Conclusions for the BUC calculation routes to be used

As follows from the preceding sections the depletion code used in a BUC criticality safety analysis should be capable of representing sufficiently accurately:

(a) The complexity of the fuel assembly design of interest (presence of integral burnable absorbers, horizontally and/or axially inhomogeneous distribution of the initial enrichment);

(b) The effect of the presence of burnable poison or CRs on the isotopic inventory of the spent fuel.

Two dimensional depletion methods are generally preferable to one dimensional depletion calculation routes, in particular when the fuel design of interest has horizontally non-uniform enrichment distributions, when integral burnable absorbers are present in the fuel, or when CRs are inserted during an irradiation period.

At the Technical Committee Meeting in 2005 [6], presentations were made giving information about the ongoing activities in developing and validating three dimensional depletion calculation procedures. Application of three dimensional depletion calculation methods in BUC criticality safety analysis will be of particular interest because of:

(1) The non-uniformity of the axial isotopic composition distribution due to the non-uniformity of axial power and hence burnup distributions;

(2) The capability of three dimensional codes to include the neutron reflection conditions at the top and bottom ends of the fuel zone which affect the axial isotopic composition distribution;

(3) The capability of three dimensional codes to represent axial enrichment zoning;

(4) The capability of three dimensional codes to take account of the fact that neither integral burnable absorbers nor burnable poison rods extend over the full active length;

(5) The capability of three dimensional codes to take account of the fact that CRs are usually not fully inserted during irradiation and that the insertion depth, if CRs are used, is usually significantly less than 80 cm.

The criticality calculation procedure should have the capability to represent:

(i) All the above mentioned complexities of the fuel assembly designs;

(ii) The complex geometries of the fuel management system of interest;

(iii) The axially and horizontally non-uniform burnup distributions.

It is state of the art, therefore, to use three dimensional criticality codes employing Monte Carlo techniques and using adequate cross-section libraries containing the nuclides to be used in BUC.

CURRENT ISSUES IN CRITICALITY SAFETY

Figure 12 shows results from a benchmark study on the effect of the use of CRs on the isotopic composition of spent fuel and hence on the fuel’s reactivity.

This benchmark study was conducted by the Expert Group on Burnup Credit Criticality Safety under the auspices of the OECD/NEA [23]. Each contributor to the benchmark used his own depletion calculation procedure (unless otherwise specified in Fig. 12) and his own criticality calculation procedure.

Figure 12 shows that, apart from contribution ‘P10’ (in this contribution a one dimensional depletion calculation procedure was used for comparison) the relative deviations of the neutron multiplication factors calculated by the contributors are within an interval smaller than –1.5%, +1.5%. This demon-strates the high quality reached in BUC criticality safety analysis.

— Benchmark cases 2 and 4 through 8: With CR insertion during depletion, cooling time 0 years;

— Benchmark cases 10 and 12: With CR insertion during depletion, cooling time 5 years;

— Benchmark cases 1 and 3: No CR insertion during depletion, cooling time 0 years;

— Benchmark cases 9 and 11: No CR insertion during depletion, cooling time 5 years;

FIG. 12. Example of the relative differences in calculated neutron multiplication factors due to the use of different depletion codes for predicting the isotopic inventory and the use of different criticality calculation codes (from Ref. [23], Table 4.1).

— Benchmark cases 1a, 2a, 3a, 4a, 9a, 10a, 11a, 12a: Actinide only BUC¹; all the other cases: Actinide + fission product BUC⁺⁾;

— Benchmark case 13b: No CR insertion, cooling time 5 years, pre-defined isotopic number densities for all participants;

— Benchmark case 14b: With CR insertion, cooling time 5 years, pre-defined isotopic number densities for all participants;

— Benchmark case 15: Fresh fuel, pre-defined isotopic number densities for all participants.

REFERENCES

[1] INTERNATIONAL ATOMIC ENERGY AGENCY, Implementation of Burnup Credit in Spent Fuel Management Systems (Proc. Advisory Group Mtg Vienna, 1997), IAEA-TECDOC-1013, IAEA, Vienna, 1998) 97–107.

[2] INTERNATIONAL ATOMIC ENERGY AGENCY, Implementation of Burnup Credit in Spent Fuel Management Systems (Proc. Technical Committee Mtg Vienna, 2000), IAEA-TECDOC-1241, IAEA, Vienna (2001).

[3] INTERNATIONAL ATOMIC ENERGY AGENCY, ARGONNE NATIONAL LABORATORY, Regional Training Course on Implementation of Burnup Credit in Spent Fuel Management Systems, 15–26 October 2001, Argonne, IL.

[4] INTERNATIONAL ATOMIC ENERGY AGENCY, Practices and Develop-ments in Burnup Credit Applications, IAEA-TECDOC-1378 CD, IAEA, Vienna (2002).

[5] SIMISTER, D.N., CLEMSON, P.D., “UK regulatory perspective on the applica-tion of burn-up credit to the BNFL Thorp Head End plant”, ICNC 2003 (Proc. 7th Int. Conf. on Nuclear Criticality, Tokai, Ibaraki, Japan, 2003) 633–638.

[6] INTERNATIONAL ATOMIC ENERGY AGENCY, Advances in Applications of Burnup Credit to Enhance Spent Fuel Transportation, Storage, Reprocessing and Disposition, IAEA-TECDOC-1547, IAEA, Vienna (2007).

[7] NUCLEAR REGULATORY COMMISSION, NRC Regulatory Issue Summary 2005–05, Regulatory Issues Regarding Criticality Analyses for Spent Fuel Pools and Independent Spent Fuel Storage Installations, NRC, Washington DC (2005).

[8] NUCLEAR REGULATORY COMMISSION, Criticality Accident Require-ments, Rep. 10 CFR Part 50.68, NRC, Washington, DC (1998).

1 Isotopes used (cf. Table I): Actinides: U-234, U-235, U-236, U-238, Np,-237, Pu-238, Pu-239, Pu-240, Pu-241, Pu-242, Am-241, Am-243; Fission products: Mo-95, Tc-99, Ru-101, Rh-103, Ag-109, Cs-133, Nd-143, Nd-145, 147, 149, 150, Sm-151, Sm-152, Eu-153, Gd-155; Others: O-16.

CURRENT ISSUES IN CRITICALITY SAFETY

[9] NUCLEAR REGULATORY COMMISSION, Guidance on the Regulatory Requirements for Criticality Analysis of Fuel Storage at Light-Water Reactor Power Plants, NRC, Washington, DC (1998).

[10] NUCLEAR REGULATORY COMMISSION, Criteria for Nuclear Criticality, Rep. 10 CFR Part 72.124, NRC, Washington, DC (1999).

[11] NEUBER, J.-C., “Use of burnup credit in criticality safety design analysis of spent fuel storage systems”, Storage of Spent Fuel from Power Reactors (Proc. Int.

Conf. Vienna, 2003), C&S Papers Series (CD-ROM) No. 20, IAEA, Vienna (2003) 404–421.

[12] OECD NUCLEAR ENERGY AGENCY, NEA Data Bank, Janis 2.1, A Java-based Nuclear Data Display Program, OECD, Paris (2004).

[13] BIDINGER, G.H., CACCIAPOUTI, R.J., CONDE, J.M., et al., Burnup Credit PIRT Report, Rep. NUREG/CR-6764, NRC, Washington, DC (2002).

[14] OAK RIDGE NATIONAL LABORATORY, SCALE, A Modular Code System for Performing Standardized Computer Analyses for Licensing Evaluation, Rep.

ORNL/TM-2005/39, Version 5 (2005).

[15] DEPARTMENT OF ENERGY, Disposal Criticality Analysis Methodology, Topical Rep. YMP/TR-004Q, Revision 01, Office of Civilian Radioactive Waste Management, Las Vegas, NV (2000).

[16] SUYAMA, K., NOURI, A., MOCHIZUKI, H., NOMURA, Y., “SFCOMPO, A database for isotopic composition of nuclear spent fuel, Current status and future development”, Storage of Spent Fuel from Power Reactors (Proc. Int. Conf.

Vienna, 2003), C&S Papers Series (CD-ROM) No. 20, IAEA, Vienna (2003).

[17] FINCK, P., TAIWO, T., GULLIFORD, J., Potential Sources of Experimental Validation for Burnup Credit, ANL/TD/00-11 (2000).

[18] ZWICKY, H.-U., LOW, J., KIRKEGAARD, J., et al., “Isotopic analysis of irradi-ated fuel samples in the Studsvik hotcell laboratory”, LWR Fuel Performance (Proc. 2004 Int. Mtg Orlando, FL, 2004).

[19] OECD/NEA NUCLEAR SCIENCE COMMITTEE, International Handbook of Evaluated Criticality Safety Benchmark Experiments, NEA/NSC/DOC(95)03/I through VII.a, OECD, Paris (1995).

[20] NEUBER, J.-C., “Burnup credit applications to PWR and BWR fuel assembly wet storage systems”, The Physics of Nuclear Science and Technology, Proc. Int.

Conf. Long Island, NY, 1998.

[21] NEUBER, J.-C., “Generation of bounding axial burnup profiles as a continuous function of average burnup”, ICNC 2003 (Proc. 7th Int. Conf. on Nuclear Criticality Safety, 2003, Tokai, Ibaraki, Japan) 672–677.

[22] NEUBER, J.-C., Specification for Phase II-E Benchmark, Study on the Impact of Changes in the isotopic Inventory due to Control Rod Insertions in PWR UO₂ Fuel Assemblies during Irradiation on the End Effect, Rev. 01 (2006).

[23] BARREAU, A., Burn-up Credit Criticality Benchmark Phase II-D, PWR UO2 Assembly, Study of Control Effects On Spent Fuel Composition, OECD/NEA Data Bank (in press).

DISCUSSION

Y. KOVBASENKO (Ukraine): In your opinion, how many isotopes are really acceptable for burnup credit calculations for PWR spent fuel management systems?

J.-C. NEUBER (Germany): It depends on the spent fuel, the spent fuel system, and the conditions you have to analyse. For instance, for a wet or water-flooded storage system you have to use the actinides uranium-235, uranium-238, plutonium-239 and plutonium-241; and you may use the plutonium isotopes 238, 240 and 242, as well as neptunium-237 and the americium isotopes 241 and 243. You may use a set of about 15 fission products, which is often called the

‘set of principal fission products’. One of these fission products is caesium-135.

Under normal operating conditions in a wet storage system, or cold conditions in a flooded system, caesium is not volatile. But in the case of a transport/

storage cask under fire conditions caesium becomes volatile, so that the distri-bution of caesium within the fuel becomes unknown. So when reflooding of the cask has to be analysed, caesium isotopes should not be included in the set of isotopes used in the burnup credit criticality safety analysis (usually, i.e. if you have no better information). This is an example. In principle, due to physics, you have to include all the nuclides with positive reactivity worth; and you may include all nuclides with negative reactivity worth — the use of which can be validated. Don’t include short lived isotopes because they usually can’t be validated.

Y. KOVBASENKO (Ukraine): So, with respect to available validation data, the number of isotopes I can use is approximately 20–30 isotopes — not more.

J.-C. NEUBER (Germany): No, in principle there are more than 30 that can be used. You will find a list of burnup credit isotopes in my paper. It should be mentioned, however, that in the USA for instance, it is allowed in the criticality safety analysis of wet storage systems to use all the fission products that can be calculated, except for xenon-135. However, this is not the European way because we have, according to our relevant standards, in particular, the German standards for the use of burnup credit, to validate the calculated number densities with the aid of experimental data.

T. SAEGUSO (Japan): Could you share with us your thoughts about how to avoid the faulty loading of unknown fuel, such as fresh fuel, into the system?

J.-C. NEUBER (Germany): As in any other accident condition, the double contingency principle is applied to the misloading event. However, there are different ways of application of the double contingency principle in the world and, in addition, the way of application may depend on the spent fuel management system one is talking about.

CURRENT ISSUES IN CRITICALITY SAFETY

Concerning borated wet storage pools inside PWR nuclear power plants, for example, the misloading of a fuel assembly which does not comply with the loading curve, i.e. which does not have enough burnup, is considered as one accident condition. By virtue of the double contingency principle a second independent, unlikely and concurrent accident event need not be assumed.

Therefore, the boron content of the storage pool may be credited — since a concurrent dilution event need not be assumed. This is the usual approach in the USA for instance, so that one assumes for a PWR wet storage pool either a misloading event or a boron dilution event. It is not necessary to combine the two events.

The German approach is different, because we take into consideration that no system can withstand the misloading of more than one fuel assembly if it cannot withstand the misloading of one fuel assembly. This is due to the fact that a misloading error results from operational errors in the fuel handling procedure or errors in the burnup information from the reactor records. In any case, if a misloading event occurs there is a great probability that this event will remain undetected despite the control checks performed during or at the end of a fuel loading campaign. If the error remains undetected, then, for example a boron dilution event taking place at a later time in a PWR wet storage system cannot be regarded as a concurrent event. The double contingency principle says that one does not need to consider two events which are independent and concurrent and which have a low probability of occurrence. But if the misloading event is not detected, a later boron dilution event is not concurrent.

So therefore, according to the double contingency principle, one has to analyse the combination of the misloading event and the later boron dilution event.

At a later time, if you again load fuel into the wet storage system for which you are using burnup credit the fault can be repeated. Then you have two undetected misloading events and you have, therefore, to combine these two misloading events with a later boron dilution event; and so on and so forth. At the end you will come out with all storage positions loaded with fresh fuel; and this scenario has to be combined with a later dilution event. This combination makes it impossible to use burnup credit.

So our conclusion was that we have to apply the double contingency principle directly to the misloading event in such a way that at least two unlikely, independent and concurrent events have to occur before a misloading event can occur. Each step of a fuel handling procedure has to comply with this application of the double contingency principle. This is laid down in our burnup credit criticality safety regulations.

With respect to spent fuel casks, in particular dry storage casks, I have to add that we have to include intentional or inadvertent reflooding of the casks with pure water in our criticality safety analysis. This makes it necessary to rule

the misloading event out by applying the double contingency principle to the misloading event directly. In other countries, as for instance in the USA, the situation is different: reflooding need not be considered and, therefore, a risk informed approach is taken consisting of estimating the probability of reflooding a dry storage cask sitting in its pad in the dry storage facility.

So you see, there are significant differences in the philosophy between different countries regarding the misloading event.

POTENTIAL BENEFITS OF