Gérard LEBASTARD
Director, International Business
COGEMA, Reprocessing Branch

Pierre KAPLAN
Deputy Director, Technical Direction
COGEMA, Reprocessing Branch

The US National Academy of Science (NAS) has called the world’s excess of weapons plutonium a "clear and present danger", and defined a standard for selecting disposition options of weapons plutonium: the Spent Fuel Standard. The goal is to make the excess weapons plutonium "as inaccessible and unattractive for retrieval and weapons use as the residual plutonium in spent fuel from commercial reactors".

The MOX option for disposition of weapons plutonium consists in fabricating a Mixed OXide fuel and burning it in commercial nuclear-power reactors thus leading to spent MOX fuel compatible with the Spent Fuel Standard. Both the United States and the Russian Federation are considering the MOX option for disposal of their weapons plutonium. While taking advantage of weapon-grade plutonium energy potential, the MOX option provides all the guarantees of safety and materials protection.

Based on the COGEMA Group MOX fuel fabrication expertise, the present paper will analyze the impact of the use of weapons plutonium on a MOX plant.

Pursuant to the Strategic Arms Reduction Treaties START I and II, many thousand of US and Russian nuclear weapons are planned to be dismantled within the next decade. The surplus to the military needs of weapons-grade plutonium (W-Pu) should then amount to about 50 mt (metric tons) on each side.

To rapidly reduce the risks posed by excess weapons plutonium, the governments of the United States and the Russian Federation have undertaken a review of the plutonium disposition options.

As reported in the FMDP^[1] General Requirements Document dated October 1, 1997, the DOE will provide for disposition of plutonium surplus by pursuing a dual-track strategy consistent with the Spent Fuel Standard . This standard is defined as "a concept to make the plutonium as unattractive and inaccessible for retrieval and weapons use as the residual plutonium in spent fuel from commercial reactors". A first option would consist in burning W-Pu as MOX fuel in existing commercial Light Water Reactors (LWRs) and potentially in CANadian Deuterium-Uranium (CANDU) reactors. The immobilization of the plutonium surplus for disposal in a repository constitutes the second track. The final decision regarding the implementation of these disposition approaches, is expected in 1998.

The Russian Federation has clearly expressed one’s support for the MOX option bringing up the considerable energy potential of W-Pu.

The MOX option, with the in-core degradation of the plutonium isotopic composition, leads to spent MOX fuel meeting the requirements of the Spent Fuel Standard. Furthermore, this option provides all the guarantees of safety and materials protection: with a thirty years experience in MOX fuel fabrication, the European MOX fuel industry and notably the COGEMA Group, is a fully operating industry with proven reliability of delivery and quality products, which operates under the control of competent national and international Authorities, in particular the IAEA and EURATOM.

The extensive experience acquired in Europe with MOX fuel fabrication by COGEMA can benefit to developing an expeditious and cost effective industrial tool for the peaceful use of all the W-Pu excess. The aim of this paper is to describe and analyze into what extent W-Pu characteristics impact on the main plant features compared to the current design of a MOX fuel fabrication plant. The following points will be discussed:

• the process for fuel pellet fabrication
• the necessary capacity of such a plant to rapidly bring down the W-Pu stockpile
• the plant safety (nuclear criticality control, shielding, confinement and cooling)
• the safeguards and materials accountability

The weapons plutonium is generally in the form of an alloy with a gallium (Ga) content up to 5 wt% (this content is generally equal or less than 1 wt%).

The first step of both MOX option and immobilization option consist in converting this metallic alloyed plutonium into plutonium oxide (PuO₂). Several conversion processes have been studied to obtain powders which are consistent with the pellet fabrication process, in particular the sintering stage.

The Russian "reference" process endorsed by the French/Russian collaboration (the AÏDA-MOX program which is now a Russian/German/French program), is a wet process mainly consisting in:

• the dissolution of W-Pu in a mixture of nitric and hydrofluoridric acids,
• the purification of W-Pu by an extraction process,
• the conversion of plutonium nitrate into plutonium oxyde by the usual oxalate process.

This process allows the elimination of gallium.

Regarding the United States, the studies conducted in the principal DOE laboratories have lead to the so-called "HYDOX" and "ARIES" dry processes. These processes combine operating sequences as follows:

• the hydridation of W-Pu,
• the oxidation of the hydride into oxide.

These dry processes lead to a partial elimination of gallium which residual content in the oxide powders is low, in general below 1%. This content has to be compatible with the in-pile fuel behavior. The first results seem to be encouraging.

The powders can possibly undergo a high-temperature heat treatment to reduce the residual gallium content.

The order of magnitude of W-Pu characteristics are given in Table 1 and compared to that of plutonium recovered from standard spent fuel.

The total Russian excess material would likely be the same as that declared by the United States (i.e. about 50 mt).

As specified in the FMDP^[2] General Requirements Document, the disposition program shall have the capability to complete the disposition of the approximately 50 mt within about 15 years. The required capacity of a MOX plant to cope with those conditions can then be estimated to about 100 tHM/y.

Such an industrial capacity level is already available at the COGEMA MELOX plant which has manufactured about 100 tHM in 1997, two years only after its commissioning.

The MIMAS process (MIcronized MASter blend), improved for the MELOX high scale MOX fabrication plant to become the A-MIMAS process (Advanced MIMAS), lead to very-high quality products manufactured from a wide range of basic nuclear materials. The impact of the utilization of W-Pu on the A-MIMAS process is analyzed hereafter.

A-MIMAS process consists in micronizing the PuO₂ powder in a ball mill with a part of the UO₂ powder to form a 30 % Pu content master blend. This first stage, which modifies the physical characteristics of the initial PuO₂ powders, tends to minimize their impact on the fabrication process, thus making easier any prospective adaptation to the W-Pu oxide originated from dry conversion processes. The master blend is diluted with a free-flowing UO₂ powder to obtain a final blend at the specified Pu content which is then homogeneized. Additives (lubricant and pore forming agent) are introduced when necessary, before the sintering stage.

Scraps (powders, green or sintered pellets) can be recycled at different stages in the process line after a specific treatment. The step of the master blend preparation is particularly suitable to introduce scraps. Indeed, the drifting effects that the scraps could trigger on the product quality before the sintering stage, are totaly inhibitated by milling the master blend and then, by diluting it with UO₂ powder. The industrial mastering of scraps recycling on significant quantities of fuels (more than 600 tHM) manufactured from a wide range of uranium powders has been largely proved ; those uranium powders are indeed originated from several conversion processes such as the ADU (Ammonium Di Uranate) and the AUC (Ammonium Uranyle Carbonate).

A-MIMAS allows a complete recycling of scraps. This efficient recycling ability is particularly suitable for a W-Pu MOX plant, as the totality of the process-charged plutonium is to be used or dry recycled.

The in-core performances of MOX Fuel Assemblies (FAs) and the in-core power distribution heavily depend on the Pu isotopic composition which has to meet very stringent requirements. A-MIMAS, through a specific software allowing an optimized choice of PuO₂ cans, is an answer to satisfy those requirements. Insofar as the isotopic range of W-Pu is more limited than that of reactor grade plutonium, the isotopic gestion to ensure the targeted energetic equivalence of the MOX fuel assemblies should be easier.

Finally, the lower heat dissipation related to W-Pu thus limiting the addditives deterioration would be favorable for the process.

As a consequence, the fabrication of MOX fuel with weapon grade plutonium would correspond, in terms of process, to the current low content fabrication campaign with reactor grade plutonium and would need obviously no process modification.

25 of 26 European "moxified" reactors are loaded with MOX Fuels manufactured in the COGEMA plants with the MIMAS and A-MIMAS processes. The accumulated experience in the utilization of those fuels state their excellent in-core behavior and performances, and demonstrates COGEMA's high capability of adaptation to meet the specifications related to various products.

With an isotopic composition close to [²³⁸Pu £ 0,05 %; ²³⁹Pu = 93,4 % ; ²⁴⁰Pu = 6 % ; ²⁴¹Pu = 0,6 % ; ²⁴²Pu » 0 %], weapon-grade plutonium is characterized by a very high proportion of fissile isotopes when compared to reactor grade plutonium. This characteristic is not perceived as a difficulty and criticality considerations when designing and operating a MOX fabrication plant dedicated to weapon-grade plutonium would not lead to disruptions compared to current concepts.

For a given MOX fuel assembly (FA) energetic equivalence, the levels of reactivity of the products are similar from the final blend onwards wether weapon-grade plutonium is processed or reactor grade plutonium. Then, the utilization of weapon-grade plutonium in the related areas of a MOX plant would not generate supplementary constraints.

This is not the case for the master blend preparation if the plutonium content of the master blend is the same for both fabrications. Nevertheless, this stage concerns a limited part of a MOX plant, and it has been demonstrated that the maximum mass of nuclear products allowed in the corresponding criticality areas and the subsequent equipment is only twice smaller in the case of weapon-grade plutonium. However, this mass limitation does not affect the production capacity: indeed, the number of MOX FAs manufactured from a W-Pu master blend batch would be almost equal to that generated during reactor grade Pu MOX FAs campaigns because of a required final enrichment much lower (for instance 3.05 % with W-Pu compared to 5.3 % with typical reactor grade plutonium).

To sum up, criticality constraints are limited to the master blend preparation stage thus entailing a reduction of the related equipment and powder batches size. But those constraints have no impact on the global production capacity.

The heat dissipation is essentially due to the presence of the ²³⁸Pu isotope. In the case of weapon-grade plutonium, the ²³⁸Pu percentage is less than 0,05%. This very low content explains the low level of W-Pu heat dissipation which is about ten times lower than that of reactor grade plutonium.

In the current MOX plants, the heat release is generally evacuated using:

• specific equipment which design allows an optimal cooling of the nuclear materials they contain,
• a permanent ventilation system.

The efficiency of the cooling technology implemented in the current MOX fabrication plants has been demonstrated for many years. The much lower thermal constraints in the case of weapon-grade plutonium could make the confinement ventilation sufficient to ensure cooling in normal situations; the permanent cooling ventilation system could then be suppressed.

This prospective simplification of a W-Pu MOX plant allowed by this lower level of constraint could contribute to further enhance the related cost effectiveness.

The glove box technology is commonly used in MOX fabrication plants to handle plutonium in all its forms (from powders to sintered pellets). The glove boxes and the related ventilation system constitutes an efficient first confinement barrier fully compatible with W-Pu handling. Indeed, very analogous to reactor grade plutonium, weapon-grade plutonium differs from it essentialy by its isotopic composition: this parameter generates less nocivity but on such a small scale that it is worth using the confinement technology well mastered in the current MOX plants.

The glove box technology used in the current MOX fuel fabrication plants prevent the workers from the contamination risk. In addition, the glove boxes have to be equipped with protections against radiation exposure. Two types of shielding are implemented:

• neutron shielding using moderators (hydrogenated materials such as polyethylene) and absorbents (boron, cadmium),
• g shielding using leaded Plexiglass or leaded glass.

As previously pointed out, the neutron emission in case of W-Pu handling is about 10 times lower than that of reactor grade plutonium. The same reduction factor is then applicable to the Dose Equivalent Rate (DER).

As shown in Table 1, the content of ²⁴¹Am isotopes which are intense g -emitters is lower for W-Pu (up to 0,6 %) compared to the maximum admissible value in the current MOX plants (3 %). This lower activity means a lower contribution to the DER but only when a small protection is needed (for instance pellets in boats, rods) because of the low energy spectrum. In the other cases, the lower activity due to Am has a much lower impact on the DER. The total activity is about 20 times smaller for W-Pu, but the g emission spectrum is tougher. As a consequence, the g DER is only 10 times lower.

A reduction factor of about 10 is generally observed when comparing the W-Pu DER to the reactor grade Pu DER, when estimated for a same source seen through the same protections.

To sum up, the efficiency of existing biological shielding is about ten times higher when considering W-Pu; radiological aspects can be then considered without any difficulty in the case of a MOX plant dedicated to weapons plutonium.

As far as weapons plutonium is handled in a MOX plant, nuclear management and control appear as a major issue.

As already mentioned, the reduction to the minimum of the personnal dose uptake is a prominent objective when designing a high-scale MOX fabrication plant. In the case of MELOX, this objective, has been achieved by means of a very high level of automation. As a first consequence, nuclear material is not easily accessible. As a second consequence, all operations are computerised including the follow up and control of operations on nuclear materials. The MELOX Safeguards Scheme was conceived by EURATOM in collaboration with the French national authorities and the plant operator at the time the plant was designed. This Scheme which is able to follow and verify on a continuous basis the internal and external flows and the inventory of nuclear materials is referred to as a "Continuous Inventory Verification" (CIV) system; it has now been operated successfully for two years. The efficiency of the implemented system has been confirmed in August 1997 with the annual nuclear material inventory: this inventory was achieved within one week with limited disturbancies in MELOX operation.

The significant feedback gained by COGEMA in the field of MOX plants operation has permitted the optimization of the nuclear material management in particular by minimizing the hold-up; this has been achieved in the MELOX case with:

• an effective gathering of dust,
• the recycling of the gathered dust.

The methodology used to design and manage this efficient system and the related equipment (neutron and g measurements systems, video cameras ...) remains valid when considering W-Pu.

The MOX fuel fabrication facility required to successfully implement the MOX option for the disposition of weapon plutonium needs industrial performances already existing and well mastered on routine operation, in modern and large plants such as MELOX.

This available technology guarantees the feasibility of the MOX option with the utmost level of safety, safeguards and cost effectiveness.

Furthermore, this unrivalled level of technical expertise allows to take advantage of the MOX option specificities:

• effective use of the considerable energy potential of weapons plutonium,
• consumption of a part of the plutonium loaded in the reactors,
• irreversible degradation of the isotopic composition of all the residual plutonium.

Worldwide peace effort is so important that it deserves the best of all available technology and experience. COGEMA is prepared to contribute to this effort.