Degradation and oxidation of B4C control rod segments at high temperatures. A review and code interpretation of the BECARRE program

The paper gives a code-based interpretation of the experimental BECARRE program carried out at the "Institut de Radioprotection et de Sureté Nucléaire" (IRSN) between 2005 and 2010, using the severe accident (SA) code ASTEC/ICARE. As part of the International Source Term Program (ISTP), the BECARRE program focuses on boron carbide (B4C) effects during SA conditions, when B4C is used as an absorber material in French PWRs. Steam oxidation of solid B4C pellets (at 1200–1800 °C), as well as oxidation of molten B4C bearing mixtures up to 9 wt% of B4C dissolution (at 1289–1527 °C) are studied, as well as degradation of 60 cm-long control rod (CR) segments representative of a PWR geometry (up to ∼2000 °C). Temperature and outlet gas releases (steam, H2, CO, CO2) measured in line, and post-test examinations (radiography, tomography, microscopic examinations) give available data to code validation and interpretation. The oxidation rates of the B4C bearing melts have been found always lower than the rates of the solid pellet oxidation in similar conditions, as modeled in ASTEC. For the degradation of the CRs, it is shown that for temperature above 1600 °C, the main effect of the B4C is more toward a mitigation of the hydrogen production rather than increasing it by additional oxidation of boron compounds. No large increases of the hydrogen release after the failure of the guide tube (GT) have been measured, due to downward relocation of the low viscosity B4C bearing melts inside the 60 cm-height CR segments. The ZrO2 oxide layer formed on the outer surface of the GT has been found very protective, leading to failure only above 1650 °C. Both isothermal and runaway thermal conditions have been used to bring about limited GT failure showing that the main mechanisms leading to failure are linked to deformation of the initial geometry, close contacts between materials, and subsequent eutectic material formations. Such a limited degradation for the GT is not modeled in the code