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Power level control of CANDLE reactor without control rods
Annals of Nuclear Energy 63 (2014) 427–431
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Power level control of CANDLE reactor without control rods Hiroshi Sekimoto ⇑, Sinsuke Nakayama Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-17, Ookayama, Meguro-ku 152-8550, Japan
a r t i c l e
i n f o
Article history: Received 1 April 2013 Received in revised form 11 August 2013 Accepted 13 August 2013 Available online 5 September 2013 Keywords: CANDLE reactor Power level Coolant flow rate Coolant inlet temperature Coolant outlet temperature 208 Pb
a b s t r a c t The power level control of CANDLE reactor without help of control rods is desirable for obtaining better neutron economy which is inevitable for CANDLE burning. In the present paper the power level is changed by adjusting its coolant flow rate and/or inlet coolant temperature. It can be demonstrated successfully to change the power level by adjusting both coolant flow rate and inlet coolant temperature from its full power to zero for SFR (sodium cooled fast reactor) and LFR (208Pb cooled fast reactor) while the coolant outlet temperature is kept at its designed value. The change of coolant inlet temperature is smaller for LFR and may be better for the heat transfer to the secondary coolant. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In conventional reactors, shim control rods inserted at the startup of operation are gradually extracted with fuel burning in order to keep the reactor critical. On the other hand, CANDLE (Constant Axial shape of Neutron flux, nuclide densities and power shape During Life of Energy production) reactors do not need this kind of control rods (Sekimoto and Ryu, 2000a; Sekimoto et al., 2001). Their burning region moves in the direction of core axis, at a speed proportionate to the power output, without changing the spatial distribution of nuclide densities, neutron flux and power density as shown in Fig. 1. CANDLE fast reactors require only natural uranium or depleted uranium as their fresh fuel. CANDLE reactors have three technical problems. The first problem is that the burning region of the initial core of their first cycle cannot be constructed using only naturally occurring nuclides. However it is rather easy problem. The CANDLE burning can be easily started from the initial core constructed by using enriched uranium where the enrichment is properly distributed in the axial direction as demonstrated in the reference (Sekimoto and Miyashita, 2006). The second problem is the extremely high burnup of the spent fuel which is about 40% (400 MWd/tHM). It causes difficult problems on irradiated materials of fuel elements, but it may be solved by recladding (Sekimoto and Nagata, 2009). The last problem is a severe requirement on neutron economy. This problem will be discussed later in this section. ⇑ Corresponding author. Tel./fax: +81 3 3929 8753. E-mail addresses: [email protected] (H. Sekimoto), [email protected] (S. Nakayama). 0306-4549/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2013.08.022
The nuclear energy has a resource problem, if we use only lightwater cooled reactors (LWR). It has also inherent difficult problems caused by radioactive materials produced in it and by employed materials and technologies tightly relating to nuclear bombs. The radioactive materials cause the problem of accident during reactor operation, and the problem of radioactive wastes after reactor operation. Finally reasonable price is usually an important requirement for base-load energy. Thus the necessary and sufficient conditions for nuclear energy utilization as primary energy in the future are considered to satisfy five requirements concerning resource, safety, waste, bomb, and economy. CANDLE reactors show much better performances than the conventional nuclear reactors for the first four requirements, resource, safety, waste and bomb as shown in the reference (Sekimoto, 2010). For the economy, though the situation seems more complicated, CANDLE has a potential to realize excellent economy performances. The power density is very sensitive to the power generation cost, and the higher power density makes the economy better. CANDLE reactors do not have blanket regions. It makes the average power density higher, when we consider the total volume of core and blanket as its denominator. However, the coolant volume fraction in the core is small as mentioned below, and the cooling performance is poor. It makes the average power density lower. CANDLE reactors have to produce fissile nuclides whose amount is more than consumed ones. However, the density distribution of each fuel nuclide through core is not optimized for this purpose. The neutrons moving downward in the core may be efficiently used for fissile production, but the neutrons moving upward will not be used efficient way. Therefore, some techniques for
428
H. Sekimoto, S. Nakayama / Annals of Nuclear Energy 63 (2014) 427–431
Fig. 1. CANDLE burning and fuel management.
increasing neutron economy compared with conventional fast breeder reactors are required for realizing CANDLE burning. This is the third problem of CANDLE reactors mentioned above. From our previous studies only fast reactors with metallic or nitride fuel can realize this burning but oxide fuel cannot (Sekimoto and Ryu, 2000c). The fuel volume fraction in the core should be larger than conventional designs (Sekimoto and Ryu, 2000b). It means the coolant channel cross section becomes smaller and then cooling performance becomes worse as mentioned above. The power density of CANDEL reactors has been increased by making its radial power distribution flatter and the core height shorter (Sekimoto et al., 2010). Using 208Pb instead of natural lead as coolant improves the neutron economy (Sekimoto, 2013) and increases the coolant channel cross section. It also increases the power density. The control rods staying in the core disturb power shape as well as decrease the fuel volume fraction. It decreases power density and deteriorates the neutron economy. In-core control rods are usually used for controlling excess reactivity with increasing burnup (shim control rods), for making shutdown and startup (shutdown control rods) and for power regulation and load following (regulation control rods). In CANDLE reactors, the excess reactivity does not change with burnup and the shim control rods are not necessary. The reactor is usually made in the cold shutdown state, while the shutdown state is maintained for a considerable period. The cold shutdown requires more shutdown reactivity worth than the hot shutdown. Usually the shutdown control rods are inserted from the hot shutdown to cold shutdown. The shutdown control rods can be replaced by properly selected several movable fuel assemblies in the core. These fuel assemblies are out of core during cold shutdown state, while no fuel burning occurs. Therefore, the fuel compositions of all fuel assemblies do not change during this process, and this method does not cause any effects on CANDLE burning. The power regulation and load following are more delicate process when regulation control rods are not facilitated. In the present paper they will be performed by adjusting coolant flow rate and/or core inlet coolant temperature.
2. Reactor designs
Table 1 Reactor core design parameters for two CANDLE reactors. SFR
LFR
Coolant Thermal power rate (MW) Electric power rate (MW) Core diameter (cm) Core height (cm)
Sodium 2350 990 402.4 166.5
208
Plenum height (cm) Pool thickness (cm) Coolant inlet temperature (°C) Coolant outlet temperature (°C)
100 Upper and lower axial 100, radial 200 395 400 550 560
Core support material Effective neutron multiplication factor
HT-9 1.0005
Pb-95%, 1315 580 352.3 141.3
207
Pb-5%
1.0003
Table 2 Fuel cell design parameters for two CANDLE reactors. SFR
LFR 235
Charged fresh fuel Smear density Zr ratio
Metallic fuel [U–Zr, 65% 6%
Cladding material Cladding thickness (mm) Fuel pin diameter, D (mm) Fuel pin pitch, P (mm) P/D
HT-9 0.51 8.56 9.46 1.105
0.51 8.56 10.72 1.252
Bonding material Volume ratio (fuel/cladding/coolant) (%)
Sodium 57.7/16.4/25.9
44.7/12.7/42.6
U-0.2%]
Table 3 Obtained characteristic values for the initial steady state for two CANDLE reactors. SFR
LFR
Speed of burning region (cm/y) Maximum coolant speed (m/s) Maximum linear heat rate (W/cm) Maximum cladding temperature (°C)
5.17 10.0 (10) 383 (500) 586 (650)
3.47 2.0 (2) 238 (500) 618 (650)
Void reactivity coefficient (Dk/k) Void reactivity coefficient ($) Loss of coolant pressure (MPa)
3.53% (–) 7.94 (8) 0.47 (–)
1.31% (–) 3.09 (8) 0.26 (–)
The values in () are the limit values imposed as constraints.
In the present paper two types of metallic fuel fast reactors are investigated, where the coolant is either sodium or 208Pb (Sekimoto, 2013). We call these reactors SFR and LFR, respectively. The size of reactor core can be more easily reduced for LFR than SFR, and the size of LFR is set smaller than SFR for this study. Main design parameters of reactor core and fuel cell for these reactors are shown in Tables 1 and 2, respectively. These values are not manufacture values but values at the operating conditions of reactors at the steady state. We tried to make each effective neutron multiplication factor more than unity and as near to unity as
possible with reasonable calculation time. Some of the important characteristic values obtained under these operating conditions are shown in Table 3. They satisfy their constraints shown in the same table. They will be used as the initial states for the power change trials in the present study. We will try to investigate the performance of methods of power level control. In these methods coolant parameters will be changed from their initial state given in these tables. The data given by these tables are for the full power operation. The power level
H. Sekimoto, S. Nakayama / Annals of Nuclear Energy 63 (2014) 427–431 Table 4 Adjusted and fixed parameters for investigated three methods. Adjusted parameters
Fixed parameters
Method 1
Coolant velocity
Method 2 Method 3
Coolant inlet temperature Coolant velocity, coolant inlet temperature
Coolant inlet temperature Coolant velocity Coolant outlet temperature
targeted by the control is lower than the initial value given in Table 1 (the design value). In the present paper only the final condition for each operation is investigated. Transient behaviors will be discussed briefly in Section 5. Details of the methods will be shown in Section 3, and the algorithm of its analysis for each method will be shown briefly in Section 4. 3. Control methods When we try to change the power level, we can choose several parameters of coolant as control variables. In the present study we
429
investigate three methods shown in Table 4. The coolant outlet temperature is considered the best parameter to be fixed for this problem (Method 3), but two simple methods (only one adjusted parameter) are also shown for comparisons (Nakayama et al., 2011). The coolant pressure drop changes when the coolant parameters are changed particularly when the flow rate is changed (Methods 1 and 3). 4. Analysis A brief flowchart of the analysis is shown in Fig. 2 for each of the three methods given in Table 4. The fixed parameters for Methods 1 and 2 do not change during the calculation, but the fixed parameter for Method 3, the coolant outlet temperature, changes for changing the adjusted parameters independently and should be converged to the designed value by adjusting two control parameters synchronously. For Methods 1 and 2 the outlet temperature changes from the designed value with changing the adjusted parameter. The differences of coolant outlet temperature at different coolant channel are not important for these cases. For Method
Fig. 2. Flowchart of analysis of the coolant control for adjusting total power.
430
H. Sekimoto, S. Nakayama / Annals of Nuclear Energy 63 (2014) 427–431
1 not uk at each coolant channel k but one parameter r is adjusted, where r is the coolant velocity reduction rate and assumed constant over all coolant channel k. The criticality is confirmed by convergence of the effective neutron multiplication factor to its initial value given in Table 1. The criteria of convergence of the coolant outlet temperature for Method 3 and effective neutron multiplication factor are shown in Fig. 2.
5. Calculation results and discussion The calculation results are shown in Fig. 3, where the outlet coolant temperature is the averaged value over all coolant channels. The results for SFR is shown in (A) and LFR in (B). For Method 1 the outlet coolant temperature increases with decreasing power level for both reactor types and reaches to its maximum permissible level. This change is more rapid for SFR than LFR, and the controllable range of power level is smaller for SFR. For Method 2 the outlet coolant temperature decreases with decreasing power level. It means the thermal efficiency decreases with decreasing power level. This effect is larger for LFR than SFR. Method 3 can be performed for whole range of power level for both reactor types.
600 Outlet temperature
Coolant temperature (OC)
550 Method 3 500
Method 2 Method 1
450
The change of inlet coolant temperature is smaller for LFR than SFR. LFR is considered better for the heat transfer to the secondary coolant. Generally speaking the transient of the power level often raises the risk of causing accidents for common power reactors. Therefore, the normal operation of power reactor should be a full power operation, so that any transients from the normal operation are always decreasing power level, which may be considered safer states than the normal operation state. In our reactors the same operation strategy is employed. The transient behavior during the power change is beyond the scope of the present paper, but some brief descriptions are presented here. Since Methods 1 and 2 adjust only one parameter, their control method is simple. On the other hand Method 3 adjusts two parameters, and then some freedom of the control is introduced. The following may be a good control strategy. In order to reduce the power level by Method 3, in the first step the inlet coolant temperature is increased slightly according to the suggestion shown in Fig. 3. The outlet coolant temperature may increase at the beginning and then it is going to decrease along decreasing power level. When the outlet coolant temperature becomes less than its design value, the flow rate is adjusted by reduction to make the coolant outlet temperature keep its design value. If the power level is still higher than the target value, the above process is going to be repeated. If the changes of the parameters are performed too quickly, some overshoots and/or oscillations of some parameters including power level may occur. The power level change operation may be divided into several steps. These changes should be kept small at each step, so that the overshoot of some parameters are small enough. The important parameters should be always monitored and checked to satisfy necessary conditions and relations.
Inlet temperature
6. Conclusions
400
350
300 0%
20%
40%
60%
80%
100%
Power
(A) SFR Outlet temperature
Method 3
Method 2
Method 1
The control rods, which consist of shim control rods, shutdown control rods and regulation control rods, disturb power shape as well as decrease the fuel volume fraction. They decrease power density and deteriorate the neutron economy. It is inevitable for attaining the desirable CANDLE burning to eliminate the control rods from CANDLE reactor core. CANDLE reactors do not need the shim control rods. When movable fuel assemblies are introduced, they do not need the shutdown control rods either. In the present study, in order to eliminate the regulation control rods, the power regulation and load following are investigated with changing coolant parameters for two kinds of CANDLE reactors (SFR and LFR). In the present paper the power level is changed by adjusting the coolant flow rate and/or inlet coolant temperature. It can be demonstrated successfully to change the power level from full power to zero with keeping the coolant outlet temperature at its designed value by adjusting both flow rate and inlet temperature for both SFR and LFR. The change of inlet temperature is smaller for LFR and may be better for the heat transfer to the secondary coolant. References
Inlet temperature
(B) LFR Fig. 3. Inlet and outlet coolant temperatures for different power level.
Nakayama, S., Okawa, T., Sekimoto, H., 2011. Power control of CANDLE reactor by coolant flow rate. Prog. Nucl. Energy 53, 891–894. Sekimoto, H., 2010. Five Requirements for nuclear energy and CANDLE Fast Reactor. AIP Conf. Proc. 1244, 3–11. Sekimoto, H., 2013. Introductions of 208Pb coolant to innovative fast reactors. In: Application of Stable Lead Isotope Pb-208 in Nuclear Power Engineering and its Acquisition Techniques. Nova Pub. (Chapter 2). Sekimoto, H., Miyashita, S., 2006. Startup of ‘‘Candle’’ burnup in fast reactor from enriched uranium core. Energy Convers. Manage. 47, 2772–2780. Sekimoto, H., Nagata, A., 2009. Core height shortening of CANDLE reactor by employing MOTTO cycle. Trans. Am. Nucl. Soc. 101.
H. Sekimoto, S. Nakayama / Annals of Nuclear Energy 63 (2014) 427–431 Sekimoto, H. and Ryu, K., 2000a. New reactor burnup concept CANDLE. In: PHYSOR 2000, Pittsburgh, USA. Sekimoto, H., Ryu, K., 2000b. Feasibility study on the CANDLE new burnup strategy. Trans. Am. Nucl. Soc. 82, 207–208. Sekimoto, H., Ryu, K., 2000c. Demonstrating the feasibility of the CANDLE burnup scheme for fast reactors. Trans. Am. Nucl. Soc. 83, 45.
431
Sekimoto, H., Ryu, K., Yoshimura, Y., 2001. CANDLE: the new burnup strategy. Nucl. Sci. Eng. 139, 306–317. Sekimoto, H., Nakayama, S., Taguchi, H. and Okawa, T., 2010. Power flattening for sodium cooled metallic fuel CANDLE reactor by adding thorium in inner core. In: PHYSOR 2010, Pittsburgh, USA.
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Power level control of CANDLE reactor without control rods Hiroshi Sekimoto ⇑, Sinsuke Nakayama Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-17, Ookayama, Meguro-ku 152-8550, Japan
a r t i c l e
i n f o
Article history: Received 1 April 2013 Received in revised form 11 August 2013 Accepted 13 August 2013 Available online 5 September 2013 Keywords: CANDLE reactor Power level Coolant flow rate Coolant inlet temperature Coolant outlet temperature 208 Pb
a b s t r a c t The power level control of CANDLE reactor without help of control rods is desirable for obtaining better neutron economy which is inevitable for CANDLE burning. In the present paper the power level is changed by adjusting its coolant flow rate and/or inlet coolant temperature. It can be demonstrated successfully to change the power level by adjusting both coolant flow rate and inlet coolant temperature from its full power to zero for SFR (sodium cooled fast reactor) and LFR (208Pb cooled fast reactor) while the coolant outlet temperature is kept at its designed value. The change of coolant inlet temperature is smaller for LFR and may be better for the heat transfer to the secondary coolant. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In conventional reactors, shim control rods inserted at the startup of operation are gradually extracted with fuel burning in order to keep the reactor critical. On the other hand, CANDLE (Constant Axial shape of Neutron flux, nuclide densities and power shape During Life of Energy production) reactors do not need this kind of control rods (Sekimoto and Ryu, 2000a; Sekimoto et al., 2001). Their burning region moves in the direction of core axis, at a speed proportionate to the power output, without changing the spatial distribution of nuclide densities, neutron flux and power density as shown in Fig. 1. CANDLE fast reactors require only natural uranium or depleted uranium as their fresh fuel. CANDLE reactors have three technical problems. The first problem is that the burning region of the initial core of their first cycle cannot be constructed using only naturally occurring nuclides. However it is rather easy problem. The CANDLE burning can be easily started from the initial core constructed by using enriched uranium where the enrichment is properly distributed in the axial direction as demonstrated in the reference (Sekimoto and Miyashita, 2006). The second problem is the extremely high burnup of the spent fuel which is about 40% (400 MWd/tHM). It causes difficult problems on irradiated materials of fuel elements, but it may be solved by recladding (Sekimoto and Nagata, 2009). The last problem is a severe requirement on neutron economy. This problem will be discussed later in this section. ⇑ Corresponding author. Tel./fax: +81 3 3929 8753. E-mail addresses: [email protected] (H. Sekimoto), [email protected] (S. Nakayama). 0306-4549/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2013.08.022
The nuclear energy has a resource problem, if we use only lightwater cooled reactors (LWR). It has also inherent difficult problems caused by radioactive materials produced in it and by employed materials and technologies tightly relating to nuclear bombs. The radioactive materials cause the problem of accident during reactor operation, and the problem of radioactive wastes after reactor operation. Finally reasonable price is usually an important requirement for base-load energy. Thus the necessary and sufficient conditions for nuclear energy utilization as primary energy in the future are considered to satisfy five requirements concerning resource, safety, waste, bomb, and economy. CANDLE reactors show much better performances than the conventional nuclear reactors for the first four requirements, resource, safety, waste and bomb as shown in the reference (Sekimoto, 2010). For the economy, though the situation seems more complicated, CANDLE has a potential to realize excellent economy performances. The power density is very sensitive to the power generation cost, and the higher power density makes the economy better. CANDLE reactors do not have blanket regions. It makes the average power density higher, when we consider the total volume of core and blanket as its denominator. However, the coolant volume fraction in the core is small as mentioned below, and the cooling performance is poor. It makes the average power density lower. CANDLE reactors have to produce fissile nuclides whose amount is more than consumed ones. However, the density distribution of each fuel nuclide through core is not optimized for this purpose. The neutrons moving downward in the core may be efficiently used for fissile production, but the neutrons moving upward will not be used efficient way. Therefore, some techniques for
428
H. Sekimoto, S. Nakayama / Annals of Nuclear Energy 63 (2014) 427–431
Fig. 1. CANDLE burning and fuel management.
increasing neutron economy compared with conventional fast breeder reactors are required for realizing CANDLE burning. This is the third problem of CANDLE reactors mentioned above. From our previous studies only fast reactors with metallic or nitride fuel can realize this burning but oxide fuel cannot (Sekimoto and Ryu, 2000c). The fuel volume fraction in the core should be larger than conventional designs (Sekimoto and Ryu, 2000b). It means the coolant channel cross section becomes smaller and then cooling performance becomes worse as mentioned above. The power density of CANDEL reactors has been increased by making its radial power distribution flatter and the core height shorter (Sekimoto et al., 2010). Using 208Pb instead of natural lead as coolant improves the neutron economy (Sekimoto, 2013) and increases the coolant channel cross section. It also increases the power density. The control rods staying in the core disturb power shape as well as decrease the fuel volume fraction. It decreases power density and deteriorates the neutron economy. In-core control rods are usually used for controlling excess reactivity with increasing burnup (shim control rods), for making shutdown and startup (shutdown control rods) and for power regulation and load following (regulation control rods). In CANDLE reactors, the excess reactivity does not change with burnup and the shim control rods are not necessary. The reactor is usually made in the cold shutdown state, while the shutdown state is maintained for a considerable period. The cold shutdown requires more shutdown reactivity worth than the hot shutdown. Usually the shutdown control rods are inserted from the hot shutdown to cold shutdown. The shutdown control rods can be replaced by properly selected several movable fuel assemblies in the core. These fuel assemblies are out of core during cold shutdown state, while no fuel burning occurs. Therefore, the fuel compositions of all fuel assemblies do not change during this process, and this method does not cause any effects on CANDLE burning. The power regulation and load following are more delicate process when regulation control rods are not facilitated. In the present paper they will be performed by adjusting coolant flow rate and/or core inlet coolant temperature.
2. Reactor designs
Table 1 Reactor core design parameters for two CANDLE reactors. SFR
LFR
Coolant Thermal power rate (MW) Electric power rate (MW) Core diameter (cm) Core height (cm)
Sodium 2350 990 402.4 166.5
208
Plenum height (cm) Pool thickness (cm) Coolant inlet temperature (°C) Coolant outlet temperature (°C)
100 Upper and lower axial 100, radial 200 395 400 550 560
Core support material Effective neutron multiplication factor
HT-9 1.0005
Pb-95%, 1315 580 352.3 141.3
207
Pb-5%
1.0003
Table 2 Fuel cell design parameters for two CANDLE reactors. SFR
LFR 235
Charged fresh fuel Smear density Zr ratio
Metallic fuel [U–Zr, 65% 6%
Cladding material Cladding thickness (mm) Fuel pin diameter, D (mm) Fuel pin pitch, P (mm) P/D
HT-9 0.51 8.56 9.46 1.105
0.51 8.56 10.72 1.252
Bonding material Volume ratio (fuel/cladding/coolant) (%)
Sodium 57.7/16.4/25.9
44.7/12.7/42.6
U-0.2%]
Table 3 Obtained characteristic values for the initial steady state for two CANDLE reactors. SFR
LFR
Speed of burning region (cm/y) Maximum coolant speed (m/s) Maximum linear heat rate (W/cm) Maximum cladding temperature (°C)
5.17 10.0 (10) 383 (500) 586 (650)
3.47 2.0 (2) 238 (500) 618 (650)
Void reactivity coefficient (Dk/k) Void reactivity coefficient ($) Loss of coolant pressure (MPa)
3.53% (–) 7.94 (8) 0.47 (–)
1.31% (–) 3.09 (8) 0.26 (–)
The values in () are the limit values imposed as constraints.
In the present paper two types of metallic fuel fast reactors are investigated, where the coolant is either sodium or 208Pb (Sekimoto, 2013). We call these reactors SFR and LFR, respectively. The size of reactor core can be more easily reduced for LFR than SFR, and the size of LFR is set smaller than SFR for this study. Main design parameters of reactor core and fuel cell for these reactors are shown in Tables 1 and 2, respectively. These values are not manufacture values but values at the operating conditions of reactors at the steady state. We tried to make each effective neutron multiplication factor more than unity and as near to unity as
possible with reasonable calculation time. Some of the important characteristic values obtained under these operating conditions are shown in Table 3. They satisfy their constraints shown in the same table. They will be used as the initial states for the power change trials in the present study. We will try to investigate the performance of methods of power level control. In these methods coolant parameters will be changed from their initial state given in these tables. The data given by these tables are for the full power operation. The power level
H. Sekimoto, S. Nakayama / Annals of Nuclear Energy 63 (2014) 427–431 Table 4 Adjusted and fixed parameters for investigated three methods. Adjusted parameters
Fixed parameters
Method 1
Coolant velocity
Method 2 Method 3
Coolant inlet temperature Coolant velocity, coolant inlet temperature
Coolant inlet temperature Coolant velocity Coolant outlet temperature
targeted by the control is lower than the initial value given in Table 1 (the design value). In the present paper only the final condition for each operation is investigated. Transient behaviors will be discussed briefly in Section 5. Details of the methods will be shown in Section 3, and the algorithm of its analysis for each method will be shown briefly in Section 4. 3. Control methods When we try to change the power level, we can choose several parameters of coolant as control variables. In the present study we
429
investigate three methods shown in Table 4. The coolant outlet temperature is considered the best parameter to be fixed for this problem (Method 3), but two simple methods (only one adjusted parameter) are also shown for comparisons (Nakayama et al., 2011). The coolant pressure drop changes when the coolant parameters are changed particularly when the flow rate is changed (Methods 1 and 3). 4. Analysis A brief flowchart of the analysis is shown in Fig. 2 for each of the three methods given in Table 4. The fixed parameters for Methods 1 and 2 do not change during the calculation, but the fixed parameter for Method 3, the coolant outlet temperature, changes for changing the adjusted parameters independently and should be converged to the designed value by adjusting two control parameters synchronously. For Methods 1 and 2 the outlet temperature changes from the designed value with changing the adjusted parameter. The differences of coolant outlet temperature at different coolant channel are not important for these cases. For Method
Fig. 2. Flowchart of analysis of the coolant control for adjusting total power.
430
H. Sekimoto, S. Nakayama / Annals of Nuclear Energy 63 (2014) 427–431
1 not uk at each coolant channel k but one parameter r is adjusted, where r is the coolant velocity reduction rate and assumed constant over all coolant channel k. The criticality is confirmed by convergence of the effective neutron multiplication factor to its initial value given in Table 1. The criteria of convergence of the coolant outlet temperature for Method 3 and effective neutron multiplication factor are shown in Fig. 2.
5. Calculation results and discussion The calculation results are shown in Fig. 3, where the outlet coolant temperature is the averaged value over all coolant channels. The results for SFR is shown in (A) and LFR in (B). For Method 1 the outlet coolant temperature increases with decreasing power level for both reactor types and reaches to its maximum permissible level. This change is more rapid for SFR than LFR, and the controllable range of power level is smaller for SFR. For Method 2 the outlet coolant temperature decreases with decreasing power level. It means the thermal efficiency decreases with decreasing power level. This effect is larger for LFR than SFR. Method 3 can be performed for whole range of power level for both reactor types.
600 Outlet temperature
Coolant temperature (OC)
550 Method 3 500
Method 2 Method 1
450
The change of inlet coolant temperature is smaller for LFR than SFR. LFR is considered better for the heat transfer to the secondary coolant. Generally speaking the transient of the power level often raises the risk of causing accidents for common power reactors. Therefore, the normal operation of power reactor should be a full power operation, so that any transients from the normal operation are always decreasing power level, which may be considered safer states than the normal operation state. In our reactors the same operation strategy is employed. The transient behavior during the power change is beyond the scope of the present paper, but some brief descriptions are presented here. Since Methods 1 and 2 adjust only one parameter, their control method is simple. On the other hand Method 3 adjusts two parameters, and then some freedom of the control is introduced. The following may be a good control strategy. In order to reduce the power level by Method 3, in the first step the inlet coolant temperature is increased slightly according to the suggestion shown in Fig. 3. The outlet coolant temperature may increase at the beginning and then it is going to decrease along decreasing power level. When the outlet coolant temperature becomes less than its design value, the flow rate is adjusted by reduction to make the coolant outlet temperature keep its design value. If the power level is still higher than the target value, the above process is going to be repeated. If the changes of the parameters are performed too quickly, some overshoots and/or oscillations of some parameters including power level may occur. The power level change operation may be divided into several steps. These changes should be kept small at each step, so that the overshoot of some parameters are small enough. The important parameters should be always monitored and checked to satisfy necessary conditions and relations.
Inlet temperature
6. Conclusions
400
350
300 0%
20%
40%
60%
80%
100%
Power
(A) SFR Outlet temperature
Method 3
Method 2
Method 1
The control rods, which consist of shim control rods, shutdown control rods and regulation control rods, disturb power shape as well as decrease the fuel volume fraction. They decrease power density and deteriorate the neutron economy. It is inevitable for attaining the desirable CANDLE burning to eliminate the control rods from CANDLE reactor core. CANDLE reactors do not need the shim control rods. When movable fuel assemblies are introduced, they do not need the shutdown control rods either. In the present study, in order to eliminate the regulation control rods, the power regulation and load following are investigated with changing coolant parameters for two kinds of CANDLE reactors (SFR and LFR). In the present paper the power level is changed by adjusting the coolant flow rate and/or inlet coolant temperature. It can be demonstrated successfully to change the power level from full power to zero with keeping the coolant outlet temperature at its designed value by adjusting both flow rate and inlet temperature for both SFR and LFR. The change of inlet temperature is smaller for LFR and may be better for the heat transfer to the secondary coolant. References
Inlet temperature
(B) LFR Fig. 3. Inlet and outlet coolant temperatures for different power level.
Nakayama, S., Okawa, T., Sekimoto, H., 2011. Power control of CANDLE reactor by coolant flow rate. Prog. Nucl. Energy 53, 891–894. Sekimoto, H., 2010. Five Requirements for nuclear energy and CANDLE Fast Reactor. AIP Conf. Proc. 1244, 3–11. Sekimoto, H., 2013. Introductions of 208Pb coolant to innovative fast reactors. In: Application of Stable Lead Isotope Pb-208 in Nuclear Power Engineering and its Acquisition Techniques. Nova Pub. (Chapter 2). Sekimoto, H., Miyashita, S., 2006. Startup of ‘‘Candle’’ burnup in fast reactor from enriched uranium core. Energy Convers. Manage. 47, 2772–2780. Sekimoto, H., Nagata, A., 2009. Core height shortening of CANDLE reactor by employing MOTTO cycle. Trans. Am. Nucl. Soc. 101.
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