Evaluation of helium impurity impacts on Spent Nuclear Fuel project processes (OCRWM)

Abstract

This document identifies the types and quantities of impurities that may be present within helium that is introduced into multi-canister overpacks (MC0)s by various SNF Project facilities, including, but not limited to the Cold Vacuum Drying (CVD) Facility (CVDF). It then evaluates possible impacts of worst case impurity inventories on MCO drying, transportation, and storage processes. Based on the evaluation results, this document: 1) concludes that the SNF Project helium procurement specification can be a factor-of-ten less restrictive than a typical vendor's standard offering (99.96% pure versus the vendor's 99.997% pure standard offering); 2) concludes that the CVDF's current 99.5% purity requirement is adequate to control the quality of the helium that is delivered to the MCO by the plant's helium distribution system; and 3) recommends specific impurity limits for both of the above cases.

Spent Nuclear Fuel (SNF) Project Product Specification

“…The helium introduced into the MCO is expected to be maintained at least 99.5% pure, which takes into account impurities added to the system during supply change-over, MCO connection operations, or maintenance activities. Sherrell and Pajunen (1999) identifies the types and quantities of impurities that may be present within helium that is introduced into the MCO by the CVD process and assesses their impacts on the entire SNF process.…”

“…This analysis, demonstrating that formation of a flammable gas mixture is precluded over the 40 year storage period in the CSB, is based on an initial backfill gas that is essentially pure helium. Sherrell and Pajunen (1999) determined that an initial backfill gas composition of 99.5% helium would not impact the results and conclusions of Plys and Duncan (1998b). Therefore, the MCO void space gas composition is specified as greater than or equal to 99.5% helium.…”

“…Sherrell and Pajunen (1999) evaluates the impact of helium impurities on gas thermal properties, particulate generation, heat generation, and hydrogen generation during CVD operation. This analysis concludes that a gas composition of 99.5% helium added to the MCO will support process safety and design assumptions.…”

Spent Nuclear Fuel (SNF) Project Product Specification

“…The helium introduced into the MCO is expected to be maintained at least 99.5% pure, which takes into account impurities added to the system during supply change-over, MCO connection opcrations, or maintenancc activities. Sherrcll and Pajunen (1999) identifies the types and quantities of impurities that may be present within helium that is introduced into the MCO by the CVD process and assesses their impacts on the entire SNF process.…”