Reduction of 14C releases from nuclear power plants

In BWRs the production of 14C contributing to gaseous releases during normal operation comes from neutron activation of 17O. In PWRs, which normally contain ppm amounts of nitrogen in the reactor coolant, also the neutron activation of 14N is important (which gives up to approximately 50 % of the 14C produced in the reactor coolant). 14C formed in fuel and structural material are not expected to be released from LWRs under normal operating conditions. By varying operational parameters, the production and releases of 14C from PWRs can be decreased (i.e. by decreasing the N2-concentration in the make-up water). Production and releases from BWRs are insignificantly affected by parameters like pH and electrochemical potential. Gaseous releases in BWRs are normally as carbon dioxide, and can be cleaned by scrubbing in alkaline media (e.g. in sodium hydroxide solution). In PWRs, a large fraction of the gaseous releases is as hydrocarbons, thus, before being scrubbed in alkali, the hydrocarbons must first be catalytically oxidised to carbon dioxide. Carbon dioxide transforms into carbonate in alkaline solution. By adding Ca2+ to the solution, carbonate will precipitate as low soluble CaCO3(s) or BaCo3(s). This carbonate precipitate solidified in cement gives a waste matrix with a low 14C leach-rate. The cement waste product is well suited for long-term storage in an SFL- or possibly in the SFR-repository. However, the long-term stability of a repository containing 14C must be evaluated for a time period of over 50 000 years, rather than the 500 years calculated for operational waste (e.g. 137Cs, 90Sr) stored in the SFR. 18 refs