Australian Safeguards and Non-Proliferation Office

The categorisation of nuclear material in the context of integrated safeguards

Victor Bragin, John Carlson and Russell Leslie, Australian Safeguards and Non-Proliferation Office, Canberra

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1. Introduction

As part of the re-examination of basic safeguards parameters, to consider their appropriateness or otherwise under Integrated Safeguards, an important area for attention is nuclear material categorisation. This is a major determinant of inspection effort and evaluation of safeguards performance. Under Integrated Safeguards, achieving credible assurance of the absence of undeclared nuclear material and nuclear activities, particularly those related to enrichment and reprocessing, in a State as a whole, would permit reductions in the current level of routine safeguards verification effort, particularly on less sensitive nuclear material. In this context matters to consider might include whether current nuclear material categorisation affects the scope for optimisation of verification effort, or whether it inhibits introduction of more rigorous verification (where and if this is desirable).

In addition to the three basic nuclear material categories uranium, plutonium and thorium the IAEA characterises nuclear material for various purposes according to criteria that can be grouped under the following headings: degree of processing (source material and special fissionable material), strategic value (direct-use material and indirect-use material), isotopic composition and radiation level (irradiated and unirradiated).

INFCIRC/153 states that the IAEA, in order to ensure optimum cost-effectiveness, should make use of "concentration of verification procedures on ... nuclear material from which nuclear weapons or other nuclear explosive devices could readily be made" Thus, the objective of refining the nuclear material categories should be, to ensure that the Secretariat has the authority to require more rigorous safeguards standards on nuclear material of a form and composition that represents a significant proliferation risk (looking at risk here purely in terms of the characteristics of the material, without taking into account the State evaluation), while at the same time having the flexibility to require less rigorous standards where the risk is lower. Judgments of relative risk should also be reflected in evaluation of safeguards performance.

In this paper we discuss weapons-grade materials as opposed to materials typical of the civil nuclear fuel cycle, current IAEA definitions of material categories and whether those need to be revised under integrated safeguards.

2. Weapons-Grade Materials

Nuclear weapons are manufactured from either weapons-grade uranium or weapons-grade plutonium:

3. Materials in Civil Programs

The weapons-grade materials described above are very different to those normally produced in civil programs, for example:

Table 1. Typical Isotopic Compositions of Spent Fuel at Discharge from Power Reactors [4]
Reactor type Fuel burn-up

Isotopic composition, %

GWd/t Pu-239 Pu-240 Pu-241 Pu-242
GCR 3.6 77.9 18.1 3.5 0.5
PHWR 7.5 66.4 26.9 5.1 1.5
AGR 18.0 53.7 30.8 9.9 5.0
RBMK 20.0 50.2 33.7 10.2 5.4
BWR 27.5 59.8 23.7 10.6 3.3
PWR 33.0 56.0 24.1 12.8 5.4

4. Current IAEA Definitions of Material Categories

As already mentioned, in addition to the three basic nuclear material categories uranium, plutonium and thorium the IAEA characterises nuclear material for various purposes according to criteria which can be grouped under the following headings: the degree of processing required or undertaken, strategic value (suitability for weapons use), isotopic composition and radiation level.

Degree of processing     Here there are two categories of nuclear material source material and special fissionable material:

Strategic value   This is a relative measure of the usefulness of a nuclear material to a potential diverter for producing nuclear explosives. There are two categories:  

Isotopic composition      It is obvious that isotopic composition is closely related to strategic value, and isotopics were taken into account in the categorisation into direct-use and indirect-use material (eg plutonium with or without a certain proportion of Pu-238, HEU as distinct from DNLEU). The isotopic composition of the material controls the relative difficulty of manufacturing a nuclear explosive with material of a specific isotopic composition or altering its isotopic composition to produce weapons-grade or weapons-useable material. Attributes that are important for determining the useability of material for weapons applications include:

In the case of uranium, the IAEA distinguishes between four categories based on isotopic composition:  

The following table illustrates the relative "distance" between different isotopic categories of uranium in terms of the required separation work.

Table 2. Separation Work (SWU) Required to Produce Uranium of Different Enrichment Levels (% U-235) and the Amount of Product (kg) Feed Material 5,640 kg of Natural Uranium
Enrichment (% U-235)

2.235 3.6 19.9 40.0 60.0 93.0
SWU

2,500 3,180 4,510 4,760 4,870 5,000
Amount of product (kg)

1,198 703 118 58 39 25

In the case of plutonium, to date the isotopic composition of plutonium has not been a major issue for safeguards, because most plutonium under safeguards is of a similar composition, ie what is termed "reactor-grade" (>19% Pu-240). Effectively the IAEA recognises two categories:

Thus the IAEA applies similar safeguards measures to all plutonium, regardless of isotopic composition, apart from an exemption for plutonium containing 80% or more of the isotope Pu‑238[10].  This is a policy position intended to reflect that all isotopes of plutonium are fissionable by fast neutrons, and that theoretically a nuclear explosive device, albeit perhaps of unpredictable yield, could be constructed using any grade of plutonium. For IAEA safeguards purposes all plutonium, even including that still in spent fuel, is defined as "direct‑use" material, ie material that can be used for the manufacture of nuclear explosives.

While the above statement reflects the common understanding of IAEA practice, there is however an interesting qualification to the formal position that all plutonium (other than Pu-238) is treated alike. In the context of substitution of unsafeguarded nuclear material for safeguarded nuclear material, the IAEA recognises that the isotopic composition of plutonium is relevant for safeguards purposes INFCIRC/66 paragraph 26(d) provides that nuclear material substituted for safeguarded material must have at least the same proportion of fissionable (ie here meaning fissile) isotopes.

Radiation Level        Here there are two categories, irradiated and unirradiated. The radiation hazard associated with the material must be taken into account at each step in the civil nuclear fuel cycle and in any process to produce a weapons-useable material. The radiation hazard is the radiation field associated with the material and the internal dose potential to humans. There are many attributes one might select to describe the effectiveness of the radiological barriers to proliferation, among them: the specific dose rates at one meter unshielded or the contact time required to accumulate the mean lethal dose.  Radiation can also complicate chemical processing. Other possible attributes could categorise the materials by the degree of remote handling required: unlimited hands-on handling acceptable, limited hands-on access acceptable, remote manipulation required, shielded facilities required.

Currently the IAEA does not apply a numerical value for the irradiation level of spent fuel the definition in the current Safeguards Criteria refers only to direct-use material containing "substantial amounts of fission products". It is our understanding that the IAEA is currently reconsidering its definition of the irradiated fuel in light of integrated safeguards. The main issue is could the radiation level of spent fuel decay over time to a point where its "self-protection" was lost? Because the handling of spent fuel having a low radiation level could require less shielding, and might not require sophisticated handling equipment, in principle at least such fuel could be more easily diverted and reprocessed. Should the intensity of safeguards be increased to reflect that such fuel may be of greater proliferation attractiveness? It is particularly pertinent to examine this issue in the context of the change in the spent fuel timeliness goal under integrated safeguards from 3 months to 12 months (discussed below).

5. Categorisation and current IAEA Criteria

It should be noted that a major impact of nuclear material categorisation on inspection effort appears to be through the concept of timeliness, and in fact categorisation and timeliness are closely related. Thus, under the currently applied Safeguards Criteria certain nuclear material categorisations are major determinants of inspection effort and evaluation of safeguards performance. The present timeliness goals, including their changes for integrated safeguards purposes, are summarised as follows. 

Table 3. Timeliness Goals under Classical and Integrated Safeguards

Table 3. Timeliness Goals under Classical and Integrated Safeguards
  Classical Safeguards Integrated Safeguards
unirradiated direct-use material (Pu, HEU, U-233) one month one month
irradiated direct-use material (Pu, HEU, U-233) three months one year
indirect-use material (DNLEU) one year one year

Under integrated safeguards a revised concept of timeliness could allow the Agency to apply timeliness goals in a less rigid manner, both in the setting and implementation of inspection frequency and in the evaluation of goal attainment. In applying timeliness goals, eg in establishing inspection frequency (which could be set above or below the current timeliness goals) it would be appropriate to reflect, inter alia:

Under this approach, the question arises whether some further development of nuclear material categories is required, or even desirable.  We believe it would be important to avoid a new scheme of categorisation leading to excessive rigidity, negating the flexibility and judgment that the new approach to timeliness is intended to establish. Under a flexible approach to timeliness, to some extent the categorisation of nuclear material can be probably sidestepped, being taken into account by the Secretariat in an informal way without the necessity to establish a formal scheme.  As against that, at the least it would seem desirable to establish guidelines to assist the Secretariat in the judgmental process, and to engender transparency in this process.

In any further development of the nuclear material categories, it is assumed there will be a continuing requirement for many of the existing categories, either for practical reasons or because they are established under the IAEA Statute, treaties and other legal instruments. These would probably include the degree of processing, strategic value and radiation level. However, some refinement of some of the categories might be desirable.

For some of the nuclear material categories, there would seem to be limited scope or purpose in refining the existing categories. This would apply to the degree of processing category, ie there seems no reason to create any categories additional to source material, special fissile material and fertile material. This would probably also apply to the category of radiation level apart from establishing a suitable definition, the distinction between unirradiated and irradiated would seem sufficient, without the need for additional categories, eg specifying degrees of irradiation.

Thus the principal scope for possible refinement appears to be in the area of strategic value/isotopic composition. Possibilities here might include:

Chemical proliferation barrier In any refinement of material categories, other factors to be taken into account might include: the physical or chemical form of the material (whether metallic or in a chemical compound), whether in a mixture, eg with material of a different category (as is the case with MOX). The chemical proliferation barrier refers to the extent and difficulty of chemical processing required to separate fissionable material from accompanying diluents and contaminants. Attributes of the chemical barrier generally relate to the degree of technical difficulty needed to refine materials into the appropriate form, be they metals or compounds. Other possible attributes include the existence of admixtures (such as those incorporated to frustrate chemical separation or denaturing), and the number of separate processing steps needed to obtain materials of sufficient purity for weapons applications. The chemical barrier effectiveness of some of the more common materials involved in the nuclear fuel cycle can be classified in the following order: pure metals; compounds (including oxides, nitrides, etc.); mixed compounds (in particular MOX fuel, and including diluents and burnable poisons; spent fuel and vitrified wastes, including fission products (highest proliferation barrier).

Material in advanced nuclear energy generating systems A further matter to address in the future is whether the current nuclear material categorisations are appropriate for the kinds of materials that may be produced through the introduction of new nuclear power generation concepts.

6. Conclusions

The concepts of material categorisation and timeliness are closely related.  A new concept of timeliness under Integrated Safeguards would allow the Agency to apply timeliness goals in a less rigid manner, both in the setting and implementation of inspection frequency and in the evaluation of goal attainment. Under this approach, the question arises whether some further development of nuclear material categories is required, or desirable.  However, it would be important to avoid a new scheme of categorisation leading to excessive rigidity. Under a flexible approach to timeliness, to some extent the different characteristics of nuclear material can be probably be taken into account by the Secretariat in an informal way without the necessity to alter formal material categorisations. As against that, it would seem desirable to establish guidelines to assist the Secretariat in the judgmental process, and to bring transparency to this process.

For some of the nuclear material categories, there would seem to be limited scope or purpose in refining the existing categories. This would apply to the degree of processing category, ie there seems no reason to create any categories additional to source material, special fissile material and fertile material. This would also apply to the category of radiation level apart from establishing a suitable definition, the distinction between unirradiated and irradiated would seem sufficient, without the need for additional categories, eg specifying degrees of irradiation. Thus the principal scope for refinement appears to be in the area of strategic value and isotopic composition. Some possibilities are discussed in the paper.

While there is understandable caution about changes to safeguards parameters relating to direct-use material, it should be recognised that the direct-use material category covers a broad range of materials which do not all have the same proliferation sensitivity. Although the Secretariat uses the term direct-use material to include MOX, this is strictly speaking not a "direct-use material" since it could not be used in a nuclear explosive without further, non-trivial, processing to separate the contained plutonium. It might be reasonable to recognise that the different characteristics of materials within the direct-use category could justify differing approaches, rather than applying the category in a simplistic manner.


[1]. See eg "Plutonium: The First 50 Years. United States Plutonium Production, Acquisition, and Utilisation from 1944 to 1994," DOE (1996). Prior to the 1970's, there were only two terms in use (by DOE) to define plutonium grades: weapons-grade (7% Pu-240) and reactor-grade (>7% Pu-240). In the early 1970's, the term fuel-grade (>7 - <19% Pu‑240) came into use, which shifted the starting point of the reactor-grade definition (19% Pu‑240).

[2]. WOHLSTETTER, A., "Spreading the Bomb Without Quite Breaking the Rules", Foreign Policy, vol. 25 (Winter 1976/1977) 88‑96, 145‑179.

[3]. HURIO, E., "DOE Program Aimed at Stretching Fuel Burnup to 100,000 MWd/MT," Nuclear Fuel, February 24, 1997, 3.

[4]. Compiled from a number of sources.

[5]. IAEA Statute, Article XX.3.

[6]. INFCIRC/153, paragraph 112.

[7]. IAEA Safeguards Glossary, Item 32.

[8]. IAEA Statute, Article XX.1.

[9]. INFCIRC/153, paragraph 36 (c).

[10]. INFCIRC/153, paragraph 36 (c).