JPS5847298A - Method of pretreating in nuclear fuel reprocessing - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pretreatment method for reprocessing nuclear fuel in a nuclear reactor. More specifically, the present invention provides pulverization of nuclear fuel by producing alkali metal or alkaline earth metal urates and plutonates in place of Us's in conventional poloxidation processes. The present invention relates to a pretreatment method for reprocessing nuclear fuel, which is characterized by efficiently removing volatile fission products.

Conventionally, various reprocessing methods have been proposed that can be roughly divided into dry and wet methods.Among these, the wet method, whose main step is the separation of fuel nuclides, fission products, and transuranium elements using a solvent extraction method, has been proposed. It has already been put into practical use. Other dry methods include the halide volatilization method, which uses differences in vapor pressure and chemical properties of fluorides or chlorides to separate them, or after dissolving spent fuel in a molten salt containing several hundred dust particles.
The possibility of a high-temperature metallurgical chemical method that attempts to perform separation by utilizing differences in electrolytic voltage, solubility, or reactivity with oxides has been proposed. By the way, the main processes before and after these main separation processes are wet.

Regardless of whether it is a dry method, dry methods are essential as pre-treatment and post-treatment steps. Examples include denitrification of uranyl nitrate, calcination, fluoridation process to UFa, or hull treatment.

Polo oxidation is one of these pretreatment steps, but it has received a lot of attention in recent years. That is, this step is located between the main steps of fuel shearing and separation, and aims to pulverize the fuel and vaporize and remove volatile fission products such as tritium. Fuel Fine Powdering 1- When the word separation step is a wet process, this is necessary to facilitate dissolution. If tritium is dissolved into light water and becomes tritiated water, it will be difficult to separate, and the economic burden of removing this nuclide will become heavy until the second requirement of "environmental safety" is met. Therefore, it is highly desirable to remove tritium by volatilization by dry methods prior to the dissolution step. In addition, ruthenium is difficult to dissolve in nitric acid (and the dissolved ions are the same as iodine (it takes various chemical forms that change slightly depending on the conditions), making separation and extraction difficult. Therefore, ruthenium is used along with tritium.

The purpose of the poloxidation process is to remove volatile fission products such as iodine, krypton, and xenon using a dry method. Poloxidation consists of cauterizing the fuel to 400-700°C in air or oxygen atmosphere. Through this operation, UO2 in the fuel is oxidized to UsOs, and due to the increase in volume accompanying the reaction, the pellet turns into powder and volatile fission products are evaporated and removed.

However, it has been found that the poloxidation method has the following problems: (1) Although 90 to 99% of tritium is removed by volatilization, carbon-14,
Krypton, xenon, iodine, ruthenium, etc. are not released sufficiently. (2) In fast neutron breeder reactor (FBR) fuel, that is, UO2-Pu02 mixed oxide fuel, if the plutonium content is less than 20%, it will turn into powder, but if it exceeds 25%, it will remain in the M2O3 phase (M = U + Pu), and the
Powderization does not proceed even if heated to 0'c. (3) When poloxidation is performed, the amount of insoluble plutonium (puo2) generated during the dissolution process increases by 4 to 5 times. In order to eliminate these drawbacks, improved methods such as high temperature poroxidation method and nitrate conversion method have been proposed, but the problem has not yet been completely solved.

The object of the present invention is to provide a dry pretreatment method that retains the advantages of the poloxidation method and does not have the drawbacks mentioned above. That is, according to the present invention, fuel pulverization and removal of volatile fission products are achieved by producing alkali metal or alkaline earth metal uranates and plutonates in place of UaOs in poloxidation. The aim is to do this efficiently.

According to the present invention, a suitable amount of an alkali metal compound or an alkaline earth metal compound is added to the spent fuel. The amount added is sufficient to produce uranate and plutonate. It is desirable that the compound to be added be one that thermally decomposes at several hundred degrees and does not produce harmful gases. It is also desirable from the viewpoint of reactivity with fuel to have the property of melting at a temperature lower than the decomposition temperature. After addition, if the mixture is heated with a stream of oxygen or air, the compound decomposes at a few hundred degrees Celsius, reacting with the fuel pellets to form uranates and plutonates. During this process, the fuel is pulverized and fission products are released into the gas phase.

According to the method of the present invention, plutonium also contains plutonate, since the volume increases (the lattice constant of plutonate is generally much larger than that of PL102).
FBR fuel has a large plutonium content, UO2-
Pu 02 mixed oxide is also easily powdered. In addition, in poloxidation, it was converted to U8'g, but this
The compound is a layered compound with strong covalent bonds, and can be regarded as a type of polymer in which uranium and oxygen atoms form a two-dimensional network structure, so that volatile fission products are formed in the crystal lattice. It is thought that the rate of release into the gas phase is low. Furthermore, the coexisting plutonium does not change from the initial Pu0z even by poroxidation, making it difficult to release the fission products trapped in the Pu0z lattice. According to the method of the invention,
Both uranium and plutonium are converted into uranate and plutonate, which have different crystal structures from the initial UO2 and Pu0z, so volatile fission products are released in high yield during this production reaction. Next, regarding the issue of solubility, PuO2 is originally known as a compound that does not dissolve easily in acids, but by heat treatment such as borooxidation, it is possible to dissolve However, as described in the literature, uranate or plutonate of alkali metals or alkaline earth metals have good solubility in acids, and the product is known to be water-soluble. The method of the present invention overcomes the difficulties previously encountered due to poor solubility.

Although the above description has focused on the case where the main separation step is a wet type, it goes without saying that the pretreatment method according to the present invention is also applicable to a reprocessing method whose main step is dry separation.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

A portion of the unirradiated UO2 pellet, approximately 200 m9, was placed in a quartz crucible and nitric acid was added.
After adding 5ml, it was dried in a dryer with several tens of dust particles.

This Na/U ratio is So17 in terms of atomic ratio. The quartz crucible was heated in a horizontal tube furnace at a rate of 3°C/min without flowing oxygen gas. Sodium nitrate melts at 320°C,
The reaction with UO2 started, and at 400 to 500°C, the pellets were pulverized and brown crystals of thorium uranate were generated. At 550-600°C, NO2 completely evaporates and the reaction ends, and Na2O (or Na202)
At the same time, bulky fine powder of sodium uranate was obtained.

Patent applicant: Japan Atomic Energy Research Institute

Claims (1)

[Claims] After adding an alkali metal salt or an alkaline earth metal salt to nuclear fuel, the two are heated in air or in the presence of oxygen gas to react, thereby converting uranium and plutonium into alkali metal or alkaline earth metal uranium, respectively. A pretreatment method for reprocessing nuclear fuel, which consists of pulverizing the nuclear fuel by converting it into acid salts or plutonate salts, and simultaneously releasing and removing volatile fission products into the gas phase.