HU216377B - Fuel element with displacement elements - Google Patents

Abstract

The invention relates to a heating element with discharging bodies, which comprises a stack of heating rods and a plurality of open-opening discharging tubes (P) arranged between the heating rods. The discharging rods (P) are fitted with fixed discharging rods (B) at the end of the operating cycle. The material of the compaction rods (B) is a material with a small cross-section for thermal females, preferably Zircalőy. In the heating element according to the invention, the heating rod bundle is rigidly and pivotally fixed together with the discharge tubes (P) in a frame structure provided with a bottom plate and a cover plate (15). Some of the heating rods are replaced by discharge pipes (P), which comprise a first filter unit (B) of the heating element, for example a Zircalőy rod, and in the operating cycles, a main shield, for example water. ŕ

Description

The present invention relates to a fuel element with displacement bodies. The fuel element of the present invention consists of a set of fencing rods and tubes placed between the running rods which contain material that does not or only weakly captures neutrons.

For different types of reactors, it may be advantageous to place tubes that do not contain fuel between the heating rods of the fuel elements. For example, hot water reactors use water filled tubes to deliver liquid water to the upper parts of the heater, whereby the amount of fuel relative to the water required as a moderator is shifted to the disadvantage of water due to an increase in steam generation. These "water poles" are open at the top and bottom so that the flow is unobstructed. The heating elements are surrounded by a heating element cabinet. The gauges and reaction controls are located outside the heater cabinet.

Most pressurized water reactors are instrumented and reactor controlled by means of suitable elements guided in the tubes between the heating rods of each element.

DE-OS 35 15469 describes a water-cooled, high-converter reactor. In the hexagonal cross-section of this reactor, the rod locations are arranged in regular hexagons between the center and the wind in the central axis of the fuel element. Some wharf bars are replaced by guide pipes, others by water bars closed above and below. The water in these water poles on the one hand improves the deceleration rate in the reactor and on the other hand this water should not be involved in the water cycle. The hydraulically effective cross-section of these water pipes is increased relative to the cross-section of the heating rods to concentrate the water flow and thereby the cooling rods.

In Soviet-type pressurized water reactors, the heating rod assembly and dashboard are surrounded by a heating element cabinet. However, the control elements are disposed axially for the fuel elements so that, for example, to slow down the nuclear reaction, a fuel element is pulled axially out of the spinning core and an associated control element is inserted.

Controlling the reaction with control elements is often helped by adding fresh fuel elements containing combustible neutron poison when replacing the fuel. This is a neutron absorber that absorbs excess neutrons at the start of the fuel cycle, thereby reducing reactor performance to the optimum value for the reactor. However, during the cycle, this neutron absorber becomes increasingly depleted, so that the reactor performance remains practically constant until the end of the cycle despite decreasing neutron emissions from the consumed fuel.

It is an object of the present invention to make better use of the fuel used for heating rods, in particular pressurized water cooled heating elements.

According to the invention, this object is solved by the heating element comprising a heater bundle and a plurality of displacement tubes disposed between the heating rods. These displacement pipes are fitted with stationary displacement rods that can be removed at the end of the operating cycle. The displacement rods are made of material with a small cross-section for thermal neutrons (for example, solid Zircaloy rods).

These displacement rods change the deceleration rate but, unlike the control rods, do not absorb neutrons. Unlike the control rods, these displacement rods are not moved during a cycle during operation.

With displacement bars inserted, almost complete deceleration occurs as long as the heating elements are relatively fresh. The neutron spectrum is relatively energy rich and some of the fuel is converted. However, when the fuel has been partially burned, at a favorable time, for example when replacing the already completely spent fuel, the displacement rods can be removed from the not completely spent fuel. This improves the deceleration rate. This will make better use of the fuel that is still commercially available and maintain the reactor's predetermined output.

The cooling rod assemblies, together with the displacement tubes, are preferably housed in a frame, especially in a cabinet. This cabinet has a headboard and a pedestal, but the displacement rods are supported only by the headboard and the displacement pipes.

Advantageously, the lower ends of the displacement pipes have openings having a cross-sectional area smaller than the cross-section of the upper open end of the displacement pipe. The displacement tubes, when removed from the displacement rods, are completely filled with the decelerating liquid water, which, however, flows only with a strong throttle. The cooling effect of the water cycle is now significantly reduced by the displacement pipes now filled with water, but adapted to the power of the heating rods.

This design is particularly suitable for the above-mentioned pressurized water reactors of the Soviet type, which have a single central tube between the heating rods. Thus, according to the invention, the heating element has no pipe other than the heating rods, the duct and the displacement pipes.

The position of the displacement pipes plays an important role in the optimal use of the fuel. In the case of heating rods having a hexagonal cross-section and having spaced bars therebetween, certain situations are particularly advantageous for both the displacement rods and the displacement pipes filled with water.

The diameter of the displacement pipes is preferably larger than the diameter of the heating rods, and preferably is approximately the distance between the geometric axis of the directly adjacent heating rods.

The material of the displacement pipes is preferably a corrosion-resistant material, in particular Zircaloy.

Preferably, at least some of the displacement pipes are provided with spacer grids.

The displacement rods are preferably solid Zircaloy rods.

Preferably, the cross-section of the heater is hexagonal, and the displacement tubes are spaced from one another to the heater.

EN 216 377 legalább at least one heating rod separates it from its geometric axis and from the edge of the heater.

In a preferred embodiment of the heating element according to the invention, the outline is hexagonal and the bar locations are arranged in the same distance between the central position on the geometric axis of the heating element and the edge in concentric regular hexagons. The first set of poles contains all of the poles in the innermost and outermost hexagons and most of the poles in between. These rods are occupied by heating rods containing fuel. The second set of rods comprises rods, six of which are directly adjacent to the first rod, and are occupied by non-fuel rods containing non-neutron absorbers or only weak absorbers. All the pole locations around the center are in the first or second group.

The invention will now be described in more detail with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a hexagonal cross-sectional fuel element used in Soviet-type reactors;

Fig. 2 is a longitudinal sectional view of the sole of Fig. 1, a

Figure 3 is a longitudinal sectional view of the heater portion of Figure 1, a

Fig. 4 fastening the displacement pipe to the heater head before removing the displacement rod,

Figure 5 is the attachment of the displacement pipe to the base of the heater after removal of the displacement rod,

Figure 6 shows another possibility of holding the displacement pipe and the displacement rod, a

Fig. 7 is another embodiment of holding a displacement tube with a displacement bar inserted,

Fig. 8 is another embodiment of holding a displacement tube without a displacement rod, a

Figure 9 is an enlarged cross-section of the displacement pipe relative to the heating rods, a

10-13. FIG. 1A is a view showing various preferred positions of displacement tubes for the heating element of FIG.

According to Fig. 1, refrigerant K flows through an axial opening into the base portion 2 of the heater, further flows inside the cabinet 3 mounted on the base portion 2 and exits through the axial opening in the header 4. The middle part of the cabinet 3 is removed in the figure so that one of the spacer grids 5 placed one above the other in the heater can be seen mist. This spacer grille 5 has a hexagonal cross-section as well as the heating element. The spacer grille forms a regular hexagonal grid with heating rods running through its grid. In the middle of the grid R0 is an instrument tube. There are additional rod locations on regular hexagonal rings between the center and the edge of the heater. The heating rods are also spaced at the same distance from each other, so that, for example, the first ring is formed by six heating rods R1, and the outer (sixth) ring is formed by 6x6 = 36 heating rods R6. Thus, the heater element comprises 126 rod locations adjacent to the guide tube. The middle part of the rods has been omitted to improve clarity.

Fig. 2 shows a better view of the bottom part 2 of the heating element inserted into the cabinet 3 and held by a screw. A base plate is placed on the base 2. This bottom plate has openings 6 through which the refrigerant is introduced into the intermediate space between each bar. 7 bridges are thus formed. The bridges 7 have holes 8 into which the lower end caps 9 of the heating rods extend. In the center of the R0 is the instrument tube, which is screwed to the bottom 10 brackets on the bridges of the bottom plate. The dashed lines indicate a mounting bar 11 that can be screwed into the lower end cap 10 of the instrument tube R0 to center the bottom plate. The displacement tube P is located in a separate rod location, the lower end of which extends through the bore in the bottom plate and is secured to the underside of the bottom plate by the nut 13.

In this embodiment, the displacement tube P, like the heating rods, rests on the grid meshes of the spacer grille 5 and extends over the bore 15 in the topsheet 15 which overlooks the heating element cabinet. The topsheet 15 has openings 17 for the refrigerant. The topsheet 15 is mounted on the head part 4 (Fig. 3). The bayonet lock formed by the meshes 18 on the head part 4 and the openings in the upper edge 19 of the heater cabinet releasably connects the head part 4 to the heater element cabinet 3 and presses the topsheet 15 towards the screw springs 16. The coil springs 16 rest on the upper ends of the heating rods (e.g., heating rods R1, R6). The base 2 together with the bridges 7 of the bottom plate, the heater cabinet 3 and the head 4 together with the top plate 15 thus form a frame which carries the heating rods and the displacement pipes P and is complemented by the spacer grid 5 supported by the displacement pipes P. The spacer grids 5 are secured to the instrument tube, and preferably to at least a few displacement tubes P.

However, the displacement rods B extending from above to the tubes for which they are intended are supported only by these tubes. The displacement rods B are thus not movable during operation of the reactor, but can be removed with a gripping tool. To do this, the B displacement rods a

As shown in Figure 4, there is a rod end 20 extending through the topsheet 15 and the displacement tubes P, which has a gripping surface. Figure 4 shows that the end of the tube passing through the topsheet 15 can be offset so that thermal expansion is accounted for.

While the heating rods are relatively new and highly reactive, the displacement rod B is inserted. This reduces the amount of water available for slowing down the neutrons, so that the number of thermal neutrons, and thus the radioactive decay and heat of decay generated, is reduced in favor of a certain proportion of fast neutrons. These fast neutrons produce conversions in the fuel. The fuel subsidy can then be selected according to the maximum power that can still be controlled with reduced deceleration during operation.

However, when the neutron current and power decrease as the fuel reactivity declines, we remove the displacement rods B and now have a larger volume of moderators to slow the neutrons down.

EN 216 377 Β available. This facilitates the decomposition of the fuel so that the reactor power is increased again and the fuel can be better utilized. Thus, when the heating rods are removed (Fig. 5), the displacement pipe P is filled with a moderator only, which can enter through the inlet openings 21 at the lower end of the displacement pipe P and exit the open upper end and flush any vapor bubbles. Thus, a bypass branch is formed inside the heating element for the decelerating refrigerant flow K. However, the pressure loss therein and the flow therethrough can be strongly suppressed by the appropriate selection of the cross-sections of the inlets 21 and adapted to the needs.

If the displacement rod B fits into the displacement pipe P and its upper opening 29 with sufficient play, no vapor bubbles will accumulate on the surface of the displacement rod B, but will be flushed out by the refrigerant. The upper end of the displacement rod B may be retained on the topsheet 15 as shown in Figs. To do this, it is supported by a side, reclined shoulder 24 from below and, by means of a screwed nut 25, from above on the topsheet 15. The displacement rod B can be removed for inspection. To do this, the bayonet lock between the head part 4 and the heating element cabinet 3 is released, the cover plate 15 is removed and the bottom plate together with the heating rods thereon is removed from the cabinet 3 by means of the mounting rod 11.

In the embodiment of Fig. 7, the end of the displacement tube P is held by the inserted displacement bar B only indirectly on the topsheet 15. When the displacement rod B is removed, this anchorage may be replaced by a centering rod 28 which, as described, is held on the topsheet 15 but extends only shortly into the displacement pipe P.

To maximize deceleration, the displacement pipes have a larger diameter than the heating rods, so that large displacement rods can be inserted.

Fig. 9 shows a displacement pipe P surrounded by six heating rods consisting of casing pipes 30 and fuel fillers 31. The "boom split" d is the shortest distance between adjacent booms. Preferably, the diameter d0 of the displacement tube P is approximately equal to the value of the heating rod spacing d.

The displacement pipes are preferably made of corrosion-resistant material. Because of its low neutron absorption, Zircaloy is particularly suitable.

In the case of a fuel element having a hexagonal edge and having bar locations which are spaced at equal distances from the central location in the geometry axis of the fuel element and in the form of concentric hexagons, the appropriate bar locations for the displacement pipes must be determined. filled with displacement rods (such as Zircaloy displacement rods) or water. In more general terms, for this fuel element, look for the pole locations on which the heating rods are replaced by non-fueled, non-neutron or at least poorly absorbent tubes.

According to the invention, although the center position R0 may be occupied by a dipstick, each of the pole positions arranged around the center position is divided into two groups. The bars in the first group are occupied by heating rods containing fuel. This includes all poles in the innermost and outermost hexagons, and most of the poles in between. The second group of rods comprises rods which are occupied by non-fuel rods, six of which are directly adjacent to the first group. According to another criterion, the second group consists of heating points only, which are on diagonals connecting opposite corners and / or on the sides of the hexagons.

The bar locations are selected according to the bar diameter and the bar spacing, and provide a favorable ratio between the surface occupied by the fuel and the cross sectional area occupied by water in terms of deceleration. Existing nuclear reactors, whose fuel elements have to be replaced completely or partially with fresh fuel elements, have predefined external diameters for each fuel element and pressure losses adapted to the cooling capacity and the amount of refrigerant flow to which the values of the new fuel elements must be adapted.

In doing so, you should try to have as many regular hexagons as possible that are concentric and equidistant around the center. Then the number of poles will be? = N x (n + l) / 2. As n increases, the bar distribution d decreases. However, due to the stability of the heating rods, a minimum diameter of the heating rod is required and account must be taken of the proportion of water required for deceleration as well as the pressure loss increasing as the rod pitch decreases. The preferred choice is n = 6.

Preferably, the bars in the second outermost hexagon are occupied only by heating rods.

10 -13. Figure 2B shows advantageous pole position distributions. Here, opposite corners are connected by diagonals D-D, and concentric hexagons have S-S side halves. These intersect at the center of the dashboard, while the six bars directly adjacent to this center form the innermost hexagon, and are also occupied exclusively by heating rods, as are the thirty-six extremes of the outermost hexagon. It is common to all the shapes illustrated that the second group of rods form the corners of equilateral triangles centered on the geometric axis of the heater and whose corners are occupied by the second group of rods and are on D-D diagonals or S-S side halves.

In Figures 10 and 11, six of the 126 beams are occupied by fuel-free tubes. They are all at the corners of the same hexagon, the second innermost hexagon (rods p2) in Fig. 10 and the third innermost hexagon (rods p3) in Fig. 11. Accordingly, these pole positions are located on the diagonals of the heating element. However, it is also possible to select pole positions on the sides of these hexagons, for example, in Figure 12, the pole positions P4 'which are in the third outermost hexagon.

Figure 12 shows a total of nine fuel-free bars. Three pivot points P2 'are in the second innermost hexagon and are also on the side bisector.

HU 216 377 Β

In addition, mixed shapes are preferred, with a portion of the second set of bars located on the bisector and a portion on the diagonal. Thus, Fig. 13 shows an embodiment in which the rods P3 in the diagonals of the third innermost hexagon and the rods P4 'on the side bisector in the nearest hexagon belong to the second group.

When the pipes are filled with water, a deceleration distribution is created which allows the fuel to be burned extensively. A weaker deceleration, or spectral shift, is obtained by occupying the second group of rods with displacement rods. This operating condition is particularly advantageous with the mechanical spectral shift rods, as long as the heating rods are relatively fresh and highly reactive.

The combination in which the displacement rods are stationary in the displacement pipes at the second group of rods during the first operating cycles of the fuel element, but excluded in the last fuel element cycles, therefore allows high fuel utilization with a coefficient adapted to the age of the heating rods.

Claims (18)

PATENT CLAIMS 1. A fuel element disposed between the rods in the fuel rod assemblies of nuclear reactors cooled by displacement bodies, in particular pressurized water, characterized in that the fuel rod assemblies comprise a plurality of displacement pipes (P) B) are placed, and the material of the displacement rods (B) is a material of low cross-section for thermal neutrons. Heating element according to Claim 1, characterized in that the heating rod assembly, together with the displacement pipes (P), is housed in a frame with a bottom plate (7) and a cover plate (15), and the displacement rods (B) are disposed on the displacement pipes (P) and (15) it is rigid and detachable. Heating element according to Claim 1 or 2, characterized in that the lower ends of the displacement pipes (P) have openings (21) which have a smaller cross-section than the open end of the displacement pipes (P). 4. Heating element according to any one of claims 1 to 3, characterized in that the displacement pipes (P) have a diameter (dO) greater than the diameter (d) of the heating rods. Heating element according to claim 4, characterized in that the diameter (dO) of the displacement pipes (P) is approximately equal to the distance (d) between the axes of adjacent heating rods. 6. A heating element according to any one of claims 1 to 3, characterized in that the material of the displacement tubes (P) is a corrosion-resistant material, in particular Zircaloy, which is a material of low cross section for thermal neutrons. 7. Heating element according to one of Claims 1 to 3, characterized in that at least some of the displacement pipes (P) are provided with spacer grids (5). 8. A heating element according to any one of claims 1 to 3, characterized in that the displacement rods (B) are solid Zircaloy rods. 9. A heating element according to any one of claims 1 to 3, characterized in that the heating rods comprise displacement rods (B), displacement pipes (P) and a single central duct. 10. Heating element according to any one of claims 1 to 4, characterized in that the heating element has a hexagonal cross-section and at least one heating rod separates the displacement pipes (P) from each other, from the geometric axis of the heating element and from the edge of the heating element. 11. A fuel element having an outline of a hexagon and the rod locations arranged in concentric regular hexagons spaced from the center (R0) in the axis of the heater and the wind, characterized in that the first set of rod locations comprises the innermost and the outermost all the poles in the hexagon and most of the poles in between, and these poles are occupied by fuel rods; a second set of rods comprises rods of which six are directly adjacent to the first rod and are occupied by non-fuel rods containing non-neutron absorbers or only weak absorbers; all the pole positions around the central position belong to the first or second group. A heating element according to claim 11, characterized in that the heating rods are surrounded on the side by a hexagonal cabinet, which is covered from the top and bottom by sheets with water openings. 13. A heating element according to claim 11 or 12, characterized in that the number of hexagons is at least six, the first set of all the rods contains the two outermost hexagons, and the center is occupied by a dashboard. 14. A 11-13. A heating element according to any one of claims 1 to 3, characterized in that the second group comprises rod locations on the diagonals of the hexagon. 15. A heating element according to any one of claims 1 to 4, characterized in that the second group comprises the bar locations on the sides of the hexagon. Fuel element according to Claim 14 or 15, characterized in that the second group consists of bars which form the corners of equilateral triangles with their center of gravity in the geometric axis of the fuel element and whose corners are on the sides and / or diagonals of the hexagon. . 17. A heating element according to any one of claims 1 to 3, characterized in that the rods of the second group are occupied by displacement pipes filled with water. 18. 11-17. A heating element according to any one of claims 1 to 4, characterized in that the rods of the second group are occupied by displacement pipes comprising stationary displacement rods, in particular made of Zircaloy, and a moderator, in particular water, in a first condition.