4. GROUP DISCUSSIONS
4.4. Regulatory aspects in burnup credit
4.4.3. Conclusions about burnup credit implementation
4.4.3.1. Burnup credit is useful and viable to develop and implement
There has been much interest by the nuclear industry in including credit for the burnup of irradiated fuel in the criticality safety analysis of spent fuel operations. It is recognized that significant benefit can be derived from the implementation of burnup credit in the storage, transport, disposal, and reprocessing of spent fuel. The benefits include economic savings, reduced risk, and the passive presence of the effect of burnup.
Under burnup credit, the capacity of spent fuel casks can be increased resulting in the need for fewer numbers of casks for dry storage and transport. Burnup credit can be used as an alternative to poison plates or other neutron absorbing components. Reduction in the loading and/or number of neutron absorbing components results in reduced cost for manufacturing spent fuel casks.
Cost savings in pool storage can occur by allowing closer spacing of fuel which increases the storage capacity of the pool. Utilizing burnup credit in the design of a disposal site can reduce the size and cost of the disposal field as well as reducing the cost of the disposal unit.
Employing burnup credit in the design of reprocessing dissolvers can result in cost savings from increased throughput, simplified design and operations, and reduced waste streams.
In addition to the above economic benefits, a reduction in the number of spent fuel casks
results in fewer loading operations for the same number of spent fuel assemblies. This
reduction in the number of loading operations results in reduced operational costs as well as a
reduction in operational exposure. Fewer casks also mean fewer spent fuel shipments. This
reduction in exposures and number of shipments, results in a general reduction in the overall
risk from activities involving spent fuel.
4.4. Upper Limits klimit of the Neutron Multiplication Factor in Burnup Credit Applications in Different Countries (Figures in italics are s) * = proposed limits, ** = no dry storage, *** = not yet decided, OM = optimal moderation, SP = single package, PA = package array Armenia Bulgaria China Czech Republic FranceGermanyHungary Lithuania Slovakia Spain SwedenUKUkraineUSA l itionsWet Storage 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 Dry Storage / Transport 0.95 0.95 0.95 0.95 SP: 0.95** PA: 0.98**
0.95 0.95 0.95 0.95 0.95 0.95** 0.95** 0.95 0.95 Disposal- - - - - 0.95* - - - - *** - - <1.0* ssolver (Reprocessing) - - - - 0.97 – 0.98 (case dependant) - - - - - - 0.92 – 0.98 (case dependant)
- - nt itions
Wet Storage 0.95 0.95 0.98 0.95 (OM: 0.98) 0.95 – 0.97 (case dependant) 0.95 – 0.98 (case dependant) 0.98 0.95 0.98 OM 0.95 0.98 0.95 – 0.98 (case dependant)
0.95 <1.0 Dry Storage / Transport 0.95 0.95 0.95 (OM: 0.98)
0.95 (OM: 0.98)
SP: 0.95** PA: 0.98**
0.95 0.98 0.95 0.98 OM
0.98 OM0.98** 0.95** 0.95 0.95 Disposal - - - - - <1.0* - - - - ***
- <1.0* ssolver (Reprocessing) - - - - 0.97 – 0.98 (case dependant)
- - - - - - 0.92 – 0.98 (case dependant)
- -
A third benefit of burnup credit to criticality safety is the fact that burnup effects are a passive feature. The effect of fuel burnup is an ever present property of the fuel and does not need any special operating procedures or administrative controls for it to be activated.
Because of these features, burnup credit is being actively pursued by many groups in a number of the member countries. Their efforts have resulted in a number of reports and documentation in the open literature. The development efforts in burnup credit include technical studies of its effects, experimental measurements, and assessments of the benefits.
As a result, the general phenomena and benefits of burnup credit are fairly well known and understood.
Based on the available information, the group concluded that the development of burnup credit has matured to the point where it is worthwhile to pursue and has great promise for implementation. Although additional work needs to be done in the development of burnup credit before its full potential can be realized, it is felt that the benefits can be significant and such development is worthwhile. Thus, continued effort in the development of burnup credit is encouraged and it is recognized that when a proper technical foundation is presented, burnup credit can be an acceptable safety measure.
4.4.3.2. Important to set standards
There is a general consensus that the application of burnup credit is a very complex issue, which requires highly sophisticated methodologies for calculating burnup and depletion values as well as challenging criticality calculations for spent fuel assemblies under a variety of conditions. Codes have to be validated, and safety margins and uncertainties of every step of an analysis must be determined. Compliance with existing safety criteria must be ensured.
Therefore it seems very advantageous, from the regulator’s point of view as well as from the viewpoint of applicants, to have standards and/or guidelines for the application of burnup credit.
The degree of detail given in a standard depends on the safety approach, the method of application, and safety philosophy. In a standard, the basic steps of a criticality analysis including burnup credit considerations should be described and safety limits should be documented. The following topics should be covered:
•
Required conditions for inventory calculations for the spent fuel
•
Specification of isotopes (Actinides and Fission Products (FPs)) allowed for consideration
•
Demonstration that the assumptions and approach are appropriately conservative
•
Specification of safety margins, which need to be determined
•
Validation of the applied codes for calculating isotopic inventory and criticality
•
Requirements concerning uncertainty analyses
•
Requirements for determining the actual burnup of the spent fuel
•
Requirements to ensure procedural compliance with the safety criteria
•
Requirements on risk informed methods, if in compliance with the safety approach of
the respective country.
Several countries have already developed regulatory and/or guidance documents on burnup credit implementation for wet and dry storage and transportation of spent nuclear fuel.
Examples are the interim staff guidance (ISG)-8 of the USA and the DIN standards 25471 and 25712 of Germany.
The group agreed that the development of international standard, guidelines or recommendations would be very helpful for those countries, which plan to allow burnup credit application in the future. Further, international guidelines could be useful for countries which have already developed regulatory documents so they can review these recommendations and, if desired, make their regulations consistent with the international guidelines.
4.4.3.3. Industry and public involvement in standards development
Regulatory guidance is often created by the regulators and supporting organizations.
Guidance development has benefited from discussions with the industry during the draft phase. However, the regulators give final approval to the guidance.
Industrial standards (such as the DIN standards) are normally created by a group of specialists from industry, the regulators, and expert organizations. Such standards are the result of discussions within this group and may not necessarily result in full agreement by every member of the group.
8)The draft of the standard is often published for comment by the public.
The existing standards and regulatory guidance establish conditions for the application of burnup credit in the country of origin. The legal state of the document will determine how closely the applicants and regulators must follow it. The standards and guidance help applicants by specifying requirements that shall be met for a successful application. On the other hand, during the preparation of the standard/guidance the knowledge of the regulators and the applicants about the process of implementing burnup credit can be significantly improved (by discussions and substantial contributions from the participants).
The discussion group believes it is important that the nuclear industry and any other interested parties are given the opportunity to comment on the proposed burnup credit standard.
It is felt that a wide consultation will lead to a greater understanding and appreciation of the issues that are important to each of the stakeholders. In turn, this will lead to the production of a burnup credit standard that meets the needs of industry as well as being acceptable to other interested parties. However, it must be recognized that some countries may have restrictions in place that would limit or prohibit public consultation.
It is also considered appropriate that once a standard is issued, it must be reviewed on a regular basis (for example, every five years may be appropriate) to ensure that its provisions continue to meet the needs of the industry, the regulators and other stakeholders. It is appropriate to leave the form of the consultation up to each individual country (a possible method could be to post a notice on a designated website).
8) Comment (J.C. Neuber): This is also true very often for non-industrial regulatory standards (as for instance
4.4.3.4. Need for more assay measurement data
Knowledge of the nuclide composition of irradiated fuel is necessary when performing a burnup credit analysis. The group agreed that there was an important need for more measurement data that are publicly available. The areas of greatest need exist for data on VVER-440/1000 fuel, high burnup fuel and MOX fuel. These data are needed to validate and benchmark the depletion codes used to provide a calculated estimate of the isotopic inventory in the spent fuel based on its irradiation history in the reactor core. Although data needs exist for validating both criticality and depletion calculations, the group recognized a particular lack of PIE assay data.
To deal with the task, the following has to be “well documented”:
•
full description of initial fuel composition (not only U235 and U238, but some of the typical impurities in fuel as U236), as well as, cladding composition and exact dimensions of pins, cladding, shroud, etc.
•
full description of irradiation history including exact power (either relative or absolutely), time length of operations and outage, and position in assembly (pin and high), for each measured sample
•
full description of measured parameters (burnup, error of measurement, cooling time, isotopic composition) for each measured sample.
4.4.3.5. Performing code validation and setting biases is necessary
While it is important to take care in performing all parts of a burnup credit analysis, the group believes that the area of code validation needs to be emphasized as requiring particular attention. Validation and a conservative assessment of uncertainties (biases, tolerances, operational data, etc.) for computer codes and their corresponding libraries is a strict requirement which is necessary for burnup credit applications.
Validation of the criticality codes as well as the depletion methodology must be performed using experimental data. Experimental data need to be of a similar fuel type to the fuel type on which the burnup credit analysis is performed. Validation of the depletion calculation methodology should be carried out against PIE assay data (isotopic composition) for the specified fuel.
Also, the validation process should determine the range of parameters for which the code is valid and can be applied for burnup credit analyses.
4.4.3.6. Use of risk informed considerations should be investigated