Any burnable poison material needs to meet a num­ber of important materials property requirements if it is to be used in nuclear fuel. Such materials must be physically stable under high-temperature conditions, they must not be susceptible to corrosion in the harsh chemical environment, and they must be compatible with the other materials present in nuclear fuel, such as UO2, Zircaloy, stainless steel, and the coolant materials water, steam, or gas as appropriate to the reactor type. The burnable poison materials must also be well behaved under the intense neutron irra­diation and gaseous releases must be accommodated. This latter consideration is particularly important for boron, because the neutron absorption leads to helium production through the reaction

10B + n!7 Li +4 He

If used in discrete poison rods, there must be suffi­cient free volume to accommodate the helium release without overpressurization. With IFBA, the helium released accumulates in the plenum volume provided to accommodate the gaseous fission products and adds to the end-of-life rod internal pressure.

Additionally, burnable poisons must be designed so that there is no possibility that the poison material could melt or slump out of the active core in an accident scenario or in any way interfere with the insertion of control rods. The migration of absorber material away from the active core would add reac­tivity during the accident scenario that could poten­tially worsen the consequences of an accident.

The zirconium diboride coating thickness used in IFBA fuel is thin enough that there is only a very small temperature drop across it and virtually no impact on fuel temperatures and fuel behavior, apart from the higher internal gas pressure due to helium generation noted earlier. In contrast, the
presence of a second ceramic phase in the fuel pellets, as is the case for gadolinia, erbia, and dysprosia, does have an impact on fuel pellet behavior, as discussed in the next section.