Patent Number: 048045144
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for neutron dosimetry and more particularly to a method and apparatus for neutron dosimetry in a nuclear power plant in applications where on-line measurement of neutron flux is necessary or desirable. 2. Background of the Invention The measurement of neutron exposure in selected regions within or outside of a nuclear reactor pressure vessel (RPV) is of interest for several reasons. Measurement of neutron exposure outside of the RPV can be used to obtain a determination of the plant operational power level. Measurement of neutron exposure outside of the RPV can also be used to determine the spatial power distribution within the RPV. Furthermore, measurement of neutron exposure within or outside of the RPV can provide dosimetry information with respect to fast neutron exposure, from which resulting vessel embrittlement may be inferred. The foregoing constitutes a broad range of applications, for which a variety of devices and techniques are currently employed. Detectors positioned outside of the reactor core (typically BF.sub.3 counters) are used to detect thermal neutrons for plant power/power distribution measurements. Passive activation samples are used for radiometric inference of vessel fluence from within vessel measurements. Passive samples containing both radiometric activation samples and solid state track recorders (SSTR) are used in outside of vessel measurements for inference of vessel fluence. Detectors positioned outside of the reactor pressure vessel are more accessible than those positioned within the RPV, but their removal can typically be effected only during a shutdown. While detectors so positioned are typically on-line devices, some are not. For example, passive counters must be transported to a laboratory in order to obtain data from them. Thus, experience has taught that exceptionally useful features in neutron dosimetry include the following: (1) for power level/power distribution purposes, sufficient sensitivity to enable meaningful inference of integrated neutron fluence over a period of minutes to hours; (2) for vessel dosimetry applications, a lifetime of years in a high radiation (neutron or gamma ray radiation) environment, with a relative insensitivity to gamma radiation; (3) also for vessel dosimetry applications, a capability of periodic on-line readout of integrated neutron dose (fluence), to provide data useful in forming decisions concerning the operation of the plant (e.g., heat up/cool down rates); and (4) capability of providing independent measurements of thermal and fast neutron flux. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a sensitive method and apparatus for neutron dosimetry on-line readout capability. It is a further object of the present invention to provide a neutron dosimeter having a long lifetime and capable of discriminating between thermal and fast neutron flux. Additional objects, 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. To achieve the foregoing and in accordance with the purposes of the invention, as embodied and broadly described herein, there is provided an apparatus for measuring intensity of neutron flux comprising means exposed to the neutron flux for generating fission fragments at a rate proportional to said intensity, means exposed to a known fraction of the fission fragments for generating light signals in response to the known fraction, means, optically coupled to the light signal generating means, for generating electrical signals in response to the light signals, and means, electrically coupled to the electrical signal generating means, for processing electrical signals to determine the intensity. In another aspect of the invention, multiple means for generating fission fragments having differing neutron energy thresholds for producing fission fragments are providing in order to obtain spectral data on neutron intensity. In yet another aspect of the invention, multiple means for generating light signals are positioned about the means for generating fission fragments, and the signals developed therein are checked for coincidence in order to discriminate events due to fission fragments from background events. Methods according to the invention are also disclosed and claimed.