D J Coates, G T Parks Department of Engineering, University of Cambridge, UK Actinide Evolution and Equilibrium in Thorium Reactors ThorEA Workshop Trinity College, University of Oxford 13 th April 2010

ADS and Critical Reactors The purpose of the accelerator: 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

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

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

Contents: 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 Thermal thorium reactor 4 Examination of characteristics and constraints governing actinide evolution in thermal 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 needed- full recycling 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 achieved within the reactor will be that of a 100% thorium reactor When operating with a 232 Th platform the underlying growth of nuclides generated from thorium provides 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; its benefit in this respect relates to the fuel cycle adopted, the device can be used for the production of significant quantities of plutonium 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

Thermal thorium reactor 4

Variations in Fission Cross-sections

Fission Fraction of Total Capture Cross-section

Actinide Evolution Pathways Where a low fission probability exists evolution outlets exist in the form of decay and neutron capture pathways A nuclide can encounter a series of capture and decay reactions before fission

Evolution of Power Full power is achieved earlier in the thermal system The thermal system runs with a lower neutron flux than a fast system for an equivalent power output Fast re-fuelling period 5 years Thermal re-fuelling period 1.5 years

Nuclide Evolution 100% Th (Thermal) The nuclides in a thermal reactor take less time to reach steady-state equilibrium

Steady-state Equilibrium Positions

Nuclide Evolution 15% Plutonium (Thermal) Includes full recycling of all actinides and replenishment of the 232 Th fuel inventory at 1.5 year intervals

Nuclide Evolution for a 15% Pu and 100% Th Reactors 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

The Dominance of the 232 Th Pathways The 232 Th growth profiles associated with 238 Pu and 239 Pu represent the lowest populations that can be achieved through irradiation

The Dominance of the 232 Th & Curium Pathways 240 Pu and 241 Pu cannot achieve the respective minimum 232 Th growth profiles in the short term, this is due to the decay pathways coming from the curium isotopes

The Dominance of the 232 Th & Curium Pathways The 242 Pu population rapidly increases to achieve a short-term equilibrium position as a result of the presence of the plutonium enrichment isotopes The 242 Pu population is slow to return to its steady-state equilibrium position Repeated enrichments will result in an increasing 242 Pu population

Power Variation With Plutonium Enrichment High plutonium enrichments would require control systems capable of achieving significant reductions in the neutron flux

3% Plutonium Enrichment (1.5 year cycle) The 242 Pu population can be reduced by: - Lower enrichment - Alternative enrichment - Shorter cycle time - No enrichment The 242 Pu increases until equilibrium is achieved relative to the enrichment isotopes

Conclusions Thermal thorium fuelled reactor systems operated over an extended period (with full recycling of actinides) achieve a balanced equilibrium between the relative abundances of the actinides. The steady-state equilibrium positions reached are independent of the starting condition, if the enrichment operation is finite the ultimate levels of abundance achieved within the reactor will be that of a 100% thorium reactor The presence of plutonium enrichment isotopes can create heightened short-term transient equilibrium levels for nuclides such as 242 Pu Repeated plutonium enrichments produce further increases in the 242 Pu population until a higher equilibrium population is reached relative to the enrichment conditions For a 100% Thorium reactor the time taken to reach full power and steady- state equilibrium is shorter than that for a fast reactor

The End