The dismantling of active plant at the Petten high flux reactor

The reactor vessel, support structure and other highly irradiated components at the Petten High Flux Reactor in the Netherlands were decommissioned over a period of seven months as part of an extensive upgrading programme. Following preparation of a detailed vessel dismantling scenario, special tools for underwater cutting, debris control and visual aid were developed in a dummy pool and were proved by dismantling a spare vessel. Radiological exposure during dismantling was minimised by preliminary studies, underwater gamma-ray measurements during maintenance outages and detailed pre-planning. Structural steel in the pool walls adjacent to the beam tubes and thermal columns turned out to be more highly activated than expected and operational radiation doses were kept to acceptable levels by a combination of permanent and temporary shielding. Almost six tonnes of medium-active waste were generated during dismantling. Part of the original thermal column has been stored intact; other waste was cut, packaged and put into long-term storage. The time spent in development of the decommissioning activities paid dividends in the successful completion of the project. Much useful data and experience has been accumulated which would be of benefit to similar projects elsewhere. (author)