Tank "passivated" to keep water "hungry"

To protect the Large Underground Xenon (LUX) dark-matter detector from stray neutrons, the detector will be submerged in a large stainless steel tank full of deionized water, commonly called ?DI water.? DI water has been purged of mineral impurities?such as ions of sodium, calcium and iron?but the danger of contamination remains. ?DI water hungers for ions,? says LUX Operations Manager David Taylor.

To make sure the DI water?s ion hunger remains unsatisfied, last week the inside surfaces of the LUX water tank were ?passivated.? Offending minerals, particularly trace amounts of iron, were removed during three days of precision chemical cleaning by technicians from a California-based contractor, Astro Pak. Sanford Lab Environmental Manager John Scheetz supervised the project.

Astro Pak technician Ralph Hill said the 4850 Level in the Sanford Lab was the most unusual place he has worked. Astro Pak?s regular customers include pharmaceutical companies, aerospace firms, NASA and Proctor and Gamble. (?Soap can get contaminated by bacteria,? Hill said.)

Hill and his colleagues made their own DI water?more than 20,000 gallons of it?to clean the tank. The first step was a simple rinse with hot DI water. The LUX tank is large?it will hold 71,600 gallons?so Astro Pak used five powerful ball-shaped nozzles capable of spraying every square inch of the tank?s interior. The water rinse was followed by an hour-long rinse with a dilute solution of sodium hydroxide. Then that basic solution was neutralized and removed. Next came a four-hour rinse with a solution of citric acid, which also was neutralized and removed. Finally, the tank was rinsed three more times with plain DI water.

During passivation, chemicals were added to the DI water through a pumping and filtration device that Hill called ?the skid.? It also heated the cleaning solutions. The Astro Pak crew tested the water every half hour for mineral content. Discharged water was pumped to the Yates Shaft sump, in accordance with a federal permit.

After the passivation, LUX scientist Dean White of the University of California, Santa Barbara removed the last traces of DI rinse water from the tank, using a squeegee and a shop vacuum. (See the photo above and the safety item below.)

Taylor said the Operations Department and Davis Campus outfitter Ainsworth Benning Construction provided essential support, including equipment delivery and setting up electrical power and a water supply. Taylor also singled out Scheetz for ?outstanding coordination? of the project.