Activity inventory of the biological shields of the Finnish nuclear reactors

As part of the decommissioning studies for 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 wellknown ORIGEN-S program using its original LWR data libraries. 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. In the Loviisa bioshields H-3, Fe-55, Co-60 and Ni-63 are the most important radionuclides at short cooling times. 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
bioshield concrete of the TVO reactors H-3 and Ar-39 are very dominant and there also Eu152 is of relatively great importance. In the latter case the activation of the reinforcing bars must be taken into account, too. Of the nuclides mentioned above only Co-60 and Eu-152 are gamma-ray emitters. The calculations reported in this article were preliminary and approximate. There are many possibilities to improve the accuracy and reliability of activity inventory estimations, for instance the more precise definition material compositions and
geometry, the use of more sophisticated computer codes and updated data libraries for activation flux calculations and the generation case-dependent activation cross section sets for final activity inventory calculations.