JPH0694876A - Nuclear fuel pellet - Google Patents

Abstract

PURPOSE:To reduce the integrated discharge amount of a gaseous nuclear fission products (FP) at a high burnup, and to suppress its rapid swelling at the output increasing time, by adding a specific amount of oxide having Ti and Si as the main component or an oxide having Ti, Si, and Al as the main component to a nuclear fuel pellet, so as to make the mean crystal grain diameter a specific value or higher. CONSTITUTION:This nuclear fuel pellet includs an additive which consists of an oxide having Ti and Si as the main component, or an oxide having Ti, Si, and Al as the main component. A titania silicate (Ti-Si-O) powder of 0.25wt% in the weight part ratio and a uranium deoxide powder are mixed to form a cylindrical compact, and it is sintered at about 1700 deg.C in a reduction ambiance for about 4h, so as to obtain a nuclear fuel pellet in which a phase 9 which consists of Ti-Si-O is precipitated on the crystal grain field of a nuclear fission substance 8. Since the mean crystal grain diameter of the pellet is made about 50mum or more, the atomic composition ratio {Ti/(Ti+Si)} is made about 0.3 or more. In the pellet of the crystal grain diameter about 50mum or more obtained in such a way, the FP gas discharge ratio is reduced lower than the conventional one, and in the pellet of the crystal grain diameter about 95mum or more, the FP gas discharge ratio is made less than half that of the conventional one.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear fuel pellet loaded in a light water reactor, and in particular, it suppresses the release of gaseous fission products (FP) and is excellent in PCI (pellet cladding interaction) performance. Regarding pellets.

[0002]

2. Description of the Related Art Currently, in light water reactors, in order to improve economic efficiency, it is planned to increase the burnup and output of fuel. At this time, there are the following problems in designing the fuel rod. (1) Increase in center temperature of fuel rod (2) Increase in amount of FP gas released in fuel rod (3) Interaction between nuclear fuel pellet and cladding tube FIG. 4 is a schematic sectional view of a conventional nuclear fuel rod. Inside the fuel rod 7, a plurality of nuclear fuel pellets 1 are stacked and filled.

When fuel pellets containing uranium dioxide as a main component are burned in a nuclear reactor, FP gases such as krypton (Kr) and xenon (Xe) are generated.

The average grain size of conventional nuclear fuel pellets is
It is 5 to 10 μm. A part of the gas atoms in the crystal grain diffuses and moves to the crystal grain boundary, precipitates as a grain boundary bubble, and grows a grain boundary bubble. Grain boundary bubble growth is a factor in fuel pellet swelling and is an important rate-limiting step in FP gas release. When the bubbles grow sufficiently on the crystal grain interface, the bubbles are connected to each other, and finally a release route of FP gas communicating with the outside of the pellet is formed. When the grain boundary is connected to the outside, the FP gas retained in the bubbles inside the grain is
Suddenly released outside the pellet. F released from pellets
P gas increases the cladding internal pressure. The fuel rod is mechanically designed with a sufficient safety margin so that the gas pressure in the cladding tube does not exceed the external pressure of the cooling water, or the integrity of the fuel rod is maintained even if it exceeds the external pressure. When increasing the fuel removal average burnup, even if the release rate of the FP gas is suppressed to a constant value, the gas release amount increases in proportion to the burnup.

For this problem, a method of increasing the plenum volume in the fuel rod and suppressing an increase in internal pressure can be considered.
However, increasing the plenum volume requires either increasing the overall length of the fuel rods or decreasing the fuel stack length,
Fuel inventory per unit core volume decreases, which is not a good idea. Increasing burnup also increases the FP gas concentration retained within the fuel pellets. If these gases are held in the form of monatoms or microbubbles, the effect on swelling is small. When the burned fuel receives an output increase, bubbles rapidly grow due to the FP gas accumulated at the grain boundaries,
Strong mechanical interaction (P) between the pellet and cladding due to burst outgassing or rapid swelling.
CI) may occur.

In order to solve these problems, it has been considered to increase the crystal grain size and precipitate a soft phase. In this regard, there are a few reports on pellets sintered by adding Al-Si-O or the like. For example, Japanese Patent Application No. 63-289054 reports the result of adding Al-Si-O and sintering in wet decomposed ammonia at 1650 ° C. for 4 hours, but the average crystal grain size is 33.
μm or less.

Further, Japanese Patent Application No. Sho 63-294863,
Japanese Patent Application No. 63-293772 discloses Mg-Si-O.
Alternatively, an example in which Mg-Al-O or Mg-Al-Si-O is added and sintered is described. However, this is also 16
Even after sintering at 40 ° C. for 7 hours, only about 30 μm was obtained. Furthermore, in Japanese Patent Application No. 63-74954, Al is used.
A method of adding a compound to increase the particle size is described, but even at 1750 ° C. for 5 hours, only a particle size of 38 μm or less is obtained.

As described above, the conventional pellets require high-temperature and long-time sintering and complicated sintering patterns in order to achieve a large particle size and high density. Further, since there is a precipitation phase having a relatively low density, the amount of FP in the same volume decreases, which is not economical.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to address such a problem, and reduces the cumulative release amount of FP gas at high burnup, and at the same time suppresses rapid swelling when the output rises. An object of the present invention is to easily provide a nuclear fuel pellet that can secure a sufficient safety margin without changing the design of the dimensions of the fuel rod, the core, etc. even when the burnup is increased.

[0010]

The structure of a nuclear fuel pellet of the present invention for solving the above-mentioned problems is obtained by stacking and filling the inside of a cladding tube of a zirconium-based alloy and comprising a nuclear fuel oxide sintered body. In the pellet, the nuclear fuel pellet is an oxide containing Ti or Si as a main component, or T
That is, the average crystal grain size of the pellets is set to be at least 50 μm by including an additive made of an oxide containing i, Si and Al as the main components. That is, by adding the above-mentioned oxide and precipitating it at the grain boundary, the grain boundary of the pellet is modified, and at the same time, the average grain size of the pellet is increased (at least 50 μm), and the precipitation of the additive is further performed. The solution is to densify more than the proportion corresponding to the volume.

[0011]

In the pellet according to the present invention, the additive or a part thereof becomes a liquid phase at the sintering temperature or lower, and due to the difference in the chemical potential existing between the uranium particles, the dissolution of uranium atoms in the liquid phase and It is considered that the so-called liquid phase sintering mechanism in which precipitation of is repeated is promoted to increase the grain size.

Furthermore, since the temperature at which the liquid phase is generated (eutectic point) is lowered by adding Ti, the liquid phase is generated at a lower sintering temperature as compared with the case where Ti is not added, and grain boundaries are formed. It is considered that it wets and further promotes increase in particle size.
In addition, a part of Ti forms a solid solution in the uranium particles (matrix) to promote the diffusion of uranium atoms in the uranium matrix, thereby further increasing the particle size. Moreover, by coating the grain boundaries at a low temperature at which sintering between uranium powders hardly progresses,
It is considered that the nuclei of pores, which normally remain as closed pores, are discharged through the liquid phase at an early stage to facilitate densification.

Such a pellet has a function of increasing the diffusion distance of the FP gas to the grain boundary by increasing the particle size of the nuclear fuel pellet and reducing the release rate of the FP gas generated in the crystal grain to the grain boundary. , The effect of reforming the grain boundary by the precipitation phase and releasing the gas released from the grain to the grain boundary to the outside relatively quickly through the precipitation phase, and increasing the density higher than the proportion corresponding to the volume occupied by the precipitation phase This also has the effect of not reducing the amount of FP gas contained in the pellet per unit volume.

The above-mentioned grain boundary modification works to increase the amount of FP gas released by the amount of gas retained at the grain boundaries, but it is retained within the crystal grains due to the increase in the grain size. At the stage where the amount of gas that has increased and combustion has progressed, the increase in the retained amount inside the grain exceeds the decrease in the retained amount at the grain boundary, and the release amount outside the pellet decreases. In addition, since the amount of gas retained at the grain boundaries during use is small, sudden release of grain boundary gas and transient swelling during output fluctuations are reduced, and fuel performance during output fluctuations is improved. In short, it is preferable that the gas held in the crystal grains is kept as much as possible inside, and the gas deposited at the crystal grain boundaries is released promptly.

[0015]

Embodiments of the present invention will be described below with reference to FIGS. FIG. 1 is an enlarged schematic view of the nuclear fuel pellet structure of the present invention. In FIG. 1, 8 is a fissile material,
9 is a grain boundary precipitation phase (Ti-Si-O). That is,
Instead of the conventional nuclear fuel pellet, a nuclear fuel pellet was produced in which the phase 9 made of Ti—Si—O was precipitated at the grain boundary of the fissile material 8 as shown in FIG. The production method is as follows. A titania silicate (Ti-Si-O) powder and a uranium dioxide powder having a weight fraction of the nuclear fuel pellets of 0.25 wt% are mixed to form a columnar molded body, and then, in a reducing atmosphere. Then, a nuclear fuel pellet was obtained by sintering at about 1700 ° C. for 4 hours. The additive was liquefied at about 1600 ° C., the additive penetrated into the grain boundaries during the sintering, and was deposited so as to cover the grain boundaries.

The concentration and composition of the additive are considered as factors that affect the crystal grain size of the pellet after sintering. FIG. 2 is a diagram summarizing the results of experiments carried out to investigate the relationship between the concentration and composition of additives that affect the average particle size of pellets. In order to set the average particle size on the vertical axis of FIG. 2 to 50 μm or more, the atomic composition ratio (Ti / Ti + Si) is about 0.3.
It turns out that it must be above. The theoretical density (TD) of each of these sintered bodies was 96.5% or more.

FIG. 3 is a graph showing the relationship between the crystal grain size of the nuclear fuel pellet of the present invention and the FP gas release rate. That is, burnup of 100 GWd of nuclear fuel pellets at a constant line output
6 is a graph showing the result of calculation of the effect of reducing the FP gas release rate when irradiated up to / tU as a function of crystal grain size. In FIG. 3, the vertical axis is the relative FP gas release rate (relative to conventional pellets), and the horizontal axis is the crystal grain size (μ
represents m). From the results of FIG. 3, among the pellets of the present invention,
When the crystal grain size is about 50 μm or more, the gas release rate (grain size 10 μm) of the conventional pellet is reduced to about 95 μm.
With the above, it is expected to be reduced to less than about half.

Next, the relative density of the pellets of this embodiment will be considered. The true density of titania silicate is about 3 g /
cm ³ , and the theoretical density of UO ₂ is 10.96 g / cm ³
Therefore, it is considered that 0.25 wt% of titania silicate accounts for about 1.5% of the total volume of the pellet. The porosity of the fuel pellets that is normally used is about 5% (relative density is 95%), but the pellet porosity of this embodiment is 3.5% or less. Due to this densification, pellets having a grain boundary precipitation phase formed were obtained without reducing the UO ₂ loading amount per unit pellet volume.

From these results, by selecting the concentration and composition of the additive, the grain boundary was modified and the crystal grain size was large,
It was found that high-density pellets can be manufactured. Although Ti-Si-O has been described above, Ti-Si-
Similar effects can be expected for Al-O.

[0020]

As described above, according to the present invention,
It is possible to suppress the release of gaseous FP from the nuclear fuel pellets and improve the PCI resistance performance of the nuclear fuel rods. Moreover, high-density pellets can be easily obtained.

[Brief description of drawings]

FIG. 1 is an enlarged schematic view of a nuclear fuel pellet structure of the present invention.

FIG. 2 is a graph showing the relationship between the average crystal grain size of the nuclear fuel pellet of the present invention, the additive composition, and the concentration.

FIG. 3 Crystal grain size and relative FP of the nuclear fuel pellet of the present invention
It is a relationship diagram of a gas release rate.

FIG. 4 is a schematic cross-sectional view of a conventional nuclear fuel rod.

[Explanation of symbols]

 1 Nuclear Fuel Pellet 2 Cladding Tube 3 Upper End Plug 4 Lower End Plug 5 Plenum Spring 6 Getter 7 Fuel Rod 8 Fissionable Material 9 Grain Boundary Precipitation Phase

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[Procedure amendment]

[Submission date] October 25, 1993

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Fig. 2]

[Figure 4]

[Figure 3]

Claims (4)

[Claims] 1. A nuclear fuel pellet comprising a sintered body of a nuclear fuel oxide, which is laminated and filled inside a cladding tube of a zirconium-based alloy, wherein the nuclear fuel pellet is Ti, an oxide containing Si as a main component, or Ti. , Si, and Al as the main components, and an additive made of an oxide, wherein the average grain size of the pellets is at least 50 μm. 2. The nuclear fuel pellet according to claim 1, wherein the amount of the additive to the nuclear fuel pellet is 0.25 wt%.
A nuclear fuel pellet characterized by the following. 3. The nuclear fuel pellet according to claim 2, wherein the nuclear fuel pellet has a relative density of 96% TD or more. 4. The nuclear fuel pellet according to claim 1, wherein the additive phase has an average atomic composition ratio (Ti / Ti + Si or Ti / Ti + Si + Al) of 0.3 or more and an additive concentration of 0.25 wt% or less. Nuclear fuel pellets characterized by.