ABSTRACT
In section 3.5 we discussed some aspects of the control and safety of critical reactors. We discuss in what respect these are modified by subcriticality.
As mentioned in section 3.5.3 a reactivity insertion of more than 1 $ in a critical reactor leads to a fast exponential excursion, equation (3.101):
WðtÞ ¼W0 exp ðpromptÞt
: ð7:1Þ
For lead cooled fast reactors n ¼ 3 108 s [7]. It follows that the power would be multiplied by 100 after 14 108=ðpromptÞ seconds. Even for ðpromptÞ ¼ 0:001, i.e. a total reactivity insertion of 0:004 (for a 233U fuelled reactor) the power is multiplied by 100 after 0.14ms! Consider now the case of a hybrid system with k ¼ 0:98, and a reactivity increase of 0:4% equal to that just considered for the critical system. The energy gain is proportional to 1=ð1 kÞ and increases by 25% only!
