JPH01123194A - Reactor core of light-water reactor - Google Patents

Abstract

PURPOSE:To enable an emergency shut down of a nuclear reactor, by placing insertion zones properly in an area with a shallow subcriticality, by shifting an area newly becoming to be of a shallow subcriticality to an insertion side of a control rod and by arranging control rods in accordance with a new situation. CONSTITUTION:For a case of a boiling-water reactor for example, three stages insertion zones 21-23 are inserted into an upper half of a fuel assembly 20 (reactor core). And a control rod which is a control rod 24 consisting of a long life zone, a high reactivity zone and a conventional zone, lined side by side from its insertion top end to its bottom end, is inserted into the reactor core, therewith, an overall deep subcriticality can be attained. In this sort of a reactor core constitution, the subcriticality is shifted axially along a direction to get shallower keeping almost constant situation, with constitution of a high burn-up typed core by upgrading an enrichment of nuclear fuel and by using a MOX fuel. Consequently, an effective subcriticality curve without any loss against the core constitution can be attained. Moreover, a shallow reactivity zone before control rods insertion is shifted to the insertion side of the control rod, therefore, a shut-down speed of the reactor at its scram increases and also a safety of a shut down of the reactor can be improved.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a light water reactor core, in particular a reactor core with a long operating cycle and a high shutdown margin, which has an intervening region in the core in a direction perpendicular to its axis. The present invention relates to a light water reactor core having a control rod formed therein and having suitable control rods arranged in such a core.

(Prior Art) In the core of a boiling water nuclear reactor, cells each consisting of one cross-shaped control rod and four fuel assemblies surrounding it are normally arranged in a regular manner. In other words, the fuel assemblies and control rods are arranged so that their axes are perpendicular and parallel to each other, and the cooling water, which functions as a moderator, flows from the bottom of the core to the top. ing. No bubbles are generated from the effective lower end of the core to the vicinity of the lower end of the heat generating section, but a large amount of bubbles are generated from the center to the upper end of the core, and the generated bubbles flow upwards of the core. When the volume ratio occupied by bubbles, that is, the void ratio increases, the neutron moderation characteristics decrease, resulting in a decrease in thermal neutron flux and a decrease in output. In order to avoid this, measures have been taken such as increasing the fission nuclide concentration, that is, the enrichment level, in areas with a high void ratio, or adding burnable poison to suppress the increase in output in areas with a low void ratio.

Therefore, in a boiling water reactor, the combustion in the upper part of the core tends to be delayed, which causes the U-235m degree to be relatively higher than in other parts, and the voids generate fissile nuclides such as PU-239. Therefore, it is well known that the margin for reactor shutdown tends to be tight in the upper part of the core. Generally speaking, efforts are being made to lengthen the operating cycle and improve fuel burn-up, with the main purpose of improving economic efficiency. In this case as well, since the degree of contraction of the fuel is inevitably increased, the margin for shutting down the reactor becomes even tighter.

Next, they are a perspective view of a conventional fuel assembly used in a boiling water reactor and a fuel assembly expected to be used in the near future, and a schematic vertical cross-sectional view of a fuel rod constituting the fuel assembly.

In Figure 8 (Sad), the fuel assembly is a water rod (not shown).
and fuel rods 2 to the upper tie plate 4. Spacer 5. It is fixed by a lower tie plate 6, fuel pellets 8 are arranged in the jacket tube 7, a spring 9 is provided in the upper gas plenum, an upper end plug 10 is provided at the upper end, and a lower end plug 11 is located at the lower end.
has been established.

FIG. 9 is a cross-sectional view of the conventional fuel assembly shown in FIG. 8. Inside the channel box 1 are 62 fuel rods 2 and 2.
The water rods 3 are arranged to form a fuel assembly. The water rods 3 suppress the shortage of water, which is a moderator, inside the reactor assembly, but since these water rods 3 are uniform in the axial direction, there is a problem that there is an excess of water below the core and a lack of water above the core. There is a point.

The fuel assembly shown in FIG. 10 was developed to improve the characteristics of the fuel assembly, and one thick water rod 5 is arranged inside the assembly to introduce non-boiling water. . However, even in this example, there is a problem in that there is an excess of water below the core and a shortage of water above the core.

The fuel assembly shown in FIG. 11 is also an improvement of the fuel assembly shown in FIG. 9, and is provided with four small channel boxes 13, with boiling cooling water in the small channel boxes 13, and between the small channel boxes 13. A non-boiling moderator water region is provided in the cross-shaped gap 14 to flatten the horizontal power distribution, but this type of fuel assembly also suffers from excess water below the core and insufficient water above it. There is a problem.

The fuel assembly shown in FIG. 12 has been developed as an improved version of the fuel assembly shown in FIG. This fuel assembly is composed of nine subassemblies 1'5, and each subassembly 15 is composed of nine fuel rods 2. A rather wide gap 16 is provided between the subassemblies 15. In the case of this fuel assembly as well, the problem of excess and shortage of water in the upper and lower parts of the core has not been resolved.

(Problems to be Solved by the Invention) As mentioned above, although bubbles are not generated at the lowest end of the fuel assembly, which is the heat generating part of a boiling water reactor (8WR>),
Bubbles are generated anywhere else in the reactor, and the bubbles flow upward (downstream) from the reactor core. Therefore, the bubble ratio (void ratio) in BWR becomes higher above the core. As a result, the neutron moderation characteristics are reduced, resulting in a reduction in the rate of nuclear fission. In other words, combustion proceeds below the core,
There will be delays above the core. Therefore, it has been proposed to increase the concentration of fission nuclides above the reactor core in order to suppress the decrease in power above the reactor core.

However, increasing the void ratio and fission nuclide concentration above the core will reduce the degree of subcriticality at the top of the core when the reactor is shut down. - On the other hand, in order to extend the operating cycle and improve economic efficiency, it is necessary to further increase the enrichment of the fuel, but this will make the subcriticality in the upper part of the core even shallower. In the end, it may become impossible to shut down the reactor. In other words, this point has become a bottleneck, and conventional nuclear reactor cores have had the problem of not being able to extend the operating cycle.

The present invention was made to solve the above problems, and
The purpose is to make it possible to shut down the nuclear reactor even if the fuel enrichment level is increased, and by suitably arranging a new means that makes it possible to shut down the reactor, the margin for shutting down in the axial direction of the reactor core becomes tight. So-called stop area (shutdown zone)
To provide a core configuration for a light water reactor that can shift the control rods to the control rod insertion side, suitably configure the control rods accordingly, achieve longer life and higher reactivity value, and increase the speed of emergency reactor shutdown. It's about doing.

[Structure of the Invention (Means for Solving the Problems)] In order to achieve the above object, the present invention provides a structure in which the axes of a fuel assembly consisting of a large number of fuel rods are arranged regularly so that they are perpendicular and parallel to each other. In a boiling water reactor, the reactor core is located at
In the core of a light water reactor, where at least bubbles are generated downstream of the coolant flow, at a distance of more than a certain distance from the upper end of the heat generating part of the core, the volume ratio of the bubbles to the space through which the coolant passes increases. In areas where the local subcriticality of the reactor becomes shallow when the reactor is shut down, the concentration of fissile nuclides is substantially increased over a certain range in the horizontal direction of the reactor core. By providing at least one intervening region of 1 tJ that significantly reduces the subcriticality, and by arranging the intervening region suitably in a portion where the degree of subcriticality becomes shallow, the portion where the degree of subcriticality newly becomes shallow is shifted to the control rod insertion side. , the control rods are suitably configured in accordance with this, thereby simultaneously achieving a long life and high reactivity value of the control rods, and making it possible to speed up the emergency shutdown speed of the reactor. be.

(Function) As described above, according to the core configuration of the present invention, the neutron interaction (coupling effect) in the upper and lower fuel regions sandwiching the region with low concentration of fissile material (intervening region) is reduced, resulting in shutdown. The subcriticality of the reactor inside can be made larger (deeper), and unnecessary excessive reactivity during reactor operation can be suppressed.
At the end of the cycle, excess reactivity disappears and the binding effect improves, making it possible to extend the operating cycle, thereby maintaining the integrity of the fuel, and making the intervening region suitable for areas where the degree of subcriticality is shallow. By this arrangement, the part where the subcriticality becomes shallower is shifted to the control rod insertion side, and the control rod is appropriately configured accordingly, thereby simultaneously extending the life of the control rod and increasing the value of high reactivity. This can be achieved and the speed of emergency shutdown of nuclear reactors can be increased.

Next, the principle of operation of the intervening region introduced in the present invention will be explained.

The effective multiplication factor of the core is k. Let it be ff. (Here, it is expressed as k and "'eff" is omitted for simplicity.) Then, the modified first group model (which is a simple and reliable description regarding k of a light water reactor) can be expressed as follows.

Here, k■...Infinite multiplication factor M2...Neutron transfer area B2...Buckling (cm-2 unit) k■ is usually set at each point (or at a certain volume point ) is treated as the ratio of the neutron emission rate and neutron absorption rate due to nuclear fission.

M2 is expressed as M2 = τ+L2. τ is the Fermi age, which is called the moving area of fast neutrons in core design.

Note that τ=τF+τE (or τ1+τ2), and τ
In this way, even faster neutrons and epithermal (ep)
(thermal) neutron, but in the present invention there is almost no need to explain them separately.

L2 is the thermal neutron transfer area (given by the ratio of thermal neutron diffusion coefficient to absorption cross section>, M=IFi is the neutron transfer distance,
=, l'''U is the thermal neutron migration distance, and B2 is the buckling (in cm-2 units), 32, , = 3,2
It is expressed as +822. 3.2 is radial buckling
13Z2 is an axial buckling.

By the way, in terms of neutron reactor physics, the value of k is defined for the entire system in the vicinity of criticality, but in the present invention, since we are from the standpoint of core design that incorporates spatial dependence into KCD, the value of k is also defined using the above formula, We will treat it as incorporating spatial dependence.

In addition, in power reactors, M2 B2 is 0.03 to 0.0
5, B2 is 0.0001 to 0.0002 (Cm-
2>, the axial direction is usually flat, and the height of the core is l
, the axial reflector savings (sometimes referred to as the axial outer end distance) are δ - mi δ. If +δ- (meaning upper side lower side), then B
′=(π)2 2+δ.

(The sign was changed in the definition of reactivity to handle subcritical systems) ? The value of is defined for the entire system in the critical vicinity in neutron reactor physics, but in this invention, we take the position of core design that incorporates spatial dependence into ■, so (ko→→?)? Spatial dependence is also taken into account and treated as if it were a value.

Therefore, the degree of subcriticality in the present invention refers to a spatially dependent degree of subcriticality for the reasons described above. If there is a place in the core system where the degree of subcriticality is low (close to criticality), it becomes an indicator that that place is likely to become critical.

Next, the operation of the present invention will be explained with reference to FIG. As shown in FIG. 8 (8), it is assumed that there are two fuel zone structures, If, each having a rectangular cross section, and a water gap of width W exists between them. Water gap W and neutron multiplication factor k at this time
. The relationship of ff is as shown in the same figure (!y). The solid line shows the dependence of keff on the width of the water gap W in the cold state (the reactor generates almost no heat, such as when the reactor is shut down), and the dashed line shows the dependence of keff on the water gap W when the reactor is cold and the void ratio is high.
The dependence of ff on the width of the water gap is shown. For ease of comparison, the lines are standardized so that W=O and the lines on both sides match. Fuel assemblies are often designed in a state where the deceleration is slightly insufficient near the optimum deceleration, that is, where the critical mass m is at its minimum.
When the water gap W increases slightly from O, keff
The value may be almost constant or slightly (+), but if W is further increased, k. The ff value becomes smaller and smaller. That is, the neutron coupling effect in the fuel regions (1) and (II) becomes noticeably smaller. As a guideline, the thermal neutron diffusion distance (~2.5 Cm in water at 20°C) can be used as a reference.

Next, if the system is high temperature and has a high void fraction, for example, BW
In the upper part of the R core, the temperature is approximately 286°C (water density: 0.74>), and the void ratio may exceed 60%.

The effective density of water in this case is ~ compared to the case of 20℃
0.74X(1-0,6) = about 0.3. As can be easily understood from the definition formula, the diffusion distance of thermal neutrons is
is proportional to the reciprocal of the change in density (1→0.3). Therefore, in this case, the thermal neutron diffusion distance is about 8 cm.

When the water gap is as wide as this, the keff value will be slightly smaller than the maximum value even when the void ratio is high at high temperature. If it is about half that length, i.e. about 4 to 5 cm, keff
The value is maximum. The reason why the maximum value appears on the broken line is because there is a shortage of moderator in high temperature and high void conditions. In other words, the moderator is in an under-moderate state, and by separately introducing water, the moderator can be reduced.
ff can be increased. This state shows that the two fuel regions I and II are in the optimum state.

The present invention is a clever application of the characteristics described above. In other words, the region where there is almost no fuel roughly corresponds to the width of the water gap (W>), and when the reactor is not producing output, the keff of the reactor is clearly lowered due to the gap and the coupling region is weakened. , k at high temperature and high void.

This method utilizes a characteristic in which the ff value either increases or hardly decreases due to the introduction of a gap. Since water is introduced into areas that lack water during high-temperature, high-void conditions, neutron moderation characteristics are improved, thereby improving output.

Next, as shown in Figure 3, the reactor core is
A case will be explained in which there are two intervening layers. In the same figure, the subcriticality PI of each core piece is P2.5
When we calculate '3, we get the following.

P+=-L--1 to 1 B12=8 20 Bz12 B22 =3, 2 + 8722 B32=8 2+Bz32 Here, δi is the reflector saving that takes into account the effects of the upper and lower adjacent core pieces, (3, 2 is common to each core piece.As the temperature of the core increases, the temperature of the moderator increases and lipoids occur.Then, k1 changes slightly and decreases on average in a light water reactor), and Mi2 is increases, and due to the increase in δi, B
112 decreases.

Appropriate selection of the intervening layer thicknesses dl and dl significantly increases δi. Then, each core piece is integrally connected, and the axial buckling is integrally formed. On the other hand, when the core temperature decreases, a feature appears in which each core piece appears to be divided by an intervening layer. This is because when the core humidity decreases, the density of water increases, and the water in the intervening layer acts to divide and shield the core into upper and lower regions.

In the present invention, this intervening layer increases the ability to separate the core pieces into upper and lower parts when the reactor is cold (when the reactor is shut down), and weakens the separation function when the temperature is high (because the density of water decreases, it actually dl
, the effect of decreasing dl appears. This can be called a bonding effect. ) using characteristics. If the values of dl and dl are appropriately selected, the effect of compensating for the lack of water (moderator) at high temperatures will appear, resulting in a lower k than in the case where there is no intervening layer (dl and dl part also has fuel). It is even possible to increase ffl1II to some extent.

The thickness of the intervening layer is optimally set to be larger than the travel distance of thermal neutrons when the reactor is cold (when the reactor is shut down), and about the same or slightly smaller than that when the reactor is at high temperatures (furthermore, when voids occur in BWR). A specific preferred range is about 3 to ac. If it is less than 2 cm, the separation function will not occur when it is cold, and the
At 0 cm or more, the splitting function at high temperatures weakens, but the effective multiplication factor of the core decreases compared to when there is no intervening layer.
This is disadvantageous because it leads to a reduction in the driving cycle. In BWR, the thickness of the inclusions is preferably 3 to 8 cm, and in PWR, the thickness of the inclusions is preferably 3 to 5 cm. In BWR, when going from cold to hot, the density of water becomes 1/3, so the neutron travel distance triples, but in PWR, when going from cold to hot. However, the density of water decreases only to about 0.65, and therefore the distance traveled by neutrons only increases by a factor of two. One reason for the difference in the thickness of inclusions is that, as mentioned above, the density of the moderator in the vicinity of the intervening layer, the volume ratio of moderator to fuel, the concentration of fissile nuclides in the fuel, etc.
This is because the coupling effect is affected.

However, in the cold state when the intervening layer exerts a separation effect, B
712 rapidly increases, the number of each core piece decreases, and P, (
subcriticality) increases. During high-temperature operation when the intervening layer exerts a binding effect, the value of B712 decreases rapidly, and in a suitable state it becomes almost equal to the state without the intervening layer, and k rapidly increases (k
t can be approximately equal to or slightly larger than without the intervening layer).

Next, a specific calculation example will be shown in which the intervening layer exhibits a separation effect and a bonding effect.

In BWR, initial average enrichment is 3.7%, burnup is 28
It is assumed that the control rod is not partially inserted in the GWd/l system.

In such a WR, the degree of subcriticality is the smallest during reactor shutdown near the 1/4 length from the top of the core, so the intervening layer is placed horizontally at that part, covering the entire core, and the thickness of the intervening layer is Calculated by changing . There are two calculation systems: cold state (20°C) and high temperature operation (286°C, with void distribution), and for each system, the effective effect of the core when the thickness of the intervening layer is set to zero is calculated. It turns out that it multiplies.

(2) In the cold state, it suddenly decreases to ~5 cm (this is the range in which the separation effect increases significantly), and then approaches saturation. This asymptotic value is k of the core section (374 parts below the core in this example). ff value.

■ At high temperatures, the keff value hardly decreases due to the intervening layer up to ≦10Cm. This is due to the bonding effect, and it actually becomes worse around 3 to 6 cm. ff is increasing. This is because the intervening layer effectively replenishes the lack of thermal neutron flux due to lack of moderator during high-temperature voids.

(Example) Embodiments of the present invention will be described with reference to the drawings.

Figure 1 shows the case where the present invention is applied to a BWR.
A) is a schematic longitudinal sectional view of an embodiment of the present invention, (B) is an axial configuration diagram of a control rod used in the reactor core of the present invention, and (C) is a schematic longitudinal sectional view of an embodiment of the present invention.
is a diagram showing the subcriticality distribution during reactor shutdown.

FIG. 1A shows an example in which three stages of intervening regions 21, 22, and 23 are inserted into the upper half of the fuel assembly (core) 20.

As a result, the degree of subcriticality (reactor shutdown margin) is significantly improved as shown from curve A to curve A in the same figure (C). The peak of curve A (where the degree of subcriticality is particularly shallow) occurs around 2/31 to 576 cm from the bottom of the core (BAF), but due to the stop margin improvement effect of the intervening region, the shutdown margin (
(subcriticality) has been improved, and the peak has now shifted to near the center of the core height at the curve entrance, and the subcriticality is deep.

Then, a control rod 24 having a configuration as shown in FIG. 8 (8), that is, a control rod 24 consisting of a long-life region, a high reactivity region, and a conventional region from the insertion tip to the insertion end, is inserted into the reactor core. As a result, the curve C becomes almost flat, and a deep subcriticality is achieved overall. In such a core configuration, increasing the fuel enrichment) I
When a high burnup core is constructed using OX fuel, the subcriticality shifts to a shallow direction (curve C) while remaining approximately constant in the axial direction. Then, a lean and efficient subcriticality curve can be obtained corresponding to the core configuration (size of enrichment, etc.). In other words, it can be said to be a high burnup core that ensures subcriticality. (In a conventional BWR, if the enrichment level is increased while the subcriticality is represented by curve A, the peak of the subcriticality curve will shift upward in the upper half of the core and become critical, causing the reactor to shut down. In addition, since the shallow subcritical portion before control rod insertion is shifted to the control rod insertion side, the reactor shutdown speed during scram becomes faster and the safety of the core is improved.

The above has explained the case where the intervening region is arranged in a conventional BWR, but below, as another example, a pressurized water reactor PWR
I will briefly explain about.

A cluster control rod that collectively operates a plurality of rod-shaped neutron absorption rods (having a diameter of about the same size or twice the diameter of a fuel rod) is inserted from above the reactor core to below the reactor core. In this case, the intervening region is arranged mainly in the lower part of the core so as to be unevenly distributed, and the part where the shutdown margin becomes tight is shifted to the upper half of the core.

On the other hand, for the neutron absorption rod, a long-R life absorbing material is placed on the insertion tip side (lower side), and a high-reactivity value absorbing material (<106 yB reduced B4 C pellets, etc.) is placed upward from the center. This configuration constitutes the core of the present invention.

Other reactors in which control rods are inserted from above include small and medium-sized reactors, natural circulation reactors, and high conversion reactors. These furnace types are still in the research and development stage, but when the present invention is applied, they will have a similar configuration to the above-mentioned PWR.

Next, using Figures 4 and 5, we will explain how the degree of subcriticality is in the axial direction of the reactor core during shutdown, and how it can be changed by implementing the present invention. As a result, we will explain what kind of control rod is suitable and how good results can be expected as a result.

FIG. 4 shows the subcriticality distribution in the axial direction of the core of the present invention and the conventional core.

The same figure (A) shows the case of a conventional core, in which the degree of subcriticality P becomes particularly shallow in the range C from 4/6 to 5/6H from the effective lower end of the core. In other words, near the effective lower end of the core, it becomes subcritical due to neutron leakage? is deep. In the subsequent partial opening, the void ratio is low, so the Pu generation resistance is low, and combustion progresses, so U
235 remaining is low and subcritical? is deep. In the range C from 4/6 to 5/6 [1 from the next core effective lower end, void vj
Since the ratio is high, there is a lot of Pu produced, and the combustion is delayed, so there is a lot of U-235 remaining, and there is little leakage, so is it subcritical? is shallow. Is the core near the effective lower end 2 subcritical due to neutron leakage? deepens.

Figure (B) shows the first stage intervening region 3/3/3 from the effective lower end of the core.
This is a case where it is placed slightly above dN. Subcriticality as shown in the figure? is significantly improved (increased) by the intervening region. However, in this example, the downward shift of the portion where the subcriticality P becomes shallow remains small.

Figure (C) shows a case where two stages of intervening regions are arranged as shown. Subcriticality in the upper part of the core active part? has increased (deepened) considerably compared to (8) in the same figure. And subcriticality? The shallower part has shifted to almost the center of the reactor core.

Figure 5 corresponds to Figure 4 (C) and shows the control rod reactivity value when the core is configured so that the subcriticality is almost constant when viewed in the axial direction. . It is a distribution map of g.

In conventional BWR, the upper part of the core, especially 3/4 from the bottom end]1~
The concept of retaining a large reactivity value in 4/4H effectively increases the control rod reactivity value and, as a result, effectively improves the reactor shutdown margin.

In this way, in the reactor of the present invention, conventional thinking no longer applies, and the control rod reactivity value is at its maximum at approximately the center of the control rod (corresponding to the center height of the reactor core) when fully inserted. The solid line in FIG. 5 indicates that it is preferable that This is achieved by separating the functions of the control rods. In other words, the tip has the longest life and the second highest reactivity. Reactivity should be prioritized in the center, and the reactivity should be appropriately large in the lower part. When a control rod with such a configuration is adopted, the safety of the reactor is improved because emergency shutdown of the reactor is quick.

q Therefore, the BWR control salvage, which is inserted upward from the lower end of the core in accordance with FIG. 5, has the following configuration.

In other words, the total length of the core is divided into approximately three equal parts, and the length from the bottom to 173 is 84C (denoted as N84G) using natural boron.
, 173~2/3 long part is 84C (
(denoted as "84 C"), the 273 to 3/3 long portion is made of a long-life type material such as Hf.

In other words, the control rod insertion tip (174 to 1/3 of the total length from the tip) receives a high amount of neutron irradiation even in the present invention, and is no different from the conventional control rod.

Therefore, it is better to use the most suitable Hf. If Hf is suitably used, a control rod can have approximately the same reactivity value as a conventional control rod (using N84 G), yet its life can be significantly increased.

In the 173 to 2/3 long part of the total length from the tip (the part using 1084G), the amount of neutron irradiation is significantly lower (for example, 273 or less) than the insertion tip, so there is almost no demand for longer life. The most important thing is that the reactivity value is large. It can be said that there is no material with higher reactivity value than boron-10 (''B) as a neutron absorber, and therefore 1084 C using 10B is preferred.

At the end of insertion, that is, at the bottom of the core during full insertion (17
3 length), the neutron irradiation dose is low and a high reactivity value is not required, so the conventionally used N84C can be used (it can also be said that the control rod is functionally separated in the axial direction).

As explained above, in the core configuration of the present invention, the portion where the degree of subcriticality becomes shallow is shifted from the upper part of the core to the center, that is, toward the control rod insertion side. Therefore, if we assume that a control rod is inserted in an emergency during a forced operation, the control rod will reach the area where the degree of subcriticality is shallow faster than before, making it possible to make an emergency shutdown of the reactor sooner. . In other words, the safety of the nuclear reactor is improved.

FIG. 6 shows a comparison between a control rod used in a conventional BWR and a control rod according to the present invention.

That is, Figure (A) is a perspective view of a conventional control rod, Figure (A) is a perspective view of a conventional control rod;
The left half of Figure B) is a cross-sectional view of the conventional control rod shown in Figure (A), and the right half of Figure B is a cross-sectional view of the control rod according to the present invention.

In FIG. 6, an SUS sheath 31 formed in a deep single character shape is fixed to a cross-shaped tie rod 32 at its opening, thereby forming a cross-shaped control rod 30.

The upper end (insertion light @) is a tip structure member 33 with a handle, and the lower end is a terminal structure member 34 integrated with a speed limiter.
It becomes. Inside the sheath 31 is a rod-shaped neutron absorbing material 35 made of a SUS tube filled with 84C powder (granules).
are arranged parallel to the control rod axis (US system)
. In addition, in the wing part, instead of arranging the absorbent material inside the sheath, a hole of about 6M is formed in a SuS plate with a thickness of about 8 mm from the outer edge of the wing in a direction perpendicular to the insertion/extraction direction toward the control rod center axis. A method of filling and sealing 84C powder in it has been developed in Sweden. Since this method can fill more 840 than the US method control rod, the reactivity value can be increased by about 10% (if the external shape is the same). This 10% is the reactor shutdown margin (usually 1
~3% Δ/k) to approximately 1% (thus 2-4% Δ/k)
).

Since it uses 84 C, it is not suitable for significantly extending the life of the American method, which is largely the same as the American method.

FIG. 6C shows two examples of conventional control rods divided into a left half and a right half. That is, the left half is uniform in the axial direction and is a typical example of a conventional control rod. The reactivity value is uniform as shown by the solid line in Figure (D). On the other hand, the right half shows a long-life control rod that the present inventors recently developed for conventional BWRs, and as shown by the dotted line in Figure CD), it extends from the effective part tip (core insertion tip) to the end. The design is such that the reactivity value decreases in four stages 36, 37, 38, and 39. A t-b plate is used as a neutron absorbing material, and water is introduced between two Hf plates to reduce the amount of dense Hf material to suppress a decrease in reactivity value. Conventional BWR
In this case, it was necessary to increase the reactivity value of the tip and to extend the service life, so such a configuration was extremely suitable (see Japanese Patent Application No. 78746/1983). .

By the way, in the nuclear reactor of the present invention, as a result of introducing the intervening region into the reactor, it is no longer necessary to greatly increase the reactivity value of the insertion tip. However, in a BWR, the control rods are inserted and removed from below the reactor core, and their tips are exposed to a large number of neutrons, which is the same as in conventional BWRs. This unique characteristic is BW
The same applies not only to R but also to PWR and other furnace types.

FIG. 7 shows the control rod characteristics necessary for the nuclear reactor of the present invention and an example that embodies them. That is, Fig. 7 (A) is a characteristic curve diagram necessary for the control rod corresponding to Fig. 5, and the same figure (B
) is a characteristic curve diagram of the control rod according to the present invention implemented as a discrete control rod based on FIG. It is illustrated with a left half and a right half. One example is shown in the same figure (
As shown in the left half of Figure C), the control rod 40 is divided into three equal parts in the axial direction. In other words, the end of the core insertion is a normal area 43 (natural B, 84G side hole filled), the central height area is a high reactivity value area 42 (10B enriched 84G side hole filled *>), and the core insertion tip is irradiated with high neutrons. Area 41 (1
-1f plate, trap shape). Note that a vertical gap 44, 45 is provided in the center of the core insertion end portion and the core insertion tip portion. In another embodiment, as shown in the right half of the figure (C), the control rod 40 is divided into three equal parts in the axial direction. In other words, the core insertion end part has a normal area 46 (an SO3 pipe filled with B4C powder and arranged in a sheath, which corresponds to the conventional type that has been used for a long time), and a center height. The part is high reactivity value area 42 (drill a horizontal hole in the SUS board and fill it with 84 G powder)
The core insertion end portion is composed of a high neutron irradiation region 41 (the Hf plate is shaped like a trap and inserted into the SUS sheath). In this example, a vertical cavity 44 is provided only in the center of the core insertion end portion.

The present inventors have been conducting research and development on control rods for BWRs for a long time, and found that the method developed in Sweden, in which a horizontal hole is made in a SuS plate and filled with B4C, has the highest reaction rate when the external shape is the same. It is suitable for realizing the degree value area. Various experimental studies have been conducted on neutron absorbing materials, and 84 C concentrated with 10s, Etl, and B6 are suitable for increasing the reactivity. Basically, B4C powder is used as the neutron absorbing material, and if necessary, B4C enriched with 1OB is used.

Since the neutron irradiation amount is high at about 11'3 of the tip of the insertion,
A Nagano umbrella-shaped neutron absorber is used. Although Hf, EtL, and other materials can be considered as the Nagano umbrella type absorbent, t-b is the most practical. 1-ff is a metal, has extremely excellent neutron irradiation characteristics and corrosion characteristics, and can be used without a covering material. The disadvantage is that the density is large and the reactivity value tends to decrease.

By using the Hv plate trap type recently developed by the present inventors (Japanese Patent Application No. 61-78746), that is, the method of introducing water between two Hf plates, a long-life control rod that eliminates the above drawbacks can be obtained. realizable. In this embodiment, from this point of view, the Hf plate is configured in a trap shape.

Roughly speaking, in the present invention, the reactivity value can be made slightly lower, so f-b is reduced near the tie rods (because the reactor shutdown margin is improved by the intervening region, the share of the shutdown margin that should be secured by the control rods is reduced). It is removed and left as a void. This alleviates the significant decrease in neutron flux near the tie rod, and as a result, the significant decrease in fuel output corresponding to this portion is alleviated. As a result, even when the control rod is inserted, combustion of the fuel in this portion proceeds, suppressing an abnormal increase in output when the control rod is inserted, and contributing to improving the soundness of the fuel.

Note that the present inventors have already disclosed many other specific plans as measures for extending the lifespan. The length of the insert end is about 173 illustrative of two embodiments.

The left half is a horizontal hole type. Since the reactivity value can be lowered in this part, the side of the tie rod is made into a void 44 (→ water).
An example is shown in which the lateral width of the absorbent filling part is reduced.

This water improves fuel integrity as described above.

The right half shows a conventional method in which absorbent material made by filling a SUS tube with B4C is arranged inside the sheath. In this example, no voids (i-water) are provided on the side surfaces of the tie rod because the reactivity value tends to be smaller than in the horizontal hole method.

In the present invention, we have explained a new core configuration of a light water reactor in which the part where the subcriticality becomes shallow is shifted to the control rod insertion side and the control rod configuration is optimized accordingly. Of course, it can also be applied.

[Effects of the Invention] As explained above, according to the present invention, the following effects are achieved.

(1) By introducing the intervening region, shutdown margin is secured even in a core using highly enriched fuel (including MOX).

(2) By suitably arranging the intervening region, the region (shutdown zone) where the shutdown margin becomes tight can be shifted, thereby shortening the time until the reactor shutdown during scram. In other words, the furnace can be shut down more safely.

(3) A control rod suitable for the above (2) in which functions are separated in the axial direction can be provided. In other words, the tip of the insertion site receives maximum irradiation, so a Nagano mortar is used, the middle part of the insertion site is made of highly reactive absorbent material, so it has a high stopping margin in cooperation with the intervening area, and the distal end of the insertion site is less important, so a conventional type is used. are doing.

[Brief explanation of the drawing]

Figure 1 shows the case where the present invention is applied to a BWR.
8) is a schematic vertical cross-sectional view of one embodiment of the present invention, and (B) is an axial groove acid (A) to (C) of a control rod used in the reactor core of the present invention.
is a diagram showing the subcriticality distribution in the axial direction of the core of the present invention and the conventional core, and FIG. 5 corresponds to FIG. 4(C), and the subcriticality of the core is approximately FIG. 6(8) is a perspective view of a conventional control rod, and FIG. 6(B) is a cross-sectional view of FIG. 6(A). The left half of the figure (C) is a cross-sectional view of a conventional control rod;
The right half of C) is a cross-sectional view of another conventional control rod. The same figure ([)) is a diagram showing the reactivity value of the same figure (C), Figure 7 (A) is a characteristic curve diagram required for the control rod corresponding to Figure 5, and the same figure (B) is the same figure. Figure (A) is a characteristic curve diagram of the control rod according to the present invention implemented as a discrete The left and right half views, FIG. 8(A) and FIG. 8(B) are respectively a perspective view of a conventional fuel assembly and a schematic longitudinal sectional view of a fuel rod constituting the fuel assembly. FIG. 9 is a cross-sectional view of the conventional fuel assembly shown in FIG. 8, and FIGS. 10 to H are all cross-sectional views of the conventional fuel assembly. 20...Fuel assembly 21, 22.23...Intervening region 24, 30.40...Control rod 31...Sheath 32...Tie rod 33...Tip structure material 34...Terminal structure Material 35...Neutron absorbing material (8733) Agent Patent attorney Yoshiaki Inomata (Idiot)
1 person) (A) No. 4 (C) Fig. 1 Gate (A An Sugimatsubu False Q7 (v,p) CB) Fig. 2 (A) (B) (C) (D) Fig. 6 (A) CB)
(Fig. 7 Fig. 9 f() Cause 1! Fig. 12

Claims (8)

[Claims] (1) A fuel assembly consisting of a large number of fuel rods bundled together in an orderly manner,
A reactor core arranged such that its axes are perpendicular and parallel to each other, and in which control rods can be inserted and removed inside the fuel assemblies or in gaps between adjacent fuel assemblies, and between each of the fuel rods. In a light water reactor core configured such that light water, which is a coolant, flows from below to above the fuel rods, a portion or the entire core is vertically divided into at least two core segments with a predetermined width. The width of the intervening region is within the thermal neutron diffusion distance during power operation of the light water reactor or twice that distance, and the width of the intervening region is within the thermal neutron diffusion distance during light water reactor power operation, or the thermal neutron diffusion distance during normal temperature shutdown of the light water reactor. A reactor core of a light water reactor characterized in that a portion where the degree of subcriticality becomes shallow during cold shutdown is shifted to the control rod insertion/extraction side by increasing the neutron diffusion distance or longer. (2) The boiling water reactor is a boiling water reactor in which a portion where the degree of subcriticality becomes shallow during normal temperature shutdown is shifted toward the bottom of the core, and control rods are inserted and removed from below the core. core of a light water reactor. (3) The pressurized water reactor is a pressurized water reactor in which a portion where the degree of subcriticality becomes shallow during normal temperature shutdown is moved upwards of the core, and control rods are inserted and removed from above the core. The core of a light water reactor. (4) This is a small- to medium-sized nuclear reactor, a natural circulation reactor, or a high conversion reactor, in which the part where the degree of subcriticality becomes shallow during cold shutdown moves toward the upper part of the core, and control rods are inserted and removed from above the core. A core of a light water reactor according to claim 1, characterized in that: (5) The control rods when inserted into the reactor core are characterized by having a strong neutron absorption structure with strong neutron absorption properties in the shallow subcriticality part, and an absorption structure with slightly smaller neutron absorption properties in other parts. A core of a light water reactor according to claim 1. (6) Strong neutron absorption structure with strong neutron absorption properties is SUS
The core of a light water reactor according to claim 5, characterized in that a horizontal hole is formed in the plate and filled with B_4C powder. (7) The core of the light water reactor according to claim 5 or 6, characterized in that the strong neutron absorbing material with strong neutron absorption properties is mainly enriched boron with an increased concentration of boron 10. . (8) The control rods for controlling the output of the reactor are:
6. The core of a light water reactor according to claim 5, wherein at least the tip portion thereof is made of a long-life neutron absorbing material such as hafnium.