Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
  • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
  • G21F9/02—Treating gases

Definitions

• the invention relates to a process for purifying traces of particularly radioactive elements generated during the storage of uranium resulting from the reprocessing of irradiated nuclear fuels.
• the uranium fuel is treated in a reprocessing plant where, after cooling, said irradiated fuel undergoes a succession of operations intended to selectively separate uranium, transuranium elements, including plutonium, and fission products.
• the uranium thus obtained separated from the nuclear reaction products is called reprocessing or ex-reprocessing uranium; it must be as pure as possible in order to be able to be reused in the usual fuel cycle with a view to its enrichment and its reintroduction into a nuclear reactor.
• this thorough purification provides a reprocessed uranium whose radioactivity level, meeting the specifications, is low and allows its use under normal conditions.
• this uranium can be stored in the form of nitrate hexahydrate, oxide, tetrafluoride, hexafluoride, etc.
• the periods range from ⁇ s to a few days, to 1, 91 years for Th 228 and 72 years for U232.
• T1 208 is particularly annoying, it emits ⁇ and has a strong irradiating power ⁇ (2.6 MeV), and becomes annoying as soon as U 232 has given birth to a sufficient quantity of Th 228 and that the descendants of this have been generated in sufficient quantity.
• this periodic purification can be done by a treatment which consists in putting the stored product in solution, then in purifying the solution obtained by conventional means such as resins, liquid-liquid exchange with solvents, selective precipitation ... before returning it to its storage form.
• the subject of the invention is a simple non-polluting process for the purification of reprocessed uranium, therefore previously rid of the products of nuclear reactions such as transuranics and fission products, said uranium having reached a high radioactivity, following storage. prolonged, making it unsuitable for any storage or any use under normal protection conditions, this purification making it possible to lower said radioactivity to very low values, for example less than a few hundred Bq per g of U again making it possible to storage and / or use of uranium ex reprocessing under normal conditions.
• Another object of the invention is to obtain uranium ex reprocessing which can be used directly in an installation for enriching and / or manufacturing nuclear fuel.
• the method according to the invention makes it possible to resume said stock periodically to purify it in a simple and inexpensive manner before it reaches a level of inhibiting radioactivity; this therefore allows a new storage period under normal conditions, pending a further purification operation according to the invention, or another use also under normal conditions of radiation protection and contamination.
• Another object of the invention is to have a process for purifying traces of radioactive elements of filiation easy to implement inexpensive and non-polluting: the impurities are recovered in compact form and not disseminated, their storage is thus easy, also inexpensive and does not generate effluent.
• the invention is a process for purifying reprocessed uranium from uranium fuel, which as such is free, following a prior separation treatment, of the elements generated during the passage of uranium fuel, in a nuclear reactor, said reprocessing uranium having been stored and having reached during this storage a sufficiently high radioactivity capable of preventing its use under normal conditions; said purification process intended to eliminate this radioactivity due to the parentage of uranium 232 present, is characterized in that said reprocessing uranium is passed in its form of liquid or gaseous hexafluoride through a chemically inert porous material.
• the purpose of the process according to the invention is therefore to eliminate the parentage products of uranium 232 which appear during storage and which cause the radioactivity to be eliminated when it becomes too high, for example greater than a value fixed by the regulation.
• the reprocessed uranium to be purified, after storage, is put into the form of hexafluoride using known methods, insofar as it is stored in another chemical form.
• the process consists in initially disposing of hexafluoride under conditions of temperature and pressure such that it is in the liquid or gaseous state. When it is stored in a container, it is enough to that of heating the latter in an oven.
• the pressure must also have a value high enough to then be able to pass the flow of hexafluoride frontally through a porous material contained in an inert confinement enclosure, maintained in temperature by insulation and / or heating; the purified flow leaving the porous material is either recovered in a water-cooled storage container, or sent directly to a processing or use facility of any kind, for example enrichment, conversion, etc.
• this process consists in circulating a main flow of UF6 inside porous tubes, called support tubes, internally covered with an active layer, or barrier layer, asymmetrical, the enrichment being made thanks to the selective tangential extraction of light isotopes from the UF6 flux, by diffusion through the barrier layer.
• the porous tube only serves as a support for the active layer and does not come into play in the diffusion phenomenon.
• the active layer is asymmetrical, that is to say that it is practically dense on the side in contact with the upstream UF6 flow so that the diffusion takes place, and has a microporosity on the other side in contact of the support tube to promote the passage of the partially enriched extracted UF6.
• the porous material can be made of fabrics stacked one on top of the other. on the others, the flow of hexafluoride passing through them perpendicular to their surface, or of a canvas wound on itself, the flow of hexafluoride passing through it parallel to its winding axis; said fabrics must be chemically inert, and withstand the conditions of pressure and temperature.
• fabrics are metallic, for example of the reps type; all inert metals can be used, for example steels, particularly stainless steel, nickel and its alloys, inconel or better still monel.
• any porous sintered materials chemically inert, ceramic, for example alumina, nitrides, carbides, etc. or preferably made of metal, or even sintered metallic felts based on fiber, can be used, the metals which can be used are the same as those mentioned above.
• the enclosure and the porous body must be chemically inert, i.e. they must resist the action of fluorine and its derivatives, HF fluorides, UF6 ...
• the quantity by weight of parentage products to be purified is minute, practically non-dosable as such, given the very small quantity of uranium 232 present at the start (a few ppb); therefore care must be taken that the sampling for analysis is always representative. Also their analysis is usually done through their radioactivity and preferably on very large samples or even all of the hexafluoride used.
• parentage products are in the form of solid or gaseous fluorides and that in the case where they are in the form of particles of solid fluorides in liquid UF6, or in gaseous UF6, these are probably of minute or even molecular size, due to the mode of generation "in situ" of parentage products.
• the porous material fixes all of the parentage products although its structure and its breaking capacity can be chosen from a very wide range.
• the latter can be chosen in a very wide range, too low values reducing the possible flow rates of hexafluoride for the same surface, too high values requiring a greater thickness of the porous material.
• the equivalent diameter of the pores must be less than 100 ⁇ m and preferably less than 50 ⁇ m. This diameter is measured by bulloscopy according to standard ISO 4003-1977 (F).
• the thickness of the porous material is generally at least 100 mm when it is canvas or metallic sintered felt.
• the thickness is generally between 0.5 and 10 mm and preferably between 1.5 and 5 mm.
• the speed of passage of the liquid or gaseous hexafluoride is usually chosen to be less than 250 m / h and preferably between 15 and 100 m / h; the contact time with the porous material is usually greater than a few hundredths of a second and preferably between 0.1 and 10 sec.
• the method according to the invention makes it possible to eliminate at least 98% of the radioactivity present in the starting hexafluoride and due to the parentage of uranium 232, and this purification even generally reaches more than 99.7%.
• the process is applicable to uranium reprocessing of any isotopic uranium 235 content; it is particularly advantageous to use it for enriched uranium which is therefore also enriched in U232 and whose storage periods at low irradiation are all the shorter the higher the U232 content.
• the emitting container containing the product to be cleaned, is connected to the purification device which itself is connected to the receiving container; these connections are made of stainless steel and are fitted with the necessary shut-off or isolation valves and pressure gauges from 0 to 6 bar, located near the containers.
• a primary vacuum pump designed to remove inert gases present in the circuit and the receiving container prior to the purification operation has been mounted on this circuit.
• the receiving container is equipped with an external water circulation cooling coil. It is installed on a weighing device, which tracks the progress of the transfer.
• the purification device consists of a cylindrical stainless steel enclosure with a diameter of 59 mm, at the ends of which open the connection tubes with the transmitter and receiver containers.
• a disc of porous material with a diameter of 50 mm is positioned transversely in the enclosure.
• a differential pressure gauge gives the pressure drop between the upstream and downstream of the porous material.
• the transmitter canister and the purification device are in the same heating oven.
• a vacuum is first made in the installation to eliminate the inert substances.
• the oven is then heated so that the transmitter canister is at about 80 ° C and its pressure is stabilized.
• the receiver can be cooled.
• valves are operated so as to effect the transfer of gaseous UF6 through the porous body.
• the decontamination of hexafluoride is measured by comparison of radiochemical analyzes of the activity of the descendants of U232 carried out on the starting UF6 and the purified UF6.
• the installation used in this example makes it possible to treat cylinders containing up to 14 t of UF6; it is similar to that of Example 1, with the exception of the cooling of the receiving container by means of a watering boom and the purification device.
• the latter consists of a cylindrical enclosure inside which are arranged, in parallel, five identical cartridges of porous material; they are cylindrical and closed at one end.
• This arrangement makes it possible to increase the access surface to the porous material which in this case is 0.5 m2, while keeping a reduced space requirement of the enclosure: diameter 30 cm and volume 65 l.
• the porous material is sintered monel, the equivalent pore diameter of which is 50 ⁇ m, the useful thickness of each cartridge is 2 mm.
• the UF6 cylinder is brought to 80 ° C., the pressure in the emitting cylinder varies between 1.2 and 1.8 bar and that of the receiving cylinder between 0.6 and 1.2 bar, during the treatment. .
• the flow rate of gaseous UF6 was varied between 57 kg / h and 331 kg / h, for a total transferred amount of 14 t of UF6.
• the purification rate obtained after counting the activity of the descendants of U 232 carried out by ⁇ spectrometry of the representative samples taken before and after treatment is 99.7%.

Landscapes

• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• General Engineering & Computer Science (AREA)
• High Energy & Nuclear Physics (AREA)
• Solid-Sorbent Or Filter-Aiding Compositions (AREA)
• Separation Of Gases By Adsorption (AREA)
• Vaporization, Distillation, Condensation, Sublimation, And Cold Traps (AREA)
• Manufacture And Refinement Of Metals (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=9354869

Family Applications (1)

Country Status (5)

Families Citing this family (4)

Family Cites Families (7)

• 1987
  • 1987-09-01 FR FR8712705A patent/FR2619952B1/fr not_active Expired
• 1988
  • 1988-08-26 US US07/236,907 patent/US4891192A/en not_active Expired - Lifetime
  • 1988-08-29 JP JP63214729A patent/JPH01138495A/ja active Granted
  • 1988-08-31 EP EP88420294A patent/EP0306420B1/fr not_active Expired - Lifetime
  • 1988-08-31 DE DE8888420294T patent/DE3866477D1/de not_active Expired - Fee Related

Also Published As

Similar Documents

Legal Events