In order to evaluate some of the possible advantages of hybrid reactors over critical reactors, we think it useful to review how these are controlled. It is essentially through the motion of neutron-absorbing control rods that the value of the criticality coefficient keff is modified. In discussions of the evolution of a reactor it is usual to use the reactivity p = (keff — 1)/keff. The time constant associated with the motion of control rods is, typically, measured in seconds. The time delay rnf between two neutron generations is much smaller, typically 10—7 s for fast reactors and 10 4s for thermal reactors [48]. Such numbers would imply a very fast evolution of the reactor, even for very small positive reactivities. As we saw in section 3.2.4, the reactor power increases exponentially with time t:

W (t) = W»“p( (1—pf (3Л01)

For p = 0:01 the power is multiplied by 2 after 70 neutron generations, i. e. less than 10 ps for fast reactors and less than 10 ms for thermal reactors! With such a fast rise in the reactor power, one might think that reactor control by control rods would be hopeless. In fact, the presence of a small fraction of delayed neutrons makes things tractable.