JPH11295464A - Control element for reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form mechanical resistance for an absorption material by providing a plurality of covering parts for absorption material, and at the same time providing a specific interval between the covering parts for absorption material surrounding other covering parts. SOLUTION: The minimum interval is provided between covering pipes 10 and 11 corresponding to the resistance of the innermost covering pipe 10. Therefore, even if the innermost covering pipe 10 fails, the adjacent covering pipe 11 is not damaged. Accordingly, the inflation of an absorption material 8 is extended to a second covering pipe 12 from the outside, and further the outermost covering pipe 13 for forming an outside covering part 7 for absorption material. Therefore, at least three covering parts for absorption material are provided, and a specific gap exists among covering parts for absorption material that surrounds other covering parts, so that each covering part for absorption material can leave a starting position when the absorption material is inflated and mechanical resistance for the absorption material can be formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an absorbent material,
A control element for a nuclear reactor comprising at least one inner and outer absorbent coating.

Here, the concept of a control element is necessary for regulating the power of a nuclear reactor, and the reactor must be able to safely shut down the reactor from any operating conditions. Control element and control rod for a nuclear reactor. These control elements absorb neutrons, thereby controlling the chain reaction,
It is inserted into or between fuel elements or nuclear fuel rods.

[0003] In the case of a boiling water reactor, the control elements are:
For example, in a large output area,
That is, on average, exactly one of the neutrons released during fission is dropped deep into the fuel element so that a new fission is induced.

When attempting to use control elements during the life of the reactor, which is about 40 years of service, the effectiveness of these control elements is not reduced by more than 10% compared to the original effectiveness. The control element can be loaded with a constant neutron flux.

In addition, these control elements must contribute to achieving the overriding protection goal of maintaining the perfection of preventing the release of radioactivity from the reactor cooling system.

It has been found that the control elements and, in particular,
The absorbent covering for accommodating the absorbent belongs to the consumable material. This is because the absorbent material expands significantly due to neutron capture, which leads to mechanical damage to the absorbent coating and subsequent washing of the absorbent material (Auswaschung). Further use of such control elements,
As a result, the local power density distribution in the core increases, which, under certain assumptions, causes even fuel rod damage, in addition to the release of tritium into the biosphere. However, depending on the predetermined arrangement of the reactor, in particular the arrangement of free gaps between the fuel element boxes in the case of boiling water reactors, and in the case of a plurality of control rod guide tubes in the case of pressurized water reactors The adaptation and optimization of the control element is limited in terms of the life of the control element. With the choice of the material of the absorbent covering and the thickness of the absorbent covering, it has already been attempted to achieve the longest possible life of the absorbent covering and thus also of the control element. However, the success of these attempts was limited.

[0007]

2. Description of the Related Art A control element of the type described in the "Field of the invention" is known from DE 39 03 844 A1. In this publication, a plurality of inner pipes for storing an absorbent material are put into one storage hole. As described in DE 41 38 030 A1, it is proposed to provide a plurality of control rods with a plurality of longitudinally extending channels, i.e. channels in which the expanding substance can expand. .

Other control elements are described, for example, in EP 0 143 661, US
4,861,544 and US 4,929,412. Control elements, especially for pressurized water reactors, are described, for example, in "Nuclear Technology 57
(1992) No. 2 (Karl Hanser, Munich), pages 84 to 89, by Siemens for pressurized water reactors and modes of operation, by L. Heins, W. Damdiets and HP Fuchs. Control Assembly Design ". A further description of this type of control element can be found in G. Festerlund et al., "ABR Control Rod," on pages 105 and 106 of Nuclear Technology 57 (1992) 2 (Karl Hanser Publishing Company, Munich). 』
It is in.

[0009]

SUMMARY OF THE INVENTION The object of the invention is to provide a control element for a nuclear reactor, which comprises one absorbent material and at least one inner and outer absorbent coating, in particular a highly localized That is, it is configured such that a large relative burnup is added.

[0010]

SUMMARY OF THE INVENTION The object of the present invention is to provide at least three absorbent coatings, and to provide a predetermined interval between the surrounding absorbent coatings. This is solved by the fact that each absorbent covering can be moved away from the starting position (Ausgangsposition) during the expansion of the absorbent and a mechanical resistance for the absorbent is formed.

[0011] Preferred refinements of the invention are set out in the dependent claims.

According to the essential basic idea of the present invention, the absorbent covering has a restricting means, which is provided substantially inside the absorbent covering. Being provided in a starting position adjacent to the material, the limiting means creates a mechanical resistance for the absorbent material and can be moved away from the starting position upon expansion of the absorbent material. To this end, a coating for the absorbent is created which, overall, prevents the uncontrolled and very rapid expansion of the absorbent by reaction forces.
In this case, the restricting means can no longer withstand the expanding absorbent during a certain expansion and bends. The restricting means can be moved away from the starting position such that the restricting means cracks, breaks, is guided out in a controlled manner and further forms mechanical resistance for the absorbent material. The concept of "can separate" (entfernbar) means that the limiting means is actually spatially separated from its starting position, or by a rupture or other partial destruction
It should be understood that it loses its direct function of limiting inflation and applying pressure inward, but the rest physically remains in the starting position. The control element thus produced can be provided with a local relative burnup of almost 100% without the burned absorbent reaching the reactor coolant.

According to the present invention, for this purpose, at least three absorbent coatings are provided, and a predetermined interval is provided between the absorbent coatings surrounding each other. Thus, each absorbent covering can move away from the starting position upon expansion of the absorbent, creating mechanical resistance for the absorbent.

The outer absorbent coating forms the outer hard jacket, while the inner absorbent coating is first adjacent to the absorbent and resists the expanding absorbent. However, at constant pressure, the inner absorbent coating may crack, causing the absorbent to expand further into the outer absorbent coating. The inner absorbent covering may, for example, form a semicircle connected to the outer absorbent covering.

According to the invention, there are provided three or more coverings for the absorbent material surrounding each other.
This is because a plurality of resistances to the expanding absorbent material can thus be made in a stepwise and fully presettable manner. These resistances in turn weaken, thus enabling a particularly long service life of the control element which can completely burn the absorbent material. Since a predetermined distance is provided between the absorbent covering portions surrounding each other, the inner absorbent covering portion may be cracked or broken by other methods at each time. However, the outer absorbent covering immediately after is not damaged. Since the predetermined interval is calculated according to the effective gradual deformation ε until the rupture of the substance used, the covering portion for the absorbent material is:
Before this absorbent coating can break,
Within a predetermined interval, it can expand under the pressure of the absorbent material. It is particularly preferred that the plurality of absorbent coatings are formed such that the outer absorbent coating completely surrounds the inner absorbent coating. These absorbent covering parts are a plurality of overlapping absorbent covering parts. They first exert pressure on the absorbent material which expands from the inside to the outside and then bend and crack under the pressure. At this time, the adjacent absorbent covering portion applies mechanical pressure to the continuously expanding absorbent, thereby effectively preventing uncontrollable expansion of the absorbent. It is particularly advantageous to form these absorbent coatings as cladding tubes. This is because such cladding is already in use and can be taken from technical experience and embodiments. According to the invention, a plurality of superposed cladding tubes which can be pushed together are provided. The diameter of these cladding tubes is so different that there is a certain spacing between the individual cladding tubes. Therefore, the inner cladding expands and cracks under the pressure of the expanding absorbent without the adjacent cladding being damaged.

[0016] Alternatively, it is theoretically possible that the limiting means consist of an elastic, heat-resistant material which is internally attached to the outer cladding tube forming the covering for the absorbent material. With such a material, pressure would first be applied inward to the expanding absorbent, but then, under the increasing pressure of the absorbent, it would bend and press outward. Another possibility consists in making the limiting means mechanically movable. Hence, the limiting means consist, for example, of movable shells, which are pressed by a mechanical element, for example a spring or several other elastic intermediate parts, onto the absorbent and start under the pressure of the absorbent It can be pushed away from the position.

In a preferred configuration of the invention, the dimensions of the inner absorbent covering or limiting means are such that the inner absorbent covering is suitable for accommodating a given sintered pellet of absorbent material. It is defined as it is. Alternatively, a powdery absorbing material can be used. B4C is preferably used as the absorbent. B4C has a good cross-section for neutron capture, especially for thermal neutrons, but, like all known absorbers, has a significant neutron-induced expansion. This expansion will destroy the absorbent coating over a long period of time. Other absorbents that can be used, for example, are AgInCd or substances containing boron and enriched in isotope B10.

It is particularly preferred to use B4 C having a theoretical density of less than 70%, especially less than 60%. This is because, in this way, the expansion of the absorbent material can first be prevented and a particularly high burnup of the substances used is achieved. However, care must be taken in optimizing the initial density of B4C that the absorbent must be sufficient to reach the target burnup in order to be able to maintain the effectiveness standard. Has 10B atoms.

Preferably, the plurality of cladding tubes are formed such that they comprise a plurality of portions. The parts of the cladding are dimensioned such that these parts of the adjacent cladding, in particular the impingement surfaces of the cladding, are offset from one another. The use of multiple parts achieves relatively easy handling, and the different dimensions of the multiple parts, or of at least one of the first used parts, allows the impingement surface between adjacent cladding tubes to be directly It happens that they are not juxtaposed and thus would create weak spots.

Preferably, such control elements are used in boiling water reactors and pressurized water reactors. In a boiling water reactor, the control element is usually assembled by four cruciform wings with a maximum of 12 absorber coatings designed as cladding tubes. In contrast, for pressurized water reactors, control elements commonly referred to as control rods, which enter the core from above, are used. The control element according to the invention can in principle be used for all types of nuclear reactors in which such absorbers are used.

The expansion of the absorbent can be accommodated by the control element of the present invention, and the outer absorbent coating will fail, and the entire control element can no longer be used. Even in such a case, it is not impossible to use almost the entire absorbing material at a plurality of positions where the neutron flux has the maximum value. The model calculations clearly show that the control element of the present invention can achieve a burnup of between 90 and 100% of the absorbent, preferably in fact 100%, when using B4C. did.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on one embodiment shown in the drawings. FIG. 1 schematically shows a core cell provided in a boiling water reactor having four fuel elements 3 each having a fuel element box 2 and one control element 1. The fuel element box 2 surrounds each fuel element 3 and is provided in a square shape.
Since there is a gap between the individual fuel elements 3, a single cross-shaped control element 1 can be moved between the fuel elements 3. There is a free water gap between the fuel element boxes 2. The fuel element box 2 is held by an upper core grid 4 and a lower core grid 5. The control element 1 has a plurality of absorbent coatings 7 filled with an absorbent 8 and is moved up and down between the fuel element boxes 2 as required, so that a controlled chain reaction (Keff -1) is achieved. ) Is maintained. The control element 1 controls the control element 1 so that the chain reaction can be immediately stopped from any arbitrary state.
Is designed so that it can be fully and immediately lowered when needed.

FIG. 2 is a cross-sectional view of the control element 1 showing how the control element 1 can be used, for example, in a boiling water reactor. The control element 1 has four wings 6 arranged in a cross shape, and these wings 6 are composed of a maximum of 21 absorbent coatings 7 filled with boron carbide (B4 C) powder. Having. B4C is preferably used as a neutron absorber due to its favorable physical and technical properties. The plurality of absorbent coverings 7 have a sheet jacket which ensures mechanical perfection. These absorbent covering portions 7 are basic components of the control element 1. This is because, unlike the initial assumptions, the absorber coating cannot be used for the entire life of the reactor and must be considered a consumable material. Already after only a few operating cycles, the neutron-induced expansion of the absorber 8 by up to 15% causes mechanical damage to the absorber coating 7 and
Subsequent washing of the absorbing substance, that is, the absorbing material 8 occurs.

FIG. 3 shows a cross section of the absorbent covering 7 of the present invention. The very expanding absorbent 8 can also be kept safe, especially for a long time, by the covering 7 for the absorbent, giving the absorbent used a nearly 100% local relative burnup, B4C thus also does not reach the reactor coolant. The absorbent material 8, which can also be used in powder form, is provided in FIG. 3 in the form of sintered B4 C pellets and is accommodated by the first and innermost cladding tube 10. The combustion reduces, on the one hand, the neutron cross section of the B4 C absorber, and on the other hand, the absorber 8 expands with expansion and thus also to local and central density changes, and of the cladding 10. It is urged that the internal pressure on the steel shell increases with combustion. However, since the creep speed of the cladding tube 10 is very small (Δε (cladding tube) / Δt << Δr (absorbing material radius) / Δt) as compared with the absorbing material growth rate, various forces acting on the cladding tube are small. , Is substantially controlled by the expansion properties of the absorber and therefore by the neutron fluence represented by:

[0025]

(Equation 1)

After exceeding the critical neutron fluence, ie, the amount of neutrons required to expand the absorber to a certain extent, and exceeding the critical strength of the cladding, the innermost cladding tube 10 fails and bursts. Thus, the pressure of the absorber 8 is first significantly reduced. More specifically with respect to neutrons, the absorber 8, and especially the ceramic zone formed by the outer regions, increases further with increasing combustion and finally reaches a further critical neutron fluence. Then, the absorbent 8 has a critical solid pressure, ie
A solid pressure is generated again which exceeds the proof stress εBruch of the second cladding tube 11 and also causes the second cladding tube 11 to fail. A minimum distance is provided between the cladding tubes 10 and 11, which corresponds to the proof stress of the innermost cladding tube 10. Therefore, even if the failure of the innermost cladding tube 10 occurs, the adjacent cladding tube 11 is intact. Correspondingly, the expansion of the absorbent material 8 is applied to the third or second cladding tube 12 from the outside,
Outermost cladding tube 1 forming outer absorber covering portion 7
Continue to 3. A plurality of coatings for absorbents connected in series,
In particular, due to the action of the cladding, the relative combustion averaged over the absorption cross-section is always increased by a constant value according to the cladding constant and the number of coatings for the absorbent material, and ultimately 100
% Is reached. Absorber 8
Is usually 7 for the highest theoretically conceivable density.
Having only an initial density of 0%, a 30% free volume can be stated. This free volume, which serves for the expansion of the absorbent, is used in the process described above to cover the absorbent coating 7.
And the constant of the material of the cladding tube of the covering member 7 for the absorbent material.

FIG. 4 shows the relationship between the critical local burnup distribution a (r) and the mean to the radius r at the time of the critical situation, ie, cladding failure, due to the large neutron fluence used. It is a diagram showing a certain burnup a. On the y-axis with reference numeral 14, the relative burnup is shown in%. The reference number 15 is attached to the x-axis, on which the radius of the absorber is indicated in mm. The line labeled 17 is the critical local burnup distribution a for the standard control element with a single absorbent coating.
(R). The covering 7 for the absorbent material is about 1.7.
It has a radius of 5 mm. Only in the outer edge area,
It is clear that a 100% combustion zone is created, i.e. the B4C absorber is baked into a hard ceramic structure, where the originally useful 30% free volume is completely depleted. Becomes Inside, the free volume is still sufficient, where useful absorbent material has not been used effectively. The solid line 16 shows the average burnup associated with the absorption cross section, which is about 50% for such an absorbent coating.

FIG. 5 is a diagram illustrating the critical local burnup distribution and the average, relevant, critical burnup for the absorbent coating of the present invention.
In this diagram, the local burn-up in% is shown on the y-axis with reference numeral 28, while the radius is shown on the x-axis 18. Upon reaching the first critical fluence and the associated rupture of the innermost cladding tube 10, a local burnup distribution, indicated by the outer line 19, results. This burnup distribution substantially corresponds to that of FIG. In this case, continuous line 2
Average, relevant critical burnup am1k, indicated by 0
A rit occurs, the burnup of which is about 50%. 0 to r
_In the region outside to ₁ ', there is 100% complete burnup. When the fluence of the second criticality is reached and, in this connection, the rupture of the second cladding tube 11, the critical local burnup distribution a (r 2), indicated by the reference numeral 21, becomes critical. Occurs in the future. At that time, the hard ceramic zone had already expanded to the range r2 '. Even in the inner region, the local burnup was even higher than at the time of the first critical fluence. Average, relevant critical burnup am2
The krit is already well 70% and is indicated by line 22. Upon bursting of the third cladding, a local burnup distribution, indicated by line 23, occurs, and the inner, relevant critical burnup distribution, indicated by line 24, is about 90%. Upon reaching the fourth critical fluence, only a few remaining zones of the absorber remain, corresponding to the line 25, in the region of the axis of symmetry 27. The absorbent has not yet been fired into a rigid ceramic structure. At this point, an average, relevant, critically close to 100% burnup, indicated by line 26, has occurred. Further, cladding tubes 10 to 13 are shown for the purpose of illustration. Line 19 in relation to innermost cladding 10
And 20 are associated with the second cladding 11 by the lines 21 and 2
2, lines 23 and 24 in relation to the third cladding 12
Lines 25 and 26 are provided in connection with the fourth cladding tube 13. Even in such a depiction, the cladding tubes 10 to 1
The required spacing ε between 3 is again shown. This interval ε
Is determined from time to time as a function of the strength of the next, inner, cladding tube.

In FIG. 6, the burnup am averaged over the absorption cross section is expressed in% on the y-axis denoted by reference numeral 30.
Then, on the x-axis 29, the theoretical accumulation of all cladding tubes (s
ummarisch) is indicated in mm. However, it was based on the fact that each covering portion for the absorbent material had a coating tube thickness of 0.1 mm. As a supplement, the number of cladding tubes is shown on the second x-axis 39. The calculation of the burnup made at this time was made based on microscopic theory. In this case, curve 31
In the above, 70% of the theoretical density of the absorbing substance, that is, B4C was assumed, whereas the curve 32 assumed 57% of the theoretical density, and the curve 33 assumed 50% of the theoretical density. B4
Here, 3 mm was assumed as the inner diameter of C. However, as a result of the calculation, the calculated value for the burnup am is B
Since it does not depend on the inner diameter of 4C, for example, 2.7 mm
It is interesting to note that a plurality of cladding tubes having an inner diameter of 3 mm have also been found to be used with the same results as 3 mm. From curve 31 it can be seen that a 0.1 mm thick first cladding tube at the location indicated by reference numeral 34 does not work when the theoretical density is 70% and the burnup is already about 45%. It is clear that this is the case. A second cladding, also having a wall thickness of 0.1 mm, would allow for a burnout of exactly 60%. This is confirmed at the location indicated by reference numeral 35. As is evident from the position indicated by reference numeral 36, the third cladding achieves a burnup of about 65%. For the fourth cladding, at location 37, about 70% of the combustion occurs, depending on the accumulated neutron fluence of about 5.95 × 10 ²¹ n / cm ² . As is evident from the line indicated by reference numeral 31, the burnup is further increased by using other cladding tubes.
When using relatively low theoretical densities, higher relative burnups are achieved, as is evident from the curves indicated by reference numerals 32 and 33. Thus, for a theoretical density of 50% (curve 33), a third cladding (position 36) achieves a burnup of 80% or more. 8.3 × 1
Depending on the accumulated neutron fluence of 0 ²¹ n / cm ² , as can be seen from the position indicated by reference numeral 37, a burnup of already 90% is achieved by the fourth cladding tube. Each cladding has a wall thickness of 0.1 mm. There is a space of about 0.01 mm between the cladding tubes.
The cladding is unevenly thick, for example, the inner three claddings are each 0.1 mm thick and the fourth outer cladding is 0.5 mm.
mm, the innermost cladding has a thickness of 0.5 mm, followed by three outer claddings each having a thickness of 0.1 mm. , Overall preferred. Due to the design according to the invention of a plurality of control rods, calculated according to microscopic theory and proven correct by measurement, the pressure loading of these control rods is
In principle, it has been found that the swelling of B4C is required. Conventional constructions allow only a burnup am of about 50% of 10B. However, in order to eliminate local neutron flux protrusions, it must be ensured that a local burn-up of up to 100% of 10B does not damage the absorber coating. In some cases, the control element of the present invention provides that the control element
In particular, a plurality of absorbent coverings can be provided only in the region above and to the wing edge zone of the control rod.

[Brief description of the drawings]

FIG. 1 is a schematic view of a core cell provided in a boiling water reactor.

FIG. 2 is a plan view of a control element.

FIG. 3 is a cross-sectional view of an absorbent covering portion of the control element of the present invention.

FIG. 4 is a diagram showing the “critical” local burnup distribution of a standard control element at the start of cleaning.

FIG. 5 is a diagram showing a burn-up distribution of the control element of the present invention.

FIG. 6 is a diaphragm to illustrate the achievable burnup.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Control element 3 Fuel element 7 Absorbing material covering part 8 Absorbing material 10 Cladding tube 11 Cladding tube 12 Cladding tube 13 Cladding tube

Claims (10)

[Claims] 1. A control element for a nuclear reactor comprising one absorbent and at least one inner and outer absorbent covering (7), wherein at least three absorbent coverings are provided. That is, since a predetermined interval is provided between the covering parts for the absorbing material that surround each other, each covering part for the absorbing material separates from the starting position when the absorbing material expands. Control element, wherein a mechanical resistance for said absorber is formed. 2. The control element according to claim 1, wherein the outer absorbent covering at least partially surrounds the inner absorbent covering. 3. The control element according to claim 1, wherein the outer absorbent covering completely surrounds the inner absorbent covering. 4. The covering part for an absorbent material is a cladding tube (10, 1).
4. The control element according to claim 1, wherein the control element is formed as (1, 12, 13). 5. The dimensions of the inner absorbent covering are such that the inner absorbent covering is suitable for accommodating a given sintered absorbent pellet. The control element according to claim 1, wherein: 6. The control element according to claim 1, wherein B4C is used as an absorbing material. 7. The cladding tube comprising a plurality of parts,
7. The cladding tube according to claim 1, wherein the cladding tubes have dimensions such that these portions of adjacent cladding tubes, in particular the collision surfaces of the cladding tubes, are offset from one another. Control element described in the above. 8. B4 C having a theoretical density of less than 70%
The control element according to any one of claims 1 to 7, wherein the control element is used. 9. The control element according to claim 1, wherein the control element is a component of a boiling water reactor. 10. The control element according to claim 1, wherein the control element is a component of a pressurized water reactor.