Hydrothermal synthesis of Ni2FeBO5 in near-supercritical PWR coolant and possible effects of neutron-induced 10B fission in fuel crud

Abstract The hydrothermal synthesis of a nickel–iron oxyborate, Ni 2 FeBO 5 , known as bonaccordite, was investigated at pressures and temperatures that might occur at the surface of high-power fuel rods in PWR cores and in supercritical water reactors, especially during localized departures from nucleate boiling and dry-outs. The tests were performed using aqueous mixtures of nickel and iron oxides with boric acid or boron oxide, and as a function of lithium hydroxide addition, temperature and time of heating. At subcritical temperatures nickel ferrite NiFe 2 O 4 was always the primary reaction product. High yield of Ni 2 FeBO 5 synthesis started near critical water temperature and was strongly promoted by additions of LiOH up to Li/Fe and Li/B molar ratios in a range 0.1–1. The synthesis of bonaccordite was also promoted by other alkalis such as NaOH and KOH. The bonaccordite particles were likely formed by dissolution and re-crystallization by means of an intermediate nickel ferrite phase. It is postulated that the formation of Ni 2 FeBO 5 in deposits of borated nickel and iron oxides on PWR fuel cladding can be accelerated by lithium produced in thermal neutron capture 10 B(n,α) 7 Li reactions. The process may also be aided in the reactor core by kinetic energy of α-particles and 7 Li ions dissipated in the crud layer.