Activity incorporation into stainless steel samples in primary coolant at Loviisa 1 during the fuel cycle 2000-2001

Increased activity incorporation in primary circuit oxides increases the risks of radiation exposure to the maintenance personnel. In order to find out the factors contributing to activity incorporation and to be able to control them, stainless steel samples with different pre-treatments are exposed to the primary coolant at the cold-leg temperature at Loviisa unit 1 since February 2000. Since then, samples have been regularly removed and replaced with new samples. The activity of the removed samples has been determined, and the oxide films on the samples have been analysed in order to find correlations between the properties of the oxide films and activity incorporation. SEM, SIMS and GDOES techniques have been used for the analysis. An additional task of the project is to find the most suitable surface analysis method for oxide films formed in VVER coolant conditions. In this report the analysis results are given together with the HTWC monitoring data obtained at the plant during the fuel cycle 2000-2001. High-temperature potential and conductivity monitoring data are in agreement with the coolant chemistry during the fuel cycle. They indicate no deviations from normal plant operation. The main contribution to the activity levels in the studied samples is due to the 124Sb and 110mAg isotopes. In addition to these two isotopes, also the activity of 60Co is of outmost importance, because of the long half life. Incorporation of 60Co, 58Co, 54Mn and 110mAg does not seem to depend on the pre-treatment of the sample. On the other hand, incorporation of 124Sb is much more pronounced into the ground samples than into the pre-oxidised samples. For 60Co, the activity increases mainly during the period including the outage, the subsequent start-up and the first five months of operation. The most significant variations in the rate of activity increase during the whole period are observed for 124Sb. SEM micrographs from the top of the samples indicate that pre-treatment has an impact on the appearance of the oxide films. The oxide films on the surfaces of ground samples exhibit a larger number of small crystals than those on the surfaces of the pre-oxidised samples. On pre-oxidised samples also some fairly large crystals exist. The results indicate that the activity levels do not directly correlate with the thickness of the film. On the other hand, the composition and distribution of alloying elements seem to have a significant impact on the activity incorporation. The enrichment of Co seems to be correlated with the presence of Cr and probably also Ni in the films. In order to obtain more information of the possible influence of the start-up period and of other stages involving significant changes in the chemistry of the coolant, sample sets have to be removed at more frequent intervals in the future. In addition, data from several fuel cycles are needed to establish reliable correlations between the trends in activity incorporation and the history of the plant over the years. Such correlations will facilitate the prediction of possible increase in activity levels on the basis of analysis data from the material sample cell. (orig.)