Storage of spent nuclear fuel (Sustainability Assessment)

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This page refers to "Safety of NFCFs".

In this section, firstly, a short description of the main processes in a storage facility for spent nuclear fuel (SNF) is given and the corresponding specific safety issues are discussed. Secondly, the assessment method will be discussed based on the corresponding criteria of the INPRO methodology in the area of safety, which have been, where necessary, adapted to the specific issues of this kind of NFCF.
The term ‘storage’ refers to the retention of radioactive waste in a facility or a location with the intention of retrieving the waste. Thus, storage is a temporary measure for which some future action is planned such as further conditioning or packaging of the waste and, ultimately, its disposal[1].
After unloading from the reactor core, SNF is usually stored for several years at the reactor site in storage pools. This initial period of storage allows a considerable reduction in the intensity of radiation fields and decay heat generation. Thereafter, SNF is usually transferred into another wet or dry storage facility away from the reactor.
Spent nuclear fuel transportation has not been considered in this manual as an independent stage of nuclear fuel cycle. The INPRO methodology implies that safety of spent fuel transportation is to be considered as part of the INPRO assessment either of the spent fuel storage facility or the spent fuel reprocessing facility.

At-reactor storage pools for SNF

At-reactor SNF storage pools are either within the reactor building or in an adjacent fuel building, which is linked to the reactor by a transfer tunnel. Access to the SNF in the storage pool is usually by means of immersing a cask in the pool, loading it with SNF and then removing the cask for lid closure, decontamination and transport. A method developed in France involves a cask loading concept with bottom access ports for transferring SNF from the pool into the cask. The advantages of this design are that contamination of the external surface of the cask by immersion in the pool is avoided, and also the requirement to lift the cask (empty and loaded) between the inlet/outlet location and the pool and a heavy duty crane is not needed. There are some cases, e.g. at gas cooled reactors (Magnox), at Sellafield in the UK and at La Hague in France, where SNF can be loaded into (unloaded from) the casks in a dry shielded cave avoiding cask immersion in water.
At-reactor SNF storage pools are constructed of reinforced concrete and usually lined with stainless steel. The pools are filled with de-ionized water with or without additive depending on the type of fuel to be stored and the adopted method of treatment. Water activity levels are maintained as low as reasonably practicable (ALARP) by either in-pool or external ion exchange systems or by limiting activity release to the bulk pool water. The pools are further equipped with leakage monitoring, as well as coolant temperature, transparency and chemical monitoring systems, and systems necessary to maintain monitored parameters within the prescribed ranges. Chemical monitoring normally involves pH measurements, soluble boron concentration measurements for criticality control where necessary, and measurements of levels of aggressive anions such as chloride and sulphate to minimize fuel degradation. Maintenance of good water chemistry provides good water clarity and usually prevents the occurrence of micro-biological organisms. If these do occur, they are treated with specific chemical dosing.
Subcriticality in SNF pools was originally maintained by spacing within the storage racks or baskets. However, with the need to store greater quantities of SNF, higher storage density has been achieved by the introduction of neutron absorbing materials in storage racks and baskets such as boronated stainless steel, boral or boraflex.

Away from the reactor site storage of SNF

Storage away from the reactor site (AFR) can be wet, in the form of centralized pools in support of reprocessing activities, secondary pools or additional pools, or most often dry, in the form of dry cask storage facilities, which may or may not have capability for off-site transport.

AFR wet storage facilities

A variety of AFR wet storage facilities are in use. A typical AFR wet storage facility has the following features:

  • Cask reception, decontamination, unloading, maintenance and dispatch;
  • Underwater spent fuel storage (pool);
  • Auxiliary services (radiation monitoring, water cooling and purification, solid radioactive waste handling, ventilation, power supply, etc.).

An AFR storage pool is a reinforced concrete structure usually built above ground or at least at ground elevation, however, one wholly underground facility is in operation. Some early pools were open to the atmosphere, but operational experience and the need to control pool water purity has resulted in the pools now being covered. The reinforced concrete structure of the pool, including the covering building, needs to be seismically qualified depending upon national requirements.
Most pools are stainless steel lined; some are coated with epoxy resin based paint. However, there has been experience with degradation of the latter after a number of years. A further option is for the pool to be unlined and untreated. In some situations, the pool may be stainless steel lined or epoxy treated only at the water line or at other locations. Regarding unlined and untreated pools, properly selected and applied concrete can be proved to have negligible corrosive ion leaching and permeability to water. The water is either a fixed quantity or a once through pond purge. Leakage from the pool is monitored, either by means of an integrated leakage collection system or via the inter-space in pools with two walls. In both cases any recovered pool water may be cleaned up and returned to the main pool. In addition to the control of activity by ion exchange or purge, some pools are operated with an imposed chemical regime (see Section 1).

AFR dry storage facilities

Dry storage of SNF differs from wet storage by making use of gas or air instead of water as the coolant (often an inert gas such as helium, or an only modestly reactive gas such as nitrogen, to limit oxidation of the fuel while in storage) and metal or concrete instead of water as the radiation barrier. SNF is normally stored in pools for several years before it becomes cool enough for dry storage to be possible.

Dry storage vaults.

In a vault, the SNF is stored in a large concrete building, whose exterior structure serves as a radiation barrier, and whose interior has large numbers of cavities suitable for SNF storage units. The SNF is typically stored in sealed metal storage tubes or storage cylinders, which may hold one or several fuel assemblies; these tubes or cylinders provide containment of the radioactive material in the spent fuel. Heat is removed in vault systems by either forced or natural air convection. In some vault systems, SNF is removed from the transport cask and moved in a shielded charge machine to its storage tube, while in others the SNF stays in the container in which it arrives, which is then placed in a transfer cask and moved by crane to its storage cylinder. Thus, vault systems typically also require cranes or fuel-handling machines.

Dry storage silos.

In a silo storage system, the SNF is stored in concrete casks, i.e. either vertical or horizontal cylinders fitted with metal inner liners or separate metal canisters. The concrete provides radiation shielding (as the building exterior does in the case of a vault) while the sealed inner metal liner or canister provides containment. Transfer casks are often used for loading of the fuel into the silos. Heat removal is by air convection. One example of a horizontal concrete silo design is the NUHOMS storage system. The system uses vertically loaded metal canisters and the horizontal concrete storage modules. The metal canisters use a double lid closure, are seal-welded, and tested for leak tightness[2].

Dry storage metal casks.

Metal casks are massive containers used in transport, storage and eventual disposal of SNF. A typical metal cask has a capacity of 4 to 26 PWR or 10 to 60 BWR fuel assemblies[3]. The structural materials for metal casks may be forged steel, nodular cast iron, or a steel/lead sandwich structure. They are fitted with an internal basket or sealed metal canister which provides structural strength as well as assures subcriticality. Metal casks usually have a double lid closure system that can be either bolted or seal welded and can be monitored for leak tightness. Metal casks are usually transferred directly from the fuel loading area to the storage site. Some metal casks are licensed for both storage and off-site transportation. Fuel is loaded vertically into the casks which are usually stored in a vertical position.

Operation of an SNF disposal facility before closure

Storage of SNF was defined above as the retention of SNF in a facility or a location with the intention of retrieving the SNF[1]. Disposal of SNF is defined as the emplacement of SNF into a facility with no intention of retrieval[4]. Commonly, SNF is envisaged to be disposed of in a deep geological depository[5].
Before SNF can be put into a disposal facility, it needs to be packed in an appropriate way for disposal, for example in special containers with long term durability. This packaging process can be performed at the site of the disposal facility or in a separate plant; again the safety issues related to packaging of SNF are comparable to those for handling SNF in a storage facility. A special safety case is necessary for an SNF disposal facility[6].
However, it is noted that before closure, i.e. during the phase where a disposal facility receives SNF, its safety issues are comparable to those of an SNF storage facility (presented in Section 4). Thus, the INPRO assessment methodology for SNF storage facilities can also be applied to SNF disposal facilities during their operational phase, i.e. before closure.
It is to be emphasized that, after closure of a disposal facility, the safety of SNF is assured by passive safety features without the need of human intervention.
Safety issues concerning potential releases of radioactive material into the environment after the closure of an SNF disposal facility are to be considered in another area of the INPRO methodology called ‘waste management’.

Safety issue in a storage facility for SNF

SNF is usually transferred to storage facilities only after an initial period of storage in the reactor pool. As stated before, this initial period of storage allows a considerable reduction in radiation emissions and decay heat. Hence, the development of conditions that could lead to potential accidents during SNF storage in the storage facilities will theoretically occur comparatively slowly and the safety of SNF storage can thus be maintained with relatively unsophisticated protective systems. However, this statement is not related to the hazards associated with handling of SNF within the storage facility and with potential external events. A comprehensive description of safety aspects in SNF storage facilities is provided in IAEA Safety Standards and other publications[7][8][9][10][11]. Various aspects of SNF management are discussed in Refs[3][12][13][14][15].
For safe operation and maintenance of SNF storage facilities, their design incorporates features to keep the fuel subcritical, remove the spent fuel decay heat, provide radiation protection, and maintain containment over the anticipated lifetime of the facilities as normally stated in the design specifications. These objectives need to be met for all anticipated operational occurrences (AOOs) and design basis accidents (DBAs) in accordance with the design basis approved by the regulatory body.

Criticality

The design of the facility needs to assure subcriticality during loading, transfer, storage and retrieval.
In case of pool storage, subcriticality needs to be guaranteed for all credible water densities including boiling, the reliance on soluble neutron poison such as borated water needs to be avoided and solid neutron absorbers such as borated steel can be used. In case of dry storage, the fuel baskets and containers are normally designed to remain subcritical in all credible situations including the introduction of a moderator due to flooding. Detailed guidance on subcriticality issues in the design of SNF storage facilities is provided in Ref[7].

Radiation exposure

Radiation exposure of workers needs to be minimized. SNF has a high radiotoxicity. For example, the beta-gamma activity in PWR spent fuel six months after unloading from reactors still amounts to about 150 TBq/tHM[3] and the dose rate at 1 m perpendicular to the centre of SNF assembly for the first ca. 10 years after discharge remains above 10 Sv/hr[16].
In case of pool storage, usually the water level above the SNF (typically at least 4 m[3]) is used as radiation shielding for the protection of workers. This level is normally maintained in all credible situations by adequate water supply systems. Another source of radiation is radioactive material released from the SNF into the pool; thus, this material needs to be controlled and removed. The clarity of the pool water and sufficient lighting can help to reduce the time that workers spend exposed to radiation during operations at the pool.
In case of dry storage, SNF loading and unloading actions need to be performed in a way that limits reflection of radiation to the workers. To minimize internal exposure of workers, the concentration of airborne radionuclides (e.g. due to fuel failures) in closed facilities are assumed to be kept within acceptable limits by ventilation with filtering of the air.
Detailed guidance on radiation protection in the design of SNF storage facilities is provided in Ref[7].
Radiation exposure to the public and environment at normal operation conditions is discussed in the INPRO methodology manual on environmental impact of stressors[17].

Heat removal

Heat is generated in SNF due to the decay of fission products and actinides. For example, one year after discharge, the decay heat from a PWR SNF assembly remains higher than 10 kW/tHM and after 10 years it is still higher than 1 kW/tHM[16]. This heat has to be removed safely to avoid overheating of the SNF and subsequent failure, but also to keep the temperature of the equipment and structures in the storage facility below design limits. Pool storage facilities need reliable active heat removal systems. Dry storage facilities are expected to use passive cooling systems to the maximum extent practicable. Detailed guidance on the residual heat removal in the design of SNF storage facilities is provided in Ref[7].

Containment of radioactive material

Apart from potential releases of radionuclides during accident conditions, the SNF assemblies may release radioactive isotopes (e.g. solid and gaseous fission products like 85Kr, 134Cs and 137Cs) due to defects in the fuel cladding. Furthermore, the outer surface of the SNF assembly claddings may be contaminated with radionuclides during the reactor operation or previous storage. To avoid exposure of workers in wet and dry storage facilities, the radionuclides released respectively in the water and in the air have to be controlled and removed.
Ref[7] provides detailed guidance for considering issues related to the containment of radioactive materials at the design and operation stages of SNF storage facilities.

External hazards

External natural phenomena and external human induced phenomena that can influence safety of the SNF storage facilities are discussed in detail in Ref[7]. SNF storage facilities need to be designed against all credible external hazards (see Section 2.1 and 2.6 of NFCF).

See also

[ + ]
Assessment Methodology
Areas of INPRO Sustainability Assessment Overview • Economics • Safety (Nuclear Reactors) • Safety (NFCF) • Waste management • Environmental Impact on Stressors • Environmental Impact from Depletion of Resources • Infrastructure
Requirements Basic Principle • User requirements • Criteria

References

  1. ↑ 1.0 1.1 INTERNATIONAL ATOMIC ENERGY AGENCY, Storage of Radioactive Waste, IAEA Safety Standards, General Safety Guide No. WS-G-6.1, IAEA, Vienna (2006).
  2. ↑ INTERNATIONAL ATOMIC ENERCY AGENCY, Operation and Maintenance of Spent Fuel Storage and Transportation Casks/ Containers, IAEA-TECDOC-1532, IAEA, Vienna (2007).
  3. ↑ 3.0 3.1 3.2 3.3 OECD/NUCLEAR ENERGY AGENCY (NEA), The Safety of the Nuclear Fuel Cycle, Third Edition, NEA No.3588, OECD/NEA, Paris (2005).
  4. ↑ INTERNATIONAL ATOMIC ENERGY AGENCY, Disposal of Radioactive Waste, IAEA Safety Standards, Specific Safety Requirements No. SSR-5, IAEA, Vienna (2011).
  5. ↑ OECD/NUCLEAR ENERGY AGENCY (NEA), Establishing and Communicating Confidence in the Safety of Deep Geological Disposal, OECD/NEA, Paris (2002).
  6. ↑ INTERNATIONAL ATOMIC ENERGY AGENCY, The Safety Case and Safety Assessment for the Disposal of Radioactive Waste, IAEA Safety Standards, Specific Safety Guide No. SSG-23, IAEA, Vienna (2012).
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 INTERNATIONAL ATOMIC ENERGY AGENCY, Storage of Spent Nuclear Fuel, IAEA Safety Standards, Specific Safety Guide No. SSG-15, IAEA, Vienna (2012).
  8. ↑ INTERNATIONAL ATOMIC ENERCY AGENCY, Risk Informed Regulation of Nuclear Facilities: Overview of the Current Status, IAEA-TECDOC-1436, IAEA, Vienna (2005).
  9. ↑ INTERNATIONAL ATOMIC ENERGY AGENCY, Predisposal Management of Radioactive Waste, IAEA Safety Standards, General Safety Requirements Part 5, No. GSR Part 5, IAEA, Vienna (2009).
  10. ↑ INTERNATIONAL ATOMIC ENERGY AGENCY, The safety Case and Safety Assessment for the Predisposal Management of Radioactive Waste, IAEA Safety Standards, General Safety Guide No. GSG-3, IAEA, Vienna (2013).
  11. ↑ INTERNATIONAL ATOMIC ENERGY AGENCY, Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management, INFCIRC/546, IAEA, Vienna (1997).
  12. ↑ INTERNATIONAL ATOMIC ENERGY AGENCY, Long term Storage of Spent Nuclear Fuel- Survey and Recommendations, IAEA-TECDOC-1293, IAEA, Vienna (2002).
  13. ↑ BUNN, M., HOLDREN, J.P., Interim Storage of Spent Nuclear fuel- A Safe, Flexible, Cost-effective, Near-term Approach to Spent Fuel Management, A joint Report from Harvard University and University of Tokyo, (2001).
  14. ↑ INTERNATIONAL ATOMIC ENERGY AGENCY, Spent Fuel Performance, Assessment and Research, IAEA-TECDOC-1343, IAEA, Vienna (2003).
  15. ↑ INTERNATIONAL ATOMIC ENERGY AGENCY, Survey of Experience With Dry Storage of Spent Nuclear Fuel and Update of Wet Storage Experience, IAEA Technical Reports Series No. 290, IAEA, Vienna (1988).
  16. ↑ 16.0 16.1 INTERNATIONAL PANEL ON FISSILE MATERIALS, Managing Spent Fuel from Nuclear Power Reactors. Experience and Lessons from Around the World. Princeton University, Princeton (2011)
  17. ↑ INTERNATIONAL ATOMIC ENERGY AGENCY, INPRO Methodology for Sustainability Assessment of Nuclear Energy Systems: Environmental Impact of Stressors, IAEA Nuclear Energy Series No. NG-T-3.15, IAEA, Vienna (2016).
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