Interactions of Carbon in a Repository Environment

Release and migration of ¹⁴C cannot be treated independently from the bulk of inorganic and organic carbon waste components in a repository. ¹⁴C is mainly produced by (n,p) reaction from ¹⁴N. Important sources of ¹⁴C is zircalloy (according ASTM standard Nmax: 65 ppm), the fuel (N contamination and ternary fission) as well as some other reactor components. In Germany, spent fuel elements from High Temperature Reactors (HTR) will contribute also to the total ¹⁴C load of a radioactive waste disposal. Additionally, organic materials containing ¹⁴C markers, metals containing ¹⁴C as carbides and other low level waste forms containing ¹⁴C in carbonates or in ashes have to be taken into account.
¹⁴C in graphite from spent HTR fuel elements is nearly immobile. Other waste forms may release ¹⁴C in a large variety of inorganic and organic species. Reactions of carbides with water/brine produce methane or acetylene gas. By microbial reactions, organic ¹⁴C can be transformed into gases ¹⁴CO₂ and ¹⁴CH₄.
Interactions of HTR fuel with brines were investigated at FZJ. At FZK-INE and FZR, various experimental and theoretical investigations have been performed in order to quantify the release of this isotope.
Limiting the concentrations of (bi-)carbonates in solution results in a low actinide solubility, especially at high pH. For this reason, at WIPP, a Mg bearing backfill material (periclase, MgO) is added to the wastes in order to keep the concentration of HCO₃-/CO₃ ^2- at a minimum. For the Asse salt mine, a backfill material was proposed by FZK-INE. This material consists mainly of brucite (Mg(OH)₂(s)) and sorel phases (Mg_2x(OH)_(2x-1)Cl·₄H₂O(s)). Various experimental and modelling studies were carried out to determine the stability of the Mg(OH)_2(s)-based material in the relevant brines, formation of the reaction products, and the effect of the material on actinide solubility in the case of carbonate-input to the brines. Experimental findings are in agreement with the modeling predictions. By precipitating carbonates from solution, an isotopic exchange effect will occur, which will control the ¹⁴C concentration in solution.
For the ERAM system, sorption of ¹⁴CO₃ ^2- onto salt concrete, sorel concrete and other backfill materials was investigated. For all materials under investigation, sorption showed distinct time dependences. The weakest sorption was observed in case of salt concrete. Sorel phases reduced the ¹⁴C concentration by more than 2 orders of magnitude during a 0.5 yr. period. Since the studied backfill materials contained to some extent solid carbonates, the retention process can be explained by isotopic exchange.
Sorption of methane (CH₄) onto the backfill materials was investigated by contacting a brine/backfill slurry with the gas. The tests were performed by monitoring the concentration of gaseous CH₄ in a mixture with a noble gas (Ar) which does not react with slurry components. The analysis of experimental results shows that the ratio of CH₄/Ar remains constant within a deviation of 5%. Consequently, the total methane sorption onto the studied backfill materials remains below 0.2 mmol CH₄/kg solid material.