removed, cut and packaged for storage as Intermediate Level Waste. Much of this work is performed remotely using specialised tooling to minimise worker interaction and reduce external dose. The tooling must be designed with radiation protection in mind to allow for safe repairs and part replacement as required throughout the work.
The primary circuits of the reactor are also highly contaminated with both beta contamination from the deposition of corrosion products and alpha contamination resulting from fuel failures during the reactor lifetime. Tool design is also considered to prevent and mitigate airborne radioactivity. One part of the retube operation is cutting of the feeders. This operation involves manual steps, thus requiring careful control of worker dose. Minimising the generation of airborne activity through tooling design can allow workers to perform the cutting operations with minimal airborne protection. Working unencumbered in this way helps reduce the overall external dose.
At the face of a CANDU reactor during the retube. Feeders( shown by yellow arrow) are the name of the primary pipework which leads heavy water to and from the fuel channels. There are 480 inlet and outlet feeders in a CANDU reactor connected to the 480 fuel channels.
After cutting out all the reactor components, the new fuel channels are placed inside the reactor and new feeders are installed. Retube of a reactor takes about eighteen months from breaker open to restart. The number of staff involved from engineering staff, operations, specialist tooling staff, etc, is between 1,500 and 2,000. The large volume of work is performed continually, using ongoing shift work, to minimise the down time of the reactor operation. The retube requires a pool of up to 120 radiation protection staff and a team of tens of health physicists to manage the work planning and oversee the operations. Specific expertise is developed using the remote tooling and employing practices which have been shown in multiple retube operations to reduce exposures ALARA( As Low as Reasonably Achievable). All operations and tooling are tested in life size mock ups, where training is conducted to ensure staff can operate the tooling efficiently and properly.
The retube usually requires a new Radiation Protection Programme to be created, to adjust to the new type of working and radiological hazards encountered during the work, compared with those found during CANDU maintenance outages. Experienced HP personnel help the sites create these programmes and new arrangements, with often new access, new survey programmes, new ALARA planning methods, different RPPE and PPE, additional and specialised RP instrumentation and changed dosimetry.
Retubing of CANDU reactors is a major radiological challenge. The CANDU industry has demonstrated that it can perform these large projects safely and cost-effectively, implementing best practices in planning and radiation protection to ensure worker dose is controlled ALARA. Working in these environments presents an exciting experience for HP and RP personnel looking to work in a fast-paced environment where high dose rates and contamination are managed and controlled.
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