Any of the spent fuel management facilities may undergo refurbishment during its lifetime due to modifications required to facilities. The need for modifications may arise various reasons: to enhance facility function, economics or safety, change in regulatory standards, etc. Some common modifications to improve facility function include changes to allow such functions as [42]:
• Storage of higher burnups or enrichments or of different types; • Improvements to spent fuel or cask handling capability;
• Additions of spent fuel treatment or conditioning systems; • Improvements of pool water treatment or cooling systems; • Improvements in structural constructions or seismic provisions.
Whatever the motive of the modification is, the facility should maintain the applicable safety criteria required for normal operating conditions, anticipated operational occurrences and design basis accident conditions for the lifetime of the facility. The implementation of modification may involve the following steps:
2.4.1. Feasibility assessment
Before starting a modification, a feasibility assessment is to be made to see the cost/benefit justification and an initial safety evaluation. For the safety assessment, a systematic analytical approach (failure mode and effect analysis) would be necessary to determine whether the modification would bring any safety impact and to check it is compliant with the regulatory conditions as approved for the facility design and operation. If appropriate, it should be reviewed by an independent safety expert. This assessment should consider the modification implementation phase as well as facility operation after its installation.
Depending on the initial safety assessment, a more detailed and comprehensive safety assessment may be needed. The extent and complexity of the additional assessment required will depend on the nature and extent of the safety impact of the modification which would fall on one of the following categories:
• Little or no safety impact within the defined safe operating limits and conditions for the facility. For this category, the licensee will generally make the decision based on the initial safety assessment.
• Significant safety impact, but not sufficient enough to require a change in the licensing basis for the facility. For this category, the licensee should prepare a detailed safety assessment and not proceed with the work until the results of a detailed assessment have been accepted by the regulatory authority.
• Serious safety impact that require a change in the license or a new licence for the facility. For this category, the detailed safety assessment will usually be subject to a formal licensing procedure by the regulatory body before acceptance and implementation.
2.4.2. Implementation of work
Having applied the necessary controls to categorization and authorization, the implementation of work needs to be subject to rigorous control. The structured consideration of the following factors is a key element in ensuring that adequate thought, assessment and control of the work are achieved:
• Exposure to radiation;
• Radioactive waste management, including transport, decontamination, and dismantling as applicable;
• Provisions required to minimize the spread of contamination; • Safe operation of the facility during the modification period;
• Industrial hazards such as high voltage, fire, use of chemicals/explosives.
Consideration should be given to the need for special temporary emergency procedures in cases where potentially hazardous situations have been identified in connection with the facility conditions during the installation.
The safety of an implemented work should be verified through a testing programme which involves checks, measurements and evaluations prior to, during implementation, and at the completion of the modification. The completion of the project must include a check that all temporary connections, procedures, arrangements, etc. that were necessary for implementation have been removed or cancelled and the facility has been returned to full operational status. Confirmation of all aspects of completion of the implementation is
important: i.e. drawings have been updated, procedures and instructions amended and any necessary training delivered. The commissioning report should cover all of these aspects.
2.4.3. Remote technology for facility modification work
The applications of remote technology in facility modifications are akin to dismantling of a part of existing facilities to replace with a new functional equivalent or additional function(s) to be built as required for the modification. In this regards, much of the work involved in the facility modification would share the implementation procedures associated with decommissioning on the one hand, and construction of additional facilities on the other. However, the interface between the two disparate types of work is a very important factor, which is applicable to neither the former nor to the latter.
The actual work to be performed by means of remote technology would of course be very much dependent on the type and design of the facility where the modification work is required. In the case of the common facility like spent fuel inspection and repair system, which might be required as an add-on function, design modification would have to give due consideration to:
• Radiological safety of the additional system to the operator;
• Potential for malfunction or mishandling of the equipment or subsequent rupture of the fuel under work and appropriate measures to recover or mitigate the consequences (e.g. underwater gas collection and local water purification systems, appropriate sealable tubes for failed fuel, etc.);
• Potential for damaging the inspection equipment due to routine operation of the storage facility (i.e. collision of fuel handling machine with the additional inspection machine installed);
• Potential for dropping of both fuel and equipment during remote handling.
Much of the work for facility modifications have common features with decontamination and decommissioning. Aspects of which have amply been covered in the relevant literature.
2.4.4. Dismantling of facilities
An area of remote technology application, which has been gaining importance in recent years, is the decommissioning of obsolete nuclear facilities of which requirement is continuing to grow with the aging facilities are due to retirements. In terms of workload, nuclear power plants represent by far the largest decommissioning challenge by the number and complexities of work. The first step for decommissioning of a nuclear power station is to remove spent fuel which represent the majority of the radioactivity inventory in the station in order to facilitate subsequent activities. The removal of spent fuel from the nuclear power stations would involve those remote technologies used for most of the spent fuel assemblies and cask operations discussed above (see 2.1.3).
Decommissioning of nuclear facilities other than reactors (mainly fuel cycle facilities) bears some distinctive technical features as well as many similarities with respect to those pertaining to reactors. The global status of decommissioning fuel cycle facilities, including spent fuel management facilities, are reported in the recent IAEA publication on decommissioning of nuclear facilities other than reactors [43].
Among the spent fuel management facilities, spent fuel reprocessing facility requires the most extensive involvement of remote technology for decontamination and dismantling work to be performed in radioactive environment. France reported the decommissioning of some old facilities including the UP1 reprocessing plant, which was closed in 1998. The French estimation of remote technology usage in the dismantling of the Marcoule plant is around 20 % of the total workload, but it might reach up to 50 % in some special cases. UK reported a similar experience from the decommissioning work of B204 plant at Sellafield, which was mothballed in 1973. There are a host of laboratory or pilot scale reprocessing facilities being decommissioned or to be decommissioned in the future [44].
In the actual harsh environment of dismantling, advanced type machines will have reliability problems and selection of appropriate technology would be critical for the successful performance of required work. Special attention is needed to take care of possible consequences from equipment failure during active work and recovery methods with economic and dose justifications