REGULATORY ASPECTS OF BURNUP CREDIT IMPLEMENTATION

J.M. CONDE

Consejo de Seguridad Nuclear, Madrid, Spain

1. INTRODUCTION

Burnup credit methodologies have been accepted for the criticality safety analysis of all the on-site wet storage of spent fuel at all the plants in operation in Spain. Some applications were licensed more than 10 years ago, and were considered acceptable at that time on the basis on the overall conservatism of the full analysis process. While some specific steps of the methodology raised doubts about their accuracy or conservatism, those doubts were duly compensated by a quantifiable high conservatism in other parts of the analysis.

The progress made in the knowledge and understanding of the phenomena involved in burnup credit, and the evolution of the codes and libraries used for the analysis have helped to solve some of the concerns present in the old analysis. However, some aspects remain to be further clarified and solved. A brief review of some of those issues is given in the following paragraphs.

2. THE VALIDATION ISSUE

Burnup credit analyses involve two main calculations: fuel depletion and management system’s reactivity. The first is aimed to determine the isotopic concentration of the different nuclides in the spent fuel as a function of burnup, given the fuel design characteristics and the depletion conditions. The criticality calculation determines the reactivity of the specific system based on the isotopic composition of the spent fuel determined in the prior step.

All the rules and regulations that apply to criticality safety demonstration ask for a detailed validation of the codes used in the process. Therefore, validation of both the depletion and the criticality code should be performed in burnup credit analysis, using experimental data covering a range coherent with the conditions to be analysed.

Validation of the depletion code should in principle be based on chemical assay data of spent fuel with the adequate burnup. The first problem found is that there is little chemical assay data available. Many interesting data is of a proprietary nature. The public data pertains mainly to major actinides, while data on fission product nuclides is very scarce.

Moreover, the majority of the public data can be considered “old”. Most of it was obtained from old fuel designs, and the burnup range is in general less than 40 GWd/MTU. Also, there is little data for enrichments above 3.5 wt%. It can be concluded that the data available is not representative enough of present-day fuel and operating strategies. An additional problem is that the lack of data impedes the determination of trends on the code behaviour when important parameters are varied (burnup, enrichment,…).

Likewise, the criticality code should be validated by comparison with critical experiments performed with spent fuel of adequate burnup values. The same problem is again found here:

no critical experiments with spent fuel are available. Validation against fresh fuel critical experiments allows the user to get confidence in the correct behaviour of the code, but it is clear that the use of the code for spent fuel analysis is not fully covered. The lack of spent fuel critical data is explained by several facts:

1. A critical state is difficult to obtain with spent fuel only. If fresh fuel is included, there will always be the doubt on whether the code bias obtained can be applied to systems containing spent fuel only,

2. The difficulties of such an experiment from a radiation protection standpoint are high, 3. The correct modelling of a spent fuel critical experiment requires either a fully validated

depletion code, what is difficult to achieve as already discussed above, or sufficient chemical assay data of the fuel used for the experiment.

Given these facts, it is not expected that the number of spent fuel critical experiments will be increased in the near future. However, some reactivity worth experiments, in which the known reactivity of a fuel system is modified by including a certain amount of a fission product, are being carried out or planned.

Other validation option is that known as “integral validation”, the best example of which is the use of reactor core critical states as reference for validation. The reactor core is modelled with the depletion code, and the core reactivity is determined with the criticality code. The comparison with the measured critical boron concentrations, control rod patterns and core power distributions provides an overall quantification of the accuracy of the complete calculation system (depletion and criticality). It is felt that this kind of work gives confidence in the adequacy and quality of the calculation system. However, it is difficult to accept this approach as a validation if not complemented with additional validation efforts, due to the following reasons:

1. The uncertainty of the full calculation system is determined, and not that of the individual codes. Undetected error compensation between the different intervening codes is possible,

2. Fuel with different burnup levels is present in the core. The dependence of the uncertainty with the calculated burnup level cannot be determined, as only core average values can be obtained. Error compensation may again occur regarding this aspect, 3. Plant measurements can have a high uncertainty, and are difficult to use as benchmark

quality data.

In addition, reactor critical conditions are different from those of spent fuel management systems, either storage of transport.

3. THE DEPLETION CALCULATION

The fuel assembly design has progressively increased its complexity during the years. The design is more heterogeneous both in the radial and axial directions, due to the inclusion of axial zones with different enrichments, axial blankets, “vanished” zones, axially varying burnable poison loading in selected rods only, annular pellet zones, water rods, etc. The trend is more remarkable in BWR fuel bundles, but the complexity of PWR fuel assemblies is also increasing.

These improved fuel designs are duly handled by commercial 2-D lattice codes, although some corrections and refinements have been needed even in the vendor codes. The problem with those codes is that their nuclear data libraries do not include all the nuclides relevant for burnup credit applications.

Codes handling enough nuclides are restricted in their geometric modelling capabilities, in the sense that some fuel characteristics relevant to the depletion calculation cannot be properly modelled. Enhancements are needed in those codes for burnup credit applications.

Other field that might need some improvement is that of the fuel operating conditions used for the depletion calculation. Usually, conservative values are considered for the different parameters (pressure, moderator and fuel temperature, void fractions, and others). These values are bounding with respect to nominal operating conditions. However, some phenomena detected in high duty fuel might indicate that the local conditions in some core zones can be rather different. These phenomena include excessive crud deposition, Boron accumulation in the crud, reduced (or increased) flow in some subchannels due to fuel distortion, very high exit temperatures in high-energy assemblies, and other. Some of these effects are not reproduced by the codes used for core analysis, or at least not completely, and they can have an effect on the calculated isotopic composition.

Likewise, the transition to higher discharge burnup values may deserve some careful consideration in burnup credit applications. Some phenomena have been demonstrated to exist in high burnup fuel, but the rod design codes are not yet prepared to model them. Therefore, the potential impact on the spent fuel reactivity cannot be assessed. As an example, the radial distribution of the isotopes within the pellet has been demonstrated to have a negligible effect on reactivity. However, as burnup is increased, the radial distribution of Plutonium is more and more skewed towards the pellet surface, and the balance between generation and consumption is modified. Whether this effect, as well as other internal to the pellet, can have an effect on the reactivity remains to be analysed.

4. THE OLD FUEL DATABASES

Most NPPs in Spain are now in the process of uprating power, once the operating cycle duration extension process that took place during the past years is ended. Much alike in most countries, the optimisation of the fuel cycle economics is leading to frequent changes in the core loading strategies and to extensions of the core operating domain, as well as to a gradual increase of the fuel discharge burnup. These changes may have two effects on the criticality safety analysis of the spent fuel storage at PWR plants:

1. The fuel operating conditions are changed (higher temperatures, harder spectrum, increased power peaking factors), so that they may lie outside the range originally considered for the depletion calculation of the burnup credit criticality safety analysis.

This effect can obviously affect also the BWR applications, and is becoming relevant due to the extensive changes in the operation strategy that these plants are making in a continuous process,

2. Some of these changes, like power uprates and the extension of the operating cycle duration, call for the need of increased fuel enrichments. A new analysis is needed in those cases.

The problem that arises here is that burnup credit analyses rely heavily on the information obtained from the spent fuel databases. It has already been said that the data available has mostly been obtained from low-enrichment and low-burnup old design fuel, which was operated under core conditions very different from those used at present. On top of that, there are some second-order effects on reactivity, such as that of the axial burnup distribution, that are specific to the plant involved. Burnup credit applications make use of the specific plant operating experience accumulated in the past, relying on information that has been gathered from fuel burnt in previous cycles. As those higher enrichments and burnups have never been used before at that plant, there is no plant data to use to evaluate these effects. The only way out from a regulatory perspective is to include a penalty in the analysis to account for this lack of knowledge, and periodically verify that the fuel behaviour under the new conditions is in agreement with the assumptions made.

5. DETERMINATION OF THE FUEL DISCHARGE BURNUP

Burnup credit analyses submittals for spent PWR fuel storage have been licensed without the need for a direct fuel burnup measurement. The fuel discharge burnup value used to verify compliance with the loading curve is determined based on the reactor records. Some uncertainties, such as that of the reactor power calculation, have been included in the plant procedures used to perform this verification.

The coherence of the criticality analysis with the plant procedures and practices has to be emphasised. Core following methods do differ widely from plant to plant, and are still rather crud in some cases. While BWR have traditionally performed a more detailed core operation follow, only some PWR plants have adopted on-line monitoring systems of a similar quality.

As a consequence, the discharge burnup calculation uncertainty is plant specific in general terms, and it is better known for BWR plants. It has to be recognised that a high uncertainty in the discharge burnup determination may invalidate a burnup credit analysis if not taken into account properly (either in the analysis or in the plant procedure), or if the reactivity margins available are not wide enough.

Given the variety of plant specific uncertainties, this problem is more relevant for the generic criticality safety analysis of spent fuel casks, which needs to be valid for fuel coming from different plants. Efforts to determine bounding burnup calculation uncertainties, at least for groups of similar plants, are needed, such as that started by EPRI at the USA.

6. BURNUP CREDIT METHODOLOGIES

Many burnup credit applications were licensed early in the 1980-decade. As already discussed above, changes in the operating conditions may affect the continued validity of the analysis.

Any departure from the operation practices and core conditions maintained in the past should be evaluated to determine if it has such an impact. These changes are sometimes difficult to identify, because of several reasons:

1. They might be introduced gradually along the cycles, instead of being a step change, 2. From the plant perspective, they are not considered a change, but only a better fuel

utilisation within the current licensed limits. As an example, it is frequent to reach burnup levels that are higher than those included in the reactor records, but still within the approved limits,

3. Some changes are not identified as affecting the criticality safety analysis, because they in fact do not affect any other plant analysis. In the BWR case, strategies such as spectral shift at the end of the cycle or feedwater temperature reduction during the cycle are usually covered in the reload safety analysis, and are applied each cycle at the discretion of the plant operator.

Continued validity of the old analysis methodology originally used can be a more important issue in some cases. It is a fact that neither the regulations, nor the calculation tools and data libraries, and for that matter, the general knowledge and understanding of burnup credit insights, were at today’s level in the early eighties. Examples can be forwarded of approved analyses that are not as conservative as we all thought they were, or even in some cases, of analyses that could be criticised from the technical point of view.

The root cause of this situation has to be tracked back to the kind of applications that were first approved. They all were storage applications, and those always have additional safety margins. In the case of PWR, because there is Boron in the pool water, and its reactivity reduction effect was not credited at the time. In the BWR case, because it’s not full burnup credit, only credit to the burnable absorber effect and depletion. The existence of those additional margins was taken into account in the licensing process.

As a consequence, a detailed development of burnup credit analysis techniques was neither performed nor required, and some sensitivities and second order effects were not really considered, because it was felt that they were duly covered by the existing margins, although it was not always explicitly stated. In the PWR case, the situation is equivalent to a taking a non-quantified credit for Boron, and something similar can be said for BWR. It has to be concluded that full burnup credit (actinides and fission products) has never been accepted on its own basis.

The problem begins when trying to apply burnup credit techniques to systems where those margins do not exist, as in the case of spent fuel transport casks for spent PWR fuel (no diluted Boron available). That is the reason why, nearly 20 years after of the original storage submittals, the international community is devoting efforts to adequately describe and quantify effects and parameters relevant to burnup credit. Those effects are of course already present in the approved applications, but they were never addressed in a detailed and correct manner. The contradiction becomes evident. The possibility of the outcome of the current efforts showing a lack of conservatism of the approved storage analysis cannot be discarded, although little safety impact should be expected.

Caution should be exercised to avoid the danger of this situation repeating itself. As an example, some actinide-only credit methodologies are covering the impact on reactivity of some second order effects claiming that the fission products provide an ample safety margin which is not credited in the analysis. As before, this is like taking a non-quantified credit for fission products, while avoiding a detailed treatment of the minor effects involved. Therefore, additional work would again be needed when trying to implement full burnup credit in the future. It could be avoided if a realistic (“best estimate”) full burnup credit reactivity was taken as a basis, determining the uncertainties and covering them conservatively in the licensing submittals. These penalties could be reduced in the future as burnup credit knowledge increases and calculation tools evolve.

REGULATORY STATUS OF BURNUP CREDIT FOR DRY STORAGE AND