Treatment of ¹⁴C in the Post-Closure Safety Assessments

  pdf NAB 24-07 Rev. 1 Treatment of ¹⁴C in the Post-Closure Safety Assessments(5.59 MB)

Abstract

The potential radiological consequences of ¹⁴C being released from the deep geological repository need to be assessed in support of the post-closure safety case for the general licence application. The present report provides an overview of the ¹⁴C treatment in the post-closure safety assessments. This radionuclide is of importance in post-closure safety assessments because: (i) it can be released both as dissolved and gaseous species in the repository system; (ii) ¹⁴C organics readily migrate in the near-field due to weak interaction with mineral surfaces in alkaline to near-neutral conditions; and (iii) ¹⁴C could be subject to substantial accumulation in the biosphere, if it does not decay in the host rock or overlying strata.

This report includes the description of:

•  the ¹⁴C source term (inventory, speciation and release rate from the individual waste materials);
•  the potential migration of ¹⁴C within the engineered structures of the repository, the host rock and confining geological units, and the deep groundwater systems;
•  and its potential migration and accumulation within the biosphere, including the exposure of potentially exposed groups to ¹⁴C.

The overall structure of the report therefore reflects the potential ¹⁴C release into spatial domains with distinct characteristics and the potential exposure pathways. For each spatial domain (i.e., waste, repository structures, host rock and confining geological units regrouped under the term containment-providing rock zone, deep groundwater systems and biosphere), a clear separation is made between the state of knowledge (including major sources of uncertainty), and the adopted approaches to safety assessment (including treatment of uncertainty).

¹⁴C is present in different types of radioactive waste and in various chemical states. The total inventory at repository closure is ~ 2.4×1014 Bq. A quarter of this inventory (i.e., ~ 24.5%) is present in low- and intermediate-level waste, mainly activated steel, contaminated materials, ion exchange resins and graphite. The majority of the initial ¹⁴C inventory (i.e., ~ 75.5%) is associated with high-level waste, mainly activated Zircaloy, spent fuel pellets and activated steel. While a period of complete containment of high-level waste is assumed to last at least 10,000 years, which corresponds to ~ 2 half-lives of ¹⁴C, no containment is assumed for low- and intermediate-level waste after repository closure. Therefore, in terms of potential releases, ¹⁴C present in low- and intermediate-level waste is more safety-relevant than in high-level waste. The mechanisms of ¹⁴Crelease from these materials and the speciation of the released compounds have been experimentally investigated in detail or, where experimental evidence was lacking, assessed by experts. For the purpose of safety assessment, the ¹⁴C-containing materials are assigned to so-called release classes, for which the expected release is modelled by means of an instant and a congruent release fraction of the respective ¹⁴C content. In addition, the various compounds to be release dare classified either as organic (i.e., reduced chemical form) or inorganic (i.e., oxidised chemical form) ¹⁴C compounds. Once released from the waste forms, the individual ¹⁴C compounds will either dissolve in pore water, migrating through the containment-providing rock zone primarily by means of molecular diffusion, or enter the gas phase and migrate along unsaturated repository structures.

The migration of ¹⁴C is modelled numerically with two complementary modelling approaches: one for the migration of dissolved ¹⁴C-bearing species, and the other for the migration of ¹⁴C-bearing volatiles. The former approach is identical to that for the migration of all other relevant radionuclides and assumes fully saturated and stationary hydraulic conditions, whereas the latter approach explicitly accounts for the generation and migration of gas within the deep geological repository. The release of ¹⁴C to the wider geological environment of the repository system may be both local at the position of underground access structures (shafts and/or ramp) and widespread at the interfaces of the confining geological units with the adjacent major aquifers above and below the deep geological repository. Depending on whether the released ¹⁴C compounds are dissolved or present in a gas phase, they may migrate further along deep aquifers to the respective discharge areas or upwards to the near-surface environment, respectively. The exact transport routes are very specific with respect to the geology, exact waste emplacements, and design of underground access structures. For the general licence application, where such information is not yet available, no credit for potential delay and dispersion of ¹⁴C during transport to the nearsurface environment is taken for post-closure safety assessments.

Upon entering the biosphere (near-surface environment), the ¹⁴C compounds would be subject to various transfer and conversion processes, leading to the accumulation of ¹⁴C in different environmental media, such as the soil, local groundwater or crops. Humans, who use these media as natural resources, would then be exposed to ¹⁴C, mostly by ingestion of contaminated food and water. The modelling approach consists of representing the most important environmental media as compartments, which are linked by transfers of stable carbon within solids, liquids and gases. The calculated equilibrium state is then used as a proxy for the equilibrium state of radioactive ¹⁴C that enters the biosphere over a prolonged period. Potential human exposure is calculated from the ¹⁴C concentrations determined in the different compartments.

Based on recent scientific knowledge from experiments and models, the treatment of ¹⁴C in postclosure safety assessments for the general licence application is more detailed, less conservative in view of the ¹⁴C inventory and source term, and more realistic regarding ¹⁴C migration in the repository near-field and biosphere than previous treatments. Moreover, Nagra’s approach is generally in line with those of other international waste disposal programmes. Differences are related primarily to waste inventories, repository concepts, stages of the disposal programmes and purposes of the respective post-closure safety assessments.