Overview of 14 C release from irradiated zircaloys in geological disposal conditions

Abstract

Carbon-14 (radiocarbon, 14 C) 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 of 14 C from irradiated Zircaloy cladding in alkaline media. 14 C is found either in the alloy part of Zircaloy cladding due to the neutron activation of 14 N impurities by 14 N(n,p) 14 C reaction, or in the oxide layer (ZrO2) formed at the metal surface by the neutron activation of 17 O from UO2 or (U-Pu)O2 fuel and water from the primary circuit in the reactor by 17 O(n,α) 14 C reaction. Various irradiated and unirradiated Zircaloys have been studied. The total 14 C 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. 14 C 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 the 14 C inventory in the oxide was significantly below the 20% commonly used in safety case assessments.