Aspects associated with burnup credit

Nuclear criticality safety was established as a discipline more than 50 years ago in response to several accidents that had occurred in nuclear weapons programmes. The importance of the safe handling of fissile materials was recognised at an early stage, both by the scientific community and the responsible authorities. Based on intensive experimentation with a large variety of configurations and materials substantial progress has been made in developing nuclear data and computer codes to evaluate criticality safety for nuclear fuel handling. The accuracy and reliability of computer code calculations has been extensively improved over the years. However, the criticality safety analysis of spent fuel systems has traditionally assumed that the fuel is fresh. This “fresh-fuel assumption” provides a well-defined, bounding approach to the criticality safety analysis. It eliminates all concerns related to the fuel operating history. However it ignores the decrease of reactivity with fuel burnup. This results in significant conservatism in the calculated value of the system's reactivity. The state-of-the-art calculation tools for criticality safety evaluations have led to reduction of the uncertainties in the evaluated neutron multiplication factors. It has allowed rational and more economical designs for manipulation, storage and transportation of fissile materials. Improved calculation methods allow taking credit for the reactivity reduction associated with fuel burnup.

The concept of taking credit for the reduction of reactivity is commonly called burnup credit.

The reduction that occurs with fuel burnup is due to the change, i.e. reduction, in concentration of fissile nuclides and the production of actinides and fission product neutron absorbers. This leads to a reduction in the analysis conservatism while maintaining an adequate criticality safety margin.

Many burnup credit applications were licensed early in the 1980S. Since the mid-1980s, the US utility industry, the US Department of Energy (DOE), and the US Nuclear Regulatory Commission (NRC) have actively considered the incentives, benefits, and obstacles associated with implementing burnup credit in the criticality safety evaluation for storage, transport, and disposal of spent nuclear fuel (SNF).

Burnup credit methodologies have been accepted for the criticality safety analysis of on-site wet storage of spent fuel at different power plants operating in several countries. Some applications, e.g. in Spain were licensed more than 10 years ago, and were considered acceptable at that time on the basis of the overall conservatism of the analysis [27, 28].

Burnup credit is defined as the consideration of the reduction in reactivity, associated with the use of the fuel in power reactors. Changes in the isotopic composition during fuel burnup, which result in a reduced reactivity can be conveniently characterized by the reduction of the net fissile content, the build-up of actinides, the increase of the concentration of fission products, and the reduction of burnable absorber concentration where applicable. In practice, the conservative use of burnup credit requires consideration of all fissile isotopes, and allows

consideration of any neutron absorbing isotopes for which properties and quantities are known with sufficient certainty. The different levels of burnup credit, which are commonly used are described as follows:

Credit for the net decrease of the fuel fissile content, taking into account both burnup and buildup of the different fissile nuclides (net fissile content level).

Credit for the net fissile content and for the absorption effect of actinides (actinide only level).

Credit for the actinides and the neutron absorption in fission products (actinide plus fission product level).

Credit for the presence of integral burnable absorbers in the fuel design (integral burnable absorber level). This credit uses the maximum reactivity of the fuel, which is often not the initial reactivity. Although not really consistent with the definition of burnup credit, it is generally considered to be a level of burnup credit because fuel depletion calculations are needed to determine the reactivity state as a function of burnup.

The incentives first emerged with spent fuel storage pools. Lack of off-site alternatives (i.e.

reprocessing, permanent disposal, or interim storage) provided significant incentives for utilities to obtain optimum use of the fixed pool storage capacity currently in place. This situation was aggravated by the demand to optimize pool storage space towards increased initial enrichments. This trend continues to the present. Thus the simple, yet conservative, assumption of using unirradiated fresh fuel for the criticality safety analysis became a significant economic barrier to continued operation of reactor power plants.

Efforts were initiated to evaluate the incentives and seek resolution of technical issues associated with the use of burnup credit in SNF storage and transport casks. In contrast to many countries where burnup credit is desired primarily to increase the allowable enrichment within existing cask designs, the USA nuclear industry is seeking to develop a new fleet of storage and transport casks that are optimized for the anticipated SNF contents. Rail casks with capacities of 32 PWR assemblies are being designed — an ~30% increase over existing storage cask concepts. These increased cask capacities can enable a reduction in the number of casks and shipments, and thus have notable economic benefits while providing a risk-based approach to improving safety. Arguments for improvement in safety have noted that the fewer shipments required with burnup credit cask designs will reduce the radiation exposure to both workers and the public as well as reducing the potential for a transport accident involving a cask.

Incentives for use of burnup credit in BWR applications have not been as significant as for PWR applications. The reason for this reduced incentive is that BWR fuels have less reactivity than PWR fuels and increased use of neutron poisons in intervening regions between assemblies have proven effective for maximizing capacities and allowing fairly high initial enrichments. Thus, the incentives are largely limited to reducing the cost of neutron poison plates and allowance for higher initial enrichment fuel (up to 5 wt% ²³⁵U).

The implementation of burnup credit has been dictated in many countries by different needs.

Some examples are as follows:

Introduction of higher enriched fuel in the existing storage, reprocessing or transport systems. In these cases, the use of burnup credit may alleviate the need for new installations or for extensive changes to existing facilities or equipment for spent fuel management activities.

Burnup credit can increase the storage capacity (both on-site and independent) by allowing smaller centre-to-centre distances in the fuel storage systems.

Burnup credit can be used for new casks, to increase cask capacities over current design capacities, to reduce the number of shipments needed.

In the area of spent fuel storage, burnup credit and its resultant capacity improvements can alleviate or minimize the environmental impacts associated with expanding or building new storage pools or dry storage facilities. Burnup credit can be used to maintain production rates at existing reprocessing facilities, even while fuel enrichments increase, thus avoiding the environmental impacts of constructing new facilities, or expanding old ones. For disposal of spent fuel, burnup credit is considered to be a necessity for any viable disposal scheme.

Ignoring the reduced reactivity from burnup credit could lead to larger disposal sites and unnecessary use of land.

Application of burnup credit requires knowledge of the isotopic inventory of the irradiated fuel for which burnup credit is taken. This knowledge is gained by using depletion codes. The uncertainty of a depletion code is controlled and established through verification of that code, usually by comparison with suitable and appropriate experiments. In-core reactor measurement data are important for verifications of depletion codes. For burnup credit applications, particular significance is attached to comparisons of calculated to measured isotopic concentrations.

Due to the depletion analysis, that has to be performed to determine the isotopic content, the results of the criticality calculations become dependent on the reactor operation conditions assumed for the depletion analysis. Due to the wide variety of fuel irradiation histories, it is necessary to look for a bounding irradiation history given by those fuel operation conditions that lead to the highest spent fuel reactivity in the criticality analysis.

The outcome of performing a burnup credit criticality analysis is the determination of a burnup credit loading curve for the spent fuel management system of interest.

The use of burnup credit implies a verification of the fuel burnup before loading for transport, storage, disposal, or reprocessing each assembly, to make sure that the burnup level achieved complies with the criteria established.

Criticality Safety Regulations for fuel storage and transport do not prohibit the use of burnup credit in the criticality safety analysis of spent fuel systems. In particular, IAEA Safety Series 116 [29] explicitly describes the implementation of burnup credit in storage facilities, and IAEA Safety Standard Series No. ST-1 [30] contains the requirements for transport. Likewise, national regulations do not prohibit the use of burnup credit for criticality safety analysis of spent fuel systems.

However, given the difficulty of validating the analysis codes used to demonstrate criticality safety and the challenge of demonstrating that all the relevant effects have been considered, no unrestricted burnup credit methodology has yet been approved. The regulatory authorities of different countries have approved the reactivity effect of a limited number of nuclides.

Which of the burnup credit levels have been approved, depends on the function and the characteristics of the given spent fuel management system.

To assist operators and dry storage and transport system designers in the implementation of burnup credit, several devices to corroborate the declared characteristic values of an irradiated

fuel assembly have been developed. Among these, the PYTHON (developed in France in collaboration between EDF and CEA), the BNFL burnup monitor, and FORK (USA) devices are the most well-known ones.

United States of America

In the USA, the regulations for transport and dry storage of SNF are promulgated in 10 CFR Parts 71 and 72, respectively. Current industry practice is to design spent fuel canisters that can be inserted into either dry storage or transport cask designs, thus eliminating the need for further handling of spent fuel assemblies once they have been loaded in a canister at the facility. The transport regulations of Part 71 provide the limiting condition for criticality safety, including consideration of water in-leakage to the canister. Although neither Part 71 nor Part 72 has any specific requirement that would prevent burnup credit from being implemented in the safety analysis, the historical practice to criticality safety analyses for transport and dry cask storage has been to assume the SNF is unirradiated, with uniform isotopic compositions corresponding to its maximum initial fresh fuel enrichment in ²³⁵U.

Because there has not been a licensing application using BUC to date, there has been no regulatory experience in the USA with licensing of a PWR or BWR transport cask. The US industry and the US DOE have supported a significant number of technical investigations, focused primarily on PWR fuel, to provide a foundation for implementation of BUC in the USA Based on this information, the SFPO issued Interim Staff Guidance 8 (ISG-8, Rev. 0), entitled Limited Burnup Credit in May 1999. Supported by initial confirmatory research, Revision 1 to ISG-8 (ISG-8, Rev. 1), entitled Burnup Credit in the Criticality Safety Analyses of PWR Spent Fuel Transport and Storage Casks, was released in late July 1999. ISG-8, Rev. 1 provides increased flexibility in the guidance for licensing of SNF casks. Some of these recommendations restrict the amount of burnup credit that can be used in the safety assessment (e.g. credit for fission products is not allowed) and some restrict the SNF population (type and range of burnup that would be allowed in a BUC cask). These recommendations were based on: 1) technical information available to NRC at the time ISG-8, Rev. 1 was issued; 2) consistency with the industry standards developed for criticality safety of fissionable material (ANSI/ANS-8.1) and LWR fuel (ANSI/ANS-8.17) in operations outside reactors; and 3) recognition that experience and additional research will provide a basis for additional guidance.

TablesX–XII provide a summary of BUC use and practice in the SPAR countries.

6.FUEL INTEGRITY