NEARCOMPLETE TRANSURANIC WASTE INCINERATION IN THORIUMFUELLED LIGHT WATER

NEAR-COMPLETE TRANSURANIC WASTE INCINERATION IN THORIUM-FUELLED LIGHT WATER REACTORS Ben Lindley

BACKGROUND • In ADSRs, transuranic (TRU) waste added to reactor with thorium. • At end of fuel cycle, reprocessed and U-233 removed. Addition thorium and TRUs added • Most waste is ultimately incinerated, but there is always some left as the isotope populations tend to equilibrium

LIGHT WATER REACTORS • U/Pu MOX allows limited recycle • 50 -75% destruction is possible using Th/Pu MOX* *Shwageraus et al. , 1995

METHOD • In this study, Th/ dirty Pu MOX is considered in a Generation III+ PWR • The TRUs are returned to the reactor after reprocessing • The U-233 is also returned to the reactor • Reloading parameters selected to give appropriate enrichments and burn-up (so note that all results are examples and ‘actual’ design may change the numbers) • One batch fuel strategy assumed (e. g. 4 batch burn~60% higher)

METHOD (2) • Analysis of single assembly performed using commercial reactor physics code WIMS 9 • Model benchmarked against MCNP calculation • Model and nuclear data library checked using IAEA benchmark

INCINERATION PERFORMANCE • Waste becomes less reactive over time in a thermal reactor. • “A fast neutron stage in the reactor appears… almost a necessity” (Rubbia et al. , 1995)

PU AND U

MINOR ACTINIDES

BURN-UP

REACTOR BEHAVIOUR • • U-233 provides required excess reactivity Faster neutron spectrum than with U-235/U-238 fuel Self shielding encourages equilibrium behaviour Fuel loaded with additional MAs can also be incinerated • Incineration tends towards ~250 kg/GWth yr (compared to 280 kg/GWth yr in ADSR)* *Rubbia et al. , 1995

PU AND U-233

MINOR ACTINIDES

REACTIVITY COEFFICIENTS • • Doppler coefficient (doesn’t change much) Void coefficient Moderator temperature coefficient 100% void coefficient

REACTIVITY COEFFICIENTS

REACTIVITY COEFFICIENTS (2)

IS A POSITIVE 100% VOID COEFFICIENT ACCEPTABLE? • In PWRs, high void fractions without emergency shutdown seems implausible • In BWRs, the void fraction at the top of the core can be 70 -80% • A negative 100% void coefficient is easier to achieve in a PWR • PWR appears preferable

REACTIVITY CONTROL • Soluble boron worth is much less • Change in reactivity over cycle is also much less (no depletion of U-235; after a large number of cycles poisoning isotopes such as Pu-240 are depleted over the cycle) • Result: little change

RELATIVE SOLUBLE BORON REQUIRED

REACTIVITY CONTROL • If coolant boils/expands amount of boron in the core is reduced • Fast neutron spectrum as coolant boils reduces boron capture cross section • Soluble boron makes the reactivity coefficients worse

MAXIMUM VS REQUIRED BORON

ALTERNATIVE CONTROL METHODS • Control rods • Burnable poisons

WHAT ELSE NEEDS CHECKING? • Reactor kinetics are different (worse than U-235/U 238) • Practicality of multiple reprocessing (also a problem for ADSR) • How much dirty Pu can be loaded in the core? (worse than ADSR) • Can the U-232 be handled and reside in the core without too much damage?

ADDITIONAL WORK • Reduced-Moderation PWR – Improved burn-up per % Pu enrichment – E. g. <16 wt% dirty Pu, 60 GWd/te 4 batch burn-up • Reduced-Moderation BWR (High Conversion) – Extensive research programme in Japan – Aim to limit TRU loading – Thorium is useful alternative to U/Pu for stability reasons – Strategic alternative to LMFBR or GFR?

CONCLUSIONS • A Generation III+ reactor can be used to achieve approaching 100% TRU incineration – – Competitive or improved burn-ups Stable Controllable Thermal-hydraulics are compatible • Low cost, low risk: new reactor designs, coolant technology and accelerator technology not required • Commercial implementation in medium term?