JPH0640142B2 - Fast breeder reactor core - Google Patents

Description

TECHNICAL FIELD The present invention relates to a core of a fast breeder reactor, and more particularly to a core of a fast breeder reactor suitable for reducing fluctuations in power distribution due to combustion.

[Prior Art] As is well known, in a fast breeder reactor, neutrons generated by nuclear fission etc. in the core of a nuclear reactor are absorbed by a fuel parent substance to cause so-called breeding in which a new fissile substance is produced. It has the feature that it can be used effectively. The core of such a fast breeder reactor is generally formed in a cylindrical shape, and the periphery of this core is surrounded by axial and radial blankets made of a fuel parent substance. The core is loaded with enriched uranium or plutonium-enriched uranium as a fuel, and the blanket is loaded with, for example, natural uranium or depleted uranium as a fuel parent substance. This fuel parent substance captures the neutrons leaking from the core to produce useful fissile material.

By the way, the upper limit of the heat output that can be taken out from the core depends on the thermal limit of the maximum temperature point. Therefore, it is possible to increase the thermal margin of the core by flattening the power distribution and reducing the maximum line power density. Therefore, JP-A-5
A core concept called a circular character-type core has been devised as described in JP-A-8-223781. The circular character type core has a low enrichment region spread on the disk in the radial direction of the core at the center of the core in the axial direction to achieve flat power distribution in the axial and radial directions.

[Problems to be Solved by the Invention] However, the above-described conventional technique does not sufficiently consider the flattening of the output distribution when the combustion of the fuel progresses. FIG. 2 shows a change in infinite neutron multiplication factor due to combustion in each region of the core region of the prior art. In the early stage of combustion, the effect of neutron leakage in each region is offset by changing the enrichment and making a difference in the neutron infinite multiplication factor.
The output distribution is flattened. Due to combustion, the fissile material density increases in the low-enrichment region inside the core and decreases in the high-enrichment region outside the core. For example, when the average burnup of fuel increases to about 120 GWd / t. The difference between the regions of infinite neutron multiplication factor becomes small and the output distribution flattening function is halved. For this reason, the output distribution fluctuates greatly, and the output distribution cannot be flattened throughout the operating period. Therefore, it is necessary to use other means such as insertion of the control rod. Further, due to the fluctuation of the power distribution, a temperature difference occurs between the coolants flowing adjacent to each other (thermal striping), and the upper core structure is subjected to periodic thermal shocks, which shortens its life.

An object of the present invention is to provide a fast breeder reactor in which fluctuations in power distribution due to combustion are reduced as compared with the above-mentioned conventional technique and which has a flat and stable power distribution throughout the operation period.

[Means for Solving the Problems] In order to achieve this object, in the present invention, a core region having a fuel substance enriched with fissile material and a core region containing a fuel parent substance as a main component are provided to surround the core region. In a fast breeder reactor core having a blanket region in which the core region has a core inner region and a core outer region surrounding it, and the core inner region having a fuel material enriched with fissile material is Has a thickness smaller than the height of the core region in the direction, and the width in the radial direction is smaller than the width in the radial direction of the core region, and the amount of the fuel substance per unit volume in the core inner region is The core structure of the fast breeder reactor is set to be smaller than the amount of the fuel material per unit volume in the region outside the core.

[Operation] FIG. 3 shows changes in infinite neutron multiplication factor in each region of the core according to the present invention due to combustion. The difference in the infinite multiplication factor of the neutron between the inner region and the outer region is almost constant irrespective of the burnup. By this, the effect of the neutron leakage in each region is offset, so that the fluctuation of the output distribution can be suppressed to a small level.

[Examples] Hereinafter, the present invention will be described with reference to Examples. FIG. 1 shows a core of a fast breeder reactor which is a first embodiment of the present invention. The core of a fast breeder reactor has a core region and a blanket region surrounding the core region. The core region is composed of an inner core 1 arranged in a disc shape at the center in the axial direction and an outer core 2 surrounding the inner core 1. In the figure, 3 and 4 are radial and axial blankets that form the blanket region. The fuel assemblies loaded in the axial blanket 3 and the core region have the same shape and the same number of fuel pins. The difference from the conventional circular-shaped core is that the inner core 1 and the outer core 2 have the same plutonium enrichment but different fuel material densities. That is, the density of the fuel substance in the inner core 1 is about 80% of the fuel substance in the outer core 2. To achieve this,
Here, the density of the fuel pellets in the portion of the inner core 1 of the fuel assembly loaded in the core is made lower than the density of the fuel pellets in the portion of the outer core 2 of the fuel assembly. That is, the amount of the fuel substance per unit length in the axial direction in the portion of the inner core 1 of the fuel assembly loaded in the core is equal to the amount of the fuel substance per unit length in the axial direction of the portion of the outer core 2 of the fuel assembly. It is less than the amount of fuel material. In such a core structure, a core in which plutonium enrichment in the core / exterior region of the prior art is varied according to the ratio of the fuel in the core / outer region per unit volume (Japanese Patent Laid-Open No. 58- 223781, the volume of the core inner region is set to 30 to 5 of the volume of the core region.
By setting it to about 0%, the output distribution is flattened through combustion. In this embodiment, since the ratio of the fuel in the core / outer region per unit volume is set to about 80%, the volume of the core inner region capable of flattening the power distribution at this time is approximately the volume of the core region. It was set to 40%. The power distribution can be flattened even if the ratio of the fuel in the core / outer region per unit volume is further reduced.
In this case, it is necessary to further increase the volume of the core inner region. On the contrary, in the core of the fuel per unit volume /
Even if the ratio of the external region is increased, the output distribution can be flattened. In this case, it is necessary to further reduce the volume of the core inner region.

Next, the result of calculating the radial power distribution variation characteristic in the core region according to the present embodiment will be described. Table 1 shows the design parameters and operating conditions of the core of the fast breeder reactor in this example. That is, the reactor heat output is about 2600 MW, the electric output is about 1000 MW, and the equivalent core diameter and core height are 30
0 cm and 120 cm. The axial and radial blanket thicknesses are 25 cm and 30 cm, respectively. The refueling interval is 15 months, the facility utilization rate is 80%, and the number of refueling batches is 3 in both the core region and blanket region.

FIG. 6 shows radial power distributions calculated at the initial stage and the final stage of the equilibrium cycle of the core of the present embodiment (FIG. 1) calculated using the core design parameters. For comparison, the results for a conventional circular core (a core in which the inner core 1 and the outer core 2 have a constant fuel density but a plutonium enrichment) is also shown in FIG. As is clear from these results,
In the core of the present embodiment, the fluctuation ratio of the radial power distribution is reduced from a maximum of about 15% to about 10% as compared with the conventional core. As a result, thermal striping The problem of will be alleviated. Further, the maximum linear power density during operation is reduced by about 3%, and the thermal margin of the core is increased. Alternatively, if the maximum linear power density is kept constant, it is possible to reduce the number of core fuel assemblies by about 3% compared to the conventional core,
The fuel production cost can be reduced accordingly.

In the second embodiment shown in FIG. 4, the region shape is optimized in order to further flatten the power distribution. The inner core 1 is thicker near the radial center of the core region and thinner around the periphery. ing. Regarding the diameter of the inner core 1, the thick portion D1 is 0.6 times the core area diameter D3, and the thin portion D2 is 0.8 times the core area diameter D3. Further, the thickness of the low-density region is 0.75 times the core region height H3 in the core central portion H1, and 0.5 times the core region height H3 in the core peripheral region H2. The volume of the inner core 1 is about 40% of the volume of the core region. With such a core structure, the power distribution is made flatter than that of the core shown in FIG. 1 throughout the combustion period, and the maximum linear power density can be reduced.

In addition to this, considering that the control rod is inserted from the upper end of the core, in order to reduce the distortion of the power distribution due to the insertion of the control rod, the inner core 1 which is a low density region is arranged in the central portion in the central axial direction of the core region. It is also possible to arrange it below. The embodiment shown in FIG. 3 is one of them.

In the above examples, the sintering density of the fuel pellets was made different in order to make the density of the fuel substance different between the regions. As a method other than the above, in order to reduce the fuel volume ratio in the inner core 1, the inner core 1 is filled with hollow pellets, and the outer core 2 is filled with solid pellets (the fuel assembly loaded in the core is filled. The fuel pellets in the inner core 1 portion of the body are hollow pellets, and the outer core 2 of the fuel assembly is
The solid pellets of fuel pellets), and the inner core 1 is filled with fuel pellets having a small outer diameter, and the outer core 2 is filled with fuel pellets having a large outer diameter (fuel assembly loaded in the core). It is conceivable that the outer diameter of the fuel pellet in the portion of the inner core 1 of the body is smaller than the outer diameter of the fuel pellet in the portion of the outer core 2 of the fuel assembly. Accordingly, the amount of the fuel substance per unit length in the axial direction in the inner core 1 portion of the fuel assembly loaded in the core is equal to the amount in the outer core 2 portion of the fuel assembly in the axial direction. Less than the amount of fuel material. It is also possible to use an oxide fuel in the inner core 1 and a metal fuel or a carbide fuel in the outer core 2. Further, in the inner core 1, a method of sandwiching a structural material pellet between fuel pellets, a method of mixing a substance having a small neutron absorption into the fuel pellets, and the like can be considered.

EFFECTS OF THE INVENTION As described above, in the core of the present invention, 1) the fluctuation of the power distribution during operation can be reduced and the effect of thermal striping on the core upper part mechanism is mitigated as compared with the conventional circular core. 2) The maximum output during operation can be reduced, and the thermal margin of the core increases. Or 3) core fuel can be reduced.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view of a core showing a first embodiment of the present invention, FIGS. 2 and 3 are characteristic diagrams showing changes in infinite neutron multiplication factor in a core region due to combustion, and FIG. FIG. 5 is a longitudinal sectional view of the core showing the second and third embodiments of the present invention. Further, FIG. 6 and FIG. 7 are characteristic diagrams showing radial power distributions of the core according to the present invention and the conventional round-shaped core, respectively. Table 1 shows design parameters in the embodiment of the present invention. 1 ... Inner core (low density), 2 ... Outer core (high density), 3 ... Radial blanket, 4 ... Axial blanket.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masaru Bando 1168 Moriyama-cho, Hitachi-shi, Ibaraki Energy Research Institute, Hiritsu Manufacturing Co., Ltd. (72) Inoue Watanabe 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Stock company Hitachi Ltd.Hitachi factory

Claims (6)

[Claims] 1. A core of a fast breeder reactor comprising a core region having a fuel material enriched with fissile material and a blanket region having a fuel parent substance as a main component and surrounding the core region, wherein the core region is A core inner region and a core outer region surrounding the core inner region, wherein the core inner region having the fuel material enriched in the fissile material has a thickness smaller than the height of the core region in the axial direction and in the radial direction. The width is smaller than the radial width of the core region, and the amount of the fuel substance per unit volume in the core inner region is smaller than the amount of the fuel substance per unit volume in the core outer region. The core of a fast breeder reactor characterized by the following. 2. The core of a fast breeder reactor according to claim 1, wherein the axial inner thickness of the core is thicker in the vicinity of the center in the core radial direction and thinner in the peripheral portion. 3. The core of a fast breeder reactor according to claim 1, wherein the axial center of the core inner region is lower than the axial center of the core region. 4. A fast breeder reactor core according to claim 1, wherein the fuel pellets in the core inner region are hollow pellets, and the fuel pellets in the core outer region are solid pellets. 5. The core of a fast breeder reactor according to claim 1, wherein the outer diameter of the fuel pellets in the core inner area is smaller than the outer diameter of the fuel pellets in the core outer area. 6. The core of a fast breeder reactor according to claim 1, wherein the density of the fuel substance in the core inner region is smaller than the density of the fuel substance in the core outer region.