GB898219A - Device for selectively detecting the occurence of fission products in gases - Google Patents

Abstract

898,219. Reactors. COMMISSARIAT A L'ENERGIE ATOMIOUE. Dec. 1, 1958 [Dec. 5, 1957], No. 38708/58. Class 39(4). In a nuclear reactor cooling-gas monitor, the short lived fission products in the gas, eg. CO 2 , are detected to indicate the fracture of fuel element sheathing. A signal corresponding to both short and long-lived fission products and a second signal representative of only the long-lived products are developed separately and applied to a comparison circuit, the output of which indicates the presence of the short-lived fission products. To reduce the effect on the fission product reading, of the short-lived radio-active isotopes produced within cooling gas itself by neutron bombardment, readings are also obtained corresponding only to the short-lived isotopes (i.e. nitrogen 16), which are then subtracted from the first reading. As shown, cooling gas is admitted to detection chamber 2 and is observed by scintillating elements 7, 7a through the windows 6, 6a. The pulse outputs from 7, 7a are applied, through pulse selectors 42, 42a in the right and left-hand channels to the comparison circuits 13, 16 and 13a, 16a. The right-hand and left-hand channels deal, respectively, with the radiations from fission products and ccoling gas isotopes. Window 6 is thinner than 6a and is transparent to fission product and the gas radiations. Selector 42 accepts only fission signals, i.e. those corresponding to energies less than 4 MeV and rejects insotope signals, i.e. corresponding to #-emission from 17, 16 and greater than 4 MeV. Window 6a is transparent only to isotope radiation signals and selector 42a accepts only these, i.e. corresponding to # energies greater than 4 MeV. Initially, gas flows continuously through the chamber 2 and as switches 14, 14a are closed the voltages 12, 15 and 12a, 15a correspond to the fission products and isotope products respectively (both short and long lived). Subsequently, valves 3, 4 are closed and switches 14, 14a are opened and only the long-lived oroducts of fission and from the gasremain in the chamber 2, to produce corresponding signals at 12 and 12a. Consequently, the potentials e and e a represent the activity of the short-lived fission products and short-lived radio-active isotopes respectively. To allow for lack of selectivity in the right-hand channel, the voltage e a is subtracted from e, the difference being indicated at 21.