and long term cooling. In the first stage, the convection of helium and the conduction inside the target are the principal heat transfer mechanisms in target; while in the second stage the heat radiation and conduction between target and reactor are the principal mechanisms. The short term response of loop parameters and target temperature distribution are shown in Fig. 15 and Fig. 16; and the long term temperature response of the target and core is shown in Fig. 17. After losing power, the stop of circulator 201 leads to the fluctuation of loop pressure, and the pressure variation rate of loop exceeds 0.03 MPa/min at 0.013 s. According to the assumption of protection system, this high
Pressure of loop (cold leg of target) Z t pðtÞdt tDt < 3:8 Mpa; Dt ¼ 0:5s Dt Pressure variation rate absolute value pðtÞpðtDtÞ > 0:03 MPa=min; Dt ¼ 0:5s Dt Mass flow of target loop Z t MðtÞdt tDt < 0:63 kg=s; Dt ¼ 0:5s Dt Target temperature at the midpoint of edge (110 mm, 216 mm, 104 mm) Z t TðtÞdt tDt > 1050 K; Dt ¼ 0:2s Dt He temperature of target hot leg Z t Tf ðtÞdt tDt > 730 K; Dt ¼ 0:5s Dt Proton beam current Z t IðtÞdt tDt > 5 mA; Dt ¼ 0:01s Dt
pressure variation rate signal ② fails. Since the decreasing of cooling capacity, heat accumulated in the target, the temperature of target continues to rise. Because of the inertia of the circulator, the mass flow rate of target loop declines slowly, and low flow rate signal ③ is triggered at 1.05 s. Then
Table 4 e Accident sequence of LOFA. Time
Event
0.0 s 0.013 s
The circulator 201 goes offline. Because of the stop of circulator 201, pressure variation rate of loop exceeds 0.03 MPa/min, but signal ② fails. Mass flow rate of target drops to 0.63 kg/s, signal ③ is triggered. The proton beam, reactor and circulator 104 are shut down, the hot spot reaches the first peak temperature of 2394 K. Mass flow rate of target drops to 0.01 kg/s, and the average temperature of the target drops to the bottom of 683 K. Mass flow rate of target drops to 0.0 kg/s, and then reverse flow occurs. Reverse mass flow rate of target reaches the maximum of 0.012 kg/s. Temperature of the target hot spot drops to the bottom of 771 K. Reverse mass flow rate of target drops to 0.001 kg/s. Because of the decay heat of reactor, the target hot spot reaches the second peak target temperature of 1050 K. Temperature of the target hot spot drops to 847 K.
1.05 s 2.05 s
20.0 s
21.1 s
24.6 s 89.6 s 132.0 s 5.03 h
240.0 h
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Fig. 15 e Parameters of the target loop (Pressure and mass flow of 202 in Fig. 9).
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Fig. 17 e Short term temperature response of target and core.
the protection system is actuated, and shuts off proton beam, reactor and circulator 104 at 2.05 s, meanwhile, the Tth (highest temperature of target) reaches the peak of 2394 K. In spite of the attenuation of helium flow, the residual heat dissipation rate is higher than the decay heat of the target, until the mass flow rate of target drops to 0.01 kg/s at 20.0 s. Therefore the Tta (average temperature of target) drops rapidly and reaches the bottom of 683 K at 20.0 s, as shown in Fig. 17. Because of the temperature difference between the hot spot and other parts of the target, the Tth continues to decline and reaches the minimum value of 771 K at 89.6 s. Due to the ratio between convective heat flux and heat capacity of target is larger than the core, the temperature drop of target is more significant than the core. Thus the Tta is lower than the Tb (the average temperature of fuel block near target) during 6 se234 s, as shown in Fig. 17. Because of the losing of heat source and the continual cooling in the cooler, the temperature and pressure of helium flattens out gradually after a fall. After the short term cooling stage controlled by helium flow, the decay heat actuates the core temperature to rise. Due to the large heat capacity and good heat transfer performance, the temperature rise of reactor is small, the Tb reaches the peak of 782 K at 8.07 h. Thereafter, the decay heat is lower than
the heat transfer between RPV and cavity cooling panels, and the temperature of core reduces gradually. For target, the decay heat of target actuates the target temperature to rise, and the Tth reaches the second peak of 1050 K at 5.03 h. As shown in Fig. 19, the target decay heat decreases to lower than the heat flux from target to core, thus the target temperature drops. As shown in Fig. 18, from 5 h to 240 h, the difference between Tta and Tb falls by 91 K, while the Tb falls by 95 K. Thus it can be seen that, the reducing of target decay heat and the declining of core temperature are two equal factor of the cooling process. The small contact area with the surrounding structures restricts the effect of thermal conduction, thus the thermal radiation is the main cooling mechanism of the target under the long term cooling stage. In the protected LOFA, because of the stopping behavior of circulator and the action of protection system, the peak temperature of target is 2394 K, which is in the safe range. Subsequent results show that, besides the LOFA, other protected accidents can also be divided into two stages: short term cooling and long term cooling. In the short term cooling
Fig. 16 e Temperature response of the target (K) (x ¼ 114 mm, z ¼ 70 mm).
Fig. 18 e Long term temperature response of target and core.
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Fig. 19 e Decay heat vs. Heat transfer of target. stage, the target temperature response of the each accident shows its individual features, since different initiating events. In the long term cooling stage, the temperature responses of the target in other accidents are similar to LOFA, because of the same heat sources and the same heat transfer mechanisms. In subsequent sections, the results of short term cooling stage will be focused on, and the results of long term cooling stage will not be repeated.
Small break loss of coolant accident This postulated event assumes the break of target cold leg 202, as shown in Fig. 9. Small break LOCA occurs at 0.0 s, the valve 211 (0.31 cm2) is opened and the helium erupts into the free volume from the target cold leg. In the case of small break LOCA, the accident sequence is listed in Table 5.
Table 5 e Accident sequence of small break LOCA. Time
Event
0.0 s 0.50 s
The target cold leg 202 ruptures. Pressure variation rate of loop exceeds 0.03 MPa/min, but signal ② fails. Pressure drops to 3.8 MPa, signal ① is activated. The proton beam, reactor and two circulators (104, 201) are shut down, the hot spot reaches the first peak temperature of 2335 K, while the mass flow rate of target drops to 0.73 kg/s. Mass flow rate of target drops to 0.01 kg/s The average temperature of the target drops to the bottom of 638 K. Temperature of the target hot spot drops to the minimum value of 726 K. Temperature of the target hot spot rises to 920 K.
69.68 s 70.68 s
88.0 s 88.3 s 128.3 s
700.0 s
Fig. 20 e Parameters of loop (pressure of 105 in Fig. 9, mass flow of 202 in Fig. 9).
Fig. 20 shows the pressure and mass flow of loop; Fig. 21 shows the temperature of hot spot. Due to the break, pressure fluctuation of loop triggers the signal ② which fails at 0.50 s. The break area is small relative to the large reactor loop capacity, that the pressure and flow rate of loop decrease slowly after breaking. The low pressure signal ① is generated at 69.68 s, then the proton beam, reactor and two circulators are shut off 1 s later, the temperature of target hot spot reaches the peak of 2335 K. After the shutdown, the temperature of target falls, and hits the bottom of 726 K at 128.3 s. The response of target in long term cooling stage is similar to LOFA.
Double end rupture loss of coolant accident At the double end rupture LOCA case, a double ended pipe rupture in the target cold leg 202 is postulated to occur at 0.0 s. The accident sequence is listed in Table 6. As shown in Fig. 22, the large break causes the large pressure fluctuation of loop, and triggers the signal ②, but the signal fails. The large leakage flow of break leads to reverse flow in target, the
Table 6 e Accident sequence of double end break LOCA. Time 0.0 s 0.002 s
0.025 s 0.123 s 1.123 s
61.8 s 70.0 s
Event The double ended pipe rupture occurs. Pressure variation rate of loop exceeds 0.03 MPa/min, but signal ② fails. The reverse flow in target reaches the maximum of 2.543 kg/s. He average temperature of outlet reaches 730 K, signal ⑤ is activated. The proton beam, reactor and two circulators (104, 201) are shut down, the hot spot falls to 1770 K. Reverse mass flow rate of target drops to 0.01 kg/s. Temperature of the target hot spot falls to 864 K.
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Table 7 e Accident sequence of beam power jump. Time
Event
0.0 s 0.004 s
The proton beam power jumps. The exceeding proton beam current is detected, but signal ⑥ fails. He average temperature of outlet reaches 730 K, signal ⑤ is activated. The proton beam, reactor and two circulators (104, 201) are shut down, the hot spot reaches the first peak temperature of 2667 K. Temperature of the hot spot falls to 697 K.
1.21 s 2.21 s
60.0 s
Fig. 21 e Temperature response of target.
Fig. 24 e Temperature response of target hot spot.
Fig. 22 e Parameters of loop (pressure of 105 in Fig. 9, mass flow of 202 in Fig. 9).
enhances the convection heat transfer in target, the temperature of target falls. The shutdown occurs at 1.123 s, in the next few seconds the temperature of target falls more quickly. The temperature of the target hot spot falls to 864 K at 70 s as shown in Fig. 23.
Beam power jump accident In the accident of beam power jump, the current of beam is postulated to jump by 50% at 0.0 s under the rated condition, with the proton energy constant. The increased beam power will lead to a 50% larger thermal power of target, which no longer matches the cooling capacity of system. Without regard to the delayed neutron and reactivity feedback, due to the increased neutron source intensity, the power of subcritical reactor will increase by 50% in the step with the target. In the case of beam power jump, the accident sequence is listed in Table 7. Fig. 24 shows the temperature of target hot spot. The hot spot reaches the peak temperature of 2667 K at 2.21 s, and then the temperature drops rapidly because the proton beam has been cut. Fig. 23 e Temperature response of target hot spot. reverse flow reaches the maximum of 2.543 kg/s at 0.025 s. Due to the reverse flow, the helium in reactor hot leg with the temperature of 864 K ingresses the loop of target and triggers signal at 0.123 s. In this process, the large reverse flow
Conclusions In order to calculate the thermal behavior of this target with the influence of surrounding core, a target-reactor coupling
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thermal-hydraulic code TRCAP for ADS was developed, in which the solid domain was simulated with a threedimensional heat conduction model and the fluid domain was simulated with a one-dimensional quasi-static model. A RELAP5 model coupling with TRCAP for the cooling system was built to simulate the transients of ADS, the decay heat of target and core was included in the model. The result of rated condition demonstrates the feasibility of the design preliminarily. Four typical accident events were studied to perform a preliminary safety assessment on the target from the perspective of thermal hydraulics. Results show that the peak temperature in the target under the transients is lower than the limiting value, they demonstrate the effectiveness of transient model and the safety margin of target. Results of transients indicates that, the stopping behavior of circulator and timely shutdown with protection system is the key factor for the target safety during short term cooling stage; while the large heat capacity and good heat transfer performance of the reactor ensure the safety during long term cooling stage.
Acknowledgments This work is supported by the special project of the State Natural Science Fund for fuel breeding and transmutation of advanced nuclear fission energy with grant number 91426301 (sub-project number: Y505030GJ0).
Nomenclature d f I l M Min Nu p pout
diameter of channel, m friction coefficient in channel proton beam current, mA length of flow channel, m mass flow rate of helium, kg s1 inlet mass flow rate of target/core, kg s1 Nusselt number of helium pressure of helium, Pa outlet helium pressure of target/core, Pa
Pr Re t T Tb Tin Tta Tth Dp DT
Prandtl number of helium Reynolds number of helium time, s temperature, K average temperature of fuel block near target, K inlet helium temperature of target/core, K average temperature of target, K highest temperature of target, K pressure drop of target/core, Pa temperature rise of He through target/core, K
references
[1] Rubbia C, Rubio JA, Buono S, Carminati F, Fietier N, Galvez J, et al. Conceptual design of a fast neutron operated high power energy amplifier. CERN/AT/95e44(ET). 1995. [2] NEAeOECD. Accelerator-driven systems (ADS) and fast reactors (FR) in advanced nuclear fuel cycles. 2002. [3] Gao Qiang. Research on the engineering simulator for HTR-PM based on THERMIX/BLAST and vPower. Doctor thesis of Tsinghua University. 2011. [4] Zhou Kefeng. Research on the simulation method of thermalhydraulic process in high temperature gas-cooled reactor. Doctor thesis of Tsinghua University. 2011. [5] Han Kyu-Hyun, Seo Kyong-Won, Hwang Dae-Hyun, Chang Soon Heung. Development of a thermal hydraulic analysis code for gas-cooled reactors with annular fuels. Nucl Eng Des 2006;236:164e78. [6] Wang Dengying. Simulation of high temperature gas cooled reactor's multi-loop system. Doctor thesis of Tsinghua University. 2011. [7] Takenaka N, Nio D, Kiyanagi Y, Mishima K, Kawai M, Furusaka M. Thermal hydraulic design and decay heat removal of a solid target for a spallation neutron source. J Nucl Mater 2005;343:169e77. [8] Draft ANS-5.1/N18.6. Decay energy release rates following shutdown of uranium-fueled thermal reactors. 1973. [9] Peng Tianji, Zhou Zhiwei, Gu Long. Thermal-hydraulic design and transient analysis of helium-cooled solid target for ADS. In: 22nd International Conference on Nuclear Engineering; 2014.
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Accident analysis of tungsten target coupled with ADS core Tianji Peng a, Minggang Lang a, Zhiwei Zhou a,*, Long Gu b a
Institute of Nuclear and New Energy Technology, Collaborative Innovation Center of Advanced Nuclear Energy Technology, Key Laboratory of Advanced Reactor Engineering and Safety of Ministry of Education, Tsinghua University, Beijing, 100084, China b Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 730000, China
article info
abstract
Article history:
The Accelerator Driven Subcritical Reactor System (ADS) is a kind of nuclear reactor which
Received 22 October 2015
can burn minor actinide waste produced from conventional reactors with inherent safety
Received in revised form
features. Because of the characteristics of a sub-critical reaction process, the fission chain
26 January 2016
reaction is maintained by additional neutrons generated by protons in spallation target.
Accepted 26 January 2016
This paper presents a concept design and the accident analysis of helium gas-cooled
Available online xxx
tungsten target for 10 MW helium-cooled experimental ADS.
Keywords:
solid domain was simulated with three-dimensional heat conduction model and the fluid
Helium-cooled tungsten target
domain was simulated with one-dimensional quasi-static model. In order to analyze the
ADS
transient characteristics of the target with cooling system, a RELAP5-TRCAP coupling
Accident analysis
model for cooling system was established, in which the decay heat of the target and core
RELAP5-TRCAP
was considered. The simulated results indicate that the peak temperature in the target is
The thermal-hydraulic model of the target coupling with core was built, in which the
lower than the limiting value under the rated operation state and during four typical transient with actuation of the safety system. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the large-scale application and development of nuclear energy, post processing of spent nuclear fuel became a subject of much attention, because of the radioactivity of fission products (FPs) and minor actinides (MAs), as well as the low utilization of fuel. Recently the Accelerator Driven System [1,2] (ADS) is introduced for transmutation of spent nuclear fuel. A concept design of 10 MW helium-cooled experimental ADS with prismatic reactor core and solid target is proposed. And a target-core coupling thermal-hydraulic code TRCAP was developed. In order to perform a preliminary safety assessment
on the target, a RELAP5 model coupling with TRCAP for the cooling system was built. The simulated results indicate that the peak temperature in the target is lower than the limiting value during typical transient scenarios with protected system.
Concept design of ADS Subcritical reactor The annular subcritical reactor is shown in Fig. 1. The subcritical reactor can be divided into the fuel zone,
* Corresponding author. E-mail address: [email protected] (Z. Zhou). http://dx.doi.org/10.1016/j.ijhydene.2016.01.164 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Peng T, et al., Accident analysis of tungsten target coupled with ADS core, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.164
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Fig. 2 e Fuel arrangement and the top view of core. The configuration of the spallation target for the gascooled ADS is schematically illustrated in Figs. 3 and 4, and the proton beam is injected in z-direction through the target window. The target is made into an integral cuboid with flow channels to bear the high pressure of helium coolant. As shown in Figs. 3 and 4, compact round flow channels inside the target increase the convective heat transfer area. To flatten the temperature distribution in the target, the cross flow mode of helium in two nearby layers of flow channels is selected. That is to say, helium flows in x-direction in the channels of odd-numbered layers, and in y-direction in the channels of even-numbered layers. Table 2 shows the parameters of the target. Fig. 1 e Configuration of reactor.
Thermal-hydraulic model and simulation permanent reflector and replaceable reflector. As shown in Fig. 2, there are four kinds of hexagonal prism fuel pin-in block type of graphite fuel assembly in the fuel zone. Since the large surface area and the low power density of the core, after reactor shutdown, the decay heat will be transferred radially through the fuel assemblies, side reflector and reactor vessel to the cooling panel by heat conduction and radiation without any active cooling system. The central part of core is occupied by the spallation target which produces neutrons to drive the subcritical core. The main parameters of the core are listed in Table 1.
Spallation target In ADS, fission chain reactions in the subcritical core are driven by the spallation target. Neutron yield and heat dissipation are the concerns in the design of target. Tungsten is an excellent spallation material for the target due to its high neutron production rate, low neutron absorption, high melting temperature, good heat conductivity and mechanical strength.
Heat transfer mechanisms Fig. 5 shows the mechanisms of heat transfer in ADS. The blue lines (in the web version) demonstrate the flow path of helium, and the red lines demonstrate the heat transfer between solid regions. Under the operation condition, the vast majority
Table 1 e Parameters of core. Geometry Layer number of fuel assembly Assembly number at each layer Height of assembly Assembly spacing Gap between adjacent assemblies Diameter of coolant hole Coolant hole spacing Operating condition Thermal power Pressure Inlet temperature Mass flow
5 30 400 mm 360 mm 2 mm 41 mm 51.5 mm 10 MW 4 MPa 480 K 5 kg s1
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Fig. 3 e Configuration of spallation target.
Math model and simulation program
Fig. 4 e Geometry of target.
Table 2 e Parameters of target. Geometry Length Width Height Number of channel Channel pitch Channel diameter Operating condition Proton energy Current intensity Beam diameter Pressure Inlet temperature Inlet velocity
216 mm 216 mm 104 mm 27 13 8 mm 5 mm
In the heat transfer problem of ADS, the temperature distribution is the main concern. Thus, the solid zone with a threedimensional heat conduction model is analyzed. Because the length of flow channel is much larger than the hydraulic diameter, the three-dimensional effect in helium flow channels can be ignored, and so the one-dimensional model is accurate enough to simulate the flow and convection heat transfer. While the inertia of helium can be negligible, so the quasi-static model is used [3]. The solid domain of the target is divided into 75,816 cubic grids with the size of 4 mm 4 mm 4 mm, shown in Fig. 6 a; helium channels are divided to 18,954 cylinders with 4 mm in length to match the solid grids, shown in Fig. 6 b. For the subcritical reactor, the one sixth of the reactor can represent the whole reactor with the periodic boundary condition, because of the rotational symmetry. The 1/6th of the reactor core is employed as the computational domain, as shown in Fig. 7, and the Reactor Pressure Vessel (RPV) is the outer
250 MeV 4 mA 200 mm 4 MPa 500 K 30 m s1
of deposition heat in target is removed by high pressure helium flow in the channels, while the rest is transferred to the reactor via heat conduction and radiation. The most of heat in core is taken away by helium flow in the fuel regions, and a very small part is removed to cavity cooling panels surrounding the reactor vessel. If the primary coolant system fails, the cavity cooling panels are the main heat sink of the reactor in the residual heat removal phase.
Fig. 5 e Heat transfer mechanisms of ADS.
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Fig. 6 e Target grids (partial).
boundary. In the vertical direction, the computational domain is divided into 12 layers, as shown in Fig. 7; in the horizontal direction, the solid computational domain is divided into 26 nodes, as shown in Fig. 8. The formula [4,5] used to calculate the frictional coefficient in the helium flow channels are shown below. 8 < 64Re1 f ¼ 0:3164Re0:25 : 0:376Re0:25
Re < 2300 Re < 2300; circular Re 2300; annular
(1)
The correlations [4,5] used to calculate the Nusselt number are depicted below.
Nu ¼
8 > > > 1 > > d 3 > > > 3:663 þ 1:663 RePr > > l > > < > 0:0214ðRe0:8 100ÞPr0:4 1 þ > > > > > > > > > > 0:0215Re0:8 Pr0:4 > :
Re < 2300 23 ! d l
Re 2300;
circular
Re 2300;
annular (2)
Based on mathematical model above, the in-house code TRCAP (Target-Reactor Coupling Analysis Program) is developed using FORTRAN language. A RELAP5-TRCAP coupling model is established to simulate the system, as shown in Fig. 9. The RELAP5 code is modified to couple with TRCAP, and the “Equivalent component method” [6] is used to simulate the target and core. Take the case of the target, the target model in RELAP5 is replaced by an equivalent componentda heating pipe with variable loss, which is modeled by “single-volume component”, “valve junction component”, and “heat structure”. The data transmission between RELAP5 and TRCAP is shown in Fig. 10.
Fig. 7 e Reactor nodes & vertical number.
The operation duration of ADS is assumed as one year. The decay heat of tungsten target is calculated according to the data in the Ref. [7], the decay heat is about 3.2 kW when shutdown. The formula in Ref. [8] is used in the work to calculate the decay heat of core. Fig. 13 shows the time variations of the decay heat of target and core.
Results of steady condition With the inlet parameters of helium and the boundary conditions for the cavity cooling panels (70 C), the results of
Heat source of ADS MCNPX code is used to calculate the heat deposition distributed in both target and core. MCNPX (Monte Carlo N-Particle eXtended) is a software package for simulating nuclear processes developed at Los Alamos National Laboratory. The heat distributions of the target and core are given in Fig. 11 and Fig. 12 respectively. The total thermal powers of the target and core are 0.8632 MW and 9.988 MW respectively.
Fig. 8 e Reactor node horizontal number.
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Fig. 9 e RELAP5-TRCAP coupling model.
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Fig. 12 e Power distribution of core.
Fig. 10 e RELAP5-TRCAP coupling calculation method.
Fig. 13 e Decay heat of target and core.
Fig. 11 e Power density distribution of target (W cm¡3).
rated condition are obtained. Fig. 14 shows the threedimensional temperature distribution. The temperature distribution profile is similar to the heat source profile. Because of the convection in the channels, the helium temperature of the downstream is higher than that of the upstream. The hot spot diverges 8 mm from the center along the direction of flow. The peak target temperature is located at (114 mm, 114 mm, 70 mm), its temperature is 2223 K, which is substantially below the melting point of tungsten, namely 3673 K.
Fig. 14 e Target temperature distribution of solid (K).
Safety protection system Without any protection, the target will melt in 9 s during loss of flow accident [9]. To ensure the safety of target, a protection system which can be triggered by low pressure, high pressure
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Table 3 e Trigger signals of protection system. Signal & condition
No. ①
②
③
④
⑤
⑥
variation rate absolute value, low flow, high temperature and other signals is adopted. When the system is triggered, the proton beam, reactor and two circulators of helium loop will be shut down. The triggering signals and triggering conditions are listed in Table 3. In order to simulate the delay of detector, all signals are the average values during last Dt (for the thermal-hydraulic parameters of gas, Dt ¼ 0.5 s; for the temperature of solid, Dt ¼ 0.2 s; for the proton beam current, Dt ¼ 0.01 s); and the execution delay time from trigging is 1.0 s. With conservative considering, assuming that the first signal fails, then the protection system is triggered by the second signal.
Accident analysis Loss of flow accident Circulator is the key equipment of the cooling system, whose reliability concerns the safety of target. Once the circulator is stopped, coolant flow rate will decrease and mismatch the thermal power. In the case of protected LOFA, the circulator 201 loses power at 0.0 s, the accident sequence is shown as Table 4. The process of LOFA can be divided into two stages: short term cooling and long term cooling. In the first stage, the convection of helium and the conduction inside the target are the principal heat transfer mechanisms in target; while in the second stage the heat radiation and conduction between target and reactor are the principal mechanisms. The short term response of loop parameters and target temperature distribution are shown in Fig. 15 and Fig. 16; and the long term temperature response of the target and core is shown in Fig. 17. After losing power, the stop of circulator 201 leads to the fluctuation of loop pressure, and the pressure variation rate of loop exceeds 0.03 MPa/min at 0.013 s. According to the assumption of protection system, this high
Pressure of loop (cold leg of target) Z t pðtÞdt tDt < 3:8 Mpa; Dt ¼ 0:5s Dt Pressure variation rate absolute value pðtÞpðtDtÞ > 0:03 MPa=min; Dt ¼ 0:5s Dt Mass flow of target loop Z t MðtÞdt tDt < 0:63 kg=s; Dt ¼ 0:5s Dt Target temperature at the midpoint of edge (110 mm, 216 mm, 104 mm) Z t TðtÞdt tDt > 1050 K; Dt ¼ 0:2s Dt He temperature of target hot leg Z t Tf ðtÞdt tDt > 730 K; Dt ¼ 0:5s Dt Proton beam current Z t IðtÞdt tDt > 5 mA; Dt ¼ 0:01s Dt
pressure variation rate signal ② fails. Since the decreasing of cooling capacity, heat accumulated in the target, the temperature of target continues to rise. Because of the inertia of the circulator, the mass flow rate of target loop declines slowly, and low flow rate signal ③ is triggered at 1.05 s. Then
Table 4 e Accident sequence of LOFA. Time
Event
0.0 s 0.013 s
The circulator 201 goes offline. Because of the stop of circulator 201, pressure variation rate of loop exceeds 0.03 MPa/min, but signal ② fails. Mass flow rate of target drops to 0.63 kg/s, signal ③ is triggered. The proton beam, reactor and circulator 104 are shut down, the hot spot reaches the first peak temperature of 2394 K. Mass flow rate of target drops to 0.01 kg/s, and the average temperature of the target drops to the bottom of 683 K. Mass flow rate of target drops to 0.0 kg/s, and then reverse flow occurs. Reverse mass flow rate of target reaches the maximum of 0.012 kg/s. Temperature of the target hot spot drops to the bottom of 771 K. Reverse mass flow rate of target drops to 0.001 kg/s. Because of the decay heat of reactor, the target hot spot reaches the second peak target temperature of 1050 K. Temperature of the target hot spot drops to 847 K.
1.05 s 2.05 s
20.0 s
21.1 s
24.6 s 89.6 s 132.0 s 5.03 h
240.0 h
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Fig. 15 e Parameters of the target loop (Pressure and mass flow of 202 in Fig. 9).
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Fig. 17 e Short term temperature response of target and core.
the protection system is actuated, and shuts off proton beam, reactor and circulator 104 at 2.05 s, meanwhile, the Tth (highest temperature of target) reaches the peak of 2394 K. In spite of the attenuation of helium flow, the residual heat dissipation rate is higher than the decay heat of the target, until the mass flow rate of target drops to 0.01 kg/s at 20.0 s. Therefore the Tta (average temperature of target) drops rapidly and reaches the bottom of 683 K at 20.0 s, as shown in Fig. 17. Because of the temperature difference between the hot spot and other parts of the target, the Tth continues to decline and reaches the minimum value of 771 K at 89.6 s. Due to the ratio between convective heat flux and heat capacity of target is larger than the core, the temperature drop of target is more significant than the core. Thus the Tta is lower than the Tb (the average temperature of fuel block near target) during 6 se234 s, as shown in Fig. 17. Because of the losing of heat source and the continual cooling in the cooler, the temperature and pressure of helium flattens out gradually after a fall. After the short term cooling stage controlled by helium flow, the decay heat actuates the core temperature to rise. Due to the large heat capacity and good heat transfer performance, the temperature rise of reactor is small, the Tb reaches the peak of 782 K at 8.07 h. Thereafter, the decay heat is lower than
the heat transfer between RPV and cavity cooling panels, and the temperature of core reduces gradually. For target, the decay heat of target actuates the target temperature to rise, and the Tth reaches the second peak of 1050 K at 5.03 h. As shown in Fig. 19, the target decay heat decreases to lower than the heat flux from target to core, thus the target temperature drops. As shown in Fig. 18, from 5 h to 240 h, the difference between Tta and Tb falls by 91 K, while the Tb falls by 95 K. Thus it can be seen that, the reducing of target decay heat and the declining of core temperature are two equal factor of the cooling process. The small contact area with the surrounding structures restricts the effect of thermal conduction, thus the thermal radiation is the main cooling mechanism of the target under the long term cooling stage. In the protected LOFA, because of the stopping behavior of circulator and the action of protection system, the peak temperature of target is 2394 K, which is in the safe range. Subsequent results show that, besides the LOFA, other protected accidents can also be divided into two stages: short term cooling and long term cooling. In the short term cooling
Fig. 16 e Temperature response of the target (K) (x ¼ 114 mm, z ¼ 70 mm).
Fig. 18 e Long term temperature response of target and core.
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Fig. 19 e Decay heat vs. Heat transfer of target. stage, the target temperature response of the each accident shows its individual features, since different initiating events. In the long term cooling stage, the temperature responses of the target in other accidents are similar to LOFA, because of the same heat sources and the same heat transfer mechanisms. In subsequent sections, the results of short term cooling stage will be focused on, and the results of long term cooling stage will not be repeated.
Small break loss of coolant accident This postulated event assumes the break of target cold leg 202, as shown in Fig. 9. Small break LOCA occurs at 0.0 s, the valve 211 (0.31 cm2) is opened and the helium erupts into the free volume from the target cold leg. In the case of small break LOCA, the accident sequence is listed in Table 5.
Table 5 e Accident sequence of small break LOCA. Time
Event
0.0 s 0.50 s
The target cold leg 202 ruptures. Pressure variation rate of loop exceeds 0.03 MPa/min, but signal ② fails. Pressure drops to 3.8 MPa, signal ① is activated. The proton beam, reactor and two circulators (104, 201) are shut down, the hot spot reaches the first peak temperature of 2335 K, while the mass flow rate of target drops to 0.73 kg/s. Mass flow rate of target drops to 0.01 kg/s The average temperature of the target drops to the bottom of 638 K. Temperature of the target hot spot drops to the minimum value of 726 K. Temperature of the target hot spot rises to 920 K.
69.68 s 70.68 s
88.0 s 88.3 s 128.3 s
700.0 s
Fig. 20 e Parameters of loop (pressure of 105 in Fig. 9, mass flow of 202 in Fig. 9).
Fig. 20 shows the pressure and mass flow of loop; Fig. 21 shows the temperature of hot spot. Due to the break, pressure fluctuation of loop triggers the signal ② which fails at 0.50 s. The break area is small relative to the large reactor loop capacity, that the pressure and flow rate of loop decrease slowly after breaking. The low pressure signal ① is generated at 69.68 s, then the proton beam, reactor and two circulators are shut off 1 s later, the temperature of target hot spot reaches the peak of 2335 K. After the shutdown, the temperature of target falls, and hits the bottom of 726 K at 128.3 s. The response of target in long term cooling stage is similar to LOFA.
Double end rupture loss of coolant accident At the double end rupture LOCA case, a double ended pipe rupture in the target cold leg 202 is postulated to occur at 0.0 s. The accident sequence is listed in Table 6. As shown in Fig. 22, the large break causes the large pressure fluctuation of loop, and triggers the signal ②, but the signal fails. The large leakage flow of break leads to reverse flow in target, the
Table 6 e Accident sequence of double end break LOCA. Time 0.0 s 0.002 s
0.025 s 0.123 s 1.123 s
61.8 s 70.0 s
Event The double ended pipe rupture occurs. Pressure variation rate of loop exceeds 0.03 MPa/min, but signal ② fails. The reverse flow in target reaches the maximum of 2.543 kg/s. He average temperature of outlet reaches 730 K, signal ⑤ is activated. The proton beam, reactor and two circulators (104, 201) are shut down, the hot spot falls to 1770 K. Reverse mass flow rate of target drops to 0.01 kg/s. Temperature of the target hot spot falls to 864 K.
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Table 7 e Accident sequence of beam power jump. Time
Event
0.0 s 0.004 s
The proton beam power jumps. The exceeding proton beam current is detected, but signal ⑥ fails. He average temperature of outlet reaches 730 K, signal ⑤ is activated. The proton beam, reactor and two circulators (104, 201) are shut down, the hot spot reaches the first peak temperature of 2667 K. Temperature of the hot spot falls to 697 K.
1.21 s 2.21 s
60.0 s
Fig. 21 e Temperature response of target.
Fig. 24 e Temperature response of target hot spot.
Fig. 22 e Parameters of loop (pressure of 105 in Fig. 9, mass flow of 202 in Fig. 9).
enhances the convection heat transfer in target, the temperature of target falls. The shutdown occurs at 1.123 s, in the next few seconds the temperature of target falls more quickly. The temperature of the target hot spot falls to 864 K at 70 s as shown in Fig. 23.
Beam power jump accident In the accident of beam power jump, the current of beam is postulated to jump by 50% at 0.0 s under the rated condition, with the proton energy constant. The increased beam power will lead to a 50% larger thermal power of target, which no longer matches the cooling capacity of system. Without regard to the delayed neutron and reactivity feedback, due to the increased neutron source intensity, the power of subcritical reactor will increase by 50% in the step with the target. In the case of beam power jump, the accident sequence is listed in Table 7. Fig. 24 shows the temperature of target hot spot. The hot spot reaches the peak temperature of 2667 K at 2.21 s, and then the temperature drops rapidly because the proton beam has been cut. Fig. 23 e Temperature response of target hot spot. reverse flow reaches the maximum of 2.543 kg/s at 0.025 s. Due to the reverse flow, the helium in reactor hot leg with the temperature of 864 K ingresses the loop of target and triggers signal at 0.123 s. In this process, the large reverse flow
Conclusions In order to calculate the thermal behavior of this target with the influence of surrounding core, a target-reactor coupling
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thermal-hydraulic code TRCAP for ADS was developed, in which the solid domain was simulated with a threedimensional heat conduction model and the fluid domain was simulated with a one-dimensional quasi-static model. A RELAP5 model coupling with TRCAP for the cooling system was built to simulate the transients of ADS, the decay heat of target and core was included in the model. The result of rated condition demonstrates the feasibility of the design preliminarily. Four typical accident events were studied to perform a preliminary safety assessment on the target from the perspective of thermal hydraulics. Results show that the peak temperature in the target under the transients is lower than the limiting value, they demonstrate the effectiveness of transient model and the safety margin of target. Results of transients indicates that, the stopping behavior of circulator and timely shutdown with protection system is the key factor for the target safety during short term cooling stage; while the large heat capacity and good heat transfer performance of the reactor ensure the safety during long term cooling stage.
Acknowledgments This work is supported by the special project of the State Natural Science Fund for fuel breeding and transmutation of advanced nuclear fission energy with grant number 91426301 (sub-project number: Y505030GJ0).
Nomenclature d f I l M Min Nu p pout
diameter of channel, m friction coefficient in channel proton beam current, mA length of flow channel, m mass flow rate of helium, kg s1 inlet mass flow rate of target/core, kg s1 Nusselt number of helium pressure of helium, Pa outlet helium pressure of target/core, Pa
Pr Re t T Tb Tin Tta Tth Dp DT
Prandtl number of helium Reynolds number of helium time, s temperature, K average temperature of fuel block near target, K inlet helium temperature of target/core, K average temperature of target, K highest temperature of target, K pressure drop of target/core, Pa temperature rise of He through target/core, K
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