I usually do not ask for this kind of stuff, but this time I really have no way to verify if this is correct(and it has to be). So here it goes: On a subcritical nuclear reactor with 100 fuel elements we read the signal of 10e-9 A. When we add 10 more fuel elements, signal jumps to 10e-8 A. How many rods do we have to add, to make the reactor critical? So, I know the relationships between reactivity/neutron flux/criticality/signal, but I have no idea how are they dependend on the number of fuel elements? Usually, I would have to solve the diffusion equation, but this exercise gives no data about reactor geometry, so.. do you guys have any idea what this could be about? ------ Could the admin move this to Homework section please? I have not seen it before..