D J Coates, G T Parks Department of Engineering, University of Cambridge, UK Actinide Evolution and Equilibrium in Fast Thorium Reactors UNTF 2010 University of Salford th April 2010

Accelerator Driven System (ADS) and Critical Reactors 1) Sub-critical operation of the reactor This provides additional re-assurance against criticality excursions – especially significant when operating with thorium and plutonium fuels in the fast spectrum 2) An additional external source of neutrons This provides an increase in the neutron population – especially significant when operating with thorium fuel in the thermal spectrum This presentation does not address the safety or neutron economy issues and, as such, the findings are equally applicable to critical thorium reactors This work forms part of a wider investigation into the advantages of ADS reactors (reactors which use an accelerator to maintain the fission reaction) The growth of actinides within a reactor is largely independent of presence of the accelerator The role of the accelerator:

Fast ADS Thorium Reactors CERN Energy Amplifier (14%Pu Enrichment) Fast ADS reactors operating with pure and enriched fuel sources have been heralded as delivering a new era in sustainable energy production The 232 Th fuel platform avoids the inclusion of 238 U in the initial fuel load providing benefits with respect to reduced plutonium generation The plutonium enrichment provides a fissile fuel source, this is burnt whilst the 233 U is generated largely from 232 Th

Benefits of the Thorium ADS Reactor “No plutonium is bred in the reactor” COSMOS magazine, “New age nuclear” Issue 8, April 2006 “(Th, Pu)O 2 fuel is more attractive, as compared to (U, Pu)O 2, since plutonium is not bred in the former” IAEA-TECDOC-1450 “Thorium fuel cycle- Potential benefits and challenges”, “The advantages of the thorium fuel cycle are that it does not produce plutonium” Thorenco LLC website “Examination of claimed advantages, (a) Producing no plutonium, This is true of the pure thorium cycle” IAEA-TECDOC-1319,”Potential advantages and drawbacks of the Thorium fuel cycle in relation to current practice: a BNFL view” “The fuel cycle can also be proliferation resistant, stopping a reactor from producing nuclear weapons-usable plutonium” Power Technology website

Overview: Validation of the models 2 Creation of two models 1 Test the claims made for the fast thorium ADS with respect to actinide production Enable rapid predictions of nuclide equilibrium and evolution Comparison of results with established code Fast thorium reactor 3 Examination of characteristics and constraints governing actinide evolution in fast reactors

Creation of the models 1

Boundary Conditions The effects of the decay and capture mechanisms from nuclides outside of the model are not accounted for within the model 33 Nuclides Included Within The Model 237 U 236 U 230 Th 231 Th 232 Th 233 Th 231 Pa 232 Pa 233 Pa 231 U 232 U 233 U 235 U 234 U 238 U 239 U 238 Pu 239 Pu 240 Pu 241 Pu 242 Pu 243 Pu 241 Am M 242 Am 243 Am 242 Am 244 Am 242 Cm 244 Cm 243 Cm 237 Np 238 Np 239 Np A simple “lumped” homogenous reactor model using averaged neutron cross-sections and ignoring spatial effects is adopted

Mechanisms Governing Nuclide Evolution 33 equations are created for the 33 nuclides in the model At steady-state equilibrium the rate of change of the nuclide populations is zero 32 of the 33 equations in the model can be set to zero The 232 Th population must be defined to avoid zero = zero solution

Steady-state equilibrium values Equilibrium values are dependent upon the size of the neutron flux applied 100 years before equilibrium Integration is needed Full recycling is assumed

Runge-Kutta Fourth Order Numerical Integration Includes full recycling of all actinides and replenishment of the 232 Th fuel inventory at five year intervals Profiles Arising from a 100% thorium Reactor

Validation of the models 2

Herrera-Martinez - Enriched Lead-cooled ADS Produced using the EA-MC code Developed at CERN by a team led by Prof. Carlo Rubbia It considers an extensive range of neutron reactions, neutron energy effects, cross-sections, materials and spatial effects Enrichment : 20% plutonium 2% americium 1.3% neptunium 0.04% curium

Comparison of Transient and Herrera-Martinez Results The value of the neutron flux applied in the transient model was adjusted to produce the equivalent reduction in 232 Th over the same 5 year period of operation

Fast thorium reactor 3

Nuclide Evolution for a 15% Plutonium Enriched Reactor Includes full recycling of all actinides and replenishment of the 232 Th fuel inventory at five year intervals

Pu, Am & Cu isotopes for a 15% Pu & 100% Th Reactor All movements in the nuclide populations are adjustments towards reaching an equilibrium position If the reactor is operated for sufficient duration all nuclides achieve steady-state equilibrium regardless of initial enrichment

Short-term Transient Equilibrium 243 Pu was not included in the initial enrichment composition By selecting an initial 243 Pu population below its long-term (and in this case short-term) equilibrium value, 243 Pu will be generated

Plutonium reduction to steady-state equilibrium By setting the initial fuel enrichments above or below the equilibrium values, nuclides can be generated or consumed as required By selecting an initial 240 Pu, 241 Pu and 242 Pu population above the long-term equilibrium values these nuclides are burnt

Influence of the 232 Th Dominant Growth Pathways The reductions in 238 Pu and 239 Pu are rapidly reversed due to the presence of the dominant growth profile arising from 232 Th

The Dominance of the 232 Th Pathways The 232 Th growth profiles represent the lowest populations that can be achieved through irradiation Each nuclide within the reactor has a unique 232 Th growth profile associated with it

Proliferation Resistance of ADS Device Unlike the case for actinide destruction, the 232 Th growth profiles do not represent the maximum limit on growth that can be achieved A 15% enrichment of 238 U will result in the short term production of plutonium (330kg, 80% 239 Pu fraction, from a 27te reactor delivering 1000MWe )

Conclusions Fast reactor systems operated over an extended period (with full recycling of actinides) achieve a balanced equilibrium between the relative abundances of the actinides The equilibrium positions reached are independent of the starting condition, if the enrichment operation is finite the ultimate levels of abundance established will be that of a 100% thorium reactor The 232 Th growth profiles provide a base line describing the lowest levels of actinide abundances that can be achieved through irradiation A fast thorium ADS (as a device) is not proliferation resistant; it’s benefit in this respect relates to the fuel cycle adopted Simplistic statements made regarding reactor operation in terms of plutonium generation do not fully represent the true nature of the mechanisms taking place which are beyond that of it being a simple burner or generator

The End