Dry air oxidation kinetics of K-Basin spent nuclear fuel

Abstract

SummaryThe safety and process analyses of the proposed Integrated Process Strategy (IPS) to move the N-Reactor spent nuclear fuel (SNF) stored at K-Basin to an interim storage facility require information about the oxidation behavior of the metallic uranium. Limited experiments have been performed on the oxidation reaction of SNF samples taken fiom an N-Reactor outer fuel element in various atmospheres. This report discusses studies on the oxidation behavior of SNF using two independent experimental systems.. 0 a tube furnace with a flowing gas mixture of 2% oxygen/98% argon 0 a thermogravimetric system for dry air oxidation.Results of the tube furnace tests show differences in the oxidation rate of samples taken fiom the damagedhorroded and undamaged regions of the same SNF element. Samples taken from the damaged/ corroded regions of the element showed an oxygen pickup rate that was time independent, indicating a linear oxidation behavior. However, the oxygen pickup by the samples taken fiom the undamaged regions decreased with increasing time that suggests a protective oxide f i l m formation on the samples.The average oxidation rate for the undamaged SNF samples in an oxygerdargon atmosphere at 250°C was 7.01 x 10" mg/cm2/min. This rate is in close agreement with the value 4.16 x l o 3 mg/cm2/min estimated from the unirradiated data of Baker and Less and reported by Ritchie (1981).(') On the other hand, the average oxidation rate of the damaged SNF samples at 25OOC (Furnace Runs 2 1 , 23 , 27) was 2.15 x 1 0-2 mg/cm2/min, which is about a factor of 5 greater than the unirradiated uranium data of Baker and Less (Ritchie 1981):') In one particular experiment, the oxidation rate of damaged SNF sample at 150°C was 1.24 x 1 0-2 mg/cm2/min, which is about 83 times greater than the value of 1.5 x 1 O4 mg/cm2/min estimated fiom the data of Baker and Less (Ritchie 1981).(*) This sample showed significantly higher hydrogen release during its drying step(a) compared to other samples. Thus, the increased oxidation rate observed for this sample may be due to an enhancement effect of uranium hydride inclusions in the sample.The data for dry air oxidation of damagedhorroded SNF samples using the thermogravimetric (TGA/DSC/MS) system showed an oxidation behavior that was complex. The data were grouped into two categories for analysis. The first group of data represents the cases when the weight change by the SNF sample followed a typical metallic material oxidation kinetics. The second group of weight change showed oxidation behavior that was very erratic due to effects that include but are not limited to 1) sample cracking, 2) rapid oxidation of the sample resulting in large volume changes, 3) spallation of the oxide film, and 4) disintegration of the sample due to rapid oxidation of particulates in the matrix. iii The shape of the weight change data for the first group could be fitted with either linear or parabolic oxidation kinetics. A linear rate equation was fitted to the data to generate a rate equation that can be co...

Oxidation rate of K-Basin spent nuclear fuel in moist air

“…The initial time-dependent oxidation rate is higher than the linear oxidation rate at the latter part of the experiment. The time period for transition from a nonlinear rate to a linear rate at the sample temperature of 198 "C is about 150 min, which agrees well with the data for the dry air oxidation at 200°C (Abrefah et al 1998a). The results at 198°C thus indicate that a protective oxide film can be formed on the SNF at (and maybe below) 198°C in a moist air atmosphere.…”

“…The three oxidation rates of the damagedcorroded SNF material in air containing 12.33 kPa water vapor partial pressure were not significantly different from the dry air oxidation rate (Abrefah et al 1998a). This observed behavior suggests that the oxidation rate of the damagedkorroded SNF uranium may not be affected by the presence of moisture in air.…”

“…In Figure 3.8, the moist air oxidation rate measurements are compared to the dry air oxidation rate equation (solid line) of SNF material (Abrefah et al 1998a). Within the experimental spread of the data, there is no significant difference between the moist air oxidation kinetics and the dry air oxidation kinetics of the damagedcorroded SNF samples.…”

“…However, because the oxidation behavior of damagedcorroded SNF material in dry air was shown to be different from unirradiated uranium in PNNL, studies (Abrefah et al 1998a), the possibility that moist air oxidation could also be different was considered. This report provides the results of experiments conducted on the SNF samples using a thermogravimetric system under isothermal conditions ranging from 198°C to 350°C and at a partial pressure of 12.33 kPa (93 Torr).…”

Uranium oxidation rates issue closure package

“…Dry air reaction rate studies at relatively high temperature (69'C to 275'C) were the initial focus of N Fuel tests, followed by moist air reaction rates in a similar temperature range (Abrefah, et al, 1998a and. These high temperature data were developed to support evaluation of accident scenarios in a hot conditioning process system.…”