Overview OF14C release from irradiated zircaloys in geological disposal conditions

Abstract

ABSTRACTCarbon-14 (radiocarbon,14C) is a long-lived radionuclide (5730 yr) of interest regarding the safety for the management of intermediate level wastes (ILW). The present study gives an overview of the release of14C from irradiated Zircaloy cladding in alkaline media.14C is found either in the alloy part of Zircaloy cladding due to the neutron activation of14N impurities by14N(n,p)14C reaction, or in the oxide layer (ZrO2) formed at the metal surface by the neutron activation of17O from UO2or (U-Pu)O2fuel and water from the primary circuit in the reactor by17O(n,α)14C reaction. Various irradiated and unirradiated Zircaloys have been studied. The total14C inventory has been determined both experimentally and by calculations. The results seem to be in good agreement. Leaching experiments were conducted in alkaline media for several time durations.14C was mainly released as carboxylic acids. Further, corrosion measurements were performed by using both hydrogen measurements and electrochemical measurements. The corrosion rate (CR) ranges from a few nm/yr to 100 nm/yr depending on the surface conditions and the method used for measurement. From a safety assessment point of view, the instant release fraction (IRF) was determined on irradiated Zircaloy-2. The results showed that the14C inventory in the oxide was significantly below the 20% commonly used in safety case assessments.