Patent Number: 046474200
Section: summary

BACKGROUND OF THE INVENTION Nuclear fission reactors use fuel pins which are loaded with pellets of fissionable nuclear fuel. The amount and concentration of fissionable or fissile material contained within the fuel pin is an important parameter for proper operation and maintenance of a nuclear reactor. Assurance of high quality and adherence to design specifications can advantageously be accomplished by inspecting fuel pins for uniformity and total content of the fissile material. It may also be desirable in certain cases to perform other types of inspections. Fuel pin scanners are currently being used to inspect nuclear fuel pins to assure proper uniformity and amount of fissile material. Current technologies do not, however, provide the production speed or level of accuracy which is now required in producing fuel pins used in liquid metal fast breeder reactors. Plutonium recycle systems used in light water reactors also have similar expected need for high production capability and accuracy in inspecting fuel pins. Fuel pin scanners are already in use in light water reactor fuel manufacturing plants. The early fuel pin scanners used passive systems which simply measured the natural radioactivity of the fuels. Such systems were very slow, thereby requiring large numbers of scanners just to inspect the output of a large light water reactor fuel plant. The economical availability of californium-252 led to the development of nuclear fuel pin scanners which activate the fissile material using radiation. Such fuel pin scanners were capable of processing up to approximately 1,000 uranium oxide fuel pins per day. Such prior pin scanners used a single pass configuration which was relatively slow and provided limited accuracy. The need for fabricating plutonium bearing nuclear fuels on a large scale arose with the liquid metal fast breeder reactor program. Scanning of plutonium bearing fuel pins used in such reactors has created special requirements which were not satisfactorily met by the prior art. Most significant of the problems was the need for greater accuracy in measuring the uniformity of fissionable material contained within the fuel pin. Liquid metal fast breeder reactor fuel pin scanners must not only detect rejectable defects but must also allow characterization of the fuel for identification purposes. Characterization of the fuel allows for the fuel to be more closely monitored during the manufacturing process. This in turn aids in the production of high quality and safe, fast reactor fuels. The typical prior art light water reactor fuel pin scanner consisted of: (1) an irradiator containing one to five milligrams of californium-252; (2) mechanisms for transporting fuel pins sequentially through the irradiator and through one or two fission product gamma ray detectors; (3) sodium iodide, bismuth germanate or plastic scintillators; (4) a gamma ray transmission device for measuring gaps and nuclear fuel density; and (5) an on-line computer for collection and processing of data. All prior art light water reactor fuel pin scanners measured fissile uniformity in a single pass of the fuel pin through the irradiator and detector. In this single pass configuration the fuel pins were passed near a irradiator containing a neutron source such as californium-252 which activates the fissile material to provide increased radioactive emissions therefrom. The activated fuel pin was then passed through a detector in a single pass. Such single pass systems were relatively slow because of the exposure time needed to sufficiently activate the fuel pin and the length over which activation occurred. Decreasing the exposure time to increase capacity required increasing the irradiation power which was not economical. Higher capacity could also be achieved through increased numbers of systems but this also was expensive and indicated the need for high capacity systems which addressed the problem in a new manner. Such single pass activation and detection was also found impractical to achieve the increased accuracy necessary in producing fuel pins used in liquid metal fast breeder reactors. Liquid metal fast breeder reactors use nuclear fuel made with mixed oxides of plutonium and uranium, rather than the uranium dioxide fuels commonly used in light water reactors. Fuel pins made with mixed oxides of plutonium and uranium are more difficult to measure for fissile uniformity because the fissile loading of the fuel pellets is much greater and the thermal neutron activation commonly used with uranium dioxide fuel pins is not effective as an activating source of radiation. Applying known light water reactor fuel pin scanner technology to fuel pins loaded with mixed oxides of plutonium and uranium requires using relatively large amounts (0.1 gram) of californium-252, as compared to 1-5 millgrams used in current light water reactor fuel pin scanners. This amount of californium-252, has a value in excess of $1,000,000 thereby rendering current fuel pin scanner technology uneconomical for fuel pins used in liquid metal fast breeder reactors. SUMMARY OF THE INVENTION It is an object of this invention to provide a nuclear fuel pin scanner which is capable of high production capacity. It is another object of the invention to provide a nuclear fuel pin scanner which accurately and reliably measures fissile uniformity and total fissile loading. It is a further object of this invention to provide a nuclear fuel pin scanner which provides extended irradiation times while maintaining a high production throughput. And it is a still further object of this invention to provide a nuclear fuel pin scanner having improved detector accuracy. 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. 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 objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the nuclear fuel pin scanning system of this invention may comprise a rotary irradiator having a source of activating radiation located centrally thereof. A plurality of spaced positions are provided for supporting a plurality of fuel pins in an arrangement wherein the fuel pins are equally spaced from the source of radiation. The source of radiation advantageously lies along the rotational axis of the irradiator and the fuel pins are slowly rotated thereabout. Means are provided for feeding and discharging the fuel pins to and from their positions in the irradiator. Means are also advantageously provided for axially oscillating the source of activating radiation relative to the fuel pins being activated in the irradiator. Further means are provided for indexing or otherwise rotating the irradiator to advance the fuel pins through a range of positions about the source of radiation. The simultaneous irradiation of a plurality of fuel pins allows for extended activation times when compared to single pass systems and the required throughput rate of the entire system. The nuclear fuel pin scanning system of this invention also includes a detector having a plurality of detector elements arranged in an axial or linear array which receives a fuel pin therein. The detector elements advantageously include a collimating shield defining an annular opening which allows radiation emitted from the activated fuel pins to strike radiation transducers. The radiation transducers can advantageously be a crystal which illuminates when struck by the radiation being detected. The crystal is optically coupled to a photomultiplier or other light transducer which produces an electronic signal representative of the level of radiation striking the crystal. The plurality of detector elements arranged in a linear array allows each element to detect a limited segment of each fuel pin. The fuel pin is oscillated repeatedly over the relative short distance equal to the spacing between adjacent detector elements. Repeated passes using multiple detectors covering short distances of the fuel pin allow detection times which are effectively much longer than if the entire length of the fuel pin was passed by a single detector either in a single pass or multiple passes. Accordingly, the production throughput can be maintained at a high rate even though accurate multiple pass detection is being performed. The multiple detector elements further allow specific accurate readings for each segment of the fuel pin. The invention further comprises a method for inspecting nuclear fuel pins to accurately determine uniformity and total amount of fissile material, while maintaining a relatively high production throughput rate. The method involves arranging a plurality of nuclear fuel pins in an arrangement about a longitudinal axis of rotation with each fuel pin spaced approximately equally from the axis. The arrangement of fuel pins is rotated about the axis of rotation which also is the location of the source of activating radiation. The fuel pins are activated over an extended period of time as the arrangement slowly rotates or is indexed about the source of activating radiation. This form of extended activation provides greater activation and hence improved accuracy in inspecting the fuel pins without decreasing the throughput rate. The source of activating radiation is preferably oscillated back and forth along the longitudinal axis of rotation thereby allowing even irradiation along the length of each fuel pin using a relatively localized source of radiation. Methods according to this invention can further include detecting radiation emissions such as gamma ray emissions from the activated fuel pins. The fuel pins are preferably positioned within a linear array of detector elements each serving to detect emissions occurring over a limited segment of the fuel pin. The fuel pins are then oscillated repeatedly over a distance approximately equal to the spacing of adjacent detector elements. Detection in this manner provides greater effective detection times for a given throughput rate.