Reply to ASK-AN-EARTH-SCIENTIST

Subject: Radioactive waste

I have read that radioactive waste can be "treated" prior to disposal to reduce its lethality or half-life. What is this treatment process, how much does it reduce the lethality or half-life of the waste in question and is it "theoretically" possible to completely neutralize radioactive isotopes so they are harmless right from a reactor?

Yours is an interesting and complex question. I can give you only a rudimentary answer, since disposal of nuclear waste is not my area of expertise.
There are two components of toxicity to humans and other organisms in radioactive materials or waste:

the radiation it emits
the parent or daughter elements, which themselves can be toxic for "chemical" reasons.

Radiation emited is a function of the amount of a radioactive isotope present, its half-life and the type of radioactive particle(s) it emits. It is typically more toxic at high concentrations (i.e., 1 gram of pure Tritium is considerably more toxic than 1 gram of Tritium dispersed in 1,000,000 liters of water). Thus, one "treatment" for nuclear waste put back into the environment is simple dilution. This is not a common technique for high-level waste. The toxicity of a radioactive isotope is very dependent on the type of particle it emits and the rate at which it emits it (these are decay pathway and half-life). These important aspects of radiation emited from various radioactive materials can not be changed; they are inherrent to the nucleus that is decaying and can not be "treated away". However, for many very short-lived isotopes, an effective "treatment" is to simply store the material (safely) for as many half-lives as it takes for the waste to be at tolerable remnant radiation levels. This is typically done for isotopes whose half-lives are considerably less than say a month or so.
The chemical form of a radiactive element is also important, in that this determines how readily it can interact with environments and the organisms they contain (and therefore, its overall toxicity). If material can be somehow rendered chemically "inert", then it will not distribute itself throughout an organism or environment in which it is placed, minimizing (but not eliminating) its toxicity (it will continue emmiting radiation from its chemically inert surface, it just can't distribute itself as easily). Some isotopes in waste material can be made less toxic by incoporating them into ceramic-type materials that are very stable in some environments found on earth. It is believed that these hybrid materials are so stable that by the time they are broken down by the effects of various geological processes, the radioactive material they contained will be essentially gone.
So, is it "theoretically" possible to completely neutralize radioactive isotopes so they are harmless right from a reactor?" NO. But it is possible to minimize their toxicity using a combination of the techniques listed above. Unfortunately, reactors usually produce a complex array of radioactive nuclides and what treatments work for some may not work for others in the mixture. This means the material will require potentially lenghty and expensive chemical purification steps. Because present policy in the US and around the world does not require in most cases the best-possible reduction of toxicity, particularly in cases where it would be expensive, the industries creating and/or using these materials typically take more minimal aproaches to waste containment and treatment.

Dr. Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822