JP2006337312A - Nuclear fuel pellet for high burnup fuel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To relax the generation interaction among nuclear fuel pellets and fuel clads due to rise in the center temperature of fuel and volume expansion of nuclear fuel pellets during high burnup. <P>SOLUTION: The nuclear fuel pellet is manufactured by sintering oxide fuel containing nuclear materials and has average diameter of bubbles formed in grains in manufacturing is 0.2 μm or smaller. With this configuration, the textural variation (rim texture formation) caused in the fuel pellets in high burnup can be suppressed, and the volume expansion of the nuclear fuel pellets in high burnup can be reduced. Accordingly, interaction between the nuclear fuel pellets and the fuel clad in high burnup is relaxed, and integrity margin for fuel at high burnup time can be improved. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a nuclear fuel pellet containing nuclear fuel material, and more particularly, to a high burnup nuclear fuel pellet used in a fuel assembly used up to a high burnup.

  FIG. 6A is an external view of the fuel rod 1 constituting the fuel assembly used in the boiling water reactor, with a part cut away, and FIG. 6B is an enlarged plan view of the fuel rod 1. Has a zirconium cladding tube 2 with a zirconium liner 2a layered on its inner surface, and a plurality of nuclear fuel pellets 3 are loaded into the cladding tube 2, and both ends of the cladding tube 2 are connected to lower end plugs 4 and A plenum spring 6 is disposed between the nuclear fuel pellet 3 and the upper end plug 4. A fuel assembly is formed by converging the multiple fuel rods 1 configured as described above.

  By the way, the soundness of the nuclear reactor fuel composed of the fuel rod 1 loaded in the light water reactor or fast breeder reactor has been confirmed up to the highest burnup experienced in the nuclear reactor. At present, increasing the burnup of nuclear reactor fuel for the purpose of improving the economics of the reactor and reducing the nuclear fuel cycle cost, the fuel integrity margin at the high burnup due to the higher burnup of the fuel May be under pressure. One specific factor of soundness pressure is an increase in the interaction (PCI) between the nuclear fuel pellet 3 and the fuel cladding 2 due to the volume expansion (swelling) of the nuclear fuel pellet 3. At the time of high burnup, it is expected that the volume expansion accompanying the rim structure formation described later will occur and the interaction between the nuclear fuel pellet 3 and the fuel cladding tube 2 will increase, especially when an abnormal event such as a reactivity accident occurs. There is a possibility that the soundness of the fuel in the future will be a problem.

  In the fuel pellets 3 irradiated to a high burnup, it is generally known that a region having a high porosity is formed as a result of microstructural changes caused by accumulation and reorganization of irradiation defects. This fine structure peculiar to high burn-up fuel is called a rim structure because it is typically recognized at the outer periphery of the pellet. As a result of the structure change, in the rim structure formation region, crystal grains at the time of manufacture are subdivided into very fine crystal grains, and coarse bubbles in which high-pressure FP gas is accumulated are formed. It is understood that the accelerated volumetric expansion of the fuel at high burnup is associated with the formation of these coarsened bubbles. Therefore, taking measures to suppress the formation of rim structure of nuclear fuel pellets is a very important proposition for improving the soundness of fuel by reducing the interaction between nuclear fuel pellets and fuel cladding at high burnup. is there.

  It has been empirically known that a method of increasing the crystal grain size at the time of manufacture (increasing the crystal grain size at the time of manufacture) is effective as a measure for suppressing the formation of the rim structure. This is interpreted for the following reason.

  As a mechanism for forming a rim structure at a high burnup, a mechanism accompanying a recrystallization phenomenon and a mechanism accompanying a polygonization phenomenon have been proposed. What is common to both is that dislocations introduced into the material along with nuclear fission play an important role in the rim formation mechanism.

  It is known that dislocations introduced into a material move in the material while consuming energy generated during fission or irradiation defects. Nucl. Mater. , 278 (2000) 54-63, the grain boundaries at the time of manufacture become obstacles to the movement of dislocations. As a result, dislocations are produced at the grain boundaries at the time of manufacture in nuclear fuel pellets that have burned to some extent. Accumulate at high density. In such a portion where dislocations are accumulated at high density, new crystal grains are formed by recrystallization or polygonization, and a rim structure is formed starting from these places. Therefore, the formation of a rim structure can be suppressed by increasing the crystal grain size at the time of manufacture, that is, by increasing the average movement distance required for dislocations to reach the crystal grain boundary at the time of manufacture.

Inhibit rim structure formed by the large grain size of the preparation during the crystal on the basis of the above principle technique not only UO ₂ fuel, UO ₂ fuel Gd ₂ added with Gd ₂ O ₃ is a burnable poison in O it is considered to be equally valid for ₃ added UO ₂ fuel. However, increasing the grain size of the crystals during production reduces the creep properties, which is one of the important physical properties of nuclear fuel pellets. This leads to an increase in the interaction between the nuclear fuel pellet and the fuel cladding at the time of high burnup. Therefore, it is not preferable to reduce the creep characteristics of the nuclear fuel pellet when considering the soundness of the fuel at the high burnup.

As a countermeasure technique, there has been devised a technique for improving the creep characteristics by precipitating a soft layer (glass phase) at a grain boundary, as represented by Japanese Patent Application Laid-Open Nos. 1-193691 and 1-22019. However, it has not yet been applied to commercial reactors. Therefore, it is desired to develop and put into practical use a technique for suppressing rim structure formation by a method other than increasing the particle size.
Japanese Patent Laid-Open No. 1-193691 Japanese Patent Laid-Open No. 1-22019

  The present invention applies the rim structure formation suppression technology devised based on the principle other than the above-mentioned increase in particle size as a technique for suppressing the formation of rim structure of nuclear fuel pellets, and the soundness tolerance of fuel at high burnup An object of the present invention is to obtain a nuclear fuel pellet for a high burnup fuel used in a fuel assembly for a nuclear reactor with improved fuel efficiency.

The present invention relates to a nuclear fuel pellet for high burnup fuel comprising an oxide fuel containing at least one nuclear fuel material of uranium and plutonium or an oxide fuel obtained by adding Gd ₂ O ₃ to the oxide fuel. The average particle diameter of the material crystal is at least 5 μm or more, and the average value of the diameter of the bubbles produced during the formation of the oxide fuel crystal grains is about 0.2 μm or less. To do.

  That is, when paying attention to the fact that the rim structure formation mechanism starts from the accumulation of dislocations, there is another effective rim structure formation suppression measure in addition to the technique for increasing the grain size of the crystal during production. Specifically, the idea is to provide many obstacles in the oxide fuel crystal to reduce the mobility of dislocations, that is, to make dislocations difficult to move.

  As a method of reducing the mobility of dislocation, (1) a method in which elements other than the constituent elements are dissolved (solid solution) in the base material crystal in the oxide fuel, and (2) in the base material of the oxide fuel. Examples include a method of precipitating the second phase particles at a high density. The former (1) is a solid solution hardening, and the latter (2) is a technique widely used in other materials in relation to precipitation hardening, but the former (1) is not preferable for the heat conduction characteristics of oxide nuclear fuel. The latter (2) may adversely affect the mechanical properties of oxide nuclear fuel. In addition to these methods, there is a method unique to a nuclear fuel, that is, a method using (3) a phenomenon in which dislocations are restrained (trapped) by intragranular bubbles, and the present invention is based on this.

  When nuclear fuel materials such as uranium fission in an oxide fuel, a number of interstitial atoms and vacancies (vacancies formed when atoms are ejected) are generated in addition to fission-generating atoms (FP atoms). Most of the vacancies disappear due to recombination with interstitial atoms within a short period of time, or disappear in sinks such as crystal grain boundaries. In this way, a very small defect complex (cluster) is formed, and gaseous FP atoms (FP gas atoms) are accumulated inside to form minute intragranular bubbles of about several nm. As schematically shown in FIG. 7, the intragranular bubbles 11 formed in this way act as an obstacle that restrains (traps) the dislocation 12 and prevents its movement. Therefore, by devising so that the intragranular bubbles 11 are densely formed in the base material of the oxide fuel, the mobility of dislocation can be reduced, thereby suppressing the formation of the rim structure at the time of high burnup. Is possible.

  In the present invention, as a specific means for realizing the above, attention is paid to bubbles during production. An oxide fuel is used in a nuclear reactor as sintered pellets obtained by sintering oxide powder containing a nuclear fuel material at a high temperature. Usually, in this sintering process, it is not completely sintered, and voids are formed as schematically shown in FIG. This is referred to as production bubble, and the size and number density of the production bubble depend on the sintering conditions such as the sintering temperature and the sintering atmosphere, and various characteristics of the oxide powder. Bubbles with a relatively small bubble diameter at the time of production are considered to disappear in the early stage of irradiation as the fission fragments pass through, and these fine bubbles at the time of production are formed as vacancies in the grain boundaries and grains. scatter. In FIG. 1, reference numeral 21 denotes a manufacturing bubble formed in the grain, and reference numeral 22 denotes a manufacturing bubble formed in the grain boundary. In fact, J.H. Nucl. Mater. 62 (1976) 138, it is reported that the number density of small bubbles having a diameter of about 0.5 μm or less is rapidly reduced in the initial stage of irradiation. In other words, small manufacturing bubbles having a diameter of about 0.1 to 0.3 μm or less act as a main pore supply source in the early stage of combustion, and function to promote the formation of bubbles in fine particles. Therefore, if a nuclear fuel pellet having an average diameter of the bubbles 21 during production formed in the crystal grains is about 0.2 μm or less, the formation of the rim structure of the nuclear fuel pellet at the high burnup can be suppressed, Therefore, it is possible to improve the fuel soundness at the time of high burnup.

  In addition, the nuclear fuel pellets in which a large number of microbubbles are formed in the crystal grains at the time of production reduces the FP gas emission that is particularly problematic at the time of high burnup as well as the interaction between the nuclear fuel pellets and the fuel cladding. Highly preferred above.

  Within nuclear fuel pellets, various types of fission-generating atoms are generated as nuclear fuel material is fissioned. Some form metallic precipitates, and some dissolve in the base material, but FP atoms of rare gases such as Xe and Kr hardly dissolve in the base material. Accumulated, accumulated in the rim structure (coarse bubbles), accumulated in grain boundary bubbles formed at crystal grain boundaries, or through a grain boundary tunnel formed by connecting grain boundary bubbles Released from the pellets into the fuel rod. When the FP gas is released into the fuel rods, the thermal conductivity in the gap between the nuclear fuel pellets and the cladding tube is reduced and the fuel center temperature is increased, and the fuel rod internal pressure is increased. When the fuel rod internal pressure significantly exceeds the hydrostatic pressure of the core coolant, that is, stress that causes the fuel cladding tube to expand, the gap between the nuclear fuel pellet and the fuel cladding tube is widened to conduct heat in the gap. The rate further decreases, leading to an increase in the fuel center temperature, which can lead to a vicious circle in which more FP gas is released. Therefore, it is very important to suppress the release of FP gas at the time of high burnup in order to improve the soundness tolerance of fuel at the time of high burnup. The FP gas generated by the fission is accumulated mainly in the intragranular bubbles, intergranular bubbles, and coarsened bubbles, but especially in the case of accidents where FP gas release is likely to occur or when the output suddenly increases. In this case, it is most preferable that more FP gas is accumulated in the intragranular bubbles, and it is intended by the present invention to encourage such a state to be formed.

  According to the present invention, the nuclear fuel pellet at the time of high burnup is obtained by having a large number of fine bubbles at the time of manufacture in which the average value of the diameter is about 0.2 μm or less in the crystal grains of the nuclear fuel sintered pellet. Rim tissue formation can be suppressed. Therefore, the interaction between the pellet and the fuel cladding at the time of high burnup can be reduced, and the fuel integrity at the time of high burnup can be improved.

  An embodiment in which the nuclear fuel pellet according to the present invention is applied to a boiling water reactor material will be described below.

  As schematically shown in FIG. 1, the nuclear fuel pellet 3 according to the present invention has a production bubble 21 formed in a crystal grain and a production bubble 22 formed in a crystal grain boundary, and has a diameter of about 0.00 mm. There are many fine bubbles in the crystal grains at the time of production of 2 μm or less.

  The principle of the present invention is particularly effective in a nuclear fuel pellet in which a large number of bubbles having a diameter of about 0.1 μm or less are formed in crystal grains as described above. On the contrary, the effect of the present invention cannot be expected so much for nuclear fuel pellets in which almost all of the bubbles at the time of manufacture have a diameter of about 0.5 μm or more. Since the frequency distribution of the diameters of the production bubbles formed in the nuclear fuel pellets approximately follows a logarithmic normal distribution, the principle of the present invention controls the median (median) of the diameters of these production bubbles to about 0.2 μm or less. Only by doing so can you expect the effect.

  2 and 3 are microstructures before irradiation of the nuclear fuel pellet A having a large number of minute production bubbles in the crystal grains and at a high burnup, and FIGS. 4 and 5 are fine production bubbles in the crystal grains. The microstructure before irradiation of the nuclear fuel pellet B which does not have and at the time of high burnup is shown typically. In FIGS. 3 and 5, dislocations are omitted in order to easily identify the difference in the formation state of intragranular bubbles.

The oxide fuel pellet A is a UO ₂ fuel pellet having a crystal grain size of about 10 μm at the time of manufacture, and the oxide fuel pellet B is a Gd ₂ O ₃ added UO ₂ fuel pellet having a crystal grain size of about 50 μm at the time of manufacture. It is. All oxide fuels are made by sintering oxide powder containing nuclear fuel material into sintered pellets, loaded into a fuel cladding tube and irradiated in a nuclear reactor to an average pellet burnup of about 70 GWd / t. Examine the tissue.

  3 and 5, in the oxide fuel pellets having a large number of minute production bubbles in the crystal grains, the intragranular bubbles 11 are formed at a significantly higher number density at a high burnup. I understand.

  Also, as a result of investigating the ratio of the rim structure formation region (region where formation of finer crystals and formation of coarsening bubbles was recognized) to the entire observation region, many minute productions were made in the crystal grains. The rim structure was formed in approximately 50% to 70% of the oxide fuel pellets having bubbles, and in 90% or more of the oxide fuel pellets having no fine bubbles during production in the crystal grains.

  Considering only the crystal grain size, the oxide fuel pellet A having a small crystal grain size at the time of production tends to form a rim structure more easily than the oxide fuel pellet B. Actually, however, the oxide fuel pellet B having a larger crystal grain size at the time of production causes a structural change over a wider range. This result is data that supports the above principle and confirms that the formation of the rim structure of the nuclear fuel pellet at the high burnup can be suppressed by means of appropriately controlling the formation state of the bubbles during production.

  With respect to uranium fuel conventionally used in nuclear reactors, as represented by FIG. 2, while relatively large production bubbles 22 having a diameter of 0.2 μm or more are mainly formed at the grain boundaries, Inside, small bubbles 21 are formed at the time of production of about 0.1 μm or less. On the other hand, in the case of gadolinia fuel, as shown in FIG. 4, almost no small production bubbles of about 0.1 μm or less are recognized, and relatively large production bubbles of about 1 μm or more in diameter are mainly formed. Yes. The difference between the two is that uranium fuel and gadolinia fuel have different sintering characteristics (easiness to sinter) and different sintering conditions, and further to adjust the pellet density during sintering. This is a difference that occurs as a result of taking into account the action of the pore former (pore former) that is intentionally mixed. The principle of the present invention is effective for all oxide fuels used in nuclear reactors, but as shown in FIGS. 2 to 5, it is particularly effective for conventionally used gadolinia fuels. Further, even when a mixed oxide fuel using plutonium together with uranium as a nuclear fuel material is used in a nuclear reactor, the effect of suppressing the formation of a rim structure by the principle of the present invention is effective, and the use of a mixed oxide fuel is It does not interfere with the effect.

Furthermore, for the nuclear fuel pellets to which the present invention is applied, the creep characteristics are improved by precipitating a soft second phase at the grain boundaries, which further increases the interaction between the nuclear fuel pellets 3 and the fuel cladding tube 2 at the time of high burnup. Desirable for mitigation. Furthermore, it is highly desirable to apply the present invention to nuclear fuel pellets having a large crystal grain size during production in order to further amplify the rim structure suppression effect according to the present invention. Conversely, making the crystal grain size at the time of manufacture smaller than that of the current material limits the effect of the present invention. The average particle size of nuclear fuel pellets is known to vary depending on various characteristics of oxide fuel powder, powder mixing state, sintering atmosphere, Gd ₂ O ₃ addition amount, etc. It is manufactured under conditions such that the average particle size is approximately 10 μm, at least 5 μm. Therefore, in order to suppress the formation of the rim structure below the present level based on the principle of the present invention, it is desirable to apply the present invention to nuclear fuel pellets having a crystal grain size of at least 5 μm or more during production.

  From the above, by applying the nuclear fuel pellet according to the present invention as the fuel material for the reactor, the formation of the rim structure at a high burnup is suppressed, thereby reducing the interaction between the nuclear fuel pellet 3 and the fuel cladding 2. It is possible to provide a fuel assembly for a nuclear reactor with improved fuel soundness at high burnup.

The schematic diagram of the fine structure of the nuclear fuel pellet by this invention. The schematic diagram which shows the microstructure before irradiation of the nuclear fuel pellet A which has a fine bubble at the time of manufacture in a crystal grain. The schematic diagram which shows the intra-cell bubble formation condition at the time of the high burnup of the nuclear fuel pellet A which has a micro bubble at the time of manufacture in a crystal grain. The schematic diagram which shows the fine structure of the nuclear fuel pellet B which does not have a fine bubble at the time of manufacture in a crystal grain. The schematic diagram which shows the intra-cell bubble formation condition at the time of the high burnup of the nuclear fuel pellet B which does not have a micro bubble at the time of manufacture in a crystal grain. (A) is a partial cross-sectional outline view of a nuclear fuel rod, and (b) is an enlarged plan cross-sectional view thereof. Schematic diagram of dislocations and intragranular bubbles observed in irradiated nuclear fuel pellets.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fuel rod 2 Cladding tube 3 Nuclear fuel pellet 4 Lower end plug 5 Upper end plug 6 Plenum spring 11 Intragranular bubble 12 Dislocation 21 Production bubble formed in the grain 22 Production bubble formed in the grain boundary

Claims (3)

In the nuclear fuel pellet for high burnup fuel comprising an oxide fuel containing at least one nuclear fuel material of uranium and plutonium, or an oxide fuel obtained by adding Gd ₂ O ₃ to this, an average of the base crystals of the oxide fuel High combustion, characterized in that the particle diameter is at least 5 μm or more, and the average value of the diameter of the bubbles formed in the oxide fuel crystal grains is about 0.2 μm or less Nuclear fuel pellet for high-grade fuel.   The nuclear fuel pellet for high burnup fuel according to claim 1, wherein a soft second phase is accompanied at a grain boundary.   The nuclear fuel pellet for high burnup fuel according to claim 1 or 2, characterized in that it is used for a fuel assembly having an average burnup of a fuel assembly of about 45 GWd / t or more.

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