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 Actinide Evolution and Equilibrium in Fast Thorium Reactors UNTF 2010 University of Salford 14 - 16th April 2010

2. Accelerator Driven System (ADS) and Critical 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 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 The role of the accelerator:

3. Fast ADS Thorium Reactors Fast ADS reactors operating with pure and enriched fuel sources have been heralded as delivering a new era in sustainable energy production The plutonium enrichment provides a fissile fuel source, this is burnt whilst the 233U is generated largely from 232Th The 232Th fuel platform avoids the inclusion of 238U in the initial fuel load providing benefits with respect to reduced plutonium generation CERN 95-44 Energy Amplifier (14%Pu Enrichment)

4. Benefits of the Thorium ADS Reactor “No plutonium is bred in the reactor” COSMOS magazine , “New age nuclear” Issue 8, April 2006 “(Th, Pu)O2 fuel is more attractive, as compared to (U, Pu)O2, since plutonium is not bred in the former” IAEA-TECDOC-1450 “Thorium fuel cycle- Potential benefits and challenges”, 2005. “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” 2002. “The fuel cycle can also be proliferation resistant, stopping a reactor from producing nuclear weapons-usable plutonium” Power Technology website

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

6. 1 Creation of the models

7. 33 Nuclides Included Within The Model Boundary Conditions The effects of the decay and capture mechanisms from nuclides outside of the model are not accounted for within the model 242Am 242Cm 243Cm 244Cm M 242Am 241Am 243Am 244Am 231U 232U 233U 234U 235U 236U 238Pu 239Pu 240Pu 241Pu 242Pu 243Pu 231Pa 232Pa 233Pa 237Np 238Np 239Np 232Th 230Th 231Th 233Th 237U 238U 239U A simple “lumped” homogenous reactor model using averaged neutron cross-sections and ignoring spatial effects is adopted

8. 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 232Th population must be defined to avoid zero = zero solution

9. 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

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

11. 2 Validation of the models

12. Herrera-Martinez - Enriched Lead-cooled ADS Enrichment : 20% plutonium 2% americium 1.3% neptunium 0.04% curium • 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

13. 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 232Th over the same 5 year period of operation

14. 3 Fast thorium reactor

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

16. 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

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

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

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

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

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

22. 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 232Th 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

23. The End