The prompt neutron lifetime is important in determining the time dependence of excursions in which the reactivity excess is so great that the reactor is critical or near critical on the prompt neutrons alone (Akctt /2) without the delayed neutron contribu­tion. Otherwise the transient is governed by the delayed neutrons and the value of / does not affect strongly the results. In multi-group space-dependent calculations the lifetime / does not appear explicitly, but the coefficients of (12.4) must be given for each energy group and region. This means that beside the parameters for static calculations, the velocity v has to be calculated (usually with spectrum codes) for each group and region. As it happens with /, those velocities do not need to be very accurate in transients governed by delayed neutrons. In the one group approximation the mean neutron lifetime / has been introduced in eqn. (12.4) but the use of this expression with one group averaged cross-sections is not very accurate so that eqn. (12.11) is often used.

The energy and space distribution of the adjoint flux can be calculated with many codes for multi-group diffusion calculations, and routines for the calculation of Л and are sometimes included in these codes.

Equation (12.11) suggests also the calculation of Л as the reactivity worth of an absorber with a 1/r cross-section.<4’5> This method allows the calculation of Л with static diffusion or transport theory codes.

A simple and accurate procedure’6’ consists in calculating the stable reactor period T corresponding to a given Ak with a space-dependent code in the absence of delayed neutrons, and to obtain / as

/ = T Ak.

Very important in the spatial averaging is the effect of the reflector, which is returning neutrons to the core with a certain delay and causes therefore a considerable increase in the prompt neutron lifetime of the reactor.