Radiation Applications, Sources, and Detection

Criticality Studies of a Neutron Multiplier, R. C. Lloyd, S. R. Bierman, E. D. Clayton. B. M. Durst (BNWL) 

There has been considerable need for neutron irradiation units with high neutron fluxrfor use in research, industry, and education. Sources of various types have been used lor this purpose for maiiy years, but have lacked the intensity and volume, with level flux, for larger sample irradiation. While the research reactor can produce the desirable neutron flux, construction and operation is rather costly. A neutron multiplier has been described. .using a subcritical lattice of enriched uranium rods,with a central cylindrical flux trap. A Cf. source is used to provide the initial neutrons to drive the unit. This lattice unit was loaded at the Critical Mass Laboratory to study aspects of criticality including keff and effects of temperature, voids, and added material. 

The experimental system consisted of a lattice assembly, reflected and moderated with water, in a cylindrical tank having a diameter of 102 cm. Measurements were made at a lattice spacing of 1.52 cm. A schematic diagram of the assembly is shown in Fig. 1. The canned fuel rods were a U -f A1 alloy having a enrichment of 93 wt% they Were 1.02 cm in diameter and 60.33 cm iii length. 

The fuel rods were safely loaded, using the critical approach method this also provided the extrapolation for the critical number of rods. At selected fuel loadings, pulsed-neutron source measurements were made to determine reactivity and prompt-neutron decay rates. The effect of voids and various materials placed in the central region and outside the loaded lattice waS measured In terms of the number of rods required for criticality. 

52-cm lattice The critical number of fuel rods at 2rc, with a S.l-cm central void and a 37-rod equivalent flux trap, was 271.4. The same system with the 5.1-cm-diam central tube half filled with water Was 276.3. When either system was heated the maximum decrease in critical number of rods, heating from 21 C, was about 2.6, while the system haying the S.l-em-diam void leveled at a somewhat lower temperature. 

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