Abstract

Stress corrosion cracking (SCC) is a documented threat to the integrity of carbon steel radioactive waste storage tanks at the Hanford Nuclear Reservation site. Presently, the slow strain rate (SSR) test is the workhorse test technique used for assessing SCC of carbon steels in nuclear waste simulants. The main advantages of the standard SSR technique are that it is relatively rapid, sensitive in detecting SCC, and relatively inexpensive. SSR test results are typically classified as "SCC" or "No SCC", depending on the ductility parameters from the test and observations of crack features. Occasionally, tests are not easily classified, which results in a third category, "Inconclusive". Inconclusive results are usually related to variations in ductility parameters compared to the control specimen, or the presence of atypical fracture facets or secondary features on the gage section. Regardless of the cause of the inconclusive result, these tests create uncertainty in assessing susceptibility to SCC. This paper describes the results of a preliminary study undertaken to develop an SCC testing protocol for evaluating inconclusive results. Three alternatives to the standard SSR technique, slower SSR (reduced extension rate), Ripple SSR, and notched SSR, were evaluated. Additionally, SCC evaluations from a crack growth technique using a compact tension specimen were compared to the results of the alternative SSR methods. The study showed that, regardless of the technique evaluated, there will be inconclusive results due to subjective aspects of the post-test evaluation. However, there appears to be a critical potential below which SCC is absent. The value of this potential varies with the test method.

Introduction

The Hanford Nuclear Reservation contains radioactive and chemically hazardous wastes arising mostly from weapons production, beginning with World War II and continuing through the Cold War. The wastes are stored in 177 carbon steel underground storage tanks, of which 149 are single-shell tanks (SSTs) and the remaining are double-shell tanks (DSTs). The U.S. Department of Energy, Office of River Protection is responsible for retrieving the tank wastes, treating them in order to encapsulate them in glass logs, and then permanently closing the tanks and associated facilities. The most recent revision of the River Protection Project System Plan proposes, models (computer simulations), and reviews multiple scenarios for transferring the wastes from the SSTs into the DSTs, vitrifying the waste and closing the tanks.1 The simulated scenarios have timelines between 30 and 50 years.1 Such a timeline places a great emphasis on understanding and maintaining the integrity of both types of tanks.

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