D J Coates, G T Parks Department of Engineering, University of Cambridge, UK

1. D J Coates, G T Parks Department of Engineering, University of Cambridge, UK 3rd Year PhD student Actinide Breeding and Reactivity Variation in a Thermal Spectrum ADSR Universities Nuclear Technology Forum University of Huddersfield 11-13th April 2011

2. Motivation for the Research: Improvements in the Sustainability of Nuclear Power Fast reactors are the subject of renewed interest due to their beneficial capability to both burn and breed transuranic actinides However despite significant investment over many years they have never been deployed in significant numbers The well proven thermal spectrum technology base may provide a more straightforward route to delivering improvements in sustainability

3. What Role Does the Accelerator Play ? In the fast spectrum: It provides additional re-assurance against criticality excursions beyond that provided by the delayed neutron fraction In the thermal spectrum: It enables sub-critical operation to be established with fuel mixtures which cannot support critical reactor operation Hence it facilitates operation with fuel cycles which would otherwise be inaccessible The Carlo Rubbia Energy Amplifier

4. Contents: 1 The Thermal Model • Brief description of the model used and thermal flux distribution 2 Validation of the Thermal Model • Comparison of the model predictions with actual PWR operating results 3 Thermal Breeder ADSR • Using the model to examine the constraints affecting thermal breeder reactors

5. 1 Thermal Model

6. Reactor Neutron Energy Distribution The neutron reaction and decay pathways are largely the same for both fast and thermal systems, the essential difference lies in the cross-sections Accurate representation of effective one-group cross-sections in the thermal spectrum can be challenging Changes in reactor geometry and self-shielding effects can have significant influences on the cross-sections The strong resonance peak which exists for 240Pu requires the capture cross-section to be continually updated as the burn-up progresses Chart taken from T. Iwasaki, N.Hirakawa, 1995

7. 2 Model Validation

8. Comparison with Takahama-3 PWR (uranium)

9. 3 Thermal Breeding

10. 238U 239U 239Np Thorium and UO2 Breeding Reactions 232Th 233Th 233Pa 233U 239Pu

11. Power Contribution of Selected Nuclides in a PWR As the U235 contribution falls away the Pu239 and Pu241 increase to provide the major contributions to the total power

12. Neutron Economy in a 3.04% 235U PWR

13. Accelerator Contribution to extended PWR Operation

14. The Thermal Thorium System An improved neutron economy can be achieved by using a Thorium fuel platform A fissile “starter” will be necessary to maintain operation over the early operating period

15. Plutonium Enriched Thermal Thorium Reactor The use of a plutonium “starter” produces an initial boost to the neutron economy but ultimately falls below that of a pure thorium fuel platform

16. Actinide Evolution Pathways The opportunities for fission before transformation into plutonium are far greater when starting from Th232 than from U238 Enrichment with heavy actinides by-passes the fission opportunities and increases the proportion of heavy actinides

17. 233U Enriched Thermal Thorium Reactor

18. Evolution of Pu, Am and Cm in a Thorium Reactor

19. Variation in Neutron Economy and Accelerator Power

20. Conclusions • It is possible to represent the evolution of actinides within a typical PWR using a lumped model • A 238U fuel platform produces a very poor neutron economy, the benefit of an accelerator is limited to extending the burn-up • The 232Th fuel platform provides a significantly improved neutron economy although insufficient for critical operation • Heavy actinide starters ultimately “choke” the reactor due to the consequential growth in the heavy actinide population • A closed fuel cycle with a thorium fuel platform would require an accelerator in the order of 20% of the generated power • An accelerator of this size would be challenging from both a practical and commercial perspective

21. The End

22. Neutron Absorption by Heavy Actinides

23. Variation Thorium Mass With Repeated Fuel Cycles

24. Variation in 233U Mass With Repeated Fuel Cycles

25. Neutron Capture Cross-sections Magnified showing 0.5(b) vertical divisions Cross-sections taken from T. Iwasaki, N.Hirakawa, 1995

26. Comparison with Takahama-3 PWR (plutonium)

27. Comparison with Takahama-3 PWR (americium)

28. Comparison with Takahama-3 PWR (curium)

29. Comparison with Takahama-3 PWR (curium)

30. Comparison with Obrigheim PWR (Am241)

31. Comparison with Obrigheim PWR (Cm242)

32. Comparison with Obrigheim PWR (Cm244)

33. Comparison with Obrigheim PWR (Pu238)

34. 49 Nuclide Model 253Es A simple “lumped” homogenous reactor model using averaged neutron cross-sections and ignoring spatial effects is adopted 249Cf 250Cf 251Cf 252Cf 253Cf 249Bk 250Bk 242Am 242Cm 243Cm 249Cm 244Cm 245Cm 247Cm 248Cm 246Cm M 242Am 232U 233U 234U 235U 236U 241Am 243Am 244Am 245Am 241Pu 242Pu 243Pu 244Pu 245Pu 231Pa 232Pa 233Pa 238Pu 239Pu 240Pu 232Th 230Th 231Th 233Th 237Np 238Np 239Np 237U 238U 239U 234Pa

35. The Accelerator Driven Sub-critical Reactor