Study of Boron Behaviour in the Primary Circuit of Water Reactors under Severe Accident Conditions - a Comparison of Recent Integral and Separate-Effect Data

Boron carbide (B4C) is widely used as an absorber material in many commercial reactors, such as boiling water reactors, Russian RBMKs and VVERs, later French pressurised water reactors, and in the European Pressurised Water Reactor. Under hypothetical 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 bundle degradation and fission product release through to the late phase, material transport in the circuit and behaviour in the containment has been studied in the nuclear-heated integral experiment Phebus FPT3 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-09 test at Karlsruhe used similar bundle geometry and had similar but less severe test conditions, and included a water reflood phase. Extensive separate-effects tests on oxidation of B4C and its interaction with surrounding materials have also been performed at IRSN Cadarache (BECARRE/VERDI) and at Karlsruhe (BOX, LAVA, QUENCH-SR). This paper compares the phenomena involving B4C observed in these experiments, concentrating on degradation, transport and deposition behaviour. 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. Possible explanations for the phenomena observed are advanced, and suggestions for further work to improve understanding are put forward.

HASTE Tim;  PAYOT Frederic;  DOMINGUEZ Cristina;  MARCH Philippe;  SIMONDI-TEISSEIRE Beatrice;  STEINBRÜCK Martin;  ZEYEN Roland; 

2011-07-21

Nuclear Society of Slovenia

JRC58138

http://www.djs.si/port2010/htm/abs/absPORT20102734.html,    https://publications.jrc.ec.europa.eu/repository/handle/JRC58138,