DE2801744A1 - PROCESS FOR SEPARATING FOCAL GASES FROM IRRADIATED NUCLEAR FUEL - Google Patents

Description

P A T E N T A N WÄ L T E 2801744

DR. KARL TH. HEGEL DIPL.-ING. KLAUS DICKEL

GROSSE BERGSTRASSE 223 2000 HAMBURG 50 JULIUS-KREIS-STRASSE 33 80OO MUNICH 6O

PO Box 5OO862 TELEPHONE (O 40) 3Θ 62 95 TELEPHONE (0 89) 88 52IO

Telegram address: Doellnerpatent Hamburg

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20OO Hamburg, the

Dr.He / mk

EXXON NUCLEJLR COMPANY, INC.

777 106th Avenue, NE
C-00777, flellevue, Washington, V.St.A.

METHOD OF SEPARATING FOCUS GASES FROM IRRADIATED THEN FUEL ¹ P

The invention relates to a method for separating volatile fission products from irradiated fuel pills with the help of NOo or a mixture of NO2 with its dissociation products, namely Op and NO.

One of the problems that arises in reusing irradiated nuclear fuel is that it is extraordinary Difficulty separating the volatile cleavage products from the

Postal checking account: Hamburg 29122O-2OB · Bank: Dresdner Bank AG. Hamburg, account no. 3 813 8Θ7.

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Reaction solutions of the heat insulation process. This is particularly important with regard to the removal of tritium, since the tritium isotope is extremely difficult to separate from aqueous solutions. The hydrogen isotoptritium has a half-life of about 12.26 years and is a ternary ^ fission product that arises in small quantities, about 29 mg per 1,000 kg of fuel. Considering the volume of irradiated fuel that will presumably have to be produced to meet the world's electrical energy needs in 2000, the amount of tritium that will be produced is substantial. Reclamation of tritium in factories of common design for "reclamation of nuclear fuel is."
practical, not possible due to the very small amounts of tritium in each ton of fuel. The tritium is namely in the form of tritium-containing water with thousands of galons
(3j785 liters) of the processing water intimately mixed in the factory. An isotope separation of the tritium from this large volume of water is required before the water can be discharged as liquid or vapor. Since isotopic separation of tritium is not practical in such volumes of water, a technique for removing volatile fission products from irradiated nuclear fuel prior to its water treatment is required
to be developed.

The problem of removing volatile fission products from irradiated Fuel occurs when irradiated uranium dioxide

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fuel and / or irradiated mixed oxides such as UC ^ and PUO2 should be worked up. The problem of elusive removal Fission products is made more difficult by the fact that the volatile Fission Produlcte within the crystal lattice of the irradiated Fuel are retained and thus cannot be removed by grinding or pulverizing the fuel are.

A method for removing volatile fission products from irradiated fuel has been developed by Oak Ridge National Laboratories, USA; it is referred to as "bulk oxidation" and is described in the report from Oak Ridge National Laboratories Sr. ORTiL-TM-3723. "Full oxidation" is a process of oxidizing irradiated fuel in the presence of oxygen at a controlled temperature to produce a very fine powder of U ^ Og. This "VoI-o-xidationsprozess" is temperature sensitive, and it has been assumed that in large installations, the reaction temperature must be kept within narrow limits expected at about 480 ⁰ C with a variation of + _ 10 ⁰ G. As a result of this critically narrow temperature range, the difficulties in working in large-scale recovery plants increase considerably. The present invention now relates to a method for removing the volatile fission products from irradiated fuel in an economically feasible time under economically feasible process conditions, with a significant reduction in costs in the.

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Reconditioning of irradiated nuclear fuel is achieved.

The present invention, broadly speaking, is a method of oxidizing irradiated nuclear fuel to remove the volatile fission products, ie iodine, xenon, krypton and tritium. Irradiated uranium dioxide or mixed oxide fuels are oxidized with nitric oxide, ie nitrogen dioxide. At the reaction temperature of this process, nitrogen dioxide is in equilibrium with its dissociation products, namely nitrogen monoxide and oxygen. Thus, the oxidizing agent can be added in the form of nitrogen dioxide or the dissociation products of nitrogen dioxide (namely, nitrogen monoxide and oxygen) or in the form of a mixture of nitrogen dioxide and its dissociation products. The oxidizing agent can be diluted with a gas such as nitrogen or nitrogen monoxide without any significant adverse effect occurring. The temperature of the oxidation reaction can be between 325 ° C and 800 ⁰ g, preferably between 350 ° and 780 ° C, are. Most preferably, the reaction temperature is adjusted to a range of 35O ° to 650 ^0C.

The oxidation reaction is carried out within a time sufficient to convert the irradiated fuel pills into a fine powder, the average particle size of which is less than 0.105 mm, preferably less than 0.045 mm. A heating ^{temperature above 800 °} C. leads to an agglomeration .... 5

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of the fine powder, and should therefore be avoided.

The irradiated fuel particles are left after removal from the. The reactor generally cools, then the fuel rods are cut or chopped up to prepare the irradiated fuel for reuse. The irradiated nuclear fuel is then placed in a reaction vessel, whereupon nitrogen dioxide alone or in a mixture with its dissociation products is passed through the reactor. The system is heated ^{to a reaction temperature between 325 0} and 800 ^{0 O.} During the oxidation reaction, the fission gases, especially tritium, escape in almost quantitative amounts with the gases flowing out. During the reaction, the UOp is oxidized to UO ^ and / or U ^ Og.

According to the present invention, an irradiated nuclear fuel, for example UOp) or a mixture of UOo and PuOp, can be introduced into a reaction vessel and oxidized here with nitrogen oxide, for example nitrogen dioxide or a mixture of nitrogen dioxide and its dissociation products, namely oxygen and nitrogen monoxide. The oxidation is carried out in such a manner that the irradiated fuel is employed in the reaction vessel and heated to a reaction temperature of about 35> O ° to 78o ⁰ G in the presence of nitric oxide. The irradiated fuel is heated for a period of time sufficient to allow the irradiated fuel to enter the

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Form of a fine powder from IUO ₀ and uü-, to be transferred. The duration of the action depends on the volume of the material in the filter, the size of the fuel, the particle size of the UOp, the temperature and the gas composition. The fine powder will have an average particle size of less than 0.105 mm, preferably an average particle size of less than 0.04 ;? mm, own.

The tritium set in freedom can be collected as tritium gas or oxidised in the form of THO, cooled and collected as a liquid, for example, TI ¹ IO ^, HTO and the like.

To demonstrate the effectiveness of the method of the invention, fuel pills were placed in an ordinary laboratory facility oxidized. The fuel particles were placed in a sample boat and placed in a combustion tube. A thermocouple was used for temperature control. An incinerator was placed around the combustion tube, to heat the system. The oxidizing agent was passed through the combustion tube in gaseous form. That Tritium set free during the oxidation was converted to THO in a copper oxide furnace and in a cold finger-shaped one Vessel caught.

EXAMPLE 1

1 g each of a sample of irradiated uranium oxide fuel (UOp) and a mixed oxide _{fuel of UO 0} and PuO ₀ was _n

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each time oxidized for four hours with UOp at ² I-OO ⁰ G. At the end of the four hour period, the irradiated fuel was in the form of a very fine powder. The powder obtained was dissolved in 15 ecm of an 8-fold molar nitric acid for between three and four hours at 100 ° C., filtered and tested for the tritium remaining in the solution. Another gram each of the sample of the irradiated fuel was immediately dissolved in 8-fold molar nitric acid for about 4 hours at 100 G without oxidation with JO _{9, filtered and tested for tritium in order to obtain a standard sample.} The remaining, filtered-off solids from both the standard samples and the test samples were again dissolved in 8-fold molar nitric acid with the addition of 0.005 molar hydrofluoric acid, and the resulting solution was tested again to determine the tritium content of the solids. The results of the oxidation of the two fuel samples are given in Table I below.

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TABLE I.

fuel Found tritium in
juci / g UO ₂ to dissolve
amount used Solids Percent tritium
iai compared toi
Standard sample Standard UO ₂ 159.0 0.05 - UO ₂ oxidizes with NO ₂ 0.17 0.12 0.2 % Standard _{sample U0 2} / Pu0 ₂ 20.4 0.11 - U0 ₂ / Pu0 ₂ oxidized with NO ₂ 0.64 0.4 5 %

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From the above results it can be seen that oxidizing the fuel with NOp prior to its dissolution is effective in removing the tritium from the fuel. The solution of the mixed oxides and the solid contained 5 % of the original tritium content.

EXAMPLE 2

A series of experiments was carried out under constant temperature conditions from 300 ° to 800 ⁰ C using Eroben fuel pills of ITCU to the Reaktionsge to determine speed. When the oxidation reaction was complete, the fuel pills were converted into a finely divided powder and the fuel sample showed a weight increase of 4 %. The results evident from]? Ig.1 were obtained.

In temperature tests between 325 and 600 0, the end product was a powder. Try
almost the same results.

product a powder. Experiments at 350 °, 500 ° and 600 ⁰ C showed

EXAMPLE 3

A series of experiments at constant heating rates were carried out to test the effect of diluting the oxidizing agent Check NOp with nitrogen. The Obtained results that can be seen from 5 "ig.2.

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The fuel samples consisted of U0 ₂ pills. It should be noted that the reaction rate of the oxidation of UO ₂ by. FiO _{2 is} relatively insensitive to dilution with nitrogen.

EXAMPLE 4-

In order to determine the influence of a dilution with ΠΟ-Gss on the oxidation of UOp, an experiment was carried out with a constant heating rate using KO2, with EO being added to the oxidation mixture. The results evident from Pig. 3 were obtained.

The oxidizing agent used consisted of a mixture of equal parts of ITO ₂ and IiO. It was observed that increased amounts of NO apparently reduce the reaction rate for an oxidation of the UO ₂ in the Ii0 _{2 stream.}

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Claims (16)

1. A method for releasing volatile fission products from irradiated nuclear fuel, characterized in that the irradiated nuclear fuel pills are treated with a nitrogen oxide oxidizing agent at a reaction temperature between about $ 25 and 800 ⁰ O for a time sufficient to convert the material into a fine powder, wherein the volatile fission products escape. 2. The method according to claim 1, characterized in that that the irradiated fuel consists of uranium oxide, plutonium oxide or mixtures of these compounds. 3- method according to claim 1, characterized in that that the reaction temperature between 350 ° and 650 ° 0 is kept. 4. The method according to claim 1, characterized in that the oxidizing agent sentlichen we consists of nitrogen dioxide. 809832/0634 ORIGINAL INSPECTED 5. / experience according to claim 1, characterized by gelcenn, that the oxidizing agent consists essentially of a mixture of nitrogen dioxide, Is made up of oxygen and nitric oxide. 6. The method according to claim 1, characterized in that the volatile cleavage products
consist of xenon, iodine, krypton and tritium. 7- The method according to claim 1, characterized in that that the oxidizing agent is diluted with a gas. 8. The method according to claim 7i , characterized in that the diluent gas consists of nitrogen. 9. The method according to claim 7 »characterized in that that the diluent gas consists of nitrogen monoxide consists. 10. Process for releasing volatile fission products from irradiated fuel, characterized in that an irradiated fuel, which consists of uranium oxide and a mixed oxide of uranium oxide and plutonium oxide, with 809832/0634 Nitrogen dioxide at reaction temperature oxidized between about 350 and about 6J? 0G will. 11. The method according to claim 10, characterized in that the nitrogen dioxide is in Equilibrium with its dissociation products, namely oxygen and nitric oxide is located. 12. Process for the release of volatile fission products, namely iodine, xenon, krypton and tritium, which are produced during the irradiation of nuclear fuels, characterized in that that one can make an irradiated nuclear fuel made from uranium oxide, plutonia oxide and mixtures These oxides are oxidized with an oxidizing agent that, from nitrogen dioxide, dissociation products of nitrogen dioxide, namely nitrogen monoxide and oxygen, and mixtures of nitrogen dioxide and its dissociation products exists, namely at tempera doors between 325 ° and 800 ° 0, whereby the The crystal structure of the fuel is changed and the volatile fission products are released be set. 809832/0634 H 2812 -φΤ 13. The method according to claim 12, characterized in that the reaction temperature is between 350 ° and 650 ⁰ C see. 14. The method according to claim 12, characterized in that the oxidizing agent with a Gas is diluted. 1ί ?. Method according to claim 14, characterized in that that the diluent gas consists of nitrogen. 16. The method according to claim 14, characterized in that the diluent gas consists of nitrogen monoxide consists. 809832/0634