At the Savannah River Plant, stress corrosion of carbon steel storage tanks containing alkaline nitrate radioactive waste is controlled by specification of limits on waste composition and temperatu...

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Control of Stress Corrosion Cracking in Storage Tanks Containing Radioactive Waste.

The stress corrosion cracking (SCC) of carbon steel in nitrate-containing environments has been known for a long time and has become important in managing the integrity of aging underground radioac...

Stress Corrosion Cracking and Localized Corrosion of Carbon Steel in Nitrate Solutions

Preparation of high level waste for vitrification involves in part the dissolution of salt cake from the carbon steel storage tanks. The salt crystals composing this cake are high in nitrate concentration with the interstitial liquid being high in hydroxide and nitrite concentration. During the salt dissolution process, a stage is reached in which the inhibitors, hydroxide and nitrite, are insufficient to prevent nitrate stress corrosion cracking (SCC) and fall outside the requirements of the corrosion control program. Additional inhibitors, which are necessary to meet the requirements, may be counterproductive to the efficiency of the process and waste minimization. Corrosion testing was initiated to better characterize the necessary inhibitor concentration for high nitrate waste during salt dissolution processing. A four-phase test program is being conducted: (1) electrochemical characterization, (2) accelerated or polarized U-bend testing, (3) long-term (non-polarized) U-bend testing and (4) vapor space U-bend tests. Electrochemical testing, which included cyclic potentiodynamic polarization (CPP), linear polarization resistance (LPR) and open-circuit potential (OCP) measurements, was performed to identify stress corrosion cracking susceptibility, to characterize pitting resistance and to determine the general corrosion rate. Polarized U-bend tests were utilized to assess the effect of minimum inhibitor concentrations and heat treatment on SCC and to determine test parameters for future long-term U-bend testing. Results from CPP, LPR and OCP tests demonstrated that carbon steel formed a protective oxide film and the potential became electropositive during exposure to the waste at all inhibitor concentrations. The tenacity of this film improved as the inhibitor concentration level was increased and the temperature was decreased. This passive film increased the resistance to localized corrosion significantly. Therefore if any of these inhibitor levels are selected for storage of dissolved salt solutions, no changes to the service life estimates that were based on general corrosion are necessary. The breakdown potential for SCC as well as the other electrochemical parameters were independent of nitrate concentration (4.5-8.5 M). The breakdown potential, however, was strongly affected by temperature (i.e., 25 and 50 C) and inhibitor concentration. These results indicate that for this nitrate concentration range a critical inhibitor level is necessary for minimizing the occurrence of SCC. The polarized U-bend tests were in good agreement with the electrochemical tests. The U-bend testing clearly demonstrated that the heat treating of the samples clearly improved the SCC resistance of A537 carbon steel even at the low inhibitor concentration (0.01 M hydroxide and 0.01 M nitrite). This concentration was insufficient to prevent cracking for any tested nitrate concentration (4.5-8.5 M). At a 7 M nitrate concentration, SCC was prevented for inhibitor concentrations as low as 0.3 M hydroxide and 0.1 M nitrite. The current inhibitor requirements for a waste containing 7 M nitrate are 0.6 M hydroxide and 1.1 M hydroxide and nitrite. Thus, a considerable reduction in the amount of inhibitor necessary may be attained. It will also be recommended that the temperature of the dissolved salt solution be maintained below 50 C.

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Determination of Corrosion Inhibitor Criteria for Type III/IIIA Tanks During Salt Dissolution Operations

The Savannah River Site produced weapons-grade materials for nearly 35 years between 1953 and 1988. The legacy of this production is nearly 37 million gallons of radioactive waste. Since the 1950s, the liquid waste has been stored in large, underground carbon steel waste tanks. During the past 20 years, the site has begun to process the waste so that it may be stored in vitrified and grout forms, which are more suitable for long-term storage. Over the history of the site, some tanks have experienced leakage of the waste to the secondary containment. This article is a review of the instances of leakage and corrosion degradation that the tanks and associated equipment have experienced since the first tanks were built. Furthermore, the activities that the site has taken to mitigate the degradation and manage the service life of the tank for its anticipated lifetime are reviewed.

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The Performance of Underground Radioactive Waste Storage Tanks at the Savannah River Site: A 60-Year Historical Perspective

Life estimation of high level waste tank steel for h-tank farm closure performance assessment

Radioactive liquid wastes are produced as a consequence of processing fuel from Savannah River Plant (SRP) production reactors. These wastes are stored in mild steel waste tanks, some of which have developed cracks from stress corrosion. A laboratory test was developed to determine the relative agressiveness of the wastes for stress corrosion cracking of mild steel. Tensile samples were strained to fracture in synthetic waste solutions in an electrochemical cell with the sample as the anode. Crack initiation is expected if total elongation of the steel in the test is less than its uniform elongation in air. Cracking would be anticipated in a plant waste tank if solution conditions were equivalent to test conditions that cause a total elongation that is less than uniform elongation. The electrochemical tensile tests showed that the supernates in salt receiver tanks at SRP have the least aggressive compositions, and wastes newly generated during fuel repocessing have the most aggressive ones. Test data also verified that ASTM A 516-70 steel used in the fabrication of the later design waste tanks is less susceptible to cracking than the ASTM A 285-B steel used in earlier designs.

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R.S. Ondrejcin

Papers

Prediction of stress corrosion of carbon steel by nuclear process liquid wastes