Radiological and non-radiological hazard inventory for safe enclosure

among other activities, spent fuel is transferred out of the facility, waste is removed and the safe enclosure structure is installed. During PSE, a hazard inventory of the facility needs to be updated or completed. The hazard inventory will consist of radiological and non-radiological hazards present in the facility. This information should be put into a database, e.g. in the form of a geographic information system. The information is required as a baseline at the time of turnover to safe enclosure and could be a combination of measurements, historical records, operational logs, interviews, videos, pictures, drawings, material samples and other documents than can be used for option studies and preparation of decommissioning plans. Today’s state of the art technology allows the use of 3-D modelling and 3-D simulations for hazardous activities, non-routine entries, characterization and other required data. Figure 9 shows an example of graphics developed in preparation for the A1 NPP decommissioning project in Slovakia.

Incidents, spills and leakages, which may have occurred during plant operations, could affect the safe enclosure and future decommissioning process. Recording this information may minimize surprises such as the unexpected

FIG. 8. Indian Point Units 2 and 3 (courtesy of the US NRC).

discovery of contaminated concrete (at some facilities contaminated floors and walls were over-painted in order to fix contamination) [21]. This information should ideally be stored in an electronic database; the data can then be migrated to new systems as electronic systems become redundant during the safe enclosure period [22, 23].

A comprehensive review of pre-decommissioning radiological characterization is given in Ref. [24].

Radiological and other hazardous substance records should ideally be updated on a regular basis. To verify the validity of the records, it is important that sampling, measurements and inventories are carried out at a predefined frequency.

A comprehensive identification of hazards during the PSE phase, and a related safety analysis, for the High Flux Australian Reactor (HIFAR) are given in Ref. [25]. The safe enclosure phase for HIFAR, including hazards and safety analyses, is described in more detail in Ref. [26].

A specific project aimed at the characterization of a reactor being prepared for safe enclosure is given in Ref. [27]. A comprehensive Electric Power Research Institute study on the characterization of reference US NPPs in view of a SAFSTOR strategy is given in Ref. [28].

More details on this topic are given in Annexes II–11, II–12, II–15 and II–17.

3.1.1. Hazardous materials

As the facility or site is being shut down, consideration will be given to hazardous materials that were deployed during the operation. The preferable option is to remove as many non-fixed hazardous materials as possible during shutdown and PSE. However, if the material cannot be removed, a programme needs to be initiated to safely maintain these items within the safe enclosure structures.

FIG. 9. Interactive Graphics Robot Instruction Program graphics developed in preparation for A1 NPP decommissioning, Slovakia (courtesy of Nuclear Power Plant Research Institute Trnava).

These materials need to be stored in suitable containers and properly disposed of as soon as possible.

Reference [29] offers a comprehensive description of hazardous materials to be dealt with during decommissioning of nuclear facilities, including commercial management or disposal strategies for these materials.

Some of the compounds listed below may be subject to changes in regulations over time that may require removal from the site prior to establishing the safe enclosure. Some materials are considered mixed waste, e.g. contaminated polychlorinated biphenyl that it is more difficult to find a disposal path for.

3.1.1.1. Asbestos

Asbestos is especially likely to be present in facilities constructed before the mid-1970s. Asbestos was used as thermal insulation on piping, walls and ceilings and in many building products, and continues to be used today in some building materials. Facilities have used asbestos or asbestos-containing products to protect electrical cables from fire. Some facilities have removed as much asbestos as possible during their operational lifetime, but a large inventory of asbestos remains in a number of facilities currently under decommissioning. Radioactively contaminated asbestos needs to be treated as mixed waste. There is a tendency at some facilities to remove as much asbestos as possible during PSE, as asbestos may deteriorate fast and release airborne fibres (Fig. 10). However, the removal of asbestos is complex and regulated, and the method of removal is crucial. The spreading of asbestos particles during removal needs to be avoided at all times. Therefore, special tents need to be installed surrounding the asbestos containing materials to be removed. The operators performing this work need special training and protective clothing. After removal, very careful checks need to be performed in order to declare an area free from asbestos. This is a costly process that can generate a lot of waste and needs to be planned for. Preparing, using and removing the infrastructure for the removal of asbestos is complicated and expensive. This work could be done during PSE or during the safe enclosure period only if the facility will be completely cleared of asbestos. During safe enclosure and final dismantling, asbestos might be found in unexpected places, and further removal work may be necessary. This makes the process even more costly. It is reasonable to remove accessible bulk asbestos materials prior to safe enclosure; however, unless there is a statutory obligation or safety drivers, full asbestos removal is generally more economical to do at the beginning of final dismantling. An asbestos management programme needs to be maintained throughout the safe enclosure period, which may require active management, e.g. heating of facilities to prevent deterioration of conditions.

FIG. 10. Asbestos removal from components at Trino NPP, Italy (courtesy of SOGIN).

Easily removable asbestos used as insulation could be removed during PSE to avoid further problems (corrosion, water accumulation, etc.). Asbestos removal remains a critical activity, as a recent case from Hanford, USA, proves [30]. A project involving asbestos abatement is described in Ref. [31].

3.1.1.2. Lead

The use of lead in nuclear facilities is widespread. It is used as a shielding material in the form of bricks, sheets, wool or shot. The physical form of the lead shielding material is dependent on the nature of its use. In addition, lead based paints and primers as well as lead sheathed cables were routinely used during the construction of many facilities. Lead is a toxic metal, and needs to be treated as chemical waste. In the case of radioactive contamination, trying to remove the lead might lead to a ‘push in’ of the contamination into the weak structure of the lead. This makes it a costly product to dispose of, as it will be categorized as mixed waste. However, there are well established lead decontamination processes. Reference [32] is one arbitrarily selected decontamination method. In fact, there are several commercial processes to recycle lead. The experience of the French Alternative Energies and Atomic Energy Commission (CEA), France, is quoted in Ref. [33]. Studsvik, Sweden, also melts low level lead comprising bricks, transport casks, radiation shielding, etc. [34].

Its high density is another reason that lead can be hard to handle and remove. Although it can be used as a very effective shielding material in disposal casks, not all countries allow the use of lead in waste containers. This is due to the environmental rules in some countries, which do not allow heavy metals to be stored in final repositories.

If lead is present in the safe enclosure, it needs to be listed and marked, and dealt with at the latest during final dismantling. The same considerations as described for asbestos apply.

3.1.1.3. Graphite

Some facilities used graphite during their operating life. Graphite was found in electrodes or electrical conductors and was used as a neutron moderator in gas cooled reactors (Fig. 11), high-power channel-type reactors, thermal columns of research reactors, and biological shielding structures. Reactor graphite may generally remain in place awaiting final dismantling and future treatment or disposal in adequate repositories. Graphite in blocks is practically non-flammable and can be left in situ without fire hazard; on the other hand, there are no provisions for graphite disposal in many countries. However, flammability due to the Wigner effect needs to be assessed as

FIG. 11. Advanced gas cooled reactor core at Windscale, UK, showing graphite blocks (courtesy of Sellafield, Ltd).

it can be a problem, especially for research reactors. A noteworthy decommissioning work at the Vandellòs gas cooled reactor in Spain during the PSE period was the dismantling of the graphite silos that were formerly used for the graphite sleeves of the nuclear fuel (Fig. 12). At the Vandellòs reactor, graphite blocks have been confined in dedicated vaults for the entire period of safe enclosure.

3.1.1.4. Chemicals

As in any industrial facility, chemicals have been used in nuclear facilities for different purposes. Chemicals present hazards to human health and the environment such as corrosion and explosion. Caution needs to be applied to chemicals that may have become unstable over time. Some compounds may accumulate in the body with serious health implications. Therefore, chemicals should be identified and removed during final shutdown operations and before safe enclosure is fully established.

3.1.1.5. Organics

Organic compounds are chemicals that contain carbon and are found in all living things. Volatile organic compounds (VOCs) are organic compounds that easily become vapours or gases (gasoline, benzene, formaldehyde, polychlorinated biphenyl, freons, solvents such as toluene and xylene, perchloroethylene (a.k.a. tetrachloroethylene), zinc bromide, aerosols, sprays, degreasers, etc.). Along with carbon, they contain elements such as hydrogen, oxygen, fluorine, chlorine, bromine, sulphur or nitrogen. VOCs are released from burning fuel, such as gasoline, wood, coal or natural gas. They are also released from solvents, paints, glues and other products that are used and stored. All VOC related hazards should be taken into account while establishing safe enclosure and related hazards should be minimized by removing these products.