JPH0374958B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Applicable Technical Field> The present invention relates to a method for adjusting the sintered density of nuclear fuel oxide sintered pellets, which are fuel for light water reactors and fast reactors, to a desired density.

<Prior Art> Nuclear fuel oxide sintered pellets such as UO ₂ or PuO ₂ -UO ₂ are produced in a range of about 85% to 96% of the theoretical density depending on the characteristics of the irradiated reactor. More specifically, low-density sintered pellets can reduce irradiation-induced swelling and reduce mechanical interaction with the cladding, making them extremely important as fuel for reactors that require high burn-up. However, in general, the porosity of the fired pellets is increased by adding an organic density-lowering agent, such as pore fluorescer, to the raw material powder, thereby reducing the density of the fired pellets. However, if this method were to be used for mass production, unless we devised a method for decomposing and distilling off the organic matter, not only would the life of the kiln be shortened due to corrosion from decomposition gases, but also the problems would occur in the pellets. It is also expected that more than the specified amount of carbon or gas produced by decomposition will be occluded.

On the other hand, high-density pellets are obtained by sufficiently pulverizing the raw material powder, molding and firing it, but in this case, in addition to the problem of impurity contamination due to wear and tear of the pulverizing equipment, it is sometimes necessary to heat the pellets to 1700°C. During the calcination process, which is sometimes carried out at extremely high temperatures, the pellets could become distorted due to unexpected rapid shrinkage or abnormal particle growth.

<Problems to be Solved by the Invention> In general, raw material powders for nuclear fuel oxide sintered pellets such as UO ₂ , PuO ₂ or PuO ₂ −UO ₂ differ in raw material base salt, preparation history, grinding conditions, etc. Even if the chemical composition is the same, sintering characteristics such as changes in shrinkage rate in the height direction and radial direction, changes in density, changes in porosity, changes in particle size, etc. when molded and sintered. are different from each other. In other words, the pulverized raw material powder has its own unique sintering properties. Therefore, it has been considered extremely difficult to reproducibly prepare pellets with desired sintering characteristics by blending two or more types of raw material powders each having different sintering characteristics, and it has also been considered extremely difficult to prepare pellets with desired sintering characteristics with good reproducibility. Until now, no measures have been proposed.

In view of the above-mentioned circumstances, an object of the present invention is to arbitrarily combine raw material powders with different sinterability, blend them, mold and fire them to form sintered pellets, and then increase the sintered density of the pellets. It is an object of the present invention to provide an adjustment method that makes it extremely easy to obtain a desired sintered density.

<Means for Solving the Problems> The present invention provides that when two or more types of raw material powders with known sintering properties are blended at a certain blending ratio, the sintering properties calculated by a calculation formula are actually It was found that the sintered pellets were in good agreement with that of the sintered pellets obtained by molding and firing, and by utilizing this property, the sintered density of the adjusted sintered pellets, regardless of whether it is high density or low density, can be adjusted. We have found that it is extremely easy to achieve the desired result.

That is, the present invention is a method for preparing sintered pellets obtained by molding and firing a mixed powder obtained by blending at least two types of raw material powders A and B, so that the sintered pellets have a desired density. , B are assumed to have known density change rates (δρ _TA , δρ _TB ) at the sintering temperature (T°C), and the blend ratio (n/m) is calculated using the formula n・δρ _TA +m・δρ _TB /n+m The purpose is to determine the value obtained from the above to be the desired density change rate of the sintered pellets.

This invention will be explained in detail below. _UO2 ,
Raw material powder such as PuO ₂ or PuO ₂ −UO ₂ is
As mentioned above, it has its own unique sintering characteristics.For example, when it is heat treated in the pre-pulverization process, the heat treatment conditions such as the heat treatment temperature, heat treatment time, heat treatment atmosphere, and added dopant are used. In addition, in the process of grinding after heat treatment or without heat treatment, the grinding conditions such as the grinding machine type, machine capacity, loading amount of grinding material, grinding time, and the grinding atmosphere in the case of vibration mill grinding, the grinding aid added, etc. In addition to the difference in UO ₂ , PuO ₂ , PuO ₂ −
If UO ₂ or a mixed powder thereof is preformed by cold pressing, and then it is crushed and sized into powders with various particle sizes or particle size distributions, the preform The sintering properties are different depending on various requirements such as different molding pressures at the time. For each raw material powder,
In this invention, a sintering property test is conducted in advance, and the results are databanked for later use, so that the sintering properties are known. When preparing sintered pellets with a predetermined density,
The raw material powder is appropriately selected from the above-mentioned raw material powders with known sintering properties.

The problem is, what blending ratio should be used for the at least two types of raw material powders A and B selected in this way to ensure that the sintered pellets obtained by molding and firing the mixed powder have the desired density. The problem lies in determining the blending ratio. In this invention, it can be easily determined from the simple equation below. That is, from the experimental results that the present inventor has conducted so far, raw material powders A and B whose density change rates at the sintering temperature T°C are δρ _TA and δρ _TB , respectively, are
When _{sintered pellets} are made by blending at the ratio of I found out this fact. Therefore, the blending ratio n/m that will give the desired density change rate is determined by the above formula, blended according to the obtained blending ratio n/m, and the mixed powder is molded and sintered at a temperature of T°C. If you bake it,
This allows pellets to have the desired density. When blending three types of raw material powders, first the calculated values for the two types and the remaining one type may be calculated according to the above formula.

Although the above has been described from the viewpoint of density among sintering characteristics, the same can be said about shrinkage rate. In other words, the shrinkage characteristics of sintered pellets obtained by blending raw material powders with known shrinkage ratios at a predetermined ratio can be estimated by the value calculated from the formula in which the density change rate is replaced with the shrinkage ratio in the above equation. .

Note that the above-mentioned dopants include Fe ₂ O ₃ ,
In addition to metal oxides such as MnO, MnO ₂ , ZrO ₂ , MgO, and SiO ₂ , their acetates, carbonates, and oxalates may also be used, as well as hydroxides and oxides such as acetylacetonal and alkoxides. It may also be a precursor of Grinding aids may be solid or liquid such as soda carbonate, lithium hydroxide, diethylamine, and piverizine.

<Example> Two types of UO ₂ powders with the same composition were prepared using completely different powder preparation processes. In order to test the sintering properties of each of the powders A and B, the green density ρ _g (%TD) and the relative density ρ _T at T°C were determined by molding and firing at various firing temperatures T°C.
(%TD) was measured, and the density change rate δρ (%) in each case was calculated using the following formula.
Figure 1 is a graph summarizing the results.

δρ=ρ _T −ρ _g /ρ _g ×100 Next, when the blend ratio n:m of the above powders A and B is 1:1, calculate the calculated value of equation (1) and plot it. As a result, a curve like the solid line in FIG. 2 was obtained. On the other hand, mold this mixed powder,
The density change rate of the sintered pellets obtained by firing was actually measured. The dotted line in FIG. 2 is a curve showing the actually measured values.

Further, when the blend ratio n:m was 1:3, the same procedure as above was carried out. the result,
Curves of calculated values shown by solid lines and measured values shown by dotted lines in FIG. 3 were obtained.

It is clear that both the two curves in FIG. 2 and the two curves in FIG. 3 are in good agreement. In other words, it was confirmed that the actual value can be estimated by the calculated value of equation (1).

<Effects> As is clear from the above explanation, the method of the present invention requires that each raw material powder be tested for sintering characteristics in advance to become known, and in this respect, it does not require some effort. do not have. However, after that, the blending ratio can be determined very easily, the molding and firing conditions can always be the same, and sintered pellets with the desired density can be prepared. Not only that, it is easy to find combinations of raw material powders that do not require the use of organic density-lowering agents just because they are low-density pellets. Because it can be estimated in advance, it is extremely advantageous in that it allows you to avoid having to worry about sudden shrinkage or abnormal particle growth only after sintering.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the sintering temperature and density change rate of raw material powders A and B. Figures 2 and 3 are graphs showing the relationship between the sintering temperature and density change rate of sintered pellets when the raw material powders A and B in Figure 1 are blended at a blend ratio of 1:1 and 1:3. , solid lines represent calculated values, and dotted lines represent actual measured values.

Claims (1)

[Scope of Claims] 1. A method for preparing sintered pellets obtained by molding and firing a mixed powder obtained by blending at least two types of raw material powders A and B to a desired density, the method comprising: Both A and B are the sintering temperature (T
It is assumed that the density change rate (δρ _TA , δρ _TB ) at the temperature (°C) is known, and the blending ratio (n/m) is calculated from the formula n・δρ _TA +m・δρ _TB /n+m. 1. A method for adjusting the density of a nuclear fuel oxide sintered pellet, the method comprising: determining a density change rate of sintered nuclear fuel oxide pellets.