Study of boron behaviour in the primary circuit of water reactors under severe accident conditions: A comparison of Phebus FPT3 results with other recent integral and separate-effects data

► We compare data on degradation of B4C control rods under severe accident conditions. ► There is good agreement amongst the integral and small-scale tests studied. ► The presence of B4C accelerates melting and relocation of nuclear fuel rods. ► Oxidation of B4C forms boric acids which can partially block primary circuit pipes. ► No significant amount of methane was detected in any experiment studied. Boron carbide (B4C) is widely used as an absorber material in many commercial reactors, such as boiling water reactors, Russian VVERs, later French pressurised water reactors, and is planned to be used in Areva's EPR™. Under severe accident conditions, B4C reacts with its surrounding stainless steel cladding, producing eutectic melts above 1200°C; remaining bare B4C and B4C/metal mixtures are then exposed to steam and oxidize highly exothermically. As well as hydrogen, gases and aerosols containing boron and carbon compounds are produced, which affect the transport and deposition of radiologically important fission products such as iodine and caesium in the circuit, and subsequent behaviour in the containment. The influence of a B4C control rod on fuel degradation and fission product release through to the late phase, material transport in the circuit and behaviour in the containment was studied in the nuclear-heated integral experiment Phebus FPT3 performed by IRSN at Cadarache. Carbonaceous gas production was monitored, and evidence for substantial deposition of B-containing compounds in the circuit was gathered. The electrically heated QUENCH-07 and -09 tests performed at Karlsruhe Institute for Technology (KIT) used similar bundle geometry and similar but less severe test conditions, with a fast cooling phase that arrested bundle degradation before the late phase. Extensive separate-effects tests on oxidation of B4C and its interaction with surrounding materials have also been performed by IRSN (BECARRE programme) and KIT (BOX, LAVA, QUENCH-SR). This paper compares the phenomena involving B4C observed in these experiments, taking the recently released data from Phebus FPT3 as a base, concentrating on transport and deposition behaviour in the circuit, with reference to degradation where necessary, e.g. as a source of carbonaceous gases. In particular, there is evidence concerning blockage formation in the hot leg of the FPT3 circuit, which affects the transport of fission products, and this is supported by similar observations in the separate-effect tests.