Chlorine-36 (Cl-36) is a key radionuclide in the long-term management of irradiated nuclear graphite. It is generated predominantly through neutron activation of Cl-35 impurities in reactor core graphite and is significant because of its long half-life, environmental mobility and bioavailability. Reliable prediction and measurement of Cl-36 inventories are therefore important to the safety assessment of graphite treatment, storage and disposal options.

Radiochemical analysis of samples collected during Magnox reactor and primary circuit characterisation provides evidence of the release, transport and re-deposition of Cl-36. Analysed levels of Cl-36 in Trawsfynydd reactor 1 core graphite are an order of magnitude lower than those predicted by activation modelling, while Cl-36 is also observed throughout the primary circuits and boilers for reactors at Dungeness A, Hunterston A and Wylfa. These observations align with international studies on the thermally-driven release of chlorine from irradiated graphite, resulting in partial loss and redistribution of the Cl-36 inventory during reactor operation.

Furthermore, there is evidence of at least two mechanisms for the deposition of Cl-36 across reactor primary circuits associated with different chemical speciation. Some Cl-36 is intrinsic to graphitic dust carried within the stream of coolant gas, but it also appears to be present in a different chemical form whose deposits are inversely correlated with temperature gradient across reactor boilers.

Different approaches to Cl-36 determination in graphite are also described, with commentary.