Metal borates and borides: a potential method for introducing boric acid into Phebus-FP

Boric acid will play an important role in determining the transport and deposition of fission products during a severe accident in a pressurised water reactor. The forthcoming Phebus-FP experiments will simulate as closely as possible severe accident conditions, and it is therefore necessary to include a representative source of boron-oxygen compounds in the programme. There are considerable technical problems associated with accomplishing this using conventional means of injection and therefore consideration has been given to the possibility of using the thermal decomposition or steam hydrolysis of metal borides to produce a representative source of the material in situ. Consideration has been given only to borates and borides of metals that will be present in the bundle region during the tests. Most borates were found not to have the required properties, and the majority of borides form vitreous surface product layers on exposure to steam, thus precluding further reactions. The exception to this was zirconium boride which forms a porous layer and therefore allows steam to diffuse to the reaction surface. A thermochemical analysis of the reaction with steam has shown that the products of the reaction at likely Phebus-FP fuel bundle temperatures will be B2O3 (gas- and condensed-phase) and HBO2 (gas-phase). Potential methods of introducing the material into the Phebus-FP bundle have also been reviewed and it was concluded that a rod of zirconium boride would be most effective. (author)