MgO–pyrochlore composite as an inert matrix fuel: Neutronic and thermal characteristics

Inert matrix fuels are an important component of advanced nuclear fuel cycles, as they provide a means of utilizing plutonium and reducing the inventory of ‘minor’ actinides. We examine the neutronic and thermal characteristics of MgO–pyrochlore (A2B2O7: La2Zr2O7, Nd2Zr2O7 and Y2Sn2O7) composites as inert matrix fuels in boiling water reactors. By incorporating plutonium with resonance nuclides, such as Am, Np and Er, in the A-site of pyrochlore, the kinf vs. burn-up curves are shown to be similar to those of UO2, although the Doppler coefficients are less negative than UO2. The Pu depletion rates are 88–90% (239Pu) and 54–58% (total Pu) when the inert matrix fuels experience a burn-up equivalent of 45 GWd/tHM UO2. Because of the high thermal conductivity of MgO, the center-line temperatures of the MgO–pyrochlore composites at 44.0 kW/m are lower than those of UO2 pellets. After burn-up, the A-site cation composition is 15–35 at.% lower than that of the B-site cations in pyrochlore (e.g., A1.84B2.17O7.00) due to the fission of Pu in the A-site and the presence of fission product elements in the A- and B-sites of the pyrochlore structure.