JPS6249943B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is characterized in that the entire wall thickness in the radial direction is formed by graphite blocks, and the graphite blocks are stacked one on top of the other to form an annular structure. This invention relates to a cylindrical side reflector for a cooled high temperature nuclear reactor.

In all reactors with side reflectors made of graphite, radiation damage can occur in the graphite facing the core after long periods of operation, or even after relatively short periods at high power densities. This may lead to the collapse of the reflective material and, in some cases, the shutdown of the furnace. In particular, the fast neutron flux associated with the power density causes dimensional changes in graphite due to the Wigner effect. First, contraction occurs, followed after a while by expansion again, and expansion continues beyond the initial state. The stress generated at that time can cause cracks in the graphite. However, side reflectors must reach an operational life of 35 years under full load conditions. In nuclear reactors using spherical fuel bodies, which pass through the core only once, the radiation load on the side reflectors is particularly high.

In this type of reactor, the desired final combustion rate is reached after one pass of the spherical fuel bodies. In this case, a gradient of the power density distribution decreases from top to bottom, with a concomitant high dose of fast neutrons in the upper third of the reactor.

Reactors with spherical fuel bodies, ie pebble bed reactors, have already been developed in which a special design of the side reflectors avoids premature shutdown of reactor operation. A nuclear reactor consisting of a deposit of uniformly formed neutron reflectors, the reflector envelope having approximately the same shape as the fuel body, is described in West German Patent Publication no.
It is known from No. 1034784. A nuclear reactor in which the side reflectors consist of a deposit of spherical graphite elements is also described in Austrian patent application A9582/63. These spherical graphite elements flow continuously into the core and are constantly replenished with new graphite spheres, so that damaged portions of the side reflectors are constantly replaced.

In order to protect the stationary side reflectors from excessive fast neutron doses, in the peripheral zone of the fuel body stack directly adjacent to the side reflectors, fuel bodies with a lower content of fissile material than the inner zone of the pile are placed in the peripheral zone. German Patent Application No. 2 352 691 discloses a device which is designed to be filled with water. It has also been proposed to configure the peripheral zone between the fixed side reflector and the fuel body pile to be very narrow and to fill it with pure graphite spheres.

Another protection measure for the most loaded parts of the side reflectors is described in German Patent Application No. 23 47 817. A countermeasure for this is to provide a neutron absorber or a neutron moderator on the ceiling reflector and the upper part of the fixed side reflector. This makes it possible to significantly reduce the fast neutron flux in radiation-damaged areas of the reflector. The above materials are rod-shaped and can be placed in a suitable cavity. In this known reactor,
The fuel body passes through the core only once.

In another nuclear reactor with a single-pass spherical fuel body, which is described in German Patent Publication No. 26 12 178, damage to the side reflectors is avoided by a special configuration of the shutoff and control system. This cut-off and control system is inter alia fed into the side reflectors and
It is equipped with a plurality of absorbent rods that can be moved inside the absorbent rod.
Absorber rods are always inserted 70% deep into the side reflector and reduce the neutron flux of this reflector.

A method is also known in which a part of the side reflector, which is a combination of rectangular parallelepiped blocks, is replaced. The function of a manipulator for removing individual reflector blocks from the core is described in DE 25 09 025 A1. This avoids having to shut down the reactor prematurely due to unpredictable damage to graphite fittings, but replacing or repairing damaged reflector sections is very time consuming and costly. Accompany.

To avoid these drawbacks, a graphite side reflector of block construction is proposed in DE 26 43 275, which makes replenishment or replacement of the reflector block unnecessary during the entire life of the reactor. Here, this is achieved in that the individual blocks, which extend continuously in the radial direction over the entire wall thickness of the reflector, have recesses consisting of a number of grooves in the inner wall surface. This measure is based on the idea that areas of the block, which are exposed to particularly high stresses due to neutron radiation and thermal effects, appear as if they were blocks of small dimensions. This means that when the inner wall surface is a very small block,
This is because stress cracking does not occur much. However, small blocks are expensive to manufacture and difficult to assemble. However, in the side reflector provided with such a concave portion, the inner surface of the block must be processed, which remains a drawback.

It is therefore an object of the present invention to ensure reliable reactor operation during the entire life of a high-temperature nuclear reactor without expensive auxiliary equipment or time-consuming measures such as partial replacement or repair. In particular, it is an object of the present invention to provide a side reflector that can be used in nuclear reactors with high power density and avoids generating high notch stress.

This object of the present invention is achieved as follows. That is, at least in a specific zone of the graphite block facing the core in the upper core area, a wear zone is provided that allows wear due to radiation, and this wear zone is not involved in the support structure of the side reflector, and the side When designing the structure of the reflective material, this wear is taken into account by increasing the wall thickness.

The present invention is based on the idea that the area of the side reflector that is exposed to extremely high doses is largely excluded from the functional area of the side reflector so that damage to this area does not affect the function of the reflector. It is based on The expected amount of graphite dust or graphite debris is similar to that of known pebble bed reactors (e.g.
It is usually difficult to distinguish from graphite dust and spheroidal fragments generated by THTR-300 nuclear reactors). Therefore, there is no risk that the safety and operational performance of the high temperature reactor will be affected by wear products.

In this way, two zones with different functions are provided in each graphite block. That is to say, the parts that play the role of support and container structure, ie the support functional area, and the wear zones, which do not have any influence on the sealing or dimensional stability of the side reflector and on the operational performance. The results of the study show that the wear zone, where the material strength drops to almost zero, is confined to a small local area of the reflector. The wear zone starts to move inside the reflector every year from a thickness of 5 mm after about 20 years of reactor operation, reaching a depth of 70 mm at the end of the reactor's operation (35 years later). It appears with a width of about 2 mm in the axial direction.

Therefore, if the dimensions of the block are appropriately designed,
It is ensured that the wear of the individual graphite blocks is limited to a specific front area. Furthermore, in this case, the effect is utilized that the strength decreases toward the inner wall surface or end surface of the graphite block, thereby preventing the spread of cracks into the interior of the block. All loading of the graphite block outside the wear zone only results in stresses that are acceptable by conventional standards.

If the penetration depth of the wear zone is 70 mm, the effect of this zone on the entire graphite block by the end of the operating period is expected to be limited to about 100 mm. It is easy to consider wear. In that case, a further safety factor of 2 or 3 can be estimated.

In order to ensure a free flow of spherical fuel bodies through the pebble bed in a pebble bed reactor, it is proposed in the present invention to adjust the maximum size of graphite fragments that should occur by appropriate measures. This measure consists, for example, in providing the wear zones of the individual graphite blocks with predetermined breaks that limit the size of the wear products that occur. The starting point is that most of the graphite inside the wear zone, which is highly stressed, is worn away by the normal flow of the spheres, but if there are large pieces of graphite that have broken off, it is likely that the expected rupture will occur. The maximum size is limited depending on the part.

Wear products consist of extremely heavily irradiated materials. If it does not flow through the pebble bed, it moves along with the spherical fuel through an area of extremely strong neutron flux where the temperature gradually increases. After a few meters of travel, the residual strength of the wear products drops to a few percent of the initial strength. This, combined with the crushing effect of the spheroid flow, results in a thorough crushing of large wear products. This reduces the load on the pebble bed and prevents an increase in pebble bed density that would otherwise occur if the debris was of insufficient size.

Preferably, the planned break in the graphite block is realized by a hole. The holes are located on the front side of each graphite block, ie in the area facing the pebble bed. A suitable distribution of the holes in the highly stressed areas of the graphite block ensures that in the event of cracking due to secondary stresses, only fragments smaller than the size of the spherical fuel bodies are produced. This can be pulled out of the reactor together with the spherical fuel body without any problem.
Small graphite fragments are filtered out by the reactor's gas cleaning system.

Providing holes in the wear zone of the graphite block also has the desirable side effect that the effects of Wigner deformation are reduced to a smaller area. In other words, it resembles a graphite block with smaller dimensions and more rigidity.

Preferably, each graphite block, at least in the upper reactor area, is provided with at least one large hole parallel to the inner wall of the graphite block. The inner wall itself remains unfinished. This hole can be closed with a graphite plug. This does not affect the remaining parts of the graphite block.

It is also possible to provide each graphite block, at least in the upper reactor area, with a large number of small holes arranged in a plane parallel to the inner wall of the graphite block. The holes can extend horizontally or vertically or both in the plane mentioned above.

In a preferred embodiment of the invention, the inner wall of the graphite block can be provided with linear slits, in which case each slit has a hole at its end, which forms the bottom of the slit, and the diameter of the hole is equal to the width of the slit. It is several times as large.

The slits and holes are within the side reflector area, ie the wear zone, which may not contribute to the sealing or shape stability of the side reflector. With these, it is also possible to realize a planned breaking section. In the side reflectors formed in this way, the stress concentration in the support area of the block is further relieved and the holes provided at the ends of the slits act like crack stops, i.e. the cracks that appear in the case of damage are prevented. does not advance to the innermost area of the block.

In addition, the side reflective material with slits and holes is
It has the advantage of facilitating the manufacture of graphite blocks having a combination of slits and holes in the inner wall.

As already mentioned, the maximum dimensions of the resulting graphite fragments can be adjusted by means of the holes. A slit made by sawing on the end face of the block,
In combination with one hole in each case at the bottom of the slit, the control of the size of the graphite fragments is further improved. That is, the maximum size of wear products can be limited to a certain range. The combination of slits and holes also reduces the Wigner effect to a smaller area. That is, it takes the form of a graphite block with smaller dimensions and good stiffness.

The distance between the hole and the inner end face of the graphite block is preferably selected to approximately correspond to the thickness of the wear zone. The plane bounded by the holes forms the boundary surface of the wear zone.

However, depending on the conditions, it may be possible to set the above distance so that the distance between the hole and the inner wall of the graphite block is a fraction of the thickness of the wear zone, and the hole is located inside the wear zone. It's convenient.

Preferably, the hole at the end of the slit has a diameter about three times the width of the slit.

In a preferred embodiment of the side reflector according to the invention, the inner wall surface of the graphite block has a grid of horizontal and vertical slits. The grid-like slits have equal spacing from each other in both directions.

For specific temperature ranges and stresses of the side reflector with slits, it is preferable to provide further holes in the graphite block in the horizontal or vertical direction or both directions. The pores smooth the temperature gradients of the individual graphite blocks and equalize the stresses.

According to the invention, it is preferred to use holes alone or especially vertical holes of the side reflector with slits and holes for cooling the side reflector. It is possible to postpone the occurrence of wear phenomena. This hole allows bypass flow of cold cooling gas through the side reflectors. The cooling gas later reenters the core and is mixed with the hot gases. The diameter of the hole at the bottom of the slit is determined according to the capacity of the cooling gas flow that will be required for sufficient cooling of the side reflector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Four embodiments of graphite blocks constituting side reflectors according to the present invention will be described below with reference to the schematic diagrams shown in the drawings.

These many blocks come together to form a ring. In the figure, the graphite block is shown as a rectangular parallelepiped for convenience. Several rings placed one on top of the other constitute a cylindrical side reflector. This side reflector is placed to surround the fuel body stack of the high temperature reactor.

FIG. 1 shows a graphite block 1. Its inner side, ie the end face 2 facing the core, is unprocessed. The graphite block 1 has vertical holes 3. The hole 3 is closed with a graphite plug 4. of the support area of the block,
These holes can significantly reduce stress concentrations due to radiation-induced deformations. The holes 3 are located inside a particular zone that does not perform a supporting function, the so-called wear zone 5. The length of graphite block 1 is 500
mm, the hole 3 has a diameter of, for example, 120 mm,
It can be arranged at an interval of 100 mm from the inner end surface 2.

FIG. 2 shows a graphite block 6 whose inner end face 7 is also unprocessed. In this case too, there is a specific wear zone 8 in the front area of the graphite block 6. Its size is taken into account when designing the structure of the side reflector. Inside the wear zone 8, a number of small holes 9, 10 are provided in several planes parallel to the inner end face 7, which serve as predetermined break points and also serve to reduce stresses. The holes 9 are arranged vertically, while the holes 10 run horizontally. Cold cooling gas can flow as a bypass flow through the holes 9 towards the deposited spherical fuel bodies to cool the graphite blocks 6.

If the length of the block is 500mm, holes 9 and 1
The diameter of 0 may be 6 mm. The distance between the holes and the distance between the holes in the front row and the inner end surface 7 can each be 40 mm.

The graphite block 11 shown in FIG. 3 has a width a=250
mm, height b = 250 mm, and length c = 500 mm. In the front area of the graphite block 1 there is a specific wear zone 12. Its size is taken into account when designing the structure of the side reflector. In this particular case for example about 70mm
It is. The wear band 12 serves no supporting function.

On the inner end surface 13 of the graphite block 11 in FIG.
By sawing, a grid 14 is provided which consists of vertical slits 14b and horizontal slits 14a. The slits 14a and 14b are spaced apart from each other by approximately 40 mm, and are approximately 40 mm apart from each other in the graphite block 11.
Since it is sawed to a depth of 40 mm, small cube-shaped individual blocks 15 are produced on the upper surface of the graphite block 11. All slits 14a, 1
The width of 4b is approximately 2 mm. If a portion of the reflective material breaks, the maximum size thereof is limited by the grating 14.

A hole 16 is provided at the end of each slit 14a, 14b, forming the bottom of each slit. The diameter of all holes 16 is three times the slit width, or 6 mm. Cold cooling gas flows as a bypass flow through the holes 16 towards the accumulating spherical fuel bodies of the high temperature reactor, thereby allowing sufficient cooling of the side reflectors.

In addition to cooling the reflector, the presence of the holes provides another desirable effect. This means that stress concentrations caused by radiation-induced deformations in the support area of the block are significantly reduced. Also, in case of breakage, the hole breaks and acts as a stopper.

FIG. 4 also shows a graphite block 2 equipped with a grid 24 consisting of horizontal slits 24a and vertical slits 24b.
1 is shown. Graphite block 21 has similar dimensions to the graphite block shown in FIG. A specific wear zone 22 is also provided in the front area. There is a hole 26 at the bottom of each slit. The grating 24 produces small cubic individual blocks 25 on the end face 23 of the graphite block 21.

Graphite block 21 has further holes 27. The hole 27 is located further inside the side reflector,
It runs vertically. As shown in FIG. 4, the holes can be arranged in a staggered manner. Graphite blocks formed in this way are used in specific temperature ranges and for specific stresses in the side reflector.
The holes 27 provide a smoothing of the temperature gradient of the individual graphite blocks, which in turn leads to stress equalization.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a first graphite block with a single hole in the wear zone, FIG. 2 is a perspective view of a second graphite block with a number of small holes, and FIG. 3 shows a combination of slits and holes. FIG. 4 is a perspective view of another graphite block having the same combination as in FIG.
Figure 3 shows a perspective view of another graphite block with additional holes. DESCRIPTION OF SYMBOLS 1...graphite block, 5...wear zone, 6...graphite block, 8...wear zone, 11...graphite block, 12...
Wear zone, 14... Grid, 21... Graphite block, 22
...wear zone, 24...grid.

Claims (1)

[Claims] 1. A gas-cooled reflector in which the entire wall thickness of the reflective material along the radial direction is formed of graphite blocks, and the graphite blocks are stacked vertically to form an annular shape, and filled with a spherical fuel body. In the cylindrical side reflector for high-temperature reactors, a wear zone is provided at least on the side facing the core of the graphite block in the upper core area to allow wear due to radiation, and this wear band serves as a support for the side reflector. A cylindrical side reflector characterized in that the wall thickness is increased by an amount corresponding to this wear zone when designing the structure of the side reflector without involving the wear band. 2. Cylindrical side part according to claim 1, characterized in that the wear zones of the individual graphite blocks are provided with predetermined breaks, thereby limiting the size of the wear products that occur. Reflective material. 3. The cylindrical side reflector according to claim 2, wherein a hole is provided in the graphite block as a predetermined breaking point. 4. Cylindrical side reflector according to claim 3, wherein each graphite block of at least the upper reactor area is provided with at least one such hole parallel to the inner end face of the graphite block. 5. The cylindrical side reflector according to claim 4, wherein the holes are closed with graphite plugs. 6. The cylindrical side according to claim 3, wherein each graphite block in at least the upper reactor area is provided with a large number of said holes, and these holes are arranged in a plane parallel to the inner end surface of the graphite block. Reflective material. 7. Linear slits are provided on the inner end surface of the graphite block, and a hole forming the bottom of the slit is provided at the end of each slit, and the diameter of the hole is several times the width of the slit. The cylindrical side reflective material according to item 2. 8. Cylindrical side reflector according to claim 7, wherein the distance between the hole and the inner end face of the graphite block corresponds approximately to the thickness of the wear zone. 9. The cylindrical side reflector according to claim 7, wherein the distance between the hole and the inner end face of the graphite block is a fraction of the thickness of the wear zone. 10 The diameter of the hole is approximately 3 times the width of the slit.
The cylindrical side reflector according to claim 7, 8 or 9, which is twice as large. 11. The cylindrical side reflector according to claim 7, wherein the slits are provided in a grid pattern running horizontally and vertically on the inner end surface of the graphite block. 12. The cylindrical side reflector according to claim 11, wherein the slits are provided at equal intervals in both the horizontal and vertical directions. 13. The cylindrical side reflector according to claim 7, wherein another hole is provided in the inner depth of the wear zone of the graphite block 21 in a horizontal or vertical direction. 14. The cylindrical side reflector according to claim 3, 7 or 13, wherein the holes provided in the wear zone of the graphite block are used for cooling the side reflector with cooling gas. .