Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including heat exchanger for reaction chamber or reactants...
Reexamination Certificate
2000-12-22
2003-08-12
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including heat exchanger for reaction chamber or reactants...
C422S159000, C422S198000, C422S224000, C422S229000, C422S233000, C422S903000
Reexamination Certificate
active
06605261
ABSTRACT:
The present invention relates to a method and apparatus for continuously transforming plutonium oxalate into plutonium oxide. More precisely, the invention relates to a method and to an apparatus suitable for continuously transforming plutonium oxalate into plutonium oxide by drying and then calcining said oxalate, in the presence of oxygen.
BACKGROUND OF THE INVENTION
The invention lies in the general context of recovering plutonium. Such recovery of plutonium, starting from purified plutonium nitrate solutions themselves coming from extraction cycles, conventionally comprises four steps:
the plutonium is precipitated from said solutions in oxalate form;
the resulting precipitate is recovered by filtering and is then drained;
it is dried and then calcined; and
it is finally homogenized.
The plutonium oxide powder obtained in this way is then packaged in compliance with the safety rules in force so as to protect man and the environment.
The operations of drying and calcining the drained plutonium oxalate (said drained plutonium oxalate generally still containing about 30% by weight water, bonded water+interstitial water) are performed discontinuously (in batches) in the prior art in distinct pieces of equipment, always under double containment for obvious safety reasons. Conventional ovens housed in glove boxes are used in succession and completely independently of each other: the first for drying said drained plutonium oxalate and the second for calcining the dried oxalate in the presence of oxygen.
OBJECTS AND SUMMARY OF THE INVENTION
The Applicant now proposes an improvement to that technique of transforming plutonium oxalate into plutonium oxide by drying and then calcining. It proposes implementing said transformation continuously, in a single apparatus.
In a first aspect, the present invention thus provides a method of transforming plutonium oxalate into plutonium oxide by drying and then calcining. In characteristic manner, in the context of said method., said operations of drying and calcining are implemented continuously in the presence of oxygen and with gas extraction in two adjacent zones of a single apparatus maintained at negative pressure (the said—negative—pressure is the relative pressure with regard to the ambient pressure).
In the invention, a single device is used, e.g. of the screw oven or conveyor oven type, that is maintained at negative pressure (for containment problems) in order to treat plutonium oxalate continuously both to dry and to calcine continuously said plutonium oxalate. The single apparatus is advantageously of the screw oven type as described in general terms below in the present specification and in more detailed manner, likewise below in the present specification, with reference to the accompanying figures. For reasons of double containment, said single apparatus is normally disposed in a glove box.
Within said single apparatus, the treated oxalate precipitate progresses continuously from a first zone in which it is dried towards a second zone in which it is calcined.
Oxygen is used within said single apparatus to transform said oxalate precipitate into oxide.
The drying and calcining of said precipitated oxalate generates gas continuously, in particular water vapor and carbon dioxide. Said gas must be extracted specifically to ensure that the single apparatus within which the method is implemented is maintained at negative pressure.
The gas is advantageously extracted under conditions optimized so that gas extraction has minimal impact on the drying and calcining process. In particular, the amount of powder entrained is minimized as is any remoistening of dried powder by the water that is generated during drying. For this purpose, said gas extraction is advantageously implemented in the drying zone of the sole apparatus within which the method is implemented. It is preferably implemented in said zone as far upstream as possible (although it must clearly be implemented downstream from the zone where the load to be treated is introduced).
In the context of the invention, as in the prior art, the extracted gas is charged with radioactive dust. Prior to being evacuated, said gas must therefore necessarily be dedusted. For this purpose, it is possible to use prior art techniques, and in particular:
said gases can be treated in a condenser-washer; and/or
they can be caused to pass through a filter element.
Such a filter element is placed on the gas extraction circuit. As a general rule n parallel-connected elements of this type are used. Conventionally, such an element is constituted by a filter medium disposed in a support and referred to as a filtering cartridge. Whatever the context of use, once the filter medium has become clogged, such a cartridge is no longer effective. In ordinary use, such cartridges need to be changed regularly. Clogging means that they should be replaced systematically, which requires the gas filtering process in question to be interrupted.
When implementing the method of the invention, the extracted gas (advantageously extracted from the drying zone) is advantageously cleaned of radioactive dust by being filtered through at least one filtering cartridge.
In the context of the present invention, it is recommended to implement said filtering under the particularly advantageous conditions of implementing said filtering without interruption and with in situ regeneration of the clogged filter medium. The method of the invention is thus advantageously implemented with the extracted gas being filtered through at least one filtering cartridge that contains a non-deformable filter medium; said filter medium is unclogged intermittently, in situ, without interrupting the filtering. Such unclogging is made possible by the filter medium in question being undeformable. To unclog the medium, it is recommended to puff a small pulse of gas in the reverse direction (relative to the filtered gas) through the filter medium in question.
Said quantity of puffed gas is clearly limited. Under no circumstances is a genuine back-flow generated; that would imply introducing a non-negligible quantity of gas into the apparatus, raising the pressure within said apparatus (which it is recalled must necessarily be at negative pressure), and could even cause dust to be blown upstream therefrom. Said quantity of gas is advantageously strictly limited to the quantity required for obtaining the desired pulse effect, which pulse effect is suitable for obtaining the intended unclogging.
Since the atmosphere of the apparatus is thus not disturbed (even for a brief interval), filtering can be carried out without interruption. Clearly, the greater the overall volume of the apparatus, the greater the pulse of gas that can be admitted. Similarly, it is appropriate to increase the number of filtering cartridges used in apparatus of small volume. Regenerating such cartridges one by one then requires only very small quantities of gas in each puff.
The looked-for pulse effect is optimized if the small pulse of gas is puffed only when a genuine layer or “cake” of dust has formed on the filter medium to be regenerated. Under such optimum conditions of implementation—i.e. when there is a layer of dust of perceptible thickness on the filter medium—the pulse of gas is subjected to maximum pressure drop and is certain not to generate any back-flow within the apparatus. In a preferred variant implementation of the unclogging method, it is therefore strongly recommended to puff a small pulse of gas only once a layer of dust has formed on the filter medium. If the gas is puffed in prematurely, two undesirable phenomena can be observed:
the filter medium in question becomes reclogged very quickly; and
a back-flow is observed within the apparatus; the puffed gas has not been subjected to sufficient pressure drop.
The person skilled in the art knows how to minimize and even avoid these undesirable phenomena; and in any event knows how to determine the minimum or optimum thickness of cake for obtaining best results, given the various parameters in question (the intern
Cohen & Pontani, Lieberman & Pavane
Compagnie Generale des Matieres Nucleaires
Leung Jennifer
Tran Hien
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