Influence of the dissolved hydrogen concentration on the contamination of the primary loop of DOEL PWR using the OSCAR code
Résumé
Normal operation of PWR generates corrosion products in the primary circuit which are activated in the core and constitute the major source of the radiation field. To minimize these risks and constraints, it is essential to understand the behavior of corrosion products by carrying out the appropriate measurements in PWR circuits and loop experiments combined with reactor contamination assessment code. The reactor contamination assessment code allows to take into account the chemical and physical mechanisms in different situations in operating reactors or at design stage. OSCAR code has been developed with this aim by CEA in collaboration with EDF and Framatome, and has actually been used since the early seventies [1]. OSCAR is a reliable tool for PWR (also used for EPR, SFR, ITER, decommissioning…) calibrated and validated with a complete database of contamination measurements in EDF water reactors. Water chemistry has an influence on corrosion of the main materials (especially nickel-based alloys), in Belgian PWR the average dihydrogen concentration used is around 30 cc/kg, which is in line with current international standards however which is not the best value to reduce stress corrosion cracking of the materials. It also has an influence on dissolution/precipitation mechanisms involved in contamination. Water chemistry control allows then to reduce significantly the radioactive contamination in the primary loop and therefore facilitates maintenance operations. In this field, hydrogen plays a critical role in limiting the presence of oxidizing species due to water radiolysis. Increasing hydrogen could also reduce core internals cracking. The aim of this study is to evaluate the influence of an increase/decrease of the Dissolved Hydrogen (DH) concentration on the contamination of the primary loop using the OSCAR code. This study presents the simulation results of a sensitivity analysis, using the 1.3 version of the OSCAR code, of the contamination in the primary loop of DOEL PWR with DH concentrations ranging between 15 and 70 mL/kg.
The evolution of the surface contamination in $^{58}$Co and $^{60}$Co were calculated on the hot legs, U tubes and Steam Generators (SG) tubing along with the mass of Ni deposited on the fuel. In order to explain those evolutions, equilibrium (concentration of the element in the coolant with respect to the considered oxide) and ionic Ni concentrations were investigated on the SG and the fuel as well as the Ni dissolution and release flux of the SG.
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