A criticality equation is derived assuming slowing-down only by hydrogen. The use of criticality experiments as a basis for calculation is described. Approximate methods are given for handling inharmonicities and multi-region reactors. Some criticality data on solutions of U03 dissolved in H3PO4 are listed in Appendix 1, and the Goertzel-Selengut equations are derived in Appendix II.

A method is presented for determining heat transfer rates and thermal stresses from the gamma-ray energy absorption of nuclear reactor shells for plane, cylindrical, and spherical geometries. Criteria for minimizing thermal stresses are developed, along with the corresponding external cooling rates necessary to minimize the thermal stress. Design charts are presented for rapid determination of approximate thermal stresses and heat transfer rates, along with a numerical example illustrating the use of the charts..

A spherical, unreflected U-235 critical assembly (52.6 kg) has been in operation since August, 1951. A remotely controlled mechanical system is used to assemble subcritical components of the sphere, and reactivity is adjusted with U-235 control rods positioned in the sphere. The maximum power level during sustained operation is about 1 kilowatt. In addition to investigations of the neutron spectrum of the assembly, observation of the changes of reactivity produced by inserting foreign materials into the assembly, and the dletermination of parameters such as the temperature coefficient of reactivity, studies have been made of the behavior of the assembly at reactivities above prompt critical.

The yield and energy distribution of the lower-energy neutrons resulting from interaction of 14-Mev neutrons with Li6 and Li7 have been measured in the energy range between 4 and 12 Mev. A coincidence spectrometer was used to detect the neutrons from a scatterer in the form of a spherical shell placed around the neutron source. By use of the spectrometer as a threshold detector, transmission measurements were also made, from which could be determined the elastic cross section for scattered neutrons above 12 Mev. The result is in agreement with the measurement of Graves of the amount of small-angle elastic scattering. Using the known total cross section of Li6, the known reaction cross sections, and the present measurements, a complete “audit” of the interacted neutrons is made. Qualitatively, one-half of the total cross section is due to small-angle elastic scattering, and all inelastic processes result in lower energy neutrons spread rather uniformly in the region from zero to 12 Mev. Measurement using a thin iron sphere gave a neutron spectrum in good agreement with previous results of Graves and Rosen.