Operational safety analysis of the Olkiluoto encapsulation plant and disposal facility

Radiation doses for workers of the facility, for inhabitants in the environment and for terrestrial ecosystem possibly caused by the encapsulation and disposal facilities to be built at Olkiluoto during its operation were considered in the study. The study covers both the normal operation of the plant and some hypothetical incidents and accidents. Release through the ventilation stack is assumed to be filtered both in normal operation and in hypothetical abnormal fault and accident cases. In addition the results for unfiltered releases are also presented. This research is limited to the deterministic analysis. During about 30 operation years of our four nuclear power plant units there have been found 58 broken fuel pins. Roughly estimating there has been one fuel leakage per year in a facility (includes two units). Based on this and adopting a conservative approach, it is estimated that one fuel pin per year could leak in normal operation during encapsulation process. The release magnitude in incidents and accidents is based on the event chains, which lead to loss of fuel pin tightness followed by a discharge of radionuclides into the handling space and to some degree to the atmosphere through the ventilation stack equipped with redundant filters. The most exposed group of inhabitants is conservatively assumed to live at the distance of 200 meters from the encapsulation and disposal plant and it will receive the largest doses in most dispersion conditions. The dose value to a member of the most exposed group was calculated on the basis of the weather data in such a way that greater dose than obtained here is caused only in 0.5 percent of dispersion conditions. The results obtained indicate that during normal operation the doses to workers remain small and the dose to the member of the most exposed group is less than 0.001 mSv per year. In the case of hypothetical fault and accident releases the offsite doses do not exceed either the limit values set by the safety authority. The highest dose rates to the reference organisms of the terrestrial ecosystem with conservative assumptions from the largest release were estimated to be of the order of 20 μGy/h at the distance of 200 m from the facility. As a chronic exposure this dose rate is expected to bring up detrimental effects to many of the reference organisms, but in all the release cases of this research the exposure duration is short. (orig.)