From 2002 interview with Philip Morrison:

There was a spill from the P9, the code word for heavy water, on the floor, so of course they rushed out with nice clean rags to try to sop it up. It was worth a dollar a gram or something. And they squeezed it onto a bucket, whereupon Zen sits down on a chair, takes off his shoes and dabbles his feet in the heavy water saying, “I believe I’m the first person in the world to wash my feet in heavy water,” carefully dried it out, put the towels into the bucket, and went away.