Study on the Neutronic Characteristics of Subcritical Reactors Driven by an Accelerated Pulsed Proton Beam

1. Study on the Neutronic Characteristics of SubcriticalReactors Driven by an Accelerated Pulsed Proton Beam  Ali Ahmad

2. Outline Motivation The Simulation Set-up Results & Analysis Conclusions

3. ADSR in question? Three main components: Accelerator Spallation target Subcritical core Possible deployment of thorium fuel cycle www.thorea.org

4. Motivation : Why pulsed beam operation? LINAC is expensive Classic cyclotron technology is mature and approaching its power limit FFAG has the potential to be both affordable and technologically capable of doing the job (Takahishi. 2001) FFAG is a pulsed accelerator The concept of an ADSR is based on the coupling between a proton accelerator and a subcritical reactor. Consequently, the accelerator technology is an issue when it comes to constructing either a research or a commercial plant. Among the accelerator options there are: 1- LINAC which is expensive 2- classic cyclotron technology is approaching its power limits 3- FFAG has the potential to be both affordable and technologically capable of doing the job As a matter of fact, FFAG is a pulsed accelerator The concept of an ADSR is based on the coupling between a proton accelerator and a subcritical reactor. Consequently, the accelerator technology is an issue when it comes to constructing either a research or a commercial plant. Among the accelerator options there are: 1- LINAC which is expensive 2- classic cyclotron technology is approaching its power limits 3- FFAG has the potential to be both affordable and technologically capable of doing the job As a matter of fact, FFAG is a pulsed accelerator

5. Motivation : cont.Why is neutronic analysis required? Pulsed proton beam means pulsed production of spallation neutrons Oscillations in the power profile are inevitable Frequent and rapid temperature transients Thermal cyclic fatigue Modeling of flux variation with time is needed One of the limiting factors in the design of nuclear reactors is the integrity of the structural materials. Operating an ADSR in a pulsed mode means pulsed production of spallation neutrons which are required to maintain the normal operation. Therefore, one would expect fluctuations in the temperature profile. As these fluctuations are repeated. A thermal cyclic fatigue might occur leading to a clad failure as shown in the picture. To examine this thermal impact, a neutronic study must preceed One of the limiting factors in the design of nuclear reactors is the integrity of the structural materials. Operating an ADSR in a pulsed mode means pulsed production of spallation neutrons which are required to maintain the normal operation. Therefore, one would expect fluctuations in the temperature profile. As these fluctuations are repeated. A thermal cyclic fatigue might occur leading to a clad failure as shown in the picture. To examine this thermal impact, a neutronic study must preceed

6. The Simulation Set-up Simulations performed using the MCNPX neutron transport code. En < 20 MeV : Nuclear data tables (ENDF/B-VI) En > 20 MeV : Nuclear models Bertini Model (Bertini 1969) Delayed neutrons and thermal treatment are included The simulations were performed using the well validated MCNPX code. In MCNPX The interaction cross sections are taken from data libraries if the energy is below 20 MeV, above this limit, several nuclear models apply. For this study, Bertini model of intra-nuclear cascade followed by pre-equilibrium model was selected. Both the effect of delayed neutrons and temperature were also included in this study. The level of core subcriticality was controlled adjusting the Pu enrichment in the fuelThe simulations were performed using the well validated MCNPX code. In MCNPX The interaction cross sections are taken from data libraries if the energy is below 20 MeV, above this limit, several nuclear models apply. For this study, Bertini model of intra-nuclear cascade followed by pre-equilibrium model was selected. Both the effect of delayed neutrons and temperature were also included in this study. The level of core subcriticality was controlled adjusting the Pu enrichment in the fuel

7. The Simulation Setup…cont Reactor Materials: - Target: Pb-208 - Fuel: Th-Pu MOX - Clad: Stainless steel - Coolant: Pb-208 / Water - Radiation shield: Lead Core arrangement: - Core geometry: hexagonal - Bundle geometry: hexagonal - Number of bundles: 125 - Number of fuel pins: 96 - Fuel active height = 202 cm This slide shows the vertical and horizontal cross section of our reactor model. The core is a hexagonal grid consists of 125 fuel assemblies, each contains 96 fuel pins. The fuel is composed of Th-Pu MOX surrounded by two types of coolants considered separately, lead and water. The lead spallation target lies at the center of the core. The reflector material used is carbon This slide shows the vertical and horizontal cross section of our reactor model. The core is a hexagonal grid consists of 125 fuel assemblies, each contains 96 fuel pins. The fuel is composed of Th-Pu MOX surrounded by two types of coolants considered separately, lead and water. The lead spallation target lies at the center of the core. The reflector material used is carbon

8. Results & Analysis (1) : Neutron spectrum evolution The first part of the results is the evolution of the neutron energy spectrum following a pulse injection. The figure in this slide shows the neutron flux evolution in a thermal ADSR at 20 ns, 50 ns and 10 microsecs. An interesting observation is that the very fast neutrons emitted in the nuclear cascade process decay between 20 and 50 ns which is short enough to be dismissed from our neutronic analysis. Therefore, installing a small and relatively inexpensive neutron source would do the same job The first part of the results is the evolution of the neutron energy spectrum following a pulse injection. The figure in this slide shows the neutron flux evolution in a thermal ADSR at 20 ns, 50 ns and 10 microsecs. An interesting observation is that the very fast neutrons emitted in the nuclear cascade process decay between 20 and 50 ns which is short enough to be dismissed from our neutronic analysis. Therefore, installing a small and relatively inexpensive neutron source would do the same job

10. Results & Analysis (1) : Neutron spectrum evolution Very fast neutrons (En > 20 MeV) decay in less than 30 ns This means that primary neutrons aren’t of interest to the core neutronics study Hypothesis: Neutron generators can be used instead of a proton beam to study neutrons’ kinetic characteristics The first part of the results is the evolution of the neutron energy spectrum following a pulse injection. The figure in this slide shows the neutron flux evolution in a thermal ADSR at 20 ns, 50 ns and 10 microsecs. An interesting observation is that the very fast neutrons emitted in the nuclear cascade process decay between 20 and 50 ns which is short enough to be dismissed from our neutronic analysis. Therefore, installing a small and relatively inexpensive neutron source would do the same job The first part of the results is the evolution of the neutron energy spectrum following a pulse injection. The figure in this slide shows the neutron flux evolution in a thermal ADSR at 20 ns, 50 ns and 10 microsecs. An interesting observation is that the very fast neutrons emitted in the nuclear cascade process decay between 20 and 50 ns which is short enough to be dismissed from our neutronic analysis. Therefore, installing a small and relatively inexpensive neutron source would do the same job

11. Results & Analysis (1) …cont 14 MeV DT-n-source and beam-operated n-source have very similar core neutronic characteristics A 14 MeV n-source can potentially replace the proton accelerator in an ADSR for research purposes Similar results have been obtained by Yamamoto & Shiroya (2003) The comparison of the neutronic behaviour between neutrons generated by a proton beam and that generated by DT-neutron source shows that they have almost the same core neutronic characteristics. The figure to the left shows the comparison in the energy spectrum and the one to the right shows the comparison in the neutron flux decay following a pulse injection. Therefore, a 14 A 14 MeV n-source can potentially replace the proton accelerator in research ADSR used to study the dynamic characteristics. The comparison of the neutronic behaviour between neutrons generated by a proton beam and that generated by DT-neutron source shows that they have almost the same core neutronic characteristics. The figure to the left shows the comparison in the energy spectrum and the one to the right shows the comparison in the neutron flux decay following a pulse injection. Therefore, a 14 A 14 MeV n-source can potentially replace the proton accelerator in research ADSR used to study the dynamic characteristics.

12. Results & Analysis (2) : Spatial variations and diffusion The diffusion of neutrons in a thermal ADSR is characterised by: - The spallation neutrons are dominant for a period t ? 10 µs - After that time, the fission neutrons become dominant The second part of the results focuses on the spatial variations and neutron diffusion. According to the neutron flux evolution curve, the spallation neutrons are dominant in a period t=10 microsecs, after that time fission neutrons become dominant. In other words the diffusion process of source neutrons is terminated in around 10 microsec The second part of the results focuses on the spatial variations and neutron diffusion. According to the neutron flux evolution curve, the spallation neutrons are dominant in a period t=10 microsecs, after that time fission neutrons become dominant. In other words the diffusion process of source neutrons is terminated in around 10 microsec

13. Results & Analysis (2) : ... cont A pulsed beam of frequency 1 kHz almost allows four orders of magnitude reduction in the neutron flux level in a fast ADSR The sharp decrease in the neutron flux in a fast ADSR is instantaneous in all assemblies The perturbation in the neutron flux due to a 1 kHz beam in a thermal ADSR is observed only in the assemblies close to the target After a few pulses, the fission neutrons become dominant elsewhere A case study where the pulsed proton beam has a frequency of 1KHz has been considered for thermal and fast systems. The following outcomes were observed: 1- the neutron flux in a fast ADSR decreases to the background neutron level, the rate of decrease is almost independent of the spatial variations. 2- the fluctuations in the neutron flux in a thermal ADSR were observed only in assemblies close to the target and once the startup phase has finished, the fission neutrons become totally dominant. A case study where the pulsed proton beam has a frequency of 1KHz has been considered for thermal and fast systems. The following outcomes were observed: 1- the neutron flux in a fast ADSR decreases to the background neutron level, the rate of decrease is almost independent of the spatial variations. 2- the fluctuations in the neutron flux in a thermal ADSR were observed only in assemblies close to the target and once the startup phase has finished, the fission neutrons become totally dominant.

14. Results & Analysis (3) : Monitoring of ADSR reactivity The ideal monitoring of core reactivity should be: On-line Accurate Simple and robust measurement technique Experiments done to measure the subcritical reactivity: MUSE (Billeboud et al. 2003) YALINA (Fernandez-Ordonez et al. 2003) None of the experimental techniques meet all of the requirements The sub-critical reactivity is an important parameter. It decides the accelerator current that will be necessary to produce the desired power as well as the margin of safety available. Measurement and continuous monitoring of this parameter in operating ADS reactors will be an essential safety requirement. The ideal monitoring of core reactivity should be: - On-line - Accurate - Simple & Robust measurement technique To my knowledge, None of the experimental techniques meet all of the requirements In this context, several experiments have been carried out, such as MUSE and YALINA The sub-critical reactivity is an important parameter. It decides the accelerator current that will be necessary to produce the desired power as well as the margin of safety available. Measurement and continuous monitoring of this parameter in operating ADS reactors will be an essential safety requirement. The ideal monitoring of core reactivity should be: - On-line - Accurate - Simple & Robust measurement technique To my knowledge, None of the experimental techniques meet all of the requirements In this context, several experiments have been carried out, such as MUSE and YALINA

15. Results & Analysis (3) : Cont. All of the proposed techniques rely on deliberate gaps in the beam to measure the subcritical reactivity The diffusion equation in a thermal ADSR: Using At a certain spatial position in the reactor (detector position): All the past experiments and porposed techniques rely on beam trips to measure reactivity by monitoring the decay of neutron flux. According to the results of neutron evolution in thermal ADSR, a simple and online measurement is potentially possible. The neutron diffusion equation in a thermal ADSR is a good starting point. To related this equation to the core reactivity we subsititute for the value of the infinite multiplication factor. Now at certain spatial position, the equation is only function of t All the past experiments and porposed techniques rely on beam trips to measure reactivity by monitoring the decay of neutron flux. According to the results of neutron evolution in thermal ADSR, a simple and online measurement is potentially possible. The neutron diffusion equation in a thermal ADSR is a good starting point. To related this equation to the core reactivity we subsititute for the value of the infinite multiplication factor. Now at certain spatial position, the equation is only function of t

16. Results & Analysis (3) : Cont. For the case when the beam is off the last equation has the solution: Using the definition of neutron life time: Then, Define a can be estimated from the flux decrease rate when the beam is off This equation has a simple interpretation if one defines the neutron lifetime as the mean time separating creation and absorption of the neutron Switching the beam off, takes the source term out, and the solution of the equation is of the form of exponential function. Now using the definition of neutron lifetime and applying natural logarithm on both sides we get a function of linear form. Now we define alpha to be the slope of this linear functionThis equation has a simple interpretation if one defines the neutron lifetime as the mean time separating creation and absorption of the neutron Switching the beam off, takes the source term out, and the solution of the equation is of the form of exponential function. Now using the definition of neutron lifetime and applying natural logarithm on both sides we get a function of linear form. Now we define alpha to be the slope of this linear function

17. Results & Analysis (3) : Cont. Once the neutron life time of a certain core configuration is known, a reactivity measurement will be straightforward To perform on-line measurement of Keff in a thermal ADSR, the separation time between pulses should be more than several tens of microseconds

18. Conclusions Experimental investigations of the neutronic transients in an ADSR can be done through a relatively cheap neutron source In a thermal ADSR the diffusion time of spallation neutrons is around 10 microseconds, while it is much quicker in a fast ADSR The flux fluctuations due to pulsed operation are almost independent of the spatial position in a fast ADSR The reactivity of a thermal ADSR can be measured on-line if the beam frequency is less than 10 kHz The next step in my research is to study the thermal issues related to pulsed operation and their consequences Thank you