FISSION ISOTOPES FOR IDENTIFICATION OF NUCLEAR WASTE SOURCES

Radioactive fission products are the major risk drivers at nuclear waste sites. Other radioactive and stable fission isotopes, of less concern from a risk perspective (e.g. molybdenum, ruthenium, palladium, xenon), can help identify source areas and distinguish contributions from different potential sources. Differing reactor conditions change the relative contributions of uranium and plutonium fission, leading to distinct fission product isotopic ratios.

Technetium-99 is an example of a long-lived isotope (213,000 year half-life) that has a high environmental mobility under oxidizing conditions. The high mobility in groundwater and long half life make it a major constituent of concern for risk assessment and site remediation decisions. However, fission technetium is essentially mono-isotopic so isotopic analogs must be used in source attribution. Ruthenium is an attractive candidate because it is chemically similar to technetium and has a low natural background. Natural abundance, uranium fission, and plutonium fission produce distinctive stable ruthenium isotopic signatures. Thus ruthenium isotope data provide additional constraints on possible technetium-99 sources.

Inductively-coupled plasma mass spectrometry (ICP-MS) provides sensitive measurements of ruthenium and other isotopes of interest. For isotopic ratio work, particular attention must be paid to the evaluation and removal of atomic and polyatomic interferences. Work is currently underway to develop separation and preconcentration techniques/methods that allow for the precise and quantitative analysis of Ruthenium-101, -102, and -104 ratios via ICP-MS. These results will be used to constrain the source term(s) responsible for technetium-99 groundwater contamination at Waste Management Area-T, Hanford Site, Washington.