V. Ragaisis

Impact of Spatial Heterogeneity of Source Term in Near-Surface Repository on Releases to Groundwater Pathway

Safety assessment of a near-surface repository of radioactive waste usually assumes a homogeneous distribution of activity within the vaults of the repository. However, in some cases there is radioactive waste, e.g., disused sealed sources, which should result in so-called “hot spots” when disposed of with other radioactive waste. An uneven distribution of waste activity is obtained in that case. The impact of heterogeneities in source distribution on radionuclide releases from a near-surface repository to the groundwater is analyzed in the paper. The conditions that stand for the application of homogeneous distribution of radioactive waste are revealed. The assessment has been performed using the methodology of the Improving Long Term Safety Assessment Methodologies for Near Surface Radioactive Waste Disposal Facilities (ISAM) study recommended by the International Atomic Energy Agency for the safety analysis of the near-surface repository. A description of the ISAM methodology and its application for the analysis of heterogeneity including a brief description of the disposal system of radioactive waste, the scenarios for the radionuclide migration, and the developed conceptual models are presented in the paper. The calculations have been carried out using the DUST and GWSCREEN computer codes, designed to simulate radionuclide transport. The modeling results for the case of homogeneous distribution of radioactive waste in the repository are considered. The impact of heterogeneities has been analyzed by comparing the modeling results of radionuclide transport for the heterogeneous case of waste distribution to the homogeneous case.

Environmental safety aspects of the new spent nuclear fuel management and storage system at Ignalina NPP

In the framework of the preparation for the decommissioning of the Ignalina Nuclear Power Plant (INPP) a new Interim Spent Nuclear Fuel Storage Facility (ISFSF) will be built in the existing sanitary protection zone (SPZ) of INPP. In addition to the ISFSF, the new spent nuclear fuel management activity will include all necessary spent nuclear fuel retrieval and packaging operations at the Reactor Units, transfer of storage casks to the ISFSF, and other activities appropriate to the chosen design solution and required for the safe removal of the existing spent nuclear fuel from storage pools and insertion into the new ISFSF. The Republic of Lithuania regulations require that the average annual dose to the critical group members of population due to operation of nuclear facility shall not exceed dose constraint. If several nuclear facilities are located in the same SPZ, the same dose constraint shall envelope radiological impacts from all operating and planned nuclear facilities. The paper discusses radiological safety assessment aspects as relevant for the new nuclear activity to be implemented in the SPZ of INPP considering specificity of Lithuanian regulatory requirements. The safety assessment methodology aspects, results and conclusions as concern public exposure are outlined and discussed.Copyright

Modelling of RBMK-1500 SNF storage casks activation during very long term storage

Existing interim spent nuclear fuel storage facility (SNFSF) at Ignalina nuclear power plant in Lithuania is fully loaded with CASTOR(®)RBMK-1500 and CONSTOR(®)RBMK-1500 storage casks. The planned lifetime of these casks is 50 years and the first loaded cask was moved to the SNFSF in 1999. The start of operation of disposal facility in Lithuania is foreseen later than the planned interim storage ends. So, the possibilities to extend the storage period over 50 years should be considered. Therefore, the casks decommissioning issues should be taken into account, as due to prolonged neutron irradiation casks materials could became activated. This paper presents modelling results of storage casks neutron activation during 300 year storage period. Modelling results show, that after 50 years of storage, side-wall and bottom of CASTOR(®)RBMK-1500 cask are activated above clearance criteria. However, for 100-300 year storage period all of the casks components could be free released.

Experience From Safety Assessment of the Existing Storage Facility for Solid Radwaste at Ignalina NPP

There is only one nuclear power plant in Lithuania — Ignalina NPP (INPP). The INPP operates two similar units with present power level of about 1250 MW(e) each. They were commissioned (first grid connection) in 1983 and 1987 respectively and provide approximately 70–80 per cent of the electricity produced in Lithuania. From the beginning of INPP operation all power plant generated solid radwaste is collected and stored in the solid radwaste facility that is located in INPP sanitary protected area. The storage facility for solid radwaste at INPP consists of four above ground buildings, namely buildings No. 155, No. 155/1, No. 157 and No. 157/1. This facility is Soviet type facility designed for the interim storage of low and intermediate level radwaste arising as a consequence of operation of NPP. The institutional waste from industry, hospitals etc., including sealed sources as well, has been stored in this facility from 1990. At present this radioactive waste storage facility is still not licensed but SAR is prepared and licensing process is going on. The paper provides information on INPP solid radwaste storage facility and discusses problems met during the safety assessment of this facility: lack of the information on the inventory data, lack of design data for the buildings etc. Modeling results of the structural analysis, radiological data, shielding analysis and proposals to improve the safety of the facility also are discussed.Copyright