Patent Number: 051184672
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 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. In FIG. 9 an inner wall IW, which may be the wall of the channels or water rods or a reinforcing wall, is shown in a longitudinal section. An arrow SS indicates the flow direction in which the water in the lower portion of the fuel assembly, and a mixture of water vapor and water droplets in the upper portion, flow along the lateral surface of the inner wall IW facing toward the fuel rods. No evaporation takes place at the inner wall IW, which protrudes out of the ler lower portion of the fuel assembly into the upper, steam-carrying space of the fuel assembly and optionally is additionally cooled by the non-boiling water flowing in the channels. Instead, water creeps upward along the inner wall IW in the form of a film F. A rib of the spacer AH which is shown in FIG. 9 has a long side with an edge AK that is constructed as a flow 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 assemblies. 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 thereof that are formed into baffles, are preceded in the flow direction by the grooves N according to the invention as seen in FIG. 10. The grooves N act as "flow trippers" for the liquid film. The grooves N preferably have edges with narrow surfaces Nk, which are at right angles to the flow direction. Accordingly, if the upwardly creeping liquid film F strikes the surfaces Nk, it is broken or separated away or detached and is entrained in the form of droplets by the medium flowing past it. This increases the proportion of droplets in the medium, so that correspondingly more droplets are also steered toward the fuel assemblies in the direction of the arrow Tr. The grooves N according to the invention may be made on a tubular wall by turning. In contrast, if an inner wall forming the water channel has planar surfaces, then the grooves may be made in the planar surfaces by milling. This is shown in FIG. 11 for a square channel cross section with rounded corners. If the corners are recessed, milling of the grooves N is easier. However, as FIG. 12 shows, it is also possible for a groove encompassing the entire channel to be milled. If the channel having the aforementioned reinforcing walls VW' is joined to the case walls, the reinforcing walls VW' can be inserted into recesses that are milled into the channel wall and can be welded at that location, as seen in FIG. 11. However, the reinforcing walls VW may also be mounted end-on on the outer surface of the channel and fastened with a welded seam S, for instance. As FIG. 12 shows, in this embodiment the contact between the reinforcing walls VW' and the channel is interrupted in the vicinity of the groove. If the inner walls carrying the grooves N are the walls of the aforementioned water rods or water channels, then only the lateral surfaces oriented toward the fuel assemblies have the grooves according to the invention. FIG. 13 shows longitudinal sections through corresponding channel walls with various groove forms. Advantageously, the grooves are formed in such a way that the channel wall tapers steadily in the flow direction until the aforementioned narrow edge surface Nk has been reached. The transition may be made virtually as a right angle, or may be rounded as in the view of FIG. 13b. If the upper groove edge is constructed in this way as a detaching edge, then a speed component aimed at the fuel rods is imparted to the droplets produced upon film detachment, so that more liquid water for boiling is delivered to the fuel rods. As shown in in FIG. 13c, the lower edge of the grooves may be constructed in this manner as well. Such grooves may be easier to mill and can already cause film detachment to occur at the lower edge. On the other hand, it must be remembered that each edge increases the flow resistance in the fuel assembly. FIG. 14 shows how corresponding grooves can advantageously be provided on both sides of a reinforcing wall. If the two grooves are each disposed at a given optimal distance ahead of the aforementioned spacers, then they are practically opposite one another. The result is a considerably reduced wall thickness at this point. FIG. 14a a shows a particularly easily manufactured embodiment of the reinforcing walls, in which the grooves disposed in the sides are offset from one another, resulting in a virtually constant wall thickness. The groove may already be made in the channel walls and reinforcing walls before these inner walls are joined together to make a corresponding "water structure". The result is a structure that is diagrammatically shown in FIG. 15. The film tripper runs over the lateral surfaces of the reinforcing walls and the remaining free outer surfaces of the channel walls as a continuous groove. From the standpoint of manufacture, it is often simpler to make the groove non-continuous. In FIG. 16, the side regions of the reinforcing walls are constructed without grooves. As a result of manufacture, the corresponding lateral ends of the grooves may be rounded. In some instances, it may be unnecessary to provide grooves in the walls of the water channel as well. Naturally, it is also possible for the inner surfaces of the case to be provided with corresponding grooves. Thus while the inner walls make it possible to provide smaller wall thicknesses of the case walls by reinforcing the case, and/or the neutron flow may be improved by forming a water channel for non-boiling water, the grooves according to the invention make it possible to avoid problematic film formation at such inner walls, and to increase the delivery of coolant to the rods, above all in the upper region, where the steam being formed lessens the cooling of the fuel rods. The fuel assembly according to the invention can therefore be constructed for increased boiling capacity or output.