Battelle, Pacific Northwest Laboratories (PNL), researchers have studied corrosion related to nuclear waste solidification processes, particularly corrosion of waste storage canisters. Since most of the PNL canister corrosion work has been conducted in support of the In-Can Melter (ICM)(1) vitrification system, we have assumed the canister goes through the reference ICM process, and is then stored in a water basin for a maximum of ten years.

The three important corrosion effects on waste canisters have been studied in this work:

  • canister oxidation by air during processing,

  • attack from the melt and melt vapors during processing, and

  • corrosion during interim water basin storage.

The most severe corrosion effect observed by us occurs during processing and is the oxidation of stainless steel canisters in contact with air. In the cases observed, the canister atmosphere was air with unrestricted flow to and from the furnace where the processing temperature was about 1075°C; the oxidation rate of 304L SS at 1075°C is about 35 mm/yr. Several techniques to eliminate this corrosion effect are currently being investigated at PNL including: 1) use an inert cover gas or coating to protect the stainless steel canister, and 2) use a material such as Inconel 601 that will not oxidize.

Corrosion due to the waste melt is not as severe as air oxidation. This effect has been studied quite extensively in connection with the development of a metallic crucible melter at PNL. Data are available on the corrosion rates of a number of materials in contact with several waste glass compositions. Long-term compatibility tests between the glass and several metals are being run up to two years at temperatures up to 700°C. In most cases, we have found the corrosion rates due to the melt or its vapor do not pose a serious problem to the waste canister. However, these rates are high enough to preclude the practical use of a metallic melter since operating life is important.

The corrosion effect that takes place during interim water storage of the canister may be a potential problem if proper corrective measures are not taken. The canister may be susceptible to stress corrosion cracking (SCC) because it will be sensitized and stressed to yield. We think the most favorable solution for preventing SCC in stainless steel (304L) involves minimizing canister sensitization and stress and providing good water quality control. We have recommended that the chloride ion be kept at a concentration below 1 ppm and the pH in the range of 9 to 12. At these conditions no 304L failures due to SCC are predicted.(2) We are also evaluating the use of alloys that are relatively immune to cracking, such as nickel base alloys with >45% nickel.

We have concluded that corrosion of a canister used during the ICM process and then stored in a water basin can be controlled. Future PNL work will concentrate on minimizing the corrosion resulting from these three mechanisms. The methods of controlling these effects will be selected on the basis of economics, operating convenience, and effects on other process parameters.

Subject
Water, Materials, Corrosion effects, Composition, Corrosion rate, Glass, Nickel based alloys, Vapors, Sensitization, Wastes, Stress corrosion cracking, Oxidation, Chloride stress corrosion cracking
© 1977 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1977
Association for Materials Protection and Performance (AMPP)
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