Functions and Requirements for the Leak Mitigation System of the Savannah River Site Double Shell High Level Waste Tanks

Abstract

The Savannah River Site (SRS) High Level Waste (HLW) storage tanks have been in service for 20 to 45 years. Degradation evaluations of DOE HLW tanks conclude that pitting and stress corrosion cracking are two primary degradation modes for the carbon steel tanks in extended service. The SRS HLW tank service experience has been that some of the tanks have suffered stress corrosion cracking that led to leakage failures. In comparison, Hanford Double-Shell Tank experience has been instances of pitting. Of the 16 total Types I and II tanks at SRS, 11 have leaked into the annular space, including all four Type II tanks. Laboratory testing has shown that the leaksites were caused by nitrate stress-corrosion cracking (SCC). Field examinations of the tanks, and laboratory testing and analysis, has shown that the cracking is confined to the residual stress field of the fabrication and repair welds. Chemistry control for HLW was developed and instituted at SRS in the late 1970's to prevent the further initiation of cracking and to avoid initiation of pitting. However, what are assumed to be pre-existing stress corrosion cracks, not observed through visual examination, have been found to contribute to leakage in the re-use of severalmore » of these tanks. A leak mitigation/crack-sealing system would allow for continued storage and planned retrieval operation without additional leakage into the secondary pans. This document provides the basis for the functions and requirements of a proposed leak mitigation system (LMS) for HLW tanks.« less