Caustic Stress Corrosion Cracking Analysis
Summary & Discussion | Wetted Parts | Pools/Tanks with Alkali Metal | Notes
Practical Experience of Creative Engineers
The principals of CEI have been practicing the superheated-steam (SHS) method of cleaning piping, drums, tanks, distillation columns, various traps and other equipment for more than 20 years. In many cases, the cleaned vessel or systems have been returned to service and continued to serve their intended purposes for many years.
Some Examples of CEI’s processing with SHS are:
- Stainless steel drums used by the maker of NaK for 50 years have been repeatedly cleaned of both residual and bulk quantities of NaK for many years and always returned to service.
- Stainless-steel potassium production columns have been repeatedly steam cleaned and then operated at temperatures as high as 850°C under stress levels that have caused permanent deformation at that temperature, yet there has been no discovery of cracking attributable to caustic stress corrosion.
- Over the years, many vessels of type 304 and 316 SS had been sent to Callery Chemical Co. from various U.S. Department of Energy (DOE) sites for disposal. The vessels consisted of sump tanks, inert-gas purifiers, feed tanks, heat exchangers, and other process vessels. They contained varying quantities of sodium, potassium, and NaK from residues to full loads. These vessels were all cleaned using the SHS process and, in many cases, were put back into service at Callery’s facility. None of the vessels leaked during or after processing.
- In 2001, CEI reacted four batches of NaK in a 316 SS tank in the span of one week. Reaction times were approximately eight hours per batch at up to 732°C pool temperature. No evidence of cracking was discovered, and the tank was pressure-tested before each batch, and pressure was used to drain the final rinse.
- At Fermi 1, a 304 SS vessel is being used to react alkali metals contained in piping and other small components. The process is the same SHS process CEI has used elsewhere. CEI participates in this processing and assisted in the initial design, safety reviews, and startup. To date, the vessel has seen over 200 cycles with temperatures routinely reaching 600°C. Part of the processing procedure calls for a pressure test before every batch.
- Fermi also had two large, cold traps that were full of sodium and sodium oxides. The vessels were processed with the SHS process and kept hot for an extended period of time.
- In January 2004, CEI used the SHS process to clean the CRCTA vessel at DOE’s Hanford site. This vessel is a mock-up of the FFTF nuclear reactor. The CRCTA vessel had approximately 90 gallons of sodium. No indications of cracking were detected, and pressure was used after processing to remove the caustic rinse waters.
This practical experience (1) gives CEI the confidence to implement the SHS process on equipment without fear of catastrophic consequences and (2) indicates that caustic stress corrosion cracking is not a significant risk.