JPH0151797B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the core structure of a nuclear reactor, and in particular to the core of a heavy water-moderated, boiling, light water-cooled, pressure tube reactor loaded with two types of fuel: slightly enriched uranium fuel and plutonium/uranium mixed fuel. This relates to improvements in the structure of the parts.

In a heavy water-moderated, boiling, light-water-cooled, pressure tube type nuclear reactor, the conventional fuel arrangement in the reactor core when two types of fuel are used is shown in FIGS. 1 to 4. Figure 1 shows a reactor core divided into two in the radial direction, with slightly enriched uranium fuel 1 loaded in the outer region and plutonium-uranium mixed fuel (hereinafter abbreviated as plutonium fuel) 2 loaded in the central region of the core. It is a two-zone core, and FIG. 2 shows a radial two-zone core with the reverse arrangement. Figure 3 also shows a radially three-area reactor core in which slightly enriched uranium fuel 1 is loaded in the inner region and outer region, and plutonium fuel 2 is loaded in the middle region of the core sandwiched between the loading regions of slightly enriched uranium fuel 1. It is. FIG. 4 shows an axially two-region reactor core in which the reactor core is divided in the axial direction, and slightly enriched uranium fuel 1 is loaded in the upper region of the core, and plutonium fuel 2 is loaded in the lower region. In FIGS. 1 to 4, 3 indicates a heavy water moderator, 4 indicates a heavy water reflector, and 5 indicates a core tank.

Next, the reactor control characteristics of the conventional reactor core described above will be explained. The nuclear parameter most closely related to the control characteristics of a nuclear reactor is the void coefficient (the amount of change in reactivity per unit change in void ratio of the coolant). In a core with a large positive void coefficient,
As the power increases, the amount of voids increases, and this increase in the amount of voids adds positive reactivity to the reactor core, further increasing the power, resulting in the problem that core control becomes difficult.

The void coefficients of the conventional core shown in Figs.
It is shown by a dotted chain line c and a broken line d. In Figure 5, the core average void fraction on the horizontal axis is about 15% and about 40%,
This corresponds to the void fraction at approximately 35% and 100% core power, respectively.

From the viewpoint of core controllability, it is desirable that the void coefficient has a small value, even if it is negative or positive.
The fuel arrangement in the core shown in the figure is not sufficient in terms of core controllability.

The present invention has been made in view of the above, and an object thereof is to provide a core structure of a nuclear reactor that can reduce the void coefficient and improve core control characteristics.

The present invention provides a heavy water-moderated, boiling, light-water-cooled, pressure tube type nuclear reactor loaded with two types of fuel: slightly enriched uranium fuel and plutonium-uranium mixed fuel. The lower region is loaded with the slightly enriched uranium fuel, and the distance from 3/15 to 1 of the effective core length from the lower end of the core is
It is characterized in that the plutonium-uranium mixed fuel is loaded in the central region of the core in the 0.5/15 region.

First, a method for reducing the void coefficient, which is the basis of the present invention, will be described. The core average void coefficient is
It is obtained from the following formula.

Here, Δk/Δv; core average void coefficient (Δk/k/%
Void) Δv _i,j,k ; Void rate change (%) in fuel segment (i, j, k) φ _i,j,k ; Neutron flux (1/cm ² ) in fuel segment (i, j, k) ) (Δk/Δv) _P ; Void coefficient of plutonium fuel (Δk/k/% void) (Δk/Δv) _U ; Void coefficient of uranium fuel (Δk/
k/% void) Δv: core average void fraction change (%) From equation (1), in order to reduce the core average void coefficient, it is necessary to It can be seen that it is sufficient to arrange fuel with a small void coefficient.

FIG. 6 is a void coefficient diagram of plutonium fuel and slightly enriched uranium fuel, where the solid line f is the void coefficient of plutonium fuel and the broken line g is the void coefficient of slightly enriched uranium fuel. It can be seen from FIG. 6 that plutonium fuel has a smaller void coefficient than slightly enriched uranium fuel. The main reasons for this are described below. Plutonium-239 has a large resonant absorption cross section when the neutron energy is around 0.3ev, so plutonium fuel absorbs more neutrons in the thermal neutron energy region (≦0.625ev) than slightly enriched uranium fuel. Therefore, even if the neutron absorption amount of the coolant decreases due to an increase in the void fraction of the coolant, the rate of decrease in the neutron absorption amount as a fuel assembly is small for plutonium fuel, and there are no factors that make the reactivity positive. This is because it is small.

Therefore, by loading plutonium fuel into a region where the amount of change in void ratio is large and a region where output is high, the average void coefficient of the core can be reduced.

Next, the void ratio distribution and the amount of change in void ratio will be considered. Figure 7 shows the core power at 90% and 100%.
This is a diagram showing the axial void fraction when . Figure 8 shows the core power from 90% to 100%.
FIG. 3 is a diagram showing the amount of increase in void ratio when changing to . From Figure 7, when the core power is 100%, the starting point of void generation is approximately 3/3 of the effective core length from the bottom of the core.
15, and it can be seen that no voids occur in the region from the lower end of the core to this position. It can also be seen that as the core power decreases, the void generation point moves upward from this position. Therefore, no matter which fuel is used in the region from the bottom of the core to 3/15 of the core effective length, the void coefficient of the core does not change.

Also, from Figure 8, the void ratio change when changing the core power reaches its maximum at a position of approximately 6/15 of the core effective length from the bottom of the core, and in the region from 10.5/15 to the top of the core, the maximum change is approximately 6/15 of the core effective length. It can be seen that the value is reduced to 60%. Therefore, by loading plutonium fuel with a small void coefficient in the region of 3/15 to 10.5/15, where the amount of change in void ratio is large, the void coefficient of the core can be reduced.

FIG. 9 is a diagram showing the axial power distribution. This figure also shows that the output is high in the region of 3/15 to 10.5/15, and it is desirable to load plutonium fuel in this region.

The present invention will be explained in detail below using the embodiment shown in FIG. 10 and FIG.

FIG. 10 is a vertical cross-sectional view of the core showing an embodiment of the core structure of the present invention. In Figure 10,
The reactor core is divided into three regions in the axial direction at 3/15 and 10.5/15 of the core effective length from the bottom end of the core, and the lower and upper regions are each loaded with slightly enriched uranium fuel 1, and the central region is loaded with plutonium fuel. 2 is loaded. 3 is a heavy water moderator, 4 is a heavy water reflector, and 6 is a core tank.

The void coefficient of the core of the example shown in FIG. 10 is shown in FIG. 5 by a solid line e. From this result, according to the embodiment of the present invention, the core void coefficient is approximately 0.16×10 ^-5 Δk even at 100% core power (approximately 40% core void ratio), which is the largest value. /
k/% void, which is a sufficient void coefficient in terms of core controllability. This makes it possible to improve core control characteristics.

In addition, in this example, the core is divided into three regions in the axial direction, so by adjusting the infinite neutron multiplication factor k∞ in the central region of the core and k∞ in the upper and lower regions of the core, the power distribution in the core axial direction can be adjusted. It is possible to flatten the core and increase the thermal margin of the core.

As explained above, according to the present invention, the void coefficient can be reduced and the core control characteristics can be improved.

[Brief explanation of drawings]

Figures 1 to 3 are horizontal cross-sectional views of the core showing conventional fuel arrangements, Figure 4 is vertical cross-sectional views of the core showing other conventional fuel arrangements, and Figure 5 is the void ratio of the void coefficient for each fuel arrangement. Figure 6 is a diagram showing the void ratio dependence of the void coefficients of plutonium fuel and slightly moated uranium fuel. Figure 7 is a diagram showing the void ratio distribution in the core axis direction. Figure 8 is a diagram showing the increase in void fraction in the axial direction of the core when the core power is increased from 90% to 100%, Figure 9 is a diagram showing the power distribution in the axial direction of the core, and Figure 10 is a diagram showing the heavy water moderation of the present invention. 1 is a vertical cross-sectional view of a reactor core showing an example of a core structure of a boiling light water cooled pressure tube type nuclear reactor. 1... Slightly enriched uranium fuel, 2... Plutonium/uranium mixed fuel, 3... Heavy water moderator, 4...
Heavy water reflector, 5...core tank.

[Claims]

1. A loop type liquid metal cooled fast breeder reactor that is part of a nuclear reactor system having a coolant pump and heat exchanger placed separately near the reactor and loop piping in fluid communication with the reactor. a flat top deck made of a flat structural material through which all the loop piping leading to the reactor enters the reactor; In a liquid metal fast breeder reactor comprising a primary vessel and a plenum disposed in the bottom of the primary vessel and connected to the loop piping for receiving pressurized coolant therefrom, a shielding cylinder is arranged in the primary vessel. a liquid metal, wherein the liquid metal is provided in an upper part of the cylinder, and is configured to form a gas-filled annular space between the shielding cylinder and the primary container, and the upper part of the primary container is insulated. Fast breeder reactor. 2. The liquid metal fast breeder reactor according to claim 1, wherein the top deck has a space for a worker to enter. 3. A liquid metal fast breeder reactor according to claim 1 or 2, wherein the loop piping is surrounded by a double-walled riser pipe having a gas space therebetween for thermally protecting the loop piping. 4 All reactor penetrations are installed on the top deck above.