JP3111382B2 - Scrap treatment method for mixed oxide fuel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for treating scrap powder generated during the production of mixed oxide fuel pellets.

[0002]

2. Description of the Related Art In the production of mixed oxide fuel (MOX fuel) used as a fuel for a fast breeder reactor or the like, for example, scrap containing a large amount of impurities generated in a pellet production process (that is, dirty scrap; It is simply referred to as scrap.) Since this scrap contains U and Pu, it has been studied to collect U and Pu in the scrap and then treat it as nuclear waste.

[0003] As a method for recovering U and Pu in the scrap, the scrap is roasted and reduced to a powder form, which is compression-molded and then sintered. A solvent extraction method of dissolving in a nitric acid solution and separately extracting U and Pu depending on the difference in partition ratio with respect to the solvent can be mentioned.

In this method, a scrap lump obtained by compression molding is sintered into a sintered scrap lump, whereby PuO ₂ crystals in the scrap are solid-dissolved in a UO ₂ matrix, and (U, Pu) O _{And 2} . (U, P
Since u) O ₂ is readily soluble in nitric acid solution, in the subsequent dissolution process and U and Pu in scrap since all moves to the nitric acid solution, so that the the U and Pu in the scrap is all recovered.

[0005]

However, in the sintered scrap mass, minute PuO ₂ crystals remain as spots without being dissolved in the UO ₂ matrix. In other words, the MOX fuel produced as a product has a spot-like PuO ₂ (hereinafter referred to as PuO ₂ spot).
There is an O ₂ spot. Since the PuO ₂ spot is hardly dissolved in the nitric acid solution, there is a problem that the PuO ₂ spot remains without being dissolved in the nitric acid solution in a dissolving step after sintering.

From the above, the present invention provides a scrap processing method capable of sintering without leaving a minute PuO ₂ spot in a scrap lump and completely recovering Pu as well as U in the scrap. It is intended to provide.

[0007]

In order to achieve the above object,
In the invention of claim 1, in order to recover U and Pu from scrap powder generated during the production of the mixed oxide fuel pellets, the scrap powder is collected into a scrap mass,
After sintering this to form a sintered body, the obtained sintered body is dissolved in nitric acid, and a mixed oxide fuel scrap processing method of extracting U and Pu, respectively, depending on a difference in distribution ratio to a solvent. A step of dissolving the sintered body in nitric acid, a step of promoting a solid solution reaction for promoting mutual diffusion and solid solution reaction of UO ₂ and PuO ₂ contained in the scrap mass. I have.

That is, in the present invention, before dissolving in nitric acid, UO ₂ (UO ₂ matrix) and PuO ₂ (PuO ₂
By promoting the interdiffusion of the _two spots) and the solid solution reaction, the PuO ₂ spot contained in the scrap mass is completely dissolved in the UO ₂ matrix, and U and Pu in the scrap are completely recovered.

The conditions for dissolving the PuO ₂ spot in the UO ₂ matrix include, for example, Al ₂ O
₃ , a substance that promotes the solid solution of PuO ₂ spots into the UO ₂ matrix, such as SiO ₂ , TiO ₂ , Nb ₂ O ₅ or a precursor of these oxides, is added to the scrap powder or added to the scrap mass. U ₄ O ₉ crystal phase or (U,
After sintering the scrap mass in an oxidizing gas atmosphere having an oxygen potential adjusted to a range where the Pu) ₄ O ₉ crystal phase is formed, subsequently sintering in a reducing gas atmosphere, and the like. No.

Under such conditions, all Pu
Since the O ₂ spot is solid-solved in the UO ₂ matrix to become (U, Pu) O ₂ and completely dissolved in nitric acid,
It is possible to collect all U and Pu in the scrap.

According to a second aspect of the present invention, in the scrap processing method for a mixed oxide fuel according to the first aspect, the solid solution accelerating step includes an accelerating substance which promotes mutual diffusion and solid solution reaction of UO ₂ and PuO _2. Is added to the scrap powder.

That is, in order to form a solid solution of the PuO ₂ spot, in the invention of claim 2, a substance which promotes the solid solution of the PuO ₂ spot, that is, a promoting substance is added to scrap powder in advance and uniformly dispersed. , The PuO ₂ spot is efficiently dissolved in the UO ₂ matrix during sintering. This method has the advantage that it is easy to carry out since only the accelerating substance is obtained and added to the scrap powder.

Further, according to a third aspect of the present invention, in the invention of the second aspect of the present invention, there is provided a mixed oxide fuel scrap processing method, wherein Al ₂ O ₃ , SiO ₂ , T
It is characterized by adding at least one of iO ₂ , Nb ₂ O ₅ , MgO, CaO and / or a precursor of these oxides.

These various accelerating substances may be used alone or in combination. It is preferable to select the most suitable ratio for each combination according to the material of the scrap each time.

The precursors of these oxides include:
Nb, Ti and the like can be mentioned, but it is better to add these in admixture with the accelerating substance rather than using them alone. In this case, the amount of addition may be determined so as to satisfy the above range in consideration of the amount of oxide generated by oxidation.

According to a fourth aspect of the present invention, in the invention of the first aspect or the second aspect of the present invention, the solid solution accelerating step includes the step of: Sintering the scrap mass in an oxidizing gas atmosphere having an oxygen potential adjusted to a range where a U ₄ O ₉ crystal phase or a (U, Pu) ₄ O ₉ crystal phase is formed in the scrap mass; And sintering in a reducing gas atmosphere.

That is, inside the scrap mass at the time of sintering,
There are a portion where the progress of sintering is fast and a portion where the progress of the sintering is fast. Since the growth of the crystal grains in the portion where the progress of the sintering is fast is fast, the crystal grains in this portion serve as nuclei for grain growth, and PuO ₂ , That is, it grows by taking in minute PuO ₂ .

According to the fourth aspect of the present invention, the conditions for sintering, that is, the oxygen potential in an oxidizing gas atmosphere, are adjusted in order to form a solid solution of PuO ₂ spots.
_{In addition} to accelerating the progress of sintering of ₂ and promoting diffusion, PuO
_It facilitates the incorporation of _two spots into the UO ₂ matrix.

This oxygen potential ΔG is defined by the following equation (1). Here, R is a gas constant, T is a Kelvin temperature, and P is a partial pressure of oxygen in an oxidizing gas atmosphere. ΔG = RTInP (1)

That is, the oxygen potential Δ at a constant temperature T
Since G depends on the magnitude of the oxygen partial pressure P in the oxidizing gas atmosphere, the value can be adjusted by controlling the oxygen partial pressure P at a constant temperature T.

The composition of such an oxidizing gas is as follows:
For example, within the concentration range of 1 × 10 ⁻³ volume% or more and 2 × 10 ⁻² volume% or less (that is, within the concentration range of 10 ppm or more and 200 ppm or less, preferably 100 ppm or more and 150 pp or less).
m) or a composition in which an oxygen gas (within a concentration range of not more than m) is mixed with a nitrogen gas, or CO ₂ and O ₂ are 5 × 10 ⁵ : 1 to 5
× 10 ² : 1 volume ratio, preferably 5 × 10 2
³ : 1 mixed by volume ratio, CO2
₂ and air mixed at a volume ratio in the range of 10 ⁵ : 1 to 10 ² : 1, preferably 10 ³ : 1 or CO ₂ and an inert gas (for example, a rare gas And nitrogen) are mixed at a volume ratio in the range of 1:10 ⁴ to 1:10, preferably 10 ² : 1.

In such an oxidizing gas atmosphere, 110
After heating in a temperature range of 0 ° C. or more and 1400 ° C. or less for 10 minutes or more and 4 hours or less, 1100 ° C. in a reducing gas atmosphere.
The heating is performed within a temperature range of not less than 1400 ° C. and not less than 10 minutes and not more than 4 hours, preferably not less than 1 hour and not more than 3 hours.

Examples of the composition of the reducing gas include hydrogen, decomposed ammonia, a mixed gas of hydrogen and nitrogen, a mixed gas of hydrogen and argon, or a mixture of these and steam.

[0024]

FIG. 1 is a flow chart showing an outline of an embodiment of the present invention. In this embodiment, both the addition of the accelerating material and the adjustment of the sintering atmosphere during sintering are performed. Needless to say, the present invention includes a case where only one of them is performed. In this embodiment, P
An experiment was conducted using scrap containing CeO ₂ having a similar behavior to PuO ₂ instead of uO ₂ (hereinafter referred to as “simulated scrap”).

First, 6.8 g of Ce was added to 120 g of UO _2.
The mixture containing O ₂ was added with carbon 5000 as an impurity.
ppm, 500 ppm of iron, 200 ppm of chromium, 100 ppm of nickel, and 200 ppm of zinc were added and mixed well to obtain a simulated scrap.

Next, the obtained simulated scrap is roasted and reduced, and Nb ₂ O ₅ as a solid solution reaction promoting substance is added to the simulated scrap so as to have a concentration of 0.1% by weight.
Press processing was performed at 4 to 7 t / cm ² to obtain a pellet-shaped scrap lump. Note that the simulated scrap roasted reduced as a comparative example, the pressing pressure 4 without adding Nb ₂ O ₅
A comparative scrap lump pressed at 7 t / cm ² was also obtained.

These scrap lumps were sintered at 1700 ° C. for 2 hours in a decomposed ammonia atmosphere as a reducing gas to obtain sintered bodies. Each of the obtained sintered bodies was examined for the presence or absence of CeO ₂ spots.

A scrap lump to which Nb ₂ O ₅ has been added;
In the comparative scrap lump without the addition of b ₂ O ₅ , there is no CeO ₂ spot in the scrap lump at all, whereas the comparative scrap lump has a CeO ₂ spot with an average diameter of about 15 μm including those having a maximum diameter of about 25 μm. The existence of many was confirmed.

From this fact, it was found that the addition of Nb ₂ O ₅ , that is, the addition of a substance which promotes the solid solution of CeO _{2 into} the UO ₂ matrix, caused the CeO ₂ spot to be completely UO _2
2 It is presumed that (U, Ce) O ₂ is dissolved in the matrix. Therefore, it can be said that all CeO ₂ spots in the simulated scrap can be dissolved and recovered in the nitric acid solution.

[0030] This is because true with regard PuO _2, the results from this example, that the addition of substances which promote the formation of a solid solution in the UO ₂ matrix completely been dissolved in UO ₂ matrix nitrate solution Therefore, it can be said that it can be recovered.

[0031]

As described above, according to the present invention, a PuO ₂ spot is dissolved in a UO ₂ matrix to form a fine PO ₂ spot.
Since the uO ₂ spots are eliminated, all the PuO ₂ spots move into the nitric acid solution together with the UO ₂ in the subsequent dissolution of nitric acid, so that all U and Pu in the scrap can be recovered.

[Brief description of the drawings]

FIG. 1 is a flowchart schematically showing an embodiment of the present invention.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G21C 19/46 G21C 3/62 G21C 21/02

Claims (4)

(57) [Claims] 1. To recover U and Pu from scrap powder generated during the production of mixed oxide fuel pellets,
After collecting the scrap powder to form a scrap lump and sintering it into a sintered body, the obtained sintered body is dissolved in nitric acid, and U and Pu are respectively extracted according to the difference in distribution ratio to the solvent. In the method for scrap treatment of a mixed oxide fuel, a solid solution for promoting mutual diffusion and solid solution reaction of UO ₂ and PuO ₂ contained in the scrap mass during a step before dissolving the sintered body in nitric acid A mixed oxide fuel scrap processing method comprising an accelerating step. 2. The method according to claim 1, wherein the solid solution accelerating step includes a step of adding to the scrap powder an accelerating substance for accelerating the interdiffusion of UO ₂ and PuO ₂ and the solid solution reaction. The scrap processing method of the mixed oxide fuel according to the above. 3. The method according to claim 1, wherein the accelerating substance is Al ₂ O ₃ or Si.
_{O 2, TiO 2, Nb 2} O 5, MgO, CaO and / or scrap processing mixed oxide fuel of claim 2, wherein the addition of at least one of the precursors of these oxides Method. 4. The solid solution accelerating step includes, when sintering the scrap mass, a range in which a U ₄ O ₉ crystal phase or a (U, Pu) ₄ O ₉ crystal phase is formed in the scrap mass. 3. The method according to claim 1, further comprising the step of sintering the scrap mass in an oxidizing gas atmosphere having an adjusted oxygen potential, and subsequently sintering in a reducing gas atmosphere. The scrap processing method of the mixed oxide fuel according to the above.