«Co-operative Programme for Exchange of Scientific and Technical Information Concerning Nuclear Installations Decommissioning Projects Decontamination ...»
Co-operative Programme for Exchange of Scientific and Technical
Information Concerning Nuclear Installations Decommissioning Projects
Used in Decommissioning Activities
A Report by the
NEA Task Group on Decontamination
NUCLEAR ENERGY AGENCY
ORGANISATION FOR THE ECONOMIC CO-OPERATION AND DEVELOPMENT
TABLE OF CONTENTSIntroduction
1. Definition and general considerations
2. Objectives and selection criteria
2.1 Objectives of decontamination technologies for decommissioning
2.2 Selection of decontamination technologies for decommissioning
3. Survey of applied decontamination techniques
4. Characteristics of selected decontamination techniques for segmented components
4.1 General considerations
4.2 Chemical decontamination
4.2.1 General considerations
4.2.2 Chemical reagents
4.2.3 Spent decontamination solutions
4.2.4 Guidelines for selecting appropriate chemical decontamination techniques.............. 18 Advantages
4.3 Electrochemical decontamination
4.3.1 General considerations
4.3.2 Chemical reagents
4.3.3 Secondary waste generation
4.3.4 Guidelines for selecting appropriate electrochemical decontamination techniques.... 21 Advantages
4.4 Mechanical decontamination
4.4.1 General considerations
4.4.2 Abrasive-blasting decontamination systems
4.4.3 Abrasive media used
4.4.4 Secondary waste generation
4.4.5 Guidelines for selecting appropriate abrasive-blasting decontamination techniques
4.5 Decontamination by melting
4.5.1 General considerations
4.5.2 Current melting practices
4.5.3 Advantages of melting as decontamination technique
4.6 Other decontamination techniques
5. Characteristics of selected decontamination techniques for building surfaces
5.1 General considerations
5.2 Basic techniques
5.3.1 Needle scaling
5.3.3 Concrete shaving
5.3.4 Hydraulic/Pneumatic Hammering
5.3.5 Dust collection
5.3.6 Production rates
5.4 Guidelines for selecting appropriate decontamination techniques for building surfaces...... 33 Conclusions
Annex 1. List of task group members
Annex 2. Questionnaire on decontamination techniques for decommissioning
Decontamination is a major decommissioning activity that may be used to accomplish several goals, such as reducing occupational exposures, limiting potential releases and uptakes of radioactive materials, permitting the reuse of components, and facilitating waste management. The decision to decontaminate should be weighed against the total dose and cost. This document presents both proven and emerging techniques which may be used to accomplish the goals stated above. The planner must familiarise himself or herself with these techniques to integrate decontamination with other decommissioning activities.
At its thirteenth meeting on October 20-23, 1992, in Rome, the Technical Advisory Group of the NEA Co-operative Programme for the Exchange of Scientific and Technical Information Concerning Nuclear Installations Decommissioning Projects established a Task Group on Decontamination in order to prepare a state-of-the-art report on decontamination in connection with decommissioning. The work focused on decontamination for dose reduction as well as for waste decategorisation or for conditional or unconditional release of materials. The decontamination of both metallic and concrete surfaces was considered.
During its early meetings, the group developed a questionnaire, which was sent to decommissioning project managers. The information requested in this questionnaire covered the technical and economical aspects of the selected decontamination techniques. Project managers were asked to complete a separate questionnaire for each specific application of a given process, including actual data on the efficiency of the process as well as data on operating and investment costs.
This overview of decontamination techniques is intended to describe some of the critical elements involved in choosing techniques to address practical decontamination problems.
This overview of decontamination techniques is based on the results of the previouslymentioned questionnaire, which was received, reviewed and summarised for this report by the Task Group. The information presented here is not exhaustive, but does represent the state-of-the-art, for the techniques mentioned, as of June 1998. This overview is intended to describe some of the critical elements involved in choosing techniques to address practical decontamination problems.
Decontamination is defined as the removal of contamination from surfaces of facilities or equipment by washing, heating, chemical or electrochemical action, mechanical cleaning, or other
techniques. In decommissioning programmes, the objectives of decontamination are:
• to reduce radiation exposure;
• to salvage equipment and materials;
• to reduce the volume of equipment and materials requiring storage and disposal in licensed disposal facilities;
• to restore the site and facility, or parts thereof, to an unconditional-use condition;
• to remove loose radioactive contaminants and fix the remaining contamination in place in preparation for protective storage or permanent disposal work activities; and
• to reduce the magnitude of the residual radioactive source in a protective storage mode for public health and safety reasons, to reduce the protective storage period or to minimise long-term monitoring and surveillance requirements.
Some form of decontamination is required in any decommissioning programme, regardless of the form of the end product. As a minimum, the floor, walls, and external structural surfaces within work areas should be cleaned of loose contamination, and a simple water rinsing of contaminated systems may be performed. The question will arise, however, whether to decontaminate piping systems, tanks and components.
A strong case may be made in favour of leaving adherent contamination within piping as well as components in a dispersed form on the internal metal surfaces rather than concentrating the radioactivity through decontamination. In most cases, decontamination is not sufficiently effective to allow unconditional release of the item without further treatment after dismantling. Therefore, savings both in occupational exposure and cost could be achieved by simply removing the contaminated system and its components and only performing certain packaging activities (e.g., welding end caps on pipe sections, using adequate equipment to cut and crimp smaller piping to reduce chances of airborne activity). However, additional cost for the disposal of materials must be weighed in this scenario.
A decontamination programme may also require a facility capable of treating secondary waste from decontamination (e.g., processing chemical solutions, aerosols, debris, etc.) The concentrated waste, representing a more significant radiation source, must be solidified and shipped for disposal in licensed disposal facilities unless properly treated in the waste reduction/recycling/ reclamation processing alternative. The optimal waste reduction configuration must be defined after an economic assessment of treatment versus transportation/disposal costs has been completed. Each of these
additional activities may increase:
• occupational exposure rates;
• the potential for a release;
• the uptake of radioactive material.
These could conceivably result in even higher doses than those received from removing, packaging and shipping the contaminated system without extensive decontamination. Resolution of this question depends on specific facts, such as the exposure rate of the gamma-emitting contamination, the contamination level, and the effectiveness of the containing component and piping (wall thickness) in reducing radiation fields in the work area.
2.1 Objectives of decontamination technologies for decommissioning There are three main reasons for considering the use of decontamination techniques.
The first reason is the importance of removing contamination from components or systems to reduce dose levels in the installations. Access to the installations could then be made easier so that it becomes possible to use hands-on techniques for dismantling rather than the more expensive use of robotics or manipulators.
A second reason is to minimise the potential for spreading contamination during decommissioning activities, especially when dealing with systems containing highly radioactive particles and actinides.
The third reason is that it may be possible to reduce the contamination of components or structures to such levels that they may be disposed of at a lower, and therefore more economical, waste treatment and disposal category or, indeed, be unconditionally released for recycling or reuse in the conventional industry or disposed of as waste exempt from regulatory concern.
Several decontamination techniques have been developed to support maintenance work in nuclear installations. With relative success, the same techniques have also been adopted when decommissioning nuclear installations and components (Table 1). Objectives differ between these applications, however.
In maintenance work, the highest degree of decontamination is sought, avoiding any damage to the component so that it may be adequately reused. In contrast, the main aim of decontamination for decommissioning is the removal of as much activity as possible, not only to decategorise waste, but to reach clearance levels so that the material from the system may be reused without radiological restrictions. In many cases, it will be necessary to remove all oxides liable to trap contaminants, as well as a thin layer of structural material in order to achieve this aim. The radionuclides indeed tend to concentrate in the intergranular regions, together with other impurities accumulated during the growth of the metal grains. Therefore, much more aggressive decontamination methods are required than those used during the service life of a plant. In this view, technical methods presenting high decontamination factors at high contamination levels do not always allow achievement of the very low levels required to release the material (e.g., inner surfaces of piping), without restrictions, provided that measurement of these very low levels is feasible.
During decontamination for maintenance, components and systems may not be damaged and the use of very aggressive decontamination methods is not appropriate. In decontamination for decommissioning, however, it is mainly the use of somewhat destructive techniques that present the possibilities of meeting the objectives to release the material at clearance levels.
Another aspect in which techniques for thorough decontamination of materials differ from maintenance or laboratory scale decontamination is the need for industrialisation. The large amount of contaminated materials produced during decommissioning procedures and available for decontamination, generally do not favour methods or techniques that are labour intensive or difficult to handle, or that present difficulties when automation is envisaged. The latter is also true in the case of full-system decontamination for maintenance.
Other factors presenting differing influences on the choice of techniques are, for example, secondary-waste production and the possibilities to recycle products from decontamination processes.
For both decontamination for maintenance and decontamination for decommissioning, these may be among the parameters for decision-making.
The absolute requirement to obtain effectively residual contamination levels that prove to be below clearance limits is also a factor of primary influence when making the choices of decontamination techniques to be used. Even if techniques for the decontamination of complex geometries (e.g., pipe bends, small diameter piping) exist, the non-accessibility of areas may prevent direct radiological measurements being used to show that the clearance levels are met.
Presently, the interest of the nuclear industry is moving from decontamination techniques for maintenance to decontamination for decommissioning. Limited data are available from decommissioning on the efficiency of usable techniques to meet the low unconditional-release criteria. In most cases, using available techniques, the clearance levels are only met in an asymptotic way. Not all methods and techniques available present the possibility of decontaminating to below the required clearance levels.
So, in some cases, decontamination is carried out in different stages, the last step specifically aiming to obtain the required objectives.
Based on these considerations, when selecting a specific technique for system and/or
component decontamination, mainly the following requirements must be considered:
• Safety – The application of the method should not result in increased radiation hazards due to external contamination of workers or even inhalation of radioactive dust and aerosols formed during its implementation; it should not add other hazards (e.g., chemical, electric, etc.).
• Efficiency – The method should be capable of removing radioactivity from a surface to the level which would enable hands-on work instead of robotics, or which would permit recycle/reuse of material or, at least, a lower waste treatment and disposal category.
• Cost-effectiveness – Where possible, equipment should be decontaminated and repaired for reuse; however, the method should not give rise to costs which would exceed the costs for waste treatment and disposal of the material, whether including replacement of the equipment or not.