JP3040756B1 - Test method for gadolinia-doped uranium dioxide pellets - Google Patents

Abstract

The object of the present invention is to establish a test method for eliminating the influence of reduction when performing a heating test for evaluating the burning stability of gadolinia-added uranium dioxide pellets, and to prevent an increase in test cost. SOLUTION: In a heating test for evaluating the burning stability of gadolinia-added uranium dioxide pellets used for nuclear reactor fuel, the relationship between the number of oxygen atoms to the number of metal atoms (O / M ratio) and the crystal lattice constant of the pellets. The effect of pellet reduction by heating test in dry hydrogen atmosphere by combining pellet density measurement before and after heating test in dry hydrogen atmosphere with O / M ratio measurement of pellet before and after heating test A test method for gadolinia-added uranium dioxide pellets, characterized in that a stoving stability value (δpv) of a pellet excluding uranium dioxide is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for evaluating characteristics of nuclear fuel pellets used in nuclear reactor fuel in a production stage, and more particularly to an evaluation of burning stability of gadolinia-added uranium dioxide pellets.

[0002]

2. Description of the Related Art Uranium dioxide pellets used as nuclear reactor fuel are obtained by press-forming uranium dioxide powder as a raw material to form a cylindrical compact, and the inside of the reactor is added with about 0 to 2% by volume of steam. In a sintering furnace with a mixed gas atmosphere of hydrogen gas or hydrogen mixed with nitrogen.
After sintering at a temperature of 0 ° C. for about 2 to 6 hours, it is usually manufactured by a method of grinding a cylindrical surface. Also, in the case of uranium dioxide pellets (gadolinia-added uranium dioxide pellets) to which gadolinium oxide (gadolinia), which is a neutron absorber, is added, a mixed powder obtained by adding gadolinia powder to uranium dioxide powder and press-mixing is formed into a cylindrical shape. This is a sintering furnace in which the inside of the furnace is a hydrogen gas or a mixed gas atmosphere in which nitrogen is mixed with hydrogen to which about 2 to 8% by volume of steam has been added, at a temperature of 1650 ° C. to 1800 ° C. for 2 hours to After sintering for about 6 hours, it is usually manufactured by grinding the cylindrical surface.

[0003] The manufactured pellets are tested and analyzed for characteristics such as size, density, impurity content, and ^U235 enrichment, and one of the characteristics is to evaluate the burning stability value. . This test evaluates the density increase value when the pellets are reheated. As a test method, the pellets whose density has been measured in advance are heated to 1700 ° C. to 1750 ° C. in a dry hydrogen atmosphere using a heating furnace. For 24 hours, and measuring the pellet density after cooling, and taking the difference between the density after the heating test and the density before the heating test.

Specifically, a pellet having no defect such as crack or chip is selected as a pellet to be a sample. First, a pellet weight Wi (g), a diameter Di (cm) and a length Li (cm) before a heating test are performed. ) Is measured, and the pellet density pi (g / cm ³ ) before the heating test is determined by the following formula: 4 · Wi / (π · Di ² · Li) Formula (1) Next, the diameter Df (cm) and the length Lf (cm) of the pellet after the heating test under the above-mentioned heating test were measured, and 4 · Wi / (π · Df ² · Lf) Equation (4) 2) After the heating test, the pellet density pf (g / cm ³ ) is obtained, and pf-pi = δp is obtained by the formula ( ³ ) to obtain a burning stability value δp (g / cm ³ ).

[0005] In this method, the weight of the pellet does not change during the heating test, and the volume (dimension) shrinks, so that the density of the pellet after the heating test increases. However, in the case of gadolinia-added uranium dioxide pellet, In particular, when the gadolinia addition rate increases, the weight of the pellet after the heating test may decrease. This is because gadolinia-added uranium dioxide pellets have the property of being relatively easily reduced, and are reduced by dry hydrogen by performing the above heating test. , The ratio of the number of metal atoms to the ratio (O / M ratio) decreases.

However, when the O / M ratio decreases, the effect of expanding the volume due to the increase of the crystal lattice constant appears, and competes with the volume shrinkage effect by heating. That is, when gadolinia-added uranium dioxide pellets are tested by the above-mentioned heating test method, they may be affected by reduction by dry hydrogen. In such a case, as a method for eliminating the influence of the reduction, conventionally, the heating test is performed in an inert gas atmosphere such as helium or argon, or in a hydrogen gas atmosphere to which the same steam as in the pellet production is added. Attempts have been made to do so. However, inert gases such as helium and argon are more expensive than hydrogen gas,
In a heating furnace for performing a heating test in a dry hydrogen gas atmosphere, when a heating test is performed in an inert gas atmosphere or a hydrogen gas atmosphere to which steam is added, the amount of oxygen generated due to mixing of a small amount of air and dissociation of steam is reduced. As compared with the case of using dry hydrogen, the life of the heater is shortened, and the test cost is increased.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to establish a test method for eliminating the influence of reduction when performing a heating test for evaluating the burning stability of gadolinia-added uranium dioxide pellets, and to establish a test method. The purpose is to prevent an increase in cost.

[0008]

The present invention has the following features. (1) In a heating test for evaluating the burning stability of gadolinia-doped uranium dioxide pellets used for nuclear fuel, the ratio of the number of oxygen atoms to the number of metal atoms (O /
(M ratio) and the crystal lattice constant, and by combining the pellet density measurement before and after the heating test in a dry hydrogen atmosphere and the O / M ratio measurement of the pellet before and after the heating test, in a dry hydrogen atmosphere Stability value of the pellet (δp
A test method for gadolinia-added uranium dioxide pellets, wherein v) is obtained. (2) In a heating test for evaluating the burning stability of gadolinia-doped uranium dioxide pellets used for nuclear reactor fuel, the ratio of the number of oxygen atoms to the number of metal atoms (O /
M ratio) and the crystal lattice constant, and the relationship between the pellet weight and the O / M ratio of the pellet, to measure the pellet density before and after the heating test in a dry hydrogen atmosphere and the pellet weight measurement before and after the heating test. By combining them, the burning stability value of the pellet (δpa) in which the influence of the reduction of the pellet by the heating test in a dry hydrogen atmosphere is almost eliminated
A method for testing gadolinia-added uranium dioxide pellets, characterized in that:

Hereinafter, the present invention will be described in detail. In the first method of the present invention, in a heating test in a normal dry hydrogen gas atmosphere, the ratio of the number of metal atoms to the number of oxygen atoms in the pellet before and after the heating test (O / M ratio) is a method of calculating the density after the heating test when the reduction of the pellet by the heating test does not occur by combining the measurements. According to the heating test, the pellet volume changes due to shrinkage due to the original sintering and expansion due to an increase in the crystal lattice constant caused by a decrease in the O / M ratio. Here, the weight and the volume of the pellet before the heating test were Wi,
Vi, the pellet weight and volume after the heating test are Wf and Vf, respectively, and the volume shrinkage rate due to the original annealing and the volume expansion rate due to the increase in the lattice constant are S and E, respectively. Is as follows: Wf / Vf = Wf / (Vi · S · E) Equation (4) Further, assuming that only the change in the pellet volume due to the original baking is brought to the pellet (there is no weight loss due to reduction and no expansion due to an increase in the lattice constant), the pellet density after the heating test is as follows: pfe = Wi / (Vi S) = E · (Wi / (Vi · S · E)) = E · (Wi / Vf) (5)

In other words, in order to obtain the test pellet density after heating when the pellet is not reduced and only the pellet volume shrinkage due to the original baking is obtained, the weight of the pellet before the test is divided by the volume of the pellet after the heating test. Density (after above-mentioned conventional heating test pellet density p
It can be obtained by multiplying f (Equation (2)) by the volume expansion rate due to an increase in the lattice constant.

On the other hand, it is known from the literature that the crystal lattice constant of gadolinia-doped uranium dioxide and the O / M ratio have the following relationship (6). αYn = 0.5947−0.01593Y−0.02379n (nm) Formula (6) where Y is the ratio of the number of moles of Gd to the total number of moles of U and Gd in the gadolinia-added uranium dioxide pellet ( G is the ratio of the number of moles of O to the sum of the number of moles of U and the number of moles of Gd in the gadolinia-added uranium dioxide pellet (O / M ratio), and αYn is Gd.
It shows the crystal lattice constant of gadolinia-doped uranium dioxide when the addition ratio Y and the O / M ratio n are n.

Therefore, the crystal lattice constant and the O / M ratio of the pellet before the heating test are set to αYni and ni, respectively, and the crystal lattice constant and the O / M ratio of the pellet after the heating test are set to αYni and ni, respectively.
Assuming that Ynf and nf, the linear expansion coefficient is αYnf / αYni
However, since the pellet expansion due to the increase in the crystal lattice constant is considered to be almost isotropic, the volume expansion rate E due to the increase in the lattice constant is as follows: E = (αYnf / αYni) ³ (7)

Utilizing these facts, the O / M ratio of gadolinia-doped uranium dioxide pellets before and after the heating test is measured, and by using equations (6), (7), and (5),
Even if the heating test is performed in a dry hydrogen atmosphere, it is possible to obtain a density after the test when the reduction of the pellets by the heating test does not occur, and the baked stability value δp when the reduction of the pellets does not occur by the heating test. (G / cm ³ ). Note that the measurement of the O / M ratio is usually a destructive test, and the heating test cannot be performed on the pellets subjected to the O / M ratio measurement. Measurement for another pellet sampled from the same pellet lot, which can be considered to have the same properties as the pellet, does not cause any disadvantage for this purpose.

The method of measuring the O / M ratio of gadolinia-added uranium dioxide pellets before and after the above heating test and obtaining the post-test density in the case where the pellets did not undergo reduction by the heating test is based on the measurement error of the O / M ratio. This method can obtain the density after the test in the case where the reduction does not occur strictly except for the error caused by the above, but involves the trouble that the O / M ratio must be measured. Thus, the second method has been established in order to more easily obtain the post-test density in the case where no reduction has occurred, albeit with some error.

According to a second method of the present invention, the O / M ratio of the pellet after the heating test can be calculated from the O / M ratio before the heating test and the weight of the pellet after the heating test. Utilizing that the / M ratio usually takes a substantially constant value, and in a heating test in a normal dry hydrogen gas atmosphere,
In this method, the density of the pellet before and after the heating test is combined with the measurement of the weight of the pellet before and after the heating test, and the density after the heating test when the reduction of the pellet by the heating test does not occur is approximately derived.

As described above, Y is the ratio of the number of moles of Gd to the sum of the number of moles of U and Gd in the gadolinia-added uranium dioxide pellet (gadolinium addition ratio), and ni is the O / M ratio of the pellet before the heating test. , Nf is the pellet O / M ratio after the heating test, Wi is the pellet weight before the heating test, WL
Is the weight of the pellet by the heating test, the atomic weights of uranium, gadolinium and oxygen are 238 and 157.
Since they are 25 and 16, the number of moles of (uranium + gadolinium) in the pellet before the heating test is Wi / ｛238 · (1
−Y) + 157.25 · Y + 16 · ni}, and the number of moles of oxygen is ni · Wi / {238 · (1-Y) +15
7.25 · Y + 16 · ni｝. The number of moles of oxygen deprived by the heating test is WL / 16.

Therefore, the pellet O / M ratio n after the heating test is n
f is nf = [mol number of oxygen before heating test−mol number of oxygen deprived by heating test] / [mol number of (uranium + gadolinium)] = {ni · Wi / (238−80.75 · Y + 16) · Ni) −WL / 16｝ / {Wi / (238−80.75 · Y + 16 · ni)} Equation (8), and the pellet O / M ratio nf after the heating test is the pellet before the heating test. It depends on the O / M ratio ni, the pellet weight Wi before the heating test, and the pellet weight reduction WL due to the heating test. However, the pellet O / M ratio before the heating test ni
Is usually 1.97 to 2.03, so even if ni is a fixed value of 2.00, ni has only a slight effect on nf.

Actually, the gadolinium addition rate Y is set to 0.15, which is the maximum value in a normal case, and the ratio of the difference between the O / M ratio before and after the heating test when ni is 1.97 and 2.00. Then, (1.97-nf) / (2.00-nf) =
0.9981, and even if ni is a fixed value of 2.00, n
It can be seen that f has only a slight effect.

Therefore, equations (6), (7), (5),
From the equation (2), an approximate post-test density pfa when the reduction of the pellets by the heating test did not occur is as follows: pfa = << [0.5947−0.01593 · Y−0.02379 · ｛2-WL · (270−80.75 · Y) / (16 · Wi)}] / (0.5471−0.01593 · Y) » ³ · {4 · Wi / (π · Df ² · Lf)}… Formula ( 9) When the weight of the pellet before and after the heating test and the diameter and length of the pellet after the heating test are measured, and the heating test is performed in a dry hydrogen atmosphere, the pellet is not reduced by the heating test. , An approximate value of the density after the test can be obtained, and the burning stability value δp when the reduction of the pellet by the heating test does not occur can be approximately obtained.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As a specific example of the present invention, a gadolinium addition rate is 4.5% by weight (a gadolinium addition rate Y).
= 0.0656) and 7.5% by weight (addition ratio of gadolinium Y = 0.1078) of gadolinia-doped uranium dioxide pellets, obtained by a heating test in a dry hydrogen atmosphere, obtained by a normal method. δpr combined with the O / M ratio measurement before and after the heating test (first method) Combination of the sintering stability value δpv excluding the effect of reduction by the heating test and the weight measurement before and after the heating test Table 2 shows the baking stability value δpa obtained by performing the heating test (second method) and substantially eliminating the influence of reduction by the heating test.

Heating test of 10 pellets each sampled from the same production lot as the gadolinia-added uranium dioxide pellets in the same atmosphere as the steam-added hydrogen atmosphere used for sintering when producing the gadolinia-added uranium dioxide pellets. The average value of the baked-in stability values obtained in Example 1 was 0.02 in the case of gadolinia-added uranium dioxide pellets having a gadolinia addition ratio of 4.5% by weight.
97 (g / cm ³ ), and 0.0188 (g / cm ³ ) in the case of gadolinia-added uranium dioxide pellets having a gadolinia addition ratio of 7.5% by weight.

From these results, when the heating test is performed in a dry hydrogen atmosphere according to the first method of the present invention, the same baking stability value as in the case where the heating test is performed in a hydrogen added hydrogen atmosphere can be obtained. In addition, it can be seen that the second method of the present invention can provide the same sintering stability value as that obtained when the heating test is performed in a hydrogen-added hydrogen atmosphere with an error of at most about 5%.

[0023]

[Table 1]

[0024]

According to the first method of the present invention, the unit cost is low and the adverse effect on the heater of the heating furnace is reduced by only slightly increasing the number of test steps of measuring the O / M ratio before and after the heating test of the pellet. The heating test in a small amount of dry hydrogen atmosphere makes it possible to evaluate the burning stability of gadolinia-doped uranium dioxide pellets without the effects of reduction.In order to eliminate the effects of reduction, helium or helium was used as the heating test atmosphere. The test can be performed at lower cost than when using an inert atmosphere such as argon or a hydrogen atmosphere to which water vapor is added.

Further, according to the second method of the present invention, if a slight error is allowed, only the increase in the number of steps for measuring the weight of the pellets after the heating test as compared with the conventional method, the unit price is low and the heating furnace A heating test in a dry hydrogen atmosphere that has little adverse effect on the heater has made it possible to evaluate the sintering stability of gadolinia-doped uranium dioxide pellets at a low cost without approximately eliminating the effects of reduction.

Claims (2)

(57) [Claims] In a heating test for evaluating the burning stability of gadolinia-added uranium dioxide pellets used for nuclear reactor fuel, the ratio of the number of oxygen atoms to the number of metal atoms (O / M ratio) and the crystal lattice constant of the pellets is determined. Utilizing the relationship, by combining the pellet density measurement before and after the heating test in a dry hydrogen atmosphere and the O / M ratio measurement of the pellet before and after the heating test, the reduction of the pellet by the heating test in the dry hydrogen atmosphere is reduced. The burning stability value δpv of the pellet excluding the influence is calculated by the following formula: δpv = pfe−pi pfe = E · (Wi / Vf) pi = 4 · Wi / (π · Di ² · Li) E = (αYnf / αYni) ) ³ αYn = 0.5947-0.01593Y-0.02379n here: pfe: pellet density after heating test in which the influence of reduction ^{(g /} cm 3) pi: pressure Pellet density before test ^{(g /} cm 3) Wi: pellet weight before heating test (g) Di: pellet diameter before heating test (cm) Li: pellet length before heating test (cm) Vf: after the heating test pellet volumes ^(cm 3) Y: gadolinia added ratio of Gd moles to the sum of U moles of Gd moles of uranium dioxide pellets (gadolinium addition rate) E: volume expansion due to the lattice constant increases ArufaYn: Gd addition rate The crystal lattice constant of gadolinia-doped uranium dioxide in the case of Y, O / M ratio n αYni is the crystal lattice constant before the heating test αYnf is the crystal lattice constant after the heating test n: Number of U moles and Gd in gadolinia-doped uranium dioxide pellet A test of gadolinia-added uranium dioxide pellets, characterized in that it is determined according to the ratio of O mole number to the total mole number (O / M ratio). Law. 2. A heating test for evaluating the burning stability of gadolinia-added uranium dioxide pellets used as a reactor fuel, the ratio of the number of oxygen atoms to the number of metal atoms (O / M ratio) and the crystal lattice constant of the pellets being determined. Relationship between pellet weight and pellet O
By utilizing the relationship of the / M ratio and combining the pellet density measurement before and after the heating test in a dry hydrogen atmosphere and the pellet weight measurement before and after the heating test, the reduction of the pellet by the heating test in the dry hydrogen atmosphere is reduced. The burning stability value δpa of the pellet, in which the influence is approximately eliminated, is calculated by the following formula: δpa = pfa−pi pfa = << [0.5947−0.01593 · Y−0.02379 · ｛2-WL · (270-80) .75 · Y) / (16 · Wi)}] / (0.5471-0.01593 · Y) "3 · {4 · Wi / (π · Df 2 · Lf)} pi = 4 · Wi / (π · Di ² · Li) where: pfa: approximate pellet density after heating test excluding the influence of reduction (g / cm ³ ) pi: pellet density before heating test (g / cm ³ ) Wi: heating test Previous pellet weight ( g) Di: Pellet diameter before heating test (cm) Li: Pellet length before heating test (cm) Y: Ratio of Gd mole number to total U mole number and Gd mole number in gadolinia-added uranium dioxide pellet ( Gadolinium addition rate) WL: Weight loss of pellets by heating test (g) Df: Pellets diameter after heating test (cm) Lf: Pellet length after heating test (cm) Test method for pellets.