Calculation of the effective delayed-neutron fraction and prompt-neutron lifetime in IBR-2M

The main parameters of IBR-2M are presented: the effective delayed-neutron fraction β eff and the promptneutron lifetime τ, calculated using the DORT two-dimensional multigroup neutron-transport compute code and the SCALE4 code with a system of multigroup nuclear constants. For a regular IBR-2M regime β eff = 0.00216 ± 0.00007, τ = (6.5 ± 0.5)·10 –8 sec, the delay-neutron value γ = 0.980, the prompt-neutron decay constant in the critical state α = 3.5·104 sec –1 . The calculations showed that the effective delayed-neutron fraction for IBR-2M is identical, within the error limits, to the measured value for IBR-2, the prompt-neutron lifetime is approximately 5% longer (β eff = 0.00216, τ = (6.2 ± 0.2)·10 –8 sec). It is shown that β eff and τ increase somewhat as the IBR-2 core size increases in the radial direction.