GB2257561A - Process and apparatus for the continuous elimination of the radioactive iodine contained in irradiation nuclear fuel elements - Google Patents

Abstract

This process consists of dissolving fuel elements in a nitric solution in a dissolver (2) at a first temperature above ambient temperature, desorbing in a first desorber (10) the iodine contained in the solution (8) leaving the dissolver at a second temperature close to the boiling point of the solution and without the injection of nitrous vapours, in order to form Pub ions and iodates from iodine species contained in the exiting solution, desorbing in a second desorber (16) the iodine contained in the solution (14) leaving the first desorber at a third temperature above ambient temperature and below the boiling point of the solution, accompanied by the injection (17) of nitrous vapours in order to reduce the iodates into volatile molecular iodine, as well as the PuO<2+>2 ions formed into Pu<4+>. Residual iodine levels are reduced by solubilisation of iodides present, the first desorber facilitating this solubilization. <IMAGE>

Description

PROCESS AND APPARATUS FOR THE CONTINUOUS ELIMINATION OF THE RADIOACTIVE IODINE CONTAINED IN IRRADIATED NUCLEAR FUEL ELEMENTS DESCRIPTION The present invention relates to a process for the continuous elimination of the radioactive iodine contained in real irradiated fuel elements, so as to minimize the residual iodine quantity leaving the reprocessing units of said fuel elements, as well as to an apparatus for performing this process. It therefore essentially applies in the nuclear field for the reprocessing of irradiated fuel elements.

It is pointed out that reprocessing consists of specially treating irradiated fuel elements which have cane fran the core of a nuclear reactor with a view to separately extracting the fission products, depleted uranium and plutonium, the two latter compounds constituting reusable recovery products.

The most widely used fuel element reprocessing process is the Purex process, which consists of dissolving the irradiated fuel elements in a nitric acid bath, followed by the chemical treatment of said solution in order to extract uranium and plutonium fran it.

Among the volatile radioelements formed in fuel elements during their stay in the reactor, a special position is held by iodine 129 due to its very long period (17.2.106 years) and the risk of radiological exposure if said product is discharged in gaseous form into the atmosphere.

Therefore very high decontamination factors are required with regards to the trapping of iodine in irradiated fuel reprocessing plants. The most reliable method is to confine most of the iodine in the dissolving gases of the fuel and then trap it on solid adsorbents with a very high efficiency, so as to move it to the uranium and plutonium extraction cycles and then spread it in different plants where its trapping would be more.difficult.

To this end, it is necessary to aid the desorption of this halogen during the dissolving of the fuel elements. Its elimination is aided by the increase of the dissolving liquid-gas ratio and by nitrogen oxides which stabilize the iodine in elementary form I2 and which is therefore volatile.

This desorption is carried out in a continuous or discontinuous dissolver boiling the nitric acid solution in which the fuel elements are dissolved and entraining the volatile iodine I2 into the nitrogen oxide and water vapour stream resulting from the evaporation of the dissolving solution.

Thus, in a discontinuous dissolver it is merely necessary to sufficiently extend the boiling period at the end of dissolving in order to complete the iodine exhaustion of the action solution, the factor limiting the desorption is the oxidation of the final traces of iodine into iodates when the solution no longer contains nitrous vapours of general formula NO and through this HNO2. Thus, the iodine concentration can drop from 10 M-3 to 10 M-4 or 10 M-6 when the vaporization level reaches 10% of the total volume of the solution.

With regards to the continuous dissolving equipment, the use thereof makes it possible to obtain high dissolving capacities with regards to the irradiated fuel. However, the iodine exhaustion of the dissolving liquor is less than in a discontinuous dissolver. Thus, there is no longer the benefit of the iodine exhaustion period due to the compli- mentary boiling occurring when operating the latter type of apparatus.

Thus, only 95% of the iodine introduced with the fuel can be desorbed in a continuously operating dissolver.

In order to obviate this disadvantage, the new continuous nuclear fuel dissolving units are constituted by two apparatuses, namely the dissolver followed by an iodine desorber operating at boiling point, into which are injected nitrous vapours to prevent the oxidation of the molecular iodine into non-volatile species. This type of apparatus is described in patent application FR-A-2 641 119 filed on December 28 1988 in the name of the Commissariat a 1 'Energie Atanique.

However, other factors can limit iodine desorption, such as the presence of organic impurities fran the gaseous and liquid reagents used or recycled during reprocessing. To the extent that these canpounds are not volatile or decomposed during dissolving, they lead to a minimum residual iodine quantity, which will condition the final decontamination factor.

Numerous studies aiming at optimizing iodine desorption have been carried out in France and abroad. However, most of these studies were carried out on nitric acid solutions or uranyl nitrate and not on the real dissolving liquors.

the article by E. ENRICH et al, Conference Management of Gaseous Wasks fran Nuclear Facilities, February 18 to 22 1980, Vienna, IAEA-SM-245/16, 1980, pp 139-156, "Improved procedures for efficient iodine removal fran fuel solutions in reprocessing plants" and that of J. Fairer et al, 21st DOE/NRC Nuclear Air Cleaning Conference, San Diego, USA, 13 to 16 August 1990, vol. 1, pp 247-258, NUREG/DP-0116, CONF-900813, "Technicalscale iodine expulsion fran the dissolver solution and balance striking for liquid and gaseous iodine fractions", pp 1-12 in particular show that it is possible to lower the initial iodine in a discontinuous dissolver to 0.1 to 0.3% by adding, at the end of dissolving, nitrous vapours canbined with a carrier oxide excess in iodate form in order to assist the isotopic exchange with the iodine species of a slightly or non-reactive nature present in solution.

French research carried out on the basis of a simulated dissolving solution has shown that under the conditions of a continuous dissolving with complimentary desorption (cf. FR-A-2 641 119), it was possible to achieve a decontamination factor of 100 and for a fuel from a light water nuclear reactor (LWR) with enriched uranium, irradiated at 33,000 MWd/t, an iodine content in the dissolving liquors of approximately 0.54 mg/l, but this is too high. However, these data have not been confirmed by continuous dissolving tests performed on real fuel elements.

The present invention is directed at a novel process for the continuous elimination of the iodine contained in real irradiated fuel elements making it possible to obviate the aforementioned disadvantages and bring the residual iodine level in the dissolving liquor down to a lower value than that which has hitherto been obtained.

With a view to decreasing the residual iodine level, the inventors decided to carry out a radiochemical analysis of the dissolving fines (prcducts not solubilized during the action of HNO3 such as metallic inclusions present in the fuel, sheath fragments, insolubilized mixed oxides, unstable or only slightly soluble fission products, etc.) recovered both in laboratories and in industrial units. This analysis has shown that under discontinuous dissolving conditions a certain quantity of iodine can be entrained with these dissolving fines, in the form of relatively insoluble metal iodides such as e.g. AgI and Pud12.

As these quantities can modify the proportions of said element entrained beyond dissolving, the inventors firstly attempted to check whether the formation of these oxides was still possible under con tinuc;l dissolving conditions.

Two tests were carried out on a laboratory scale using REP UOX fuels irradiated to 55,000 MWd/t under conditions as close as possible to those of an industrial installation, in order to follow the behaviour of the iodine in the dissolver and complimentary desorber and quantify the fraction of said halogen remaining in solution in soluble and insoluble form.

During these tests, the influence of certain operating parameters on the desorption of iodine was also studied, as is shown in the following table.

TABLE I Test No. 1 2 Duration of the test 40 h 70 h Dissolver nominal capacity* 125 g of oxide/h 125 g of oxide/h fuel supply semicontinuous continuous Solution residence time 4.8 h 4.8 h Solution temperature boiling (107 C) 100 C Desorber 1: Solution residence time 2h 2h Solution temperature boiling (107 C) 100 C NOx quantity injected 2.7 moles/mole 7.5 moles/mole of U + Pu of U + Pu Desorber 2: ** Solution residence time 4h Solution temperature 100 C Nox quantity injected 7.5 moles/mole of U + Pu *The ccmposition of the solution on leaving the dissolver was: - (U) + (Pu) = 225 g/l - (H ) = 3N.

** The second desorber was only added to increase the solution residence time.

Tests 1 and 2 are in accordance with the process described in FR-A-2 641 119. The main results are given in the following table II. TABLE II Test Apparatus Solution Redox PuO22+ Residual iodine in solution (%) Residual Total No. temperature potential (total Pu %) lodates Organoiodine Total iodine residual of species in fines iodine solution (%)* (%)* (V/SHE) Dissolver boiling 1.12 5 0.30 0.57 0.87 0.65 1.47 1 (# 107 C Desorber boiling 1.14 20 ind. ind. 0.45 0.22 0.67 (# 107 C Dissolver 100 C 1.07 #0.5 0.18 0.36 0.54 2.17 2.70 2 Desorber 1 100 C 1.08 #0.7 0.06 0.27 0.33 0.69 1.02 Desorber 2 100 C 1.1 #2 0.02 0.16 0.18 0.44 0.62 * The residual iodine percentages are given relatve to the initial iodine introduced with the fuel.

The main results of these tests revealed that: the formation of metallic iodides in dissolving fines was possible under continuous dissolving conditions; the iodine decontamination factor after desorption was essentially limited by the colloidal or precipitated iodides and by the "organo icdine" species present in solution (very slow mineralization of these compounds) and not by the other mineral species (mainly iodates); the precipitated iodine fraction in the fines leaving the dissolver is much lower at boiling (107"C) and in the presence of a large quantity of iodate and 2+ (test 1) than at a temperature of about 100"C where the dissolving medium is much less oxidizing (test 2), which would appear to indicate that the boiling of the solution can limit the precipitation of metallic iodides and favour their dissolving.

On the basis of these results, the inventors developed a novel process for the elimination of the iodine contained in irradiated fuel elements and which, contrary to the strategy used at present in continuous dissolving units consisting of obtaining a high nitrous acid concentration in the complimentary desorber, essentially aims at increasing the oxidizing power (i.e. the potential) of the solution leaving the dissolver and favouring the formation of hexavalent plutonium and ipso facto iodates so as to solubilize the precipitated iodides in the dissolving fines or present in colloidal form by the reaction of these compounds with the iodates present in the medium and improve the mineralization of the "organoiodine" compounds.

This process is based on the use of two desorbers instead of one, which are placed in series at the outlet fran the dissolver and operate under different conditions.

More specifically, the present invention relates to a process for the elimination of the radioactive iodine contained in irradiated fuel elements, which inter alia contain plutonium and comprising the following stages:" a) continuously dissolving in a nitric solution in a dissolver fuel elements at a first temperature above ambient temperature, b) first desorption in a first desorber of the iodine contained in the solution leaving the dissolver at a second temperature close to the boiling point of the solution and without the injection of nitrous vapours, in order to form hexavalent plutonium so as to increase the oxidizing power of the solution leaving the dissolver and thus favour the formation of iodates fran the iodine species contained in the solution leaving the dissolver, c) second desorption in a second desorber of the iodine contained in the solution leaving the first desorber at a third temperature above ambient temperature and below the boiling point of the solution, accanpanied by the injection of an adequate quantity of nitrous vapours to reduce the plutonyl ions and iodates formed in the dissolver and the first desorber respectively into Pu ions and volatile molecular iodine.

The process according to the invention is performed at a pressure close to atmospheric pressure.

The dissolving stage can take place at the boiling point or at a lower temperature.

The reduced temperature dissolving stage a) leads to the reduction of the vaporization ratio of the solution and to an increase in the internal reflux ratio. This makes it possible to maintain an adequate iodine fraction in the nitric solution. In this case, the first temperature is chosen in such a way that the solution leaving the dissolver contains 2 to 15 and preferably 5t of the iodine quantity initially present in the fuel elements.

In particular, the first temperature is chosen approximately 5 to 15% below that of the boiling point of the solution. The tents temperature close to the boiling point of the solution is understood to mean a temperature equal to the boiling point or between the boiling point and a temperature, 10 0C below the latter.

The third temperature is a function of the maximum PU02 concentration in the solution, which is acceptable in the extraction cycles, and 6+ chosen so as to have a Pu content below 20% in the total plutonium and ensure the desorption of the volatile molecular iodine formed. In particular, the third temperature is approximately 5 to 10 C below the boiling point.

The first desorption stage permits the solubilization of the iodides according to the remutation reaction (1):

These iodides essentially result from metallic iodides such as silver iodides, palladium iodides (we2), rhodium iodides (RhI3), etc., with which they are in equilibrium according to the equation:

in which M is a metal and y the valency of said metal.

In > case of silver iodide the complete remutation reaction satisfies the relation (2):

The molecular iodine formed during this reaction (2) can then desorb in gaseous form according to the reaction (3):

or slowly oxidize according to the reaction (3 bis):

Taking into account the hydrolysis reaction of the No2 formed, satisfying the following equation (4):

the overall reaction (5) is obtained:

The thus formed iodates can interact with the iodides present in the medium in order to transform them into volatile molecular iodine according to equation (1).

In order to speed up the solubilization of the metallic iodides formed, it is advantageous to add iodate ions in excess during the first desor 127 ption, in which the iodine is not radioactive (typically 103). In this case, the dissolving stage in the dissolver can take place at boiling point.

During the first desorption, the nitrous acid HNO2 formed during the dissolving reaction of the fuel elements rapidly decomposes or is consumed by reacting with the hexavalent plutonium formed, according to reaction (6) and (6 bis):

This leads to a concanitant increase in the Redox potential of the solution.

Thus, in a dissolving liquor, the Redox potential is imposed by the pairs HNO2/N03 and Pu4+/PU6. (cf. equations (15) and (16)).

During this second desorption, the reduction of the iodate ions into molecular iodine by nitrous vapours is governed by equation (7):

The injection of nitrous vapours during the second desorption also makes it possible to reduce the Pu2 ions formed in the first desorber into 4+ 2 Pu ions.

The iodides in solution, in equilibrium with the metallic oxides, can also be oxidized in a nitric medium and this takes place more rapidly if the dissolving and/or desorption temperatures are close to the boiling point of the solution, in accordance with reaction (8):

Taking account of reaction (4), we obtain reaction (9):

The nitrous acid produced in the solution can also oxidize the metallic iodides according to equation (10):

Nevertheless, this intermediate acid is rapidly destroyed on boiling according to reaction (6) and finally we obtain equation (ill):

or for M=Ag equation (12)::

The invention also relates to an apparatus for performing the above pro cess, essentially comprising a dissolver for continuously dissolving the fuel elements in a solution, fuel element and nitric acid supplies to the dissolver, first and second desorbers connected in series with the dissolver, separate heating means for the dissolver and for each desorber, a first pipe for supplying the solution from the dissolver to the first desorber, a second pipe for supplying solution from the first desorber to the second desorber, a nitrous vapour supply pipe to the second desorber, a discharge pipe for the solution from the second desorber and discharge pipes for the gases fran which the molecular iodine has been formed, mounted on the dissolver and the desorbers.

The invention is described in greater detail hereinafter relative to non-limitative embodiments and with reference to the attached drawings, wherein show: Fig. 1 Diagrammatically an installation permitting the performance of the process according to the invention.

Fig. 2 Diagrammatically the HNO2 concentration variations, expressed in molarity, as a function of the Redox potential E in volts relative to a standard hydrogen electrcde (SHE) of a solution of uranyl nitrate and plutonium present in degrees of oxidation (+EV) and (+VI), for a temperature of approximately 100 c.

Fig. 3 The variations of the Pu2 concentration as a function of the HNO2 concentration, expressed in molarity, of a solution of uranyl nitrate and plutonium for a temperature of 100 C.

According to the invention, the process illustrated by fig. I operates continuously. In addition, the dissolver and the two desorbers are arranged in series.

With reference to fig. 1, the process according to the invention firstly consists of continuously dissolving in a known dissolver 2 containing the dissolving solution and the irradiated fuel elements. The fuel elements are continuously introduced at 4 in the form of powder or fragments using known means at the top of the dissolver 2 and nitric acid is introduced in known manner at 7.

The dissolver 2 is heated by a system 5 to a temperature of approximately 95"C, so as to maintain an adequate iodine fraction in solution of approximately 52. The volatile molecular iodine formed during dissolving escapes in the form of gas through a pipe 6 fitted at the top of the dissolver. It is entrained by the water vapour and nitrous vapours resulting fran the heating of the solution and the dissolving reaction.

The species essentially present in the nitric solution contained in the dissolver 2 are metallic iodides (particularly AgI), colloidal iodides, organoiodine compounds formed fran impurities introduced by the recycling of the fuel reprocessing solutions, iodate ions, dissolved molecular iodine and other fission products Pu4cand P1202 ions, nitrate ions, H+ ions and uranyl ions. The PuO22+ ions represent less than 1% of the total pultonium.

The solution leaving the dissolver 2 by the pipe 8 enters a first desorber 10 heated to a temperature of approximately 1050C (close to the boiling point of 107"C of the solution) using a heating system 13.

This desorber 10 can be constructed like that described in FR-A-2 641 119. Into the said desorber 10 is introduced, if necessary, using a 127 -1 supply pipe 11, inactive iodate ions, It3 , in order to favour the remutation reaction 1). The volatile iodine formed during desorption is discharged at the top of the desorber 10 by a pipe 12.

The nitric solution leaving the desorber 10 by a pipe 14 more particularly contains iodate ions, organoicdine species in trace form and metallic iodides in trace form. The solution also contains uranium, plutonium and fission products in nitrate form. Up to 50% of the total plutonium are in the form of plutonyl nitrate. The residence time in the first desorber is a function of the iodine quantity initially present in the fuel elements and the iodine proportion in the form of precipitated iodides, colloidal compounds and/or organoiodines. In all cases a minimum time of 2 hours is necessary.

The solution leaving the desorber 10 undergoes a second desorption in a second desorber 16 having a design identical to the first desorber and heated to a temperature of approximately 100 C with the aid of a heating system 15. Into the second desorber are introduced, within the solution, nitrous vapours of general formula NOX using a supply pipe 17 fitted at the top of the second desorber, so as to reduce the locate ions of the solution into volatile molecular iodine in accordance with equation (7).

Moreover, these nitrous vapours transform the plutonyl ions into Pu ions according to equation (6 bis). The thus formed iodine is discharged at the top of the desorber 16 through the pipe 18. For the second desorber a residence time of approximately 2 hours is adequate.

The nitric solution collected at 20 on leaving the second desorber no longer contains iodine. The purified solution leaving at 20 can then be supplied to uranium and plutonium clarification and extraction units.

With a real enriched uranium LWR fuel irradiated at 55,000 MWd/t, using the process according to the invention, there was a better iodine decontamination factor than that obtained with the prior art simulations dissolving an irradiated fuel at 33,000 MWd/t. Therefore the real fuel contains more iodine than in the prior art and the decontamination factor - s passed from 100 to 160 integrating both the iodine in solution and the iodine contained in the dissolving fines. The residual iodine content in the case of the real fuel is below 2.4 mg/Kg of U+Pu.

Therefore the process according to the invention applies to real fuel elements containing particular chemical forms of iodine and not the simulated solutions as in the prior art.

It is generally useful to check the HNO2 and PuO22+ content at different points of the process. Thus, the satisfactory operations of the units for extracting the plutonium and uranium contained in irradiated fuel elements positioned downstream of the dissolving and clarification units, requires the HUN02 and Put2 contents formed in the nitric solution are as low as possible, which leads to the interest of the continuous, in situ checking thereof.

Moreover, one of the features of the invention has consisted of proving that it was possible to continuously measure these two species by locally measuring in each apparatus the Redox potential of the solution using adapted Redox potential electrodes.

The inventors therefore measured the HN02 and PuO2 concentrations and the Redox potential of a solution of uranyl nitrate and plutonium, at a temperature of approximately 100 CC, where the concentrations (U), (PU), (NO3) and (H+) were constant.

The operating conditions for the tests were as follows: (H+) = 3N, (U)+(PU) = 0.96M, (N03) = 4.92M, T = 100 C.

In this medium, the following Redox reactions were used: and

which leads to the equation (6 bis).

At equilibrium, the potential of the solution can be described either by the pair NO3/HNO2, so that:

or by the pair PuOv /Pu4+, so that:

E01 and E02 being normal potentials at 100 C of the two considered pairs.

On the basis of the curve E=f((HN02)) plotted in fig. 2, we experimentally obtained the equation: E1 = 1.02 - 0.035 log (HNO2) (17) which is in gocd accordance with Nernst's law.

In the same way, it is possible to deduce from the experimental curve (PuO22 )/(Pu )=f((HN 2)) designated A in fig. 3, the relation:

The ratio (PuO22+)/(Pu4+) is therefore inversely proportional to the HN02 content, which confirms the fact that the governing species is HNo2 (cf. reaction (6 bis)).

The experimentally determined curve B of fig. 3 gives the variations of the PuO2 concentration, expressed as a percentage relative to the total Pu, as a function of the HN02 concentration.

By replacing (HN02) in (18) by its expression taken fran (17), we finally obtain:

Consequently, the measuranent of the Redox potential of the dissolving solution makes it possible to continuously obtain a representative quantity of the HNO2 content and the PUO2 level of the solution in situ.

This method is used for determining the HNO2 and PuO2 concentrations in the second desorber according to the invention, in order to adjust the NOx flow injected into said apparatus, so as to obtain the desired contents before the extraction of the uranium and the plutonium. The higher the temperature in the second desorber, the larger the injected NOx quantity.

Claims (11)

1. Process for the elimination of the radioactive iodine contained in irradiated fuel elements, which inter alia contain plutonium and com- prising the following stages: a) continuously dissolving in a nitric solution in a dissolver (2) fuel elements at a first temperature above ambient temperature, b) first desorption in a first desorber (10) of the iodine contained in the solution (8) leaving the dissolver at a second temperature close to the boiling point of the solution and without the injection of nitrous vapours, in order to form hexavalent plutonium so as to increase the oxidizing power of the solution leaving the dissolver and thus favour the formation of iodates fran the iodine species contained in the solution leaving the dissolver, c) second desorption in a second desorber (16) of the iodine contained in the solution (14) leaving the first desorber at a third temperature above ambient temperature and below the boiling point of the solution, accompanied by the injection (17) of an adequate quantity of nitrous vapours to reduce the plutonyl ions and iodates formed in the dissolver and the first desorber respectively into pu4+ ions and volatile molecular iodine.

2. Process according to claim 1, characterized in that addition (11) takes place of icdate ions in excess, in which the iodine is nonradioactive, to the first desorber (10) in order to oxidize the metallic icdides precipitated during dissolving.

3. Process according to claim 2, characterized in that the fuel elements are dissolved at the boiling point.

4. Process according to claim 1, characterized in that the first temperature is below the boiling point of the solution in order to limit the vaporization ratio of the solution and therefore the desorption of iodine.

5. Process according to any one of the claims 1 to 4, characterized in that the second temperature is below the boiling point of the solution.

6. Process according to any one of the claims 1 to 5, characterized in that the second temperature is between the boiling point and 10 C below the latter.

7. Process according to any one of the claims 1 to 6, characterized in that the third temperature is approximately 5 to 10 C below the boiling point.

8. Process according to any one of the claims 4 to 7, characterized in that the first temperature is approximately 5 to 15"C below the boiling point.

9. Process according to any one of the claims 4 to 8, characterized in that the first temperature is chosen so that the solution leaving the dissolver contains approximately 2 to 15t of the iodine initially present in the fuel elements.

10. Apparatus for the continuous elimination of the radioactive iodine contained in irradiated fuel elements, essentially comprising a dissolver for dissolving these elements in a nitric solution, supplies (4, 7) for fuel elements and nitric acid to the dissolver, first and second desorbers connected in series to the dissolver, separate heating means (5, 13, 15) for the dissolver and each desorber, a first pipe (8) for supplying the solution fran the dissolver to the first desorber, a second pipe (14) for supplying the solution from the first desorber to the second desorber, a pipe (17) for supplying nitrous vapours to the second desorber, a pipe (20) for the discharge of the solution from the second desorber and pipes (6, 12, 18) for discharging gases frcm which the molecular iodine has been formed and fitted to the dissolver and to the desorbers.

11. Apparatus according to claim 10, characterized in that it also can- prises a pipe (11) for supplying iodate ions to the first desorber, in which the iodine is non-radioactive.