Nuclear Criticality Safety: A Subject of Growing International Importance


*Ms. Michaele Brady-Raap works for Duke Engineering & Services, Inc.(Hanford, Washington, USA) and Mr. Blair Briggs for the Idaho National Engineering & Environmental Laboratory (Idaho Falls, Idaho, USA). Mr. Enrico Sartori is a member of the NEA Nuclear Science Division/Data Bank.


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 number of documented criticality accidents in "western" facilities over this period is slightly under 50. Information has recently been published concerning accidents that occurred in the ex-Soviet Union and more data is in the process of being released. About 20 per cent of these accidents occurred in production plants, 10 per cent in working reactors (Chernobyl being the best known and the most dramatic accident) and the remainder in critical facilities where the properties of the assemblies themselves were being investigated. The rate at which these accidents occur has strongly decreased over the years. Such events have now become very rare.

The importance of the safe handling of all fissile materials was recognised at an early stage both by the scientific community and the responsible authorities. At the beginning, intensive experimentation with a large variety of configurations and materials took place in order to establish a basis of knowledge of such systems. At the time, computational methods and basic nuclear data were either not yet properly developed or had not reached sufficient sophistication to reliably predict the critical status of fissile materials.

Over the years, substantial experience has been gained in both experimentation and in data and code development. This state-of-the-art knowledge in criticality safety also has an economic impact. The reduction of uncertainties in safety margins allows improved and more economical designs for manipulation, storage and transportation of fissile materials.

New fuel cycles, handling of excess fissile materials from the weapons programmes and its possible use for civil energy applications make new demands on method development, experimentation and regulations.

This article briefly describes the work carried out by the NEA in this field and outlines perspectives for the future.

Overview of activities

The following activities have been carried out by the NEA on reactor fuel cycle safety:

Criticality safety

Work has concentrated on the following studies of interest to Member countries:

Criticality of nuclear fuel packages

A first study examined the ability of various computational methods to accurately compute criticality for systems which have been measured as experimentally critical. A procedure which evolved from this work allows to assess whether a given computational method produces "valid" results. This procedure provides a basis for acceptance of computational results by regulatory authorities on an international basis.

Criticality of fuel undergoing dissolution

Eighteen experimental configurations were studied. In addition three calculational benchmarks on criticality codes for dissolving fissile oxides in acids have been completed. A particularly difficult problem proved to be the treatment of fuel double heterogeneities (solid fissile material surrounded by fissile material in solution). A reference calculational method for this type of problem was identified and adopted.

Burn-up credit

Burn-up credit is a term that applies to the reduction in reactivity of burned nuclear fuel due to the change in composition during irradiation. Conventional reactor codes and data used for in-core physics calculations can be used to evaluate the criticality state of burned, light water reactor (LWR) fuel. However, these codes involve complicated models and have large computational and data requirements. In reactor applications, these detailed analyses are required for the efficient operation of specific reactors. In away-from-reactor applications such as the design of casks (flasks) for the transportation of spent nuclear fuel, the candidate fuel for use in the cask may come from any reactor and it is desirable that the design allow for the inclusion of as much of the existing and expected fuel inventories as safely possible. In other words, for reactor operations the objective is to use most effectively very specific fuel in a specific application. For away-from-reactor applications the objective is a general design for a wide variety of fuels.[2]

Traditionally, established away-from-reactor codes have been used for applications such as the design of storage and transportation (S/T) casks. In this type of analysis, the fuel is usually assumed to be at its full initial enrichment to provide a large safety margin for criticality safety analyses. The incentives for pursuing burn-up credit over the current, fresh fuel approach are widely recognised: the approach can extend enrichment limitations for existing S/T containers, and may contribute to the development of higher capacity S/T systems that would result in fewer fuel shipments and therefore decreased risk to the public. There is also potential application to criticality safety in dissolvers for fuel reprocessing as well as for timely and efficient transport to and from reprocessing facilities.

However, before such an approach can be approved by licensing agencies, it would be necessary to demonstrate that the available criticality safety calculational tools are appropriate for application to burned fuel systems and that a reasonable safety margin can be established. To this end, a suite of burn-up credit criticality benchmarks have been established by the NEA. The benchmarks have been selected to allow a comparison of results among participants using a wide variety of calculational tools and nuclear data sets. The nature of the burn-up credit problem requires that the capability to calculate both spent fuel composition and reactivity be demonstrated. The benchmark problems were selected to investigate code performance over a variety of physics issues associated with burn-up credit as described in Table 1.

The focus here is the comparison of the results submitted by each participant to assess the capability of commonly used code systems, not to quantify the physical phenomena investigated in the comparisons or to make recommendations for licensing action. Participants use a wide variety of codes and methods using both deterministic and stochastic (Monte Carlo) techniques. Nuclear data were taken from several sources – the Evaluated Nuclear Data Files (ENDF/B), the Japan Evaluated Nuclear Data Libraries (JENDL) and the Joint Evaluated Files (JEF).

Table 1. Summary of recent burn-up credit, benchmark problems addressed

Primary objective
Phase I • Examine effects of 7 major actinides and 15 major fission products for an infinite array of pressurised water reactor (PWR) rods at different burn-ups and cooling times.

• Compare computed nuclide concentrations for depletion in a simple PWR pin-cell model to actual measurements at different burn-ups.

Phase II • Examine the effect of axially distributed burn-up in an array of PWR pins as a function of initial enrichment, burn-up and cooling time.

• Repeat study in a 3-D geometry representative of a conceptual burn-up credit transportation container.

Phase III • Investigate the effects of moderator void distribution in addition to burn-up profile, initial enrichment, burn-up and cooling time sensitivities for an array of boiling water reactor (BWR) pins. 

• Compare computed nuclide concentrations for depletion in a BWR pin-cell model.

Phase IV • Investigate burn-up credit for mixed oxide (MOX) spent fuel.


Experimental Data Relevant to Criticality Safety

In support of evaluation and validation of methods for criticality safety, several databases have been established:

Spent Fuel Isotopic Composition Database

A database of light water reactor (LWR) spent fuel assay data that has been compiled and contains isotopic inventory data collected from 13 LWRs, including 7 pressurised water reactors (PWRs) and 6 boiling water reactors (BWRs) in Europe, the United States and Japan as well as axial burn-up profiles.

International Handbook of Evaluated Criticality Safety Benchmark Experiments

This handbook (ICSBEP) contains criticality safety benchmark specifications that have been derived from experiments that were performed at various nuclear critical facilities around the world. The benchmark specifications are intended for use by criticality safety engineers to validate calculational techniques used to establish minimum subcritical margins for operations with fissile material. At present the handbook comprises about 2 000 critical configurations and is available on CD-ROM.[3]

Criticality Benchmark Experiments

There have been several activities that involve experiments applicable to burn-up credit.

Working Party on Nuclear Criticality Safety

During a recent experts’ meeting on Needs for Critical Experiments, it was concluded that, in view of the limited number of operational critical facilities and the international scope of the needs for criticality safety technology, the NEA should encourage the performance of new critical measurements on a multilateral, international basis with regard to the sharing of facilities, staff expertise and funding resources. Also, given the absence of some experimental capabilities in many countries and the near-unique capabilities in others, the NEA should, through its Member countries’ representatives, recommend to their sponsoring agencies that certain facilities with unique capabilities be made available for international measurements programmes. This policy would reduce the need for redundancy in capabilities and promote stable funding for maintaining staff and equipment. With the expanding number and scope of NEA activities on criticality and safety, a Working Party for Nuclear Criticality Safety was established to provide guidance and overall co-ordination of these activities. This working party deals with technical, away-from-reactor, criticality safety issues relevant to fabrication, transportation, storage and other operations in the fuel cycle of nuclear materials. Specific Task Forces have been set up:

Additional areas and items of activity comprise:


If nuclear energy is to play an important role in our economies in the future (it currently represents 25 per cent of electricity production in the OECD area), fissile materials must be handled safely over the whole fuel cycle. Several fuel cycle options exist, and their advantages and disadvantages are being hotly debated at the technical, economical, political and public level. One of the questions being asked is: should plutonium be considered as a liability or an energy source?

In the coming years there will likely be further clarification of potential nuclear fuel cycle strategies, each one with its specific needs in criticality safety. Although a wealth of information is available from more than 50 years of cumulative knowledge acquired, case-specific analyses will be needed and will dominate criticality safety.

The release of fissile materials from some countries’ weapons programmes to the civil nuclear fuel cycle will influence future work according to available options. Open or closed fuel cycles, once-through or recycle will dominate the debate and research.

Diversity in national policies is expected to persist in this area in the short term. In the long term, and in order to leave the options open for future generations, it is worth sharing criticality information beyond national policies. It is for that reason that the Working Party on Nuclear Criticality Safety was set up, addressing issues that are of common interest to all countries with fuel cycle facilities. An international conference is scheduled for 1999 in France to address these themes as they relate to criticality safety, and to cover in particular the different fuel cycle options such as once-through, recycle and actinide partitioning, burning and transmutation.

In conclusion, criticality safety calls for constant support and attention. A sound understanding and correct application of the principles of nuclear criticality safety are vital to the nuclear industry. The objective is to pursue an accident-free goal, while keeping in mind the repercussions that an avoidable criticality excursion could have. Current activities and future initiatives will obviously build upon past accomplishments. Events have shown that criticality safety is an international issue. It is therefore in the interest of all that information be widely shared and disseminated, notably through the NEA.


[1] The Safety of the Nuclear Fuel Cycle, OECD, 1993 (ISBN 92-64-13824-2).

[2] M.C. Brady et al.: "International Studies on Burnup Credit Criticality Safety by an OECD/NEA Working Group, Proc. International Conference on the Physics of Nuclear Science & Technology, Long Island, NY, USA, 5-8 October 1998

[3] J.B. Briggs ed.: "International Handbook of Evaluated Criticality Safety Benchmark Experiments", NEA/NSC/DOC(95)03 Rev. 3, September 1998.