JPS5928875B2 - control rod assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in fuel assemblies for nuclear reactors, and relates to a control rod assembly comprising a spider and a burnable poison rod (control rod) removably attached to the spider.

In order to create useful power from a nuclear reaction process, it is necessary to assemble fissile uranium or other suitable materials into a sufficiently dense array to maintain a continuous energy-producing chain reaction. It is.

The heat obtained by the fission reaction of the fuel assembly or core is transferred to a working fluid, which is generally water, and pressurized water is passed through the core at high velocity.

Because the heat, vibration, and radiation generated within the core of a nuclear reactor create a harmful environment, it is essential to maintain the structural integrity of the reactor core components.

Therefore, the core of a pressurized water reactor is usually constructed by arranging a plurality of fuel assembly groups in a substantially perfect cylindrical shape.

Each fuel assembly consists of an array of approximately 200 elongated fuel rods spaced parallel to one another.

Each fuel rod contains a stack of cylindrical uranium distributary pellets, the uranium providing the fissile fuel for the reactor.

These fuel assemblies support not only fuel rods but also several other elements.

For example, instrument tubes for observing temperature and neutron flux conditions within the core, end fittings and fuel element retention grids for stably fixing each component of the fuel assembly, and fission induction within the core. Control rods and control rod guide tubes for regulating power output from the reactor by selectively absorbing neutrons also form part of the fuel assembly.

As is well known, the distribution of neutrons varies depending on the location within the reactor core.

For example, the density of neutron-generating uranium is lower near the periphery of the core than in the center of the core, so the number of neutrons at the periphery of the core is smaller than in the center of the core.

Furthermore, since neutrons "escape" more easily from the periphery of the core than from the center of the core, the concentration of neutrons near the periphery of the core tends to decrease even further.

Since the amount of heat produced in any part of the core depends on the concentration of neutrons in that part, there is a clear tendency for the core to be hotter than the periphery.

This tendency to develop local maximum temperatures in different regions within the core is generally undesirable in several respects.

First, although nuclear reactors are designed to accommodate core operation below a certain temperature, once this maximum core temperature is reached at one or a few localized locations within the core, the Total latent heating value cannot be realized.

This is because in order to prevent a specific region, or "hot spot" that has reached its maximum design temperature, from exceeding its design temperature, the temperature of other regions within the core must be suppressed to a lower value. It is.

Therefore, by suppressing the neutron density (and therefore the calorific value) in the center of the core and increasing the neutron density in the relatively large volume of the peripheral annular region of the core, the output of the reactor as a whole can be increased. can be done.

This averaging of the power distribution within the core results in a lower core than if the neutron density, temperature, and power were allowed to reach their maximum in the center or elsewhere in the core. The amount of power generated can be increased.

In order to achieve "uniform averaging of the power distribution," it has been customary to insert burnable poison rods into fuel assemblies.

Burnable poison rods are typically tubes filled with a substance that has very high neutron absorption.

A suitable material for this purpose is, for example, an alumina matrix in which boron carbide is dispersed and sintered.

Neutrons absorbed by the material within the poison rod are effectively excluded from the fission and power generation process.

Therefore, in order to "unify the power distribution of the core,"
Burnable poison rods are concentrated within a fuel assembly in the center of the reactor core.

In addition, in view of a variety of sensitive factors, it may be desirable to concentrate burnable poison rods in other parts of the core where design or operational features result in locally high neutron concentrations. be.

Fuel assemblies must support all of the various structural components within the hazardous environment of the reactor core, while also meeting the somewhat contradictory requirements of allowing rapid and easy disassembly. Must be.

For example, fuel assemblies become radioactive when exposed within a working reactor core.

This radioactivity is so powerful that inspection and repair of the fuel assembly can only be carried out using remote control equipment behind adequate radioactivity shielding.

Therefore, since disassembly of a fuel assembly takes a long time and is expensive, it is commercially important that the fuel assembly be a structure that is both robust and easy to disassemble.

A burnable poison rod mounted within a fuel assembly is part of the structure of the assembly.

Generally, in the prior art, the poison rod is mounted so that it can move in a direction parallel to the length of the fuel rod.

The means for longitudinally movably connecting the poison rods to the rest of the fuel assembly is often from a central bub and a plurality of arms extending radially from the bub. A “spider” consisting of

The reason that the poison rod must be mounted so that it can be moved longitudinally is that the reactor operator must move the poison rod in and out in order to equalize the power output of the reactor.

The used poison rods, still attached to the spider, must be removed in bulk, placed in a radiation-shielding container, and transported to an appropriate location for disposal.

In order to effectively utilize the volume of this shipping container, it is desirable to remove each poison rod from the spider.

Unfortunately, however, the operation of removing the poison rod from the spider is complex and dangerous for several reasons.

Inside the core of a nuclear reactor, a poison rod that has been sufficiently exposed to radiation generates internal gas pressure.

Also, the sheath or tube enclosing the burnable poison has become considerably weakened as a result of exposure to radiation over a period of time.

A poison rod is typically connected to each spider arm by a threaded wire fitting.

Under such conditions, the most common technique for removing the poison rod from the spider is by shearing or sawing.

When sawing a poisonous rod, the rod can be handled gently (handling gently is an important factor when considering the gas pressure inside the rod), but in this case, the rod can be sawed with radioactive materials. Sawdust is produced.

On the other hand, the cutting method by shearing does not cause the problem of sawdust, but the rod must be handled roughly.

Therefore, the burnable poison rod is coupled to the spider arm in a manner that can withstand the environment of the reactor core, and the poison rod is coupled to the spider arm in a manner that allows for easy, quick, and gentle removal. There is a need for a means to connect the two.

According to the invention, a pin or shaft is fixed to one end of the burnable poison rod, and the shaft is installed in the hole of the spider arm.

Various means can be used for fixing this shaft to the spider arm, but it is preferable, for example, to upset the end of the shaft so that it is pressed against the wall of the hole in the spider arm.

The poison rod attached to the spider according to the invention is
The shaft can be easily removed from the spider arm by forcing it out of the hole in the spider arm by applying a force in a direction coincident with the longitudinal axis of the rod and the shaft connected thereto.

That is, the application of a longitudinal force forces the upsetting portion of the shaft into the hole in the spider arm and ultimately out of the chain hole.

In this case, a considerable force is applied to force the shaft out of the spider's pore, but the force is applied longitudinally to the shaft and toxin rod.

This feature of the invention not only protects the fragile, internal gas pressured poison rod from breakage and other damage that may occur during shear cutting operations, but also protects the poison rod from sawing. It also avoids the radioactive sawdust produced when separated from the spider.

The invention provides many other advantages.

For example, the toxic rod attachment device of the present invention is less costly to manufacture than conventional threaded attachments.

In addition, according to the present invention, the bonding force between the shaft of the poison rod and the spider arm is also excellent under normal operating temperature conditions in the core of a nuclear reactor.

For example, the coefficient of thermal expansion of the spider arm, which is typically made of stainless steel, is greater than that of the Zircaloy shaft of the poison rod, so the resulting axial stress must be compensated for by some means. According to the invention, this is achieved by creating a radial slack between the shaft and the spider arm, reducing the possibility of undesired thermal stresses in that area.

Without this automatically achieved stress relief, the shaft could shear and the poison rod could be released from the spider and into the core.

The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.

Referring to FIG. 1, a spider 10 made of stainless steel or the like includes a central cylindrical bub 11 having a longitudinal axis 12 and a central cylindrical bub 11 extending radially from the bub in a plane perpendicular to the axis 12. Multiple spider arms (first
The figure shows spider arms 13, 14, 15, 16, 17, 2.
0,21 are shown).

One layer of burnable poison rods 22, 23, 24, 25° 26.
Attach 27.30 to each corresponding spider arm.

These poison rods are elongated tubes, the axis of which is arranged parallel to the axis 12 of the bubble 11.

Referring to the poisonous substance rod 30 shown in a partially cutaway cross-sectional view, the outer surface of the poisonous substance rod is connected to a hollow tube 31.
one end of the tube has a solid plug 3 attached by welding or other means within the open end.
It is sealed by 2.

A stack of short cylindrical burnable poison pellets 33 is loaded into the tube 31 and placed on the plug 32 .

The individual pellets 33 of this stack are pressed longitudinally by springs 34 which abut against the uppermost pellets of the stack.

The outer end of the spring 34 is held by a plug 35 inserted into the other end of the tube 31 and fixed thereto.

As seen in FIG. 3, the plug 35 is formed with a chamfered end 36 which constitutes an insertion guide for the cylindrical portion 37 thereof.

The inner diameter of tube 31 matches the outer diameter of cylindrical portion 37, providing a snug fit.

As shown, the upper end of the cylindrical portion 37 is articulated with an enlarged cylindrical portion 41 forming a radially projecting shoulder 40 therebetween, which shoulder is attached to the end of the tube 31. Engage and weld.

In the illustrated embodiment, the outer diameter of the tube 31 and the outer diameter of the cylindrical portion 41 are used to planarly articulate the exposed cylindrical portion 41 of the plug 35 that projects beyond the end of the tube 31 and the outer surface of the tube 31. make them equal.

The upper end of the exposed cylindrical portion 41 of the plug 35 is formed with a transverse flange or shoulder 42 with a shaft or pin 43 extending longitudinally from the center of the shoulder.

The shaft 43, which is largely formed of solid metal, is provided with a longitudinal bore 44 extending from its free end to about half its length.

A portion of shaft 43 is shown in FIG. 2 as manufactured and prior to being fully attached to spider arm 46.

The shaft 43 therefore has a generally straight cylindrical profile, and the hole 44, which extends from the free end of the shaft to about half its length, also has a shallow conical bottom. It is cylindrical.

The shaft 43 is inserted and attached to the spider arm 46.

For this purpose, spider arm 46 is formed with a generally cylindrical opening 50 having a longitudinal axis 51 coinciding with the longitudinal axis of poison rod and shaft 43 .

According to the present invention, the enlarged diameter recess 52 is formed within the opening 50.

The recess 52 overlaps a portion of the entire length of the hole 44 of the shaft 43.

Continuation of the recess 52 is via a frustoconical surface 53 into the hole 54 which is the remainder of the opening 50 .

As shown in FIG. 2, the hole 54 has a diameter slightly larger than the outer diameter of the shaft 4=3 inserted into the opening 50.

In use, shaft 43 is inserted into aperture 50 and mechanically deformed by hydraulic forming or mechanical expansion or other suitable means to form frusto-conical surface 53 of aperture 50 and enlarged diameter recess 52. contour to form a contemplated "point of weakness" to facilitate removal of the poison rod, as shown in FIG.

The poison rod is thus secured to the spider arm without the need for expensive and cumbersome welded fittings or the like.

Although the material of the spider arm and the material of the tube of the poison wand are often different metals, the frusto-conical surface 55 (FIG. 3) of the hole 44 of the plug 35 of the poison wand is designed to accommodate thermal expansion. Provides a tapered surface that reduces the effects of coefficient differences.

That is, the degree of expansion of the stainless steel spider arm, which expands axially as the temperature increases, is typically that of Zircaloy (Z
It is thicker than the expansion degree of the end plug 35 of the poison rod formed by IRCAL TOY.

Therefore, normally axial stress will occur, but
In the configuration of the invention, both parts, namely the spider arm 46 and the plug 35, are loosened (displaced) in the radial direction, so that
Axial stress is reduced.

That is, the spider arm also expands more radially than the plug 35, so that the conical surface 53 of the opening 50 of the spider arm 46 and the deformed portion or conical surface 55 of the shaft 43 of the plug adjacent thereto The shaped interface is displaced downward in the axial direction.

Both parts thus remain in close contact both radially and axially, but the stress in the shaft 43 is
This is significantly reduced compared to the stress that would normally occur.

Thus, the poison rod structure of the present invention maintains a tight bond between its spider arm and the shaft of the plug over the entire operating temperature range of the nuclear reactor, yet still maintains the poison rod structure as detailed below. Provides a predetermined "point of weakness" to facilitate removal of the material rod.

Referring to FIG. 4, the hole 44 of the shaft 43 of the plug 35 is enlarged over about half of its total length by expansion or upsetting, as described above. The outer peripheral surface of the spider arm 43 is brought into pressure contact with the enlarged diameter recess 52 of the spider arm 46.

Hole 44 has a small diameter hole portion 66 that communicates through a frusto-conical tapered portion 55 .

Further, the small diameter hole section 66 terminates in a conical section 67.

When removing the burnable poison rod from the spider arm 46,
Push the mandrel 70 into the hole 44 of the shaft 43 in the direction of arrow 71.

Mandrel 70 is preferably configured as an elongated cylindrical metal shaft, the diameter of which is approximately equal to the small diameter bore portion 66 of shaft 43.
Make it even smaller.

The longitudinal axis of the mandrel 70 is the axis 51 of the toxic substance frame 30.
almost matches.

To remove rod 30 from spider arm 46 without undesirable shearing or sawing techniques, simply move mandrel 70 in the direction of arrow 71 with sufficient force to force shaft 43 through small diameter hole portion 54. Just push it in the direction.

Although not shown in the figure, a group of mandrels are arranged so as to align with the hole in the shaft of the plug of each toxic substance frame attached to the spider, and the mandrels are fixed together to a support plate. Can be done.

Then, all the poisonous substance frames can be removed at once by aligning and pushing these mandrels into the holes of the corresponding plug shafts.

Thus, the present invention avoids the conventional dangerous, costly, and time-consuming process of shearing or sawing each poison frame one by one to separate it from the spider.

[Brief explanation of the drawing]

FIG. 1 is a side view of a typical spider and toxic substance frame combination; FIG. 2 is a cross-sectional view of a portion of a shaft formed in accordance with the present invention; and FIG. 3 is a partially cutaway view of the shaft attached to the spider. FIG. 4 is a partially cutaway cross-sectional view illustrating the method of the present invention for removing a toxic substance frame. 10...Spider, 30...Poisonous substance frame, 35...Plug, 43...Shaft, 4
4...hole, 46...spider arm, 5
0... Opening, 52... Expanding diameter recess, 53
・・・・・・Truncated conical surface, 54・・・・Small diameter hole portion, 55・・・・・・Truncated conical surface, 66・・・
・・Small diameter hole portion, 67・・・・Conical portion, 70・
...Mandrel.

Claims (1)

[Claims] 1. A control rod assembly comprising a spider 10 having a plurality of spider arms and a burnable poison rod, wherein at least one spider hole 54 is formed in the plurality of spider arms. , the spider hole has an enlarged diameter recess 5
2, a small-diameter hole portion, and a truncated conical portion 53 constituting a transition portion from the enlarged-diameter recessed portion to the small-diameter hole portion; , and a shaft 43, the plug has a cylindrical portion 37 that fits tightly within the tube, one end of the cylindrical portion being formed as a chamfered end 36 and the other end of the tube. a radially projecting shoulder 40 engaging and welded to the end face, the shaft projecting longitudinally from the center of the shoulder and extending approximately half its length; a hole 44 in the direction, and at least the portion of the shaft defining the hole 44 extends radially outwardly from the shaft for removably attaching the poison rod to the spider arm. a truncated conical portion of the spider hole by deforming it into;
fixed within an enlarged diameter recess and a reduced diameter hole portion, the shaft longitudinally moves the poison rod relative to the spider aperture, thereby maintaining the structural integrity of the poison rod and the spider. A control rod assembly characterized in that it is configured to substantially return to its original shape and permit extraction of the poison rod from the spider arm without destroying its structural integrity. 2. The control rod assembly according to claim 1, wherein the spider is made of a material having a higher coefficient of thermal expansion than the material of the shaft. 3. The control rod assembly according to claim 1, wherein the material of the spider is stainless steel. 4. The control rod assembly according to claim 1, wherein the material of the edict is Zircaloy.