Index


O9705

Plutonium Isotopics - Non-Proliferation And Safeguards Issues

John Carlson, John Bardsley, Victor Bragin, John Hill

Australian Safeguards Office, Canberra ACT, Australia

IAEA-SM-351/64


Abstract

Current developments - including the increasing stocks of excess fissile material from weapons dismantlement, plans to utilise this material in power reactors or dedicated plutonium burning reactors, and the prospect of separation of weapons-grade plutonium from fast breeder reactor blankets - draw attention to the position of low burn-up plutonium in the nuclear fuel cycle. This is not just an issue for the future - there are already at least 100 SQs of very low burn-up plutonium under safeguards. Hence it is timely to examine safeguards and non-proliferation issues related to this material.

The situation which arose with the DPRK highlights the fact that production of separated weapons-grade material by a non-nuclear-weapon State should not be accepted as a normal activity. Even for nuclear-weapon States, the proposal for a convention on the cut-off of production of fissile material for weapons purposes has implications in this regard. A proscription on the production - or separation - of plutonium at or near weapons-grade would be an important confidence-building measure in support of the disarmament and non-proliferation regime.

Although low burn-up plutonium, if available, would be of greatest interest to a diverter, current safeguards practice does not provide for the application of more rigorous safeguards measures to such plutonium. There would seem a good case to take account of the greater attractiveness of low burn-up plutonium and to apply safeguards measures accordingly. Although the quantity of plutonium currently under safeguards in this category is not insignificant, it is a relatively small proportion of total safeguarded plutonium, and a more intensive regime for such plutonium does not appear to have major resource implications for the IAEA or facility operators.

The paper also touches on more general issues, such as implications of the use of remote monitoring and the relevance of timeliness goals.

1. INTRODUCTION

In response to the various demands and expectations placed upon it, the IAEA is giving attention to the re-orientation of safeguards resources to areas recognised as having highest priority, and to a greater use of technology in order to enable the most efficient use of available inspection resources. In this context it is timely to examine safeguards issues related to 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 "reactor-grade". 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 [1]. 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.

This policy position is underscored by reference to the announcement by the US in 1977, that in 1962 it had successfully conducted an underground test of an explosive device made from "reactor-grade" plutonium [2]. Additional information concerning the test, including the fact that the yield was less than 20 kilotons, was provided by the US Department of Energy (DOE) in June 1994 [3]. In accordance with DOE policy not to reveal the actual isotopic composition of plutonium used in specific weapons or tests, the US has never revealed the isotopic quality of the plutonium used in the 1962 test. At the time of this test the definition of "reactor-grade" plutonium was substantially different to the contemporary definition, which encompasses an intermediate category, "fuel-grade", recognised since the 1970s - ie the definition of "reactor-grade" used in the 1960s had an isotopic content of just over 7% Pu-240 as its lower boundary, compared with the current definition which has an isotopic content of 19% Pu-240 as its lower boundary. There are suggestions that the material used in the 1962 test was what would now be termed "fuel-grade," probably closer to the weapons-grade end of the fuel-grade range [4].

The point of this discussion is, not to contend that a nuclear explosive device could not be made from reactor-grade plutonium, or that reactor-grade plutonium is unattractive for potential proliferants, but rather to note that the argument about the efficacy of reactor-grade plutonium has obscured the case for a more rigorous approach to plutonium having an isotopic composition much closer to that actually used in nuclear weapons.

2. ISSUES DISCUSSED IN THIS PAPER

This paper argues that from the non-proliferation perspective, clearly it is preferable to avoid the production of significant quantities of plutonium at or near weapons-grade, even if under safeguards. This is not to imply that safeguards are inadequate, but rather to recognise that such a development could engender fears about what might happen to the material in the future, eg if the State concerned were to renounce the NPT. Thus the production of plutonium at or near weapons-grade could undermine the confidence which safeguards are intended to provide. The point can be illustrated by reference to the DPRK - had that State been in full compliance with its safeguards agreement, it might now be accumulating weapons-grade material with legal impunity.

Fuel cycle technologies have inherently differing levels of proliferation risk, which affect the assurance required from international safeguards measures [5]. From the safeguards perspective, this paper argues it should be recognised that if diversion of plutonium were attempted, low burn-up material would be the most attractive target. Hence it is prudent to apply more rigorous safeguards measures to such material. This could be done in the context of a general re-orientation of safeguards resources to areas recognised as having greater priority. Practical aspects of doing this are discussed below.

Although not specifically covered in this paper, similar considerations arise in the case of uranium-233, which is a product of the thorium fuel cycle. U-233 is a fissile material which theoretically could be used for nuclear weapons, and as such is subject to the same safeguards requirements as U-235. The authors suggest that the non-proliferation and safeguards aspects of the thorium fuel cycle, particularly the separation of U-233, should be examined in line with the discussion in this paper of plutonium issues.

The paper does not attempt to canvass the non-proliferation issues related to storage, conversion, stabilisation and disposition of weapons-grade plutonium released from dismantled weapons and declared excess to national security needs [6].

3. DEFINITIONS OF PLUTONIUM GRADES

Before proceeding any further, it will be useful to take up some questions of definition. Without wishing to prejudge the definitions of "high burn-up" and "low burn-up" plutonium which might be adopted for future safeguards/non-proliferation purposes, attention is drawn to the following DOE definitions which are in general use [7]. 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 (³19% Pu-240).

"Weapons-grade" plutonium (WGPu) contains no more than 7% of the isotope Pu-240. WGPu is produced in heavy water- or graphite-moderated production reactors fuelled with natural or slightly enriched uranium. All production reactors are on-load refuelled to allow for short fuel irradiation times. Within weapons-grade there is the sub-category of "super-grade" plutonium (SGPu), containing no more than 3% Pu-240.

Another way to produce WGPu is through irradiation of U-238 by fast neutrons. Such are the conditions in the (natural or depleted uranium) blanket of a Liquid Metal Fast Breeder Reactor (LMFBR). The composition of plutonium produced in the blanket of a LMFBR (about 4% Pu-240) places it in the WGPu category.

WGPu can inadvertently be produced in power reactors. In the early 1970s, this happened, for example, in the US when leaking fuel rods caused the utility operating the Dresden-2 reactor to discharge the entire initial core containing a few hundred kg of plutonium with 89-95% Pu-239 [8].

"Fuel-grade" plutonium (FGPu) contains more than 7%, but less than 19%, of the isotope Pu-240. FGPu is produced in some nuclear reactors that have a spent fuel burn-up lower than that resulting in reactor-grade plutonium, but higher than that resulting in WGPu. For example, FGPu is often produced in tritium production reactors. FGPu can also be produced in power reactors, in initial core loads and in damaged fuel discharged after one year's irradiation.

"Reactor-grade" plutonium (RGPu) is produced in power reactors and contains 19% or more of the isotope Pu-240. In general, plutonium derived from current commercial light- and heavy-water reactors contains around 50-65% Pu-239, the remainder being largely Pu-240 and heavier isotopes of plutonium. As there are many types of power reactors, and differences in fuel composition, coolant and moderator system and burn-up level, plutonium commonly called RGPu can have various isotopic compositions, as illustrated in Table I. For the current generation of fuel, 60,000 MWd/t is seen as the limit, but eg DOE budget documents for FY 1998 show that the Department hopes to develop an advanced LWR fuel capable of reaching burn-ups of 100,000 MWd/t with enrichment levels of 5% U-235 [9].

Table I. Typical Isotopic Compositions of Spent Fuel at Discharge from Power Reactors [10]

Reactor type

Fuel burn-up
GWd/t

Isotopic composition, %

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

It should be noted that the plutonium isotope composition in the reactor core is not evenly distributed. Hence the figures discussed here for plutonium composition are average figures for discharged fuel. Even though fuel assemblies are moved around during refuelling, some parts of fuel rods will have a plutonium isotope composition closer to that of WGPu.

There is an additional sub-category: "MOX-grade" plutonium (MGPu), containing about 30% or more Pu-240 (MOX = mixed-oxide, ie a uranium and plutonium mix). MGPu is recycled plutonium, the plutonium in irradiated MOX--fuel made of RGPu.

4. ATTRACTIVENESS OF LOW BURN-UP PLUTONIUM

Irrespective of the arguments on what is theoretically possible with RGPu, there is no doubt that plutonium at a suitably low burn-up level is extremely attractive for nuclear weapons purposes, and that RGPu is less so.

The higher plutonium isotopes (especially Pu-240 and Pu-242) have substantially higher spontaneous fission rates than Pu-239, hence are prone to cause "pre-initiation" during a nuclear explosion (adversely affecting reliability and yield). Decay of the higher isotopes is also a cause of radiation and heat problems. A further problem, from the weapons perspective, is the Pu-238 content of higher burn-up plutonium. A number of commentators have noted that the isotopic composition of plutonium has a marked influence on its effectiveness for first generation weapon designs [11].

The particular attractiveness of WGPu is illustrated by former US practices and programs (no doubt similar examples can be found in the military programs of other weapon States):

The point can be made, that if the foremost nuclear-weapon State was unwilling to produce weapons from FGPu, neither reactor-grade nor even fuel-grade would be the material of first choice for a would-be diverter. Certainly there would be a premium on obtaining plutonium of the lowest possible Pu-240 content, if possible of weapons-grade or at least at the lower end of fuel-grade.

Reduced Shielding Requirements
Apart from questions of weapon performance, there is another reason for lower burn-up plutonium to be more attractive for diversion. Current safeguards approaches take into account the fact that spent fuel elements in reactor storage ponds are highly radioactive. This consideration led to the assumption that the conversion time (ie the time which would be required to convert the material concerned into the metallic components of an explosive device) for irradiated fuel in clandestine reprocessing activities would be increased considerably by the need for shielding. In addition to such activities being slower than for unirradiated material, the facilities would be more expensive and require sophisticated remote handling equipment.

Plutonium from low burn-up spent fuel, however, particularly if it has also had a long cooling time, would have substantially reduced shielding requirements [12], needing a much less massive facility than current reprocessing plants. For such material, after one cycle of solvent extraction (or ion exchange) downstream processing requirements could possibly be reduced to glove-box systems. Clandestine facilities of this kind could be extremely difficult to detect.

5. PLUTONIUM CURRENTLY UNDER SAFEGUARDS

At the end of 1996, there were some 586.4 tonnes of plutonium under IAEA safeguards. 528.2 tonnes were contained in irradiated fuel, 53.7 tonnes comprised separated plutonium outside reactor cores, and 4.5 tonnes were in the form of recycled plutonium in reactor cores [13]. These figures include plutonium in reactor cores. A significant fraction of the plutonium undergoing irradiation in reactor cores is of low burn-up, though this situation will have changed by the time the fuel is discharged.

Currently almost all the plutonium arising in the civil fuel cycle is from the normal operation of "thermal" reactors. Such plutonium is of high burn-up, well outside the range defined as weapons-grade. There are some exceptions, however, where the production of irradiated fuel containing low burn-up plutonium will be unavoidable - for LWRs, the examples of initial core loads and damaged fuel have been mentioned already. The content of higher plutonium isotopes in such fuel, while normally above that of weapons-grade material, is sufficiently low to warrant special safeguards attention.

The normal operation of on-load refuelling reactors (eg certain gas-graphite and heavy water reactors) can also result in some low burn-up fuel. Defective fuel is removed from PHWRs using the same normal refuelling procedures that are in place for removing intact fuel, and is generally treated no differently from normally discharged fuel [14]. However, defective fuel at some stations is stored in a special location. Reinsertion of failed fuel is not practised at PHWRs, for economic reasons. Apart from damaged fuel, fuel assemblies from the outer fuel channels would have lower burn-up levels.

Burn-up levels from the operation of various reactor-types are illustrated in Table II.

Table II. Reactor burn-up levels (in GWd/t)

Reactor type

Burn-up level corresponding to WGPu (7% Pu-240)

Burn-up level in typical initial cores

Burn-up level from normal operation

PWR

4.8

14

33.0

PHWR

1.6

n/a

7.5

RBMK

2.0

n/a

20.0

GCR

1.0

n/a

3.6

The IAEA does not publish figures showing how the plutonium is distributed by category, or at least by reactor type. But recently the information indicated in Table III was made available to the authors.

Table III. Distribution of plutonium in spent fuel of LWRs under safeguards by category [15] (as of 31.10.96)

Burn-up range, GWd/t

1.0-5.0

5.0 - 10.0

10.0 - 15.0

>15.0

(corresponding to WGPu)

(corresponding to FGPu)

(corresponding to RGPu)
Amount of plutonium, t

0.06

0.74

4.20

152.4
Fraction % of total

0.04

0.47

2.67

96.82

It can be seen from Table III that, of the civil plutonium currently under IAEA safeguards, there are at least 800 kg in the very low burn-up category (<10.0 GWd/t - corresponding to weapons-grade and the lower end of fuel-grade), and a further 4.2 tonnes in the upper range of fuel-grade. It should be noted these figures relate only to LWR fuel, and do not take into account low burn-up plutonium which may exist elsewhere.

At present there are limited quantities of separated weapons-grade plutonium under safeguards. There are small quantities in laboratory use, and there are critical assemblies which use such material in the form of coupons. The major source of weapons-grade plutonium under safeguards is excess plutonium from dismantled weapons. Currently this material is confined to the nuclear-weapon States, though there have been some suggestions that ex-weapons plutonium might be transferred to non-nuclear-weapon States for use in MOX fuel.

In the future, another major source of low burn-up plutonium will be the blanket material from fast breeder reactors (FBRs). FBR blankets will contain plutonium well within the weapons-grade range, even of "super-grade" (around 3% Pu-240). While it is commonly assumed this is not an immediate issue, because there has been a slow-down of FBR development, there are currently a number of FBRs and FBR demonstration projects. The Japanese reactor Monju is one example - depending on when Monju is restarted, blanket material could be being reprocessed within the next 3-4 years. It is reported that France has obtained WGPu from reprocessing blankets from the Rapsodie and Phénix prototype FBRs at Marcoule [16].

6. NON-PROLIFERATION ISSUES

The Canberra Commission on the Elimination of Nuclear Weapons was an international group of experts on security and disarmament issues convened by the Australian Government to develop ideas and proposals for a concrete and realistic program to achieve a world totally free of nuclear weapons. One of the conclusions in the Commission's Report, presented in August 1996, was that:

"A prohibition on production of all nuclear material at or near weapons grade may prove a practical step of considerable value in support of the eventual elimination of nuclear weapons and could be included in the proposed cut-off convention or a complementary international agreement" [17].

A proscription on the production of plutonium at or near weapons-grade would be an important confidence-building measure in support of the non-proliferation regime. As noted earlier, in the case of production of significant quantities of weapons-grade material the application of safeguards measures, though technically sound, might not provide the requisite degree of assurance about the future intent of the State concerned.

Weapons-grade plutonium has very limited use in civil nuclear activities, and there is no legitimate civil (or military) requirement for such materials which could not be met from existing stocks. Therefore a proscription on the production of low burn-up plutonium should not cause practical difficulties. There is a clear case for an international norm to this effect.

In addition to current operating situations where production of low burn-up plutonium cannot be avoided, potentially there will be large-scale arisings of low burn-up plutonium in the blanket material from fast breeder reactors. Since in the future production of blanket material will be the major reason for operating FBRs (ie to obtain plutonium for recycle), obviously it is not practicable to proscribe the production of such plutonium in irradiated blanket material. The real sensitivity over very low burn-up plutonium arises where it exists as a separated product, in other words if it is reprocessed so as to recover unirradiated low burn-up plutonium.

The concerns relating to the unavoidable production of low burn-up plutonium can be alleviated if there were an undertaking not to reprocess so as to separate such plutonium in unirradiated form. Where reprocessing of low burn-up material is proposed, arrangements could be put in place to ensure that it is reprocessed in stream with high burn-up material, such as FBR core fuel or LWR fuel, so that the resultant product will have a sufficiently high proportion of the higher plutonium isotopes. Obviously there is a need to adopt a definition which will avoid undue practical problems for industry while meeting non-proliferation objectives. Definitional issues are discussed further in Section 8 of this paper.

It is recognised there may be some economic penalty in the reprocessing arrangements outlined here, but the States concerned should be prepared to accept such costs in the broader interest of the international security environment.

Japan, which is proceeding with an FBR development and demonstration program, has indicated in-principle commitment to the approach outlined above, through a policy of blending, with reactor-grade plutonium, low-burn-up plutonium recovered during experimental reprocessing of FBR blanket material [18]. While commitments by individual States are important, clearly multilateral commitments will have greater effect. Consideration should be given to mechanisms for achieving this. As the Canberra Commission has suggested, one avenue is to address a general proscription on the separation of low burn-up plutonium as part of, or in parallel to, the development of proposals for a cut-off convention.

7. SAFEGUARDS ISSUES

Since it is a primary objective of safeguards to address the possibility of diversion, and it is difficult to escape the conclusion that low burn-up material would be most attractive to a would-be diverter, the assurance provided by safeguards would be enhanced if they were to place particular emphasis on such material.

If low burn-up plutonium is to receive special safeguards attention, what kind of measures might be taken? In terms of current safeguards practice, two parameters are particularly relevant in determining the inspection regime: timeliness and detection probability. If the current safeguards system were to continue unchanged, the authors would propose changes to both these parameters in order to recognise the sensitivity of low burn-up plutonium.

Timeliness
The current concept of timeliness is intended to reflect possible conversion time, ie the time which would be required to process plutonium into weapon components. For unirradiated plutonium, the timeliness goal is one month. The adequacy of this goal is debatable, especially where material more attractive for diversion is involved. For example, it is noted that the Agency has considered, though not introduced, a shorter timeliness goal, two weeks, for unirradiated plutonium in metallic form. Under the current concept of timeliness, isotopic composition is not taken into account, as it is not considered directly relevant.

However, isotopic composition can be relevant to timeliness. As discussed in Section 4, low burn-up fuel has significantly lower radiation levels, resulting in reduced shielding requirements. A shorter conversion time may well be possible compared with the diversion assumptions on which the current three months timeliness goal for irradiated material is based. Accordingly, in principle it can be argued that the timeliness goal for low burn-up material should be less than three months. Rather than propose a new timeliness goal, say six weeks, which would be necessarily arbitrary, the authors suggest this might be the same as for unirradiated plutonium, ie one month (assimilating low-irradiated fuel to fresh MOX fuel).

Another question is whether the timeliness goal could be extended for very high burn-up fuel, say above 30% Pu-240 (essentially, plutonium recycled in MOX fuel), for example six months instead of three. This is difficult to justify under the current concept of timeliness, which is based on conversion time. The radiation levels of high burn-up fuel are not sufficiently different to those of normal burn-up fuel to have a significant effect on shielding requirements, hence conversion time. Further, since high burn-up fuel is likely to be stored with fuel of normal burn-up, the practical benefit of a longer timeliness goal is not clear. On the other hand, this issue might be examined further when we reach the point where MOX fuel is being reprocessed. Plutonium which has undergone two or more cycles will have a very high Pu-240 content, and it is questionable whether the current timeliness requirement of monthly inspections for unirradiated plutonium would be warranted for plutonium of this quality.

Detection probability
This refers to the probability, if diversion of a given amount of nuclear material has occurred, that verification activities will lead to detection. The Agency's current detection probability goals for plutonium depend on the form of the plutonium and on the particular circumstances - for separated plutonium it is "high", 90%; for spent fuel under INFCIRC/153 safeguards, without containment and surveillance (C/S), it is "medium", 50%; where there is satisfactory C/S then in some situations the detection goal will be "low", 20%, or verification might consist only of an item count.

Detection probability is based largely on sampling and measurement plans. If low burn-up plutonium were to receive special attention, for a start it would be necessary to specifically identify it as such, so that it could be subjected to a specific sampling and measurement plan. Because at present there are limited holdings of unirradiated low burn-up plutonium, identification should be straightforward. In the case of irradiated low burn-up plutonium, it would be necessary to identify the particular fuel elements concerned. The verification task would be simplified if these fuel elements were grouped at a particular place in the spent fuel pond.

Low burn-up plutonium having been specifically identified, the authors suggest that its attractiveness be recognised by increasing the relevant detection probability goal by one level, ie where "low" would otherwise apply the goal should become "medium", and so on.

The Strengthened Safeguards System (SSS)
Under current safeguards procedures, prima facie the adoption of a reduced timeliness goal would require more frequent inspections. Likewise, achievement of a higher probability goal would require more inspection effort. The SSS however will provide the opportunity to obtain the additional assurance appropriate to low burn-up plutonium and at the same time to make substantial efficiency gains.

In particular, remote monitoring has the capability to allow the monitoring of events in or close to real-time, thereby achieving much shorter timeliness targets than can be attained through regular inspections. Thus remote monitoring can reduce inspection costs while at the same time increasing safeguards effectiveness. For these reasons the IAEA is working towards the widespread application of remote monitoring technologies.

Remote monitoring has major implications for current concepts of timeliness. It can be argued that if a remote monitoring system is well-designed and reliable, the current concept of timeliness would cease to have any practical application. If it is considered that some additional assurance is needed as to the ongoing integrity of the remote monitoring system (eg that it has not been defeated in some way), this could be provided through unannounced inspections. Unannounced inspections will form an important part of new safeguards approaches, serving a number of purposes, one of which could be to complement remote monitoring in this way. Whether the concept of timeliness should be a factor in determining the incidence of unannounced inspections would seem to merit further study.

Practical implications for safeguards
As indicated at Table III, the proportion of plutonium currently under safeguards derived from burn-up of 15,000 MWd/t or less is around 3% (note this figure applies only to LWR fuel). Information available to the authors indicates there are currently some 120 LWRs (ie ¾ of those under safeguards) with start-up fuel or spent fuel which has been unloaded after 1 year, which would fall into this category.

Key features of a safeguards approach suggested for low burn-up spent fuel might include the following:

A strengthened safeguards approach for unirradiated low burn-up plutonium might include dual C/S remote monitoring with real-time or near real-time data transmission and review, and a higher incidence of unannounced inspections.

Conclusions
Two points which emerge from the foregoing discussion are: that there is not necessarily a direct correlation between the current concepts of timeliness, detection probability and the strategic value of different nuclear material; and that in any event timeliness will have much less relevance where remote monitoring is deployed.

Assuming that low burn-up plutonium will not normally be produced or separated (in accordance with the proposals in this paper), but that more rigorous safeguards arrangements would apply where this does occur, then it does not appear that the increased inspection load on the IAEA will be substantial. It is recommended that the Agency make a detailed study of the disposition of low burn-up material and practical measures to provide adequate assurance with respect to this material.

8. DEFINITION OF "LOW BURN-UP" FOR NON-PROLIFERATION/SAFEGUARDS PURPOSES

As mentioned earlier, it will be important to determine a criterion for "low burn-up" which does not cause undue practical difficulties for facility operators and the IAEA, while at the same time meeting non-proliferation concerns. Is the appropriate dividing point fuel-grade, ie just under 19% Pu-240, or might some lower figure be considered?

It is understood one figure which has been looked at informally within the IAEA is 17% Pu-240. This would still be well outside the weapons-grade range, and may be a satisfactory figure for the purposes discussed here. Application of the 17% figure would have some practical benefit, in reducing the total quantity of plutonium which would require more rigorous safeguards procedures under the "low burn-up" category. From the limited information available, it would appear the total number of fuel elements falling within the "low burn-up" category might be halved. On the other hand, the additional safeguards effort involved does not appear to be onerous, and any saving might be considered marginal compared with additional assurance derived from applying the higher (19%) figure.

9. CONCLUSIONS

As a consequence of most plutonium under safeguards to date being "reactor-grade", the isotopic composition of plutonium has received only limited attention. Events in the DPRK serve to highlight an issue which, in the absence of appropriate action, can be expected to assume increasing importance - that the production and possession of significant quantities of plutonium at or near weapons-grade has the potential to undermine the confidence on which the non-proliferation regime is built. Accordingly, the authors argue that such material should be subject to the most rigorous control - the most effective measure being to limit its production and separation to the greatest possible extent.

There is a view that safeguards and non-proliferation measures should not differentiate between plutonium grades, because this might be seen as minimising the proliferation risks of reactor-grade plutonium and could lead to pressure to reduce controls on such plutonium. A reduction in controls on reactor-grade plutonium is by no means a natural consequence of differentiating between plutonium grades, however, and such a reduction is not advocated in this paper. Rather, the concern is that ignoring the different degrees of attractiveness resulting from isotopic composition could be counter-productive to non-proliferation objectives.

The authors suggest that an outline of a non-proliferation approach satisfactorily addressing the issue of low burn-up plutonium might contain the following elements:

(a) States would refrain from any avoidable production of low burn-up plutonium, eg through abnormal operation of reactors;

(b) reprocessing would not be undertaken so as to obtain low burn-up plutonium as a separated product (low burn-up plutonium would be reprocessed in stream with high burn-up material);

(c) where possible, accumulation of low burn-up fuel would be avoided - where a State uses reprocessing (by itself or by another State) as part of its spent fuel management strategy, low burn-up fuel would be given priority (with reprocessing carried out in accordance with (b) above);

(d) where significant quantities of unirradiated weapons-grade plutonium are held, eg for critical assemblies, consideration would be given to the possibility of replacing that plutonium by non-weapons-grade plutonium (similar to the international program to convert research reactors from HEU fuel);

(e) existing stocks of unirradiated low-burn-up plutonium would be diluted, or given priority for fuel fabrication, or permanently disposed of.

While non-proliferation arrangements along these lines can limit the production of low burn-up plutonium, where such plutonium does exist there is the question whether current safeguards measures are appropriate. If diversion of plutonium from safeguards were contemplated, it seems reasonable to assume that low burn-up plutonium would be of greatest interest to the diverter. The assurance derived from safeguards would be enhanced if safeguards approaches took this into account.

Where there are significant holdings of unirradiated plutonium, it is debatable whether the current timeliness goal of one month is appropriate as far as low burn-up plutonium is concerned, and remote surveillance with real-time, or near real-time, reporting to the IAEA would represent a considerable improvement. It can also be argued that the current timeliness goal of three months for irradiated plutonium is not appropriate in the case of low burn-up plutonium - while this conclusion might suggest the need for more frequent timeliness inspections, these could be obviated through the introduction of remote monitoring. In the case of both unirradiated and irradiated plutonium, unannounced inspections would be an important part of the safeguards approach. In addition, an increased concentration of verification activities is suggested for low burn-up plutonium.

Having regard to the relatively small proportion of plutonium currently under safeguards which is in the low burn-up category, a more intensive regime for such plutonium would not appear to have significant resource implications for the IAEA, nor to create any particular difficulty for facility operators.

REFERENCES

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

[2] See eg GILLETTE, R., "US Test Shows Nuclear Bombs Can Be Made From Low--Grade Plutonium", Washington Post, September 13, 1977.

[3] "Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium", DOE Facts (1994) 186-7.

[4] See for example, the French Government publication "L'énergie nucléaire en 113 questions" (1996) 113.

[5] PERSIANI, P.J., "Comments on Fuel Cycle Concepts and Impacts on Non-Proliferation and Safeguards Concerns", Proceedings of 19th ESARDA Annual Symposium (1997).

[6] See eg JAEGER, C., et al., "Joint US/Russian Plutonium Disposition Study. Nonproliferation Issues", 37th Annual Meeting Proceedings of the Institute of Nuclear Material Management, Naples, Florida (1996) 884-889.

[7] See eg "Plutonium: The First 50 Years. United States Plutonium Production, Acquisition, and Utilisation from 1944 to 1994," DOE (1996).

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

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

[10] Compiled from a number of sources.

[11] MEYER, W., et al., "The Homemade Nuclear Bomb Syndrome", Nuclear Safety, vol. 18, no 4 (1977) 427-418; VON SEIFRITZ, W., "Remarks on the Plutonium-240 Induced Pre-Ignition Problem in a Nuclear Device", Nuclear Technology, vol. 54, no. 3 (1981) 431; CARSON MARK, J., "Nuclear Weapons Technology", in FELD, B.T., et al. eds., "Impact of New Technologies on the Arms Race: A Pugwash Monograph", MIT Press (1971) 133-138; "Can Terrorists Build Nuclear Weapons", in LEVENTHAL, P., YONAH, A., eds., "Preventing Nuclear Terrorism: The Report and Papers of the International Task Force on Prevention of Nuclear Terrorism," Lexington Books (1987) 91-103; "Reactor-Grade Plutonium's Explosive Properties," Nuclear Control Institute, Washington DC (1990); GARWIN, R.L., "Explosive Properties of Various Types of Plutonium", paper presented at NATO Advanced Research Workshop "Managing the Plutonium Surplus: Applications and Options," Chatham House, London (1994).

[12] BRAGIN, V., ONG, L., "Radiation Levels Associated with Nuclear Reactor Spent Fuel", IAEA, STR-287 (1992).

[13] IAEA Annual Report for 1996, 74.

[14] MACDONALD et al., "Detecting, Locating and Identifying Failed Fuel in Canadian Power Reactors," Report AECL 9714.

[15] Information received from the IAEA.

[16] ALBRIGHT, D., BERKHOUT, F., and WALKER, W., "World Inventory of Plutonium and Highly Enriched Uranium 1992," SIPRI, Oxford University Press (1993) 44.

[17] "Report of the Canberra Commission on the Elimination of Nuclear Weapons", Department of Foreign Affairs and Trade, Australia (1996) 93.

[18] "Outline of 1993 White Paper on Nuclear Energy, Atomic Energy Commission General Remarks", Japan (1993) 11.