In a power station's core, uranium fissions at a steady rate, month after month. In the same fuel with nothing absorbing the spare neutrons, the number of fissions would grow explosively. The difference between those two outcomes comes down to one count.
You have seen that each fission releases two or three neutrons, and that a chain reaction keeps itself going when nearby nuclei absorb those neutrons and split in turn. The next question is how fast the chain grows. The answer turns on one number: how many of each fission's neutrons go on to cause another fission.
Each fission releases two or three neutrons.
Not every released neutron has to trigger a new fission; something can absorb a neutron first.
The count that decides everything: on average, how many neutrons from each fission go on to cause the next fission?
If, on average, exactly one neutron from each fission causes another, each round of fissions is the same size as the last, and energy is released at a steady rate. We call this a controlled chain reaction.
If more than one neutron from each fission causes another, each round has more fissions than the last, and the number of fissions grows explosively. We call this an uncontrolled chain reaction.
In a reactor, control rods absorb the surplus neutrons, so on average exactly one neutron per fission goes on. The reactor runs controlled.
The same fuel with nothing absorbing neutrons runs away: two or three neutrons per fission go on, and the chain grows uncontrolled.
Some people picture a nuclear power station as a slow bomb. It is not. The station's control rods hold the chain at exactly one continuing neutron per fission, so the energy release stays steady instead of growing.
In a lump of fissioning fuel, each fission releases three neutrons. On average, two are absorbed by control rods and exactly one goes on to cause another fission. Is this chain reaction controlled or uncontrolled?
The deciding count is the average number of each fission's neutrons that cause another fission.
Here that count is exactly one.
Exactly one continuing neutron per fission is controlled.
This chain reaction is controlled: each round of fissions is the same size as the last, and energy is released at a steady rate.
The control rods are now lifted out of the same fuel, and nothing absorbs the spare neutrons. On average, all three of each fission's neutrons go on to cause new fissions. Is the chain reaction controlled or uncontrolled now?
The deciding count is the same question asked again: how many of each fission's neutrons cause another fission?
Here that count is three, which is more than one.
More than one continuing neutron per fission is uncontrolled.
This chain reaction is uncontrolled: each round has more fissions than the last, and the number of fissions grows explosively.
In a lump of fissioning fuel, on average exactly one neutron from each fission goes on to cause another fission. Is this chain reaction controlled or uncontrolled?
Orla says: 'A nuclear power station is just a slow bomb; the two run the same kind of chain reaction.' Is Orla correct?
Blanka says: 'In a controlled chain reaction, on average exactly one neutron from each fission goes on to cause the next fission.' Is Blanka correct?
In a lump of fissioning fuel, each fission's neutrons cause, on average, two new fissions. What happens to the number of fissions from one round to the next?
A reactor runs its chain reaction controlled. What do its control rods do?
What separates a controlled chain reaction from an uncontrolled one?