Radiological safety aspects: draining of heavy water during preparatory phase of proposed decommissioning of Cirus

CIRUS reactor, the nucleus of nuclear programme of India was a 40 MWth research reactor, commissioned in July 1960. After five decades of valuable service to the nation, reactor was permanently shut down on 31st December 2010. The reactor used natural uranium as fuel and was cooled by demineralized water in a closed loop. The Heavy water (D2O) was used as moderator and helium gas was used as cover gas in the reactor vessel. Moderator system removed about 5% heat of the reactor power. This heat in turn was removed by sea water in heat exchangers. System tritium activity before permanent shutdown of Cirus reactor was 5.0 Ci/l. Tritium is the main radioactive isotope present in the moderator system which can cause internal hazards. During preparatory phase of decommissioning activities heavy water was drained from reactor vessel, heavy water heat exchangers, ion exchangers, different loops and instruments. Drained heavy water was transferred to storage tank no. 1 for interim storage. On the spot cold finger sampling was done to monitor tritium activity in air during and after each job related to handling of heavy water. Maximum tritium activity measured in air was 213 DAC. Internal dose due to tritium for all the jobs was 2.65 pmSv. This paper gives details about the radiological safe procedures followed, monitoring of tritium activity in air and internal dose assessment due to tritium during draining of heavy water from reactor moderator system. Implementation of operational practices and radiological safety coverage provided during the job resulted in low internal dose due to tritium and negligible rise in stack releases. (author)