JPH05196770A - Nuclear fuel pellet - Google Patents

Abstract

PURPOSE:To prevent the density drop of a fissile material, by adding at least one of aluminium oxide and silicon oxide to the fissile material. and beryllium oxide, and by keeping the porosity of a pellet not more than a specified value. CONSTITUTION:In a nuclear fuel pellet obtained by sintering a fissile material, when at least one of aluminium oxide and silicon oxide, and beryllium oxide are added to the fissile material, and at least one of aluminium oxide and silicon oxide is added to the nuclear fuel so that the porosity of the pellet is at most 5%, these are deposited in grain boundaries of the pellet, boundary slips become easy to occur, and the stress of the mutual action between the pellet and a coated pipe is reduced. Further, when beryllium oxide of high heat conductivity is added, the heat conductivity of the pellet is improved. In the pellet, the density reduction of the fissile material can be prevented, and when the added volume of the additive is less than the reduction volume of the porosity, inversely the density of the fissile material can be increased. By the reduction of the porosity, the heat conductivity of the pellet is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high burnup nuclear fuel pellet used in a nuclear reactor.

[0002]

2. Description of the Related Art In light water reactors, higher burnup of nuclear fuel is being promoted in order to improve economic efficiency. However, in the case of fuel at high combustion, the core temperature of nuclear fuel pellets rises and the fission product gas in nuclear fuel rods ( The increase in the amount of FP gas released and the occurrence of interaction (PCI) between the nuclear fuel pellet and the cladding tube are major problems.

Conventionally, in order to add ductility to an oxide having low thermal conductivity and brittleness to increase thermal conductivity, a metal rich in ductility and high thermal conductivity was added and precipitated in a sintered body. Cermet material is used. As a nuclear fuel pellet, there is known one in which metal fibers, beryllium oxide fibers and whiskers are deposited in a ceramic constituting the nuclear fuel pellet (see Japanese Patent Laid-Open No. 53-16198). However, when pellets are manufactured by sintering uranium dioxide powder and fibrous substances having different shapes, it is considered difficult to increase the density of the pellets. A decrease in pellet density leads to a decrease in fissile material density, which is not preferable for high burnup.

JP-A-55-27942 describes a nuclear fuel pellet containing 0.2 wt% to 5 wt% of at least two kinds selected from aluminum oxide, beryllium oxide, calcium oxide, magnesium oxide, silicon oxide, sodium oxide and phosphorus oxide. However, when calcium oxide or magnesium oxide is added to uranium dioxide and sintered, these additives form a solid solution with uranium dioxide and reduce the thermal conductivity of the nuclear fuel pellets. Therefore, it is desirable that the additive does not form a solid solution with the oxide containing the fissile material.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and an object of the present invention is to lower the central temperature of a nuclear fuel pellet, reduce the amount of FP gas released, and improve PCI. And to reduce the porosity of the pellets and prevent the reduction of the fissile material density to provide a nuclear fuel pellet.

[0006]

The present invention has been made to achieve the above object, and in a nuclear fuel pellet formed by sintering a fissionable material, aluminum oxide and silicon oxide among aluminum oxide and silicon oxide are used as the fissionable material. At least one and beryllium oxide are added, and the porosity of the pellet is 5% or less.

In the above, both aluminum oxide and silicon oxide and beryllium oxide are added, and the total amount of aluminum oxide and silicon oxide is about 0.001 to about 0.25% by weight, and beryllium oxide is about 1.5. When the content is less than or equal to% by weight, it has a particularly excellent effect. Here, aluminum oxide and silicon oxide are also included when they react to form a compound.

Further, the above is characterized in that the average crystal grain size of the crystal grains excluding the precipitation phase is 20 μm or more.

[0009]

When at least one of aluminum oxide and silicon oxide is added to the nuclear fuel, these are precipitated at the crystal grain boundaries of the nuclear fuel pellet. These precipitated phases are more likely to be deformed than the parent phase and are more likely to undergo grain boundary slip, the creep rate of nuclear fuel pellets is increased, and the stress during PCI is reduced. Furthermore, by adding beryllium oxide having high thermal conductivity, the thermal conductivity of the nuclear fuel pellets is improved, the core temperature of the nuclear fuel pellets is reduced at the same linear output as the conventional linear output, and the temperature distribution inside the pellets is It is smaller than the nuclear fuel pellets of.

Further, when at least one of aluminum oxide and silicon oxide and beryllium oxide are added, they react with each other to form a liquid phase at least at a part of the sintering temperature, and are burned while wetting the crystal grain boundaries of the fissile material. Bonding progresses (liquid phase sintering) or precipitates at triple points. In such a sintering process, the growth rate of crystal grains and the moving speed of pores are accelerated, so that the crystal grains are increased in size and reduced in porosity.

Therefore, even if the density of the fissile material is reduced by the addition of at least one of aluminum oxide and silicon oxide and beryllium oxide, it can be compensated for by the reduction of the porosity. Therefore, the nuclear fuel pellet of the present invention. Can prevent a decrease in fissile material density. On the contrary, when the amount of the additive added is smaller than the volume of the reduced porosity, the fissile material density can be increased. In addition, the thermal conductivity of the pellet can be improved by reducing the porosity. Further, as the crystal grain size of the nuclear fuel pellet becomes larger, the diffusion distance of the FP gas generated in the crystal grain becomes longer and the FP gas becomes longer.
The amount of gas released is reduced.

Next, the case where the three elements of aluminum oxide, silicon oxide and beryllium oxide are added in the above-mentioned predetermined proportions will be described. In this case as well, basically as described above, the effect of improving the creep rate is greater when both aluminum oxide and silicon oxide are contained. The aluminum oxide-silicon oxide compound is precipitated at the crystal grain boundary or at the grain boundary triple point so as to cover the crystal grains of the mother phase as a glass phase or a crystalline phase. Is more likely to be deformed than the parent phase, the creep rate of the nuclear fuel pellet is increased, and the stress during PCI is reduced.

Next, the amounts of aluminum oxide and silicon oxide or their compounds and beryllium oxide added will be described. The addition amount of aluminum oxide and silicon oxide or its compound is preferably in the range of about 0.001 to 0.25 wt%. This range is about 0.001 wt.
% Or less, it was experimentally confirmed that the additive cannot effectively cover the grain boundary of the uranium dioxide pellet, and if it is about 0.25 wt% or more, the additive may be significantly precipitated on the pellet surface. It was confirmed experimentally. The addition amount of beryllium oxide is about 1.5 wt%
The following are preferred. This is because when the amount of beryllium oxide added exceeds this range, the fissile material density decreases.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. (Embodiment 1) FIG. 1 is a view showing a fine structure of a nuclear fuel pellet which is an embodiment of the present invention. In the figure, the matrix phase is the fissile material 1, and the hatched portion is the precipitation phase 2 of at least one of aluminum oxide and silicon oxide and beryllium oxide. In this example, 0.3 wt% of aluminum oxide-silicon oxide compound powder and 0.9 wt% of beryllium oxide powder were added by weight fraction of nuclear fuel pellets, and sintered at 1700 ° C. to 2000 ° C. As shown in 1, a low porosity pellet having a porosity of about 2% or less was obtained.

[0015]

[Table 1]

In this case, when aluminum oxide-silicon oxide reacts with beryllium oxide, it has a eutectic point that becomes a part or all of the liquid phase at about 1520 ° C. or higher. Therefore, when this powder and the fuel powder are mixed and sintered, at least a part of these becomes liquid at a temperature higher than the temperature at which sintering is usually performed to wet the grain boundary of the uranium oxide or the mixed oxide, or A low porosity is achieved due to the triple point precipitation. Since the porosity of normal nuclear fuel pellets used in light water reactors is 4 to 5%, in the present embodiment, addition of aluminum oxide, silicon oxide and beryllium oxide suppresses the decrease in fissile material density. On the contrary, it was found that the fissile material density could be increased.

FIG. 2 is a diagram showing the relationship between the thermal conductivity at 1000 K and the beryllium oxide concentration when spherical beryllium oxide is deposited on a conventional nuclear fuel pellet, and with the same porosity (porosity 5%). Compared. In the figure λ / λ ₀
Is the ratio of the thermal conductivity λ of the pellet on which beryllium oxide is deposited and the thermal conductivity λ ₀ of the conventional nuclear fuel pellet. With the nuclear fuel pellet of the present invention, a low porosity is achieved, so that FIG.
The thermal conductivity is higher than that shown in. For example, when the pellet peripheral temperature is set to 500 ° C. and the linear output is obtained from the integrated thermal conductivity of the pellet, at a linear output of 40 kW / m,
In a nuclear fuel pellet in which 0.9 wt% of beryllium oxide is deposited, the increase in the thermal conductivity lowers the pellet center temperature by about 200 ° C.

(Example 2) 0.1 wt% to 0.3 wt%
Silicon oxide and 0.9 wt% beryllium oxide are added to the fissile material and sintered at 1700 ° C to 1900 ° C,
As shown in Table 2, nuclear fuel pellets having a porosity of 1.1% or less were obtained.

[0019]

[Table 2]

In this case, when silicon oxide reacts with beryllium oxide, it has a eutectic point which becomes a part or all of the liquid phase at about 1670 ° C. or higher. Therefore, when the powder and the fuel powder are mixed and sintered, at least a part of silicon oxide-beryllium oxide becomes a liquid at a temperature higher than the temperature at which the sintering is usually performed, and a grain boundary of uranium oxide or a mixed oxide. Low porosity is achieved due to the wetting or precipitation at triple points.

(Example 3) 0.1 wt% to 0.3 wt%
Aluminum oxide and 0.9 wt% beryllium oxide were added to the fissile material and sintered at 1800 ° C to 2100 ° C to obtain nuclear fuel pellets having a porosity of 0.9% or less as shown in Table 3. ..

[0022]

[Table 3]

When aluminum oxide reacts with beryllium oxide, it has a eutectic point which becomes a part or all of the liquid phase at about 1800 ° C. or higher. Therefore, when this powder and fuel powder are mixed and subjected to high temperature sintering, at least a part of aluminum oxide-beryllium oxide becomes a liquid and wets the crystal grain boundary of uranium oxide or mixed oxide or precipitates at triple points. Therefore, low porosity is achieved.

(Embodiment 4) In this embodiment, the weight fraction of nuclear fuel pellets is 0.025 wt% (about 0.1 vol%).
Aluminum oxide-silicon oxide compound powder of 0.56
Beryllium oxide powder of wt% (about 2 vol%) was added to uranium dioxide powder and sintered at 1700 to 1780 ° C. for 2 to 6 hours to give a porosity of 1.4.
% Low porosity pellets were obtained.

[0025]

[Table 4]

Since the porosity of a normal nuclear fuel pellet used in a light water reactor is about 5%, in the nuclear fuel pellet of this embodiment, the density of fissile material is lowered by the addition of aluminum oxide, silicon oxide and beryllium oxide. Can be prevented and the fissile material density can be increased.

With respect to the uranium dioxide pellets having a porosity of 5% which is usually used, the thermal conductivity K _P when the porosity is decreased by P% is given by the following equation (1). K _P = K ₉₅ [100- (5-P) α] / (100-5α) (1) where K ₉₅ is the thermal conductivity of the uranium dioxide pellets having a porosity of 5%, and α is the following formula (2) ) Is the coefficient. α = 2.6−0.5 × 10 ⁻³ T (2) Here, T is temperature (° C.).

On the other hand, the thermal conductivity K _C when V% beryllium oxide is spherically dispersed in the uranium dioxide pellets having a porosity of 5% in a volume ratio of the pellets and is deposited without changing the porosity.
Is given by the following equation (3). _{100-V = (K 95 /} K C) 1/3 (K B -K C) / (K B -K 95) ... (3) where, K _B is the thermal conductivity of the beryllium oxide.

When K ₉₅ and K _P and K _C given by these equations are compared at an integrated thermal conductivity of 500 to 1700 ° C., the porosity is reduced by 1% with respect to the integrated thermal conductivity of K _95. And the ratio of the increase in integrated thermal conductivity due to Beryllium oxide to the nuclear fuel pellet is 1v
It is shown that the rate of improvement in thermal conductivity due to ol% precipitation is almost equal. Therefore, when 1 vol% of beryllium oxide is deposited and the porosity is reduced by 1%, the thermal conductivity improving effect is almost twice as high as the thermal conductivity improving effect of the uranium dioxide pellets due to the reduced porosity.

In this embodiment, the integrated thermal conductivity at 500 to 1700 ° C. is increased by 10% or more with respect to the uranium dioxide pellets having a porosity of 5%. Due to this increase in thermal conductivity, the core temperature of the nuclear fuel pellet decreases with the same linear output as the conventional linear output, the temperature distribution in the pellet becomes smaller than that of the conventional nuclear fuel pellet, and the FP gas generated in the pellet is reduced. Since the diffusion rate is low, the amount of FP gas released is reduced,
Since the thermal expansion of the pellet becomes small, PCI is reduced, and the center temperature of the pellet is lowered, so that the fuel melting margin is increased. In order to prevent the density of the fissile material from decreasing, it is sufficient that V ′ ≦ P (4) where V ′ is the volume ratio of aluminum oxide, silicon oxide and beryllium oxide to the nuclear fuel pellets. The addition amount of beryllium oxide that satisfies such conditions is 1.5% or less, preferably 1.2% or less, in weight ratio with respect to the nuclear fuel pellets. Further, the weight ratio of beryllium oxide in aluminum oxide, silicon oxide and beryllium oxide is preferably 3% or more.

In the nuclear fuel pellets of this example, those having a crystal grain size of about 20 μm or more were obtained. The crystal grain size is obtained by polishing the pellet cross section, immersing the polishing surface in a solution that selectively corrodes the crystal grain boundaries, and taking an optical micrograph of the center portion of the surface where the crystal grain boundaries are corroded. Unit length / number of unit length crossed by grain boundaries is 1.
Multiply by 56). The addition amount of aluminum oxide and silicon oxide for adjusting the crystal grain size to 20 μm or more is 0.001 to 0.25% by weight ratio to the nuclear fuel pellet, and preferably 0.001.
Is 0.10%, and more preferably 0.001
It is 0.05%. Since the crystal grain size of a normal nuclear fuel pellet used in a light water reactor is several μm or less, in the nuclear fuel pellet of this example, the diffusion distance of FP gas in the crystal grain is lengthened to release FP gas to the outside of the pellet. Can be reduced.

[0032]

As described above, according to the present invention,
Since the thermal conductivity of the nuclear fuel pellets can be improved, the central temperature of the nuclear fuel pellets can be lowered, the amount of FP gas released can be reduced, and PCI can be reduced.
Moreover, since the creep speed of the nuclear fuel pellets can be increased, PCI can be further reduced. Further, since the porosity of the nuclear fuel pellet can be lowered, it is possible to compensate for the decrease in the fissile material density due to the addition of aluminum oxide, silicon oxide or beryllium oxide, or conversely, to increase the fissile material density. .. Further, since the crystal grain size of the nuclear fuel pellet can be increased, the amount of FP gas released can be further reduced.

[Brief description of drawings]

FIG. 1 is a view showing a fine structure of a nuclear fuel pellet which is an embodiment of the present invention.

FIG. 2 is a diagram showing thermal conductivity at 1000 K when spherical beryllium oxide is deposited on a conventional nuclear fuel pellet.

[Explanation of symbols]

1 ... Fissile material, 2 ... Precipitated phase composed of at least one of aluminum oxide and silicon oxide and beryllium oxide.

Claims (3)

[Claims] 1. A nuclear fuel pellet obtained by sintering a fissile material, wherein at least one of aluminum oxide and silicon oxide and beryllium oxide are added to the fissile material, and the pellet has a porosity of 5% or less. Nuclear fuel pellets characterized in that. 2. A nuclear fuel pellet formed by sintering a fissionable material, wherein the fissionable material contains 0.001 to 0.25 wt% of aluminum oxide and silicon oxide (including aluminum oxide-silicon oxide compound). Beryllium is added up to 1.5 wt% and porosity is 5%
The nuclear fuel pellet according to claim 1, wherein: 3. The nuclear fuel pellet according to claim 2, wherein the average crystal grain size of particles excluding the precipitation phase composed of aluminum oxide, silicon oxide and beryllium oxide is 20 μm or more.