Trends in near-surface environmental tritium concentrations at chalk river site following shut down of the NRU reactor

Abstract

Recovery phase following the shutdown of the National Research Universal (NRU) nuclear reactor at Canadian Nuclear Laboratories' (CNL's) Chalk River site is studied here. In 2009, NRU was shut down for a period longer than a year during reactor vessel repair and then was shut down permanently in 2018. In both cases, tritium was sampled in the atmosphere, precipitation, vegetation, and soil. Tissue free water tritium (TFWT) and organically bound tritium (OBT) were collected in plants and in tree leaves at 1 m and 2 m above the ground. Two transects connecting dry uphill locations and neighbouring swamps downhill were sampled. These study transects are located away from the reactor site and waste management areas, and as such, are expected to represent pure recovery due to dilution of local tritium in the regional geophysical compartments of atmospheric water vapour and near-surface groundwater (aided by incoming precipitation). Trends in environmental tritium at CNL following the NRU reactor shutdown were assessed and the environmental half-life of tritium was derived for near-surface strata (atmosphere, biosphere/plants, and rhizosphere/groundwater). It appears that in 4 years, pure local (pure recovery-phase) dilution of historical tritium has reached the level at which it is now tangibly affected by remote atmospheric tritium plumes coming from CNL waste management areas with southeast (SE) winds. Exclusion of episodes associated with SE wind and with remote tritium plume results in more coherent trends across sampled near-surface geophysical strata. Trend-line exponents provide T_1/2 = 1.30 ± 0.24 years of environmental half-life of tritium. The role of microclimate may require further investigation, but insignificant differences between wet and dry locations observed in TFWT already imply applicability of the environmental half-life of tritium to other sites in recovery.

Features of the site-specific recovery process at CNL include insensitivity of OBT and groundwater HTO trends (half-life exponents) to exclusion of samples collected under the remote tritium plume, whereas atmospheric HTO and especially precipitation HTO trends (exponents) appear affected by as much as 49 % and 267 %, respectively. The sensitivity of trend for TFWT (environmental half-life exponent) was less tangible, because of only moderate 35 % decrease in exponent after exclusion of remote plume episodes. In summary, in 4 years after the NRU reactor shutdown, the drivers of tritium transfer (atmospheric HTO and precipitation HTO) are already under substantial influence of the tritium plume from remote sources at CNL, but recipient compartments of environmental tritium are either not, or are only slightly affected by the remote tritium plume. This implies recovery phase completion in the CNL atmosphere (and in precipitation washing tritium out of it), but yet unfinished recovery of recipient compartments of OBT and groundwater HTO where quasi-stationary phase has not yet started.