Patent Number: 040627250
Section: summary

BACKGROUND OF THE INVENTION This invention pertains to control rods for the control of nuclear reactors and more particularly to a part length rod useful for controlling power oscillations and for contributing to reactor shutdown. DESCRIPTION OF THE PRIOR ART It is well understood in the art of nuclear reactor power generation that larger reactors exhibit unstable oscillatory distortion of the neutron flux in the axial direction. These power oscillations occur as a result of local increases in the neutron flux leading to the "burn-out" of xenon 135 (produced by radioactive decay of iodine 135) which increases reactivity, and leads to a further flux distortion and so on. In the course of time the concentration of xenon 135 begins to build up because of the higher flux level, and the whole process is reversed. Various attempts have been made by the prior art to provide specially designed control rods for the purpose of controlling such xenon produced power oscillations. One such earlier attempt is illustrated in U.S. Pat. No. 3,081,248 issued to P. J. Grant on Mar. 12, 1963. This attempted solution to the control problem proposed the provision of a control rod or control means comprising a pair of neutron absorber members adapted to be inserted into different parts of the reactor core and linked or coupled whereby movement of one member into, or out of, the reactor core was accompanied by a corresponding movement of the other member out of, or into, the reactor core, the arrangement being such that by the differential movement of the two members, the fluxes in the two parts of the reactor core are relatively adjusted without substantially effecting the total flux. This solution, however, was fraught with difficulties which rendered the solution generally unacceptable to the power industry. One of the difficulties was that the special control rod had a length in excess of the length of the active portion of the reactor which necessarily tended to increase the length of the pressure vessel surrounding the reactor core. Unnecessary increases in pressure vessel size are extremely expensive and are avoided if at all possible. A second difficulty was that each of the special control rods had to be driven with a special control rod drive mechanism which could not be scrammed when a rapid shutdown of the reactor was required. A second prior art solution to the control of power distribution oscillations within the core is the use of a part length control rod. Such rods are controlled independently of the main control rods and generally contain the neutron poison in the lower portion of the rod, with the upper portion of the rod being substantially non-neutron absorbing. In operation the poison portion is normally positioned in the central region of the core. If these rods are allowed to scram when a rapid shutdown of the reactor is required, the rods drop to the bottom of the reactor and the poison could be removed from a position of higher control worth to a position of lower control worth. This would tend to increase the reactivity of the reactor just at a time when every effort is being made to reduce the reactivity. Accordingly, utilization of the prior art linear motion devices for part length control rods of commercial nuclear power plants did not result in a fail safe system, since a simultaneous trip of several part length control rods could result in an undesirable increase in the reactivity unless all the full length rods are tripped at the same time. Hence, the use of these prior length rods requires the use of a different type of linear motion device which is incapable of scramming the part length control rods. One such non-scrammable part length control rod drive mechanism is disclosed in U.S. Pat. No. 3,825,160 issued to Lichtenberger et al. on July 23, 1974 and assigned to the present assignee. As can be appreciated from the above discussion, the prior art solutions to the control of power oscillations required two distinct control rod drive mechanisms; a scrammable drive mechanism and a non-scrammable drive mechanism. In addition to the increased cost associated with providing two distinct types of drive mechanisms on each nuclear reactor, the prior art solutions have the effect of decreasing the control flexibility of the reactor. This follows since the positions of the part length rods and their drive mechanisms become fixed once the drive mechanisms are welded to the reactor pressure vessel. Hence, the reactor designers do not have the flexibility of relocating the part length rods from these initially fixed positions without expensive and complex disassembly and relocation of the drive mechanisms. Thus a need is felt for a part length rod which may be scrammed into the core when a rapid reactor shutdown is necessary so that additional flexibility in the positioning of the part length rod may be achieved by simply moving the part length rod from one scrammable control rod drive mechanism to another identical scrammable control rod drive mechanism. SUMMARY OF THE INVENTION A part length rod is provided which may be mounted on a scrammable control rod drive mechanism and which may be scrammed into the reactor core when a rapid reactor shutdown is required. The part length control rod of this invention has first and second ends with a first neutron absorbing material at its first end, a second neutron absorbing material at its second end spaced from the first neutron absorbing material by a distance less than the length of the core, and a third intermediate portion connecting the first and second neutron absorbing materials, the intermediate material being substantially non-neutron absorbing. The first neutron absorbing material is a material of high macroscopic neutron absorption cross-section. The second neutron absorbing material has a smaller macroscopic neutron absorption cross-section than the first neutron absorbing material. The second neutron absorbing material is normally positioned in the central region of the core for control of power oscillations. The first neutron absorbing material is normally positioned outside of the reactor core where it has little or no effect on the neutron flux of the reactor core. Upon the requirement for a rapid reactor shutdown, the part length control rod is scrammed or inserted into the core so that both first and second ends of the control rod are simultaneously positioned within the core at opposite ends of the core or so that at least a fraction of each end of said first and second ends are simultaneoulsy positioned within the core. In greater detail, the reduced macroscopic absorption cross-section of the second neutron absorbing material is obtained by providing a material with a high number density but having a low microscopic neutron absorption cross-section. Such a material is Inconel 600 or Inconel 625. In such a control rod it is desirable for the length of the second neutron absorbing material to be in the range of 25 percent of the length of the core and 55 percent of the length of the core. An optimum length is 50 percent of the length of the core. The first end of the part length control rod containing a high worth neutron poison is preferably not more than 20 percent of the length of the core and may consist of boron carbide (B.sub.4 C) as the poison material. The third or intermediate portion of the control rod is relatively non-neutron absorbing relative to the first and second ends of the control rod and is an Inconel tube filled with a hydrogeneous material such as water.