JPH0631762B2 - Light water reactor core - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention has a core of a light water reactor, in particular, a long operation cycle, and has an intervening region in the core in a direction perpendicular to its axis as a high stop margin type core. The present invention relates to a core of a light water reactor in which such a core is formed and suitable control rods are arranged.

(Prior Art) In the core of a boiling water nuclear reactor, cells composed of one cross-shaped control rod and four fuel assemblies surrounding the cross control rod are regularly arranged. That is, the fuel assemblies and the control rods are arranged so that their axes are vertical and parallel to each other, and the cooling water having a function as a moderator is configured to flow upward from below the core. . No bubbles are generated near the lower end of the effective core, that is, the lower end of the heat generating part, but a large amount of bubbles are generated from the center to the upper end of the core, and the generated bubbles flow above the core. When the volume ratio of bubbles, that is, the void ratio, increases, the moderating characteristics of neutrons deteriorate, so the thermal neutron flux decreases, and the output decreases. In order to avoid this, it has been dealt with by increasing the fission nuclide concentration, that is, the enrichment at the site with a high void ratio, or by adding a combustible poison to suppress the output increase at the site with a low void ratio.

Therefore, in a boiling water reactor, combustion in the upper part of the core is likely to be delayed, which causes the U-235 concentration to be relatively higher than other parts, and voids generate fissile nuclides such as Pu-239. It is well known that the reactor shutdown margin tends to be severe in the upper part of the core. Furthermore, efforts are being made to prolong the operating cycle and improve the burnup of fuel, mainly for the purpose of improving economic efficiency. In this case as well, the fuel enrichment is inevitably increased, and the shutdown margin of the reactor becomes even more severe.

Next, typical examples of the conventional fuel assemblies used in the boiling water reactor and the fuel assemblies expected to be used in the near future will be described with reference to the drawings.

8A and FIG. 8B are a perspective view of a conventional fuel assembly and a schematic vertical cross-sectional view of a fuel rod forming the fuel assembly, respectively.

In FIG. 8 (A), in the fuel assembly, a water rod (not shown) and a fuel rod 2 are fixed by an upper tie plate 4, a spacer 5 and a lower tie plate 6, and the outside thereof is surrounded by a channel box 1. Is configured. Fuel rod 2 is the same figure
As shown in (B), a fuel pellet 8 is provided in the cladding tube 7, a spring 9 is provided at the upper part of the gas plenum, an upper end plug 10 is provided at the upper end, and a lower end plug 11 is provided at the lower end.

FIG. 9 is a cross-sectional view of the conventional fuel assembly shown in FIG. In the channel box 1, 62 fuel rods 2 and 2 water rods 3 are arranged to form a fuel assembly. The water rod 3 suppresses a shortage of water as a moderator inside the assembly. However, since the water rod 3 is uniform in the axial direction, there is an excess of water below the core and a shortage of water above. 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 water is excessive below the core and water is insufficient above the core.

The fuel assembly shown in FIG. 11 is also an improvement of the fuel assembly shown in FIG. 9, and four small channel boxes 13 are provided. Boiling cooling water is provided in the small channel boxes 13 and also between the small channel boxes 13. By making the non-boiling moderator water region in the cross-shaped gap 14, the horizontal power distribution is flattened, but this type of fuel assembly also has excess water below the core and insufficient water above. There is a problem.

The fuel assembly shown in FIG. 12 was developed as an improved version of the fuel assembly shown in FIG. This fuel assembly is composed of nine subassemblies 15, and each subassembly 15 is composed of nine fuel rods 2. A relatively wide gap 16 is provided between the subassemblies 15. Even in the case of this fuel assembly, the problem of excess and deficiency of water above and below the core has not been solved.

(Problems to be Solved by the Invention) As described above, although no bubbles are generated at the lowermost end of the fuel assembly which is the heat generating part of the boiling water reactor (BWR),
In other parts, they are generated everywhere, and the generated bubbles flow upward (downstream) of the core. Therefore, the bubble ratio (void ratio) of the BWR becomes higher toward the upper part of the core. As a result, the neutron moderating property deteriorates, and the fission rate decreases. That is, combustion proceeds below the core,
It will be delayed above the core. Therefore, in order to suppress the decrease in power above the core, it has been proposed to increase the fission nuclide concentration above the core.

However, increasing the void fraction and increasing the fission nuclide concentration above the core will reduce the subcriticality above the core when the reactor is shut down. On the other hand, in order to prolong the operation cycle and improve economic efficiency, it is necessary to further increase the fuel enrichment, but this means that the subcriticality in the upper part of the core will become shallower and eventually It is possible that the reactor cannot be shut down. That is, this point becomes a bottleneck, and there is a problem that the operation cycle cannot be extended in the conventional reactor core.

The present invention has been made to solve the above problems,
The purpose is to enable the reactor shutdown even if the fuel enrichment is increased, and by suitably arranging a new means for enabling the reactor shutdown, the shutdown margin in the direction of the core axis becomes severe, so-called Stop area (shutdown zone)
To the control rod insertion side, and correspondingly configure the control rod to achieve a long life and high reactivity value, and to create a core structure of a light water reactor that can accelerate the reactor emergency shutdown speed. To provide.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a fuel assembly in which a large number of combustion rods are regularly bundled so that their axes are vertical and parallel to each other. And a core in which the control rods can be inserted and removed in the interior of the fuel assembly or in a gap between adjacent fuel assemblies, and between the fuel rods, the fuel rods extend from the bottom to the top of the fuel rods. In a core of a light water reactor configured so that light water as a coolant flows, a part or the whole of the core is vertically divided into at least two core sections, and the subcriticality is reduced and the fissile nuclide concentration is reduced. The intervening region is arranged at a position significantly lower than the fuel above and below, and the width of the intervening region is larger than the moving distance of thermal neutrons when the light water reactor is shut down and smaller than the moving distance of thermal neutrons during light water reactor power operation. Then, the part where the subcriticality becomes shallow during normal temperature stop is moved to the control rod insertion / extraction side, and the control rod has its function formed in three regions in the axial direction, and the neutron absorption characteristics in the middle region are higher than those in the lower region. However, the neutron absorption lifetime of the upper region is longer than that of the lower region.

(Operation) As described above, according to the core configuration of the present invention, the neutron interaction (coupling effect) in the upper and lower fuel regions with the region of low fissile material concentration (intervening region) interposed therebetween is reduced, and as a result, the fuel cell is stopped. The subcriticality of the reactor can be made larger (deeper), and unnecessary excess reactivity during reactor operation is suppressed,
The binding effect when the excess reactivity disappears at the end of the cycle is improved, and as a result, the excess reactivity is restored and the operating cycle can be extended, and the fuel integrity is maintained, and the intervening region is By suitably arranging the part where the subcriticality becomes shallower, the part where the subcriticality becomes shallower is newly shifted to the control rod insertion side, and the control rod is suitably configured correspondingly, whereby the control rod It is possible to achieve longer life and higher reactivity value at the same time, and to speed up the emergency shutdown speed of the reactor.

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

The effective multiplication factor of the core is K _eff . (Here, it is represented by K for simplification, and “eff” is omitted.) Then, the modified first group model (simple and reliable as a description of K of a light water reactor) can be represented as follows.

Here, K ∞ ... infinite multiplication factor M ² neutron moving area B ²・ ・ ・ Buckling (cm ⁻² unit) K ∞ is usually due to nuclear fission at each point (or constant volume point) in the core design. It is treated as the ratio of the neutron emission rate to the neutron absorption rate.

M ² is represented by M ² = τ + L ² . τ is the Fermi age, which is called the moving area of fast neutrons in core design.

It should be noted that τ = τ _F + τ _E (or τ ₁ + τ ₂ ), and τ may be further divided into fast neutrons and epithermal neutrons as described above, but in the present invention, it is divided up to this point. There is little need to explain. L ² is a thermal neutron migration area (given by the ratio of thermal neutron diffusion coefficient and absorption cross section), Is the neutron travel distance, Is a thermal neutron transfer distance, and B ² is a buckling (cm ⁻² unit), which can be represented by B ² = B _r ² + B _Z ² . _Br ²
Is a radial buckling and B _Z ² is an axial buckling.

By the way, the value of K is defined as the whole system in the vicinity of criticality as neutron reactor physics, but in the present invention, since it is in the position of core design that incorporates spatial dependence in K∞, the above K value is also used. , I will treat it as one that incorporates spatial dependence.

In a power reactor, M ² B ² is about 0.03 to 0.05,
² is about 0.0001 to 0.0002 (cm ^-2 ), the axial direction is usually a flat plate, the core height is Z, and the axial reflector saving (sometimes called the axial outer end distance) is δ≡δ ₊ + δ ₋ (Upper side + lower side) Given in.

(The sign is changed in the definition of reactivity to handle the subcritical system) The value of ρ is defined for the entire system in the vicinity of criticality in neutron reactor physics, but in the present invention, the spatial dependence is incorporated into K∞. Since we are in the position of the core design, we will treat it as if (K∞ → K → ρ) ρ value also incorporates spatial dependence.

Therefore, the subcriticality in the present invention discusses the space-dependent subcriticality for the above reason. If there is such a place in the core system where the degree of subcriticality is small (close to criticality), it is an index that indicates that it tends to become critical.

Next, the operation of the present invention will be described with reference to FIG. Same figure
As shown in (A), there are two fuel regions I and II having a rectangular cross section, and a water gap having a width W exists between them. Water gap W and neutron multiplication factor K _{eff at} this time
The relationship is as shown in FIG. The solid line shows the dependence of K _eff on the width of the water gap W in the cold state (the state in which the reactor hardly generates heat like when the reactor is shut down), and the broken line shows K when the void ratio is high at high temperature. _It shows the dependence of _eff on the size of the water gap. For easy comparison, standardization is performed so that both curves match when W = O. Since the fuel assembly is often designed to be slightly decelerated in the vicinity of the optimum deceleration, that is, the critical mass is minimized, the water gap W becomes O.
The value of K _eff may be almost constant or may be (+) when the value becomes slightly larger than K _eff.
The value becomes smaller and smaller. That is, the fuel region (I)
The neutron coupling effect of (II) becomes noticeably smaller. The thermal neutron diffusion distance (~ 2.5cm in water at 20 ° C) can be used as a guideline.

Second, if the system is hot and has a high void fraction, for example BW
At the upper part of the R core, the void ratio may exceed 60% at about 286 ° C (water density of about 0.74). In this case, the effective water density is about 0.74 x (1-0.6) = 0.3, compared to the case of 20 ° C. The diffusion distance of thermal neutrons is almost proportional to the reciprocal of the density change (1 → 0.3), as can be easily understood from the definition formula. Therefore, in this case, the thermal neutron diffusion distance is about 8 cm. When the water gap is wide like this, the K _eff value becomes slightly smaller than the maximum value even at high temperature and a high void ratio. The K _eff value becomes maximum when it is about half, that is, about 4 to 5 cm. The maximum value appears on the broken line because the moderator is insufficient in the high temperature and high void state. That is, the moderator is in a shortage state (under-moderate state),
By introducing water separately, K _eff can be increased. This state indicates that the two fuel regions I and II are in the optimum state.

The present invention skillfully applies the above characteristics. That is, the region where fuel is scarcely present corresponds to the width (W) of the water gap, and the Keff of the reactor is obviously reduced by the gap when the reactor is not producing power (state where the coupling region is weakened). ), At high temperature and high void, the K _eff value is increased by the introduction of the gap, or the characteristic is in the range where it hardly decreases. Then, since water is introduced into the portion where water was insufficient at the time of high temperature and high void, the neutron moderation characteristic is improved, and thereby the output is improved.

Next, as shown in FIG. 3 (A), the case where the core is divided into three in the axial direction and two intervening layers are present will be described. In the figure, the subcritical degrees ρ ₁ , ρ ₂ , ρ ₃ of each core piece are obtained as follows.

Here, δ _i is the reflector saving that incorporates the influence of the upper and lower adjacent core pieces, and B _r ² is common to each core piece. When the core temperature rises, the moderator temperature rises and voids occur.
Then, K∞i is (decrease in average in light water reactors) slightly changed, Mi ² increases, _B due to an increase in [delta] _i Z i
² decreases. Proper selection of the thicknesses d ₁ and d ₂ of the intervening layer significantly increases δ _i . And each core piece is integrally connected, the axial buckling is integrated, Becomes On the contrary, when the core temperature drops, the feature that each core piece is divided by the intervening layer appears. This is because the water density increases as the core temperature decreases, and the water in the intervening layer appears to have a function of vertically shielding the core.

According to the present invention, the intervening layer increases the function of separating the core pieces into upper and lower parts when the light water reactor is stopped, and weakens the function of separating the core pieces during the light water reactor power output operation (since the density of water decreases, d ₁ , d _The effect that ₂ becomes smaller appears. This can be said to be the coupling effect.) The property is used. If the values of d ₁ and d ₂ are appropriately selected, an effect of compensating for the effect of water (moderator) shortage during light water reactor power operation is also exhibited, and there is no intervening layer (d ₁ ,
It is even possible to increase the Keff value a little more than in the case where the d ₂ part also has fuel).

It is optimal that the thickness of the intervening layer is larger than the moving distance of thermal neutrons when the light water reactor is shut down, and is about the same as or slightly smaller than that during the light water reactor power operation (more voids occur in BWR). A preferable range as a concrete value is about 3 to 8 cm. If it is less than 2 cm, the separation function does not occur when it is cold.
If it is more than cm, the dividing function during light water reactor power operation is weakened, but it is disadvantageous because the effective multiplication factor of the core is reduced and the operation cycle is reduced compared to when there is no intervening layer. B
For WR, the thickness of inclusions is preferably 3-8 cm.
Then, the thickness of the inclusions is preferably 3 to 5 cm. That is BW
In R, the water density becomes 1/3 when the light water reactor is stopped (cold state) to when the light water reactor is in output operation (high temperature), so the neutron migration distance is tripled, but in PWR the pressurized reactor is stopped (cold state). ) Et al., The density of water decreases only to about 0.65 even when the pressure furnace is operated at high temperature (high temperature), and therefore the neutron migration distance increases only within twice. One of the reasons why the inclusions have different thicknesses is that they are separated or separated according to the moderator density near the presence of the intervening layer, the moderator to fuel volume ratio, the fissionable nuclide concentration in the fuel, etc., as described above. This is because the coupling effect is affected.

Then, when the light water reactor in which the intervening layer exerts the separation effect is stopped, B _zi ² increases rapidly, so that K _{i of} each core piece decreases,
ρ _i (subcriticality) increases. During light water reactor operation in which the intervening layer exerts a coupling effect, the value of B _zi ² decreases sharply, and in a suitable state, it is almost equal to the state without an intervening layer, and K _i rapidly increases (K _i is the same as when there is no intervening layer). Can be approximately equal or slightly larger).

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

In BWR, initial average enrichment 3.7%, burnup 28 GWd / t
In this system, the control rod is not partially inserted.

In such a BWR, the subcriticality becomes the smallest during the reactor shutdown at about 1/4 length from the upper end of the core, so the intervening layer is placed horizontally across the entire core at that part and the thickness of the intervening layer is Was calculated by changing. There are two types of calculation system, one is when the light water reactor is stopped (20 ℃) and the other is at the light water reactor output operation (286 ℃, with void distribution).
The effective multiplication factor K _eff of the core when the thickness of the intervening layer was set to zero was used as a reference for both systems. An example of this calculation is shown in the graph of FIG. The graph shows the following.

When the light water reactor is stopped, it suddenly decreases to ~ 5 cm (this is the range in which the separation effect remarkably increases), and then approaches saturation. The asymptotic value is a K _eff value of (core lower 3/4 parts in this example) the core sections.

During light water reactor power operation, the _Keff value hardly decreases due to the intervening layer up to _≤10 cm. This is due to the bonding result, and K _eff rather increases in the vicinity of 3 to 6 cm. This is because the effect of replenishing the thermal neutron flux shortage due to the moderator shortage during the high temperature void from the intervening layer is effective.

(Example) The Example of this invention is described with reference to drawings.

FIG. 1 shows the case where the present invention is applied to BWR.
(A) is a schematic vertical cross-sectional view of an embodiment of the present invention, (B) is an axial configuration diagram of a control rod used in the core of the present invention, the same figure (C) is a subcritical distribution during reactor shutdown It is the figure shown.

FIG. 1 (A) shows an example in which three stages of intervening regions 21, 22, 23 are inserted in the upper half of the fuel assembly (core) 20. As a result, the subcriticality (furnace shutdown margin) is greatly improved from curve A to curve B in Figure (C). The peak of curve a (the subcriticality is particularly shallow) occurs around 2 / 3H to 5 / 6H from the core bottom end (BAF), but due to the effect of improving the stop margin in the intervening region, the stop margin is greatly increased like curve b. (Subcriticality) has been improved, and the peak has already shifted to the center of the core height in the curve B,
Moreover, the subcriticality is deep.

Then, a control rod having a structure as shown in FIG. 3B, that is, a control rod 24 composed of a long-life type region, a high reactivity type region, and a conventional type region is inserted into the core from the insertion tip to the insertion end. As a result, it becomes almost flat as shown by curve C, so that a deep subcriticality is achieved overall. In such a core configuration, if the fuel enrichment is increased or a high burnup type core is constructed using MOX fuel, the subcriticality shifts in the shallow direction (curve C) in a substantially constant axial direction. . Then, it is possible to obtain an efficient subcriticality curve with no waste corresponding to the core configuration (magnitude of enrichment, etc.). That is, it can be said that the core has a high burnup type with a subcriticality secured.
(In the conventional BWR, if the subcriticality is represented by the curve a and the enrichment is increased, the peak of the subcriticality curve shifts upward in the upper half of the core and becomes critical. Moreover, since the shallow part of the sub-criticality before the control rod is inserted shifts to the control rod insertion side, the reactor shutdown speed at the time of scram is increased, and the safety shutdown capability of the core is improved.

Although the case where the intervening region is arranged in the conventional BWR has been described above, the pressurized water reactor PWR will be described below as another example.
Will be briefly described.

A cluster control rod for collectively operating a plurality of rod-shaped neutron absorbing rods (having a diameter of about a fuel rod or about twice that thereof) is inserted from above the core toward the bottom of the core. In this case, the intervening regions are mainly arranged so as to be unevenly distributed below the core, and the portion where the stop margin becomes severe is shifted to the upper half of the core.

On the other hand, for the neutron absorbing rod, the long-life absorber is arranged on the insertion tip side (lower side), and the high-reactivity value absorber ( ¹⁰ B enriched B ₄ C pellets or the like) is arranged from the central part upward. This configuration constitutes the core of the present invention.

Other reactors in which control rods are inserted and removed from above include small and medium-sized nuclear reactors, natural circulation reactors, and high conversion reactors. These reactor types are still in the stage of research and development, but when the present invention is applied, they have the same configuration as that of the PWR.

Next, referring to FIG. 4 and FIG. 5, how the subcriticality during the reactor shutdown is in the axial direction of the core, and how it is changed by carrying out the present invention, As a result, what kind of control rod is preferable and how good results can be expected will be described.

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

FIG. 6A shows the case of the conventional core, and the subcritical region ρ becomes particularly shallow in the range of 4/6 to 5 / 6H from the lower end of the effective core.
That is, the subcriticality ρ is deep near the lower end of the core effective portion due to neutron leakage. In the subsequent part B, since the void ratio is low, the amount of Pu generated is low and combustion proceeds, so that U235 remains little and the subcriticality ρ is deep. 4/6 from the bottom of the next effective core
In the range c of ~ 5 / 6H, since the void ratio is high, the amount of Pu produced is large and the combustion delay causes a large amount of U-235 remaining, so there is little leakage.
The subcriticality ρ is shallow. Near the upper end of the effective core, the subcriticality ρ deepens due to neutron leakage.

In the same figure (B), the one-stage intervening region is arranged slightly above 3 / 4H from the lower end of the effective core. As shown in the figure, the subcriticality ρ is significantly improved (increased) by the intervening region. However, in this example, the downward shift of the part where the subcriticality ρ becomes shallow is small.

FIG. 6C shows a case where the two-stage intervening regions are arranged as shown in the figure. The subcriticality ρ in the upper part of the effective core is much larger (deeper) than that in Fig. 2B. Then, the part where the subcriticality ρ becomes shallower is shifted to almost the central part of the core.

FIG. 5 corresponds to FIG. 4 (C) and is a distribution diagram of the control rod reactivity value ρ _CR when the subcriticality of the core is configured to be substantially constant in the axial direction.

In the conventional BWR, having a large reactivity value from the upper part of the core, especially from the lower end to 3 / 4H to 4 / 4H effectively enhances the control rod reactivity value, and as a result, effectively improves the reactor shutdown margin. It was a way of thinking.

Thus, in the reactor of the present invention, the conventional idea no longer holds, and the control rod reactivity value is the maximum at almost the central portion of the control rod (reacting to the central height of the core) at the time of full insertion. The solid line in FIG. 5 indicates that it is preferable that This is to separate the functions of the control rods. That is, the tip has a long life first and a reactivity second.
(Long life region), reactivity priority in the central part (high reactivity value region), lower part if there is an appropriate amount of reactivity (passing region). When the control rod having such a structure is adopted, the safety of the furnace is improved because the emergency shutdown of the furnace is early.

Therefore, the BWR control rod inserted from the lower end to the upper end of the core corresponding to FIG. 5 has the following structure.
That is, the total length of the core is divided into 3 equal parts, and from the lower end to 1/3 length, B ₄ C using natural boron (referred to as ^N B ₄ C), 1 /
The 3 to 2/3 long part is composed of B ₄ C using concentrated boron (referred to as ¹⁰ B ₄ C), and the 2/3 to 3/3 long part is composed of a long-life type, for example, Hf.

That is, the control rod insertion tip portion (1/4 to 1/3 of the entire length from the tip portion) has a high neutron irradiation dose in the present invention and is no different from the conventional control rod. Therefore, it is preferable to use the most suitable Hf. When Hf is preferably used, the reactivity value can be almost the same as that of the conventional control rod (using ^N B ₄ C), and the life can be greatly increased.

From 1/3 to 2/3 of the total length from the tip part (the part where ¹⁰ B ₄ C is used), the neutron irradiation dose is significantly lower than that of the above insertion tip part (for example, 2/3 or less), so that the life is extended. There is almost no demand for it, and high reactivity value is of the utmost importance. Boron -10 ⁽¹⁰ B) has high reactivity worth than a neutron absorber may be said that none, therefore, ¹⁰ B
¹⁰ B ₄ C using is preferred.

Insertion end, that is, the lower part of the core (1/3 from the bottom
Long), the neutron irradiation dose is low and high reactivity value is not required, so that the conventionally used ^N B ₄ C can be used (it can be said that the control rod is functionally separated in the axial direction). .

As described above, in the core structure of the present invention, the part where the subcriticality becomes shallower is shifted from the upper part of the core to the central part, that is, to the control rod insertion side. Therefore, assuming that the control rod is urgently inserted during the full pull-out operation, the control rod reaches a portion where the subcriticality becomes shallower earlier than in the conventional case, so that the reactor can be stopped urgently earlier. That is, the safety of the nuclear reactor is improved.

FIG. 6 is a comparison between the control rod used in the conventional BWR and the control rod according to the present invention. That is, (A) is a perspective view of a conventional control rod, (B) is a cross-sectional view of the conventional control rod shown in (A), and the left half of (C) is the conventional control rod. The vertical cross-sectional view of the rod, and the right half of the same figure (C) is a vertical cross-sectional view of the control rod according to the present invention.

In Fig. 6, a SUS sheath 3 formed in a deep shape
The opening 1 is fixed to a cross-shaped tie rod 32 at its opening to form a cross-shaped control rod 30. The upper end (insertion tip) is a tip structural member 33 with a handle, and the lower portion is a terminal structural member 34 integrally formed with a speed limiter. Inside the sheath 31, a rod-shaped neutron absorbing material 35 formed by filling B ₄ C powder (grains) into a SUS tube is arranged in the direction parallel to the axis of the control rod (US method). In the wing part, instead of arranging the absorbers in the sheath, a hole of about 6 mm is formed in the SUS plate of about 8 mm thickness from the outer end of the wing toward the central axis of the control rod in the direction perpendicular to the insertion / extraction direction. A method of filling and sealing B ₄ C powder therein has been developed by Sweden. This system has more B ₄ than the American type control rod.
Since C can be filled, the reactivity value can be increased by about 10% (when the outer shapes are the same). This 10% is about 1% of the reactor shutdown margin (usually 1-3% △ K / K) (hence 2-4%).
ΔK / K) can be improved. Since B ₄ C is used, it is almost the same as the US type that it is not suitable for a significantly long life.

FIG. 6 (C) shows two examples of conventional control rods divided into a left half portion and a right half portion. That is, the left half is axially uniform and is a typical example of a conventional control rod. The reactivity value is uniform as shown by the solid line in FIG. On the other hand, the right half is a long-life control rod that the present inventors have recently developed for conventional BWRs, and as shown by the dotted line in the same figure (D), from the effective end (core insertion end) to the end. It is designed to reduce the reactivity value in four stages 36, 37, 38, 39. An Hf plate is used as a neutron absorbing material, water is introduced between the two Hf plates, and the Hf material having a high density is reduced to suppress a decrease in reactivity value. In the conventional BWR, it is necessary to increase the reactivity value of the tip portion and to extend the life thereof, and thus such a configuration is extremely suitable (Japanese Patent Application No. 61-61).
-78746, etc.).

By the way, in the nuclear reactor of the present invention, as a result of introducing the intervening region into the nuclear reactor, it becomes unnecessary to increase the reactivity value of the insertion tip. However, in the BWR, the control rod is inserted and removed from below the core, and the tip end receives a large amount of neutron irradiation, which is no different from the conventional BWR. Such characteristics are
The same applies to PWR and other furnace types as well.

FIG. 7 shows the control rod characteristics required for the nuclear reactor of the present invention, and an example embodying them. That is, FIG. 7 (A) is a characteristic curve diagram necessary for the control rod corresponding to FIG. 5, and FIG. 7 (B) shows the control rod according to the present invention implemented discretely based on FIG. The characteristic curve diagram and FIG. 7C show two examples embodied on the basis of FIG. 2B in the left half part and the right half part of the figure. In one embodiment, the control rod 40 is axially divided into three equal parts, as shown in the left half of FIG. That is, the core insertion end part is the normal region 43 (natural B, B ₄ C lateral hole filling), the central height part is the high reactivity value region 42 ( ¹⁰ B concentrated B ₄ C lateral hole filling), and the core insertion tip part is high. It is composed of a neutron irradiation area 41 (Hf plate, trap shape). The core insertion end portion and the core insertion tip portion are provided with longitudinal voids 44 and 45 in the central portion. In the other embodiment, as shown in the right half of FIG. 6C, the control rod 40 is roughly divided into three equal parts in the axial direction. That is, the core insertion end portion is usually in the area 46 (SUS tube is filled with B ₄ C powder,
These are arranged in the sheath, corresponding to the conventional type that has been used for a long time), and the central height part has a high reactivity value region 42 (a horizontal hole is made in a SUS plate and filled with B ₄ C powder) The tip portion of the core insertion is composed of a high neutron irradiation region 41 (the Hf plate is trapped and inserted into the SUS sheath). In this example, a vertical void portion 44 is provided only in the central portion of the core insertion tip portion.

The present inventors have long been conducting research and development of control rods for BWRs, but when the outer shapes are the same, the method of opening a horizontal hole in the SUS plate developed in Sweden and filling it with B ₄ C is most suitable. It is suitable for realizing high reactivity value areas. We have also conducted various experimental studies on neutron absorbers,
B ₄ C or EuB ₆ in which ¹⁰ B is concentrated is suitable for increasing the reactivity.

Therefore, in FIG. 7 (C), the horizontal hole filling method is basically used, and B ₄ C powder and, if necessary, B ₄ C enriched with 10 B are used as the neutron absorbing material. Since the neutron irradiation is high at about 1/3 of the insertion tip, a long-life type neutron absorber is used. Hf, Eu, etc. can be considered as the long-life type absorbent, but Hf is the most practical. Hf is a metal, has excellent neutron irradiation characteristics and corrosion characteristics, and can be used without a coating material. The drawback is that the density is large,
The point is that the reactivity value tends to decrease.

Recently developed by the present inventors (Japanese Patent Application No. 61-78746)
No.) If a Hf plate trap type, that is, a system in which water is introduced between two Hf plates, a long-life control rod that solves the above drawbacks can be realized. In this embodiment, the Hf plate and the trap are configured from this point of view.

Further, in the present invention, the reactivity value may be set to be slightly low, so that Hf is eliminated in the vicinity of the tie rod (the sharing of the stop margin to be secured by the control rod is reduced because the furnace stop margin is improved by the intervening region). It is a void. As a result, a large decrease in the neutron flux near the tie rods is mitigated, and as a result, a large decrease in the fuel output corresponding to this portion is mitigated. As a result, combustion of the fuel in this portion progresses even when the control rod is inserted, and an abnormal increase in output when the control rod is pulled out is suppressed, which contributes to the improvement of fuel integrity.

Note that the present inventors have already disclosed many concrete proposals as measures for long life. About 1/3 length of the insertion end shows two examples.

The left half is a horizontal hole system. Since the reactivity value may be lowered in this portion, an example is shown in which the lateral width of the absorbent filling portion is reduced by forming the void 44 (→ water) on the side surface of the tie rod. This water improves the fuel integrity as described above.

The right half shows a conventional method of arranging an absorbent material, which is configured by filling a SUS tube with B ₄ C, in a sheath. Since the reactivity value tends to be smaller than that of the horizontal hole method, no void (→ water) is provided on the side surface of the tie rod in this example.

In the present invention, the part where the subcriticality becomes shallow is shifted to the control rod insertion side, and the core configuration of the novel light water reactor in which the configuration of the control rod is made suitable has been described, but in principle, it is also applicable to fast reactors and the like. Of course, it can be applied.

Further, although the case of the water gap has been described in the present invention, it is needless to say that even if natural uranium-depleted uranium is used, the same effect as in the case of the water gap can be obtained.

[Effects of the Invention] As described above, according to the present invention, the following effects can be obtained.

(1) Highly enriched fuel (including MOX) by introducing the intervening region
The shutdown margin can be secured even in the core using.

(2) By appropriately disposing the intervening region, the region (shutdown zone) where the stop margin becomes severe can be shifted, and the time to shut down the furnace during scram can be shortened. That is, the furnace can be shut down more safely.

(3) The control rod is formed in three regions in the axial direction of its function, the neutron absorption characteristic of the middle region is higher than that of the lower region, and the neutron absorption life of the upper region is longer than that of the lower region.
A control rod suitable for the above (2) can be provided.

[Brief description of drawings]

FIG. 1 shows the case where the present invention is applied to BWR.
(A) is a schematic vertical sectional view of an embodiment of the present invention, (B) is an axial configuration diagram of a control rod used in the core of the present invention, (C) is a subcritical distribution during reactor shutdown 2A and 2B are views for explaining the operation of the present invention, and FIGS. 3A and 3B.
4 is a diagram for explaining the characteristics of the core according to the present invention, FIG.
Figures (A) to (C) are diagrams showing core axial direction subcriticality distributions of the core of the present invention and the conventional core, and Fig. 5 corresponds to Fig. 4 (C). Control rod reactivity value distribution diagram when the criticality is configured to be almost constant in the axial direction, FIG. 6 (A) is a perspective view of a conventional control rod, and FIG. 6 (B) is the same diagram (A). ), A left half of the figure (C) is a vertical cross section of a conventional control rod, a right half of the figure (C) is a vertical cross section of the control rod according to the present invention, and a figure (D) is Diagram showing reactivity value of Fig. 7 (C), Fig. 7
(A) is a characteristic curve diagram necessary for the control rod corresponding to FIG. 5, and (B) is a characteristic curve diagram of the control rod according to the present invention which is implemented discretely based on the same diagram (A). Figure (C) is the same figure (B)
FIGS. 8 (A) and 8 (B) are views showing a left half portion and a right half portion, respectively, of two embodiments embodied based on FIG. FIG. 9 is a cross-sectional view of a conventional fuel assembly shown in FIG. 8, and FIG. 9 is a cross-sectional view of a conventional fuel assembly. It is a figure. 20 ... Fuel assembly 21, 22, 23 ... Intervening region 24, 30, 40 ... Control rod 31 ... Sheath 32 ... Tip structural material 34 ... End structural material 35 ... Neutron absorbing material

Claims (8)

[Claims] 1. A fuel assembly in which a large number of fuel rods are regularly bundled is arranged such that their axes are vertical and parallel to each other, and a gap is formed inside the fuel assembly or between adjacent fuel assemblies. In a core in which control rods are arranged so that the control rods can be inserted and removed, and in the core of a light water reactor in which light water, which is a coolant, flows between the fuel rods from below the fuel rods upwards, a part of the cores is provided. Alternatively, the whole is vertically divided into at least two core sections, and the intervening region is arranged at a position where the subcriticality becomes shallow and the fissionable nuclide concentration is significantly lower than that of the upper and lower fuels. If the width is larger than the moving distance of thermal neutrons when the light water reactor is shut down and smaller than the moving distance of thermal neutrons during light water reactor output operation, and the part where the subcriticality becomes shallow during normal temperature shutdown is moved to the control rod insertion side. The control rod is characterized in that its function is formed in three regions in the axial direction, the neutron absorption characteristic of the middle region is higher than that of the lower region, and the neutron absorption life of the upper region is longer than that of the lower region. The core of a light water reactor. 2. A boiling water reactor in which a portion where the degree of subcriticality becomes shallow when the light water reactor is stopped is moved to the lower side of the core and control rods are inserted and removed from the lower side of the core. The core of the light water reactor described in paragraph. 3. A pressurized water reactor in which a portion where the degree of subcriticality becomes shallow when the light water reactor is stopped is moved toward the upper side of the core, and control rods are inserted and removed from the upper side of the core. The core of the described light water reactor. 4. A small-to-medium-sized reactor, a natural circulation reactor or a high conversion reactor, in which a portion where the subcriticality becomes shallow when the light water reactor is shut down is moved to the upper part of the core and control rods are inserted and removed from the upper part of the core. The core of a light water reactor according to claim 1, characterized in that 5. The control rod during core insertion has a strong neutron absorption structure having strong neutron absorption characteristics in the portion where the subcriticality becomes shallow,
The core of the light water reactor according to claim 1, characterized in that the other part has an absorption configuration having a neutron absorption characteristic slightly smaller than that. 6. A strong neutron absorption structure having strong neutron absorption characteristics
The core of a light water reactor according to claim 5, characterized in that the SUS plate is formed with horizontal holes and is filled with B ₄ C powder. 7. The light water reactor according to claim 5 or 6, wherein the strong neutron absorbing substance having a strong neutron absorbing property is mainly boron enriched boron having a high concentration of boron 10. Core of. 8. A power control rod for controlling power output of a nuclear reactor, wherein at least a tip portion thereof is made of a long-life type neutron absorbing material such as hafnium. The core of a light water reactor according to item 5.