Modelling of radiolytic production of HNO₃ relevant to corrosion of a used fuel container in deep geologic repository environments

Abstract

Copper-coated steel containers are part of the engineered barrier system to permanently store Canadian nuclear fuel waste in a deep geological repository. This work models the dose rates (D Rs) at the container surfaces as a function of fuel age. It also utilises a humid-air radiolysis model to study the effects of D R and humidity on radiolytic oxidant production for conditions where unexpected early water intrusion reaches clay seal materials. Radiolysis of humid air produces HNO 3. The HNO 3 production rate in a condensed water droplet formed on a container surface was conservatively estimated by assuming that every •OH produced by primary radiolytic processes was immediately converted to HNO 3 in the gas phase and that all of the HNO 3 was absorbed in the water droplet. Also assuming that all of the nitric acid absorbed in the water droplet is consumed in corroding copper and using a hemispherical water droplet geometry, the corrosion depth of the copper coating induced by humid-air radiolysis is conservatively estimated to be 9.4 μm over the permanent storage time.

The kinetics of copper corrosion in nitric acid

“…[ 17 ] Radiolysis model calculations show that significant amounts of nitric acid could be produced at dose rates relevant to DGR conditions. [ 14 ]…”

“…[7][8][9][10][11] However, a model that assesses the influence of radiolysis on the corrosion of the copper coating does not yet exist. [6] As humid air radiolysis is a short-term process that could cause some corrosion damage without leading to container failure, [12][13][14] it may be sufficient to determine the nature and extent of corrosion damage, leading to the specification of a corrosion allowance for this process. To achieve this, a thorough understanding of the mechanism of the corrosion process is required.…”

“…Many reactive species can result from the gamma radiolysis of humid air, with one of the most important species being nitric acid. [12][13][14] The influence of radiation on copper corrosion in humid air has been studied and discussed. [15] More recent studies have shown that whereas minimal corrosion was observed at the low gamma radiation dose rates anticipated in a DGR, [16] substantial corrosion occurred at much higher dose rates.…”

“…[17] Radiolysis model calculations show that significant amounts of nitric acid could be produced at dose rates relevant to DGR conditions. [14] In a previous work, [15] the corrosion process in both aerated and deaerated nitric acid solutions was investigated by following the pH and the concentration of dissolved copper in a small volume of the solution designed to simulate the limited amounts of water that will condense on the container surface. These studies showed that negligible corrosion occurred in anoxic nitric acid, but the corrosion rate increased markedly when oxygen was added.…”

The Effects of Cathodic Reagent Concentration and Small Solution Volumes on the Corrosion of Copper in Dilute Nitric Acid Solutions

“…Under these conditions the radiolysis of the aerated vapor will produce HNO 3 , 10-12 which will be absorbed into condensed H 2 O in contact with the aerated vapor. [13][14][15] The effect of radiation on the corrosion of Cu has been studied in both aqueous solutions [16][17][18][19][20][21][22][23] and in humid air, 24 usually at dose rates well in excess of those anticipated on a container surface, but with contrasting observations. In both aerated and deaerated chloride solutions (150°C, 27 Gy/h), radiation appeared to be beneficial, leading to the formation of an apparently protective Cu 2 O film.…”