EP0306420A1 - Process for the purification of traces of radioactive elements generated during the stockpiling of uranium resulting from the processing of irradiated nuclear fuels - Google Patents

Abstract

Process for the purification of uranium resulting from reprocessing, previously separated from the fission products generated in a nuclear reactor, consists in eliminating the descendants of U232 appearing during storage, by passing said uranium, in its hexafluoride form, to through a chemically inert porous material.

Description

TECHNICAL AREA

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.

STATE OF THE ART

After combustion in a nuclear reactor, 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. Before reuse, this uranium can be stored in the form of nitrate hexahydrate, oxide, tetrafluoride, hexafluoride, etc.
However, this purification carried out by chemical means, however advanced it may be, does not eliminate the isotopes of uranium and in particular U232 emitterα which remains in trace amounts, a few parts per billion (ppb), in the stored uranium which is not dangerous in itself.

On the other hand, during storage, it generates annoying parentage products, which by successive disintegrations of generally short periods give α and β emitters which ultimately lead to stable Pb 208.

These descendants are successively made up of: Th 228, Ra 224, Rn 220, Po 216, Pb 212, Bi 212, Tl 208, Po 212, Pb 208. The periods range from µ s to a few days, to 1, 91 years for Th 228 and 72 years for U232. Among them, T1 208 is particularly annoying, it is an emitter and with a strong irradiating power γ (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 the latter have have been generated in sufficient quantity.

In practice, after 2 years of storage, half of the equilibrium radioactivity is obtained; in absolute value, the level of radioactivity reached obviously depends on the quantity of U232 present in the uranium ex initial reprocessing.

For example uranium having the following characteristics after reprocessing
U232 = 1.15 ppb / U
activity of descendants of U232 = 100Bq / g of U
is found, after a storage of 3.25 years, with an activity of 4000 Bq / g of U, value which poses very serious problems for the user.

Thus we see that it is necessary to periodically eliminate the descendants of U232 during prolonged storage to reduce the activity of uranium to its starting value observed after reprocessing.

When in general the uranium ex reprocessing is stored in the form of solid nitrate, oxide or tetrafluoride, 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.

In addition to their complexity, these treatments have the drawbacks of contaminating the installations with the products or reagents used in a worrying manner and of generating radioactive effluents which it is necessary to treat before discharge.

OBJECT OF THE INVENTION

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.

It thus aims to eliminate the traces of radioactive parentage generated by uranium 232, and to provide uranium ex reprocessing which can be handled, used or again stored under usual conditions not requiring protective means heavy.

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.

When the uranium must be stored over a long period, 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 new 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.

DESCRIPTION OF THE INVENTION

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. However, it is particularly advantageous to store it in the form of hexafluoride in the usual containers provided for this purpose, so that the purification process according to the invention can then be applied to them in a simple manner.

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 at 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.

Such treatment of the entire flow of hexafluoride is different from the isotopic enrichment phenomenon of UF6 which occurs in enrichment plants by gas diffusion.

Indeed, 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. Thus a stream of UF6 is introduced at one end of the porous tube, internally covered with its active layer, of which a part enriched in light isotope is extracted by diffusion through the active layer, and of which the most important remaining part, depleted spring at the other end of the tube. Only a slight pressure prevails inside the porous tube.

On the contrary, according to the invention, the entire flow of UF6 crosses frontally the porous material, which is itself active, the process thus appearing more to filtration than to diffusion, but the material n not acting exclusively as a filter, as will be seen later.
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. Preferably they 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.

Likewise, 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 ...

Their corrosion must be low so as to avoid clogging of the porous body and pollution of the outgoing hexafluoride.

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.

It is likely that the major part of these parentage products is 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.

Thus, surprisingly, the porous material fixes all of the parentage products although its structure and its breaking capacity can be chosen from a very wide range.
In particular, 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.

In practice, in order to have good purification efficiency, the equivalent diameter of the pores must be less than 100 μm and preferably less than 50 μm. This diameter is measured by bulloscopy according to standard ISO 4003-1977 (F).
At the same time, the thickness of the porous material is generally at least 100 mm when it is canvas or metallic sintered felt. In the case of sintered porous materials 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.

It is seen from these parameters that the flow rates can be high, which is particularly advantageous when using gaseous hexafluoride, and that at constant speed or residence time it is more advantageous to treat the liquid hexafluoride.

We see that we can also treat large flow rates of hexafluoride while using porous bodies whose access surface is relatively small, for example for flow rates of 500 kg UF6 / h, an area of the order of 0.5 m2 may be sufficient, this illustrates the reduced size of the purification device.

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%. These results are obtained by comparing before and after treatment either measurements of γ irradiation made in contact with the container or the pipes, or preferably spectrometry measurements of the entire container or a representative sample.

When the porous material is sufficiently contaminated by the products which it has fixed during the treatment, which is noted by measuring the irradiation in contact with its confinement enclosure, said enclosure and its contained porous material are then replaced by a new set. Because of its compactness, the assembly is easy to handle, even at a distance, easily eliminable in a controlled landfill, as it is or after packaging, or better storable to allow its radioactivity to decrease before re-use.

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.

We see that the process is particularly simple and advantageous to implement when the reprocessed uranium is stored in UF₆ form and that it allows its storage over very long periods, since it suffices to make a transfer from time to time from a transmitter container to a receiver container via a porous body according to the invention.

EXAMPLES

The following nonlimiting examples will make it easier to see how to implement the invention.

EXAMPLE 1

We start with a reprocessing uranium hexafluoride stored in an Al-based alloy container; the receiving container is identical.

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.

Different porous bodies will be tested.
To conduct the purification according to the invention, 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. At the same time, the receiver can be cooled.

Once the pressures have stabilized, the 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 purification rates, representing the ratio of these two measurements and obtained with the various porous bodies used are grouped in the following table: Test No. Porous body Gaseous current Transmitter Receiver Purification rate Type Nature material Thickness Access surface Pore diameter Debit Surface flux Gas velocity Temperature Pressure Temperature Pressure mm cm2 µm kgUF6 / h kgUF6 / hcm2 m / h ° C bar ° C bar % 1 sintered stainless steel 2 20 50 1.2 0.06 50 80 1.8 17 1.8 99.6 2 sintered stainless steel 2 3 50 0.9 0.3 233 80 1.8 17 1.8 99.1 3 Knitt stamp monel 2 20 1.8 0.09 75 80 1.5 17 1.5 98.6 4 Sintered monel 2 20 50 1.8 0.09 75 80 1.5 17 1.5 99.5

EXAMPLE 2

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 availability 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 μm, the useful thickness of each cartridge is 2 mm.
As before, 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 speed correlatively varied from 36 m / h to 82 m / h and the residence time in the porous material from 0.6 sec to 0.26 sec.

Under these conditions, 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%.

Claims (7)

1. Process for the purification of reprocessed uranium from which the fission products generated in a nuclear reactor have been separated beforehand, said uranium having been stored for a long period during which its radioactivity has reached a high limit value, characterized in that , in order to eliminate this radioactivity due to the parentage of uranium 232 present, said reprocessed uranium is passed in its liquid or gaseous hexafluoride form through a chemically inert porous material. 2. Method according to claim 1 characterized in that the porous material has an equivalent pore diameter less than 100 µm and preferably less than 50 µm. 3. Method according to any one of claims 1 or 2 characterized in that the porous material is preferably a porous sintered material and more particularly a sintered metal. 4. Method according to claim 3 characterized in that the sintered metal is stainless steel, nickel or its alloys, or monel. 5. Method according to claim 3 characterized in that the porous sintered material has a thickness between 0.5 and 10 mm, preferably between 1.5 and 5 mm. 6. Method according to any one of claims 1 to 5 characterized in that the hexafluoride is treated in gaseous form. 7. Method according to claim 6 characterized in that the speed of passage of the gaseous hexafluoride through the porous material is less than 250 m / h and preferably between 15 and 100 m / h.