Information on the spent fuel to be stored

Technical information on the spent fuel to be stored is the very basis for the design of the facility and associated systems. It would include, among others, inventory, locations, types of fuel, technical characteristics, etc. There is a vast and increasing array of fuel designs which are still evolving as advanced reactor systems are coming into service and high burnup fuels, mixed oxide (MOX) and other new fuel designs are becoming common place.

The need for spent fuel data depends on the use of the required data. There are several levels of users of the spent fuel as following.

• Utility (generator) and organizations assigned with spent fuel management,

• National level regulator and stakeholders, and

• International organizations.

Because of the current status of institutional arrangement based on the “polluters pay”

principle adopted by the majority of countries, the primary users of spent fuel inventory data are the utilities which supply relevant data to the national regulator as required. There have

10 Also known as Bids Invitations Specifications (BIS).

also been several international cooperation initiatives on spent fuel management on regional level, but so far without any tangible results.

In general practice, the information on nuclear fuel is passed over from one operator to another in the fuel cycle (sometimes attached with the fuel shipment as a manifest) together with the physical and legal transfer of management responsibility and in many instances, ownership of the spent fuel.

The information on spent fuel may be broken down into the following categories:

3.2.1.1. Spent fuel arising and storage projection

The key information required for the determination of the throughput of the AFR facility and the establishment of its operating scheme includes the annual production rate and the cumulative amount of spent fuel taking into consideration the factors mentioned in previous section. Based on this information, a projection of needs for the spent fuel storage is made and the total capacity required is estimated including later potential expansion of the facilities.

It may not be possible or necessary to foresee fully the envisaged AFR spent fuel storage capacity since nuclear programs in most Member States are continuously changing. Decisions will have to be made according to available projections of spent fuel arising and remaining pool capacity at the nuclear plants. Allowance would be required at the AR pools for contingencies such as removal of reactor fuel load in emergencies (referred to as core discharge) and pool operational contingencies. Allowances would also be required to deal with potential project delays in planning AFR storage. A staged, modular approach may well be more appropriate to satisfy immediate needs (i.e. several years of storage) of capacity building and for planning provisions for future extensions. Future requirements could be included at the initial design stage at a preliminary conceptual level and refined at the time of modular expansion of the AFR storage systems.

3.2.1.2. Spent fuel types

There are several types of spent fuel including pressurized water reactor (PWR) fuel, boiling water reactor (BWR) fuel, mixed oxide (MOX) fuel, Canada Deuterium Uranium (CANDU) fuel, other pressurized heavy water reactor fuels and advanced gas cooled reactor (AGR) fuel.

Fuel types differ not only among reactor types, but also among various vendors who manufacture different fuel for different reactor types (such as Babcox and Wilcox, Combustion Engineering, Westinghouse, Framatome and Russian reactors) who use customized fuel designs of differing enrichment and burnups¹¹.

It would be necessary to recognize the impact of individual fuel types on the AFR storage in cases where an AFR storage system is to be designed for multiple use of spent fuel from many reactor types.

11There are various types of research reactor fuels, experimental assemblies, reactivity booster assemblies, spent fuels from the earlier reactor types, and fast breeder reactors (FBRs), which may not fall into the above categories but may require to be considered for AFR storage.

3.2.1.3. Spent fuel characteristics

Spent fuel is characterized by the changes that occur during the in-service operation of the nuclear fuel in a reactor. These include depletion of the fissionable isotope, such as ²³⁵U and concentration of several hundred fission product nuclei in the fuel. The degree to which such changes occur depends on the burnup of the fuel, i.e. amount of energy produced by the fuel per unit mass of the fuel (expressed usually in MWd/kgU). All of the nuclei are subject to radioactive decay, some of which take hundreds of thousands of years or longer. These fission products are normally contained within the ceramic fuel matrix in the containment envelope provided by the fuel cladding. With suitable shielding of the external radiation in the spent fuel, adequate protection can be provided during handling and storage. In the case of defective fuel, however, leakage of radionuclides from the fuel would be an additional consideration [33], [34]. Heat production in the spent fuel is a direct consequence of the radioactive decay and is a significant factor to be considered in the design of storage systems.

All spent fuel related factors affecting the storage system should be determined. Key safety objectives that require accurate spent fuel data are sub criticality assessments, heat removal assessments and radiation shielding calculations. Characteristics of the spent fuel assemblies to be stored should include at least the following parameters:

• Assembly/bundle identity (serial number of assembly/bundle),

• Physical description (fuel and clad type and geometry, post-irradiation form, mass),

• Initial enrichment and discharge burnup (composition, materials, isotopes, etc.),

• Irradiation history (residence times in the core, linear power rating, reshuffling schemes etc.),

• Age of spent fuel after removal from the reactor,

• Information on defective or leaking fuel, with possible logging of water (important for long term safety requirement), and

• Any unusual features of particular fuel assemblies (experimental assemblies, boosters etc.).

Spent fuel characteristics are generally tracked by highly developed and complex codes during in-service operation and information is generally available to a high degree of sophistication. Further evolution of spent fuel characteristics during the storage period (decay heat, radioactivity reduction, radiation-induced effects etc) as well as monitoring technologies for spent fuel integrity/degradation during storage is also reaching a mature level of understanding. The above information is generally compiled on an individual assembly basis, since some of these factors could widely vary from fuel to fuel depending on its power history. For this purpose, a readable number embossed on it by the manufacturer is generally used to identify the fuel assemblies and to correlate their data. Based on this information, and using appropriate computer codes, various other information needed for the storage system design and for various assessments can be further developed for the fuel assembly, such as decay heat output in the fuel, fission and actinide product inventories, external radiation data, and detailed database required for meeting safeguards requirements.

It is important to recognize that spent fuel is made of reactive materials and will be subject to physical and chemical changes over time. These changes may affect the overall safety and integrity of the spent fuel in storage and therefore the overall safety of the storage system.

Adequate provisions must be made to take account of these changes that may arise both during irradiation and following discharge from a reactor.

Overall, some effort may be required to define acceptance conditions for spent fuel in the AFR storage facilities such that AFR storage design specifications can be developed compatible with the received spent fuel. This will require cooperation between the NPPs and the project staff such that any extraordinary technical difficulties can be identified in advance and resolved in the best possible manner. NPPs will also be the keepers of operational information on stored fuel at the reactor sites, particularly information on fuel failures or damages during in-service and later on during handling and storage at the reactors, which would be necessary in customizing the AFR design to spent fuel condition.

It should be noted that fuel characteristics change as nuclear fuel and reactor technology advances in various countries, such as from increased burnup of fuels, use of recycled plutonium in fuels (MOX and mixed carbide fuels), achievement of higher densities and other improvements, leading to trends in spent fuel characteristics different than those for current reactor fuels. Based on the characteristics of the spent fuel existing at the time of the selection of the AFR storage facility, some allowance may have to be made to make room for the future development of the fuel used in the reactors. The modular approach for AFR storage will allow the required flexibility to take into consideration any unforeseen changes that could take place in the future including changes to fuel characteristics, containers, regulatory requirements, and the knowledge base of storage systems. A modular approach as well as adaptive measures incorporated during design will also allow future improvements in the storage systems themselves to be accommodated based on the lessons learned in the initial stages and from feedback from the storage operations.

3.2.1.4. Defective fuel

Defective fuel may require special attention in terms of canning them prior to storage in an AFR (if it is not done already at the reactor pools). Operational objectives and safety approaches for defective fuel may differ for NPPs and for AFR storage facilities. What may be considered generally acceptable for at-reactor storage may not be suitable for AFR storage due to potential for contamination during transportation and long term storage. This may require special size containers for storage as well as transportation if they do not fit into standard containers. An agreed upon criteria (usually based on sipping procedures) would be established to identify defective fuel that need placement in a sealed can.

Adequate provision of information on the defectiveness (with possible logging of information) is particularly important for design of dry storage systems, as evidenced by the spent RBMK fuel storage project in Nuhoms system at Chernobyl [31].

At the AFR storage specific detection systems would be considered to confirm NPPs’ data as spent fuel is received, confirm fuel integrity during further storage, and ultimately confirm fuel integrity at the time of fuel retrieval.