M-F. Debreuille, A. Lavenu, B. Kniebihli
COGEMA
2-4 rue Paul Dautier 78140 Vélizy, FRANCE

J-C. Broudic
CEA
BP 171 30207 Bagnols/Cèze, FRANCE

Ph. Bretault
SGN
1 rue des Hérons 78182 Saint Quentin cedex, FRANCE

Generation of scraps and waste are inherent in any nuclear activity. They can be minimized but not totally avoided. The plutonium handling facilities generate solid waste that are difficult to decontaminate due to the resistance of plutonium oxide to the conventional dissolution processes.

Using silver II dissolution is an elegant and efficient method for treatment of scraps and plutonium bearing waste. The Centralized Alpha Waste Treatment unit (UCD) has recently started active operation on COGEMA La Hague site. It has already yielded encouraging results in decontamination of solid waste. Today, UCD Facility demonstrates that the leaching process using electrogenerated Silver II ions for the decontamination of alpha solid waste has reached its operational dependability. This unit is part of the solid waste management implemented in COGEMA facilities in accordance with the ALARA principle.

Generation of radioactive waste is the unavoidable consequence of any nuclear activity. In addition, the specifications of the end-products of this industry are so stringent that generation of rejected products (scraps) can not be avoided either.

The final disposal of solid waste containing long-life radio-elements has become one of the key issues of the nuclear industry from both technological and economical point of view. Long term storage in deep repositories, although very expensive, appears to be the only suitable solution. It is therefore of major interest to reduce the volume and radiotoxicity of waste to be disposed of in deep repositories. In that way, decontamination of solid waste to enable their surface storage is very attractive.

Plutonium handling facilities such as conversion or MOX fuel fabrication generate solid waste and scraps that are difficult to treat by conventional leaching and dissolution processes.

Since the beginning of the eighties, the silver II dissolution process has been developed in France for both:

• dissolution of PuO₂ or U/Pu oxide,
• and, decontamination of PuO₂ contaminated solid waste.

Since 1981, more than 10 units using silver II dissolution process have been implemented either in CEA (Atomic Energy Commission) research centers or in COGEMA plants. A distinction has to be made according to the function of the process:

• dissolution of PuO₂ or U/Pu mixed oxide under any physical form, powder or MOX pellets,
• decontamination of solid waste or Pu recovery from contaminated waste (metallic waste, plastic waste, incinerator ashes...).

In both cases, a suitable pretreatment has to be implemented according to the feed characteristics. All the applications of the silver II process have common features:

• they are all based on the use of silver II ion which has been selected for its strong oxidizing properties. This makes it possible to dissolve PuO₂ at ambient temperature in a nitric acid medium.
• to minimize the required amount of reagent, silver II ions are continuously regenerated by electrolytical process.

• they all use one of the two standard electrolysis cells:

• the main equipment are always made of zirconium or titanium, selected for their high corrosion resistance, and have a critically safe geometry.

Several pilot electrolysers were designed and operated by CEA, leading to the construction of the industrial applications presented hereafter. Two industrial dissolution processes are implemented on La Hague site.

In UP3 plant, dedicated to the reprocessing of foreign spent fuels (Japanese, German, Swiss, Belgian, Dutch...), T4 is the plutonium purification/conversion building. The silver II dissolution unit includes a small size dissolver designed to recover plutonium from off-specification PuO₂ batches, PuO₂ sample excess and PuO₂ powder from glove-box sweeping. Its capacity is 1 kg of PuO₂/batch. Since the active start-up of UP3 in 1989, 30 kg of PuO₂ have been dissolved in this unit.

In UP2-800 plant, dedicated to the reprocessing of French spent fuels, R1 is the shearing/dissolution building. The URP unit has a dual purpose:

• dissolution of aged PuO₂ powders for americium removal prior to using for the MOX fuel fabrication - capacity 2.5 t/year,
• dissolution of MOX fabrication scraps (MOX powders and rejected MOX pellets after crushing) - capacity 8 t/year.

The application of silver II dissolution to the decontamination of solid waste has been developed by CEA during the eighties in ELISE and PROLIXE Facilities (Fontenay-aux-Roses Research Center). These units are still currently used for the decommissioning of the radiochemistry building. Today, CEA uses a new unit for leaching and embedding alpha solid waste (LEDA) in its new ATALANTE Facility on Marcoule site.

After this development work, the process was industrially applied on COGEMA La Hague site:

• in the Specific Waste Treatment unit (UTD) implemented in UP3/T4 building,
• in the large scale Centralized Alpha Waste Treatment unit (UCD) belonging to the UP2-800 plant.

The ELISE Facility is designed to decontaminate solid alpha waste and has a capacity of 10 to 20 kg of waste per batch. It is installed in glove boxes. Following the silver II leaching process, the waste are rinsed, dried and conditioned for surface storage. Since the active commissioning in 1987, ELISE has processed more than 4 tons of solid waste.

The PROLIXE Facility is designed to treat alpha, beta, gamma waste. It has the same capacity as ELISE but is installed in a shielded hot-cell. Since the active commissioning, in 1988, PROLIXE has processed more than 4 tons of solid waste.

This unit has the same function as PROLIXE. It is used to decontaminate the solid waste generated in ATALANTE Facility. PROLIXE has also played an important part in the Research and Development works for the UCD unit.

This unit has been designed recently for the specific needs of UP3/T4 building. It will be able to decontaminate plastic waste (after shredding if necessary), metallic waste and specially metallic PuO₂ filters. This unit is now under commissioning, active start-up is expected in the near future.

The Centralized Alpha Waste Treatment unit is composed of three lines which have the following functions:

Line B of the Centralized Alpha Waste Treatment unit is composed of three main parts:

• sorting and mechanical pretreatment of the waste,
• silver II leaching,
• counting of the plutonium content of the waste drums before and after treatment.

Sorting and pretreatment are gathered in a single large size cell fitted with two opposite working faces:

• on one side, the normal operations are performed with light telemanipulators,
• the opposite side is fitted with gloves for the direct maintenance operations.

In this cell, the 120-liter-waste-drums are received through an air-lock and the waste are sorted into two categories:

• cellulosic waste,
• metallic parts with a poor corrosion resistance in the leaching solution.

• the large size metallic parts are cut into small pieces,
• the plastic waste are shredded.

Silver II leaching is performed in a 200-liter-vessel connected to the electrolyser where silver II ions are continuously regenerated. The initial leaching solution is a mix of nitric acid and silver nitrate (AgNO₃). The leaching solution is circulated between the leaching vessel and the electrolysis cell by means of a turbine in the circulation pot. Decontaminated waste are then rinsed, dried and packaged in new 120-liter-drums.

Counting of the plutonium content is performed for every drum entering and leaving the unit. According to the French regulation for long term storage, the maximum alpha activity in final package for surface storage is 0.1 Ci/t (0.37 10¹⁰ Bq/t). This limit corresponds to about 200 mgPu. As a consequence, every drum leaving UCD with more than 200 mgPu are not suitable for surface storage. The global efficiency of UCD line B can therefore be estimated with the following ratios:

UCD unit was licensed for active operation in July 1996. Line B of UCD started active operation in February 1997. Up to now, six decontamination campaigns have been performed. This first year of active operation was considered by the operator as a training and a learning period. The main objective was to demonstrate the ability of UCD to decontaminate a wide range of alpha contaminated waste liable to be generated on a reprocessing site. Over this period, almost 100 drums containing various types of alpha solid waste have been treated. Figure 3 shows that the capacity has steadily increased over that period.

Today, the experience gained with UCD line B can be summarized with the following figures:

Following lessons were learnt from the first year of active operation:

• Optimization of sorting:

• Efficiency of the decontamination process:

• The decontamination efficiency is excellent for most of the metallic parts ; especially for the empty PuO₂ cans which represent the bulk of the metallic waste generated by reprocessing and MOX fabrication activities. Considering waste drums containing only empty PuO₂ cans, following performances were reached:

• Decontamination efficiency of the plastic waste greatly depends on the efficiency of the shredding operation:

Thanks to the extensive Research and Development work on silver II processes performed by CEA, an innovative and comprehensive management for any kind of alpha solid waste issuing from Pu handling facilities has been implemented by COGEMA. The industrial applications of silver II dissolution in URP and UCD units, designed by SGN, are the key components of this new system. UCD unit is now able to reduce volume and radiotoxicity of alpha solid waste which are either accumulated or to be generated in COGEMA facilities. This perfectly meets the objectives of the ALARA principle applied to solid waste.

The first year of active operation of UCD has demonstrated the industrial feasibility and the efficiency of the silver II leaching process. This experience has also confirmed the necessity of sorting and pretreatment operations and their influence on the global performances.

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