Nuclear Fission — Chain Reaction

Aim: To investigate whether there is a rod position that keeps the reaction steady.

Part A — Does the rod position affect the reaction?

1. With the rods fully inserted (100%), press Start Reaction and watch the meter for about ten seconds. What does it read?
2. Slide the rods out to 0% and watch for another ten seconds. What happens?
3. Push the rods back to 100%. Does the reading come back down? Write one sentence saying whether rod position affects the neutron rate.

Part B — Which settings make it rise, and which make it fall?

1. Press Reset Reaction, set the rods to 0%, press Start Reaction and let the meter climb to its maximum.
2. Set the rods to 20%. Wait about ten seconds, then record the insertion, the rate you read, and your judgement — rising, steady or falling — in the table.
3. Repeat for 40%, 60%, 80% and 100%, using a new row each time.
4. Look down your table. Where is the boundary between settings that make the rate rise and settings that make it fall?

Part C — Pinning down the steady setting

1. Using your boundary from Part B, test insertions in smaller steps (5%, then 1%) until you find a setting where the meter holds steady — within its normal flicker — for at least 20 seconds. Record it in your table.
2. Set the rods a few percent below your steady setting and, starting from a low rate, write down the meter reading every 5 seconds for 30 seconds. Does the rate go up by the same amount each time, or by the same factor?
3. Now set them a few percent above the steady setting and take readings every 5 seconds starting from a high rate. What pattern do your numbers follow on the way down?

Part D — Runaway growth and emergency shutdown

1. With the rods at 0%, time how long the meter takes to climb from background to its maximum reading. This meter stops at its top value — a real reactor's fission rate would not. Using the pattern you found in Part C, describe what would happen to the energy released each second, and explain why that kind of growth is dangerous.
2. Each fission releases energy and sets free two or three more neutrons. Why does "same factor" growth run away so much faster than "same amount" growth?
3. Insert the rods to 100% and watch where the reading settles. Why does it not fall all the way to zero?
4. In an emergency, a reactor drops its rods fully in. Watch the diagram at 100% insertion and look at what happens to neutrons that hit a rod. Use that to explain why full insertion always shuts the chain reaction down, whatever the rate was before.

Part E — Zooming in: what one fission looks like

1. Switch to the Nuclear Scale tab and click a nucleus near the neutron source to fire at it. Watch the very first hit closely, then write down everything that comes out of the nucleus when it splits.
2. Press Reset Nuclei and fire again, watching the Energy Released meter and the fissions counter. By how much does the energy rise for each extra fission? Where does this energy go in a real reactor?
3. Reset and fire one neutron, then leave the simulation alone until the neutrons in flight reach zero. Note the number of fissions and the number of nuclei left. Repeat twice more. Do you get the same numbers each time? Suggest why some chains die out early even though plenty of nuclei are left.
4. Each fission sets free 2 or 3 neutrons, so one fission can lead to two or three, then four to nine, and so on. Use this to explain the "same factor" growth you measured in Part C — and what the control rods in the Reactor tab must do to these neutrons to stop the numbers multiplying.