THE DESIGN OF NEUTRON RADIOGRAPHY FACILITY AND ITS APPLICATION IN SPRR-300

The neutron radiography is one of non destructive test technologies. The neutron radiography facility has been designed in SPRR-300 to check the inner structure of some components. The facility is made up of collimator system (including the shutter), the table for sample, the imaging system, the controlling system and so on. In the collimator system, The graphite is used as a moderator, and the bismuth as a gamma filter and the material made of lead, boron, polyethylene and cadmium as the diaphragm. The interior structure of the collimator is like a labyrinth in order to reduce its weight and absorb effectively scattered neutrons. For improving the shielding performance and the controlling precision of the exposure time, a shutter is used. The table for sample is used to modify the tested part and orientation of the sample. There are two methods of neutron radiography in SPRR-300, one using films (including direct imaging and indirect imaging) and the other using CCD camera. The converters are Gd foil and Gd2O2S foil for direct imaging, and In foil, Dy foil for indirect imaging, and NNU-2(6LiFZnS(Ag)) for CCD imaging. To improve Signal-to-Noise in CCD imaging system, the double thermocouple refrigeration is used. To improve resolution power, the lens with small visual angle is used. To improve dynamic range, the image grabbing, processing and analyzing technology of 14bit data is used. In the controlling system, PLC(Programmable Logic Control) is used to improve the facility automation and the radiation shielding of researchers and instruments. The neutron radiography has been utilized in recent years to detect internal structures or defects of some components in SPRR-300, and some good effects were obtained. Introduction: Neutron radiography is an imaging method that the objects are penetrated by neutron and information is obtained about the inner structure (non-destructive) .Its principle is the same as X-ray Radiography. But compared with X-ray radiography, because of the difference of the interaction with a atom neutron radiography has some merits which X-ray radiography has not, such as penetrating some dense materials , for example lead, bismuth, uranium and so on and getting high contrast of light materials enveloped in these dense materials, distinguishing between elements of similar atomic weight and isotopes of the same element and avoiding the interference of gamma radiation of the irradiated materials by using the indirect imaging method, so neutron radiography is a only available non destructive method in some situations(1, 2) SPRR-300 is a Swimming Pool Research Reactor(3.5MW). It is a light-water moderated and cooled, graphite and beryllium reflected thermal reactor. The maximal thermal neutron flux in the core of SPRR-300 is about 6.5×1013 n.cm-2.s-1(3). Neutron radiography has been developed in the reactor since 1980s, but with a stagnation of a long period. Now, neutron radiography has been re-developed during these years, and a new neutron radiography facility has been designed and set up in SPRR-300. There are two methods of neutron radiography: a method of using films and a method using CCD. The former has better power resolution and lower time response, and more difficulty to be digitaled and the later is contrary. Results: The structure of the neutron radiography facility in SPRR-300 is showed in Fig.1(2). The neutron for neutron radiography is drawn from a radial thermal neutron channel, which diameter is 150mm. The 55mm lead plate in front of the channel is used to filter γ-rays from the reactor core. Graphite plug is placed in front of the channel to moderate neutron. The value of cadmium ratio (Rcd) obtained for gold foil on the orifice of the channel may adjusted from 4 to 150 if the length of graphite plug is changed from 0cm to 100cm. An interior collimator like a taper after the graphite plug is used to prevent the divergence of neutron flux. The interior structure of the interior collimator is like a labyrinth. On the one hand, the structure can reduce the weight of the