La France nucleaire/Nuclear France: TREATMENT OF RADIOACTIVE WASTE(ENGLISH)

Radioactive wastes are treated by various widely different measures, and new technologies are always being developed. We discuss here briefly only one aspect of this complex subject: volume reduction.

The first part of the CEA’s Spin program was Puretex, which grouped research tending to decrease the volume and the activities of the waste, in particular secondary wastes coming from reprocessing [Bataille 96]. Moreover producers of waste work to reduce the volume of waste that they must send to storage or disposal centers in order to reduce the cost of disposing of these wastes.

There are at least five methods of approaching the problem of radioactive wastes:

Rigorous sorting prevents the mixing of waste that is lightly contaminated with waste that requires treatment and/or costly storage. Practiced by most operators today, it is the only means of avoiding the creation of secondary waste, waste that is a byproduct of the treatment of waste.

Presses designed to reduce the volume of solid wastes are located at most major production sites. The risks are minimal, but, depending on the waste compacted, the wastes may disperse gaseous effluents and liquids that must be trapped and packaged.

Used since the creation of nuclear sites, decontamination consists of such treatments as coprecipitation of contaminated liquids, the sanding of metals to eliminate surface contamination, and the soaking of metallic wastes in a chemical bath. Secondary wastes are always created-the sludge from precipitation, contaminated sand, contaminated liquids . . . Today, decontamination is used to change the category of given wastes. Thus, one can begin with medium-activity solid wastes and end up with solid wastes said to be of low activity and a great quantity of effluents that are themselves treated to become sludge also described as of low activity or effluents of low activity that are then released into the environment.

Called “recycling”, this method saves nuclear materials. However, the recovery of nuclear materials creates secondary wastes including radioactive effluents and sometimes consists of operations that clearly increase the risks for workers. Jean-Luc Lamy, a health physicist explained: “Before, campaigns [by the wet method] were long and voluminous, homogeneous, clean. Now more and more often, one finds relics, discards, old stocks more or less known or listed. It is necessary to eliminate them, to recover them, while avoiding being trapped by a package that has been badly or not at all marked. A small, unfortunate package of PuO2, an incorrect package and it’s the plague. Therefore the ‘varied’ campaigns, with multiple lots, true radiological lotteries are regarded with horror” [Lamy 93a].

This method, which encompasses a wide variety of processes, is increasingly used:

--Melting metals: the CEA melted lead at Marcoule and at Saclay in the sixties and seventies. More recently, it experimented with the melting of metals at Saclay, and the melting of hulls, especially at Marcoule. A furnace, located near the closed G2 and G3 reactors at Marcoule, melted contaminated metal from the two reactors and from other CEA centers. Other furnaces were located at Alès and at Fos-sur-Mer. Centraco, near Marcoule, is melting metal from the CEA, EDF, Cogéma, and other companies. The CEA in 1995 even announced that “fusion could later be envisaged for high activity metals” [CEA 94].

--Evaporation: the evaporation of liquid effluents is carried out at Saclay, Cadarache, Valduc, Marcoule, La Hague, and perhaps Grenoble. Marcoule and La Hague have long concentrated solutions of fission products by means of evaporation. Marcoule installed an evaporator for other liquid effluents in the eighties, and La Hague recently installed new evaporation units to treat liquids formerly treated by coprecipitation.

--Incineration: the treatment of wastes by incineration is not an innovation. In the sixties, the CEA put into operation radioactive-waste incinerators at Cadarache, Fontenay, Marcoule or Grenoble. But incineration is becoming increasingly prominent. Old incinerators at Cadarache, Fontenay, and Grenoble, it seems, are operating after overhauls. FBFC at Romans operates an incinerator for uranium wastes, and Melox at Marcoule, and Valduc each operate an incinerator for alpha-contaminated wastes. In the future Cadarache will have such an incinerator. Centraco is equipped with an incinerator designed to burn 5000 t of low-level contaminated waste from Cogéma, EDF, and other producers.

--Other procedures: at Cadarache the CEA stabilizes old UNGG fuel in a high-temperature furnace to permit its reprocessing or storage. Cogéma at La Hague treats organic solvents by distillation and pyrolysis. Following the example of Le Bouchet and Malvési, Annecy and the FBFC factory at Romans treated, and in the case of Romans at least, still treat discards by grilling. Vitrification, used at La Hague and Marcoule, is more a method of packaging than of treatment, but this method also includes the heating of waste.

Each time that wastes are treated by heat, whether to reduce the volume or for another reason, there is a risk that radionuclides and/or other toxic materials accompanying the radionuclides, will become volatile and escape into the environment. Filtration systems are never 100% effective. A loss of a minimal percentage of burned material can be dangerous. The release of one or two grams of uranium oxide, for example, represents the dispersion of a hundred million million (10E+14) particles of uranium, which can cause biological damage. Heat never destroys radionuclides, but it can help to distribute them.

We discuss here, as an example, only incineration. The majority of the incinerators mention above, let us recall, incinerate wastes that are not only chemically toxic but also radioactive, “mixed” waste, as one says in the United States .

The incineration of mixed waste is accompanied by the following problems:

--Release of chemical products: the incomplete combustion of waste, which can be produced, for example, by incinerating together a mixture of materials each one of which requires different conditions for optimum combustion, creates “products of incomplete combustion” or PIC, according to the American terminology. These PIC form in the part of the incinerator where combustion gases are cooled. The most dangerous are thought to be the halo-carbons including dioxin. A way to create dioxin is to incinerate together PVC and a material containing carbon (wood, paper). One cannot eliminate, but only decrease, releases of dioxin and similar compounds by injecting active carbon into the system.

--Release of metals: several metals become volatile in combustion chambers and escape as fine particles. The filters, when they function well, can retain up to 99.9% of the metal particles, except for mercury which is more difficult to trap.

--of radionuclides: they are released in the gaseous effluents or remain in the ashes, the filters, or the waste water. In an incinerator functioning normally the majority end up in the ashes. The carbon 14, the tritium, and several iodine isotopes escape very easily and the totality of these radionuclides passes into the atmosphere if special filters are not used, which is usually not the case. The uranium, the plutonium, and the cesium-137 are less volatile but can nevertheless pose a problem. A study by the US Environmental Protection Agency showed that a typical incinerator of mixed waste could lead to an exposure of the public to plutonium exceeding federal norms [USEPA 1991].

--Creation of solid secondary waste: the treatment of combustion gases with water and the treatment of that water create contaminated sludge that must be stored and “weakly” contaminated washing water that is released. The ashes from incinerators for plutonium and uranium waste pose a particular problem. The CEA has developed a procedure permitting the recovery of “98% or more” of the plutonium and americium in the ashes [Moulney 94; Marc 98]. However, the 98% do not permit the declassifying of the ashes for surface storage. Such a declassifying would require a decontamination factor of 10,000 to 100,000 [Cécille 93]. The FBFC plant at Romans has experienced difficulties in the treatment of uranium wastes but we do not have recent information concerning those wastes.

--Need for maintenance: the effectiveness of an incinerator depends on the manner in which it is maintained as well as on what is burned there. Numerous accidents or incidents occur because of the complexity of gas purification systems, which means that maintenance is essential.

--Temptation to use the incinerator improperly: once an incinerator has been constructed, there is a temptation to use it to get rid of any type of waste, including waste coming from other installations. The radioprotection technician Jean-Luc Lamy notes in this respect: “The responsibility of the sending installation is always on the line, for analyses are most often general, which means that there is “forgetfulness” in regard to impurities in the form of transuranics, fission products, and other contaminants. . . . An omission, certainly, but how devasating, when they are your men, your installations that are trapped, harmed” [Lamy 93a].

For the public, the only means of insuring a posteriori that an incinerator or another thermal treatment does not contaminate human beings is independent monitoring of the radioactivity around the site.