Patent Number: 046577262
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. "An Improved Water Displacer Rod Spider Assembly For a Nuclear Reactor Fuel Assembly" by Trevor A. Francis; U.S. Ser. No. 595,154; filed Mar. 30, 1984. 2. "Control Rod For Nuclear Reactor" by Trevor A. Francis and John F. Wilson; U.S. Ser. No. 556,576; filed Nov. 30, 1983. BACKGROUND OF THE INVENTION The present invention relates generally to nuclear reactors, and more particularly is directed to an apparatus used with a fuel assembly for controlling the nuclear reactivity by varying the volume of the moderator/coolant associated with the fuel rods of the assembly and, at the same time, adding a burnable poision gas in thereby improving the fuel utilization, thus allowing for lower fuel enrichments. In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by frameworks referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower transversely extending core support plates. Conventional designs of these fuel assemblies include a plurality of fuel rods and hollow tubes or guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the guide thimbles. The guide thimbles are structural members which also provide channels for neutron absorber rods, burnable poison rods or neutron source assemblies which are all vehicles for controlling the reactivity of the reactor. Top and bottom nozzles on opposite ends thereof are secured to the guide thimbles in thereby forming an integral fuel assembly. Generally, in most reactors, a moderator/coolant such as water, is directed upwardly through aperatures in the lower core support plate and along the various fuel assemblies to receive the thermal energy therefrom. An example of such a fuel assembly structure can be seen in U.S. Pat. No. 4,326,419; granted to Donald J. Hill. Since the nuclear industry's inception, core component design improvements have evolved in response to changes in regulatory requirements, manufacturing considerations, and power generation costs. Increasingly, utilities and fuel suppliers have focused ever more strongly on neutron economy and reduced power generation costs. These effects have been motivated by increased fuel and fuel enrichment costs. In response to these demands, designers have been working hard in developing new designs and in modifying existing designs to improve fuel utilization, as well as, in increasing safety margins in reactors. It is known that improved fuel economy can be achieved in a PWR (Pressurized Water Reactor) by initially operating with a reduced H/U (hydrogen/uranium) ratio and then returning the ratio to normal somewhat later in the core cycle. The initial H/U reduction has the effect of increasing the epithermal part of the neutron spectrum at the expense of the thermal part. This results in increased breeding and decreased fission and fuel depletion rates. Since reactor fuel starts off with excess reactivity, this spectral shift represents no problem early in the core life; however, if the decrease in H/U were maintained through the entire core cycle, nothing would be gained because the higher fertile material absorption and lower fission rate would more than balance the gains from the increased breeding and lower burnup. Consequently, in order to properly take advantage of the increased breeding and lower burnup, it is necessary to return the H/U ratio back to its normal value. The net result would allow a reactor to be operated for a full core cycle with a reduced initial uranium enrichment. One of the ways of altering the H/U ratio which has been investigated involves the use of displacer rods. As the name implies, these rods are placed in the core to initially displace some of the moderating water and decrease the H/U ratio, and then, at some point during the core cycle, the displacement associated with these rods would be removed. 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 the above cross-referenced copending application of Trevor A. Francis, entitled "An Improved Water Displacer Rod Spider Assembly For A Nuclear Reactor Fuel Assembly". Among other unfavorable conditions, movable control mechanisms are expensive. 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. The basic idea makes use of a small heating element surrounding a specially indented end cap on the hollow displacer rod. At an appropriate time, the heater is turned on and the indented part of the end cap is weakened to the point where the external water pressure opens the end cap and fills the rod with water. The basic idea was expanded to include a manifold for each fuel assembly which would be constructed very similar to the spider-like control rod clusters presently used in reactors. All the rods in the cluster would be controlled by a single end-cap in the cluster head. The end-cap on each cluster would have an external plug connected to the heater inside. The procedure for changing the H/U ratio during a reactor cycle would be as follows: first, the reactor would be reduced to lower power or placed in a hot shut-down condition; the heaters in all the displacer rod clusters would be activated through heater power cables until all the end-caps have blown; and then, the reactor would be started up again. With the increased reactivity resulting from the higher H/U ratio, an elevated concentration of boron shim would have to be reintroduced into the primary coolant. Some of the problems anticipated with such an approach would be the reliability of the connectors and wiring when exposed to the pressure and corrosive capabilities of the reactor water, the potential failure of the rods themselves, what to do with the burst displacer rods after use since they are contaminated and thus inconvenient and impractical to transport and/or discard, and lastly, there is concern as to what would happen to such a displacer rod system in the case of LOCA (Loss of Coolant Activity) or other reactor problems. The present inventors were aware of the teachings of the above described works and their shortcomings when they developed their alternative approach which is the subject of the present invention. SUMMARY OF THE INVENTION The present invention provides a moderator control apparatus for a nuclear reactor fuel assembly so as to improve fuel utilization and thereby reduce fuel cycle costs. The apparatus is designed to displace a portion of the moderator/coolant for a reduced H/U ratio at initial start-up and then later, on a gradual basis, remove the displacement in shifting the energy spectrum by returning the H/U ratio to normal. The displacer rods are initially filled with a gaseous burnable poison to prevent large positive moderator temperature coefficients so as to insure a negative moderator temperature coefficient and to help in power shaping. Near the end of the cycle, with the reintroduction of the moderator/coolant, any remaining burnable poison gas is released into the system and taken away in the off gas system. The design is such that the removal of the moderator/coolant displacement, as well as, the release of the burnable poison gas is carried out on a slow and independently controlled basis in thereby insuring safety against accidental release or large change of reactivity during any single occurrence or transient. The system also increases an operator's flexibility in relieving unexpected power tilts during the operating cycle. This spectral shift, burnable poison, apparatus additionally alleviates the utilities concern over disposal of spent burnable poisons that are presently used to hold down excess reactivity. Further, the design of the apparatus allows for last minute power distribution adjustment. Since the poison gas can be loaded at the plant site and therefore it is possible to change the poison loading up until the time when the apparatus is placed in the core. Such flexibility greatly aids in finding acceptable loading patterns if after shutdown a utility decides not to use assemblies previously planned to be loaded. Still further, the design is such that maintenance and repair can easily be performed in the spent fuel pit. Another highly advantageous feature of the invention is that the control apparatus is reusable simply by recharging or refilling with a poison gas. This refilling operation can easily take place on-site, thus eliminating the expense of discarding the rods or the inconvenience and high costs in transporting them off-site due to their contaminated condition. Accordingly, the present invention sets forth in a fuel assembly for a nuclear reactor including an organized array of upstanding fuel rods, a number of elongated guide thimbles strategically located within the fuel rod array, and a moderator/coolant flowing upwardly along the fuel rods, an apparatus to control the nuclear reactivity for improved fuel utilization in thereby reducing fuel cycle costs. The control apparatus includes: (a) a plurality of hollow displacer rods adapted to be inserted into respective ones of the guide thimbles for displacement of a predetermined volume of the moderator/coolant associated with the fuel rods to decrease the hydrogen/uranium ratio from a given level; (b) a manifold adapted to be disposed on the top of the fuel assembly and having a plurality of inlet ports and a plurality of exit ports connected to and in fluid flow communication with respective ones of the displacer rods and with each of the inlet ports being in fluid flow communication with at least one of the exit ports; and (c) valve means operably associated with the manifold inlet ports for selectively controlling the flow and non-flow of the moderator/coolant into the displacer rods, flow of the moderator/coolant through the inlet ports and into the displacer rods increasing the moderator/coolant volume to thereby shift the hydrogen/uranium ratio back to its given level. The invention further includes the displacer rods being filled with a burnable poison gas which is released into the moderator/coolant as it flows into the rods so as to insure a negative moderator temperature coefficient and to assist in power shaping. More particularly, in the preferred embodiment, the manifold takes on the form of a hub defining a central opening and a plurality of hollow tube-like vanes mounted on and extending radially outwardly from the hub. The inward ends of the vanes defining the inlet ports which are circumferentially spaced about and disposed adjacent to the central opening. The valve means is selectively movable between a non-flow mode, wherein all inlet ports are closed to the flow of moderator/coolant therethrough, and a sequential flow mode, wherein some of the inlet ports are open to flow while other ones are closed to flow of the moderator/coolant therethrough. In the preferred embodiment, the valve means is in the form of an elongated hollow pipe or stem having at least one aperature or orifice, and preferably two, defined in the lower section thereof. The valve stem is rotatably mounted in the manifold with its lower orificed section being disposed in the central opening of the manifold and with its exterior wall being in abutting contact with the inlet ports so as to prevent flow of the moderator/coolant therethrough. Flow of the moderator/coolant, and the release of the burnable poison gas simultaneously therewith, through the inlet ports only occurs when the orifices are aligned with the inlet ports which is accomplished through rotation of the valve stem. 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.