Activity inventory of the biological shields of the Finnish nuclear reactors

As part of the decommissioning studies of the four Finnish nuclear reactors (two 465 MWe VVER-440 type PWRs at Loviisa and two 735 MWe BWRs at Olkiluoto) the activation products inventories of the biological shields of the reactors have been estimated. First, material data were updated and improved by analyzing samples taken from the bioshields. Then the radial irradiation flux distribution was calculated with the one-dimensional ANISN transport code using cross sections taken from the BUGLE-80 data library. Finally, the activation and the subsequent cooling of the bioshield materials were calculated with the well-known ORIGEN-S program using its original LWR data libraries. The main result was the total radioactive inventory and its nuclide-wise distribution as a function of the cooling time at different radial locations of the bioshields. Due to the different geometries (shielding) the radioactive inventory at the inner surface of the biological shield of a Loviisa unit will be about two orders of magnitude greater than that of an Olkiluoto (TVO) reactor. To some extent the nuclide-wise distributions are different, too. In the case of the Loviisa bioshields H-3, Fe-55, Co-60 and Ni-63 are the most important radionuclides from the point of a prompt dismantling, which at present is the preferred option for the Loviisa reactors. In the long-term the activity will be dominated by Ca-41 and to a smaller extent C-14, Cl-36, Ar-39 (if it remains in the concrete) and Ni-59. In the bioshields of the TVO reactors H-3 and Ar-39 are very dominant and there also Eu-152 is of relatively great importance. Of the nuclides mentioned above only Co-60 and Eu-152 are gamma-ray emitters. (author)