A disposal-MOX concept for plutonium disposition

Abstract

In case it is desirable to dispose of inventories of separated civil PuO₂ that have no further use, a suitable immobilisation matrix is required, prior to disposition in a geological disposal facility. Conversion of Pu into a mixed oxide (MOX)-type material with characteristics suitable for disposal has previously been suggested, but not yet demonstrated at laboratory or industrial scale. We here demonstrate the feasibility of different synthesis routes for simulant “disposal-MOX”, using Th⁴⁺ as a Pu⁴⁺ surrogate and containing Gd³⁺ in a suitable quantity to ensure criticality control. Compositions of (U_(1−(x+y))Th_xGd_y)O_2−δ, where x = 0.1, 0.2 and x : y = 10 : 1 or 100 : 1, were synthesised by a solid state route mimicking the industrial MIMAS (MIcronized MASterblend) MOX fuel fabrication process, or through an oxalic wet co-precipitation method. Both synthesis routes gave a single phase fluorite structure upon heat-treatment at 1700 °C, with a grain size similar to (Pu,U)O₂ MOX fuel. The relative density of the sintered pellets was >90% but was highest in co-precipitated materials, with Th⁴⁺ and Gd³⁺ additions more homogenously distributed. Though no unincorporated ThO₂ or Gd₂O₃ was observed in any sample, Th and Gd-rich regions were more prevalent in materials produced through solid state synthesis, in accordance with MIMAS MOX fuel microstructures. The incorporation of Gd³⁺ within the fluorite lattice, which is favourable from a criticality control perspective in a Pu wasteform, was found to be charge balanced via the generation of oxygen vacancy defects, but not U⁵⁺. These results demonstrate feasible synthesis routes for a disposal-MOX wasteform product via both solid state and wet co-precipitation fabrication routes.