An expression is developed relating fission product residence time to fission product mass number for a hypothetical gas core vortex reactor. Utilizing the expression and considering that the fission products, Kr-87, Br-87, Kr-93, I-137, arid I-138, are also delayed neutron precursors, a modified effective delayed neutron fraction is calculated for U-233, U-235, and Pu-239 fueled cores. Temperature coefficients for the referene cores are developed as functions of reflector temperature, assuming a Maxwellian neutron spectrum. Utilizing these power coefficients and the modified effective delayed neutron fractions, the consequences of reactivity perturbations in the reference cores are analyzed. It is shown that optimally moderated gaseous fission cores should possess a high degree of inherent stability.
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MacLain et al. (1962) studied this question.
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