JPS5810678A - Fuel assembly for heavy water moderated pressure tube type reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fuel assembly for a heavy water moderation pressure tube type nuclear reactor.
In particular, the present invention relates to a fuel assembly suitable for improving the core characteristics of a heavy water-moderated boiling water-cooled pressure tube nuclear reactor and at the same time improving the combustion characteristics of the fuel.

Conventional fuel assemblies of this type have generally been cluster-type fuel assemblies in which a large number of small-diameter rod-shaped fuel rods are arranged concentrically and bundled together. FIG. 1 shows a schematic diagram of a heavy water-moderated pressure tube nuclear reactor using this conventional fuel assembly. A general characteristic of this type of reactor is that the coolant void coefficient has a fairly positive value during reactor startup, especially in the low power range from high-temperature standby where coolant voids begin to occur to turbine entry. If this occurs, the neutron flux and steam drum water level may fluctuate, making it difficult to control the reactor.

The cause will be explained below. In FIG. 1, at low power, relatively cold water is sent from the water supply pipe 1 to the steam drum 2, passes through the downcomer pipe 3 and the inlet pipe 4, and enters the coolant in the pressure pipe 6 in the reactor core 5. As the water cools, the voids in the coolant collapse and the water level in the steam drum 2 falls. When the coolant void coefficient has a large positive value, this tendency is accelerated through changes in the neutron flux, and the reactor becomes susceptible to scram. The coolant void coefficient is positive when using plutonium-uranium mixed fuel, and the above problem does not occur, but when uranium fuel is used, the coolant void coefficient is positive, causing the above problem. It's getting better.

Recently, as a method to improve the coolant void coefficient, as proposed in Japanese Patent Application Laid-Open No. 55-135784, a neutron absorbing material, which is a burnable poison, is added in the fuel material or on the outside of the fuel rods in parallel with the fuel rods. A method is known in which the entire length of the fuel rod is loaded.

This method utilizes the effect that thermal neutron flux inside pressure tube 6 increases as voids occur, whereas thermal neutron flux decreases greatly when there are no voids in the coolant inside pressure tube 6.
This is an application of the fact that placing a neutron absorbing material in the pressure tube 6 produces a negative reactivity injection effect on voids.

However, this method has the disadvantage that once the burnable poison has disappeared, the effect of making the coolant void coefficient more negative is lost. In order to maintain the above effect throughout the combustion of the fuel, it is conceivable to load the neutron absorber in a concentrated manner so that the neutron absorber remains until the final stage of combustion. The problem is that the achieved burnup is lower and more fissile material is consumed.

The present invention has been made in view of the above, and its purpose is to make it possible to make the coolant void reactivity coefficient at low power even more negative, and at the same time to further improve the achieved burnup of the fuel. An object of the present invention is to provide a fuel assembly for a heavy water moderation pressure tube m nuclear reactor that can be used in a nuclear reactor.

The present invention was developed by focusing on the fact that changes in core reactivity due to changes in coolant void ratio in heavy water-moderated boiling water-cooled pressure tube reactors occur downstream of the point at which boiling starts in the coolant flow path. The main feature of this system is that the neutron absorbing material is loaded within the range of 2/3 of the coolant downstream of the effective length of the core of the fuel assembly, which is downstream from the starting point.

The present invention will be described below with reference to the embodiments shown in FIGS. 2 and 3, and the embodiments shown in FIGS.
This will be explained in detail using FIGS. 6 through 6.

FIG. 2 is a longitudinal cross-sectional view of the center portion of an embodiment of the fuel assembly of the present invention. The fuel assembly shown in FIG. 2 is a cluster-type fuel assembly in which a large number of fuel rods are arranged concentrically, and includes a large number of small-diameter rod-shaped fuel rods consisting of a fuel pellet 7 and a cladding tube 8 surrounding the fuel pellet 7. A fuel spacer 9 that bundles the cladding tubes 8, an upper tie plate 10 and a lower tie plate 11 that fix the upper and lower parts of the fuel rods, respectively, and a spacer tie rod 12 that is arranged in the center and maintains the spacing between the fuel spacers 90. It consists of: In the embodiment shown in FIG. 2, a neutron absorbing material 13 such as boron carbide powder is housed in the upper part of the spacer tie rod 12.

FIG. 3 is a longitudinal sectional view showing one embodiment of the spacer tie rod 12 of FIG. 2. FIG. The spacer tie rod 12 has two circular tubes 14 made of Zircaloy alloy screwed together to a connecting end plug 15, and tie plate fixing end plugs 16 are welded to the upper and lower parts of the two circular tubes 14, respectively. In the upper circular tube 14, there is a neutron absorbing material made of boron carbide 17 (
13) is housed in a neutron absorbing material storage tube 18. Note that the boron carbide 1 in the storage tube 18
7 is sealed by the end @19 and the storage tube 18
Inside, a space for gas generated from boron carbide 17 is secured by a spring 20. In this way,
The length of the part containing 17t of boron carbide, which is a neutron absorbing material, is about 1/2 of the effective length of the reactor core, and the length is about 1/2 of the effective length of the reactor core, and it is stored on the upper downstream side in the axial direction. There is a structure in which the coolant flows inside the lower circular pipe 14,
Furthermore, a projection 21 for holding a spacer is welded to each of the upper and lower circular tubes 14.

FIG. 4 is a diagram showing the relationship between the core output and the boiling start point position of the coolant in a boiling water cooled pressure tube reactor, and the boiling start point position is shown in terms of the effective distance in the axial direction of the core. According to FIG. 4, the coolant starts boiling at an effective axial distance of 1850 cm when the core power is approximately 20%. That is, at low core power of 20% or less, changes in coolant voids occur in the upper half of the core in the axial direction, and changes in reactivity due to changes in coolant voids also occur in the upper half of the core.

Therefore, when the core power is about 20% or less, cooling is possible when the neutron absorber is loaded over the entire effective length of the fuel assembly, and when it is loaded on the upper half of the effective length as in the above-mentioned nine flow side. The effect of making the material void reactivity coefficient more negative is the same.

Figure 5 is a diagram showing the relationship between the core output and the coolant void reactivity coefficient difference in the case of uranium fuel. The atomic number density of a certain boron isotope (mass 10) is approximately 1 x 1611 atoms/Kokuyi when the fuel is new, and the figure shows the value at the time of combustion of approximately 130 WD/T, which corresponds to the final stage of the combustion cycle. In FIG. 5, curve 1 is when the neutron absorber is not loaded, curve 6 is when the neutron absorber is loaded over the entire effective length, and curve C is when the neutron absorber is loaded in the axial direction as in the embodiment of the present invention. It is shown as a half-loaded field standard (zero). In addition, the atomic number density of the neutron absorber (boron) at the time of combustion of approximately 130WD/T in this figure is approximately 2X1
0".pieces/cm".

As shown in FIG. 5, it can be seen that loading the neutron absorber makes the coolant void reactivity coefficient negative by about 4X10-'Δ compared to not loading it. Furthermore, up to about 20% of the core power, the difference in the coolant void reactivity coefficient is exactly the same when the neutron absorber is loaded over the entire effective length and when it is loaded in the upper half in the axial direction, and the core power is reduced to 20%. Even if it is above, it can be seen that the difference between the two is slight up to about 40%. In addition, the core power is approximately 4
If it is 0% or more, the weight of the coolant void reactivity coefficient with respect to the output coefficient becomes small, so even if the coolant void reactivity coefficient difference goes in the positive direction somewhat, it does not pose a particular problem in controlling the increase in the power of the core.

Next, it will be explained that loading the neutron absorbing material in the upper half in the axial direction as in the embodiment of the present invention is more advantageous than loading it across the entire effective length.

Figure 6 is a relationship diagram between the atomic number density of the neutron absorbing material (boron) in the center of the fuel assembly at the time of fuel removal and the difference in average burnup of the fuel assembly. In this case, curve 6 is the characteristic curve when the upper half in the axial direction is loaded. As shown in Fig. 6, when the neutron absorber is loaded over the entire effective length, the difference in the average burn-up of the extraction is improved when the neutron absorber is loaded in the upper half in the axial direction as in the present invention. When the atomic number density of the neutron absorber is 2X1 (F@pieces/cm), when the neutron absorber is loaded over the entire effective length, the burnup decreases by about &5GWD/'r, whereas in the axial direction When loaded in the upper half, it is approximately 1.60
Although the WD/T only decreases, the burnup improves by about 20 WD/T.

As described above, according to the embodiment of the present invention, the coolant void reactivity coefficient at low power during reactor startup can be made more negative, and operation control during startup can be facilitated. Further, the achieved burnup of the fuel can be further improved with the same enrichment as before.

In the above embodiment, boron carbide powder was used as the neutron absorbing material, but sintered boron carbide pellets or borosilicate glass may be used, and similar effects can be obtained.

In addition, although boron carbide 17, which is a neutron absorbing material, is stored in the upper circular tube 14 of the spacer tie rod 12 at the radial center of the fuel assembly, neutron An absorbing material may be mixed, however, in this case, gadolinium oxide is used as the neutron absorbing material. Even in this case, the same effect as the above embodiment can be obtained. In the embodiment, the neutron absorbing material is loaded in the upper half in the axial direction, but the same effect can be obtained if the neutron absorbing material is loaded within the 2/3 region on the downstream side.

As explained above, according to the present invention, the coolant void reactivity coefficient at low power can be made more negative, operation control at reactor startup can be easily controlled, and the achieved fuel combustion This has the effect of further improving the level of performance.

[Brief explanation of drawings]

FIG. 3 is a longitudinal sectional view of the central part showing one embodiment of the body; FIG. 3 is a longitudinal sectional view of the spacer tie rod of FIG. 2;
Figure 5 is a diagram showing the relationship between core power and coolant boiling start point position, Figure 5 is a diagram showing the relationship between core power and difference in coolant void reactivity coefficient, and Figure 6 is a diagram showing neutron absorbing material. A diagram of the relationship between the atomic number density and the average burnup difference is shown. 7... fuel pellets), 8-... cladding tube, 9... fuel spacer, 12... spacer tie rod, 13-...
Neutron absorbing material, 14...Circular tube, 15...Connection end plug, 17...Charcoal door (upper blade (7#)) pC output (#+42

Claims (1)

[Claims] 1. In a fuel assembly for a heavy water moderated pressure tube type nuclear reactor consisting of a large number of fuel rods, neutron absorption occurs within the range of the coolant downstream side 273 of the core effective length of the scissor fuel assembly. A fuel assembly for a heavy water moderation pressure tube type nuclear reactor that is loaded with materials and made into mold. 2. A patent in which the neutron absorbing material is arranged along the fuel rod, a glass material containing boron carbide or boron is used as the neutron absorbing material, and the neutron absorbing material is surrounded by a cladding tube. A fuel assembly for a heavy water moderating pressure tube type nuclear reactor according to claim O@s. 3. The fuel assembly for a heavy water moderating pressure tube type nuclear reactor according to claim 2, wherein the neutron absorbing material is loaded at a central position of the fuel assembly. 4. The fuel assembly for a heavy water moderated pressure tube type nuclear reactor according to claim 1, wherein the neutron absorbing material is added to the fuel material of the fuel rod, such as oxidized neutron JIL rim.