Patent Number: 050646036
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a nuclear reactor sensing system; and more particularly to a hydroball string sensing system that determines the location and velocity of a hydroball string. To control the operation of a nuclear reactor the neurron flux within the reactor core region is monitored. To monitor this neutron flux, numerous tubes are positioned within the core. These tubes house strings called hydroball strings that include stainless steel beads on a flexible stainless steel cable. Each of the stainless beads includes a material such as manganese that absorbs neutrons while the stainless steel bead is in the core region. After the strings have been in the core for several minutes, the strings are removed from the core region and the decay time of the activated manganese in each bead is measured. This measurement indicates the neutron flux to which each bead was subjected while within the reactor core region. The hydroball strings are moved into and out of the reactor core region by pumping water through the tubes in the direction of desired hydroball string movement. With this technique for approximating the neutron flux in a reactor core region, it is critical: (1) to know how long the hydroball string spends in the reactor core region, (2) to know how much time elapses between the string leaving the core region and the start of measuring the radiation emitted from each hydroball, and (3) to confirm the location of the hydroball string at various times as it is moved into and out of the reactor core region. Because the hydroball string is housed within a thick walled stainless steel tube that contains reactor core fluid at the same pressure as within the reactor core, it is difficult to achieve these critical objectives. SUMMARY OF THE INVENTION It is an object of the present invention to provide a hydroball string sensing system which can accurately determine the time spent by a hydroball string within a reactor core region. It is another object of the present invention to provide a hydroball string sensing system capable of operating on tubes which are maintained at 600.degree. F. or above. It is a further object invention to provide a hydroball string sensing system which can accurately locate a hydroball string. It is still another object of the present invention to provide a hydroball string sensing system that can accurately determine the amount of time that the radiation from a particular hydroball is sensed. It is still a further object of the present invention to provide a hydroball string sensing system capable of detecting and controlling the velocity of a hydroball string within a tube. To achieve the above and other objects, the present invention provides a hydroball string sensing system for a nuclear reactor having a core containing a fluid at a fluid pressure, the system comprises: a tube connectable to the nuclear reactor core so that the fluid can flow within the tube at a fluid pressure that is substantially the same as the fluid pressure of the nuclear reactor core; a hydroball string including a string member having objects positioned thereon with a specified spacing, the objects include a plurality of hydroballs and bullet members positioned at opposing ends of the string member; and a first sensor means positioned outside of a first segment of the tube, for sensing one of the objects being positioned within the first segment, and for providing a sensing signal responsive to the sensing of the first sensing means. The present invention also provides a hydroball string sensing system for a nuclear reactor having a core containing a fluid at a fluid pressure, the system comprises: a tube connectable to the nuclear reactor so that the fluid can flow within the tube at a fluid pressure that is substantially the same as the fluid pressure of the nuclear reactor core; a hydroball string including--a string member having objects positioned therealong with a specified spacing, the objects including a plurality of hydroballs, and bullet members positioned at opposing ends of the string member; first ultrasonic transducer sensor means, positioned outside a first segment of the tube, for sensing one of the objects being positioned within the first segment, and for providing a sensing signal responsive to the sensing of the first sensor means; second ultrasonic transducer sensor means, positioned outside a second segment of the tube being spaced a given distance along the tube from the first segment, for sensing one of the objects being positioned within the second segment, and for providing a sensing signal responsive to the sensing of said second sensor means; timing means for determining an amount of time between the first sensor means sensing the one of the objects within the first segment and the first sensor means sensing another one of the objects within the first segment; means for determining a velocity of the objects based upon the specified spacing and the determined amount of time; means for adjusting the rate of the fluid flow so as to make the determined velocity substantially equal to a desired velocity; and detector means, positioned outside of the tube and between the first and second segments for counting gamma rays emitted from one of the objects while the object is between the first and second sensor means.