BG99135A - Fuel element with driving out bodies - Google Patents

Abstract

The fuel element, especially such with water under pressure of Soviet design type, is used in the nuclear power industry. It performs spectral shift where part of the fuel stems (R1 & R6) are replaced by hollow pipes (P) each of which, during the first operational cycles of the element, containing a driving out body (B), e.g. zircalloy stems, and the last operational cycles containing liquid retarder. The positions with hollow pipes occupied by material having no fuel substance which does not absorb neutrons, are suitable for fuel elements having hexagonal cross section. The described positions are separated from the centre of the fuel stem.

Description

The invention originates from a thermal nuclear reactor and relates to a fuel element having the features in the limiting part of claim 1 (EP-A-O54234, EP-A-iU183O69). In this case, fuel-containing and non-electron-absorbing stems are inserted, embedded, and the invention also relates to a hexagonal cross-sectional fuel element and positions for receiving stems of this kind.

For different types of reactors, it may be advantageous to arrange hollow tubes between the fuel stalks of the fuel element, which do not contain any fuel material. For example, in boiling water reactors, water filled pipes are used to feed the flowing water to the upper part of the fuel element, where due to the increasing steam generation, the quantitative ratio of the fuel to that required as a retarder / moderator / water is adverse to water. These water stalks are opened up and down to allow water to flow freely. Moreover, the fuel elements are surrounded by the housing and the fuel cells, the measuring instruments and reaction controls being located outside the housing of the fuel cells.

In most pressurized water reactors, the instrumentation and control of the reactor is accomplished by means of appropriate elements which are inserted into the hollow tubes located between the fuel stalks of the individual elements.

D13-OS 35 15 469 describes a fuel cell for a water-cooled high-converter reactor having a hexagonal cross section with stalk positions that are spaced equidistant / on the correct hexagons between e> e at a central position in the axis of the fuel element and the edge of the right hexagon. In this case, some positions of the fuel stems are replaced by guide tubes and others by closed top and bottom water pipes. The water of these water stalks should, on the one hand, improve the retarder / moderator / ratio in the reactor and, on the other, not participate in the water circulation. The hydraulic cross-section of these water tubes thus acting is enlarged relative to the cross-section of the fuel stems in order to concentrate the water flow and thereby cool the fuel stems.

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In Soviet-type pressurized water reactors, the fuel stems and tooling tubing are covered by the fuel element housing, however, the absorber elements are axially mounted on certain fuel elements, such as, for example, a fuel element to control the reduction of the fuel element. the nuclear reaction is located externally outside the hot nuclear zone and then the connecting control element is inserted within the zone.

The management of the reaction by absorbing elements most commonly _w is based on the fact that the replacement of the fuel element is inserted fresh fuel elements which have able to burn neutron poison. In this case, it is a neutron absorber that absorbs excess neutrons at the start of the fuel element cycle and thus maintains the reactor power to an optimum value for which the reactor is calculated, but consumes increasing during the cycle so that, despite the decreasing neutron emission of fuel consumed, reactor power remains practically constant until the end of the cycle.

EP-A-0054234 describes a reactor fuel element is water iodine pressure, in which tubes are inserted at the locations of several fuel stems into which ubiquitous nuclear response control stems (control stems) and partially eject stems are inserted . The ejection stems contain neither combustible material nor absorbent material, but only eject the cooling water used as a retardant. According to EP-A-O183O69, stems containing blank pellets for fuel may be used in place of these stems. In both cases, it is intended that these stems be removed from the reactor core when the reactivity weakens after a longer duration of use. Through this, the delay ratio and then also the energy spectrum of the emerging neutron radiation are modified such that the losses of reactivity are compensated by at least a partial (spectral shift). In addition, the reactor contains mechanical devices that push the ejection stems into positions corresponding to the current state of operation and its optimal energy spectrum. A single reactor cannot be further equipped by replacing existing fuel elements with fuel elements with ejection stems for spectral displacement within the meaning of these publications, rather wide-ranging changes to the reactor structure are required.

EP-A-0153240 relates to an epithermal nuclear reactor, the interior of which contains fuel cells, which in addition to said fuel stems also comprise stems containing nuclear fuel capable of reproduction (natural uranium). The epithermal nuclear reactor is so much incompletely delayed that the resulting energy-rich neutrons turn nuclear fuel capable of reproduction into a high-quality combustible substance (eg, plutonium). This publication does not contain an indication of how these reproductive fuel stalks are secured in the fuel cell and whether they remain in it for only a few reactor cycles.

The invention seeks to make better use of the fuel substance required for the fuel stalks of a fuel element, in particular a water-cooled iodine pressure fuel element, which can only be achieved by changes in the fuel element without the need for separate fitting of the reactor to the altered fuel cells.

This task is accomplished by a fuel element comprising the features of claim 1. The fuel element ia the thermal nuclear reactor has, under one corner plate, a bundle of fuel stems and several hollow tubes open between the fuel stems into which the ejector stems of material are inserted. , which lacks fuel, with a small effective cross section for thermal neutrons. These ejection stems are stationary and thus supported by the hollow tubes and the cover plate so that they can be removed .. before starting the service cycle.

Preferably, the hollow tube containing the fuel stem bundle extends sideways from a housing housing the bundles of fuel stem and hollow tubes and is supported by a frame having a roof plate and support plate. The pushing stems are fixed firmly but detachably from the hollow tubes and the roof plate. Better use of the combustible material is achieved by placing hollow tubes open between the combustion bundles, open hollow tubes which, in the sense of said spectral displacement, are stationary.

These ejection stems change the delay ratio, but unlike the control stems do not absorb neutrons. Contrary to the control stems, they do not move within one cycle during operation.

While the fuel elements are relatively fresh and the ejection stems are embedded, only incomplete deceleration takes place. The neutron spectrum is relatively energy rich and some of the fuel substance is converted. However, if the fuel substance is partially burned, at the appropriate opportunity, for example by replacing already completely burned fuel elements, the expelling stems may be removed from the incompletely burned fuel elements, thereby improving the retarding ratio. This utilizes the better used fuel substance and maintains the preset reactor power.

Preferably, the hollow tubes have their lower end openings with less cross-section than the open top openings of the hollow tube. When the ejection stems are removed, they are completely filled with slow-flowing water, which flows strongly throttled through the hollow tube. The cooling effect of the water circulation is not significantly reduced by the already filled with water pipes, but is adjusted to the capacity of the fuel stems.

The design is particularly suitable for the aforementioned Soviet-type pressurized water reactors, where a single, centrally located instrumentation pipe is provided between the fuel stalks. However, according to the invention, no other tube is provided in the fuel element other than the fuel stalks, the tooling tube and the hollow pipes with the pushing stems.

The position of the hollow tube plays an essential role for the optimum use of the fuel substance. However, certain positions, both for the ejector stems and for the water filled tubes, are particularly advantageous for fuel stems with hexagonal cross-sections and spaced positions of the stems.

Other suitable solutions of the invention are characterized in the additional claims and are explained in more detail by more exemplary embodiments and 13 figures.

They show:

Fig. 1 shows the principal construction of a hexagonal cross-sectional fuel cell as used in Soviet structural type reactors

FIG. 2 and FIG. 3 are longitudinal sections through the heel and head of one such fuel element

FIG. 4 and 5 show the attachment of a hollow tube to the head of the fuel element before and after removal of the ejector stem. FIG. 6 - another possibility of maintaining the hollow pipe and ejection stem;

Fig.7 to Fig.8 - Another exemplary embodiment of mounting a hollow tube with and without an embedded ejection stem

Fig. 9 - cross section of a hollow tube, expanded compared to the fuel stems

FIG. 10 to 13 - different preferred positions of the hollow tubes for the fuel element of FIG

According to FIG. 1, the coolant K enters through an axial opening in the support portion / heel / 2 of the fuel element, but runs the length of the interior of the housing mounted on the heel 3 and exits through an axial opening in the upper part / head / 4. The middle part of the housing 3 is removed to allow one to see one of the many above each other in the fuel element remote holders 5. This remote holder has, as well as the housing of the fuel element, a hexagonal cross section and forms a regular hexagonal grille through whose openings / on grating / Are routed fuel rods. In the center of the grille one can see a pipe for tooling R0. Other bar positions lie

- Ί - on regular hexagonal rings between this center and the edge of the fuel element. The fuel stems are spaced evenly over these equally spaced rings so that e.g. the first ring is formed from 6 positions of the fuel stalks Rl, and the outer / sixth / ring is formed from G x 6- 36 positions of the fuel stalks R6. Together with the guide tube, the fuel element contains 126 rod positions, and for the sake of clarity, the middle portion of the rods has been removed.

Figure 2 shows a well-mounted fuel element housing and supported by a screw pe4a 2 on which a support plate is mounted. This support plate has through holes 6 through which the coolant is introduced into the intermediate space between the individual positions of the stems. For this purpose, necks 7 are provided which have openings 8, in which the lower connection segments 9 engage the fuel stems. In the center R0, a tooling tube can be seen which is screwed down with the bottom end segment 10 to the neck of the support plate, showing, through the dashed lines, an assembly rod 11 that can be screwed into the lower end segment 10 of the tooling tube. equipment for manipulating the support plate. With P, a hollow tube is positioned at a particular position on the stem, the lower end of which enters the opening of the support plate by means of a nut 13.

In this embodiment, the hollow tube as well as the fuel stems are supported by the openings of the spacer holder 5 and the hollow tube protrudes from its upper open end through an opening of the top of the cover of the fuel element 4 of the cover plate 15, which carries the through holes 17 for the cooling means and is mounted on the head 4 (FIG. 3/). The bayonet closure, formed by the thumb 18 of the head 4 and the corresponding window in the upper edge 19 of the fuel element housing, connects dismountingly the head of the fuel cell bush8 and compresses the cover plate 15 to the helical springs 16, which are broken at the upper ends. fuel stems / e.g. stem positions R1 and R6./ The heel 2 with the necks 7 of the support plate, the housing of the fuel element 3, the head 4 with the cover plate 15, and the center $ 0. tooling tubes form a single frame (skeleton), which HQxni fuel stems and hollow tubes and is supplemented by the remote holders that support the fuel stems and hollow tubes. The spacers are attached to the tooling tube, preferably at least to one of the hollow pipes.

The pushing stems B, which are inserted from above into the hollow tubes, in this case are only supported by the hollow tubes. During the operation of the reactor, they are stationary, but can be removed using a special clamping tool. As shown in FIG. 4 they have a protruding over the roof plate and the hollow tube end 20, with a special clamping plate. Figure 4 shows that the end of the hollow tube passing through the roof plate 15 is movable to take into account thermal extensions.

While the fuel stems are new and highly reactive, the ejection stem is embedded. This reduces the volume of neutron retardation available to slow the neutrons so that the number of thermal neutrons and thus the radioactive decay and the resulting decay heat are reduced in favor of a significant portion of fast neutrons that lead to conversion / conversion / fuel substance. The addition / addition (s) of the combustible substance may be selected according to the maximum cooling capacity still present in the reduced delayed operation state.

If, as the reactivity of the fuel substance decreases, the neutron flux and power decrease, then the ejector rods are removed, and then a larger volume of the retarder is available to slow down the neutrons. This creates favorable conditions for the decomposition of the combustible substance so that the power of

the reactor is raised again and the fuel substance can be used better. With the ejection stems removed (FIG. 5/), the hollow tube is only filled with a retarder (moderator), which enters through the small passage openings 21 at the lower end of the tube and is pushed out and exits through the open upper end, which may eventually form bubbles. steam. Bypass flow of refrigerant K, a bypass occurs inside the fuel element, whose loss in pressure and flow can be greatly reduced by appropriate selection of the cross-section of the openings 21 and adapted to the needs.

If the ejection stem is located in the hollow tube and its upper orifice 29 with sufficient clearance, no burrow bubbles can be collected on the surface of the ejection stem, and they are flushed out by the coolant. In order to secure the ejection stem in the hollow tube, its upper end can be further secured to the roof plate according to FIGS. 6 and 7. This can be supported by a lateral frame 21 from below and by a screwed nut 25 from above to the roof plate. Plate 15. The ejector stem may be withdrawn due to inspection arrangements, whereby the bayonet joint between the head and housing of the fuel element is released, the roof plate is removed and the support plate with the fuel stems therein is withdrawn from the housing through the mounting rod 11.

In the exemplary embodiment of FIG. 7, the upper crane of the blank space P is held only indirectly to the roof plate above the pushed ejection bar. Tons of crying.> Tag: n ».

the ejection stem may be replaced by a centering stem 28, which retains the description of the ejection stem toward the roof plate in a manner that only protrudes a short end inward into the hollow tube.

In order to achieve the greatest possible delay variation, it is advisable that the diameter of the hollow tube be larger than the diameter of the fuel stem so that large ejection stems can be inserted.

Fig. 9 shows a hollow tube P surrounded by six consisting of hollow tubes 30 and fuel material fillers 31 of the fuel stems. The division of the fuel stalks, i. E. the smallest distance between adjacent fuel stalks. Recommended ^and - ι mended diameter dO of tube P is approximately equal to the value of the division of fuel stems d. The hollow tubes are preferably made of corrosion-resistant material, and, because of the negligible absorption of neutrons, is particularly suitable for circus.

For a hexagonal edge fuel element and stem positions that are evenly spaced between one central position 1'0 / center / in the axis of the fuel element and the edge on regular hexagons, the appropriate stem positions for the hollow tubes are firmly established, which exploits are filled with ejection stems / e.g. from the compass / / or water. To summarize, this fuel element seeks positions on the walls where the fuel stems are replaced by tubes with a non-combustible material that does not absorb or at least slightly absorb neutrons. According to the invention, the central position CO is advantageously occupied by a tooling pipe, so that all positions around the center position R0 are stepped into two groups. The positions of the first group stems are pi-mat

from fuel-containing fuel stems. It includes all positions of stems from the innermost and outer hexagons, as well as a plurality of stalk positions lying between them. The second group consists of stalk positions occupied by non-combustible tubes and whose six immediately adjacent positions the stems belong to the first group. According to another criterion, the second group consists only of positions of the fuel stalks lying at opposite angles of the diametrically connected diagonals and / or on the medians of the hexagons. The positions are consistent with the diameter of the stems and the division of the stems and provide a proportionately delayed ratio between the surface occupied by the fuel substance and the surface occupied by water in cross section. In the case of existing nuclear reactors, the combustion elements of which must be completely or partially replaced by fresh combustion elements, the outer diameters are predefined and the coolant flow and the flow of the coolant flow out of the pressure of the individual combustion components to which the corresponding values of new fuel elements.

In doing so, we must aim for as many as possible η concentric, evenly spaced around the central position 1 × 0 of regular hexagons, from which the number of stalk positions n = n x (n + 1) / 2 follows. Increasing η decreases the separation of fuel stalks d, however, for reasons of stability of the fuel stalks, a minimum diameter of the fuel stalks is required and the water part required for delay is taken into account and the pressure losses which increase with decreasing separation of the fuel stalks. The choice of n- 6 is recommended.

For the top, it is appropriate if the positions of the swamps are on the woof

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most outer hexagons are only occupied by fuel stems. Figures 10 to 13 give again the appropriate position distributions. In this case, dd ^{denotes the} connecting opposite angles and ^{s_s denote} the sides of the concentric hexagons. They intersect in the central position occupied by the tooling tube, while six, directly adjacent to that center, the positions form the innermost hexagon and are occupied exclusively by fuel stems, as well as the 36 edge positions that form the outermost hexagon. The common thing in all the configurations shown is that the positions of the stems of the second group form the angles of equilateral triangles whose centers of gravity lie on the axis of the fuel element and whose angles are occupied by the positions of the stems of the second group and lie diagonally 0-1 ) m / nl of the medians of the S ^-S countries

In Figures 10 and 11, six of the 126 stalk positions are occupied by tubes with non-combustible material. They all lie on the angular points of the regular hexagons, namely in Fig. 10 the second innermost hexagon (position P2) and in Fig. 11 the third of the innermost hexagons (position P3). These positions lie on the diagonals of the fuel element. Positions can also be selected on the medians of the sides of these hexagons, e.g. positions P4 'in Fig. 12, which lie on the third of the outermost hexagons.

Fig. 12 shows all nine unoccupied stems, with three positions P2 'on the second innermost hexagon lying respectively on the medians of the sides of the hexagons.

Mixed forms also have advantages, in that one part of the positions of the second group lie on the angular loops, the other part lie on the diagonals. Thus, Fig. 13 shows one embodiment in which, in the third of the innermost hexagons lying on the diagonals of the hexagon, the positions P3, in the next hexagon, lying on the medians of the sides of the positions P4 'belong to the second group.

If the pipes are filled with water, a delay distribution occurs, which allows a particularly long combustion of the combustible substance. A weaker delay, i. E. energy spectrum shifted to faster neutrons / spectral displacement / is obtained if the positions of the stems of the second group are occupied by ejecting stems, and this state of operation with mechanically embedded ejection stems / mechanical spectral displacement / has particularly great advantages while the fuel stems are relatively fresh and highly reactive.

One combination, in which, in the first service cycles of a fuel element, the expelling stems are stationary in the hollow tubes at the positions of the stems of the second group, but in the last cycles of the fuel element is removed, the full utilization of the combustible substance with a timely manner is stimulated. coefficient

L of fuel stem operation, and favorably discharged * at 0 power stick.

Claims (2)

Patent Claims 1. Fuel element with sheaf of fuel stems /R1.R6/ and several hollow tubes (P) open between the fuel stems, open at the top, in which stationary push-out stems / B / of small material are inserted before starting the service cycle effective cross section for thermal neutrons. 1. Fuel element for a thermal nuclear reactor, located below the cover plate / 15 / joints of fuel stems / R1.R6/ and several located between the fuel stems, open hollow tubes / P /, in which are pushed out stems / B / of material lacking fuel is a small effective cross-section for thermal neutrons, characterized in that the ejection stems (B) are stationary, and before the start of the operating cycle they are positioned, securing the possibility of removal from the hollow tubes / P9 / and the roof plate / 15 /. 2. Fuel element according to claim 1. characterized in that it has a side beam of hollow body fuel bundles (P / casing / 3), in which a bundle of fuel stems with the hollow tubes and a bundle of fuel stems with the hollow tubes are housed. a frame with a support plate / 7 / and a cover plate / 15 /, in which the ejectors attach the stems / rigid, stable and detachable / with the possibility of disassembly / from the hollow tubes / P / and the cover plate / 15 /. A fuel element according to claim 1 or 2, characterized in that the hollow tubes (P) have at their lower end openings (21) a smaller cross-section than the open top edges (29) of the hollow tubes. A fuel element according to any one of claims 1 to 3, characterized in that the diameter (h) of the hollow tubes is the greater of the diameter (4) of the fuel stalks. 5. Fuel element that the diameter / o'0 / / o! / Between the axes of the immediately adjacent fuel stalks. 6. Fuel element CONSIDERING that the hollow tubes are made of a durable material. Fuel element according to one of Claims 1 to 6, characterized in that at least one of the hollow tubes is provided with a remote holder / 5 /> Fuel element according to one of Claims 1 to 7, characterized in that the ejection stems are massive circus stalks A fuel element according to any one of claims 1 to 8, characterized in that there is no other pipe between the fuel stems except the hollow tubes with the pushing stems thereof, centrally located for the tooling. A fuel element according to any one of claims 1 to 9, characterized in that it has a hexagonal cross section, wherein the hollow tubes are separated from each other, from the axis of the fuel element and from the edge of the fuel element by at least one fuel element stem. 11. Hexagonal edge fuel element and stem positions arranged between one central position (R0) in the axis of the fuel element and edge evenly on concentric regular hexagons, in particular according to one of claims 1 to 10, characterized by that a / a first group of stem positions containing all stem positions on the innermost and outer 1 hexagon and the plurality of stem positions lying between them and it / group / is occupied by a fuel-containing combustible stem. b / one second group contains stem positions, the six immediately adjacent stem positions belonging to the first group and are occupied by stems without / containing / no combustible substance, containing or containing at most a stratum-absorbing neutron material. c / all located around the nth / ez / w /.; - .. n ·. <:> the stems belong to the first or second groups. 12. A fuel element according to claim 11, characterized by a hexagonal housing enclosed side by side with the fuel stems, which is covered with water holes through the top and bottom plates. 13. Fuel element according to claim 11 or 12, characterized in that the number of these hexagons is at least six, the first group of all positions of the stems comprises the two outermost hexagons and the central position (R0) is occupied by a pipe for tooling. A fuel element according to any one of claims 11 to 13, characterized in that the second group contains positions of the stems that lie on the diagonals of the hexagons 15. Fuel element according to one of Claims 1 to 14, characterized in that the second group comprises positions of the stems lying on the sides of the hexagonal sides. 16. A fuel element according to claim 15 or 16, characterized in that the second group consists of positions of fuel stalks that form the angles of equilateral triangles whose f centers of gravity lie on the axis of the fuel element and whose »» * it angles lie on the medians of the sides m / nx on the diagonals of, / hexagons. /. ' Fuel element according to one of Claims 11 to 17, characterized in that the stems from the positions of the second group are filled with hollow tubes of water. $ 9. Fuel element according to one of Claims 11 to 17, characterized in that the stems on the positions of the second group are hollow tubes, which in one first operating mode contain fixed pushing stems but no more than zirconia, and in the second operating mode contain a funnel (moderator), nose water. PATENT CLAIMS / old version / Fuel element according to claim 1. characterized in that the bundle of fuel stems with the hollow tubes is supported in a skeleton with a support plate (7) and a cover plate (15) and that the ejection stems are secured firmly, firmly and detachably from the hollow pipes / P / and the roof plate / 15 /.