Methodological Approaches to Measurement of Carbon-14 for Control of its Radiation Impact on the Personnel and the Public

Purpose: Analysis of the current regulatory and methodological framework on control of doses from intake of 14C for the personnel and the public living in the control area of the nuclear power plant (NPP). Identifying the most informative methods of controlling radiation impact of 14C on a human being. Material and methods: Research literature on radiation impact of naturally occurring 14C; 14C entering the environment as a result of nuclear weapon tests; and 14C entering workplaces and the control area of NPP has been reviewed. Dose coefficients and other radiation characteristics of 14C provided in IAEA, ICRP and UNSCEAR publications have been summarized. Results: According to UNSCEAR, annual radiation burden caused by global 14C is the highest one (about 80 %) among radiation burdens associated with four critical naturally occurring cosmogenic radionuclides: 3H (0.01 µSv/year), 7Be (3.0 µSv/year), 14C (12 µSv/year) and 24Na (0.2µSv/year). The main way of 14C intake is the alimentary one when this isotope enters the human body with food. Dose from this kind of intake of global 14C can reach 40 µSv. The annual dose caused by aerogenic (inhalation) way of intake of global 14C does not exceed 1 µSv. The most informative methods of dose assessment for the personnel of NPP and the public living in the control area involve measurement of content of 14C in top soil, vegetation and food products. Conclusions: Significant amount of 14C enters the environment within the control area during operation of NPP, which causes the public radiation dose exceeding the dose from global 14C. The most informative objects characterizing content of technogenic 14C in the control area of NPP are top soil (humus) and vegetation. The liquid scintillation spectrometry involves sample preparation by burning of samples in oxygen with capturing of generated carbon dioxide and its transfer into organic solvent. This is the most technologically viable method for mass control of 14C content in samples of top soil and vegetation.

Reconciling the Apparent Absence of a Last Glacial Maximum Alpine Glacial Advance, Yukon Territory, Canada, through Cosmogenic Beryllium-10 and Carbon-14 Measurements

We present a new in situ produced cosmogenic beryllium-10 and carbon-14 nuclide chronology from two sets (outer and inner) of alpine glacier moraines from the Grey Hunter massif of southern Yukon Territory, Canada. The chronology potential of moraines deposited by alpine glaciers outside the limits of the Last Glacial Maximum (LGM) ice sheets potentially provide a less-ambiguous archive of mass balance, and hence climate than can be inferred from the extents of ice sheets themselves. Results for both nuclides are inconclusive for the outer moraines, with evidence for pre-LGM deposition (beryllium-10) and Holocene deposition (carbon-14). Beryllium-10 results from the inner moraine are suggestive of canonical LGM deposition, but with relatively high scatter. Conversely, in situ carbon-14 results from the inner moraines are tightly clustered and suggestive of terminal Younger Dryas deposition. We explore plausible scenarios leading to the observed differences between nuclides and find that the most parsimonious explanation for the outer moraines is that of pre-LGM deposition, but many of the sampled boulder surfaces were not exhumed from within the moraine until the Holocene. Our results thus imply that the inner and outer moraines sampled pre- and post-date the canonical LGM and that moraines dating to the LGM are lacking likely due to overriding by the subsequent Late Glacial/earliest Holocene advance.

Depth to water table correction for initial carbon-14 activities in groundwater mean residence time estimation

Carbon-14 (14C) is routinely used to determine mean residence times (MRTs) of groundwater. 14C-based MRT calculations typically assume that the unsaturated zone is in equilibrium with the atmosphere, controlling the input 14C activity. However, multiple studies have shown that unsaturated zone 14C activities are lower than atmospheric values. Despite the availability of unsaturated zone 14C data, no attempt has been made to generalise initial 14C activities with depth to the water table. We utilise measurements of unsaturated zone 14C activities from 13 studies to produce a 14C–depth relationship to estimate initial 14C activities. The technique only requires the depth to the water table at the time of sampling or an estimate of depth to water in the recharge zone to determine the input 14C activity, making it straightforward to apply. Applying this new relationship to two Australian datasets (113 14C measurements in groundwater) shows that MRT estimates were up to 9250 years younger when the 14C–depth correction was applied relative to conventional MRTs. These findings may have important implications for groundwater samples that suggest the mixing of young and old waters and the determination of the relative proportions of young and waters, whereby the estimated fraction of older water may be much younger than previously assumed. Owing to the simplicity of the application of the technique, this approach can be easily incorporated into existing correction schemes to assess the sensitivity of unsaturated zone 14C to MRTs derived from 14C data.