Patent Number: 047284803
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Moderator Control Apparatus For A Nuclear Reactor Fuel Assembly" by D. B. Lancaster et al, assigned U.S. Ser. No. 623,744 and filed June 22, 1984 now U.S. Pat No., 4,657,726. 2. "Nuclear Fuel Assembly With Improved Specral Shift Rods" by H. M. Ferrari, assigned U.S. Ser. No. 638,333 and filed Aug. 6, 1984. 3. "Spectral Shift Apparatus And Method For A Nuclear Reactor Fuel Assembly" by Frank J. Baloh et al, assigned U.S. Ser. No. 701,051 and filed Feb. 12, 1985 now U.S. Pat. No. 4,683,103. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with an apparatus and method employed in refurbishing a spectral shift mechanism for use in a nuclear fuel assembly. 2. Description of the Prior Art In the conventional designs of pressurized water reactors (PWR), an excessive amount of reactivity is designed into the reactor core at start-up so that as the reactivity is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. However, since an excessive amount of reactivity is designed into the reactor core at the beginning of the core life, steps must be taken at that time to properly control it. One technique to control reactivity is to produce an initial spectral shift which has the effect of increasing the epithermal (low reactivity) part of the neutron spectrum at the expense of the thermal (high reactivity) part. This results in production of fewer thermal neutrons and decreased fission. Then, as fission decreases during extended reactor operation, a reverse shift back to the thermal part of the neutron spectrum at the expense of the epithermal part is undertaken. Such control technique is primarily accomplished through the use of displacer rods. As the name implies, these rods are placed in the core to initially displace some of the moderator water therein and decrease the reactivity. Then, at some point during the core cycle as reactivity is consumed, the displacement associated with these rods is removed from the core so that the amount of moderation and therewith level of reactivity in the core are increased. One approach considered for removing this displacement is through the use of movable mechanisms, similar to those associated with control rods. Such an approach is described in a U.S. patent application entitled "An Improved Water Displacer Rod Spider Assembly For A Nuclear Reactor Fuel Assembly" by Trevor A. Francis, filed Mar. 30, 1984 and assigned U.S. Ser. No. 595,154 now abandoned. Another approach contemplated for removing the displacement is to have membranes provided on the ends of the displacer rods which are penetrated at some point in time to allow the rods to be filled with water. A small heating element surrounding a specially indented end cap on the hollow displacer rod is turned on at an appropriate time. The heat weakens the indented part of the end cap to the point where the external water pressure opens the end cap and fills the rod with water. A further approach used to remove the displacement is the provision of at least one rod in the fuel assembly filled initially with helium or other suitable gas. Then, as reactor operation proceeds, the gas-filled rod expands and increases in length until it engages a spike mounted on the adjacent portion of the top nozzle. The spike pierces the upper end plug of the rod and permits the rod to fill with water. Such approach is described in U.S. Pat. No. 4,371,495 to Marlatt. Still another approach to displacement removal is to withdraw water displacer rods at the desired time by using a drive mechanism. This approach is described in U.S. Pat. No. 4, 432,934 to Gjertsen et al. Yet another approach utilizes a rotatable valve disposed in the central opening of a manifold in flow communication with a plurality of water displacer rods. Burnable poison gas initially fills the rods and is slowly released therefrom by operating the valve to progressively open inlet ports to the rods in a sequential pattern. At the same time the poison gas is slowly released from the rods, the displacement is removed by filling the rods with coolant. This approach is described in the first patent application cross-referenced above. While all of the above-cited prior approaches operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, a need still exists for an alternate approach to the implementation of the spectral shift concept in currently operating nuclear power plants in a cost-effective manner for achieving greater utilization of nuclear fuel being placed in such plants. SUMMARY OF THE INVENTION The present invention together with other components, some of which comprise the invention claimed in the third U.S. patent application cross-referenced above, provide a combination of refurbishable equipment and reactor operational transient adjustments for implementing the spectral shift concept in a manner designed to satisfy the aforementioned needs. While the present invention herein and the invention of the third cross-referenced application are particularly adapted for working together to implement the spectral shift concept on a reusable basis, it is readily apparent that each invention may be incorporated either singly or together in a nuclear reactor. However, both inventions are illustrated and described herein for facilitating a complete and thorough understanding of the present invention. The combination of refurbishable equipment and reactor operational transient adjustments disclosed herein implements the spectral shift concept in a relatively simple way which is reusable and can be made highly reliable and immune to false actuation. It utilizes known technology and is not dependent on development of "high risk" features. Although substantial reactivity is inserted into the fuel assembly during the removal of moderator displacement, this is accomplished with the reactor in a shut-down condition. No special wiring nor plumbing, which would entail extensive modifications to the reactor internals and increase refueling time, is required to actuate the spectral shift. Instead, a reactor system operational variable (i.e., temperature, pressure, flow, etc.) is used to actuate the removal of displacement. The actuation occurs on demand by the operator and not as a result of inaccurately predictable phenomena such as fissile material burnup, or material swelling or creep. The equipment can be fabricated economically and is easily refurbished for continued use. In summary, therefore, as compared to previous approaches to implementation of spectral shift, the equipment disclosed herein is less complicated, requires no extensive modifications to existing hardware and is not prone to inappropriate actuation. Accordingly, the present invention is directed to an apparatus and method for refurbishing a spectral shift mechanism for contiuned use in the same fuel assembly or reuse in another fuel assembly wherein the spectral shift mechanism has at least one elongated hollow water displacer rod being substantially filled with a moderator/coolant liquid, such as water. The refurbishing apparatus basically includes: (a) first means disposed in flow communication with an upper portion of the interior of the hollow displacer rod; (b) second means disposed in flow communication with a lower portion of the interior of the hollow rod; and (c) means interconnectable with the first means and second means so as to provide an entrance path for air under pressure to the upper portion of the hollow rod via the first means and an exit path for water from the lower portion of the hollow rod via the second means, the exit path being separate from the entrance path. Therefore, the introduction of air under pressure along the entrance path into the upper portion of the hollow rod will force the liquid within the hollow rod to flow along the exit path from the lower portion of the rod until the level of the liquid within the rod lowers to the level of the second means in the lower portion of the hollow rod. More particularly, the refurbishing apparatus is associated with a spectral shift mechanism having a plurality of displacer rods and includes: (a) a generally cylindrical member having an opening defined at one end thereof, first port means and second port means being displaced from one another and a central plenum communicating with the end opening and the first and second port means; (b) a plurality of first conduits extending between the cylindrical member and respective ones of the displacer rods, each of the first conduits interconnecting the first port means of the member and an upper portion of one of the rods in flow communication; (c) a plurality of second conduits extending between the cylindrical member and respective ones of the displacer rods, each of the second conduits interconnecting the second port means of the member and a lower portion of one of the rods in flow communication; (d) a generally cylindrical charging tool having defined therein first and second flow passageways with corresponding first and second flow openings, the tool being insertable within the central plenum of the cylindrical member through the end opening therein; and (e) means on one of the cylindrical member and the charging tool and adapted to seal with the other of the member and the tool so as to place the first and second flow openings of the tool in separate flow communication respectively with the first and second port means of the cylindrical member, whereby the introduction of air under pressure through the first passageway of the charging tool and through the first conduit to the upper portion of the displacer rod will force liquid within the rod from the lower portion of the rod through the second conduit and the second passageway of the charging tool until the level of the water in the rod lowers to level of the second conduit therein. The method for refurbishing the spectral shift mechanism which has at least one hollow water displacer rod substantially filled with liquid comprises the operative steps of: (a) establishing flow communication with an upper portion of the interior of the hollow rod to provide an entrance path for air under pressure to the upper portion of the hollow rod; (b) establishing flow communication with a lower portion of the interior of the hollow rod to provide an exit path for the liquid from the lower portion of the hollow rod, the exit path being separate from the entrance path; and (c) introducing air under pressure along the entrance path into the upper portion of the hollow rod so as to force the liquid within the hollow rod to flow along the exit path from the lower portion of the hollow rod until the level of the liquid within the hollow rod lowers to the level in the rod at which flow communication was established with the lower portion of the rod. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.