Patent Number: 050646028
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the present invention, flux-trap control rod 100 comprises four wings 102, two of which are shown in FIG. 1. Control rod 100 is accessed from above via handle 104, and is coupled from below to a hydraulic control rod drive with coupling socket 106, which may be released with coupling release handle 108. The hydraulic control rod drive raises, or lowers, control rod 100 to control the reactivity of the reactor core. Each wing 102 comprises an outer stainless steel sheath 110 with openings 112 that correspond to the openings in the hollow absorber tubes inside the sheath. When control rod 100 is in place in the reactor, water along its length is exchanged between the outside of the control rod and the inside of the absorber tubes. Openings 114 allow water to enter the bottom and leave the top of the absorber tubes through sheath 110. The flux-trap control rod 100 of the present invention has a cruciform configuration (looks like a "plus" sign (+) from above), as can be seen from FIG. 2. Each wing 102 contains three absorber tubes 200. These define four quadrants where fuel assemblies are located, indicated by dotted lines 202. A neutron-absorber tube 200 has water inlets 300, flux-trap control rod flow diverters 302, and an attachment portion 304, as shown in FIG. 3. The flow diverters 302 are made by cutting tabs out of absorber tube 200 while leaving integral attachment portions 304. The tabs are bent inwards until they touch the opposite wall of absorber tube 200, thus forming flow diverters 302, as well as inlets 300. In a preferred embodiment of the present invention, the absorber tube 200 is made of hafnium, and the inlets 300 and flow diverters 302 are rectangular. The size of the inlets 300 and flow diverters 302 may be varied to divert any, or virtually all of the water flowing up absorber tube 200. Neutron absorber tube 200 has outlets 400, as shown in FIG. 4. The outlets 400 oppose inlets 300, and are located directly beneath the point where flow diverters 302 touch the inner wall of absorber tube 200. The spacing and size of flow diverters 302 are such that the water flowing through absorber tube 200 is diverted through outlets 400 before it has a chance to boil. In a preferred embodiment of the present invention, the outlets 400 are circular. At least a portion of the water traveling up absorber tube 200 is diverted through outlets 400 by flow diverters 302. Inlets 300 are about the same size as outlets 400, which reduces pressure gradients. Control rod 100 has hafnium absorber tubes 200 with flat opposing sides and rounded ends as shown in FIGS. 3 and 4. The tubes are about 12' long, and made in two 6' sections with an expansion joint in between to accomodate temperature changes. The hafnium in the top section is 0.07" thick. In the bottom section it is half as thick, 0.035". Both sections have the same outer dimensions. The absorber tube 200 has a width of 1.35", and a thickness of 0.22". The radius of curvature at each of the corners is about 0.11". Inlets 300 are rectangular with widths of 0.4", and heights of 0.5" in the bottom section and 0.6" in the top section. Flow diverters 302 are 0.4" wide and 0.5" high in both sections. The inlets 300 in the top section are slightly larger than the flow diverters 302 because the hafnium is thicker than in the bottom section, and doesn't leave as large of an opening when bent to the opposite wall. Outlets 400 are circular with a 0.4" diameter. Inlets 300 and outlets 400 are spaced at 6 inch intervals along the length of the absorber tube 200, with the top of the outlets 400 located at the same spot the tab diverters 302 touch when bent. Rectanglular inlets 300 are preferred because the resultant flow diverters 302 remain flush with the opposite wall in an absorber tube 200, shaped as shown in FIGS. 3 and 4. Circular outlets 400 are preferred because they have the most area for a given perimeter, leaving absorber tube 200 more intact. FIG. 5 shows a cross-section of a single wing 102 of control rod 100 when operational, i.e., inside the reactor core, with the resultant water flow. Sheath openings 112 coincide with, and are slightly larger than, inlets 300 and outlets 400 in absorber tube 200. The difference in size is shown by the overlap portion 500 of absorber tube 200, and is there to take into account the difference in thermal expansion between hafnium and stainless steel. A small gap 504 also separates sheath 110 and absorber tubes 200 for this same reason. Water flow, indicated by arrows 502, is essentially parallel to flux-trap control rod 100. External water between control rod 100 and the fuel assemblies (located at dotted line 202) enters control rod 100 through inlet openings 300, and flows up absorber tube 200 to the next flow diverter 302, where it is at least partially diverted back outside through outlet openings 400. The active divergence of the water to the exterior of control rod 100 through outlets 400 causes more water to be drawn through the corresponding inlets 300. This is where the improvement lies when compared to the prior art passive exchange of water through simple openings. As can be seen in FIG. 5, flow diverters 302 are bent inwards until they touch the opposing wall of absorber tube 200 while remaining attached at attachment portion 304. Since the internal water is substantially replenished with external water at every flow diverter 302 as it travels up the flux-trap control rod 100, it does not reach saturation and boil. Thus its effectiveness as a neutron moderator is assured. In turn, absorber tube 200 has increased effectiveness as a neutron absorber, and the flux-trap control rods 100 of the present invention work reliably with a constant efficiency in a boiling water reactor operating at full power. FIG. 6 provides a general overview of the waterflow in a reactor pressure vessel equipped with control rods of the present invention. Inside pressure vessel 600 is reactor core 602. Waterflow 604 travels up through core 602 (using pumps, which are not shown) where it is partially converted to steam 606. Steam 606 goes through outlet 608, and is used to turn turbines which turns a generator to produce electricity. The steam is passed through a cooler, or "heat exchanger", to condense it to water, which enters pressure vessel 600 through inlet 610, where it rejoins reactor water 604. The reactor water that was not vaporized travels down the outside periphery of core 602 after it has emerged from the top, and repeats the cycle. Regions 612 in core 602 depict two control rod configurations (not to scale), each consisting of a control rod and its four associated fuel assemblies. Extensions 614 below pressure vessel 600 house the hydraulic drives that raise and lower the control rods. Diagonals 616 represent the locations of each set of twelve flow diverters along the length of each control rod configuration 612. The waterflow at the inlets and outlets associated with flow diverters 616 is shown respectively by arrows 618 and 620. Water 622 entering the bottom of control rod configurations 612 is continually exchanged between the interior and exterior of the twelve absorber tubes in each configuration 612 as it flows upwards, and boiling inside the absorber tubes is prevented. Although inlets and outlets are shown as rectangular and circular, respectively, other shapes can be employed. Moreover, rectangular flow diverters can have rounded vertical sides so the shape of flow diverters more closely approximates the internal cross-section of the absorber tube. Such an arrangement forces the exchange of more hot internal water for cooler external water at each of the intervals where inlets and outlets are located. Inlets can be trapezoidal, with a wide attachment portion to strengthen the flow diverters. Inlets and outlets with circular or oblong shapes tend to maximize area while minimizing perimeter, and thus tend to leave the absorber tube more intact for a given surface area. One advantage of the present invention is the amount of latitude there is in fitting the flux-trap control rod to the specific engineering and environmental constraints it may be exposed to inside the BWR. To avoid internal boiling under a given set of circumstances, the flow diverters may be made larger, relative to the internal cross-sectional area of the absorber tubes, and spaced further apart. Alternatively, the flow diverters can be smaller or spaced closer together. The ease with which the flow diverter configuration can be made to fit individual situations is inherent in the design of the present invention. An additional advantage of the present invention is the facility with which one can modify the flow diverters to accomodate variations in the internal and external water temperatures along the length of the absorber tubes. The spacing, shape, and/or size of the flow diverters can be varied along the length of each tube individually. The flow diverters may also be utilized with hafnium absorber tubes that have varying thicknesses along the length of the tubes, of the type mentioned in U.S. Pat. No. 4,882,123, and described above. The present invention thus provides a flux-trap design control rod for a boiling water reactor that is both effective in preventing internal boiling, and adaptable to external conditions. Flow diverters may also be utilized with absorber tubes that have various internal cross-sectional shapes. The present invention also provides for a range of embodiments not described above. The neutron absorbing material need not be hafnium, but can be any other suitable material. Likewise, moderators other than water are used in alternative embodiments. Flux-trap control rods of the present invention can be made with any combination of a liquid moderating material and bendable absorbing material. These and other modifications to and variations upon the described embodiments are provided for by the present invention, the scope of which is limited only by the following claims.