Chapter 51: Predisposal of Radioactive Waste – Storage | A Book: Nuclear Energy

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on April 22, 2013.

Within the context of predisposal management of radioactive waste, storage refers to the placement of radioactive waste in a nuclear facility where appropriate isolation and monitoring are provided. In radioactive waste management, storage may take place between and within the basic radioactive waste management steps. Storage may be used to facilitate the next step in radioactive waste management, to act as a buffer within and between radioactive waste management steps, or in awaiting the decay of radionuclides until authorized discharge, authorized use or clearance can be allowed.Radioactive waste may be stored in solid, liquid or gaseous form or as raw, pretreated, treated or conditioned waste. The intention of storage is that the waste will be retrieved for authorized discharge, authorized use or clearance or for processing and/or disposal at a later time. The criteria for acceptance of waste packages in a storage facility shall therefore take account of the known or likely requirements for subsequent radioactive waste disposal. Safety requirements for the protection of human health and the environment shall be met by appropriate design, construction, operation and maintenance of the respective facilities, including provision for the eventual retrieval of the waste.

There are many reasons why it may be appropriate to store radioactive waste for varying periods of time. The following three examples usually encountered at small storage facilities for radioactive waste, where storage is incidental to the primary purpose of the facility:

To allow for the decay of short lived radionuclides to a level at which the radioactive waste can be released from regulatory control (clearance) or authorized for discharge, or recycling and reuse;
To collect and accumulate a sufficient amount of radioactive waste prior to its transfer to another facility for treatment and conditioning; and
To collect and accumulate a sufficient amount of radioactive waste prior to its disposal.

Whereas the following two examples are usually associated with larger storage facilities undertaking the treatment and storage of waste from nuclear fuel cycle facilities and from centralized facilities at which the waste from many small users of radioactive sources is collected and processed:

To reduce the heat generation rate of high level radioactive waste prior to its disposal and, in some cases, prior to steps in its predisposal management; and
To provide long term storage of radioactive waste in those States lacking a suitable disposal facility.

The period of storage may be highly variable and may be only a few days, weeks or months in the case of storage for decay or storage prior to the transfer of waste to another facility. Extended storage periods of many years may be necessary in the case of the storage of high level waste for cooling or for the long term storage of radioactive waste for which there is no disposal option yet available.The storage facility may be located at the facility generating the waste, such as a nuclear power plant, a hospital or a laboratory, or it may comprise a separate entity such as a centralized facility or a national treatment and storage facility. Storage facilities can range from secured cabinets and closets at laboratories up to large facilities built for serving a nuclear power plant.

1. SMALL STORAGE FACILITIES:

Small inventories of radioactive waste comprising radionuclides with relatively short half-lives are typically handled at small waste storage facilities. Simple waste treatment activities such as low force compaction may also be carried out at small waste storage facilities. In addition, sealed sources of various types may be handled at small waste storage facilities.

Some types of radioactive waste may be stored for periods specifically to allow the radioactivity of the waste to decay to levels that permit its authorized discharge or removal from regulatory control (i.e. clearance). Storage may also be necessary for operational reasons, for example, to accommodate the off-site transfer of waste to a waste treatment facility at specified time intervals.

The storage of waste in centralized facilities rather than in a multitude of on-site facilities should be considered, since there will be opportunities to adopt more stringent safety standards and at the same time to realize economies of scale.

For most small waste storage facilities, simple design features together with correspondingly simple operating procedures will be appropriate. The stored radioactive waste should be characterized (e.g. by radionuclide type, inventory, activity concentration, half-life and the physical, chemical and pathogenic properties of the waste) and the results should be documented in an inventory log. If pathogenic radioactive waste is to be stored, it should be deactivated before its placement in storage.All waste packages and their documentation should be identified with a unique code for the purposes of tracking. In most cases a simple indelible weather resistant tag and logbook will be sufficient. Where biomedical radioactive waste is produced in large volumes, it should be considered whether a separate storage area for biomedical radioactive waste is necessary. Waste that is to be shipped to a centralized waste management facility should be packaged in accordance with the waste acceptance criteria of the receiving facility.

Radioactive waste should be packaged in such a manner that it is not accessible to pests such as insects or rodents as they can present a serious threat to its containment. This is particularly relevant for the storage of biohazardous radioactive waste or where waste may be stored in plastic bags.

1.1 Operation of Small Storage Facilities:

1.1.1 General Operational Considerations:

The operational activities of a small storage facility for radioactive waste should be described in and carried out in accordance with facility specific procedures. The operational activities may include: receipt, storage and retrieval of waste; labeling of waste packages; inventory control; package inspection; radiation protection; monitoring and surveillance; record keeping; and preparation of waste packages for dispatch to another facility. The scope and level of detail of the procedures should be commensurate with the radioactive inventory, the associated hazards and the extent of the storage activities. The procedures should be such as to ensure compliance with the operational limits and conditions approved by the regulatory body.

1.1.2 Radiation Monitoring:

Radiation monitoring should be conducted routinely to determine the external radiation levels and surface contamination levels inside the waste storage facility, along the boundaries of the storage facility and on the surface of waste packages. In facilities where loose waste is compacted and repackaged for storage or transport, monitoring for airborne contamination may be appropriate.

In storage facilities where there is a potential for surface contamination, fixed or portable instruments for detecting external contamination on workers should be provided at exit points from the area.

Monitoring instruments should be periodically tested and calibrated. The energy response and the measurement range of the instruments should be appropriate for the radionuclide composition of the waste and the expected ranges of radiation levels and contamination levels.

1.1.3 Radiation Protection:

A radiation protection programme should be developed as part of the application to the regulatory body for authorization.

The operation of most small storage facilities for radioactive waste will require only a very simple and limited programme of periodic maintenance, testing and inspection (this could be as simple as checking the effectiveness of the locks at the facility). The maintenance, testing and inspection records should be subject to periodic review.

1.1.4 Security

Owing to the limited waste inventory and low hazard levels encountered in most small storage facilities for radioactive waste, the measures for security and access control may consist simply of a locked door or cabinet (e.g. a locked storage area with authorized key holders and an access log). However, security for disused sealed sources of high activity may require additional consideration.

1.1.5 Decommissioning:

Decommissioning involves the removal of all stored waste, followed by a survey to determine the residual surface contamination levels and external radiation levels. The facility may require decontamination and the removal of contaminated materials and equipment.

2. LARGE STORAGE FACILITIES:

A large storage facility for radioactive waste may receive a wide variety of waste types from a number of different origins. The design and operation of a large waste storage facility should be commensurate with the potential hazards associated with it.

A tracking system for waste packages should be developed and maintained. For large waste storage facilities, the use of a computerized system for tracking packages should be considered. A storage plan that shows the configuration of the emplaced waste packages, including the zoning for the level of hazards, should be prepared and maintained.

Procedures should be developed for the safe operation of a large waste storage facility. The extent and the degree of detail of specific procedures should be commensurate with the safety significance of the particular subject of the procedures and should cover, where applicable:

Operations, including all necessary limits and conditions;
Commissioning;
The management system;
Maintenance, inspection and testing;
Training;
Modifications made during design, construction, commissioning and operation;
Recording, reporting and investigation of events;
Radiation protection and safety performance;
Contingency and emergency arrangements;
Safeguards;
Security measures;
Control of radioactive discharges to the environment; and
Acceptance criteria for waste packages.

2.1 Operation of Large Storage Facilities:

2.1.1 General Operational Considerations:

Typical operational activities associated with waste storage are the routine operations of receipt, processing, emplacement, storage and retrieval of waste packages and their preparation for disposal. Supporting activities include: radiation protection; monitoring and surveillance; testing and examination of waste packages; inspection of the components of the storage facility; maintenance and repair; labeling of waste packages and record keeping.

Storage facilities should be operated in accordance with written procedures. These procedures should be such as to ensure compliance with the operational limits and conditions approved for the storage facility by the regulatory body.

Modification of the storage conditions should be subject to specific plans and procedures and accompanied by appropriate authorizations from the regulatory body. The impact of any modifications on the safety of the stored waste should be considered in each case. Operational practices and administrative controls appropriate to the level of hazard should be applied. Examples of these include:

The use of pre-work assessments and training mock-ups to minimize exposure during operation and maintenance activities;
The application of remote handling technologies for operation and maintenance;
Establishment of contamination controls when items are transferred or removed from areas of higher contamination to areas of lower contamination; and
Appropriate planning for, and careful conduct of, storage activities so as to minimize exposure during operation and maintenance activities.

A system for tracking waste packages should be developed and maintained. For large storage facilities, a computerized system for tracking waste packages should be considered. Where practicable, a detailed storage plan showing the configuration of the emplaced waste packages, including the zoning for the level of hazards, should be prepared and maintained.2.1.2 Operational Limits and Conditions:

Storage facilities should be operated in accordance with a set of operational limits and conditions that are derived from the safety assessment of the facility to identify the safe boundaries of operation. Operational limits and conditions set out specifications relating to waste packages, safety systems and procedures, radiological criteria and requirements for personnel. Operational limits and conditions for storage facilities should be developed by the operator and should be subject to approval by the regulatory body. Operational limits and conditions should be revised as necessary in the light of experience from commissioning and operation, modifications made to the facility and changes in safety standards. Reference [31] provides guidance for the development and implementation of operational limits and conditions for nuclear power plants; much of this guidance is applicable to storage facilities for radioactive waste.

The risks posed by the waste and the conditions of its storage should be taken into consideration in determining the operational limits and conditions. The operational limits and conditions will be specific to each storage facility. The operator may wish to set administrative margins below the specified limits as an operational target to remain within the operational limits and conditions.

Operational limits and conditions for the storage of waste should include, as appropriate:

Specifications for waste packages (waste form, radionuclide content and container characteristics) consistent with the waste acceptance criteria for the storage facility;
Concentration limits for liquid waste, e.g. to prevent the precipitation of solids;
Requirements for safety systems, e.g. requirements for ventilation, heat removal, tank agitation and radiation monitoring, including requirements for the availability of these features in normal and abnormal conditions;
Periodic testing of equipment, especially backup systems that need to be available in emergency conditions;
Maximum radiation dose rates, especially on container surfaces;
Maximum levels of surface contamination for containers;
Requirements for training and qualification of personnel and minimum staffing levels; and
Limits on the cumulative radionuclide inventory.

The initial operational limits and conditions should normally be developed in cooperation with the facility designers well before the commencement of operation to ensure that adequate time is available for their assessment by the regulatory body.2.1.3 Operational Procedures:

Procedures should be developed for managing and operating the storage facility under normal conditions, in incidents and under postulated accident conditions. Responsibilities for the approval of any necessary deviations from procedures for operational reasons should be clearly defined. Any deviation from the approved operational procedures should be justified and its implications for safety should be determined.

In accordance with the management system, arrangements should be in place for the review and approval of operating procedures and for the communication to operating personnel of any revisions. Periodic reviews should be undertaken to take account of operational experience. Any revisions should be adopted only after they have been reviewed to ensure compliance with operational limits and conditions, approved by authorized persons and documented.

2.1.4 Radiation Protection:

The objectives of the radiation protection programme are to ensure that the radiation doses to workers and to members of the public arising from the normal operation and the possible abnormal operation of the storage facility do not exceed regulatory limits and that radiation protection is optimized. Releases of radioactive material to the environment should also be controlled in accordance with the requirements of the regulatory body.

Additional procedures for radiation protection may be necessary for application to non-routine activities for waste storage, such as the movement of waste through passages and areas commonly used by personnel, the handling of packages with undocumented characteristics and the purging of areas where ventilation is intermittent.Radiation dose rates should be specified for waste packages, shielding surfaces and other locations and radiation levels should be monitored at intervals sufficient to alert the operator to any changes due, for example, to the unexpected and undetected buildup of radioactive material or the degradation of shielding.

2.1.5 Maintenance, Testing and Inspection:

Before the start of operations, the operator should prepare a programme of periodic maintenance, testing and inspection of systems that are essential to safe operation. The need for maintenance, testing and inspection should be addressed from the design stage. Non-intrusive testing and inspection that incorporates the diagnostic assessment of performance as part of normal in service activities is to be preferred. Testing and inspection should establish and verify correct function, performance and conditions against acceptance criteria.

The programme should be periodically reviewed with account taken of operational experience. Systems and components that should be considered for periodic maintenance, testing and inspection may include:

Waste containment systems, including tanks and other containers;
Waste handling systems, including pumps and valves;
Heating and/or cooling systems;
Radiation monitoring systems;
Calibration of instruments;
Ventilation systems;
Normal and standby systems for electrical power supply;
Utilities and auxiliary systems such as systems for water, gas and compressed air;
The system for physical protection;
Building structures and radiation shielding; and
Fire protection systems.

The frequency of maintenance, testing and inspection should be such as to ensure that the reliability of equipment remains high and that the effectiveness of systems remains in accordance with the design intent for the facility. The reliability of systems should not be significantly affected by the frequency of testing.

Suitably qualified, trained and experienced personnel should be deployed in the approval and implementation of the maintenance, testing and inspection programme and in the approval of associated procedures. Test procedures should normally include test acceptance criteria.

Records should be kept of activities for maintenance, testing and inspection. These records should be subject to periodic review to establish trends in system performance, the reliability of components in the system and the effectiveness of the maintenance programme. The reviews should include the identification of appropriate corrective measures.

2.1.6 Security and Access Control:

Access to areas in which waste is stored should be controlled to ensure safety and the physical protection of materials. In meeting operational requirements to control access, a zoned approach, working inwards towards areas having more stringent controls, may be applied. There should be provisions for detecting any unauthorized intrusion and for taking countermeasures promptly.

3. INTERIM STORAGE OF RADIOACTIVE WASTE PACKAGES:

Conditioning of radioactive waste involves those operations that transform radioactive waste into a form suitable for handling, transport and disposal. However, if for some reason disposal of waste packages cannot be made immediately after conditioning (e.g. if a disposal facility is not available or if radioactivity in waste packages must decay to lower levels), interim storage of waste packages is required and must be arranged in such a way as to ensure the integrity of radioactive waste packages and their suitability for further disposal after retrieval from a storage facility.Storage is an integral part of the waste management process. While the storage of conditioned waste is normally described as interim storage, for some countries this will probably be fairly long term, even to the point of de facto disposal. Somewhere between ten and fifty years will most likely be required for storage until a repository can be constructed and licensed, or until radioactivity has decayed to a sufficiently low level for disposal as cleared waste.

Radioactive waste may exist in several forms when it passes through the treatment and conditioning processes. It may exist sequentially as raw, treated, immobilized and fully conditioned waste. While in storage it should be expected to retain its form and suitability for transport and disposal for up to 50 years without subsequent reconditioning. This is accomplished through the interaction of three sets of criteria:

The waste acceptance criteria (WAC);
The waste form and container specifications; and
The design and operating requirements of the storage facility.

The operations to be carried out in a storage facility will be limited to receipt, emplacement, integrity control (if required), retrieval, and preparation for dispatch of waste packages. The interim storage operations are essentially passive for the long period of time when waste packages are pending retrieval (most probably in bulk) until the repository facility is established. All operations concerned with storage must be carried out within the written authorized procedures.3.1 Receipt and Emplacement:

The waste receipts should be programmed in advance. The store manager must examine the information to confirm that the waste package is acceptable for storage (e.g. correct packaging standard, radiation levels within limits). If the package is unacceptable, the details are to be recorded and the documents returned to the consignor with an explanation or a request for further information. Package acceptance qualification conditions should include, but not be limited to: maximum allowable weight per package; mechanical resistance for the stacking of packages; satisfactory corrosion resistance of the container material; sufficient resistance to a standard fire test; and no loss of integrity after a drop test from a height related to the package transport condition. The above tests are designed to confirm the adequacy of the standard packaging design and should be performed occasionally during waste package production.

In the case of external contamination, the package must be decontaminated and rechecked before interim storage is authorized.

On acceptance, the equipment required for transport of the waste package to the store should be selected, and the store operator should proceed with this equipment and prepare the appropriate documentation to store the waste. The operator must be trained in the appropriate methods of radiological protection and in the use of radiation protective equipment if needed during handling of the waste package. At the store, a suitable location for the waste package should be identified and the location details recorded. The waste package would be placed in the chosen location. Segregation of waste types is desirable to facilitate a planned retrieval for further treatment or any unplanned retrieval that is revealed as necessary by periodic inspections for possible degradation of waste containers, and in case there are categories of waste to be placed eventually in particular repository locations.

The information provided by the consignor and the storage location of the package are incorporated in the central store records.

3.2 Integrity Control:

Adequate conditions for safe storage of waste packages should be maintained during the storage phase to avoid deterioration. Proper radiation protection measures must be applied to ensure that exposure to workers and the public is kept as low as reasonably achievable and that there is no contamination of the store or the waste packages. Monitoring should be undertaken to ensure that contamination has not occurred. The frequency of the monitoring will depend upon the quantity and type of the waste packages.

The records of the store inventory should be kept up to date, and the store contents periodically checked against the records.

3.3 Retrieval and Dispatch:

Following receipt of a request to retrieve a package from storage, the store manager should obtain the details of the particular waste package from the store’s records and pass them to the appropriate party. If the details are in order the package may be accepted for removal from the storage facility. Once the store manager has authorized the release of the waste package, it is retrieved from the store and taken to the dispatch area. Here, the package should be monitored for radiation levels before it is released. The details of the waste package will be transferred to the transport records and the waste packaged for transport in accordance with the requirements of the transport regulations. The package storage records should be amended to record the date of dispatch and the receiving party.

Here are some recommendations for the interim storage facilities articulated by the IAEA:

It is recommended that interim storage facilities should not accept any new waste that is not adequately characterized. Every package must comply with the acceptance requirements of the storage facility;
Non-immobilized wastes emplaced in containers for storage should be segregated in the storage facility to the extent practical. The packages should also be segregated by waste type and storage duration;
Facilities for the storage of waste should have a design life that is independent of the emplaced packages;
The safety assessment should support the suitability of the facility for the duration of its intended use. Storage of low contact dose rate Low Intermediate Level Waste (LILW) should allow for visual inspection of packages. For prolonged storage it should be recognized that surveillance requirements may need to be modified during the period of storage. For the storage of high contact dose rate LILW, High Level Waste (HLW), and spent fuel, indirect controls may be put in place to inform the operators in the event of a failure;
If the waste is characterized and conditioned properly within an adequate quality assurance programme, failure of the package is not expected. But in the event of package failure, for any reason, provisions for retrieval of a package must exist in the storage facility; and
Packages might be damaged if they fall during handling in a storage facility, and in that case the storage facility must provide for retrieval of the package as well as decontamination of the area, if necessary. If a package failure occurs during storage owing to the waste form or a widespread external corrosion of the container, then further action to eliminate the hazard for all packages of the same type should be evaluated. Packages that fail during storage and cannot be retrieved and remediated should be isolated in place, whenever possible, to protect personnel, the environment and neighbouring waste packages from contamination.

4. LONG TERM STORAGE OF RADIOACTIVE WASTE:

Prior to the authorization of a facility for the storage of radioactive waste, the operator should provide the regulatory body with plans for the long term management of the radioactive waste being stored. The public, especially communities located near the storage facility, should be informed of these plans.

Long term storage of radioactive waste refers to situations in which waste is stored for periods that exceed the original design life of the containers and storage facilities, for example, owing to the disposal of the waste being delayed or postponed. Long term storage can also refer to situations in which the waste packages and storage facility are designed for relatively long periods of storage (e.g. 100 years).

Additional technical considerations for long term storage of waste are:

Engineered systems, facilities and institutional controls should be more robust or should be more actively maintained. If possible, passive safety features should be used;
Information should be retained in a readable and understandable form for future generations. For long periods of time, the deterioration of records (whether material or electronic) will be more significant; and
Inadvertent or deliberate intrusion into waste storage facilities may be more likely over longer time periods and intrusion should be considered in the safety assessment.

For storage beyond the original intended period of storage, the design life of the storage facility and that of the waste packages may be exceeded. This should prompt a re-evaluation of the storage strategy, which may include a re-evaluation of the initial design, operations, the safety assessment and other aspects of the waste storage facility.

For storage beyond the original intended period, testing, examination or evaluation may be necessary to assess the integrity of waste packages. Potential problems with waste packages should be considered in advance of the need for physical action (such as overpacking or placing the waste into new waste packages). In some cases it may be justified to move waste packages into a more robust storage facility rather than to overpack or replace them.

For unanticipated long term storage, consideration should be given to mitigation of the consequences of potential changes in the stored radioactive waste. Changes in the stored waste may include:

The generation of hazardous gases caused by chemical and radio lytic effects (e.g. the generation of hydrogen gas by radiolysis) and the buildup of overpressure;
The generation of combustible or corrosive substances;
The corrosion of metals (e.g. carbon steel); and
Degradation of the waste form.

These considerations are especially important for long term storage for which small effects may accumulate over long periods of time. Uncertainties in parameters and models should be considered in analytical approaches used to evaluate processes over long periods of time.

Resources:

Chapter 52 …