JPH0251094A - Fuel assembly, fuel rod and reactor core - Google Patents

Abstract

PURPOSE:To increase an allowance for the stoppage of a reactor with a reduction in the degree of reaction between the periods of operation and stoppage by dividing a fuel assembly into two areas downstream from a point a half of a fuel effective length along the direction of the flow of a coolant. CONSTITUTION:The weight of <233>U contained in an upper one-third area of this fuel assembly is for example 3.3kg and the weight of a fissile substance, <235>U, contained in this area is -1.3kg. Therefore, a weight ratio of the <233>U is -0.72 with respect to the total fissile substance (<233>U to <235>U). On the other hand, as the weight of the <235>U is -6.6kg and that of the <233>U 0 in a lower two-third area. In this area, a weight ratio of <233>U is 0 with respect to the total fissile substance. As a result, in this fuel body, the weight ratio of the <233>U with respect to the total fissile substance becomes larger in the upper area than that in the lower area. When the fuel assembly thus arranged is mounted in an outer area 9, a change in the degree of reaction can be reduced between the periods of operation (high temperature) and stoppage (cold temperature) thereby improving an allowance for the stoppage of a reactor.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a fuel assembly used in a boiling water nuclear reactor,
In particular, the present invention relates to a fuel assembly suitable for ensuring reactor shutdown margin.

[Conventional technology]

In a boiling water reactor, the density of water, which is a moderator, reaches a high temperature of approximately 280° C. during operation, and voids are generated, so the density is lower than that at room temperature (approximately 20° C.). Therefore, the potential reactivity of the core is generally greater at room temperature than during operation.

On the other hand, in the design of nuclear reactors, from a safety perspective, it is required that the reactor can be reliably shut down at room temperature even if the control rod, which has the highest reactivity value, becomes unable to be inserted. That is, it is necessary to ensure a sufficient difference between the effective neutron multiplication factor in the state where the control rod with the highest reactivity value is not inserted and the effective neutron multiplication factor in the critical state of 1.0.

In particular, in order to achieve high burnup, it is necessary to use highly concentrated fuel, and in this case, securing margin for reactor shutdown becomes an important issue. As a means to ensure margin for reactor shutdown, (1) changing the fuel assembly arrangement, (2) increasing the reactivity value of control rods, and (3) using fuel rods containing burnable poison have been adopted.

In (1), the fuel is shuffled so that the reactivity values of the control rods are averaged. However, this procedure has the disadvantage that it takes one hour and reduces plant capacity utilization.

(2) is to secure reactor shutdown margin by increasing the reactivity control ability of insertable control rods. but,
Surplus reactivity control during operation has drawbacks such as increased distortion in the power distribution caused by the control rods.

In (3), by adding a neutron absorbing material such as gadolinia to the fuel, the reactivity of the fuel is suppressed, and surplus reactivity control and reactor shutdown margin are ensured. especially.

From the viewpoint of securing margin for reactor shutdown, a fuel assembly with a region containing many burnable poisons above the fuel was proposed in Japanese Patent Laid-Open No. 59-10218.
It is presented in Publication No. 8. The reason why countermeasures to the upper part of the fuel are effective for the reactor shutdown margin is that (a) in a normal boiling water reactor, where water as a coolant flows in from below the fuel and voids are generated, the fuel has a high density of water; The output tends to be higher in the lower part, and the combustion of fuel in the upper part is slower than in the lower part, so more fuel substances remain in the upper part.

(b) In the upper part of the fuel where the void ratio is large, the difference in reactivity between operation and room temperature is larger than in the lower part.

For this reason.

[Problem to be solved by the invention]

However, in the above conventional technology, since a large amount of burnable poison is added to the upper part of the fuel, the reactivity in the upper part of the fuel, which is lower than that in the lower part during operation, becomes even smaller.

For this reason, the power distribution is further distorted toward the lower side of the fuel, which is undesirable from the viewpoint of flattening the power distribution.

An object of the present invention is to increase the shutdown margin of the furnace without deteriorating the axial power distribution during operation.

[Means to solve the problem]

The above object is achieved by reducing the change in reactivity between operation and room temperature. For this reason, in the fuel assembly of the present invention, the fuel containing 283U is used in the upper part of the fuel where the atomic ratio of water to fuel changes greatly between operation and room temperature, and the reactivity changes greatly. Alternatively, a fuel containing other nuclides with similar properties to 233U, or nuclides that convert to such nuclides by neutron absorption, is used in the upper part of the fuel.

[Effect] The present invention will be explained in detail below.

FIG. 2 shows changes in the regeneration rate (average value of the number of newly generated neutrons when η neutrons are absorbed) for '85U and 233U with respect to neutron energy. As shown in the figure! 86 U is the resonance energy region (1 eV to I
The value of ηR in the thermal energy region (1 eV or less) is smaller than the value 9 in the thermal energy region (1 eV or less). On the other hand, ZIH
For U, the value of ηR in the resonance energy region is almost the same as the value in the thermal energy region.

On the other hand, in the upper part of the fuel, the void ratio increases during operation and the water-to-fuel atomic ratio decreases, so the neutron spectrum becomes hard and the proportion of reactions in the resonance energy region increases. On the other hand, at room temperature, no voids occur and the atomic ratio of water to fuel increases, so the neutron spectrum becomes softer and the proportion of reactions in the thermal energy region increases.

Therefore, a fuel containing 233U is used instead of a fuel containing 235U in the upper part of the fuel, where the void ratio is high during operation, the difference in the atomic ratio of water to fuel between operation and room temperature is large, and the neutron spectrum changes greatly. This makes it possible to reduce the difference in fuel regeneration rate (η) during operation and at room temperature. That is, this makes it possible to reduce the change in reactivity between operation and room temperature, thereby increasing the margin for reactor shutdown.

Furthermore, since there is no need to reduce the reactivity during operation, there is no need to deteriorate the output distribution in the axial direction.

〔Example〕

Hereinafter, the fuel assembly of the present invention will be explained in detail using Examples.

FIG. 1 shows a fuel assembly according to an embodiment of the present invention. The fuel assembly 1 has a rectangular shape, and the channel box 2
, 62 fuel bodies 3 and two water rods 4. In addition, in this fuel assembly, the water to fuel volume ratio is ~3
．． 0, which is a soft system of neutron energy spectrum. The fuel body 3 has an enrichment of 1.8 in the upper 1/3 region.
5w/.

233U is mixed with natural uranium. On the other hand, in the lower two-thirds region, uranium fuel with an enrichment of 3.70w10 is loaded.

The weight of “u” included in the upper 1/3 area is 3.3
kg, and the fissile material contained in this area2
The weight of 33U is ~1.3 kg. Yotte”U (
7), the weight ratio to the total integral R substance (''U 235u ) is ~0.72. On the other hand, the lower 2
The weight of 28IIU included in the /3 area is ~6.6kg
, the weight of z8δU is O, so 288 in this region
The weight ratio of U to total fissile material is O. Therefore, the present fuel assembly has 283 points in the upper region than in the lower region.
The weight ratio of U to total fissile material is increasing.

FIG. 3 shows the relationship between the infinite neutron multiplication factor and the void fraction for this fuel assembly. The fuel assembly of the present invention has an upper 1/3
By loading 233U into the fuel assembly, the infinite neutron multiplication factor during cold shutdown is improved compared to a fuel assembly composed only of JullU enriched fuel, without significantly changing the neutron infinite multiplication factor at a void rate of 70% during operation. , can be as low as/to ~5% Δ. As a result, the reactor shutdown margin was reduced to ~1.
It can be increased to/by 2%Δ.

As explained in Fig. 2, when using z38U, the difference in reactivity between operation and cold shutdown can be reduced in the resonance energy region (1 to 10'eV), as explained in Fig. 2.
This is because the regeneration rate ηR value is almost the same as ηT in the thermal energy region (1 eV or less).

That is, during operation, the average neutron energy is high, and the regeneration rate is the value ηR in the resonance energy region, and during cold shutdown, the average neutron energy is low, so the regeneration rate is the value ηT in the thermal energy region. In the case of 238U, since ηR and ηT are almost the same, the difference in reactivity between operation and cold shutdown is also small.

Therefore, the same effect as in the example of FIG. 1 can be expected even if another fissile material is used instead of 233U.

However, the fissile material has a resonance energy region of ηR
The ratio of the value to η in the thermal energy region is ηR in the energy region of 235U, which is a typical fissile material.
235 and ηT235, that is, it is necessary to satisfy the following relationship: ηR/ηT> ηR233/ηT23!1.

233U used in the first example is! 32Th of (
n+ γ) The product produced by the reaction is reprocessed and used.

However, as in the embodiment of FIG. 4, 232T1. You may also use This fuel assembly 101 contains 3.0 w of uranium with an enrichment of 3 and 7 w/o in the upper 1/3 region.
10" Th51 is added. On the other hand, the lower 2/3
The area is loaded with uranium with an enrichment level of 3.7w10.

Also, with combustion, 234T), the following nuclear reaction,
zazTh(n, y) 233U is generated and 233U is accumulated in the upper part, so that the effect of improving the reactor shutdown margin can be obtained.

In addition, the surplus reactivity at the initial stage of combustion is reduced by neutron absorption by 232Th. Also, as combustion progresses, 2
Because LI U accumulates higher reactivity.

Another advantage is that excess reactivity can be easily controlled.

FIG. 5 shows a third embodiment of the invention. In the fuel assembly of this example, fuel 5, which is a mixture of natural uranium and z33 U with an enrichment of 1.85w10, is placed in the upper 1/3 region, and uranium fuel with an enrichment of 3.7w10 is placed in the lower 2/3rd region. There are 36 fuel rods 3 loaded with 233U.
A fuel rod 32 loaded with fuel 6 having an enrichment of 3.7w10 and 36U is composed of six eggs.

Generally, the gap water outside the channel box 2 and the water in the water rod 4 are not boiling. Therefore, a state of good neutron moderation occurs locally around these non-boiling waters, and a peak in output tends to occur. Considering this point,
In this example, in order to flatten the power distribution within the assembly, fuel rods not containing z33 U were arranged around these gap waters and water rods. This makes it possible to improve the radial power distribution.

FIG. 6 shows an example in which the fuel assembly of the present invention is applied to the core 7 of a 1100 MWe class boiling water reactor.

As shown in this figure, an assembly of 764 bodies is divided into an inner core area 8 of 492 bodies, an outer peripheral area 9 of 180 bodies, and an outermost peripheral area 10 of 92 bodies. Then, the fuel assembly of the third embodiment is loaded. Generally, in the core of a nuclear reactor, the output tends to be lower in the outer region 9 of the core, and combustion tends to be delayed. Particularly in the upper part of the reactor core, neutron leakage and a hard neutron spectrum result in low output and delayed combustion, making it difficult to afford reactor shutdown. However, if these fuel assemblies are installed in the outer region 9, the change in reactivity during (high-temperature) operation and cold shutdown can be reduced, so that the margin for reactor shutdown can be improved.

〔Effect of the invention〕

According to the present invention, the difference in reactivity between operation and shutdown can be reduced, so the reactor shutdown margin can be increased.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view (a
) and a vertical cross-sectional view of the fuel rod (b), Figure 2 shows the relationship between neutron energy and regeneration rate η, and Figure 3 shows the change in the neutron infinite multiplication factor (koo) during operation and cold shutdown. figure,
FIG. 4 is a cross-sectional view (a) of a fuel assembly according to a second embodiment of the present invention and a vertical cross-sectional view of a fuel rod (b), and FIG. 5 is a fuel assembly according to a third embodiment of the present invention. FIG. 6 is a cross-sectional view of a reactor core loaded with a fuel assembly of the present invention. L, 101... Fuel assembly, 2... Channel box, 3, 31.32... Fuel rod, 4... Water rod,
Uranium fuel containing 5...233U, 6...23
3 Uranium fuel not containing U, 7...core, 8...
...Inner region of the core, 9...Outer region of the reactor core, 10.
...The outermost region of the core, 51, . 23zTh Muuran fuel containing 61, . Uranium fuel that does not contain 2327h.

Claims (1)

[Scope of Claims] 1. A fuel assembly configured such that a large number of fuel rods and water rods are surrounded by a channel box, through which a coolant flows along the longitudinal direction of the fuel rods, comprising: Divide the assembly into two regions downstream from the 1/2 point of the effective length of the fuel along the flow direction of the coolant, and calculate the total nuclear fission of ^2^3^3U included in the downstream region. 1. A fuel assembly characterized in that the weight ratio of the active substance to the upstream region is larger than the weight ratio of the upstream region. 2. In claim 1, the average value η of the number of newly generated neutrons when one neutron is absorbed at a neutron energy of 1 to 10^3 eV
The ratio of _R and η_T below 1 eV is η_R^2^3^ of ^2^3^5U in the above neutron energy range.
η_R/η_T> for the ratio of 5 and η_T^2^3^5
At least one type of fissile material that satisfies the relationship η_R^2^3^5/η_T^2^3^5^2^3^
A fuel assembly characterized by replacing 3U. 3. In a fuel assembly configured such that a large number of fuel rods and water rods are surrounded by a channel box, and a coolant flows through the channel box along the longitudinal direction of the fuel rods, It is divided into two regions downstream from the 1/2 point of the fuel effective length along the flow direction, and the weight ratio of ^2^3^2Th contained in the downstream region to the total fissile material contained in the region is determined. , a fuel assembly characterized in that the weight ratio is larger than that of the upstream region. 4. Divide the fuel loading area into two regions with different lengths,
A fuel rod characterized in that a weight ratio of ^2^3^3U contained in a short region to all fissile material contained in said region is larger than said weight ratio of other regions. 5. In claim 4, the ratio of the η_R at a neutron energy of 1 to 10^3 eV and the η_T at a neutron energy of 1 eV or less is ^2^3 in the energy range.
For the ratio of η_R^2^3^5 and η_T^2^3^5 of ^5U, η_R/η_T>η_R^2^3^5/η_T
A fuel rod satisfying the relationship ^2^3^5, characterized in that at least one type of fissile material is replaced with ^2^3^3U. 6. Divide the fuel loading area into two regions with different lengths,
A fuel rod characterized in that the weight ratio of ^2^3^2Th included in the short region to the total fissile material contained in the region is larger than the weight ratio of other regions. 7. Claims 4, 5, or 6, characterized in that fuel rods are arranged at locations other than the outermost periphery of the cross section of the assembly facing the channel box and the vicinity of the water rods. fuel assembly. 8.Claim 1, 2, 3 or 7
2. A nuclear reactor core according to paragraph 1, characterized in that the region excluding the outermost periphery of the reactor core is divided into two regions in the radial direction, and fuel assemblies are loaded in the outer region.