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Ali Ahmad

Study on the Neutronic Characteristics of Subcritical Reactors Driven by an Accelerated Pulsed Proton Beam. Ali Ahmad. Motivation The Simulation Set-up Results & Analysis Conclusions. Outline. Three main components: Accelerator Spallation target Subcritical core

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Ali Ahmad

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

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

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

  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

  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

  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

  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

  8. Results & Analysis (1) : Neutron spectrum evolution

  9. 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

  10. 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)

  11. 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 τ  10 µs - After that time, the fission neutrons become dominant

  12. 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 Thermal Fast

  13. 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

  14. 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):

  15. 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 • α can be estimated from the flux decrease rate when the beam is off

  16. 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

  17. 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

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