This invention relates to radiography by neutrons and gamma rays.
The Transient Reactor Test (TREAT) Facility is an apparatus for performing safety tests on various phases of nuclear reactor operation. It includes a nuclear reactor to apply transient bursts of neutrons to irradiate a sample located within the reactor core. The sample typically consists of one or more fuel pins enclosed within protective test sections. It is of considerable interest to know what happens to such fuel pins and their enclosing structures upon the incidence of a sudden burst of neutrons. A hodoscope is used to determine this. In the hodoscope, a neutron collimator is placed near the test specimen to pass neutrons preferentially in straight lines toward an array of neutron detectors. Each neutron detector is aligned with a channel in the collimator and responds primarily to neutrons that have proceeded from a sample such as the fuel pins through the collimator to the neutron detector. A gamma detector is normally placed in front of or behind each neutron detector to provide additional information. For measurements involving fuel pins, it is useful to detect the fast neutrons resulting from fission of fissionable material in the fuel pins as a result of the incidence of a burst of neutrons generated by TREAT. However, operation of the hodoscope is not limited to reactor bursts. It is possible to use the hodoscope in the manner described to obtain distinctive static neutron or gamma radiographs of any test sample having properties that will affect the incident flux of neutrons by reflecting, deflecting, or absorbing portions of the neutron flux, or by generating gamma rays in response to the flux of neutrons. In the following, a radiograph is defined as an image obtained by using information from the effects of a target upon incident radiation, including that of gamma rays and neutrons. For studies of fuel pins that contain fissionable material, it is simpler to detect the fast neutrons by using neutron detectors that are responsive selectively to fast fission neutrons and respond little or not at all to neutrons characteristic of the incident flux that may enter the collimator.
The studies that have been carried out at the TREAT Facility have concentrated on observations of damage to fuel pins subjected to transient bursts. If a fuel pin is damaged by such a burst, it typically assumes a deformed shape. It is of interest to record when, where and how it is deformed. One way of doing this is to connect each of the neutron or gamma detectors to a digitizing system which is connected to an indicator such as a neon bulb. The neon bulbs are flashed periodically at frequencies of several thousand cycles per second to indicate the detection of neutrons. The patterns of lights are recorded on film in a high-speed framing camera that is synchronized with pulses of the bulbs. Alternatively, each detector can be connected to produce a record on magnetic disks or tape. The typical neutron detector has a cross-sectional area that permits the stacking of detectors in an array such that detectors are spaced approximately 38 mm apart in one direction and 22.5 mm apart in the orthogonal direction. This spatial resolution of detectors is then the spatial resolution that is obtained of the neutron time-resolved image, and of the gamma rays, when a gamma detector is disposed ahead of or behind each neutron detector. The combination of this relatively crude spatial resolution with the relatively high time resolution afforded by pictures taken as frequently as one millisecond apart provides an excellent picture of failure modes in nuclear fuel rods. There are, however, situations in which it would be useful to forgo the time resolution and to gain better spatial resolution. For example, it might be desirable to obtain a detailed measure in situ of the quality of the cladding in a fuel rod or the placement of individual fuel pins within a fuel rod to be sure that the later observations of failure of such a fuel rod are not functions of an anomalous geometry. It might be desirable to measure a failed rod in situ to eliminate any further damage upon removal of the rod for external radiography. In addition, it may be desirable to obtain a neutron or gamma radiograph of relatively high resolution of structures other than fuel rods. An example is accumulation of steel resulting from destructive tests performed on clad fuel rods.
It is an object of the present invention to adapt the hodoscope to provide high-resolution neutron and gamma radiographs.
It is a further object of this invention to perform in-situ neutron and gamma radiography with high spatial resolution.
Other objects will become apparent in the course of a detailed description of the invention.