JPH03249595A - Nuclear fuel pellet - Google Patents

Abstract

PURPOSE:To decrease the amt. of the released gaseous FP during combustion by specifying the crystal grain size of a structure to >=20mum and a combustion density to 94 to 97% TD and increasing the pores dispersed in the crystal grains to the quantity larger than the quantity of the pores existing at the grain bound ary. CONSTITUTION:This nuclear fuel pellet is formed by molding raw material powder essentially composed of UO2 and sintering the molding and is formed to >=20mum crystal grain size of the structure and 94 to 97% TD sintering density. The pellet is formed with the pores dispersed in the crystal grains in the quan tity larger than the quantity of the pores existing at the grain boundary. The gaseous FP generated in the pellet is accumulated into the independent pores in the crystal grains in this way on progression of combustion. In addition, the gas is less released through the grain boundaries and, therefore, the gaseous FP holding power higher than the gaseous FP holding power of the conventional pellets having about the same porosity is obtd. and the amt. of the FP to be released during the combustion is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to nuclear fuel pellets that have excellent retention of fission product gas during irradiation.

"Conventional technology" Recently, a so-called high burnup plan for using nuclear reactor fuel for a longer period of time is being considered, but in order to realize it,
It is important to prevent fission product gas (FP gas) generated by nuclear fuel from being released outside the nuclear fuel pellet as much as possible.

The mechanism by which FP gas is released outside the pellet is generally considered as follows. First, FP within the crystal grains of the pellet
Gas is generated, and this gas forms bubbles within the grains or at the grain boundaries. When the number of bubbles generated at the grain boundaries reaches a certain level, tunnels are finally formed along the grain boundaries, and the FP gas present at the grain boundaries is released to the outside of the pellet through this tunnel.

From this, even if it is not possible to suppress the generation of FP gas itself, it is possible to increase the grain size in the sintered pellets and increase the distance that the FP gas generated within the grains reaches the grain boundaries. , it is thought that the gas is confined within the pellet, and as a result, the amount of FP gas released can be reduced. For this reason, the idea of using pellets with large crystal grain sizes as nuclear fuel for high burnup is becoming popular. Although the optimum crystal grain size is not yet clear, according to studies conducted by the present applicant on burnup, FP gas release rate, etc., it is thought that 20 μm or more is suitable.

Conventional methods for producing large-sized pellets include adding Nisai Hia (Nbto5) to the raw material UO and powder, and sintering the compacted powder in an oxidizing atmosphere such as CO. A method using highly active UOI powder with a high crystal grain growth rate as a raw material has already been proposed.

However, with methods that use additives, the effect on physical properties such as the melting point of nuclear fuel pellets is not always clear;
Further, the method of sintering in an oxidizing atmosphere has problems such as a very complicated manufacturing method and high cost. For this reason, the method of forming pellets using highly active UO2 powder as a raw material has the least problems. From this point of view, the present applicants first filed a patent application filed in 1983. -142506, patent application 1986-19
No. 0079, Japanese Patent Application No. 127934/1983, Japanese Patent Application No. 1983
-1.27935 and U.S. Patent Application No. 139447
In this issue, we proposed a method for producing large-sized pellets using highly active UO powder.

However, it has been confirmed that when nuclear fuel pellets are produced using such highly active U2O powder as a raw material, the crystal grain size becomes large and, as a side effect, the sintered density becomes higher than the desired value. The crystal density is set at 94% to 97% TD according to the standards for product pellets, and if it exceeds this standard, problems will occur.

Therefore, in order to prevent the sintered density from becoming too high when using highly active powder, the applicant filed a patent application in
In No. 0340, a method was proposed in which a bore former such as ammonium oxalate was added to the raw material powder to generate pores in the structure of the nuclear fuel pellet, thereby offsetting the increase in sintered density due to the increase in particle size.

``Problem to be Solved by the InventionJ'' By the way, judging from the FP gas release mechanism described above, it is thought that it is preferable for this type of pore to exist independently within the grain rather than at the grain boundary. In other words,
The independent pores within the grains have the function of effectively confining the FP gas, but the pores at the grain boundaries act as tunnels.
This is because, on the contrary, it will promote the release of FP dregs.

From this point of view, when looking at the crystal structure of large-grain pellets obtained by the method of adding a bore former mentioned above, pores exist evenly within the grains and at the grain boundaries, or they exist in clusters at the grain boundaries. It was found that the FP gas retention force may be kept low accordingly.

On the other hand, through subsequent experiments, the present inventors discovered that U s Os powder could be used as a new bore former. When U s Os powder is added to UO powder and subjected to molding and sintering, pores are generated in the U and Os particles due to the difference in shrinkage rate during sintering with UO powder, which increases the sintered density. can be lowered.

However, as a result of further investigation, we discovered a new phenomenon in which when the amount of U s Os powder added was within a certain range, unlike in the case of conventional bore formers, more pores were dispersed within the crystal grains. did.

The present invention was made based on the above findings, and an object of the present invention is to provide nuclear fuel pellets that can obtain higher FP gas retention power even with the same sintering density.

"Means for Solving the Problems" Hereinafter, the nuclear fuel pellets according to the present invention will be specifically explained.

The nuclear fuel pellet of the present invention is formed by molding and sintering raw material powder whose main composition is U Ot, and the crystal grain size of the structure is 20 μm or more, preferably 30-100 μx,
The sintered density is 94 to 97% TD, and is characterized in that there are more pores dispersed within the crystal grains than pores present at the grain boundaries.

When the grain size is less than 20 μm, the distance for the FP gas generated within the grains to reach the grain boundaries is shortened, and the distribution density of the grain boundaries increases, so the proportion of pores in contact with the grain boundaries increases.
Due to these synergistic effects, sufficient FP gas retention force cannot be obtained. Moreover, if the crystal grain size is larger than 100 μl, the mechanical strength of the pellet may be reduced. Further, if the sintered density is less than 94% TD or more than 97% TD, the product pellet cannot meet the specifications.

The average diameter of the pores is desirably 5 to 100 μl, especially 10 to 50 μl; if the diameter is less than 5 μl, the pellet may shrink due to disappearance of pores during irradiation, causing a problem of 100 μl or less.
Compared to μX, dogs have a problem in that the open porosity of the pellet surface increases.

In nuclear fuel pellets with the above structure, there are more pores dispersed within the crystal grains than pores existing at the grain boundaries, so as combustion progresses, FP gas generated within the pellet is stored in independent pores within the crystal grains. Moreover, since gas is less likely to be released through grain boundaries, the amount of FP gas released can be significantly reduced compared to conventional berate having a similar porosity.

Next, an example of a method for producing such nuclear fuel pellets will be described.

In this method, a powder of highly active UO' having a specific surface area of 3x'/9 or more, preferably 5 to 15x'/9 is used as the main composition of the raw material powder. If it is less than 3x''/y, the crystal grain size of the pellet cannot be made sufficiently large, and the retention of FP gas will deteriorate.If it is larger than 15x''/y, the grain size may exceed the above-mentioned upper limit. Note that the specific surface area herein is defined as a value measured by the BET method.

Such highly active UO powders can be used in ADt, i-method or AU
In Method G, it can be easily produced by controlling the precipitation conditions. The details of the technology have already been disclosed by the present applicant in each of the above-mentioned applications. In addition, highly active UO and powder can be obtained by highly pulverizing ordinary inert UO and powder to increase the specific surface area, or by performing oxidation-reduction treatment on inert UO and powder to make it highly active. It is also possible to obtain it by

Next, add 10 to 20 wt to such highly active UO powder.
Add % U,08 powder. When the amount added exceeds 20 wt%, the crystal grain size becomes small and the distribution density of grain boundaries becomes large, so the proportion of pores existing at grain boundaries increases significantly, and the PP gas retention force decreases relatively. Also, 10w
If it is less than t%, the sintered density of the pellet will exceed 97% TD.

The particle size of U30s powder is related to the average diameter of pores,
Desirably, the average particle size is about 5 to 100 microns. If it is less than 5 μm, it will be smaller than the lower limit of the above-mentioned average pore diameter, and if it is 100 μm, it will exceed the upper limit for dogs.

Next, this mixed powder is molded with a press mold, and then sintered in a hydrogen stream or a humidified hydrogen stream to obtain nuclear fuel pellets. The conditions may be the same as before. After sintering, U30. Since the shrinkage rate of the portion is smaller than that of U Ot, pores are formed, and these pores are present within the crystal grains.

The reason why pores are uniformly distributed throughout the tissue is not yet clear, but the following speculations can be made. That is, U3O
Since the U30.8 powder has a high affinity with the UOx powder that forms the matrix, the U30.8 powder has a high affinity with the U30. When the amount of powder added is 20 wt% or less, grain boundaries move relatively freely past pores during sintering and are dispersed independently of each other. However, U
When the amount of s Os powder added is greater than 20 wt%,
As the distribution density of pores increases, the force that prevents grain boundary movement can no longer be ignored, and the probability that grain boundary movement stops in contact with pores increases, so the proportion of pores remaining at grain boundaries increases. This is expected to increase.

"Example" Next, examples will be given to demonstrate the effects of the present invention.

UO,P, prepared by dissolving UO,F, in water, is reacted with ammonia water to produce ADU.
After filtration and drying, DU was roasted and reduced to produce a highly active UO powder with a specific surface area of about 10x'/9.

To this powder, U3O8 powder and ammonium oxalate were added as a bore former and mixed uniformly to produce three types of raw material powders. U3O, as a powder, particle size force '1
0 μx was used. And 3 tons of these raw material powders
/cm'', and each green compact was molded at 17 cm in a hydrogen stream.
Sintering was performed at 50°C for 4 hours.

The three types of nuclear fuel pellets thus obtained were cut, polished and etched, and then photographs of their structures were taken using an optical microscope. Figure 1 is a reproduction of a photograph of the structure of a nuclear fuel pellet containing 20 wt% of U 30 s powder, and Figure 2 is a reproduction of a nuclear fuel pellet containing 20 wt% of U 30 s powder.
This is an example in which 30 wt% of s Oa powder was added, and an example in which 30% or ammonium oxalate was added.

In the pellet to which ammonium oxalate was added, the crystal grain size was 49 μl, but pores were evenly distributed mainly at the grain boundaries and inside the grains. On the other hand, 20% of U306 powder
In the example where wt% was added, the crystal grain size was 48 μl, and most of the pores were present within the crystal grains. However, U 30
When the amount of a powder added increases to 30wt%, the grain size increases to 2
It was reduced to 5 μl, and the proportion of pores at the grain boundaries was significantly increased.

"Effects of the Invention" As explained above, according to the nuclear fuel pellet of the present invention, there are more pores dispersed within the grains than pores present at the grain boundaries, so as combustion progresses, the number of pores inside the pellet increases. The generated FP gas is stored in independent pores within the crystal grains,
Moreover, since gas is less likely to be released through grain boundaries, it is possible to increase the holding power of FP gas and reduce the amount of FP gas released during combustion compared to conventional pellets having a similar porosity.

[Brief explanation of drawings]

FIG. 1 is an enlarged view of the structure of a nuclear fuel pellet according to an example of the present invention, and FIGS. 2 and 3 are enlarged views of the structure of nuclear fuel pellets of comparative examples, respectively.

Claims (1)

[Claims] A nuclear fuel pellet made by molding and sintering nuclear fuel material powder, which has a crystal grain size of 20 μm or more, a sintered density of 94 to 97% TD, and has pores dispersed within the crystal grains. A nuclear fuel pellet characterized by more pores than grain boundaries.