Overview OF¹⁴C release from irradiated zircaloys in geological disposal conditions

Abstract

Carbon-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.

Overview of CAST project

“…The experimental works conducted on the corrosion of Zircaloy in CAST tends to confirm the data reported in the literature with corrosion rates in the order of a few nm/yr at the most at low temperature, in alkaline or neutral conditions [Gras 2014 (CAST D3.1); Necib et al 2018 (CAST D3.20)]. CAST allowed progress in the knowledge of the corrosion mechanism of Zircaloy.…”

14C content in CANDU spent fuel cladings and its release under alkaline conditions

“…Total 14 C content measured with this analytical method is in the same order of magnitude as the value estimated by modeling for an average burn-up of 7 MWd/kg U (1.78E + 04 Bq/g of Zy-4). Details on the modeling of the 14 C accumulation during CANDU fuel irradiation are given in CAST D3.13 report (Necib et al 2016).…”

14C Exposure from Disposal of Radioactive Waste Compared to14C Exposure from Cosmogenic Origin

“…The corrosion rates from measured adsorbed and released hydrogen are as low as corrosion rates determined from released non-radioactive nickel and chromium: below 1 nm per year. The smaller corrosion rates measured with zirconium are attributed to the small solubility product of zirconia (Necib 2018). Assuming a value of 1 nm per year and a 14 C content of 10 4 Bq/g, a reasonable 14 C release rate is in the order of 10 9 14 C molecules per cm 2 per year at start of disposal i.e.…”

“…The nitrogen content in metals, neutron irradiated in a plant, is frequently not specified or measured but the potential 14 C inventory in metals can be bounded. Examples of measured nitrogen contents in CAST are 17 ppm and 25 ppm in PWR Zircaloy-4 specimens from two Belgian nuclear reactors Tihange and Doel (Necib 2018) and 0.04% for stainless steel from a Finnish surveillance capsule (Mibus 2017). Values twice of three times as large are reported within the CAST project e.g.…”

“…The identified carbon species released from steel are low molecular weight organics up to C5 (Wieland 2015). At alkaline reducing conditions, dissolved organic carbon compounds have been measured to be the major 14 C species released from neutron irradiated steel, except for specimens with the highest calculated 14 C activity concentrations (Mibus 2018), neutron irradiated Zircaloy (Necib 2018) and neutron irradiated graphite (Toulhoat 2018). A quantification of released organic carbon into carboxylic acids, alcohols and aldehydes has not been completed within CAST but this quantification could reduce the conservatism in assessments since organic carbon species are assumed to be non-retarded species in safety assessments (Capouet 2017) but conjugate bases of carboxylic acids are charged species in which at least one of the four just described mitigation mechanisms within the barriers could act.…”