JPS6027958B2 - Pebble bed reactor and method of operating this reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention consists of a graphite structure consisting of a top reflector, a side reflector and a bottom reflector formed substantially from a deposit of spherical fuel bodies through which a heat transfer medium flows. A preferably spherical fuel body formed with an encapsulation of graphite is passed from above to below through the core surrounded by a reflector of in the portion of the upper reflector wall facing the fuel body deposit and/or within the upper portion of the side reflector wall, in a bevel bed reactor, to reach the desired final combustion loss after passage of
The present invention relates to the above-mentioned bubble bed nuclear reactor and a method of operating the reactor, characterized in that a substance that absorbs neutrons or reduces the velocity of additional neutrons is used at or near the same.

Nuclear reactors in which spherical fuel bodies are continuously supplied and removed after the combustion loss of the reactor, and in which the fuel bodies in the reactor are present in the reactor as deposits of these spherical fuel bodies, are known in the art. belongs to the level.

In such reactors, which are referred to as spherical pile bed reactors, the deposits of the fuel body are present in a cylindrical vessel during operation, and the fuel body deposits are present on the walls of this vessel. is covered with a layer of graphite about one skin thick. The graphite layer used to slow down the fast neutrons released during fuel splitting is called a reflector. According to a very advantageous operating method of this nuclear reactor, the fuel injection is carried out in such a way that the fuel body reaches its final combustion loss during one pass through the core. This results in a power density distribution that decreases from top to bottom over the height of the reactor. This has the advantage that, in this mode of operation, the heat exchange of the cooling gas flowing downward through the reactor is optimal. Moreover, even taking into account the limits to be determined from various technical points of view, it is found that at an average power density of 9 to 1 W/approximately 100
It also has the advantage that an average gas outlet temperature of 0 qo is achievable.

However, it is a disadvantage that the graphite of the reflector surrounding the reactor core is damaged due to the active fast neutrons in the energy range from 1 to 107 eV. As a result of the increasing neutron dose, the phenomenon of shrinkage first appears, and an associated expansion and weakening of the graphite of the reflector occurs. This damaging process occurs more rapidly the higher the temperature of the irradiation. To avoid this, conventionally the permissible rapid dose of about 4.7 x 1 p2-2 should not be exceeded. However, the permissible loading of the fuel assembly allows an increase in average power density up to lamW/〆, which is an undesirable power density limitation. The limit of this world's power, that is, the limit before damage to the graphite of the reflector still appears, was conventionally about 7 to 10 MW/. It is therefore an object of the present invention to create the conditions for the possibility of increasing the average power density and therefore the total power of a reactor supplied with such - spherical fuel bodies over the same remaining lifetime. It is. The invention proceeds from the recognition that it consists in avoiding rapid doses in the most heavily loaded areas of the reflector.

The invention also proceeds from the recognition that in particular the internal limiting layer of the top reflector and the upper part of the side reflectors are exposed to high dose loads. The invention further provides that the lower region of the reflector is exposed to a relatively small dose, but this takes into account that the cooling gas flowing through the core is heated to a temperature of over 850°C. This is also based on the recognition that if the distortion of the workpiece material used due to high temperatures is not taken into account, it will not limit the power output of the reactor. The above-mentioned problem is solved in the case of a reactor of the first mentioned type according to the invention, in that part of the wall of the top reflector and/or of the upper part of the wall of the side reflectors facing the fuel body deposits. The solution is to use a substance that absorbs neutrons or reduces the velocity of neutrons in or near it.

A significant reduction in the fast neutron radiation in the region affected by the dose of the reflector is thereby achieved. Insofar as the measures according to the invention are used, depending on the circumstances, the neutron east is thereby as above 30 to 4
It is reduced within a limited range of 0%, or 20% to 80% if necessary. Since it has the effect that the dose damage is also reduced to the same extent,
It is immediately possible to increase the power density of the reactor accordingly. A very advantageous embodiment of the nuclear reactor according to the invention provides that on the wall of the top reflector and/or on the top of the wall of the side reflectors, a substance absorbing neutrons, such as boron, hafnium or the like, is added. It consists in containing coated particles with a diameter of several hundred meters, which contain the substance.

Other advantageous measures which may be used instead of or in conjunction therewith include holes, cavities or the like in the wall of the top reflector and/or in the upper part of the wall of the side reflectors containing boron, hafnium, etc. It consists in that rods containing a substance that absorbs neutrons, such as or the like, are arranged. Due to the high absorption of neutrons within the reflector achieved by these means, it is clear that within a distance of about 80 degrees from the wall of a reflector, the thermal neutron concentration within the spherical pile is significantly reduced. This has the effect that, as a result of the associated reduction in the splitting rate, the influx of fast neutrons into the interface of the reflector region, where the above measures are carried out, is reduced by 80% by the neutron-absorbing material in a dose-dependent manner. . If necessary, it may also be expedient to reduce the dose in the lower part of the reflector, if very high gas outlet temperatures are desired or if particularly dose-sensitive workpieces are used. Instead of adding absorbent material in coated form,
It is of course also possible to provide the reflector wall in the area mentioned in other ways, for example in the form of uncoated granules.
In addition to boron and hafnium, other substances suitable as absorbents can of course also be used, such as manganese, iron, nickel and gadolinium, with suitable mixtures of absorbers also being possible if desired. In that case,
The fact that these materials have different working areas and that the concentrations of manganese, iron, and iron do not change substantially during decades of reactor operation can be fully exploited. Boron, gadolinium and hafnium have a high absorption area. Therefore, it is necessary to add them only at very low concentrations. However, they are combustible and each must be renewed at intervals of several years. The build-up of the absorber on the reflector wall means that in this part of the reflector the temperature of the helium flowing towards the reflector wall is approximately 100 degrees lower than in the case of no loading on the reflector wall of the workpiece material absorbing neutrons. , and 150°C lower. This significantly increases the lifetime of the reflector. By this measure the power density is further increased in the inner region of the core. As a result, the amount of fast neutron radiation that accumulates on the reflector interface during the operating period of the reactor is reduced. Another means to increase the power of a bubble bed reactor with a deposit of spherical fuel bodies is to contain substances that absorb neutrons and/or reduce the velocity of neutrons, It consists in feeding the nuclear reactor from above with spherical fuel bodies that act only over a predefined distance during passage. Adding a small amount of boron to the fuel body and supplying the fuel body in such a way that the fuel body so treated is distributed over the entire surface of the spherical deposit is already similar to the addition of boron to the upper reflector. has a similar effect. Another highly preferred measure according to the invention, which can be used together with or instead of it, is to provide a nuclear reactor with 20 to 4 spherical fuel bodies containing substances that absorb neutrons and/or substances that reduce the velocity of neutrons. It consists in supplying V to the side green band in contact with the side reflective material inside. In this case, it may be expedient to add hafnium, boron or the like as absorbent to the spherical fuel bodies which are introduced laterally into the batch. Preferably, a gradually burning absorbent material is added to the fuel body that is fed close to the sidewalls of the reactor. In this case as well, the absorbent material can be contained within the fuel, either contained within it or contained within another element. The addition of neutron absorbing material can be carried out to the spherical fuel body containing the fuel or into a separate spherical element. The supply of elements containing combustible neutron-absorbing substances is particularly highly advantageous if this measure is carried out in the upper half of the reflector and preferably in the upper third of the reflector. .
Instead of using combustible absorbers, it has also proven advantageous to use plutonium isotope mixtures with a high proportion of Pze4o as neutron absorbers.

This is advantageous since Pu2 has a very high absorption cross section in the neutron spectrum present during nuclear reactor operation. So it practically acts like a combustible poison. Furthermore, the plutonium isotope Pu
The use of isotope Pu241 by neutron capture
It has the advantage of converting into Since this stuff is a fissile material, it leads to a very economical way of operating a nuclear reactor. Furthermore, because fissile material formation in spherical fuel bodies occurs relatively deep in the core, the power output corresponding to a relatively high fissile material introduction is achieved despite the relatively low introduction of fissile material into the fuel body. It is thereby possible to obtain density.
The Pu side is usually the isotope Pu90P〆400Pu241
Since it exists in the mixture of 10P〆42, when a plutonium mixture is used, it overlaps with the configuration that occurs in the middle of P〆4o, which becomes Pu9 due to neutron capture. This actually has the effect of retarding the combustion on the Pu side.
If a plutonium mixture with a composition corresponding to the case required is charged into the reactor from the side walls of the reflector, the total amount of side reflectors can be reduced in a simple way by reducing the overall upper third. This is accomplished in 1. The fast dose is 20 and 6 according to the amount of plutonium used respectively.
It was revealed that the amount was reduced by 0%. Another highly preferred measure according to the invention is the use of neutrons as neutron absorbing material.

Thereby it is achieved to capture neutrons in the upper part of the reactor and fully utilize the breeding effect of Th females in its lower part. Moreover, since the active cross-section of the m2 shipboard is relatively small, only a portion of the Th232 burns, ie, only about 10%. By using Th phosphorus,
Thermal neutrons at the top of the reflector are only slightly reduced. However, the advantage of this measure lies in the reduction of the concentration of fissile material in the upper part of the core, and by means of this measure the power density in the upper region of the reflector and its transfer to a lower region is achieved. Moreover, a reduction in the dose in the upper part of the reflector is associated therewith. The ratio of Th2 is 2 per spherical fuel body.
It has been found that 0 to 30 pm is preferable.
The measures according to the invention are such that in the reactor 0, in particular in the upper part of the area adjacent to the wall of the reflector, there is a high proportion of the substance that reduces the velocity of the neutrons, i.e. a low proportion of fuel, and now almost no fuel is present. The addition of a fuel body exhibiting a high proportion of neutron moderating material, namely graphite, is very effectively supported by a very advantageous variant of the method of operating a nuclear reactor according to the invention.

If necessary, pure graphite spheres can be added. A spatial displacement of the power density is thereby obtained, such that the power density decreases over the entire height of the peripheral band. That leads to a reduction in the heating of the cooling gas in this region. With the resulting reduction in temperature near the wall of the reflector, a reduction in the neutron dose is thereby also achieved at the same time. However, this measure is primarily used in conjunction with other measures according to the invention. The advantage of the embodiments of the nuclear reactor according to the invention and of the method of operating this reactor consists in its simplicity and high adaptability to the respective existing situation. Therefore, these measures themselves can be adapted very precisely to other situations during the course of operation of the nuclear reactor. That is,
For example, the charging method of a nuclear reactor is such that the effect of high-speed flow on the outer reflector is below the average level of the reactor during the first 5 or 1 operating period of the reactor; It is possible to change the way the reactor is inserted, such as displacing it in the upper quarter of the reactor. In this way a uniform distribution of the dose to the outer reflector is obtained. Since this displacement is actually based on the displacement of the output, it follows that: That is, during the first few years of operation the high velocity dose acts at a relatively low temperature in the upper part of the reflector, so that the loss of graphite in the reflector is very small during this initial period. Then, in the lower part of the reflector, during the operating period associated with itself, at low currents and high temperatures, partial healing increases the radiation damage that has occurred to some extent. The measures according to the invention can be supported by a purposeful utilization and/or by an additional arrangement of absorption rods controlling the operation of the nuclear reactor. The accompanying drawings show some comparative examples to illustrate the measures according to the invention.

FIG. 1 shows the various effects of using the measures according to the invention on the fast neutron east along the axis of the reactor, as well as on the fast neutron east, i.e. along the interface between the reactor and the outer reflector. This is a chart showing high-speed annual doses.

In FIG. 1, each curve represents the following.

Curve 1: shows the neutron flux along the reactor axis.

Curve 0: at the top of the reflector its total cross-sectional area 2a =
Figure 3 shows neutron east along the axis of the reactor when an absorber for thermal neutrons that is 0.0016 arc-1 is deposited.

Curve m: the axis of the reactor when, in the upper part of the reflector, a heat absorber is deposited with its total action cross section, i.e. 2a > 0.1 skin - 1 until complete blackening. Shows neutron east along.

Curve W: Shows neutron east along the interface of the internal wall of the side reflector.

Curve V: 2a = 0.0016 The neutron east along the interface at the internal wall of the side reflector when a heat absorber is deposited on top of the reflector with a total cross-sectional area of -1. show.

FIG. 3 is a schematic explanatory diagram showing one embodiment of a nuclear reactor according to the present invention.

The neutron east shown in FIG. 1 relates to a nuclear reactor with a one-zone core in which spherical fuel bodies are continuously supplied and withdrawn and are present as a deposit of spherules within the reactor.

A nuclear reactor with a one-zone core is one in which the fuel body supply is single and corresponds to fuel bodies of the same type, that is, in the simplest case, with the same overall fuel body charge. It means a nuclear reactor like that.

The power density of this reactor is OMW/〆. The curve labeled 1 shows fast neutron east along the axis of the reactor.

At the top of the reflector its total cross-sectional area 2a = 0.00
Since the heat absorber, which is 16 skin-1, was introduced, the heat flow east and therefore the power density in the lower region of the core varied. At that time, fast neutrons east follow the course shown by curve b. The high velocity dose at the inner twill of the top reflector was reduced by 28%. The equilibrium operating limit set in this case was nevertheless reduced by only 0.4%. Correction was immediately made by increasing the initial accumulation from 6.50% to 6.60%. When the active cross-section of the absorber above the reflector is increased to complete blackening, the neutron east takes the shape shown by curve m. It is 82 at the interface
%Diminished. Curve N shows the fast neutron east along the interface between the core and the side reflectors without the measures according to the invention. The course of curve V shows the displacement of the power density when the side reflector is lightly poisoned within about 80 skins of the upper side of the spherical deposit. In this case, the cross-sectional area of action is 0.
It was 0016 Kay-1. As is clear from the course of the curves, the power density in the core was shifted by using the measures according to the invention in such a way that the fast dose was reduced by about 30% in the region mentioned.

In order to achieve the action area 2a = 0.001&ne-1,
0.15% manganese was mixed with the graphite of the reflector.

This manganese addition is 0.72% by volume F or 0.3
This corresponds to a mixture of Ni in group volume %. The relationship shown in FIG. 2 is such that spherical fuel bodies are continuously supplied and removed;
and 2, such as those present as spherical deposits in nuclear reactors.
A nuclear reactor with an obijo core was studied. The power and power density of this nuclear reactor was reduced to W/. A nuclear reactor with a two-zone core means a reactor with two fuel body supplies.

Charge in the marginal zone adjacent to the side reflector of the spherical deposit (
In this case, the dose of fast neutrons at the reflector interface over the length of the reactor corresponds to the course of the curve indicated by A (Th2321360 and 35
It contained 1 Madarayu.

For economical reasons, spherical fuel bodies and pure graphite spheres were mixed in a ratio of 75.5:24.5, so that the content of breeding and fissile substances in the fuel bodies used was U2 salt was 2.1 pm.
For the fast neutron east or annual dose shown by curve B, each Th2 approximately 1 per 100 spheres.
It contained 890 yen and U2SI22 ta. No pure graphite spheres were added with the fillers. The amount of fissile material in the sphere itself was reduced. These spheres contained Th23,218.9 and 351.2, respectively. As is evident from FIG. 2, the fast neutron dose in the upper third of the reflector was reduced by 24% on average by the measures shown for curve B.

Indeed, in the lower region of the reflector the fast neutron dose increased slightly due to the enhanced multiplication effect. However, this does not reach the technically permissible limit in this area anyway, so we can live with it. The superiority of using the measures according to the invention is also explained by the fact that the costs for the circulation of the fuel are practically the same as without these measures.
Referring now to FIG. 3, reference numeral 1 designates a reactor pressure vessel according to the invention, which contains a fuel deposit la charged from above in the direction indicated by arrow 2. . This reactor pressure vessel 1 has a supply pipe 3 for the usual fuel elements and, furthermore, a supply for such fuel elements as additionally containing neutron absorbing substances and substances reducing the velocity of neutrons. It has a tube 4. Further, the reactor pressure vessel 1 has a checkpoint 5 for inflow of cooling gas introduced through the fuel element introduction path 5a and the inflow conduit 14.
A reflector structure is provided which encompasses the furnace ceiling 5 with b.

The cooling gas passed through the conduit 14 first enters the upper cooling gas storage chamber 1, and then passes through the inlet 5b. The above reflector structure further includes a leg reflector 6 and a bottom reflector 7.
The reflective material 7 has a cooling gas storage chamber 11 described below and a flow path 7a for passing cooling gas discharged from the chamber 11 through a conduit 15. Section 8 of the top reflector 5 and section 9 of the side reflectors 6 contain neutron absorbing material embedded therein in the form of bullet-shaped grains or rods.
The reactor pressure vessel 1 also includes a fuel element removal pipe 12 and a fuel element storage device 12a. Finally, it is noteworthy that the outer peripheral green zone 16 of the preferably pole-shaped deposit la contains neutron-absorbing substances and/or substances that reduce the velocity of neutrons. Of course, the invention is in no way limited to the particular embodiments shown in the accompanying drawings, but also encompasses any modifications falling within the scope of the claims.

The gist of the present invention is the description of the claims, but
Examples of implementation include the following. '11 In the wall of the top reflector and/or in the upper part of the wall of the side reflector there are coated particles with a diameter of several hundred mm containing a substance that absorbs neutrons, Nuclear reactor according to claim 1. ■ Claims in which rods containing neutron-absorbing substances are arranged in holes, cavities or the like provided in the walls of the top reflector and/or in the upper part of the walls of the side reflectors. Range 1 and above (
1} nuclear reactor.

'3' Patent for supplying 20 to 40 spherical elements containing substances that absorb neutrons and/or reduce the velocity of neutrons to the reactor core in a marginal zone adjacent to the leg reflector. A method according to claim 2.

'4} Claims and above {3'
'Koyoru method. '51 A method according to claim 1 or '3' above, which uses a plutonium isotope mixture having a high proportion of P〆40 as a neutron absorbing material.

'6} Using Th23 as a neutron absorbing substance,
A method according to claim 2 or the above legs. '71
The method according to [6}' above, wherein the ratio of Th232 is between 20 and 30 per spherical element.

'81 A method according to claim 2 and '3} above, in which graphite is used as a substance that reduces the velocity of neutrons.

[Brief explanation of the drawing]

FIG. 1 shows the various effects of using the measures according to the invention on the fast neutron flux along the reactor axis and the fast neutron flux along the interface between the reactor and the outer reflector, i.e. This is a chart showing high-speed annual doses. Figure 2 is a diagram showing the fast neutron east or annual dose along the internal boundary of the leg reflector. FIG. 3 is a schematic explanatory diagram showing one embodiment of a nuclear reactor core pressure vessel according to the present invention. In FIG. 3, the main parts are shown with reference numerals as follows. 1... Furnace pressure vessel, la... Fuel deposit, 2... Fuel charging direction, 3... Normal fuel element supply pipe, 4...
・Joint tube of fuel elements containing also a neutron absorbing substance and a neutron moderating substance, 5... Upper reflector, 5a...
...Fuel element introduction path, 5b...Cooling gas introduction opening, 6...Side reflector, 7...Bottom reflector, 7a...Cooling gas Outflow channel, 8...
The neutron absorbing material part of the top reflector, 9...
・Neutron absorbing material part of side reflector, 10...
- Upper cooling gas storage chamber, 11... Lower cooling gas storage chamber, 12... Fuel element extraction pipe, 12a.
...Fuel element storage device, 14...Cooling gas inlet pipe, 15...Cooling gas outlet pipe, 16...
The outer edge band of fuel deposits. Figure 3 of the map

Claims (1)

[Scope of Claims] 1. A reflector made of graphite consisting of a top reflector, a side reflector and a bottom reflector, which is substantially formed from a spherical fuel body deposit and at the same time has a heat transfer medium flowing through it. A spherical fuel body formed with an envelope of graphite is passed from above to below through the core surrounded by In a pebble-bed nuclear reactor, in the portion of the wall of the top reflector facing the fuel body deposit, and/or within or near the top of the wall of the side reflector, reaching a final combustion loss of The pebble bed nuclear reactor described above, characterized in that a substance is used that absorbs neutrons and reduces the velocity of additional neutrons. 2. A spherical fuel body containing a substance that absorbs neutrons and/or a substance that reduces the velocity of neutrons, and which acts effectively only over a predefined distance as the fuel body deposit passes through the reactor core. A method for operating a nuclear reactor according to claim 1, characterized in that the reactor is supplied from above.