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

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

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 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 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

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

Thermal Model 1

The neutron reaction and decay pathways are largely the same for both fast and thermal systems, the essential difference lies in the cross-sections Reactor Neutron Energy Distribution Chart taken from T. Iwasaki, N.Hirakawa, 1995 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 240 Pu requires the capture cross-section to be continually updated as the burn-up progresses

Model Validation 2

Comparison with Takahama-3 PWR (uranium)

Thermal Breeding 3

Thorium and UO 2 Breeding Reactions 238U239U239Np 239Pu 232Th233Th233Pa 233U

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

Neutron Economy in a 3.04% 235 U PWR

Accelerator Contribution to extended PWR Operation

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

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

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

233 U Enriched Thermal Thorium Reactor

Evolution of Pu, Am and Cm in a Thorium Reactor

Variation in Neutron Economy and Accelerator Power

Conclusions It is possible to represent the evolution of actinides within a typical PWR using a lumped model A 238 U fuel platform produces a very poor neutron economy, the benefit of an accelerator is limited to extending the burn-up The 232 Th 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 It is possible to represent the evolution of actinides within a typical PWR using a lumped model A 238 U fuel platform produces a very poor neutron economy, the benefit of an accelerator is limited to extending the burn-up The 232 Th 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

The End

Neutron Absorption by Heavy Actinides

Variation Thorium Mass With Repeated Fuel Cycles

Variation in 233 U Mass With Repeated Fuel Cycles

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

Comparison with Takahama-3 PWR (plutonium)

Comparison with Takahama-3 PWR (americium)

Comparison with Takahama-3 PWR (curium)

Comparison with Obrigheim PWR (Am241)

Comparison with Obrigheim PWR (Cm242)

Comparison with Obrigheim PWR (Cm244)

Comparison with Obrigheim PWR (Pu238)

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

The Accelerator Driven Sub-critical Reactor