Surface storage

Engineered storage facilities for DSRSs may be different by design due to radiation risk involved. An engineered storage facility for Category 3–5 DSRSs may be of simple construction, for example a building on a concrete base pad with a steel frame construction and corrugated metal sheets covering the walls and the roof (Fig. 78). Alternatively, a warehouse type construction with no arrangements for package handling and heating or ventilation is widely used.

FIG. 78. A light construction waste storage facility.

More sophisticated engineered stores with full engineered features are more appropriate for longer period storage of the DSRSs exhibiting high surface dose rates. The storage facilities may include arrangements for package handling, shielding with concrete (or equivalent), remote inspection, ventilation, temperature control, effluent collection, and building surfaces prepared to facilitate decontamination. The minimum construction standard for these stores is an adequately shielded, warehouse-type building with a solid floor and adequate safety provisions for waste package inspection (Fig. 79).

FIG. 79. A robust waste storage facility.

Another type of area storage is the placement of DSRSs in the larger container which, at least, provides better physical protection of the stored packages. An example of a simple area storage facility (ISO shipping/storage container) is given in Fig. 80.

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FIG. 80. Large scale ISO freight containers.

The container can be placed in a suitable place, i.e. at a centralized collection site, in a small nuclear research centre, a nuclear power plant, or a guarded area under government control. Depending on the size, between 40 and 70 200 L containers can be stored inside. Later, when a repository is available, the containers can be transported directly without requiring additional reloading steps.

At the end of the storage period, it is important to ensure that identification, retrieval and transport of the conditioned sources to the disposal facility are possible. Packages need to be stacked in a systematic manner to allow easy access and retrievability, if required, taking into consideration the use of forklift or crane access to all locations, as well as the ability to reach and retrieve any package.

9.5.3. Problems encountered and lessons learned 9.5.3.1. Subsurface storage

Underground wells (shallow boreholes and vaults) have been used for the storage of DSRSs in many Member States for many years. Generally speaking, the underground facility is favourable in terms of shielding and security.

However, this storage option has been reconsidered in some Member States. Originally, some facilities were intended for disposal of DSRSs and the retrievability of sources was not considered. The cost of retrieval of these sources today, coupled with the risk involved, has demonstrated that this option is not a prudent one, especially for long term storage of DSRSs (conditioned and unconditioned). Sub surface storage may be considered where storage time is very short and climatic conditions are favorable i.e. dry climates and location remote from inhabited areas.

9.5.3.2. Surface storage Location of the storage facility

Some storage facilities were located in areas with a potential risk of flooding (low lying area). This resulted in the degradation of containers with DSRSs.

In some cases, the storage facilities have been constructed too close to the site boundary, thereby restricting the dose rate of disused sources stored in the facility and/or requiring additional shielding within the facility. This also restricts handling high activity sources in the vicinity of the storage facility. In other cases, storage facilities appeared to be located too close to the national borders because of political changes leading to the creation of new independent states.

Beyond the technical and political aspects to be considered in the siting of storage facilities are the issues of acceptance by the local population of these facilities. The acceptance of the facility and plans play a major role and may have a relevant influence in the technical field.

Design features

Environmental storage conditions

Past experience demonstrates that in some facilities waste packages with DSRSs could be stored without any degradation during tens of years but over the same time period revealed deterioration of waste packages in other facilities after a certain period of storage. Degradation of waste packages over time becomes a very important issue, particularly when the storage time needs to be extended.

Experience shows that the atmospheric conditions inside the storage facility have a major influence on the longevity of the packages. Depending on the climate, air quality control systems could be installed inside. Cooling or dehumidifying equipment could be installed to avoid or minimize external corrosion/degradation of the waste container. In the design, not only the conditions in a completely loaded facility should be considered but also situations where the facility is partially loaded: air conditions, air flow and humidity conditions might be very different under partial and full load conditions.

Failure of DSRS packages during storage has occurred sometimes owing to mechanical damage of the container during handling. The damages ranged from paint scratches which accelerated corrosion of the container material to destruction of the container. Container defects tend to manifest themselves early, and this is a good reason to segregate sealed sources by the date of conditioning. In this way, systematic problems with container integrity can be avoided. The longer the DSRS is stored, internal or external influences can make package failure more likely.

Experience also shows that many containers that had been designed for transport of sources are inadequate for extended storage. If transport regulations are not an immediate concern, i.e. for extended storage, then alternative and less costly storage solutions for containers should be considered.

Past experience has also shown that the design of the storage facility should allow for flexibility, modification and expansion, particularly when disposal options (including waste acceptance criteria) are not finalized and the storage time needs to be extended. In this regard, it is advisable to design and construct modular extendable storage facilities, allowing for expansion if necessary. Such modular storage facilities already exist in some countries including the United Kingdom and Slovakia.

Smaller source containers are usually stacked to enhance the capacity of storage buildings. Older sources were often packed into non-standardized containers, often not qualified for stacking. It is practicable to put those irregularly shaped packages into standardized (temporary) overpacks to facilitate stacking and thus increase storage efficiency.

As result of different operational storage activities, or as result of penetrating water into the storage facilities, some small quantities of usually very low level secondary liquid radioactive waste aroused. This confirms the necessity of the installation of an appropriate system for collection and treatment of such waste. Similarly small amounts of solid waste result from monitoring activities (e.g. probing for surface contaminations on packages and the inner wall and bottom) and from handling activities (e.g. protective clothes). Measures for its collection and treatment need to be proposed by design.

Retrievability

Even if carefully evaluated, the failure of single packages or groups of packages cannot be excluded during storage. In early periods of operation, DSRSs were placed into boreholes or concrete bunkers which were subsequently backfilled with sand and then covered with concrete. Inspection of these sources was simply impossible. This has significantly complicated the process for retrieving these sources, and the addition of the sand has significantly increased the volume of radioactively contaminated waste that will need to be placed into a disposal facility. This practice is not considered appropriate, either for short term or for long term storage.

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Maintenance

In some cases building structures of storage buildings have failed in the past. Reasons for those failures were the use of building materials with inappropriate ageing properties, missing quality control, wrong static building layout, underestimation of settling properties of the structures or underground, etc. Replacement of those failed structures is difficult, costly and may require temporary removal of waste containers to another location.

Experience shows that storage facilities must have high quality in design and constructive materials and techniques in order to reduce the need of these operations. During operation of the storage facility building structures need to be inspected closely and settling should be regularly measured.

Control measures may include measurements of the structure and foundation and regular inspections and they need to be planned in advance. Installation of monitoring equipment in high radiation areas is hazardous and should be suggested in the design stage. Frequently almost all space in storage facilities is filled with containers, making finding problems difficult.

Other attributes of importance to be monitored include the operability of cooling equipment where heat generating sources are stored, as well as the operability of monitoring instrumentation.

Access to store and packages

In some cases, old storage facilities have been designed and constructed with the same access for operational staff and for transport vehicles loaded with radioactive waste. This was the reason for some accidents and it infringes the radiation protection rules. Taking into account this lesson, the storage facility should include double and easy access.

The storage facility must be designed to guarantee access to all waste packages. This is very important to facilitate placing the containers but also assure regular control and inspection of packages.

Communication

Many storage facilities are located away from other operating nuclear facilities and are often visited by a single individual, emphasizing the need for communication. In the facility design a proper level of internal communication should be ensured between any staff performing works and the central control room.

Operational procedures Inspection of packages

Although past practices have not always permitted the periodic surveillance of DSRS packages in storage, inspections and monitoring of the contents of the storage facility are now required unless the packages are subject to a comprehensive management system (former the quality assurance) from the time of generation.

Containers should allow proper manipulation, monitoring, inspections and repackaging of sources. Stacking in arrangements, such as columns, facilitates access and ease of handling and permits inspection. Handling and lifting devices require inspection and monitoring, as they are just as susceptible to corrosion as the waste containers themselves. Sufficient funding and staff must be available for this purpose.

The storage shed is usually filled from the back to the front. At the time of retrieving the containers, a corridor or a back door can allow to retrieve the older containers first. Otherwise, one must to empty the whole shed to reach the older containers.

The accumulation of dust inside the storage facility has been observed that may complicate the access to the packages and demand cleaning operations which can be costly in terms of radiological protection.

Environmental monitoring

Despite efforts during design and operation to make structures leak-tight and watertight, water ingress has occurred. Significant efforts have been spent determining the source of water in storage structures. An extensive

groundwater monitoring programme is needed to identify unanticipated releases from waste encased in, for example, concrete. In other cases, it has provided evidence that storage facilities are functioning well after many decades.

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