JPH07198574A - Method of determining heating holding conditions in measurement of oxygen/metal atom ratio by oxidation/ reduction weight method of gadolinia-containing uranium dioxide fuel body and method measuring therefor - Google Patents

Abstract

PURPOSE:To provide a highly accurate measuring method of oxygen/metal atom ratio (O/M ratio) of sintered pellet of uranium dioxide containing gadolinia. CONSTITUTION:An atmosphere of 4 to 8X10<-7> atom is formed based on a balanced oxygen partial pressure of 2 to 3X10<-14> in a stoichiometric composition corresponding to a temperature range of 600-1, 200 deg.C as heat holding temperature for oxidation/reduction thermal treatment and an oxidation/reduction thermal treatment of a sample with a known weight thereof is performed to measure a weight difference with a heat balance 17 before and after the oxidation/ reduction thermal treatment. Then, heat holding time is determined in a time range of 1-24h. The O/M ratio is measured from a change in the weight between the before and after the oxidation/reduction thermal treatment under heat holding conditions of the oxidation/reduction thermal treatment to reach 2.00 in the O/M ratio of a fuel body of gadolinia-containing uranium dioxide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inspecting an oxide nuclear fuel containing gadolinia (gadolinium oxide) as a combustible poison used in a nuclear reactor for power generation, etc.
/ M ratio (oxygen to metal atom ratio).

[0002]

2. Description of the Related Art Oxide nuclear fuel pellets must satisfy all manufacturing specifications in order to guarantee performance including irradiation behavior as a nuclear fuel. It is necessary to ensure that one of these important specifications for oxide nuclear fuel pellets has a stoichiometric composition. As an inspection method for ensuring that the oxide nuclear fuel pellets have a stoichiometric composition, that is, the presence or absence of excess oxygen in the oxide nuclear fuel pellets and the quantification thereof, and the presence or absence of insufficient (deficient) oxygen and its amount It is essential to establish a measurement method that enables the quantification of In the case of measuring the O / U ratio of the oxide nuclear fuel pellets of UO ₂ simple substance, it was confirmed that the single-phase U ₃ O ₈ was produced by the oxidation heat treatment at 800 ° C. in the air. is doing. When measuring the O / M (M = U + Pu) ratio of mixed oxide nuclear fuel pellets (a solid solution of UO ₂ and PuO ₂ , referred to as MOX), 6% hydrogen / helium with a dew point of 0 ° C. as described in ASTM C698. 800 ° C / in mixed gas
Since it was confirmed that the O / M ratio reached 2.000 in 6 hours, the oxidation / reduction gravimetric method has been established. on the other hand,
O / M of oxide nuclear fuel pellets containing gadolinia (M = U
In the case of the measurement of the + Gd) ratio, there is a possibility that the preconditions of the oxidative gravimetric method may not be established and the accuracy of the chemical analysis is a problem as described later. It may be less than the case of a single unit and MOX.

Conventional gadolinia (gadolinium oxide)
Oxygen-to-metal atomic ratio of the filled uranium dioxide (Oxygen
-To-Metal Ratio: called O / M ratio) can be measured by the following method described in ASTM C968-83.

The measuring method described in ASTM C968-83 basically adopts the oxidization gravimetric method as in the case of the sintered pellet of UO ₂ alone, and the sintered pellet of uranium dioxide containing gadolinia is used. A sample crushed to an appropriate size was pre-oxidized by heating at 575 ° C./2 h in air, and then subjected to an oxidation treatment by heating at 900 ° C./2 h to convert the U-Gd-O phase into (U, Gd) ₃ O ₈ ( M ₃ O
It is based on the idea that M = U + Gd), which is called ₈ .

Therefore, in this measuring method, the weight of the sample before and after the oxidation treatment is measured, and the O / M ratio of the sintered pellet of uranium dioxide containing gadolinia is given by the following equation.

O / M = 2.667 − {(W ₂ −W ₁ ) / W ₂ } (1
7.554538-0.00190E) + 0.01G

Where W ₁ = initial sample weight W ₂ = sample weight after oxidation E = concentration (%) G = Gd ₂ O ₃ addition (%)

For example, Japanese Unexamined Patent Publication (Kokai) No. 2-50692 based on the measuring method described in this ASTM C968-83.
The method for measuring the O / M ratio of sintered pellets of uranium dioxide containing gadolinia disclosed in US Pat.
It is set to 000 ° C.

[0009]

However, if the amount of Gd ₂ O ₃ added is increased, even _if the heat oxidation treatment of MO ₂ ± _x is devised, all the M ₂ O ₈ is not reached and the MO ₂ phase remains. T.
Yato et al. ANS Winter Meeting
(1986).

Thus, the above ASTM C968-8
3 and the oxidative gravimetric method, which is a dry method of the O / M ratio of the sintered pellets of uranium dioxide containing gadolinia, disclosed in Japanese Patent Laid-Open No. 50692/1990, with the recent extension of the operating period of a nuclear reactor, When the amount of gadolinia added is increased, there is a problem that even if the oxidation heating method is devised, MO _{2 which} is the premise of the oxidation weight method does not completely reach M ₃ O ₈ .

This fact is also due to the fact that the X-rays of the sintered pellets of uranium dioxide containing gadolinia after the thermal oxidation treatment of the present inventors.
It was also confirmed in the line diffraction experiment. Therefore, this A
It is considered that the measurement method described in STM C968-83 is not currently supported by the findings obtained as described above.

On the other hand, in addition to the dry weight method, which is a dry method, a conventional chemical analysis method that is a wet method, that is, sinter pellets of uranium dioxide containing gadolinia are dissolved with nitric acid or the like,
The content ratio of U and Gd is obtained by the chemical analysis method, and the balance is O
Although the O / M ratio can be obtained as the content rate, the present wet method is inferior to the dry method in that the chemical analysis is troublesome, the analysis time is long, and the accuracy is low.

From the above, the variation is small,
It is desired to develop a technique which is highly accurate, easy to operate, and capable of measuring the O / M ratio in a short time. The present invention has been made in view of the above circumstances, and is a method for sintering gadolinia-containing uranium dioxide that overcomes the problems of the preconditions of the dry weight method, which is a dry method, and the accuracy of chemical analysis, which is a wet method. It is an object to provide a method for measuring the O / M ratio of pellets.

[0014]

To solve this problem, the method for determining the heating and holding conditions in the measurement of the oxygen-to-metal atomic ratio of the gadolinia-containing uranium dioxide fuel body of the present invention by the oxidation / reduction gravimetric method is the gadolinia-containing method. A method for determining the heating and holding time when the oxygen-to-metal atomic ratio of the fuel body of uranium dioxide becomes approximately 2 by oxidation / reduction, and corresponds to the temperature range of 600 to 1200 ° C as the heating and holding temperature for the oxidation / reduction heat treatment. Equilibrium oxygen partial pressure of stoichiometric composition 2 ~
Oxidation / reduction of a sample of known weight is performed by forming an atmosphere from 3 × 10 ^-14 to 4-8 × 10 ^-7 atm.
The difference between the weights before and after the reduction treatment is measured by a thermobalance, and the heating and holding time is obtained in the time range of 1 to 24 hours as the heating and holding time. Further, the method for measuring the oxygen to metal atomic ratio of the gadolinia-containing uranium dioxide fuel body of the present invention is carried out under the heating and holding conditions of the oxidation / reduction heat treatment at which the oxygen to metal atomic ratio of the gadolinia-containing uranium dioxide fuel body becomes 2.00. Therefore, the feature is that the oxygen to metal atomic ratio of the fuel body is measured from the weight change before and after the oxidation / reduction heat treatment.

[0015]

[Function] The heater keeps the heating and holding temperature in the heating container at 6
The temperature range is set to 00 to 1200 ° C., and the heating and holding time is set to 1 to 24 h. Further, the atmosphere in the heating container is set to equilibrium oxygen partial pressure of stoichiometric composition 2-3 × 10 ^-14 to 4-8.
In order to obtain × 10 ^-7 atm, two kinds of mixed gas are supplied into the heating container from two kinds of gas cylinders. that time,
The two kinds of mixed gas are supplied to the heating container after being adjusted to a predetermined pressure by the decompression control valve and then accurately controlled by the gas flow rate regulators of both of them to be well stirred by the gas mixer. To be done. In the container thus formed, a sample such as a sintered pellet of uranium dioxide containing gadolinia is subjected to oxidation / reduction treatment, and the weight difference before and after the oxidation / reduction treatment is measured with a thermobalance.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the drawings showing an embodiment. First, a measuring device used in the measuring method of the present invention will be described. In FIG. 1, 1
Is a measuring device. The measuring device 1 includes a gas supply device 2, a heating device 3, a weighing device 4, and a gas discharge device 5. The gas supply device 2 includes gas cylinders 6 and 7, pressure reducing control valves 8 and 1.
1, the gas flow rate regulators 12 and 13, and the gas mixer 14 are provided and are connected to the heating device 3. The gas cylinders 6 and 7 are for forming an atmosphere having a predetermined oxygen partial pressure in the heating device 3 under a predetermined temperature.
The gas cylinder 7 and the gas cylinder 7 contain different kinds of gas.

The heating device 3 has a heating container 15 and an external heat type motor 16. The heating container 15 is a thermobalance 17.
Can be accommodated. As the weighing device 4, a thermal balance 17 is used in this embodiment. The gas exhausting device 5 includes an outlet sample gas valve 18, an outlet oximeter 21, and a bubbler 22.

The O / M ratio of the nuclear fuel of uranium dioxide containing gadolinia is measured by using the measuring apparatus having the above-mentioned structure as follows. In this example, sintered pellets were used as a nuclear fuel for gadolinia-containing uranium dioxide, but in addition to this, dried gadolinia-containing uranium dioxide powder, or (U, P
The same operation can be used to measure the O / M ratio even with a u, Gd) O ₂ , (Th, Pu, Gd) O ₂ based sintered body or a powder subjected to a drying treatment.

As a premise, it is generally said that an oxide is in an equilibrium state under the oxygen potential determined by the temperature and oxygen partial pressure in the system, and the stoichiometric composition thereof is determined.
Based on this assumption, FIG. 2 shows a relationship diagram between the equilibrium oxygen partial pressure and the heating temperature in the case of the sintered pellet of uranium dioxide containing 10 wt% gadolinia having the stoichiometric composition of the present invention. The data used to create this FIG. Une and
M. Oguma, J .; Nuc1. Mater. ，
110 (1982) 215-222, U
It depends on the model that the valence of U in _1-y Gd _y O _{2 + x} is balanced by the existence of tetravalent and pentavalent. This Figure 2
Then, the temperature range of 600 to 1200 ° C is set as the oxidation / reduction temperature, and U _0.86 is set according to these heating and holding temperatures.
_2-3x10 as equilibrium oxygen partial pressure of Gd _0.14 O _2.000
^-14 to 4-8 × 10 ^-7 atm.

FIG. 3 is a conceptual diagram showing the reduction behavior under equilibrium oxygen partial pressure in the case of sintered pellets of uranium dioxide containing 10 wt% gadolinia having a superstoichiometric composition of the present invention. It is generally said that the time required for the oxide to reach an equilibrium state in the system, that is, the oxidation rate / reduction rate of the oxide, is high on the high temperature side. In general, the oxidation rate and the reduction rate of an oxide are affected by the physical form of the oxide having the same composition, that is, the density, the open porosity, and the specific surface area of the oxide that contributes to an effective reaction.

Therefore, in the present invention, in consideration of the density of the sintered pellet of uranium dioxide containing gadolinia, which is the object of O / M ratio measurement, the heating and holding temperature is set to 600 for heating and holding time of 1 to 24 hours. Set within a temperature range of up to 1200 ° C. If the characteristics of the sample are significantly different, it is necessary to grasp the heating / holding temperature / time by the thermobalance shown in Fig. 1, but if it is within the range of normal sample characteristics, the heating / holding temperature / time once set is set. The sample may be weighed before and after the oxidation / reduction heat treatment under the conditions.

Next, the O / M ratio is measured. Oxidation / reduction is performed under the above-mentioned temperature holding conditions, and the weight is measured by a thermobalance. That is, the case where the example of the sintered pellet of uranium dioxide containing gadolinia is used as the sample 10 will be described. First, there are provided two kinds of gas cylinders 6 and 7 for forming an atmosphere having a predetermined oxygen partial pressure at a predetermined heating and holding temperature, and a decompression control valve for a predetermined gas for obtaining these predetermined pressures. Through the gas mixer 14 to satisfactorily stir the two kinds of mixed gas, and is supplied to the heating container 15.

[0023] Here, two types of combinations of gases which form an atmosphere having a predetermined oxygen partial pressure, ie, the oxygen potential at a predetermined temperature shown in FIG. 2, CO ₂
/ CO, CO ₂ / H ₂ or H ₂ O / H ₂ is used. Predetermined gas flow rate regulators 12 and 13 are provided to secure a predetermined oxygen partial pressure, and the predetermined oxygen partial pressure of the mixed gas is constantly measured by the inlet oxygen meter 23, and the measured value signal is It is fed back to the gas flow regulator, and the gas flow can be accurately controlled.

Next, the container can be heated, held, and cooled in a predetermined manner by the externally heated heater 16. The heating container is equipped with a thermobalance so that the rate of reduction / oxidation of the sample, that is, the time to reach the equilibrium state can be grasped. Once the measurement conditions for the sample O / M ratio of 2.00 are known, there is no need for this thermobalance.
It can be omitted. FIG. 4 shows a conceptual flow chart of this O / M ratio measuring operation.

The calculation of the O / M ratio of the sintered pellet of uranium dioxide containing gadolinia is as follows. O / M ratio _{= 2.000- (W 2 -W 1)} F / (W 2 -W
_T )

Where W ₂ is the weight of the sample and the boat after the oxidation / reduction heat treatment, W _{1 is} the weight of the sample and the boat before the oxidation / reduction heat treatment, F is the molecular weight of the oxide / 16 W _T , and is the tare weight of the boat. is there. Finally, a bubbler 22 is provided at the gas exhaust portion to prevent air from entering the container.

[0027]

As is apparent from the above description, according to the present invention, a sintered pellet of uranium dioxide containing gadolinia having a stoichiometric composition is desired. / M ratio can be measured, and therefore, the sintered pellet of uranium dioxide containing gadolinia confirmed to have an O / M ratio of 2.00 can be guaranteed as a nuclear fuel.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing an O / M ratio measuring device.

FIG. 2 is a diagram showing the relationship between equilibrium oxygen partial pressure and heating temperature in the case of a sintered pellet of uranium dioxide containing 10 wt% gadolinia having the stoichiometric composition of the present invention.

FIG. 3 10% by weight with superstoichiometric composition of the invention
The conceptual diagram which shows the reduction | restoration behavior under equilibrium oxygen partial pressure in the case of the sintered pellet of uranium dioxide containing gadolinia.

FIG. 4 O / M of uranium dioxide containing gadolinia of the present invention
The conceptual flow chart of the ratio measurement method.

[Explanation of symbols]

 1 Measuring Device 2 Gas Supply Device 3 Heating Device 4 Weighing Device 5 Gas Discharging Device 6 Gas Cylinder 7 Gas Cylinder 8 Pressure Reduction Adjustment Valve 10 Sample 11 Pressure Reduction Adjustment Valve 12 Gas Flow Regulator 13 Gas Flow Regulator 14 Gas Mixer 15 Heating Container 16 External heat type heater 17 Thermobalance 18 Outlet sample gas valve 21 Outlet oxygen meter 22 Bubbler

Claims (4)

[Claims] 1. A method for obtaining a heating and holding time at which the oxygen-to-metal atomic ratio of a fuel body of gadolinia-containing uranium dioxide becomes approximately 2 by oxidation / reduction, and the heating / holding temperature for oxidation / reduction heat treatment is 600 to Equilibrium oxygen partial pressure of stoichiometric composition corresponding to temperature range of 1200 ° C 2-3 × 1
Oxidation / reduction treatment of a sample of known weight is performed by forming an atmosphere of ^0-14 to 4-8 × 10 ^-7 atm, and the weight difference before and after the oxidation / reduction treatment is measured with a thermobalance, and the heating holding time is Oxidation of uranium dioxide fuel containing gadolinia, characterized in that the heating holding time is obtained in the time range of 1 to 24 hours.
A method for determining heating and holding conditions in the measurement of oxygen-to-metal atomic ratio by the reduced weight method. 2. The fuel body is a sintered pellet of uranium dioxide containing gadolinia, a powder of uranium dioxide containing gadolinia subjected to a drying treatment, or (U, Pu, Gd) O ₂ ,
The oxygen-to-metal atom by the oxidation / reduction gravimetric method of the gadolinia-containing uranium dioxide fuel body according to claim 1, which is a (Th, Pu, Gd) O ₂ -based sintered body or a dried powder. Method of determining heating and holding conditions for measuring ratio. 3. The weight change before and after the oxidation / reduction heat treatment under the heating and holding conditions of the oxidation / reduction heat treatment in which the oxygen-to-metal atomic ratio of the fuel body of uranium dioxide containing gadolinia obtained by the method of claim 1 becomes 2.00. The method for measuring the oxygen-to-metal atomic ratio of a gadolinia-containing uranium dioxide fuel body is characterized by measuring the oxygen-to-metal atomic ratio of the fuel body. 4. The gadolinia-containing uranium dioxide powder as the fuel body of gadolinia-containing uranium dioxide is a sintered pellet of gadolinia-containing uranium dioxide or a dried gadolinia-containing uranium dioxide powder. Measuring method of oxygen to metal atomic ratio of fuel body.