JPS6143673B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to ceramic nuclear fuel pellets.

The useful life of a nuclear reactor fuel element depends on the interaction of the fuel element sheath with the nuclear fuel contained therein. These interactions may be chemical, such as corrosion of a zirconium alloy sheath due to moisture in the fuel, or mechanical, such as expansion and compaction of the fuel. Probably too.

Sintered uranium dioxide fuel pellets used as nuclear fuel undergo dimensional changes as a result of both expansion and densification. These dimensional changes have been found to have an undesirable impact on the integrity of the fuel element sheath and therefore the performance of the fuel element. This leads to (1) densification of the pellet as residual fine sinter porosity is removed by irradiation or heat treatment, and (2) solid and gaseous fission products accumulate as inclusions and bubbles. Efforts have been made to reduce the dimensional instability that is the result of two mechanisms: pellet expansion due to

Such efforts to produce dimensionally stable uranium dioxide pellets have previously been attempted by balancing the pressure of the gas contained in the pores in the pellet against the surface tension, which would otherwise The aim has been to stabilize the pores in the pellets (resulting in a reduction in sintering time) (so that they are not eliminated even with longer sintering times).

When uranium dioxide powder is compressed to form pellets, which are then sintered, the density of the pellets increases as sintering progresses, ie, the fuel pellets become denser and shrink. The driving force for this densification is provided by the reduction in surface energy that accompanies the contraction of voids. Densification is therefore brought about by a reduction in the size of the pores within the pellet, and in some cases by the complete disappearance of some pores. Even if the sintering process continues, some pores will eventually remain as closed pores in the pellet, and the pressure exerted by the gas contained in these closed pores will increase the size of the closed pores. It will balance the surface tension that you are trying to reduce, and it will no longer change. That is, equilibrium is reached and each independent pore is stabilized.
In this state, the pellet density reaches its upper limit. This upper limit density is the heating equilibrium value. The heating equilibrium values of pellets produced in this manner are generally high, typically greater than 95% of the theoretical density (10.40 g/min).
^cm3 or more).

On the other hand, nuclear fuel specifications often require porosity as high as 10% to minimize expansion due to fission product accumulation. To provide this porosity, sintering is stopped before the densification limit is reached, reducing the density of the uranium dioxide pellets below the heating equilibrium value, resulting in more (or larger) pores than at the densification limit. leave. When the nuclear fuel obtained by this method is further heat-treated, it becomes denser, approaches the heating equilibrium value, and therefore shrinks.
Furthermore, upon irradiation, the shrinkage exceeds the heating equilibrium value. This is because in a sintered equilibrium uranium dioxide pellet, most of the pores are less than a few microns in diameter, and the pores must contain gas at a pressure of more than 10 atmospheres (to stabilize). Moreover, these small pores are easily removed during irradiation. Thus, pellets that densify under reactor conditions will generally not only reach the heating equilibrium value, but also reach the theoretical density value unless prevented by other mechanisms, such as fission product-induced expansion. Keep getting closer. As such, the pores in conventional uranium dioxide fuel pellets are unstable, resulting in dimensional changes during reactor operation. The present invention seeks to provide uranium dioxide fuel pellets with reduced dimensional instability.

According to one of its aspects, the present invention provides sintered nuclear fuel pellets of uranium dioxide or plutonium dioxide or both uranium dioxide and plutonium dioxide, which are typically observed when sintered without pore-forming additives. having voids in the form of irregularly distributed closed pores created by a removable pore-forming additive that increases the average pore size substantially greater than the average pore size;
All of those closed pores are in a state close to equilibrium where the gas pressure and surface tension within those closed pores are balanced at a given fuel density, and any point in the fuel pellet is at most a few grains away from the closed pores. The pellets are only minutes apart.

According to the present invention, dimensional changes due to subsequent heat treatment can be minimized. Also, the increased pore size makes it difficult for irradiation to remove the pores, and even after long irradiation times, voids are still visible throughout the pellet so that the expansion of the fission products can be accommodated. Although the pores are stable to heat treatment, they are unstable in the presence of compressive stress caused by the expansion of the fission products and contract again to reach equilibrium. This allows expansion to be accommodated without significant dimensional changes in the fuel pellets. The effectiveness of this mechanism depends not only on the presence of pores, but also on the fact that the spacing between each pore is small enough to easily accommodate expansion. Therefore, it is ideal that each crystal grain of the dioxide has at least one side adjacent to the pores.

According to a second aspect of the invention, the sintered nuclear fuel pellets of uranium dioxide or plutonium dioxide or both uranium dioxide and plutonium dioxide have an average pore size of 6 to 100 μm.
, and has voids with substantially no continuous pores.

The presence of open pores can be tested by measuring the soaked density and geometric density of the pellet.
If these two values do not match, continuous porosity is present.

A pore size that satisfies both the criteria of good resistance to densification and good adaptability to expansion at a typical specification fuel density is consistent with the grain size and pore irregularities typically observed during irradiation. In case of distribution, it is in the range of 6-100 μm. The absence of open pores results in pellets whose surface area, after working to their final size, is determined solely by the physical dimensions of the surface. Conventionally produced pellets having a density of 93% or less of the theoretical density usually contain a large number of open pores, and the number of open pores increases rapidly as the density decreases. Finished pellets expose a greater surface area to the atmosphere and moisture absorption increases significantly with decreasing density. Therefore, strict drying procedures are often required. However, in the pellets produced according to the second aspect of the present invention, moisture absorption is small;
It has little to do with density. For example relative humidity 40
% air, humidity levels can easily reach 5 μg/g UO ₂ even at densities as low as 70% of the theoretical density, as measured by gas extraction in vacuum at 1000°C.
At 90% of theoretical density, humidity levels reached as low as 2 μg/g UO ₂ .
Thus, a simple drying step aimed only at removing surface water after wet finishing is appropriate over the entire fuel pellet density range. The absence of open pores in the pellets according to the invention also increases mechanical strength and toughness since essentially round pores result in lower stress concentrations than a corresponding amount of open pores. This reduces severe cracking of the pellets during handling and finishing, and the reduction in both cracking severity and frequency is significant.

Dimensional instability of nuclear fuel pellets in accordance with the present invention is reduced by reducing the number of pores with a size of about 5 μm or less to 2 1/2% or less of the pellet volume and by reducing the number of pores of about 5 μm or less in size to about 20 μm or less. It can be further reduced by creating a large number of pores of larger size.

As an example, a fuel element was fabricated from pellets of uranium dioxide of controlled porosity at 92.5% of theoretical density in a type of sheath that followed the movement of the pellets therein. The closest pore/distance between pores in the range of 6 to 25 μm to the pellet is 15 μm for 99% of the total.
Created pores smaller than m. 186efpd (effective
After irradiation at a maximum rate of 28 watts/g for full power days (=5200 MWD/teU), these fuel elements showed no appreciable diameter reduction. This irradiation is close to the calculated amount at which maximum shrinkage occurs and isotropic linear shrinkage of up to 2.5% would be expected without porosity adjustment according to the invention. In the hot center of the fuel pellet, the uranium dioxide is more flexible and equiaxed grain growth occurs, pushing its adjacent grains into larger sized pores to accommodate the expansion of the fission products. There was clear evidence. The density of the end surface of the fuel was measured using a mercury hydrometer method, and an average density of 0.03 g/cm ³ was obtained. This corresponds to an isotropic linear shrinkage of 0.1% or less.

By using ammonium oxalate, sugar or starch as a removable pore-forming agent when mixed with uranium dioxide powder of a size smaller than the particle size of the additive to produce a homogeneous mixture, A method has been developed to give the structure

Below are examples of the use of such additives in the method.

Example 1 Pellets of uranium dioxide giving a density of 10.81 g/cm ³ when sintered at 1625°C for 4 hours were prepared under the same conditions but with ammonium oxalate by weight with particle sizes ranging from 8 to 30 μm. When processed at 2%, it gave a density of 9.97 g/cm ³ . After wet polishing and drying through an air oven at 125℃,
These pellets were measured after reaching equilibrium in air at 20°C and 40% relative humidity, with a concentration of 1.5 μg/
Had a humidity level of gUO ₂ . After sintering for another 90 hours at 1625℃, the density of the sample is
10.85 g/cm ³ and 10.01 g/cm ³ , each corresponding to an isotropic linear shrinkage of about 0.12%. Other non-pore-controlled uranium dioxide pellets sintered under the same conditions to 9.97 g/cm ³ and had a humidity level of 50 μg/g UO ₂ measured in a similar manner. After 13 hours in air at the above temperature, these pellets had a density of 10.61 g/ ^cm3 , which is approximately
Corresponding to an isotropic linear contraction of 2.0%.

Example 2 1.9% by weight of sucrose with an average particle size of approximately 30 μm is added to uranium dioxide which would normally sinter to 98% of its theoretical density.
% density pellets were produced. The average pore size after sintering the pellets for 4 hours was 25 μm.

Example 3 1.0% by weight of starch with an average particle size of 40 .mu.m was added to uranium dioxide which would sinter to 98% of theoretical density by conventional methods to produce pellets having a density of 94% of theoretical density. The average pore size of the sintered pellets was 32 μm.

Example 4 Uranium dioxide powder and plutonium dioxide powder were dry mixed in a ball mill with 0.4% by weight of starch with a particle size of 20-40 μm, then granulated with a binder and compressed into pellets.
The pellets were debindered in carbon dioxide at 800°C and then sintered at 1650°C in a 4% hydrogen/argon mixture. The density of the sintered pellets was 94% of the theoretical density and the average pore size was 30 μm.

Example 5 Uranium dioxide powder and plutonium dioxide powder were dry mixed in a ball mill. The resulting mixture has a particle size range of 20 to 1.7% by weight.
Mix by hand with ~55 μm ammonium oxalate;
It was then granulated with a binder and compressed into pellets. The pellets were debindered in carbon dioxide at 800°C and then sintered at 1650°C in a 4% hydrogen/argon mixture.

The density of the sintered pellets was 91.5% of the theoretical density and the average pore size was about 30 μm.

Controlled porosity pellets prepared according to all examples were examined for the substantial presence of open porosity. As a result, no continuous pores were observed. Furthermore, it was observed that the pores were irregularly distributed, with each point of the pellet no more than a few grains (not more than 5) away from a given pore.
These results can be achieved by selecting the particle size of the dioxide to be smaller than the particle size of the additive, preferably less than 1/5, and by making the combined mixture of the dioxide and the additive sufficiently homogeneous. It is considered that it was obtained.

Claims (1)

[Claims] 1 Nuclear fuel powder containing uranium dioxide and/or plutonium dioxide is completely volatilized at high temperatures to form discontinuous pores in the sintered pellets, and a solid pore-forming agent having a particle size larger than that of the nuclear fuel powder particles is used. A method for producing sintered tube fuel pellets after mixing, in which a mixture of nuclear fuel powder and a pore-forming agent is compressed to create raw material pellets, and then the pellets are sintered, and the volatilized pore-forming agent is sintered. In a method for producing sintered tube fuel pellets in which discontinuous pores are left in the pellets, a pore-forming agent selected from the group of ammonium oxalate, sugar and starch is used as a pore-forming agent, and when no pore-forming agent is present, The raw material nuclear fuel powder has a density of at least 97.5% of the theoretical density by sintering, and the pore-forming agent has a particle size such that the sintering results in an average pore size in the range of 6 to 100 microns. and the density of the sintered pellets is 90% of the theoretical density.
A method for producing sintered tubercle fuel pellets, characterized in that a pore-forming agent is present in an amount greater than %.