RU2499306C1 - Method of cleaning irradiated nuclear fuel - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: separation of electropositive fission products from fission of alkali metal chlorides is carried out by chemically reducing the electropositive fission products on molybdenum metal. Separation of a portion of the electropositive fission products (zirconium and niobium) is carried out based on an exchange mechanism to form zirconium and niobium dioxides. Molten molybdenum is removed in form of molybdenum pentachloride by bubbling chlorine gas through the salt melt.

EFFECT: invention provides high percentage of extraction of electropositive fission products, a simple apparatus scheme, cheap process of treating nuclear fuel.

2 cl

Description

The present invention relates to the field of creating non-aqueous methods for processing irradiated nuclear fuel (SNF), the most promising of which are pyroelectrochemical processes carried out in molten mixtures of chloride salts. The technology for selecting components of SNF largely depends on the type of fuel (oxide, metal, nitride) and the method of its dissolution in salt melts based on various mixtures of lithium, sodium and potassium chlorides. So the oxide fuel used in almost all nuclear power reactors is dissolved, as a rule, by chlorination with gaseous chlorine, resulting in the initial technological melt composition: solvent + UO ₂ Cl ₂ (UCl ₄ ) + PuCl ₃ (PuCl ₄ ) + chlorides of fission products ( PD) and transplutonium elements (TPE). One of the most important technological problems is the extraction of electropositive fission products (ruthenium, rhodium, palladium, niobium, zirconium) from such melts in order to purify uranium and plutonium from them and to obtain a high-quality uranium-plutonium product in subsequent technological operations.

An analysis of the prior art in this field indicates the existence of different methods for the extraction of electropositive PD (Ru, Rh, Pd, Nb. Zr) from uranium chloride melts, for example:

1. Vasin B.D., Ivanov V.A., Raspopin S.P. On the interaction of zinc-based alloys with uranium and zirconium-containing chloride melts. - Melts, 1997, No. 2, p. 47-50.

The possibility of a sufficiently deep selective extraction of zirconium (up to residual concentrations of thousandths of a mass%) from a melt of (Na-K) Cl _equiv + UCl ₃ + ZrCl ₄ into a liquid alloy based on zinc (Zn-U) is shown. The achieved separation coefficients of uranium and zirconium closest to it in terms of electrochemical properties are about 10 ³ . A significant drawback in the processing of oxide fuel is the need for preliminary conversion of U (VI) to U (III), which often complicates the technological scheme.

2. Bychkov A.V., Vavilov S.K., Skiba O.V. Pyroelectrochemical processing of irradiated uranium-plutonium oxide fuel for fast neutron reactors. - Sat Proceedings of SSC RIAR “Closed fuel cycle: pyroelectrochemistry, technology of vibration compaction, fuel elements”. Dimitrovgrad, 1994, issue 1, pp. 13-20.

The possibility of purifying uranium and plutonium from Ru, Rh, Pd, Nb, Zr due to their extraction from oxyhalide systems obtained after dissolving spent oxide fuel by chlorination is shown. At the same time, metallic Ru, Rh, Pd are electrochemically released at the cathode together with uranium dioxide. At the same time, niobium and zirconium oxides are transferred chemically (via exchange reactions) to the cathode deposit of uranium dioxide. A significant drawback of this method is the complex control system for the electrolysis process with the release of a large (25%) amount of uranium in the form of dioxide at the cathode.

Closest to the claimed technical solution is a method for purifying nuclear fuel based on uranium dioxide (GP patent No. 1084340 "Process for the Purification of Nuclear Fuels") dissolved in a melt of alkali chlorides. Hydrogen gas is used as a reducing agent at a low feed rate to the melt. The precipitation of uranium dioxide is carried out in two or more stages. At the first stage, about 1% of uranium precipitates in the form of dioxide, while the precipitation product also contains electropositive fission products. Thus, the remaining uranium in the melt is purified from Ru, Rh, Pd, Nb, Zr.

The main disadvantages of this method is the use of gaseous hydrogen, which, when mixed with atmospheric oxygen, can produce an explosive mixture, which makes it necessary to comply with certain safety measures.

The problem to which the claimed invention is directed, is to create a method for cleaning irradiated nuclear fuel in chloride melts containing products of dissolution of oxide fuel by chlorination, by phased extraction of electropositive fission products, characterized in that the extraction of electropositive fission products is carried out by immersion in a plate melt metal molybdenum and its exposure in contact with the melt for 2-5 hours, followed by purification of the melt from dissolved molybdenum.

The essence of the proposed method consists in contacting a technological melt based on alkali metal chlorides containing the products of dissolution of irradiated oxide fuel by chlorination with metallic molybdenum immersed in a melt in the form of a plate. Metallic molybdenum reduces part of uranium (VI) to UO ₂ , and together with this, ions of ruthenium (III), rhodium (III) and palladium (II) are reduced to a metallic state. Niobium and zirconium, which have a very high affinity for oxygen, pass into the solid phase by a purely chemical process through the exchange reaction with UO ₂ in the form of oxide compounds.

) при обработке расплава хлором.Molybdenum (III) formed in the melt during oxidation-reduction reactions is removed by converting it into a volatile molybdenum compound ( $$MoC{l}_6^{-}$$

) when processing the melt with chlorine.

The result of these operations is the purification of fissile materials (uranium and plutonium) in the melt from electropositive PD.

Example

A melt based on a eutectic mixture of sodium and cesium chlorides (NaCl-2CsCl) containing zirconium, niobium, and palladium ions in various proportions at a temperature of 550 ° C was taken. Palladium was chosen as the most electronegative element from the group of platinum metals.

During the experiments, the mass of the molybdenum plate introduced into the melt decreased, and a precipitate formed on it. Qualitative analysis of the precipitate, performed on an X-ray fluorescence spectrometer, showed the presence of a substrate, in addition to molybdenum, uranium, zirconium, niobium and palladium in it. The solid phase precipitated on molybdenum, most likely, consists of metallic palladium and oxides of uranium, niobium and zirconium.

The table shows the results of the experiments, in particular, data on the initial and final concentrations in the salt phase of uranium and PD simulators (Pd, Nb, Zr), as well as the contact time of the melt with metal molybdenum.

, который можно удалить, барботируя через расплав газообразный хлор. После 90 мин барботирования хлором молибден полностью удаляется из расплава в виде летучего соединения MoCl₅ (частично сконденсировавшегося в верхней холодной части ячейки). Далее расплав в течение 5 минут вакуумируется для удаления растворенного хлора из расплава. Результаты химического анализа замороженного образца расплава свидетельствуют об отсутствии в нем молибдена.During the ongoing reactions, Mo (III) is formed in the melt in the form of a chloride complex $$MoC{l}_6^{3-}$$

which can be removed by bubbling gaseous chlorine through the melt. After 90 minutes of bubbling with chlorine, molybdenum is completely removed from the melt in the form of the volatile compound MoCl ₅ (partially condensed in the upper cold part of the cell). Next, the melt is evacuated for 5 minutes to remove dissolved chlorine from the melt. The results of chemical analysis of the frozen melt sample indicate the absence of molybdenum in it.

Thus, as can be seen from the table, there is a fundamental possibility of a fairly deep purification of technological chloride melts coming from the stage of SNF dissolution from electropositive PDs at different initial component ratios in one stage.

Claims (2)

1. The method of purification of irradiated nuclear fuel in chloride melts containing products of dissolution of oxide fuel by chlorination, by phased extraction of electropositive fission products, characterized in that the extraction of electropositive fission products is carried out by immersing a plate of metallic molybdenum in the melt and holding it in contact with the melt in for 2-5 hours, followed by purification of the melt from dissolved molybdenum. 2. The method according to claim 1, characterized in that the molybdenum dissolved in the melt is removed by purging the melt with gaseous chlorine.