Improvements in devices for detecting breaks in fuel element jackets in a nuclear reactor

984, 641. Monitoring nuclear reactors. COMMISSARIAT A L'ENERGIE ATOMIQUE. March 19, 1961 [May 20, 1960], No. 18287/61. Heading G6C. A device for determining the proportion of fission products in a gaseous stream comprises a rotatable metallic drum on which fission products may be deposited electrostatically when a postive ion repulsion electrode is energized; the drum comprising a number of areas on which ions may be deposited, one area on which ions are not deposited, and a recessed area for trapping a given volume of gas to be tested; a radio-activity detector disposed opposite the drum at some distance from the ion repulsion electrode; and calculator means operatively connected with the detector for giving, in response to the measurements made by the detector for the respective areas of the drum, the concentration of shortlived fission products in the gas stream, the concentration of long-lived fission products, and the total activity of the gas stream. Referring to Fig. 5 gas to be tested passes through a filter 6 into a delay chamber 17 and then to the space between the ion repulsion electrode 11 and the drum 10. As shown in Fig. 2 the drum 10 has a number of peripheral areas n on which ions may be deposited, a special area y on which ions are not deposited, and a recessed area x for trapping gas. A scintillation detector 12 and its associated photomultiplier 13 are arranged near the periphery of the drum and the drum is surrounded by a gastight casing 10a; carbon dioxide is passed through a duct 8 to the space between the detector 12 and the drum during measurements, except those made on area x, to reduce the effect on the detector of any active gas which may be present. There may be 20 areas on the drum and the detector may be spaced 3 area away from the electrode 11. The drum is rotated of an electric motor 23, Fig. 5, through magnetic coupling 22 and gearing having a ratio 3:20 so that one complete revolution of motor 23 causes a given area of the drum to move from electrode 11 to detector 12 or vice-versa according to the direction of rotation. The shaft of motor 23 carries a cam 24 which opens contact 24a once per revolution, and the shaft of drum 10 carries three cams 18, 19, 20 which open contacts 18y, 19 y-1 and 20 x respectively when the drum is stopped with area y opposite the detector, with area y-1 (the area preceding area y in order of scanning) opposite the detector, and with area x opposite the detector. These contacts select the appro