Decommissioning experiences: the case of small research reactors

Over the last decades a wealth of decommissioning experience has been accumulated. A considerable number of projects related not only to nuclear power plants or research reactors, but also to small research laboratories or medical facilities have been successfully completed. The diversity in the types and sizes of such facilities poses different technical issues and several uncertainties. The nuclear industry is unique in its documentation of experiences in decommissioning. No other industry puts on much effort into recording experiences and lessons learned from the shutdown of a facility, dismantling and site remediation. Decommissioning experiences have revealed some best practices to be followed, mistakes and associated lessons learned, that are translated into improved planning for subsequent

decommissioning projects.
For small research reactors, the decommissioning strategy can be significantly simplified if the final removal of the nuclear fuel from the facility is planned during normal operations and carried out just after the shutdown. Once the fuel has been removed, most of the remaining activity will be associated with highly irradiated parts and components e.g. reactor vessels and internals, or with components contaminated by strong gamma and/or alpha emitters, which require additional shielding. The decontamination and dismantling of small research reactors produce amounts of waste materials of various categories. Most materials released from dismantling are either non-radioactive waste or potentially contaminated

material that requires measurement before release at clearance levels⁽¹⁾. The necessary tools and equipment for remote handling and for shielding, as well as the requirements for their use, need to be identified within the decommissioning strategy. Careful consideration needs to be given to access routes during decommissioning of reactors, such that people and materials are subject to the appropriate levels of control coming into and going out of the area. The radioactive waste management policy and the waste acceptance criteria⁽²⁾ need to be established before starting decommissioning of the reactor, as waste accumulation on site can delay progress once active dismantling is underway.

Case studies and lesson learned from past decommissioning activities
During the demolition and removal of concrete structures at the 10 kW North Carolina State University research and training pool reactor in 1983, concrete dust was distributed throughout a three-storey building. The concrete and dust were found not to be radioactive, so clean up was straightforward, but the ventilation system and controls for the decommissioning work area clearly needed improvement. This led the project team to recommend that initial demolition only involved removal of non-active concrete. This permitted the improved ventilation and sealing arrangements to control the dust to be assessed and implemented before active concrete demolition got underway. The important lesson learned was that successful control of dust requires careful planning and provision of adequately designed dust containment systems.
When decommissioning small research reactors, one of the common difficulties reported is the lack of funds to complete the project, hence highlighting the need for tight project management from the outset. For the

decommissioning of the Greenwich Argonaut research reactor in London (1996-1999), before the project could proceed, a new safety case for the reactor had to be written, taking account of the latest guidelines and industry experience gained from decommissioning of other low power reactors. Writing the safety case required a considerable time and resource commitment from the project manager and the site licensee: this could have been considerably reduced if the existing safety case had been more comprehensive and up to date. Three relevant lessons learned derived from this project:

• The importance of regularly reviewing and updating the existing site or facility safety case: it should remain consistent with current national and international guidelines and it should take due cognisance of the experience gained during decommissioning of similar facilities.
• The importance of having a single expert external and independent peer review team for the project documentation.
• The site licensee encountered difficulties in keeping track of his responsibilities as part of safety management: the necessary conditions for compliance were contained in a number of different documents.
• Instead they should be reported in a non-executive nuclear safety management framework document, to be produced before commencing the decommissioning project, to focus the site licensee on the various nuclear safety, conventional safety and quality related responsibilities.

When decommissioning a 10 kW Argonaut-type Jason research reactor at Slough in the UK further lessons learned were reported:

• During maintenance activities, advantage should be taken of the opportunity to make radiological characterisation and materials sampling, saving this data as historical records to facilitate future decommissioning.
• Participation in an international co-operation project can be extremely helpful for discussion, interchange of information and learning from the experience of others.
• Quantitative neutron-activation analysis⁽³⁾ might be a useful technique when measuring materials composition at trace levels (ppm)