Patent Number: 048790869
Section: summary

BACKGROUND OF THE INVENTION This invention is directed to a reactivity control system for a light water breeder reactor (LWBR). In particular, the LWBR reactivity control comprises a stationary seed-blanket core arrangement comprising a radial arrangement of the fuel into a pattern of discrete seed regions that contain ThO.sub.2 -UO.sub.2 fuel pellets and discrete blanket regions that contain pure thorium dioxide fuel pellets. The United States Government has rights in this invention pursuant to Contract No. DE-AC12-76-SN00052 between the U.S. Department of Energy and the General Electric Company. The continuous, world-wide growth of nuclear power based on current light water reactors will deplete readily obtainable supplies of the fissile fuel isotope uranium-235. While authoritative studies may disagree on the expected timing of this event, there is a general agreement that, to pursue nuclear power as a major energy source, the nuclear fuel cycle must take advantage of the potential energy available in the abundant fertile fuel isotopes, uranium-238 and/or thorium-232. Reactor technology in coming decades must shift away from the current light water reactor with a once-through fuel cycle toward more fuel efficient concepts, including fuel recycle, high energy converter reactors and breeder reactors. Light water moderated converter reactors or breeder reactors using the thorium-232/uranium-233 fuel cycle are looked upon as attractive options for future nuclear reactors. The attractiveness of the thorium fuel cycle in a light water reactor derives from three major considerations; (1) the core, reactor equipment, primary system and balance of the plant are all based on the well-established technology of light water reactors; (2) fuel utilization is better than for the uranium/plutonium fuel cycle in a similar light water reactor application with recycled fuel and the thorium fuel cycle can achieve a self-sustaining breeder reactor system; (3) existing pressurized water reactor plants could be refitted with thorium fuel cycle converter cores, although the highest level of fuel utilizations are probably not achievable at full plant power ratings. The atomic Energy Commission and its successor governmental agencies, ERDA and the Department of Energy, has attempted to demonstrate the potential of the thorium fuel cycle in light water moderated reactors from the mid-1970's. Various concepts are being explored, including pre-breeder, converter and advance breeder reactors including a scale-up of the Shippingport reactor operated by the Duquesne Light Company. A very recent development in this technology has advanced the concept that a practical commercial scale breeder may be made which does not rely upon a separate source, such as a pre-breeder and converter reactor to provide its initial load of fissile uranium-233. The concept includes a breeder reactor plant which becomes its own pre-breeder by using fuel elements of different dimensions for the initial pre-breeder core cycles. The present invention is directed to a reactivity control system for the breeder concept of this type of pre-breeder/breeder reactor system. The reactivity control system for the breeder concept proposed in this application must perform all the functions of the reactivity control system in a commercial pressurized water reactor (PWR) but, in addition, it must perform those functions while minimizing the loss of neutrons to neutron poisons or other parasitic materials. For example, a typical present generation commercial PWR control system consists of soluble boron used in the primary coolant and poison control rod assemblies used for shutdown, regulating, and axial power shaping. This control system has the advantage of good axial and radial power distributions due to the uniform poisoning effect of soluble boron, but has the disadvantage of poor neutron economy due also to the presence of a relatively large amount of soluble boron. Accordingly, this type of control system is not conductive to a light water breeder reactor concept because of poor neutron economy present when one uses large quantities of boron. The Shippingport light water breeder reactor control system consists of a movable fuel control. In this system, each active module of the core contains a central movable fuel assembly surrounded by a stationary blanket assembly. Reactivity control is accomplished by varying the axial position of the movable seed assemblies relative to the surrounding stationary blanket assembly. For a typical module, the movable speed volume is approximately 30% of the total active module volume. The Shippingport LWBR control system has the advantage of good neutron economy because it adjusts reactivity by using variable seed positions rather than a poison material. However, due to the movable fuel, the Shippingport LWBR system results in a higher axial power peaking not present in the commercial PWR. The reactivity control system of the present invention is able to achieve the neutron economy of the movable fuel Shippingport LWBR while maintaining the axial power peaking properties of commercial PWRs. Accordingly, the reactivity control system of the present invention is able to incorporate the advantages of the PWR control system and the LWBR control system of Shippingport without their attendant disadvantages. SUMMARY OF THE INVENTION It is the primary object of the present invention to provide a reactivity control system for a LWBR which meets all the control system requirements set forth by present PWRs. It is another object of the present invention to provide a reactivity control system for a LWBR which maximizes absorption in fertile material and minimizes the absorption in poison material. It is a further object of the present invention to provide a reactivity control system for a LWBR which minimizes axial and radial power peaking. It is still another object of the present invention to provide a reactivity control system for a LWBR which maximizes the use of current light water reactor technology. Additional objectives, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, as embodied and broadly described herein, the reactivity control system of the present invention comprises a reactor core having a stationary seed-blanket arrangement comprising a plurality of symmetrical, contiguous, substantially hexagonal-shaped regions. Each of these regions has a central and peripheral blanket area juxtapositioned an annular seed area and bounding the seed area, The central and peripheral blanket areas contain a plurality of blanket fuel rods wherein the blanket fuel rods contain thoria fuel pellets. The annular seed area contains a plurality of seed fuel rods and a plurality of movable thoria shim control rods. The seed fuel rods contain a mixture of thoria and urania fuel pellets. The term stationary seed-blanket arrangement is used to signify that all seed and blanket fuel rods are fixed in the core as are the fuel rods in a typical, commercial PWR core. These rods do not move as do the seed rods in the Shippingport LWBR. In the preferred embodiment of the reactivity control system of the present invention, the cross section of said reactor core contains about 37 contiguous, symmetrical, substantially hexagonal-shaped regions. In a further preferred embodiment of the reactivity control system of the present invention, the movable thoria shim control rods are located substantially in the center of the annular seed area of the nuclear reactor core. In a still further preferred embodiment of the present invention, the reactivity control system also includes poison shutdown rods, poison regulating rods, axial power shaping rods, and a soluble coolant containing boron. It is, of course, understood that each of these components in the reactivity control system performs in its known conventional manner. The reactivity control system of the present invention combines the advantages of the PWR control system and the Shippingport LWBR control system previously described. The reactor control system of the present invention, like the commercial PWR control system, has the advantage of good axial and radial distributions, but does not have the disadvantage of poor neutron economy. The reactivity control system of the present invention has the advantage of good neutron economy similar to the Shippingport LWBR system but because of the stationary seed-blanket core concept of the present invention can nearly flatten the variation in the lifetime activity and, in itself, almost eliminates the need for soluble boron at the full power operation. Accordingly, it can be seen that the reactivity control system of the present invention incorporates the best features of the pressurized water system and the Shippingport light water breeder reactor system without their attendant disadvantages. The reactor system of the present invention has good neutron economy while maintaining axial power peaking similar to a commercial pressurized water system.