Techniques pyrochemical

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

The NaCl-KCl eutectic is used when the pregnant extraction salt is to be processed by aqueous recovery (this is the salt currently used at Rocky Flats because calcium follows americium in the present aqueous recovery process). The NaCl-CaCl system is used when the salt is processed by pyrochemical means to recover the americium and residual plutonium. When the pyrochemical recovery technique is used, the NaCl-CaCl2-MgCl2 salt is contacted with liquid calcium metal at approximately 850°C in a batch extractor. The calcium reduces A111CI3,... 

The electrolyte salt must be processed to recover the ionic plutonium orginally added to the cell. This can be done by aqueous chemistry, typically by dissolution in a dilute sodium hydroxide solution with recovery of the contained plutonium as Pu(OH)3, or by pyrochemical techniques. The usual pyrochemical method is to contact the molten electrolyte salt with molten calcium, thereby reducing any PUCI3 to plutonium metal which is immiscible in the salt phase. The extraction crucible is maintained above the melting point of the contained salts to permit any fine droplets of plutonium in the salt to coalesce with the pool of metal formed beneath the salt phase. If the original ER electrolyte salt was eutectic NaCl-KCl a third "black salt" phase will be formed between the stripped electrolyte salt and the solidified metal button. This dark-blue phase can contain 10 wt. % of the plutonium originally present in the electrolyte salt plutonium in this phase can be recovered by an additional calcium extraction stepO ). 

The Los Alamos National Laboratory has had a very active program for the production of high purity plutonium metal for both Laboratory and national needs for many years using pyrochemical techniques. 

Phil Horwitz asked me to comment on what I saw as potential disadvantages of the various plutonium pyrochemical processes extolled by speakers in the Tuesday sessions. I, too, am a fan of pyrochemical techniques. I recognize that pyrochemical processes for Pu processing are just in their infancy - on batch plant-scale. To be truly useful, such processes need to be operated on a continuous basis. Scientists and engineers concerned with such technology need to develop equipment and procedures required to operate pyrochemical processes in a cost-effective, continuous manner."... 

The production of the light actinide metals is usually accomplished through the reduction of tri- or tetrafluorides with an electropositive metal, for example, Ca, Zn, or Mg. The heavy actinides are typically made through the direct reduction of oxide phases. An alternative method to access the actinide metals is through pyrochemical methods. In this technique, actinides in high-temperature molten salts, for example, NaCl-KCl or LiCl-KCl eutectics, are electrolyzed... 

Open-minded as evidenced by the splendid series of articles by Crass and articles on the subject in the encyclopedias, though reliable formulas are less prominent than mechanical techniques. What the reader will find in Chapter 12 of this book is undoubtedly the most ex.tensive study on the pyrochemical aspects of the subject. 

In chloride media, the back-extraction of the actinides seems to be feasible. This technique is well adapted to the core of pyrochemical process developed by the CEA and thus was selected for further investigation. 

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