Possibilities of Thorium U tilization in VVER-1000 Reactor

1. Possibilities of Thorium Utilizationin VVER-1000 Reactor Jan Frybort, NRI in Rez

2. Content • Thorium as a nuclear fuel • Analysis fundamentals • Fuel assembly design • Fuel cycle calculation • Fuel depletion calculation • Fuel toxicity and decay heat • Proliferation resistance • Fissile material breeding • Conclusions 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

3. Thorium as a nuclear fuel • Thorium is an alternative fertile material • 232Th can be utilized in nuclear reactors to breed fissile isotope 233U • Breeding capabilities of thorium in thermal reactors are better than 238U • Nuclear characteristics of fissile 233U surpass those of 235U – lower ratio of capture to fission, larger reproduction factor in a wide range of energies 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

4. Analysis fundamentals • The main purpose of the analysis was to compare thorium fuel assemblies with different fissile material support with reference uranium assemblies for VVER-1000 reactor • Characteristics of thorium assemblies with 235U enriched uranium, reactor grade plutonium and 233U were calculated in HELIOS 2D lattice code • Prepared data in form of libraries were supplied to 3D advanced nodal code ANDREA • ANDREA allows to calculate fuel cycle with assemblies loading and reshuffling 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

5. Fuel assembly design • Design of fuel assemblies with 2 enrichment zones were prepared • Geometry of the thorium fuel is identical to standard uranium fuel • Two assemblies with different average enrichment were prepared for each supporting fissile material 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

6. Thorium fuel assembly enrichment • Two types of fuel pins were used in all assemblies • Thorium was supported by reactor-grade plutonium, uranium enriched by 20 % of 235U and pure 233U • Composition of the reactor-grade Pu was calculated by HELIOS for typical VVER-1000 fuel (burnup 40 MWd/kg, 30 years after discharge) 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

7. Fuel cycle calculation • An equilibrium loading for 320 EFPD in a 5-year cycle was calculated • No burnable absorbers were used in thorium assemblies in contrast to reference fuel • Acceptable uniform power distribution among assemblies and fuel rods were achieved in all fuel types except for 233U fissile material • Graphs show hot channel peaking factor and maximum vertical relative fuel pin power 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

8. Fuel depletion calculation reactivity coefficients • Time dependence of reactivity coefficients were calculated for all higher enriched fuel types • The first graph shows fuel temperature coefficient (Doppler) • Similar results are observable for all assemblies • Time dependence of moderator feedback coefficient is plotted in the second graph • Fuel supported with 233U fissile material has much lower moderator feedback coefficient 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

9. Fuel depletion calculation fuel composition • Fuel composition changes during fuel burnup to 72 MWd/kg • Actinide composition of calculated assemblies were compared • Fuel composition was expressed relative to initial mass of heavy metal • The first graph shows burnup dependence of 239Pu mass – thorium assembly with plutonium support is great in Pu burning and thorium presence limits Pu breeding • The second graph shows 233U breeding and consumption 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

10. Fuel toxicity and decay heat • Toxicity and decay heat of the thorium fuel was calculated relative to the reference uranium fuel • Thorium utilization has positive effect on both toxicity and decay heat • Graphs for toxicity are presented, time dependence of residual decay heat is similar • Toxicity of the thorium fuel is lower due to limited or almost negligible Pu breeding (233U case) • In the long term there is an increase, because of higher activity of 233U 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

11. Proliferation resistance • There are many aspects of nuclear fuel proliferation resistance – material, technical and institutional barriers • Thorium utilization influences material barriers • Proliferation resistance related changes: • Critical mass of Pu increased • Increased spontaneous neutron generation • Higher heat generation rate from 238Pu • Presence of 232U in thorium fuel – affects radiological barriers and detectability 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

12. Fissile material balance 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010

13. Conclusions • Thorium utilization in VVER-1000 reactor is feasible • Important advantage of thorium fuel is low production of transuranium isotopes resulting in low overall toxicity and decay heat • Conversion capabilities of LWR reactors are limited, but still large fraction of power is released from bred fissile material • The most viable option appears to be thorium mixed with low enriched uranium – it has similar properties to standard uranium fuel with flat power distribution even without burnable absorbers 12th session of the AER Working Group F and 3rd Meeting of INPRO Project RMI, Liblice April 7, 2010