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Conference Article: Unloading of the reactor core and spent fuel management of THTR 300

Abstract

Following granting of License 7/12a on October 22, 1993 and preparatory work, unloading of the THTR pebble bed reactor core was initiated on December 7, 1993. Achieving the state 'plant free of nuclear material' was one prerequisite for implementation of further preparatory activities to establish safe enclosure. To reach this target, it was necessary to remove approx. 670,000 operating elements (approx. 84% of which were fuel elements). Basically, unloading of the core was implemented in the same way as removal of the operating elements during duty operation, however, process engineering modifications to the charging system were required due to replacement of the primary gas helium with nitrogen and air and reduced temperature and pressure as compared to duty operation. During unloading operation, the operating elements were sorted by means of the burn-up measuring system and were transferred into operating element containers (steel cans), 2,100 elements per container. Insertion of absorber rods and addition of unirradiated absorber elements ensured clearly subcritical conditions at any moment during unloading of the core, which was confirmed by the measured values of neutron flux density. The residual inventory of fissile material remaining in the reactor pressure vessel after completion of core unloading activities by December 1994 is 0.976 kg and is thus significantly lower than the required value of 2.5 kg. Due to the limited storage capacities of the plant, it was necessary to ship the fuel element containers simultaneously with core unloading, In a remote-controlled process, the fuel element containers were transferred from the spent fuel store to a shielded loading station, loaded into one transport and storage cask of the CASTOR THTR/AVR-type each, which was then sealed with the primary lid. Following leak testing and definitive sealing by staff working on a working platform outside of the loading station, the transport and storage casks were transferred to six-axle purpose-designed railway wagons and shipped to the Ahaus fuel element interim storage facility (BZA). By April 1995, a total number of approx. 620,000 fuel elements had ben transported from THTR to BZA in 57 shipments, on general 6 transport and storage casks on 2 railway wagons per shipment. Due to actual burn-up of the THTR fuel elements falling below the design values (mean Burn-up per fuel element container max. 85,000 MWd/t HM) and the long cooling-down period, dose rates on the casks were very low. Neutron dose rate measurements taken on a loaded transport and storage cask showed results of < 1 muSv/h at the cask surface. After loading the cask on the transport wagon a gamma dose rate of 1-2 muSv/h at the closed transport hood and of 0.5 muSv/h in a distance of 2 m from the transport wagon was measured. The dose load received by the personnel was very low during the complete cask handling. The evaluation of the official dosemeters did not either show any of relevant exposure the employees (0.0 mSv/month effective dose).

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Reference:

Technical committee meeting on technologies for gas cooled reactor decommissioning, fuel storage and waste disposal. Juelich (Germany) 8-10 Sep 1997

International Atomic Energy Agency, Vienna (Austria)

IAEA-TECDOC--1043, pp:143-150