DME-221 Thoria Fuel: Fabrication, Irradiation Testing and Post-Irradiation Examination

Abstract

Thoria (ThO₂) has several advantages over Urania (UO₂) as a nuclear fuel; its higher thermal conductivity will result in lower operating temperatures, and its higher melting point and chemical inertness promise better stability under postulated accident conditions. Thorium is a “fertile” material, breeding fissile ²³³U as it is irradiated, making it more proliferation resistant. Thorium is also abundant as a natural resource. Thoria fuel is therefore an attractive future energy source for Canada and the world, being sustainable, safe and secure. Past irradiation tests have shown that pellet microstructure greatly influences the irradiation performance of thoria fuels (e.g., fission-gas release is strongly influenced by the fuel microstructure). The “DME-221” test, conducted at AECL’s Chalk River Laboratories, was designed to demonstrate the superior irradiation performance of thoria fuels characterized by homogeneous, high-density microstructures. Eighteen DME-221 fuel elements were irradiated in the NRU loops in a 36-element demountable bundle. Six of the elements had pellets comprised of natural thoria (ThO₂); twelve comprised thoria blended with 1.0 wt.% or 1.5 wt.% ²³⁵U (Th, U)O₂. Various power histories were achieved as a result of the varied initial fissile component. To date, DME-221 thoria fuel has demonstrated excellent fuel performance to burnups of over 900 MWh/kgHE (38 MWd/kgHE); fission-gas release is substantially lower than that expected from UO₂ fuels experiencing similar operating histories. This paper highlights the fabrication, irradiation testing and post-irradiation examination of twelve DME-221 fuel elements. The remaining six DME-221 elements are currently planned for irradiation in NRU to burnups of 1000-1500 MWh/kgHE.