G. Maurin and P. Suru
SGN

F. Chotin
COGEMA
1 rue des Hérons, Montigny-le-Bretonneux
78182 St-Quentin Yvelines Cedex - France

Within the framework of the volume reduction strategy of the waste issuing from the spent fuel reprocessing in its UP3 and UP2 800 plants of La Hague, COGEMA has decided to build a new facility intended for the compaction of hulls and end pieces (ACC).

The ACC facility will enable the compaction of not only the hulls and end pieces, but also the technological waste issuing from the reprocessing operations.

The waste are compacted under a pressure of 200 MPa, they are then conditioned in Universal Canisters ( Standard Canister of Compacted Waste : CSD-C ) also used for fission products glasses.

The waste volume is reduced by a factor 4. The design of the ACC facility allows the conditioning of hulls and end pieces equivalent to 2600 TU/year and of technological waste equivalent to 200 Universal Canisters.

A significant R&D program has been carried out both for the process aspect (compaction) and for safety aspect as regards the facility and the final product.

ACC facility will be operational at the beginning of the year 2000.

In order to improve the way waste are treated and to benefit from the excellent overall result of the reprocessing operations, COGEMA has developed a new advanced waste management concept. The ACC facility is the more visible part of this new management.

This COGEMA's strategy is focused on decreasing the overall volume of high activity waste stored in underground repositories to approximately 0.5 m3/tU significantly less than 1980 design estimations which reached more than 3 m3/tU.

Today, different type of high and intermediate activity waste arise from the reprocessing activity :

• fission products, vitrified and conditioned in glass universal canisters, known as CSD-V,
• hulls and end pieces, separated from the irradiated fuel in the head-end of the reprocessing plant

These structural waste were conditioned by cementation, a process which was licensed by French regulators.

Since mid 95, they are stored in an interim storage facility in drums filled with water, waiting for compaction and conditioning in compacted waste universal canister known as CSD-C.

• technological waste, generated not from the fuel itself but from the reprocessing operations.

All the technological waste were conditioned in a single type of fiber concrete canister.

A drastic minimization has already been obtained by comparison to the design waste volume, by relying on the high equipment reliability and improved sorting, performed at the entrance of the solid waste processing facility (AD2). Sorting allows to send more than 80% of the technological waste to surface disposal.

These gradual improvements have given COGEMA the possibility to set up another stage in the waste management strategy. This new step consists, for hulls and end pieces and high activity technological waste, in both :

• significant reduction of the waste disposal volume by compaction,
• standardization of the packaging for deep disposal in universal canisters.

The implementation of this strategy requires the construction of a new facility for compaction. This facility, known as ACC, will compact hulls and end pieces and high activity technological waste and will place the compacted discs in CSD-C.

The compaction process required a significant R&D program.

It was developed in 4 phases :

• preliminary tests in 1982 conducted by CEA,
• inactive hulls compaction process development by KFK from 1982 to 1990,
• inactive full scale hulls, end pieces and component waste compaction process development led by SGN and COGEMA in 1993 and 1994,
• compaction tests with active hulls at a reduced scale ( 1/4 ) led by KFK in 1994 within the framework of the COGEMA-GNS cooperation contract.

The 2 latest development phases of these tests are described hereafter.

These tests were conducted in France by SGN under COGEMA contract.

An optimal pressure of 200 MPa was confirmed, enabling product densities in the range of 65% to 69% of the theoretical density .

The volume reduction ratio of the raw waste is higher than 4 .

• Handling behaviour of the disc

• ×Dropping the disc from a height of 2 m.
  2 m is a conservative value for a disc dropping height in the ACC compaction project.
  The integrity of the disc has been checked.
• Lid pull-off test
  These tests demonstrated good overall strength with a force up to 50 kN to tear off the lid of the can.
• Uniformity of discs

This test phase was carried out in two steps :

• inactive tests
  to allow the comparison with the full scale inactive tests.
• active tests
  in order to predict the results at full scale and in active conditions.

The simulant material is the same as that used for the full scale inactive tests.

Several analyses regarding the quality of the product were carried out:

• effect of lubrication,
• effect of can height,
• effect of compaction speed,

At reduced scale, compaction was performed on the hulls alone without end piece. The size of the can did not allow the incorporation of end pieces.

The volume reduction ratio is lower than full scale inactive results.

Split tensile test on compacted disc have also been performed.

The tests demonstrated good overall strength with a force of about 50 kN required for the dislocation of the disc.

Three types of hulls were compacted. They originated from the following fuels reprocessed in UP3 by COGEMA :

• BWR Brunsbuttel 26000 MWd/t
• BWR Gundremmingen 9000 MWd/t
• PWR Grafenrheinfeld 39000 MWd/t

The following points were analyzed for the different fuels :

• effect of burn up,
• effect of the amount of fragments present in the waste among the hulls,
• variation in density as a function of compaction pressure,
• analyses of gases emitted during compaction.

The compacted discs obtained from active tests were tough and their density was slightly lower than 60% of that of zircaloy. Taking into account the negative effect of the reduced scale, the best industrial estimate of the compaction ratio for irradiated hulls is higher than 60%.

The burn up, in the range processed, was found to have a very small influence.

After visual inspection of the discs and of their content (by removing the lid), it appeared that no significant production of fines from zircalloy hulls had occurred during compaction. The initial presence of fragments mixed with the hulls had no effect on the result.

After removal of the lid, one to two hulls were extracted with great difficulty using a remote handled tool (chisel type), confirming the excellent mutual cohesion of the hulls.

The activity release, relative to activity inventory, was measured in the off gas after compaction. This release was small for gas (Kr85, Tritium), and very small for dust (Co60, Ru106, Sb125, Cs137, Cs134 ). Co60 is an impurity of Zircaloy base metal of the hulls, others are mainly fission products.

The measurements showed a very limited escape of fine zircaloy particules, so called fines.

The mechanical process for the hulls and end pieces is composed of the following steps (see figure) :

• reception and unloading of the drums of hulls and end pieces,
• separation of hulls and end pieces and controlled filling of the can,
• drying under inert gas,
• compaction by a 25000 kN press,
• filling of the CSD-C container with the compacted discs and welding of the lid,
• control of non fixed surface contamination and decontamination (if needed),
• activity measurement and fissile material counting,
• CSD-C containers shipment to interim storage.

Concerning the technological waste, the functions are the following ones :

• component waste reception,
• segmentation and can filling,
• loading of the can with hulls if wanted,
• the further operations are the same as described before for the hulls and end pieces : drying if necessary, compaction...

The compaction process achieves a volume reduction by a factor about 4.

The design of ACC allows the conditioning of waste equivalent to 2600tU/year.

A schematic representing the CSD-C design is shown on the following figure.

The outer geometries of compacted waste and vitrified waste universal canisters are similar, allowing the use of the same handling system for both canisters.

The CSD-C is the end product of medium-level long-lived wastes and high-level short- and long-lived wastes conditioning, as classified by French regulations.

Radiological characteristics of typical CSD-C resulting from a reference PWR fuel (initial enrichment 3,7%, burn up 45000MWd/t, cooling time 8 years ), assuming mixed structural elements (90% by weight) and technological wastes (10% by weight) are given hereafter :

Fission products glass CSD-V typical characteristics are given for comparison.

Canister lids are equipped with a filter so that off gas release resulting from radiolysis and evaporation of any residual water, or organics liable to be present in technological wastes are made possible without any other material leakage.

The choice of the canisters material has been carried out in order to comply with several mechanical and corrosion resistance requirements. Stainless steel with low carbon content is the best candidate, some additives being involved in local corrosion risk prevention.

Containment and handling possibility are preserved in case of dropping.

Canisters are sealed by a plasma arc welding process adapted to their geometry.

This welding process has been qualified through a test program and the successful results obtained :

• full penetration, no lack of fusion,
• no gas pores,
• mechanical characteristics in accordance with those of base metal
• limited temperatures inside the canister :

Full-scale drop tests were conducted, aimed to check the behaviour of a canister dropped during handling.

Drop tests were performed in two design configurations accounting for all possible situations in La Hague facilities :

• 9 m drop of a canister on a rigid floor,
• 7.7 m drop of a canister on another canister.

After these tests, the canisters analysis confirmed the satisfactory design of the CSD-C :

• no crack or incipient crack,
• limited deformation allowing easy handling of the canister.

Zircaloy, the material used in fuel element clads, has pyrophoric properties approaching those of zirconium metal. Its ignition risk is contingent to the state of division of the metal (particle size and specific surface area), its arrangement (in thin or thick layers, in clusters), which condition the specific reaction area and the oxygen diffusion, and on the heat extraction capacity of the environment.

Literature contains a large bibliography on the pyrophoricity risks of zirconium.

KFK conducted R&D tests on small zirconium hull particles (<100µm) produced with a disk saw. Since these particles are not completely representative of the materials handled in COGEMA's installations, COGEMA, with the technical assistance of the CEA, decided to conduct a major R&D program based on products representative of those present in the CSD-C.

Taking into account our ignition tests, confirming bibliography, 3 grain size classes A, B, C have been conservatively defined :

Adapted mechanical processes have been developped in order to produce A, B, C simulants.

Active hulls and fines, produced by different shearing machines and from various fuels in UP2 and UP3 head end facilities have been sampled and characterized.

Analysis have shown that Zr fines grain size distribution is dependent on the shearing machine; a significant proportion of the smallest Zr particles do not follow hulls and end pieces : they are sent to vitrification with fission products; moreover, analysis of Zr fines of diameter < 50 m m revealed that they were already made of 50% ZrO2.

A conservative reference Zr fines mixture has been defined : 7% A, 60% B, 33% C for safety analysis. These fines represent only 5% of the total Zr mass mainly constituted by hulls.

· Ignition tests on fines

· Ignition tests on discs

• 8 different discs, 7 filled with reference fines mixtures and 1 with a conservative fines mixture (27%A, 47%B, 26%C ) have been heated up to 500°C, without ignition. Two of these discs contained conservatively 5% water and gave the same satisfactory result.
• Two discs weighting 1.6 kg, filled with reference fines mixtures have been submitted to impact tests representing the design shock energy density corresponding to load drop accident: no ignition was observed.

The results obtained by the comprehensive Zr pyrophoricity R&D program performed for the ACC facility are the basis for CSD-C safety demonstration. These favourable results should not be opposed to the bibliography.

Previous tests have been reproduced giving consistent data. Zr fines ignition risk is dependent on the very characteristics of fines and process conditions. Representative fines mixtures are less pyrophoric than the conservative simulant used in the past. Specific tests have been adapted to each step of the process, in order to define the appropriate preventing measures.

As it will be shown hereafter, CSD-C discs are in very favorable conditions as far as pyrophoricity safety is concerned.

Finally, the risk of ignition can be ruled out in the discs for the following reasons :

1. Conservative and representative fines mixtures inside discs do not ignite at temperature up to 500°c.

These results are due to compaction which prevents oxygen diffusion in the waste and improves thermal diffusion.

2. The maximum layer of Zr fines on a disc, inside the CSD-C, is not sufficiently thick to ignite.

The ACC press is specially designed for drawing up fines which might get out of discs.

3. Ignition sources are not present :

• Design temperatures are low (T<100°c) because of a low heat release and sufficient thermal conductivity
• Discs would not ignite in case of maximum design load drop
• Sparks due to static electricity may be ruled out because CSD-C and its content are at the same electrical potential 

The qualification program for compacted hulls, end pieces and technological wastes Universal Canisters called CSD-C has been conducted by SGN for COGEMA with technical support from CEA and is nearing completion.

Sufficient information is available today to guarantee the quality and safety of these canisters which will be produced by ACC facility in 2000.

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