Gerald Senentz
COGEMA
La Hague, France

Renaud Liberge
SGN
St Quentin en Yvelines, France

An active laboratory liquid effluent treatment unit was started in 1995, in La Hague - UP3 nuclear reprocessing plant.

The purpose of this facility is to decontaminate active liquid effluents containing corrosive ions, and to rout the activity towards vitrification. The treatment is based on precipitating actinides and separating them from the main stream by ultrafiltration.

Results for the two first years of operation have confirmed expected decontamination factors and flow rate. In more than 10,000 hours of operation only one of the two ultrafiltration modules which operate simultaneously was changed due to irreversible fouling.

The laboratory of UP3 reprocessing plant for process control analyses generates specific effluents. Unlike process effluents, their salt content may be high, combined with a substantial activity and plutonium concentration. Moreover, these effluents contain a significant amount of corrosive chemicals, forbidding concentration in evaporators.

Laboratory effluents thus require a specific treatment. They were bituminized till 1995 when a specific treatment unit started operation. This unit is aimed at decontaminating effluent stream and routing separated activity towards vitrification. It resulted in a drastic reduction of the waste volume.

The purpose of this paper is to describe the treatment unit and discuss results achieved.

UP3 commercial reprocessing plant is operating since 1990. It reached nominal capacity (800 tHM/year) in 1995.

The initial design of liquid waste management was based on vitrifying high level effluents, bituminizing low level and medium level effluents, and treating and discharging very low level effluents.

As the activity and volume of low level and medium level effluent generated throughout the plant were below expected values, COGEMA launched an ambitious programme in order to further minimise waste volume. The aim of this programme was to suppress bitumen conditioning and maintain only two outlets for effluent streams: treatment and discharge for very low level effluents and vitrification for other effluents.

Liquid effluents streams initially intended for bituminization were sorted into three categories: 1) very low active (VLA) streams; 2) active streams with none or very low salt and corrosive ions content, which can be concentrated by evaporation and then routed to the vitrification facility; and 3) other streams with both a substantial activity and a significant salt and/or corrosive chemical content, which need a specific treatment as glass specifications limit salt content, and protection of equipment against corrosion require a very low content of corrosive ions.

The third category exclusively consists of active laboratory effluents including sample excess, rinsing effluents and chemical reagents. The treatment implemented splits the incoming stream in: i) a decontaminated stream with most of the chemicals, and ii) an active stream with very low salt and corrosive ion content which is suitable for vitrification.

The laboratory analytical lines are sorted according to the chemical nature of chemicals used and effluents generated are segregated in order to reduce the volumes to be treated: effluents from salt-free analytical lines are directly sent to concentration units and then to the vitrification facility, and effluents from analytical lines using unwanted chemicals are sent to the laboratory effluent treatment unit.

Thanks to the development of alternative analytical methods free of unwanted chemicals, the volume of active effluents not suitable for concentration and vitrification diminished. The annual capacity of the treatment unit is about 100 m³.

To comply with VLA effluent classification requirements and vitrification specifications technical performances (design values) of the treatment unit are:

• a decontamination factor greater than 1,000,

• diminution of salt content by 500.

b decontamination factor is of no importance because of the very low b activity of the analytical effluents to be treated.

The process selected in light of performance requirements hereabove listed is ultrafiltration enhanced by co-precipitation. It consists in (Fig. 1):

• adding ferric iron and soda to the effluent stream, in order to precipitate ferric hydroxide;
• absorbing a emitters by ferric hydroxide floc (co-precipitation);
• concentrating the resulting precipitate through an ultrafiltration loop (volumic concentration factor of 50), and flushing out most of soluble ions with the permeate;
• washing the retentate using a weak alkaline solution, to further reduce the ion concentration of the retentate by ten (discontinuous diafiltration);
• dissolving the retentate by adding nitric acid;
• transferring the retentate to concentration units;
• transferring the permeate to the VLA effluent stream after control.

Inactive and active laboratory tests, on various effluents were carried out in order to optimise the amount of iron and soda added to the solution, as well as operating conditions like transmembrane pressure and temperature. The operating parameters of the unit are:

The laboratory effluent treatment unit of La Hague - UP3 plant comprises (Fig. 2):

• a reception tank, where effluents are analysed;
• a feeding tank, from which effluents are transfered to the precipitation tank;
• a precipitation tank (stirred tank), where iron and soda are added and co-precipitation occurs;
• a feeding tank (stirred tank), which is used as a lag tank to transfer effluents to the ultrafiltration loops;
• two parallel ultrafiltration loops, each of them containing a concentration tank, a self-priming centrifugal pump (called "PAAC") of 8 m³/h capacity, an ultrafiltration module, a heat exchanger and a pressure - regulating valve;
• a permeate reception tank;
• a retentate re-dissolution tank.

Although the capacity of one ultrafiltration loop suffies to treat effluents, both loops are operated simultaneously in order to minimise operating time (thus allowing the operator of this unit to cope with other tasks).

The ultrafiltration module was adapted to specific requirements, especially to comply with the maintenance principles implemented in La Hague reprocessing plants.

Each module (Fig. 3) consists of a fixed tank under a concrete slab floor housing a removable subassembly containing 18 membranes creates a filtration surface area of 0.2 m². Inlet and outlet fluid pipes (feeding, permeate, retentate) are connected to the tank.

One module - subassembly and housing - is very compact (outer diameter < 0.2 m, length < 1 m). The subassembly, like other elements being subject to maintenance (pumps, valves...) and installed under the shielded slab, can be removed using a mobile shielded cask.

The ultrafiltration modules are fitted with commercial mineral tubular membranes from French manufacturer Tech-Sep (Carbosep membranes). These membranes are made of a carbon support and a thin zirconia layer ensuring required filtering performances.

Effluents are pumped and circulate in the ultrafilter cylinders. Liquid that permeates through the side membranes, flows out into the permeate reception tank.

The laboratory effluent treatment facility is operating since August 1995. As of October 1997, the two ultrafiltration loops have accumulated more than 10,000 working hours and the volume of effluents treated is 135 m³.

Pu concentration measured in the feeding tank is generally around few mg/l. In the permeate reception tank, after treatment, it is generally lower than the detection limit (1 µg/l). Plutonium decontamination factor is then about a few thousands, sometimes up to 10,000. Few peaks of Pu concentration (about two per year) were observed caused by insufficient neutralisation: pH is a key process parameter. In these cases effluents are recycled.

Substantial decontamination factors have been measured for uranium and ruthenium (several dozens), while caesium decontamination factor is negligible. Significant decontamination factor has also been obtained with antimony (up to ten).

In January 1997, one ultrafiltration module has been replaced because of low throughputs due to irreversible fouling. The very low ambient irradiation level (few hundreds of nSv/h) allowed the replacement of the old module without using the mobile shielded cask: vinyl bags ensuring the containment were sufficient.

The other module is still in operation.

Comparing throughputs of the original module still in place (cumulating more than two years of operation) and the new one shows the efficiency of periodical chemical cleaning with only a slight evolution of performances with time (Fig. 4) :

• about 3.5 x 10^-5 m³ .s^-1.m^-² (125 l. h^-1.m-²) for the new module,
• about 2.1 x 10^-5 m³ s^-1.m^-² (75 l.h^-1.m^-2) for the old one.

Today, the load factor of one ultrafiltration loop calculated with the design value of effluent volumes to be treated would be about 60 %, which meets the expected value. However, thanks to the continuous optimisation of analytical methods leading to a reduction of laboratory effluents, and because both loops are operated simultaneously, the actual load factor is only 20 %.

Chemical regeneration of the membranes is performed at the end of each batch in order to recover the initial permeation flow rate. It consists of: i) rinsing the loop with a diluted alkaline solution in order to mechanically remove most of the deposit without dissolving activity; ii) further cleaning the loop using acid in conditions of low pressure across membranes, in order to chemically dissolve the remaining traces of precipitate without draining activity into the permeate circuit.

The efficiency of the cleaning is checked by comparing throughput of acid with reference values. If necessary, the cleaning is repeated, every time with new fresh acid solution. A final alkaline neutralisation of the loop is carried out just before starting a new treatment.

The treatment unit for active laboratory liquid effluents has been operating successfully since 1995: no more laboratory effluent is sent for bitumen conditioning since its start-up. It contributed to minimising the total volume of high-level and transuranic waste without any increase of activity released to the sea. The corresponding activity has been directed to the vitrification of high level waste.

The treatment unit is part of the new advanced waste management concept launched by COGEMA which is aimed at reducing the high activity waste volume to approximately 0.5 m³/tHM by the year 2000. The 1980 design estimations reached more than 3 m³/tHM. Today the volume of high activity waste produced is already below 1 m³/tHM.

After more than two years of industrial operation, enhanced ultrafiltration has achieved very good performances with regards to alpha decontamination of effluent containing soluble unwanted chemicals. Expected flow rates have been reached. The dreat reliability and long lifetime of mineral membranes have been demonstrated.