CROSS CUTTING ISSUES

A number of cross cutting safety and technical issues were identified. These were placed into the following four categories:

• Fuel design ramifications;

• Compatibility of fuel with acceptance criteria for present and future phases;

• Compatibility of packaging and equipment between shipping and receiving facilities;

• Knowledge management/adequacy of records.

3.2.1. Interfaces between front-end of fuel cycle and BEFC

Fuel design requirements should include BEFC performance considerations. Fuel design and irradiation conditions affect the spent fuel characteristics which in turn affect individual safety assessments, risks, available options and costs of handling, transportation, storage, reprocessing, and disposal. For example, although a move to higher fuel burnup typically reduces front-end costs, it adds cost on the back-end by increasing storage times and the potential for personnel exposure necessary storage, handling, reprocessing, and disposal. High burnup has been found to exacerbate all phases of the management of spent fuel, particularly through increased heat loads and increased susceptibility to degradation [2]. Another example is the effect of fuel design and burnup on cladding condition and its susceptibility to failure during storage and transportation.

Although fuel design decisions have significant impacts on the available options and costs in spent fuel management, BEFC considerations are often not explicitly included in design criteria of nuclear fuels. Fuel design and reactor operations decisions should ensure compatibility with all steps of the BEFC, i.e. storage, transport, reprocessing, and final disposal. Similarly, BEFC facility operators and facility/equipment designers should consider a range of fuel design parameters with the objective of accommodating evolving fuel designs.

Discussion and negotiation are necessary in order to evaluate how specification changes could affect BEFC safety assessments, risks, costs or available options, and to reach a consensus that satisfies the requirements of the fuel vendor, nuclear power plant (NPP) operator and of BEFC management.

A successful BEFC is a key to growing the industry. Hence, it is in the interest of fuel vendors, nuclear power plant (NPP) operators, and BEFC facility owners/operators to work together to ensure that decisions are made in view of their impacts on the entire fuel cycle

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TABLE 1. THE PHASE – PHASE MATRIX Wet storage

(AR and AFR) Transport Reprocess and disposal direct disposal Front end of fuel cycle

and reactor operations See section 3.2 See section 3.2

Wet storage

(AR and AFR) See section 3.3.1 See section 3.3.2 See section 3.2 Dry storage

(AR and AFR) See section 3.3.1 See section 3.3.2 See section 3.2

Transport See section 3.3.3 See section 3.2

Reprocess and disposal

direct disposal See section 3.2

3.2.2. Maintaining compatibility of fuel with acceptance criteria for present and future phases

Receiving facilities should specify acceptance criteria at the earliest opportunity in order to establish requirements for fuel condition and equipment compatibility as well as the information and records needed to demonstrate compliance.

The fuel owner is responsible for ensuring that the spent fuel and any associated packaging fulfil all relevant requirements such as:

• Physical compatibility (size, weight, structural integrity, etc.);

• Compatibility with safety basis (thermal, radiological, criticality, etc.);

• Compatibility with handling, transport and storage requirements, including suitability for retrieval and transport after the anticipated storage period;

• Known or likely requirements for subsequent disposal or other management aspects included in the owner’s spent fuel management strategy, such as the need for further treatment or conditioning of spent fuel;

• Damaged fuel must be identified and placed into a condition that meets acceptance criteria.

The following are major considerations relative to ensuring compatibility with acceptance criteria.

3.2.2.1. Thermal load and dose limits Cooling pool

Cooling pool design should ensure adequate removal of decay heat likely to be generated by the maximum inventory and heat load of spent fuel anticipated during operation. The design of heat removal systems should include an additional margin of heat removal capability to account for processes likely to degrade or impair the system over time. In determining the necessary heat removal capability, the post irradiation cooling interval and the burnup of the

fuel to be stored should be a major consideration. Redundant or diverse heat removal systems should be provided.

Timing

Thermal load and dose limits for the transport cask will determine the length of time that the fuel has to cool before it can be transported. In some cases, cask thermal limits for storage may be more restrictive than for transport. Thermal loading often establishes the timing when fuels may be moved between subsequent phases. Thermal loads are affected by fuel burnup and fuel type (i.e. MOX fuel). Higher heat loadings and/or cask loading constraints may require longer cooling times. As a result, additional cooling pool and heat rejection capacity may be required to maintain sufficient cooling pool capacity for defueling. Alternatively, additional storage casks and/or storage casks with additional heat rejection capacity may allow shorter cooling periods and thus offset a need for additional cooling pool capacity.

Higher heat loads will also affect timing, throughput, and dose during transportation and subsequent reprocessing and disposal.

Issues associated with cask thermal loading have become increasingly important due to higher burnup fuels and larger capacity casks.

MOX fuel

The increased source term for MOX fuel from reprocessing increases both heat loading dose during fuel fabrication and handling. This affects reactor safety analyses, cooling time, cask loading limitations, etc. In addition, the presence of increased quantities of plutonium in the fuel may necessitate more rigorous safeguards requirements during transport.

3.2.2.2. Cladding integrity

Events during storage or transport may change the spent fuel integrity so that it might require remediation, repackaging, or other design solutions to ensure acceptability for post storage transportation, reprocessing, or disposal.

Fuel design and burnup history affect fission gas production and resultant stresses in fuel and cladding. Fuel cladding creep and rupture temperature is related to cladding stress state, which may be a factor in cask and facility safety analyses. Stress state of the fuel may also influence the tendency for hydride reorientation in the cladding. Some current fuel designs result in excess hydrogen in cladding that, at high temperatures, may result in hydride reorientation.

The resulting loss of ductility may limit the fuel’s ability to withstand transportation accident scenarios.

Failed fuel should be identified and documented to ensure that proper accommodations are made during subsequent phases. Assessment of spent fuel integrity may be necessary prior to acceptance and placement in storage. Damaged fuel or spent fuel debris must be identified and placed into a condition that meets acceptance criteria. Suitable equipment shall be provided to identify failed fuel bundles for the purpose of segregated storage and subsequent reprocessing. These include equipment such as sniffing facility, periscope, etc. for direct or remote inspection of fuel bundles and other irradiated components.

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3.2.2.3. Dryness

When fuel is loaded into a cask, characterization may be needed to confirm compliance with acceptance criteria (dryness, thermal, etc.) before fuel is accepted. Drying and demonstration of dryness is normally done at the wet storage facility where fuel is loaded. This may impose requirements on the receiving and/or the shipping facility to demonstrate or evaluate that criteria are satisfied and/or to accommodate any discrepancies. The evaluation of the moisture content is an important step in the demonstration of the absence of hydrogen build-up in the cask, and degradation of the fuel due to corrosion and galvanic action. It therefore requires adequate equipment and tools, accurate procedures, maybe adapted facilities, and exacting quality control.

3.2.2.4. Undefined or changing disposal criteria

Acceptance criteria need to be known in order to enable storage conditions, packaging, and other decisions that assure criteria can be reasonably met. However, acceptance criteria for the disposal facility are often not available and/or may change after storage decisions are made. In many countries, disposal time is far in the future and the criteria for acceptance of spent fuel on the disposal site are not well defined. In some cases development of different final disposal concepts for spent fuel could lead to different types of casks or canisters. The fuel owner may have to demonstrate that storage conditions did not jeopardize the compatibility of his fuel with final disposal criteria (e.g. cladding may or may not be credited in repository safety basis). Casks loaded with the objective of optimizing dry storage may not meet disposal criteria. Repackaging, which increases cost and exposure, and/or the need to select a repository site and design disposal facilities for existing sub-optimum packages could be avoided by thoroughly identifying the interfaces and addressing the issues in advance.

Until final disposition is defined and implemented, it is not possible to determine if spent fuel can meet the acceptance criteria without further conditioning. Hence, efforts should be made to preserve flexibility for future options.

3.2.3. Ensuring compatibility of packaging, equipment, and operational controls between shipping and receiving facilities

3.2.3.1. Compatibility of packaging for damaged fuel

If damaged fuel is present, special equipment, packaging, or processes may be required.

Storage packaging may or may not be designed for transportation (and vice versa). Likewise, the packaging may or may not permit monitoring/inspection of fuel condition, which may affect transportation decisions. For damaged fuel, perform additional safety analyses is required to determine the need to modify the transport package and/or its safety basis and authorization.

3.2.3.2. Compatibility of equipment and facility

Cask and fuel handling and other equipment interfaces must be designed and managed to ensure the cask can be handled at both shipping and receiving facility. Similarly, other facility constraints such as overhead clearances and crane capacities at the receiving facility, shielding, etc. must be compatible with the package being shipped.

3.2.3.3. Contamination controls

Contamination controls on the cask and transport conveyance must be effectively understood and addressed by the shipper and receiver.

3.2.4. Knowledge management and adequacy of records

The safe and cautious management of the spent fuel records and information is necessary to demonstrate compliance with facility acceptance criteria and to enable transfer of ownership.

Records from manufacturing and reactor operation must include sufficient information on handling and developments during storage to demonstrate that the acceptance requirements for subsequent phases of the BEFC are satisfied (preferably without the need for re-opening the cask) and records management must have sufficient controls to ensure their acceptability.

Adequate records are also necessary to support the decommissioning of storage and other BEFC facilities. Some examples of good practices are summarized below:

• If the acceptance criteria for the spent fuel endpoint are not clearly specified, preserve comprehensive information in order to preserve the ability to demonstrate compliance with future acceptance criteria;

• To support future transfer of ownership, maintain a complete information chain of the spent fuel history (fabrication, operations, and storage);

• Establish and undertake an operating experience feedback (OEF) programme to collect, screen, analyse, and document operating experience and events at the storage facility in a systematic way. Consider also relevant operational experience and events reported by other facilities;

• Gather, safeguard, and periodically update digitalized media containing any information considered useful for the future management of the spent fuel.

3.3. EVALUATION OF THE PHASE TO PHASE MATRIX