DE1592129A1 - Process for the reprocessing of fuel and / or breeding elements - Google Patents

Description

1592139

Remote research facility Jülich of the state of Mordrhein-Westphalia-e.V.

Process for the reprocessing of Fuel and / or breeding elements

The invention relates to a method for the reprocessing of fuel and / or breeding elements for nuclear reactors, in which the fuel and / or breeding material as Uranium-thorium mixed oxide or as uranium-thorium mixed carbide is present. The fuel and / or breeding material can be in a casing made of metal, graphite or some other material. It can also be used in advance of Shell grains, which are also known under the name coated particles, can be used. There are those predominantly spherical fuel cores to hold back the radioactive nuclides produced during the fission process, old dense shells made of pyrolytically deposited carbon, silicon carbide or some other suitable carbide coated. Fuel and / or breeding elements of the aforementioned type are in particular ift High temperature reactors used.

In the case of reprocessing, it is important that the » and / or breeding material from the 'during the cleavage process

η mn .:

■ - ■ '■■ - 3 -

20 * 111/0414

BATH

to free the fission products arising during the operation of the reactor and - as far as uranium-thoriura elements are concerned - to separate the fissile uranium from the thorium. The substances separated and freed from the reactor poison are then processed again into fuel and / or breeding elements in a further process.

As far as this is necessary, at the beginning of the reprocessing process to start the burning and / or brood material certain envelopes are often removed mechanically. The removal of the coatings from spherical fuel and / or debris particles Pyrolylytic carbon or carbides either by separation in a stream of oxygen (J.T. Roberts, L.M. Ferris, E.L. Nicholson, R.H. Rainey and CD. Wateon; ORNL-TM-1139 (1965); H.O. Witte, ORNL-TM-1411 (1968)) or by chemical oxidation in alkali salt melts (H. J. Riedel and £. Merz; Nukleonik (1967)). As a further measure for the reprocessing of fuel assemblies the solvent extraction with tri-butyl phosphate has already been used in an aqueous way »To increase the economy There is a possibility of reprocessing on uranium-PIutoniun Elements also used fluoride distillation (R.K. Steunenberg and. R.C. Vogel; Reactor Handbook, 2nd Edition, Vol. II, Interscience Publishers, (1961), chapter 6). Up to now, however, fluoride distillation has been restricted to thorium-containing fuel and / or brood elements not applicable.

For example, if the fuel and / or breeding material is in the form of coated particles, and it is used so-called head-end process to remove the coating of fuel and / or breeding material particles the measure

209811/04U

- ό -

BATH ORIGINAL

to burn the particles in the fluidized bed, the starting materials for further reprocessing measures are uranium-thorium-oxide mixtures in the presence of uranium-thorium mixed carbides. If coated uranium-thorium mixed oxides are present, the sintered cores are retained. It has been found that the reactivity of the mixture of ThO ₀ and IJ ₃ Og, which is obtained when carbidic nuclear fuels are present, can be fluorinated, but the time required is relatively high even at temperatures between 420 ° to 520 ° C, so that even after fluorination over a period of 45 minutes, only a uranium yield of about 93 could be achieved (REBlanco et al .; Nucl. Science and Eng. 20, 13 (19S4)) A cyclic fluorination-pyrohydrolysis can be achieved, but here too long fluorination times are required. However, fluorination of the generally highly sintered uranium-thorium mixed oxides has so far been almost impossible. The cores of coated particles were completely retained during the combustion process but only poorly volatile thorium tetrafluoride, which forms a cover layer on the fuel and / or debris particles, ie e has only very low diffusion rates. A fluorination of highly sintered uranium thorium mixed oxide cores has therefore not been economically feasible up to now (ELNicholson, IM Ferris and J "T. Roberts; EUR-278C β. 145-1G6 (19S5)). Attempts have also been made to first grind the cores of highly sintered uranium-thorium mixed oxides and then subject them to cyclic fluorination-pyrohydrolysis. However, it has not yet been possible to separate uranium even in these ways

209811 / 04U ο .. - ⁴ -

Construction IiGiNAl

the thorium matrix is sufficient to achieve a yield that is sufficient from an economic point of view. To overcome these difficulties, the fuel and / or brood material was ground and then brought into homogeneous solution by hydrofluorination with HF at a temperature of about 650 C in a fluoride salt melt and then tried to find the fuel and / or brood material to fluorinate elemental fluorine from the melt (AA Jonke, Atonic Energy Review, Vol. 3, No 1 pp. 3-60, Int. Atomic Energy Agency, Vi en na 1965). A major disadvantage of this measure, however, is the extremely strong corrosion of the equipment used to carry out this process. Another disadvantage is that the _{thorium dissolved in the melt as ThF 4 is} practically no longer economically usable except in the molten salt reactor. It must therefore be accepted that it will be stored as radioactive waste together with the molten salt.

The object of the invention is therefore a process for Wiederaufarbeitu »g of oxide and carbide uranium Thorioa combustion and / or fertile elements to provide, in an economical manner dea separation is öglich by fluorination".

This object is achieved according to the invention in that the fuel and / or brood material consisting of uranium-thorium mixed oxides, after removal, the possibly provided coverings and / or layers of layers that were previously - preferably when the fuel and / or Brutetoff is in particle form - applied «orden» reri, is dissolved in a pyrosulphate melt. the invention is included in the cutting

- 5 -

209811/0414

BATH ORIGINAL

ze solid alkali hydroxide introduced in such an amount, that the solution reacts alkaline.

In a further process stage according to the invention is then according to the invention that after the precipitation by filtration or centrifugation in a mixture with Alkali uranate produced by thorium oxide, i.e. alkali monouranate and / or Akl ^ aidiuranat fluorinated and that resulting uranium hexafluoride is distilled off.

Method according to the invention is also on

Mixed fuels and / or breeding materials, which are present as uranium-thorium carbides, can be used. But in this case it is necessary to convert the fuel and / or breeding material into a mixture of U ₃ Og and ThO ₂ before the fuel and / or breeding material is used in the pyrosulphate melt, for example by burning it off in the oxygen stream. If the fuel and / or breeding material is present in the form of uranium-thorium mulled carbides, it can also be advantageous to dissolve the fuel and / or breeding material in oxidizing alkali salt melts known per se after removal of the casing and, if necessary, the coating on it the precipitate formed to separate de? Fluorinating uranium. Melts of NaOH + Na 0 ^, Na ₉ CO ₃ + K ₂ CO ₃ or the like have proven to be very suitable melts.

The following measures are suitable for carrying out the fluorination of the accumulated precipitate. The alkali urate obtained in the precipitation in a mixture with thorium oxide is released by the action of gaseous Hydrogen fluoride HF at temperatures between 450 ° and

209811 / (HU

BAD OWOlNAL - β -

550 C converted into a mixture of UO ₂ Fr, and alkali fluoride and then fluorinated by elemental fluorine _m optionally in a mixture with inert gases such as nitrogen or the like at temperatures between 350 and 550 ° C. The conversion of the resulting potassium auranate into uranium ether cafluoride thus takes place according to the reaction equations

K ₂ U ₂ O ₇ + 6 HF ⁴⁵ ° " ⁵⁵ ° ^C » 2 UO ₂ F ₂ + 2 KF + 3 H UO..F «+ 2 F" _ ₊

According to another very zweclcmäßigen measure for performing the method according to the invention the accumulating in cases mixed with thoria Alkaliuranat by the action of a mixture of hydrogen fluoride and a gaseous reducing agent such as SO ₀ or NH ₃ zwisKhen at temperatures 400 ° and 500 ° C in a mixture of Uranium tetrafluoride and alkali fluoride converted, whereupon in a further process stage at a temperature between 350 ° and 550 C the uranium tetrafluoride is converted into TJranhexafluorid by the action of elemental fluorine, optionally in a mixture with an inert gas such as nitrogen or the like. The reaction then takes place according to the equations

Na ₂ UO ₄ + S HF + SO ₂ UF ₄ + 2 HaF + 3H ₂ O + SO ₃ .

or _oo

3 S ₂ U ₂ O ₇ + 30 HF + 4 NH ₃ 3 UF ₄ + 6 KF + 21 Η.Ό + 2 N,

and _rt

3SS ° «b24 or.

_{il +} , 3SS ^ b24 or.

⁴ 2O9811 / 0 * U BAD ORIGINAL

»7 -

1882129

2iacli a further advantageous modification of the measures for implementing the method geaäß the Erfinduag is the resulting in the precipitation ira mixed with thoria All: aliuranat after separation from the melt 'at a temperature of about 450 ⁰ C by means of elemental fluorine fluorinated in the fluidized bed furnace. The fluorination takes place. thus according to the reaction equation

? -. SSO P

-. SSO P

Na ,, UO ₄ + 4 F., '- up "+ 2NaF + 2 O ₀

The thorium diatia present is given at the respective i also took to fluorinate the alkali metal urate, however only partially fluorinated. However, it has been shown that this does not disturb the quantitative formation and volatilization of uranium hexafluoride. The great The advantage of the method according to the invention is that the uranium is removed from the precipitate formed in the melt easily and completely as volatile uranium hexafluorici detachable 1st ..

Examples:

1 · In 30 g of a potassium pyrosulphate melt 10 g intact uranium-thorium mixed oxide particles entered, where the atomic ratio of uranium to thorium was 1: 5, and in a closed reaction vessel 700 ° to 800 ° C quantitatively brought into solution within a period of two and a half hours. Afterward 30 g of solid NaOH were added to the melt at about 600 ° C added. The precipitated precipitate of alkali metal uranium and alkali metal diuranate as well as thorium oxide was by filtration through a sintered filter from the peeling

209811/0414 ^M0MMt

-S-

ze separated, in Ans .3 chip because the mixture in the fluidized bed furnace was placed for one hour with elemental fluorine at 500 C for Realst ion and depressed the distilled uranium hexafluoride in a kept at minus 60 ⁰ C cold trap. As the subsequent analysis showed, the uranium had been volatilized quantitatively as U-anhexafluoride, while the thorium remained in the residue as a partially fluorinated product. The uranium content found in the thoriura residue was 0.05 %.

2. _{A precipitate of K 9} U ₀ O ₇ + ThO formed during precipitation from a Pyrosuifat.schmelze, the atomic ratio between uranium and thorium was 1: 5, was in a fluidized bed furnace, the furnace body consisted of pure sintered corundum, at a furnace temperature between 500 ° and 55O ° C treated with hydrogen fluoride for two and a half hours. The uranium in the mixture used was quantitatively converted to UO..F. The reaction material was obtained in powder form. The subsequent volatilization of the uranium as uranium hexafluoride took place in the same facility through an inlialb-hour reaction with elemental fluorine. The uranium yield was greater than 99.5 %.

3. A precipitate from a melt of HaQIf and Νε- ₂ Ο ₂ of Na ₉ UO ₄ + ThO ₀₁ in which the atoite ratio between uranium and thorium was 1:30 was never in the previous example for four hours in a fluidized bed furnace at 500 ° C with a mixture of HF and SO _n , in which the mixing ratio 2: 1

209811/0414

BAD OffiGINAL

fraud, treated. The reaction material was obtained as a loose powdery product. Sales at TJF- was
quantitatively, as by a chemical analysis of the
Product has been confirmed. The subsequent volatilization of the uranium with I * proceeded in the same way as in the previous examples.

BAD OFnGiNAL
209811/0414

- ίο -

Claims (2)

Claims 1. Process for the remanufacturing of burning and / or Breeding elements for nuclear reactors where the fuel and / or the breeding material is present as uranium-thorium mixed oxide or uranium-thorium mixed carbide, characterized; that the fuel and / or the breeding material after removal of the possibly intended for its inclusion Wrapping and / or of the fuel and / or breeding material in known "Ifeise wrapping" coatings, as far as it is in the form of carbides, after prior conversion into one when burned off in the Oxygen flow resulting mixture of U ,, CU and ThO., Is dissolved in a Pyrosulfatsehraelze, whereupon Solid alkali metal hydroxide is introduced into the melt in such an amount that the solution becomes alkaline reacts and that by precipitation and subsequent filtration or centrifugation in a mixture with Thorium oxide as alkali uranium obtained by fluorination and distillation as uranium hexafluoride is separated. 2. Process for reconditioning fuel and / or Breeding elements for nuclear reactors, in which the fuel and / or the breeding material is a mixture of uranium and 209811/0414 ^BAD Thoriuracarbiden is present, d a d u. R ch ge mark eh η et that the fuel and / or the breeding material nn.ch the removal of the possibly envelope provided for its inclusion and / or from fuel and / or breeding material in on known manner enveloping coatings iri. an oxidizing alkali salt melt known per se is dissolved and that by precipitation and anscMiefsenclos Filtration or centrifugation in the Mixture with thorium oxide as alkali urate Urnn by fluorination and distilling off as uranium fluovid is separated. Veri'ahron according to claim 2, characterized in that in the form of üran-thorium Misehoarbiden present combustion and / or Brntstoff in melting of NaOH + Na "0 _{o, o} C0 Na" + K _n CO or the like to form a precipitate is converted by alkali uranium and thorium oxide. Process according to Claims 1 to 3, characterized in that the alkali metal urate obtained during precipitation in a mixture with thorium oxide is converted into a mixture of UO ₉ F by the action of gaseous hydrogen fluoride at temperatures between 450 ° and 550 ° C ₉ and alkali fluoride and then fluorinated by demenaj'trefi fluorine, optionally in a mixture with inert gases such as nitrogen or the like, at temperatures between 350 and 550 ° C. 5. The method according to claims 1 to 3, characterized -12 - BAD ORGiNAL 209811/0414 marked levels that when felling alkali uranium obtained in a mixture with borium oxides by, exposure to a mixture · of hydrogen fluoride and a gaseous reducing agent such as SOr, or NII..J at »temperatures between 400 ° and 500 ° C in Mixture: of uranium tetrafluoride and alkali fluoride is converted, whereupon ir. a further process Pt at a temperature between 350 ° and 550 ° C the uranium tetrafluoride by the action of elementary Fluorine, optionally = in a mixture with one k inert! Gas such as nitrogen or the like in Uranium hexaf1uorid is converted. 6. Procedure na «. · !! Claims 1 to 3, characterized in that the alkali uranium obtained in the precipitation in a mixture with thorium oxide after separation from the melt at a temperature of about 450 C by means of eggs *
shaft furnace is fluorinated. of about 450 C by means of elemental fluorine in the vortex 2 0 9 8 1 W 0 A 1 / * BATH ORIGINAL