JPH0833471B2 - Method for adjusting creep speed of nuclear fuel body and nuclear fuel body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a nuclear fuel body, and in particular, it has good FP (fission product) gas retention required for high burnup utilization and good PCI. The present invention relates to a creep speed adjusting method for a ceramic nuclear fuel body having (pellet-clad tube interaction) characteristics and the nuclear fuel body.

(Prior art) In order to use the high burnup of nuclear fuel, the fuel pellet must have a good retention of FP gas (fission product gas) because it is necessary to suppress the increase in internal pressure of the fuel rod. Since it does not connect, good PCI (mechanical interaction between pellet and cladding) property is required, that is, PCI can be reduced, and larger crystal grain size is required for the former.
For the latter, various measures have been taken in consideration of making the pellet as soft as possible.

Regarding the former method of increasing the crystal grain size, it is considered necessary to increase the crystal grain size as described above in order to retain FP gas. The creep speed decreases in inverse proportion to the square of the particle size (D ² ). In other words, the pellet becomes hard and the damage to the cladding tube during PCI increases.

Therefore, as a solution for this, in addition to the addition of Al ₂ O ₃ or SiO ₂ to the oxide of at least one element of uranium or plutonium or the nuclear fuel substance obtained by adding gadolinia to the substance, Nb ₂ O ₅ or TiO ₂ to increase the diffusion rate of atoms (see JP-A-55-27942, JP-B-63-16716, JP-A-63-73189, etc.), or between large particles. Although it has been proposed that a small particle size be inserted in to make it slippery, those with Nb ₂ O ₅ added have a large particle size and are good in terms of PCI characteristics, while Nb ₂
Due to the remaining O ₅ , TiO ₂ etc., the release rate of FP gas increases,
Further, in the method of sandwiching the small particle size between the large particle size particles, the FP gas release rate in the small particle size portion becomes large, and it is difficult to improve the FP retention property in any case.

By the way, in the above-mentioned conventional UO ₂ and other ceramic fuels, the pellet hardness, in other words, the damage to the cladding during PCI, and the high temperature creep rate (υ) that affects the FP gas retention are one. It is an important parameter, and is usually summarized by the following equation, where D is the average diameter of crystal grains.

υ = 1 / D ² f (T, ρ, σ) where T is temperature, ρ is density, and σ is pressure.

Although the test results are in good agreement with this equation, in the conventional case, the diameter obtained by the intercept method or the equivalent circular area method is applied as the average diameter in the equation.

(Problems to be Solved by the Invention) However, as a practical problem, the fact that the average diameters (D) obtained by the above method are the same and the creep speeds do not necessarily match and is different has been found as a result of the experiment.

That is, generally speaking, if the average diameter (D) obtained by the above method is the same, naturally the creep rate must be the same, but in the case where the crystal grain shape is different, that is We found the experimental fact that they did not necessarily match and that the creep rates were different.

That is, it was found that the creep rate was smaller when the crystal grains were closer to a circle even if the average diameter (D) was the same.

Such a thing cannot be explained by the conventional high temperature creep rate equation based on the average diameter, and in the future as a crystal structure of a nuclear fuel body that requires improvement of FP gas retention characteristics and PCI characteristics from the viewpoint of high burnup utilization, This is a point that needs to be examined.

In contrast to the actual situation as described above, the present invention aims to newly review the creep behavior of a nuclear fuel body based on the high temperature creep rate equation based on the above average diameter, and particularly the high temperature creep behavior is an action on each surface of each crystal. From the above, it was found that the surface area rather than the diameter regulates the creep behavior, and the surface area is further added as its function.
The purpose of this is to define the crystal structure of a nuclear fuel body having good FP gas retention required for high burnup utilization of nuclear fuel and good PCI characteristics.

(Means for Solving the Problem) That is, the feature of the present invention which meets the above-mentioned object is that, basically, in a nuclear fuel body containing UO ₂ , the crystal grain shape of the fuel body crystal grains is changed. The purpose is to change and adjust the creep behavior by changing the surface area per unit volume or unit weight of crystal particles.

Further, the invention described in claim 2 is a more specific form of the above invention, and in a nuclear fuel body containing UO ₂ , the equivalent circular area diameter is D as a particle diameter to be included in the creep rate equation.
And the relationship between the two is d = nD, where d is the diameter based on the particle surface area or perimeter, and the creep evaluation diameter (dc) defined by the formula dc = (1 / n) ² D is the particle size. It is characterized by being applied and changing and adjusting the creep behavior.

Furthermore, the invention described in claim 3 relates to a ceramic nuclear fuel body produced by applying the above method, and in particular, in the creep rate equation of the fuel body, the equivalent circular area diameter is D, and the diameter based on the particle surface area or the perimeter is Assuming that the relationship between the two is d = nD, the creep evaluation diameter (dc) defined by the equation, dc = (1 / n) ² D, is applied as the particle size to adjust the change in creep behavior. It is done and n in the formula is set in the range of n = 1.1 to 4 U
It is a ceramic nuclear fuel assembly containing O ₂ , (U, G _d ) O ₂ , and (P _u , U) O ₂ .

Hereinafter, the present invention will be described in more detail.First, as described above, according to the present invention, since the high temperature creep behavior of the ceramic fuel body containing UO ₂ is an action on each surface of each crystal, the intercept method or the equivalent circular area is used. It is not enough to use only the particle diameter obtained in 1. as a reference, but rather, it is assumed that the surface area controls creep behavior rather than the same diameter, and the surface area is used as a reference.

That is, when the particle size (diameter) based on the surface area as the creep effect particle size is d, the number of crystal particles (N) in a constant volume (V) is N = V / 4 / _3π (d / 2) ³ = 6V / πd ³ . Therefore, the total surface area (S) of those particles
Is S = N × 4π (d / 2) ² = 6V / πd ³ × 4π (d / 2) ² = 6V /
It becomes d.

Therefore, when this is actually applied, the equivalent circular area diameter (Do) and the surface area reference diameter (d _o ) are obtained for a particle whose shape is not a regular octagonal concave as shown in FIG. Is approximately equal to d _o D _o , and the crystal grain cross-sectional area (S) and the equivalent circular cross-sectional area (S _o ) are approximately equal to S = S _{o and the} crystal grain circumferential length (l)
Is almost equal to the equivalent circumference length (l _o ), and l = l _o .

At this time, the relation between the diameter (Dr) of the crystal grain on the cross section and the circumferential length (l) as a substitute value of the surface area is Dr = 1 / π × l = 0.32l.

Naturally, in this case, D _o = D _r .

On the other hand, in the case of particles having a concave surface as shown in FIG.
Assuming that the equivalent circular area diameter is D _i and the surface area reference diameter is d _i , naturally d _i > D _i .

That is, the circumference length (l _o ) of the crystal grain cross section calculated from the equivalent circle area diameter is l _o = πD _i , and therefore, the particle perimeter is assumed to be n times larger than the perimeter length (l _o ). Then, the area (S _i ) becomes S _i = (n) ² S _o .

Therefore, from S _o = 6V / d _o , S _i = 6V / d _i , d _i = (1 / n) ² d ₀ (1 / n) ² D ₀ … (1) From the equivalent circle area diameter (D _o ) (1 / n)
^The surface area reference diameter (d _i ), which is a particle size smaller by ² , is expected to affect the creep behavior.

Of course, in the case of FIG. 1, since it is n1, d _i
= D _o , which is included in the conventional creep formula.

Thus, as specified in the present invention, it is more effective to correct the creep rate formula based on the surface area of each crystal, and the prediction of the creep rate after the grain size change can be more accurately obtained. Become.

(Example) In order to further prove the suitability of the present invention, UO ₂ fuel pellets having different crystal grain shapes are prepared as specific examples, and the creep speeds of the respective pellets are compared and described.

3 and 4 are UO ₂ pellets having a gold phase in which the crystal grains produced by controlling the sintering conditions based on the prediction of the present invention are not regular polygons but have concave surfaces, and conventional 3 is a crystal structure of a UO ₂ pellet in which n is 1.0, which is close to a regular polygon formed by the manufacturing method. After making the UO ₂ pellets in Fig. 3, 1
400 ° C., carried out high-temperature creep test In 45MP _a, to obtain a ^{_{υ 1 = 3 × 10 -3 /}} Hr as the creep rate of the density after correction (υ _1).

On the other hand, when the creep rate was experimentally determined for UO ₂ pellets having a crystal form as shown in FIG. 4, the rate (υ ₂ ) was υ ₂ = 1 × 10 ⁻³ / _Hr .

On the other hand, according to the calculation from the metallographic structure, UO _{2 in} Fig. 3
The equivalent circle area diameter (D ₁ ) of the pellet is 10 μm, but the average perimeter is 1.5 of the perimeter calculated from the equivalent circle area diameter.
Times, that is, n = 1.5.

On the other hand, the equivalent volume diameter (D ₂ ) of the pellet shown in FIG. 4 was about 8 μm. In this case, n = 1.0.

Therefore, this value than the effective diameter d _1, d ₂ is _{d 1 = D 1 / (n} ) 2 = 1 / (1.5) 2 D 1 d 2 = 1 / (1.0) 2 D 2 where the creep rate upsilon ₂ Considering υ ₁ as a standard, ν ₁ / υ ₂ = (d ₁ / d ₂ ) ^-2 = (1 / (1.5) ² × 10/1 / (1.0) ² × 8) ^-2 = 3.2, υ ₁ is about 3 times υ ₂ , which is in good agreement with the results of the above experiment.

That is, according to the present invention, even if the crystal grain shapes are different, the creep rate can be accurately grasped, and the conventional grasping of the high temperature creep rate based on the average grain size of the crystal grains can be sufficiently corrected. I understand.

(Effects of the Invention) The present invention provides a nuclear fuel body containing UO ₂ as described above,
The high-temperature creep rate was conventionally summarized based on the average particle size of the crystal particles, and based on the viewpoint that the surface area of the crystal regulates the creep behavior, the surface area is used as a reference. And considering the action on each surface of the crystal,
The creep rate of UO ₂ pellets can be calculated more accurately, and according to this idea, not only simply changing the particle size in the past, but also changing the particle shape, the crystal particle surface area per unit volume or unit weight It becomes possible to adjust the creep speed by changing the particle size, and it is possible to easily improve the PCI characteristics of the nuclear fuel body by changing the particle shape as described above, and to have the FP gas retention required when using high burnup. Very effective for improving the PCI characteristics related to creep speed.

The invention according to claim 2 proposes a means for predicting the creep speed to make the change adjustment of the creep behavior more concrete, and the invention according to claim 3 is the high combustion produced in this way. It is a nuclear fuel that is effective for repeated use.

[Brief description of drawings]

FIG. 1 is a diagram showing the grain shape of a normal crystal grain and the state of each cross section of its equivalent circle, and FIG. 2 is a diagram showing the grain shape of a crystal grain having a recess and its cross section of its equivalent circle. The figure is a micrograph showing the metallographic structure of a sample (n = 1.5) having a large perimeter, and FIG. 4 is a micrograph showing the metallographic structure of a standard sample (n = 1.0).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (71) Applicant 999999999 Chugoku Electric Power Co., Inc. 4-33 Komachi, Naka-ku, Hiroshima City, Hiroshima Prefecture (71) Applicant 999999999 Japan Nuclear Power Co., Ltd. 1-16-1 Otemachi, Chiyoda-ku, Tokyo No. (71) Applicant 999999999 Nuclear Fuel Industry Co., Ltd. 3-23-5 Nishishinbashi, Minato-ku, Tokyo (72) Inventor Kazutoshi Watami 7-12-12-302 Ueno Shiba-cho, Sakai City, Osaka Prefecture (72) Inventor Takano Tetsuya 3-1, 7-1-714, Aramaki Honzawa, Aoba-ku, Sendai-shi, Miyagi Prefecture (72) Inventor Takeshi Tamakoshi 1 Higashi-shinmachi, Higashi-ku, Nagoya-shi, Aichi Chubu Electric Power Co., Inc. (72) Hirobumi Nakajima Chiyoda-ku, Tokyo 1-6-1, Otemachi Nihon Nuclear Power Co., Ltd. (72) Inventor Kiyoshi Kasai 2 of 46 Konoike, Higashi-Osaka City, Osaka Prefecture (56) References JP-A-55-27942 (JP, ) Patent Akira 63-73189 (JP, A) JP Akira 62-280688 (JP, A) Tokuoyake Akira 63-16716 (JP, B2)

Claims (3)

[Claims] 1. A nuclear fuel body containing UO ₂ , wherein the creep behavior is changed and adjusted by changing the crystal grain shape of the fuel body crystal grains and changing the surface area per unit volume or unit weight of the crystal grains. Method for adjusting creep speed of nuclear fuel assembly. _{2. In} a nuclear fuel body containing UO ₂ , the equivalent circular area diameter is D as the particle diameter to be included in the creep rate equation, and the diameter of the particle surface area standard or the perimeter standard is d, and the relationship between them is d = nD. Then, the creep evaluation diameter (dc) defined by the formula dc = (1 / n) ² D is applied as the particle diameter, and the creep behavior is changed and adjusted to adjust the creep speed of the nuclear fuel body. 3. A ceramic nuclear fuel body containing UO ₂ , (U, Gd) O ₂ , and (Pu, U) O ₂ , wherein the fuel body has an equivalent circular area diameter of D in the creep rate equation and a particle surface area. Assuming that the relation between the two is d = nD, where d is the diameter of the standard or the perimeter, the creep evaluation diameter (d is defined by the formula dc = (1 / n) ² D
The change in creep behavior is adjusted by applying c) as the particle size, and n in the formula is n = 1.
A nuclear fuel body characterized by being set in a range of 1 to 4.