Patent Number: 048896845
Section: summary

BACKGROUND OF THE INVENTION This invention relates to the fuel bundles of boiling water nuclear reactors and more particularly relates to an improved interface between the lower tie-plate and fuel bundle channel. Outline of the Problem Boiling water nuclear reactors generate steam in their core. This core is composed of an array of side-by-side vertically upstanding square sectioned fuel bundles. These fuel bundles divide the core region of the reactor into the so-called core bypass region, exterior of the fuel bundles, and the core region interior of the fuel bundles. The flow region interior of the fuel bundles is at a higher pressure than the bypass region. Typically, water is forced to circulate through the fuel bundles by pumping. The flow region exterior of the fuel bundles is the core bypass region. This region contains nonboiling water and is used to provide increased presence of water for the moderation of high speed neutrons to low speed neutrons so that the chain reaction in the boiling water reactor can continue. In order for this invention to be completely understood, the construction of a typical fuel bundle must be understood. Thereafter, the operation of such a fuel bundle during normal online operation of the reactor will be set forth. The problem of creep and pressure induced deflection of the channel in the vicinity of the lower tie-plate will be set forth. It is this deflection problem to which this invention is addressed. Then the problem of reflood of the core during a loss of coolant accident will be discussed. The participation of the lower tie-plate in such reflood will be set forth in preparation for an improvement in reflood of the disclosed invention. Fuel bundle construction can be summarized in a simplified format sufficient for the understanding of this invention. A fuel bundle consists of a group of fuel rods between an upper tie-plate and a lower tie-plate. The upper tie-plate and the lower tie-plate and the fuel rods extending therebetween are provided with a polygon section, which section is preferably square. This section is surrounded by a water impervious channel which forms a water tight boundary from the lower tie-plate to the upper tie-plate. The lower tie-plate consists of a plate which supports the lower ends of the fuel rods and an integral tubular structure which channels flow from below the lower tie-plate to the bottom of the lower tie-plate. The plate has openings between the fuel rods, and flow passes through these openings and into the fuel bundle. The lower tie-plate has four purposes. First, it supports the heavy fuel rods. Second, it forms a mechanical connection between the upper tie-plate and the lower tie-plate by threaded connection between some of the fuel rods. Third, the lower tie-plate allows water inflow from below the fuel bundle into the interior of the fuel bundle. Finally, the lower tie-plate in cooperation with the fuel bundle channel restricts leakage flow from the interior of the fuel bundle to the bypass region. This invention provides an improved method for restricting the leakage flow. There are two avenues of water flow through the lower tie-plate. The first avenue of flow is through openings between the fuel rod locations, which allow water used for both neutron moderation and steam generation to flow under pressure upwardly through the fuel bundle. This flow enters and passes from the lower tie-plate in the form of pure water. Steam is generated within the fuel bundle and passes out through the upper tie-plate in the form of a steam water mixture. The second avenue of flow is from the major aperture in the tie-plate through the side of the tie-plate to the so-called core bypass region. This flow occurs through small metering apertures, some of which are formed in the side of the lower tie-plate. During normal operation, these apertures supply the core bypass region with low pressure water. During a loss of coolant accident, these same apertures in the lower tie-plate permit so-called "reflood" of the interior of the fuel bundles from the core bypass region. Having discussed in general terms the construction of the fuel bundle and its relation to the core, the function of the fuel bundle during normal operation can be set forth. The problem of pressure acting on the channel of the lower tie-plate can be understood. During normal operation, water is introduced in forced circulation from the reactor and in effect pumped through the lower tie-plate of the fuel bundle. Water around the fuel rods is confined along a path by the fuel channel. A water steam mixture exits the top of the fuel bundle. After exit at the top of the fuel bundle, the water steam mixture passes on to steam separators with the water being recirculated and the steam being separated for power generation. The core bypass region also has a flow. This flow occurs among other places through the collective apertures in the sides of the lower tie-plates of all of the fuel bundles. Water is metered to the core bypass region at a reduced pressure. Thus, there is a substantial pressure differential at the lower tie-plate across the fuel bundle channel. The effect of this pressure differential on the fuel channel is easy to understand. The high water pressure from the inside of the fuel bundle acts towards the low water pressure in the core bypass region to the outside of the fuel bundle through the fuel channel wall. The square section channel is subjected to pressure forces that in the absence of resistance would cause the square sectioned channel to become cylindrical. Responsive to this pressure difference, the channel deflects away from the lower tie-plate. During reactor operation, the channel is subject to a neutron flux. The neutron flux, in combination with the stresses due to the pressure loading, causes the channel to creep so that the channel deflection increases with time. It is known to place a reinforcing band around the bottom of the channel at the lower tie-plate to prevent leakage. It will be appreciated that the interstitial volume between fuel bundles defines the volume for control rod excursion and control of the reaction. As far as bands used for channel reinforcement extend into this region, their added dimension is not desired. Further, insofar as such reinforcement adds to the neutron absorbing mass of the channel, the resultant reaction causes a loss of efficiency. SUMMARY OF THE PRIOR ART In the prior art, the lower tie-plate has been provided with indentations passing along the area of overlap of the channel. These indentations accommodate side-by-side spring biased fingers. These fingers are spring biased from the tie-plate outwardly to and towards the channel. These spring biased fingers occupy the interstitial volume between the channel and the tie-plate. As the channel deflects away from the tie-plate, the fingers move into the increasing interstitial volume and block fluid flow. Thus, the expansion due to both pressure differential and radiation induced creep does not cause excessive leakage. Unfortunately, such springs are themselves a contributor to the undesired expansion of the channel at the lower tie-plate, since the springs apply a load to the channel. SUMMARY OF THE INVENTION In a nuclear boiling reactor, an improved lower tie-plate and fuel channel interface for a boiling water reactor fuel bundle is disclosed. The fuel bundle has a lower tie-plate for supporting fuel rods and permitting the introduction of fluid interior of the fuel bundle. An upper tie-plate maintains the lower tie-plate supported rods in side-by-side relation and has apertures for discharging a mixture of water and steam. The fuel rods extend between the tie-plates for the generation of steam with some of the fuel rods forming a threaded connection fastening the tie-plates together. A polygon sectioned channel, preferably square, surrounds the tie-plates and fuel rods for the confining of fluid flow between the tie-plates interior of the bundle. The interface of the channel as it surrounds the lower tie-plate is reconfigured. This reconfiguration includes means for inducing a rapid pressure drop from the interior juncture of the lower tie-plate and channel to and towards the exterior juncture of the lower tie-plate and channel. This rapid pressure drop leaves the bottom portion of the square sectioned channel without a pressure load. In one embodiment, a labyrinth seal configuration is made in the lower tie-plate consisting of intermittent interruptions of an otherwise constant flow area between the lower tie-plate channel. The labyrinth seal is disclosed as configured either in the lower tie-plate or channel. In a preferred embodiment, a venturi flow configuration with diffuser is provided so that pressure drop in accordance with Bernoulli's principle effects reduced pressure between the lower tie-plate and channel. This reduced pressure reinforces the unstressed and lower portion of the channel with a (negative) hydraulic force applied to counter the (positive) force of outward channel bowing. Reverse venturi flow channel with diffusers are configured at the corners of the channel and adjacent lower tie-plate. These reversed venturi flow channels provide a resricted metered flow during reactor operation from the interior of the fuel bundle to the exterior core bypass region. At the same time and during a loss of coolant accident, a low pressure flow path for reflood of the fuel bundles is provided. Other Objects, Features, and Advantages An object to this invention is to disclose a reconfiguration of the lower tie-plate at the tie-plate channel juncture which hydraulically reinforces the channel at the tie-plate. According to a first embodiment of this invention, a labyrinth seal is configured to the periphery of the lower tie-plate. This labyrinth seal consists of intermittent horizontal pockets interrupting an otherwise constant section flow path configured between the tie-plate on one hand and the channel on the other hand. This labyrinth seal effects an immediate drop in pressure of water trying to pass from the relatively high pressure region interior of the fuel bundle to the low pressure region in the core bypass region exterior of the fuel bundle. An advantage of the labyrinth seal configuration is that the lowermost portion of the channel is left in an unloaded configuration. In this unloaded configuration, it suitably reinforces the overlying portion of the channel subjected to a hydraulic differential force. Thus, the overlying portion of the channel subjected to a high pressure interior from within the fuel bundle and a low pressure exterior in the core bypass region is in effect reinforced by the lower unloaded portion of the channel. According to a second and preferred embodiment of this invention, a venturi flow region is deliberately configured in the lower tie-plate immediate the lower portion of the fuel bundle channel. This venturi channel includes a diffuser for inducing a favorable pressure distribution over the lower portion of the fuel bundle channel. An advantage of this aspect of the invention is that the lower portion of the fuel channel experiences a (negative) hydraulic force with respect to the core bypass volume. This negative hydraulic force in addition to the unloaded portion of the fuel channel coacts upon the channel to maintain improved proximity to the lower tie-plate. A further advantage of this latter configuration is that the greater the tendency of the channel to bow away from the lower tie-plate, the greater the velocity of the flow through the venturi. The greater the velocity of the flow through the venturi, the stronger the hydraulic forces acting on the lower portion of the fuel channel. Thus, the increased hydraulic forces on the fuel channel maintain its proximity to the lower tie-plate. An improved seal results under all conditions. A serendipitous effect follows the reinforcement of the channel by fluid flow. Since most reactors at the lower tie-plate contemplate a small amount of flow from the interior of the channel at the lower tie-plate to the exterior of the channel in the core bypass region, the required leakage for the channel reinforcement provides this required flow. Such provision of required flow to the core bypass region has the supplemental result of the hydraulic reinforcement as set forth above. A further advantage of this invention is that no reconfiguration of the channel at the lower tie-plate is required. Rather, by reconfiguration of the lower tie-plate alone, the flow as set forth in this invention can be achieved. A further object to this invention is to illustrate a reconfiguration of the lower tie-plate which permits fuel bundle reflood from the core bypass region in the event of a loss of coolant accident. According to this aspect of the invention, the lower tie-plate at the corner is configured to form a passage having a venturi and a diffuser. The diffuser is aligned to provide energy efficient water flow in the direction of the interior of the fuel bundle from the core bypass region. In normal operation and due to reverse flow through the diffuser only a small and metered flow is experienced from the interior of the fuel bundle to the core bypass region. During reflood, a venturi diffuser assisted low friction flow path is established from the core bypass region to the interior of the fuel bundle. Consequently, efficient reflood can easily occur.