DEVELOPMENT OF NEW PROCESSES FOR
FRENCH CEA RESEARCH CENTER WASTE VOLUME
REDUCTION AND CONDITIONING MATRIXES IMPROVEMENT
Gerard Naud
Atomic Energy Commission (CEA)
Cadarache Nuclear Research Center, France
Rosemarie Atabek
Atomic Energy Commission (CEA)
Fontenay-aux-Roses Nuclear Research Center, France
Amaury De Buzonniere
Technicatome
Saclay Nuclear Research Center, France
ABSTRACT
The management of research center waste is a priority program. One of the objectives of the CEA is to renovate those of its facilities that process the wastes of various sites. This challenging objective implies R&D efforts to define and develop new processes for effluent treatment and waste conditioning.
INTRODUCTION
As the management of radioactive wastes is a priority program in France, the objective of the CEA is to renovate the treatment of these wastes with the aim to:
The production of wastes, both in volume and characteristics, together with the specifications of releases, varies according to the sites. Waste management structure must therefore be adapted to each situation and, as a result, differs according to the site. However, CEA general policy with respect to wastes has led the research and development teams to define and develop new processes able to provide solutions applicable to these various situations.
EFFLUENT TREATMENT
The production of effluents from a Research Center is not constant with time. The effluents are produced by research laboratories, their chemical and radiochemical composition and their flow rates vary within a wide range. This characteristic means that the processes to be implemented have to be highly flexible.
Effluent radioactive release authorizations are very restrictive. The decontamination processes that have to be developed must therefore be extremely effficient. Release specifications also have constraints with respect to admissible anion and cation concentrations.
These characteristics impose sorting the effluents at their source and a specific management so as to adopt a processing strategy that permits attaining the objectives that are set.
The variety of waste flows and the performances to be obtained mean that we have had to envisage setting up different treatments. The presence of one or several evaporators in the treatment systems is essential in order to obtain the desired decontamination factors. The aim of the other techniques to be implemented is to:
In any cases, the wastes produced have to be compatible with the projected conditioning processes.
Faced with this problem, the CEA, in order to renovate its effluent processing plants before 2005, has chosen to study and develop chemical treatments associated with membrane techniques for solid/liquid separation. The use of a cross-flow membrane technique provides numerous advantages: improvement of the decontamination factors, reduction of reagent doses, and stability of the flow rates to be treated.
Beforehand, a strategy for the management of the various streams enables identifying the reference effluents on which feasibility studies will be performed. Table I gives an example of the types of effluent to be treated in a future processing plant.
The decontamination factors to be obtained in this case are 1000 for a emitters and between 100 and 500 for b g emitters.
TABLE I. Chemical and Radiochemical Characteristics of Research Center Effluents
|
Characteristics |
MAS Effluent |
MA/FA Effluent |
EVA Effluent |
|
pH Total salinity (g/l) NaNO3 (g/l) Other anions |
acid 20 15 |
neutral 5 to 20 4 PO43- SO42- |
neutralized 400 250 to 300 PO43- SO42- |
|
Major radionuclides |
Pu, Am |
Pu, Am Cs, Co, Sr, Ru, Sb |
Pu, Am Cs, Co, Sr, Ru, Sb |
|
Minor radionuclides |
Cs, Co, Sr, Ru, Sb |
|
|
The laboratory feasibility studies showed that decontamination could be performed with a single treatment for the three types of effluents. These performances can be obtained by adapting the reagent doses: ferrous and ferric hydroxides, copper hydroxide, barium sulfate, mixed nickel and potassium ferrocyanide.
The choice of a membrane technique depends on several parameters. The immediate interest of industrialist is to select a membrane with the greatest pore radii so as to benefit from a high filtration flow rate. This will enable reducing the surface of the membrane that is installed and thus minimizing investment. However, the proposal must take into account the drop in selectivity and the risk of in-depth plugging when the diameter of the particles to be retained is close to that of the membrane pores.
In this case, our choice was microfiltration that allows high filtration flow rates. However, a definitive selection will be confirmed after testing on representative active effluents.
With this aim, a test bench has been installed in a shielded cell. This bench can receive several types of membrane: microfiltration, ultrafiltration, and nanofiltration membranes. In the last case, the size of the pores is of the order of the nanometer, which permits stopping organic compound molecules with a molar mass of more than 300g/mol and partially stopping multivalent ionized salts. Thus, sodium nitrate, a preponderant compound in evaporation concentrates, can be partially eliminated using this technique. The active application of this technique has been made possible by the evolution of the structure of the membranes allowing reduced operating pressure (around 5 bar).
WASTE CONDITIONING
Current Status
Wastes from low-level and medium-level effluent processing are, at present, conditioned by cementation and bitumen embedding.
Ashes from the incineration of dry active waste (DAW) and organic liquids are stored temporarily in drums while awaiting their ultimate conditioning.
DAW that cannot be incinerated is compacted, then placed in drum or containers.
Various wastes are stored temporarily while awaiting processing.
The R&D objective is to propose:
There are three secondary objectives:
Evaporator Concentrate Conditioning
The effluents produced by the laboratories of the Saclay Research Center are concentrated in an evaporator. After an insolubilization treatment, the concentrates are embedded in bitumen.
These liquid wastes contain about 300 g/l-1 of mineral salts, mainly sodium nitrate. Their radiochemical activity is of the order of 0.1 Gbq l-1, essentially due to Cs137 (see Table II).
Table II. Example of Evaporation Concentrate Composition
|
CHEMICAL COMPOSITION
|
|
|
Element |
% |
|
Na K Ca NO3 SO4 PO4 |
32 2.1 0.75 35 1.8 9.3 |
|
Total 278 g/l of salt |
|
|
RADIOCHEMICAL COMPOSITION
|
|
|
Element |
% |
|
Cs137 Co60 Sr90 Pu238 H3 Other |
82 7.2 3 0.3 5 2.5 |
|
Total 80 Mbq/l |
|
Among the processes considered for the replacement of bitumen embedding, ceramic processing (Ref. 1) has been submitted to evaluation. The choice of the conditioning matrix was nepheline (NaAlSiO4), for the following reasons:
The process being studied includes a pretreatment with a view to:
The first objective is reached using an oxalic acid treatment performed at 70/80°C to eliminate the resulting nitric acid.
The second objective is met by means of calcination after the introduction of additives (Al2O3 and SiO2).
The material is consolidated by sintering at 1100° C.
The different phases were refined in the laboratory on simulated concentrates, then on real concentrates. These tests enabled demonstrating the feasibility of the process and verifying the qualities of the ceramic produced .
This study was completed by testing different stages at pilot-study scale.
Two orientations were envisaged for the pretreatment:
The first orientation has proved promising and its development is in progress.
In fact, this new conditioning process would enable meeting the objective of reducing waste volumes. The treatment of 3 000 m3 of effluents per year results in the production of 34 m3 of bitumen waste forms, whereas the « ceramization » of concentrates would result in only 8 m3 of ceramic waste forms.
Isoflash
This apparatus is a dryer in which the liquid to be treated is sucked up by a flow of hot air that has a helicoïdal movement. The liquid is pulverized in fine, evenly-sized droplets. The apparatus is designed to reach high temperatures.
Isoflash is thus a simple tool, with no moving parts, able to dry and calcine an effluent. As mentioned above, it has been successfully used to produce powders that were sintered. Other applications are envisaged in the waste management field, such as the treatment of effluents loaded with organic products. Isoflash is under development at Cadarache in collaboration with Technicatome.
The Plasma Torch
The diversity of the wastes awaiting treatment incited the CEA to develop a flexible treatment process. In accordance with its strategy of waste volume reduction, the CEA acquired and tested a Plasma Arc Centrifugal Furnace at Cadarache (Ref. 2, 3, 4).
The plant includes a furnace equipped with a 150 kW transferred arc plasma torch, secondary combustion chamber, and gas treatment. The plant can process 20 Kg.h-1 of waste per hour.
The tests were performed on inactive wastes to which tracers were added, simulating radionuclides such as cerium, cobalt, and cesium. The plant allows testing the whole process, including the high temperature gas filtration system, on a ceramic filter that can be cleaned.
The vitrified slag of the simulated wastes was characterized:
The gas filtration system proved highly efficient and, at the end of the tests campaign, the ceramic candles were recycled in the furnace.
The feasibility of incinerator ash vitrification was demonstrated: the volume reduction factor range from 3 to 5.
The processing of mixed wastes was successfully tested (ashes + oil, zeolite mixture, resins, etc.).
In view of the first tests, the reconditioning of old wastes (bitumen waste forms, cement waste forms, etc.) seems possible.
Tests are continuing on different types of wastes, including various debris. The current objective is to make the plant entirely nuclear so as to be able to test the treatment of real active wastes in the year 2001.
CONCLUSION
All the processes presented here should enable providing appropriate solutions in order to modernize and complete waste management at the CEA.
REFERENCES