A cleanup of nuclear waste from the cold war era threatens to eliminate the supply of an obscure isotope that shows great promise in cancer treatments, according to a report by the Energy Department’s inspector general.

But the department has concluded that the material, uranium 233, which does not exist in nature, is too expensive and risky to keep. The dispute is a rare instance where environmental cleanup and human health are in direct opposition.

The isotopes have a convoluted history. Uranium 233 very slowly breaks down into thorium 229, half of it making the conversion over 159,000 years. There is a significant amount of thorium 229 only because the Energy Department has had tons of uranium sitting around for decades.

The thorium is not medically useful, but its half-life, the time for half to convert, is 7,340 years, and it decays into a radium isotope, and then into actinium, which has a 10-day half-life. (Short half-lives are desirable in this kind of cancer treatment, because such materials deliver their dose promptly.) One of the “daughters” of actinium is bismuth, which has a half-life of only 45.6 minutes. Researchers are testing both the actinium and the bismuth as therapeutic drugs.