Pilot Plant for UF6 Conversion
Development, Assembling and Testing of Pilot Plant for Conversion of Uranium Hexe-fluoride Depleted on Isotope U-235 to Uranium Metal and Anhydrous Hydrogen Fluoride
Tech Area / Field
- ENV-RWT/Radioactive Waste Treatment/Environment
3 Approved without Funding
Kurchatov Research Center, Russia, Moscow
Project summaryAll the modern applications of nuclear technique and technology connected with use of uranium have their basis in using isotope U-235 which percentage in naturally occurring uranium is in all of 0.7204 %. Percentage of isotope U-238 in natural uranium is of 99.2739 %; the bulk of uranium extracted from naturally occurring uranium ores by now is localized at stockpiles of the uranium enrichment plants as depleted (or conventionally depleted) uranium hexafluoride.
It is well known that uranium isotopes separation technologies enables to extract the main mass of U-235 from uranium hexafluoride leaving as a waste depleted UF6. When an enrichment of uranium is of 90 %, ~ 0.9 t UF6 from each ton supplied to an enrichment plant arrive to a dump. The process of storing depleted uranium at the countries experiencing in uranium isotope separation technology has been continuing since 1950. Million tons of depleted UF6 has been stored at the dumps all over the world.
Today's topicality of the depleted uranium hexafluoride problem is determined by economic and ecological factors. Ecological problems have resulted from potential and actual hazard for human health when large masses of volatile radioactive substance are stored in steel containers in the open outdoors. Economic problems are determined by three factors at least:
- large resources of fluorine are dumped despite this product is expensive and in short supply: each ton of uranium hexafluoride contains 0.324 t. fluorine;
- depleted uranium is of considerable value as an ingredient of some nuclear fuel compositions in reactor engineering, as component of alloys of unique and universal use.
- storing depleted uranium requires considerable running cost for maintenance work and spread of stockpiles.
It is known at least one purposeful approach to resolution of a problem of depleted uranium accompanied by large-scale scientific research and development works; these works have been conducted under the leadership of the leader of this project. The technology has a basis in plasma steam conversion of depleted uranium hexafluoride. The process has been experimentally tested on four levels: laboratory research, verification at an experimental stand, testing the process at a pilot plant, development and test of equipment at an industrial plant. Technical results are summarized as follows:
- capacity - 0.14 - 0.15 t. UF6/h;
- power of the plasma reactor - 0.26 MW;
- mole ratio of (H-OH) / UF6 = 3-4;
- HF concentration in a gas stream before entering the condenser - to 88.5 %;
- HF concentration in a gas stream after the condenser - up to 95.4%;
- output of fluorine to target products(anhydrous hydrogen fluoride and hydrofluoric acid) - 96 - 98 %;
- content of residual fluorine in uranium oxides - up to 2 %;
- expenditure of energy - within 1.7 - 1.9 kWh/kg UF6.
The process and the equipment for its performing have been prepared for application in 1989 but industrial application has not been carried out because of general crisis of USSR nuclear industry in that time. Then economic problems aggravated the situation.
By now one can see some disadvantages of this technology, namely:
- incomplete extraction of fluorine into anhydrous HF (~ 90 %);
- uranium oxides are acceptable but inefficient compounds for storing uranium and especially for utilization of depleted uranium itself; moreover optimization of the process from point of view of maximal extraction of fluorine results in declining physical and chemical properties of uranium oxides hindering any application of these products in nuclear fuel cycle.
Nowadays specialists from USA make an attempt to develop a plasma hydrogen concept of conversion of depleted uranium hexafluoride where the most acceptable products of conversion are anhydrous hydrogen fluoride and uranium metal; the latter corresponds more to requirements of utilization and even storing of depleted uranium. This concept has some experimental support but contains disadvantages hindering its large-scale industrial realization:
- yield of element uranium is comparatively rather low - less than 30 % of theoretical one; it means that one must extract uranium from common mass of the product obtained and create once more technological line for reducing ~ 70 % uranium remained;
- uranium is obtained as a pyrophoric powder susceptible to autooxidation in the air and even to self-ignition; the latter is of great potential danger at creating such large-scale production as processing of depleted uranium hexafluoride;
- at the processes of such kind, especially at using quench in converging/perging supersonic nozzles one deals with micron and submicron powders which can not be collected in cyclones.
This project pursues an object of complex processing depleted uranium hexafluoride terminating in obtaining compact uranium, total utilizing anhydrous hydrogen fluoride, increasing safety of the process. For solution of this problem we offer a method having a basis in hydrogen reduction of uranium hexafluoride at high temperatures carried out at one technological apparatus and consisting of four consecutive - parallel stages.
The first stage is reduction of uranium from uranium hexafluoride to lower uranium fluorides. This intermediate aim is attained by generation of an electrical discharge in a flowing mixture of UF6 and H2; during this procedure the mixture of UF6 + H2 transforms to (U-F-H)-plasma. If at this operation temperature of plasma is of 6000 K, the main portion of uranium is present as U-atoms; it means that U is reduced completely. But when (U-F-H)-plasma goes out of electrical discharge region, intensive recombination of uranium fluoride proceeds followed by powerful light radiation and condensation of UF6 - fragments non-volatile under usual conditions: UF4, UF3 and U. Recombination can be followed by partial formation of volatile fluorides: UF5 and even UF6.
At the second stage the reduction products of UF6 are transferred into condensed phase where recombination processes are decelerated, and the reduction process of uranium completes up to formation of liquid uranium. For it the (U-F-H)-plasma generated at the first stage is directed to a surface of uranium tetrafluoride melt which is obtained as follows. The UF4 batch is placed into a water-cooled sheath transparent to frequency electromagnetic field, stable to corrosion action of uranium fluoride melt. The sheath aforesaid is inserted into a coil of a frequency generator coaxially with the electrical discharge chamber where the (U-F-H)-plasma has been generated. The plasma flow interacts with the surface of the UF4 charge and melts an upper layer of the latter. The coil is supplied with frequency voltage, the UF4 melt zone interacts with frequency field and, as a result, all the charge is heated rapidly because of direct induction heating and melts.
At the surface of the UF4 melt at the interaction with the (U-F-H)-plasma uranium and lower fluorides of uranium condense; the latter disproportionates simultaneously with formation of uranium. As the reduction of uranium proceeds, intensive mass exchange in the condensed phase occurs conditioned by ratios of melting temperatures and densities of the products obtained. Because of great difference in densities of uranium and uranium fluorides the former precipitates and the latters float up to the surface layer subjected to plasma action while UF4 floats up in UF3.
Thus, uranium reduces completely both from UF6 and from the primary batch of UF4 within several minutes under action of plasma flow and of frequency direct induction heating. Decrease of UF4 at the batch is compensated with uranium fluorides from the (U-F-H)-plasma. During this process fluorine is bond into gaseous hydrogen fluoride which volatilizes from uranium reduction zone.
The third stage carried out simultaneously with two former ones is removal of liquid uranium from a bottom of the reactor - sheath and pouring into cooled ingot molds under protective gas.
The forth stage carried is removal and collection of the second commercial product -anhydrous hydrogen fluoride. The removal of HF is realized through a filtration module consisting of multilayer cermet filters collecting submicron powders from the process gases and providing safety of the process from uncontrolled penetrations of pyrophoric products beyond limits of the technological zone. Then the flow of hydrogen fluoride cleaned is condensed, collected into transport reservoirs and the product is sent either to realization or to feed maintenance of fluorine electrolyzers.
In the course of realization of the project the following results are expected:
(1) the stand for conversion of depleted UF6 to uranium ingots and anhydrous hydrogen fluoride having power within 200-300 kW will be developed and assembled; this stand at appropriate increment of power supplies and at correction of geometric sizes of the reactors is brought to a power of an industrial apparatus (1000-1500 kW);
(2) all four consecutive and parallel stages composing the process will be researched in detail: primary reduction of uranium in a plasma reactor; complete reduction of uranium in a frequency reactor of direct induction heating; removal of liquid uranium from the frequency reactor and pouring the metal into ingot molds; removal and collection of the second commercial product - anhydrous hydrogen fluoride;
(3) the research and development works at the main elements of the stand apparatus will be conducted: plasma reactor, frequency reactor, layout for removal and pouring liquid uranium, cermet recleanable filters on the basis of multilayer filter elements supplied with ejection regeneration system;
(4) materials technology researches of the product obtained will be conducted and application fields of these ones will be determined;
(5) an 1000 kW industrial module for conversion of depleted uranium hexafluoride will be calculated;
(6) ecological and economic consequences of application of the new technology will be evaluated.
The research and development aforesaid will enable to formulate principles of the plasma hydrogen conversion of uranium hexafluoride concept and will result in creature of technical and commercial basis for its realization.
Potential participation of foreign collaborators implies exchange by positive and negative experience in carrying out separate stages and the whole process, development of new approaches in use of depleted uranium in economy, joint publications, new patents to modification of the process itself and to its design. It is probable aforesaid will result in more speedy resolution of this problem which, as a matter of principle, is an international problem, at the countries-collaborators.
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