Patent Number: 051436914
Section: description

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a boiling water nuclear reactor having a pressure vessel Pp, in which a reactor core with vertically oriented fuel assemblies BE is disposed. A steam outlet line DA leads to a steam turbine DT, which drives a generator G. Water condensed in a condenser C is delivered through a feed water pump P to a water input line WE of the pressure vessel. Unevaporated water in the fuel assemblies is also recirculated through a water cycle or circuit WC and a coolant pump WP. The fuel assemblies BE located in the pressure vessel contain vertically disposed rods ST shown in FIG. 2, which are held at the bottom in a base or base part Ft and at the top in a cap or head part K and are laterally surrounded by a water case, box or duct WK. The cap part K has outlet openings O for a water/steam mixture, which are connected through other non-illustrated components, such as for drying the steam, in a closed cycle or circuit with the steam turbine DT. Corresponding inlet openings on the base part cannot be seen. The fuel rods are fixed in the case by spacers AH, which extend transversely between the fuel rods. While FIG. 2 shows only one spacer, normally from 5 to 7 such spacers are disposed in succession in the case, at approximately equal intervals. A water channel or duct CAN for non-boiling water preferably extends longitudinally relative to the case and is connected to the cycle or circuit of the pumps P and WP through corresponding inlet openings in the base Ft and a corresponding outlet opening O' in the cap K. In the cross section through the water case WK shown in FIG. 3, the fuel assemblies are disposed in the meshes or mesh openings of a regular, rectangular grid that has 9.times.9 positions for the fuel rods ST. However, instead of a fuel rod, the center of the fuel assembly has a water channel or duct, which is a so-called "water rod" WS and in this instance is formed of a tubular inner wall in the case. In FIG. 4, the water case WK, for which a polygonal cross section has advantageously been selected, also has a square cross section. In this instance, however, a plurality of inner walls have been provided with corresponding water rods WS, WS'. In this case, only 9.times.9-5 positions for the fuel rods ST remain. A water channel CAN having the square cross section already shown in FIG. 2 has proved to be particularly advantageous. In FIG. 5, 9.times.9-9 fuel rods ST can be accommodated with the water channel CAN. FIG. 6 shows a different preferred embodiment, in which opposed walls of the case are joined to one another by inner walls, that are each parallel to case walls if a polygonal cross section is used. In the quadratic form of FIG. 6, the result is a cross-shaped structure of reinforcing inner walls VW. Such reinforcing inner walls VW allow the fuel assembly to have a high feed pressure for the water, with an increased flow speed which can thus lead to increased steam production, despite relatively thin case walls WK. In order to compensate for pressure differences in the various quadrants of the case, perforations or other openings may be provided in the reinforcing walls VW, which extend longitudinally over practically the entire length of the case. In the cross section of FIG. 7 as well, inner walls are provided in the interior of the case, but some of them form a water rod WS", which in this instance is relatively large, while some are constructed as reinforcing walls VW', which join the opposed case walls together through the water rod. In contrast to FIGS. 3-5, the water channel formed by the water rod WS" is not disposed strictly centrally within the case but rather is shifted somewhat to the side. FIG. 8 shows a central water channel CAN formed by some of the inner walls, which is joined to the case walls through another group of inner walls that serve as reinforcing walls VW', as in FIG. 7. FIG. 9 shows a longitudinal section of a wall of the fuel assembly case WK. An arrow Ss indicates a flow direction in which the water in the lower portion and a mixture of water vapor and water droplets in the upper portion of the fuel assembly flow along the lateral surface of the wall of the case WK facing toward the fuel rods. No evaporation takes place at the wall of the case WK, which protrudes out of the cooler lower portion of the fuel assembly into the upper, steam-carrying space of the fuel assembly and is cooled by the non-boiling water outside the fuel assembly. Instead, water creeps upward in the form of a film F. The spacer AH has a rib or web shown in FIG. 9 with a long side having an edge AK that is constructed as a flow tab or baffle and protrudes into the steam flow in such a way that droplets TR contained in the flow are diverted from their horizontal flow direction and spun into the direction of the fuel rods. The result is a partial separation of a droplet flow Tr from the water vapor flowing in the direction of an arrow Dp and more liquid water being supplied to the fuel rods. This separation action is increased if the spacers AH, or the edges AK of the ribs thereof which are formed into flow tabs or baffles, are preceded by flow trippers, as seen in the flow direction. As seen in FIG. 10, the profile on the inside of the case walls extends rectilinearly in an alignment Kk--Kk parallel to the flow direction, upstream and downstream of the flow trippers, as seen in the flow direction. The flow tripper of the prior art shown in FIG. 1? is constructed as a groove N which, as seen in succession in the flow direction, firstly includes a ramp or sloped surface machined into the case wall having a ramp alignment Rr, then enlarges in cross section, and finally ends in an edge Nk perpendicular to the flow direction. In order to ensure that the liquid film will in fact detach and be entrained as droplets by the coolant flow Ss, the loped surface with the alignment Rr must be sufficiently wide. Otherwise, the coolant flow in the region of the flow tripper would hardly be diverted at all from its normal longitudinal direction. In the least favorable case, the grooves would fill with liquid alone, and in the most favorable case, although droplets would form, they would soon be deposited on the wall again in the form of a film, since a speed component oriented into the interior of the case, or in other words aimed at the fuel rods, would hardly be imparted to them. Accordingly, in order to attain the improved cooling output sought, the grooves must be sufficiently wide and deep. However, although this is entirely desirable with a view toward widening the flow cross section and lessening the pressure drop for the coolant liquid, it is highly undesirable for the mechanical strength of the case. According to the invention, as shown in FIG. 11, the profile on the inner wall of the case is curved inward into the interior of the case in the vicinity of the flow tripper, thereby reducing the inner cross section of the case that is available for the coolant flow. However, the "front sloped surface" (that is, the side of the inwardly curved profile of the inner surface of the case that faces into the liquid flow) is constructed as an impact surface Pf that is perpendicular to the liquid flow. Even if this impact surface Pf that extends perpendicular to the longitudinal direction is relatively narrow, the coolant flow at that location causes detachment or breaking away of the film F, and the resultant droplets TR are given an increased speed component in the direction of an arrow Tr' at the impact surface Pf. While the steam is diverted in the direction of an arrow Dp' at the sloped surface of the profile facing away from the coolant flow, the droplets TR continue to be spun into the interior of the case, resulting in effective separation of steam and droplets at the flow tripper. Flow trippers of this kind are provided above all in the upper portion of the fuel assembly, where steam production occurs, or in other words approximately in the vicinity of the upper third of the fuel rods. It is unnecessary for the sloped surface that faces away to be as flat as is shown in FIG. 11. FIG. 12 shows that the sloped surface, which is constructed as the impact surface Pf, can extend around the entire inner cross section of the fuel assembly case WK. A remote, "rear" sloped surface Pb, at which the temporarily narrowed flow cross section widens again, may belly outward. It is merely advantageous for the transition between the two sloped surfaces to form an impact edge Pk that is rounded as little as possible and instead is sharp (for instance at a right angle or acute angle). This promotes detachment or separation of the flow and reinforces the perpendicular flow component Tr' of the droplets. The details of the construction of the flow tripper can be varied to meet the needs of economical manufacture. For instance, the fuel assembly case of FIG. 12 may be manufactured with trippers extending transversely to the alignment in the form of corresponding inwardly oriented bulges of a semi-finished part, and the impact surface Pf having the impact edge Pk can be made by subsequent mechanical retouching. It is also possible, for instance, to first make a case in which wall parts that are intended for the flow trippers have essentially constant thickness. The bulges are made by bending these wall parts, with the impact surfaces being made in them subsequently by removing the excess wall material from the sloped surfaces B facing into the coolant flow, as shown in FIG. 13. In a case having a square cross section and rounded corners, bumps (or dimples) DP created by bending may extend over the entire square cross section, which has proved to be advantageous. However, depending on the manufacturing process, the region of the rounded corners may be spared instead. The parts of the wall carrying the bumps have practically a constant thickness d in FIG. 13, which is reduced only on the side facing into the flow by the incorporation of the impact surface. This reduced wall thickness is virtually no impairment in terms of mechanical strength, i.e., in terms of accommodating problematic bulging of the case in response to a pressure difference between the interior and exterior of the case, because firstly even a slight removal of material causes turbulence in the liquid film and secondly the wall at these points already has increased mechanical strength because of the inwardly oriented bending. In the configuration of FIG. 14, additional wall material has been applied to the corresponding wall parts having the constant thickness d. This is most simply accomplished by welding on the additional wall material. In this case, the additional wall material itself is formed of welding material. Accordingly, in FIG. 14, transversely extending weld seams SN are applied to the inside of the case. The disturbing or undesirable sloped surfaces B are removed by mechanical machining, and the corresponding impact surfaces Pf are formed. In FIG. 15, reinforcing metal sheets WB are welded to the wall parts having a constant thickness as additional wall material, and the bulges are formed by a thickening of the reinforcing sheets. As shown in FIG. 16, ribs SG can also be welded perpendicularly to the wall, in other words perpendicularly to the alignment, as additional wall material. The additionally applied wall material also reinforces the case which has a wall thickness that can therefore be selected to be relatively small. In order to lend sufficient rigidity to the entire case despite such a wall thickness, the reinforcing sheets VW and VW' shown in FIG. 6-8 may be provided. In order to improve the neutron flow, the water and channels mentioned in conjunction with FIGS. 3-5 may also be advantageous. In this regard, inner walls that extend between the fuel rods and are parallel to the case wall may also be provided with impact surfaces extending perpendicularly to the flow direction. These additional impact surfaces on the inner walls may be constructed as shown in FIGS. 13-15. In the case of impact surfaces that protrude into the interior of the case and would reduce the flow cross section, if difficulties should arise because of an increased pressure drop and/or an increase in the space available, then flow trippers of the type shown in FIG. 10 may be provided for these inner surfaces. In each case, the flow trippers are at their most effective if they are disposed upstream of corresponding spacers, as seen in the flow direction and as seen in FIG. 11.