Role of tritiated hydrogen in formation of HTO and OBT in soil and vegetation in vicinity of a tritium processing facility

Abstract of the technical paper/presentation presented at:
4th International Conference on Radioecology and Environmental Radioactivity (ICRER 2017)
September 3–8, 2017

Prepared by:
M. Ilin and M. Rinker
Canadian Nuclear Safety Commission

Abstract

Tritium is released from nuclear facilities in two chemical forms: as tritiated hydrogen (HT) and as tritiated water (HTO). Most tritium released from nuclear reactors in Canada is in the form of HTO, whereas tritium processing facilities (TPFs) emit a larger fraction of HT to the environment. Although HT is up to 25,000 times less harmful than HTO if ingested, it is transformed to HTO in various reservoirs such as air, soil, plants and animals. The release of tritium can also result in elevated concentrations of another important form of this radionuclide as organically bound tritium (OBT), which is strongly incorporated into organic compounds such as sugars, proteins, starches, lipids, cell structural materials and amino acids. Therefore, OBT has significantly longer retention time in biological organisms and systems than HTO. This paper provides results of field research initiated and funded by the Canadian Nuclear Safety Commission in 2008–09 in order to address the potential for accumulation of tritium in human foods or in the environment from routine atmospheric releases of this radionuclide from nuclear facilities.

Interestingly, soils and vegetation samples collected near a TPF in 2009 generally demonstrated a systematic increase in HTO and OBT levels as well as in OBT/HTO ratios relative to the previous year. This systematic increase was statistically significant and unique among all nuclear sites. In order to understand this, the relevant TPF emission data were analyzed. It was found that the 2009 sampling campaign was preceded by a 4-week period of high HT emissions compared to the previous year, whereas a considerable reduction of HTO emissions was observed during a 12-week period before the sampling in 2009. Therefore, the elevated formation of HTO and OBT in both soil and vegetation in 2009 was strongly linked to atmospheric releases of tritiated hydrogen from the TPF. While the dose consequences of tritium releases from the TPF were well below regulatory limits, these results suggest that the role of tritiated hydrogen in formation of HTO and OBT in soil and vegetation, its mechanisms and rates need to be further clarified in the field and/or laboratory studies (e.g., using direct measurements of HT and HTO concentrations in the air at the soil and vegetation sampling locations). This would help inform continuous improvements regarding regulatory dose assessments and environmental monitoring programs for nuclear facilities that routinely emit tritium into the surrounding environment.

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