Tritium dynamics in soils and plants grown under three irrigation regimes at a tritium processing facility in Canada

An abstract of the journal article published in the Journal of Environmental Radioactivity, March 2016.

Authors: Steve Mihok ¹, Monika Wilk ², Anthony Lapp ², Nadereh St-Amant ¹, Nana Kwamena ¹, Ian Clark ²

¹ Canadian Nuclear Safety Commission
² Department of Earth Sciences, University of Ottawa

Abstract

The dynamics of tritium released from nuclear facilities as tritiated water (HTO) have been studied extensively, with results incorporated into regulatory assessment models. These models typically estimate organically bound tritium (OBT) for calculating public dose since OBT itself is rarely measured.

Higher-than-expected OBT/HTO ratios in plants and soils are an emerging issue. To support the improvement of models, an experimental garden was set up in 2012 at the SRB Technologies (SRBT) tritium processing facility in Pembroke, Ontario to characterize the circumstances under which high OBT/HTO ratios may arise. Soils and plants were sampled weekly to coincide with detailed air and stack monitoring. The design included a plot of native grass/soil, contrasted with sod and vegetables grown in barrels with commercial topsoil under natural rain and either low- or high-tritium irrigation water. Air monitoring indicated that the plume was present infrequently at concentrations of up to about 100 Bq/m³ (the garden was not in a major wind sector). Mean air concentrations during the day on workdays (HTO 10.3 Bq/m³, HTO 5.8 Bq/m³) were higher than at other times (0.7–2.6 Bq/m³). Mean tissue-free water tritium (TFWT) in plants and soils and OBT/HTO ratios were only very weakly or not at all correlated with releases on a weekly basis. TFWT was equal in soils and plants and in above- and below-ground parts of vegetables. OBT/HTO ratios in above-ground parts of vegetables were above 1 (1.5–1.8) when the main source of tritium was from high-tritium irrigation water. Ratios were below 1 (0.4–0.6) in below-ground parts of vegetables when irrigated with high-tritium water, and above 1 (1.3–2.8) in vegetables rain-fed or irrigated with low-tritium water. In contrast, OBT/HTO ratios were very high (9.0–13.5) when the source of tritium was mainly from the atmosphere. TFWT varied considerably through time as a result of SRBT’s operations; OBT/HTO ratios showed no clear temporal pattern in above- or below-ground plant parts. Native soil after ∼20 years of operations at SRBT had high initial OBT that persisted through the growing season; little OBT formed in garden plot soil during experiments. High OBT in native soil appeared to be a signature of higher past releases at SRBT. This phenomenon was confirmed in soils obtained at another processing facility in Canada with a similar history.

This study’s insights into variation in OBT/HTO ratios are of regulatory interest, and should be incorporated in assessment models to help design relevant environmental monitoring programs for OBT.

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