Advanced Candu Systems For Plutonium Destruction

Abstract

High neutron economy, on-line refuelling, and a simple fuel-bundle design result in a high degree of versatility in the use of the CANDU® reactor for the disposition of weapons-derived plutonium. CANDU mixed-oxide (MOX) fuel is a near-term, technically achievable, economic option. Studies led by AECL show that four Bruce A reactors could consume 50 te of plutonium in less than 12.5 years. The symmetry in the simultaneous drawdown of excess weapons-derived plutonium from both the United States and Russia in Canada was an important consideration in the recent US Record of Decision, which includes the CANDU MOX option for further evaluation.

The CANDU versatility enables advanced options for plutonium destruction. One such option is the use of an inert matrix, non-fertile material as the carrier for weapons-derived plutonium. Mixing the plutonium with inert SiC in a standard 37-element CANDU bundle would result in destruction of 93% of the fissile plutonium (²³⁹Pu and ²⁴¹Pu). Fuel management studies were conducted, confirming that fuelling rates and maximum powers are well within limits. Because of the very high thermal conductivity of SiC, fuel temperatures would be very low, and negligible fission-gas release is expected.

The Pu-ThO₂ cycle would also achieve a very high efficiency in plutonium destruction. With ∼2.6% weapons-derived plutonium in ThO₂ in a modified CANFLEX bundle (a large central graphite displacer surrounded by 35 fuel elements in the two outer fuel rings), a burnup of 30 MWd/kg heavy element (HE) can be achieved, and >94% of the fissile plutonium destroyed. Good neutron economy is the key to high efficiency in plutonium destruction with ThO₂. Of course, ²³³U is produced, through neutron capture in ²³²Th and subsequent β-decay, and partially burned in situ. This material is safeguarded in the spent fuel and is not attractive for weapons because of contamination with ²³²U. The spent fuel could be stored until an economical and proliferation-resistant means of recycling the ²³³U is developed.