Understanding the behaviour of absorber elements in silver–indium–cadmium control rods during PWR severe accident sequences

Abstract

In the case of a hypothetical severe accident in a Pressurized Water Reactor (PWR), Silver–Indium–Cadmium (SIC) control rod failure occurs early during the sequence. Release of absorber melt could induce early fuel rod degradation by interaction of molten SIC alloy with fuel rod cladding, and the absorber materials could interact with the fission products, affecting significantly their speciation and transport in the primary circuit as well as their behaviour in the containment. This paper summarises the experimental and modelling progress made on this topic within SARNET over the whole project. Following a review of the status of knowledge, including the modelling in the main severe accident codes (ATHLET-CD, MAAP4, SCDAP, MELCOR, ASTEC), detailed calculations of the specific EMAIC and integral PHEBUS FPT2 experiments were performed. Accurate calculation of vapour pressure of the molten absorber elements is needed, requiring reliable values of the activity coefficients. The importance of accurate reproduction of the control rod degradation was shown, with the behaviour of absorber elements at rupture being critical as well as the thermodynamic data and speciation of the system Ag–In–Cd–Zr–H–O. The QUENCH-13 bundle experiment (FZK), conducted with a realistic integral geometry composed of 20 electrical heated rod simulators and one central SIC absorber rod, has helped to characterize the behaviour of absorber elements at the time of rod rupture, with measurements of the SIC release, using impactors (AEKI) and electrical low-pressure impactor and Berner low-pressure impactors (PSI). Coordinated pre and post-test calculations using SCDAP/RELAP5 based codes (PSI), MAAP4 (EDF), ATHLET-CD (GRS), ASTEC (IRSN) helped in defining the test and in its interpretation. Before this experiment, five tests were performed on small-scale SIC control rod samples using different designs and conditions. They helped in defining the conditions for the QUENCH-13 experiment. Five additional tests on similar small-scale samples are foreseen to help interpretation of the QUENCH-13 results. In QUENCH-13 the on-line aerosol measurements with electrical low-pressure impactors indicated control rod failure in the range 1550–1600 K; the test was terminated later at 1813 K by water reflood. Analysis of aerosols measured at sample points in the off-gas line showed significant Cd and In transport after rod failure with a smaller amount of transported Ag. Relocated SIC melt in the form of rivulets was detected in the lower part of the bundle. Investigation of SIC material properties (solidus, liquidus) by further analysis of data from QUENCH-13 is also presented. In parallel, an exhaustive review of activity coefficients of the elements in the SIC melt, including the effect of Zr was began (IRSN with the CNRS Marseille).