Accounting for variation in plant operating history, fuel design and spent fuel operation

Variations in plant operating history, fuel design and the spent fuel operation being evaluated have the potential to introduce perturbations to the neutron energy spectra – either during operations or in the spent fuel environment. These effects can alter the nuclide inventory at a given burnup and consequently the reactivity of the spent fuel condition. Likewise, conditions of the spent fuel application can directly impact reactivity. Any best estimate analysis of BUC

will need to assess these affects in detail. Lesser degrees of BUC require analyses detailed enough to ensure that claims of “bounding” or “conservative” can be justified. One can expect that the relative importance of these parameters will change for specific scenarios. Examples of concerns related to problem specific impacts include:

• Assess the most reactive assembly type and condition for each scenario;

• Assess impact of combining different fuel designs as complicated by BUC;

• Assure normal and off-normal effects are included in operating history;

• Assess the relative importance of nuclides for the following applications:

- UOX, - MOX,

- Storage and transportation, - Repository,

- Reprocessing.

• Depletion parameters (e.g. moderator density):

- PWR vs. BWR.

It is necessary to demonstrate an in-depth understanding of the system being evaluated and to consider that even local effects may impact some parameters in a non-conservative direction related to generic conditions accepted as bounding.

3.2.2.1 Axial and horizontal burnup profiles

The influence of the axial burnup profiles is generally called the “end effect” because the top and bottom ends of the fuel assembly have lower burnup mainly due to neutron leakage.

Differences between the top and bottom are due to the use of control rods and moderator density differences for PWRs. For BWR, the profile is influenced additionally by enrichment and void fraction profiles.

The axial burnup profile must be considered because keff calculated with a real axial profile may be higher than with the assumption of a flat one (i.e. assuming the average burnup for the assembly). The importance of axial burnup profile is dependent of the specific configuration and could be increased when these less burned ends come close together in an array configuration.

The axial burnup profile depends on the reactor operational conditions. In order not to be overly conservative, a database of axial profiles specific for these reactor-operating conditions should be available from measurements (in-core and/or out-of-core measurements of profiles).

A bounding profile or reactivity correlation (based on minimum burnup or preferably reactivity effect) may be derived from this database in such a way that using this bounding condition (i.e. profiles or reactivity correlations) ensures sub-criticality at the required level of confidence. The important issue is to have measurements (either from a database or specific measurements) to verify that the assumed axial distribution used in analyses is satisfied in reality. If not, the most reactive state of the fuel should be assumed. If the assumption about the spent fuel requires the definition of a bounding axial profile (i.e. the assumption of a flat profile is demonstrated not to be conservative), also this axial burnup profile must be shown to be conservative by a pre-shipment measurement. The details of the measurement process and requirements will be addressed by the working group investigating safety assessment and implementation.

Some accident conditions may change the geometry of the transport or storage system. During the criticality analysis these changes have to be considered. Particular attention has to be paid to the less burned ends.

Generally the influence of the horizontal variation of the burnup is much less than the influence of the axial variation. Significant horizontal burnup profiles are possible for fuel assemblies that may have been inserted close to the periphery of the reactor core, next to a different type of fuel (MOX or UO2) or in the case of the WWER, close to a control assembly.

The effect of these horizontal distributions should be addressed for these situations.

Every criticality study relies on both a conservative assumption about the composition of the spent fuel and the spatial variation of this composition.

3.2.2.2 Criticality analysis modeling (consideration beyond fresh fuel assumption)

Issues associated with the criticality analysis modeling are principally the same as for the analyses using the fresh fuel assumption for out-of-core spent fuel management. Established good practices for developing nuclear criticality safety evaluations should be followed. There are two notable differences that require special attention in these evaluations. First, due to the axial distribution of burnup and therefore nuclide inventories across the fuel, there is a greater tendency for BUC calculations to suffer complications related to source convergence issues.

Since the fuel is underburned at both ends, the problem is one of a loosely coupled system.

The standard methods of addressing source convergence should be used as needed. These methods include careful observation of the results to assure convergence, increasing the number of neutron histories and varying the starting distribution of neutrons. Secondly, the increased complexity of the material descriptions required to represent the nuclide inventories require additional modelling detail of fuel regions to account for burnup distributions. In some cases this is best addressed by developing a systematic approach to interface the criticality code with the depletion code. Updates and improvements to the nuclear data libraries may also be required to accommodate use of these nuclides. In fact, even the determination/verification of which nuclides are important to be considered in the analyses may be specific to the application (e.g. curium isotopes are extremely important in evaluating BUC for MOX fuel and have little or no importance for UO2 fuel in LWRs).

3.2.3 Conclusions and recommendations

BUC is neither the classical out-of-reactor criticality analysis nor is it a typical fuel management assessment. Both of these technical areas are well established and have many lessons learned to offer to the analyst/engineer assessing BUC. Bringing these two engineering disciplines together is a scientific problem requiring study and understanding of the associated physics. The basic requirement is to demonstrate an understanding of the problem that is to be solved. The level of detail required to do this is related to the amount of credit one is seeking to obtain as well as that which can be justified based on independent experimental and operational experience.

The key issues addressed in this document are based on the experience and lessons learned from a number of experts with considerable experience in BUC and/or plant operations. These key issues should provide some insight to the BUC analyst as to the level of detail that would be required in a best estimate analysis of BUC. Analytical experience in specific applications is the best guidance in establishing an acceptable technical defense for conservative models

seeking a lower level of BUC (i.e. fissile depletion, or some form of partial BUC such as actinide only, etc.). Based on the experience of the analyst and/or regulator, either qualitative or quantitative arguments are acceptable in the justification of the relative importance of these parameters to the overall assessment of BUC.

The working group concluded that training workshops for BUC should continue to be organized and presented. There was discussion among the group that it might be reasonable to organize a meeting jointly with experts from fuel cycle management and in-core fuel analyses to discuss issues associated with the depletion parameters. Some felt that this could be problematic if one tried to separate the topics of depletion and criticality since qualifying the importance of the depletion parameters must be kept within the context of the criticality analysis.

The working group also concluded that a document should be produced summarizing and making available the BUC regulatory requirements in different member states (e.g. for storage), and transportation including status of any requirement for measurements). This report could be based on the current information compiled in the questionnaire circulated prior to this meeting and confirmed during the working groups that indicates where member countries have approved BUC or are considering BUC.