On 26 June 1954 the reactor in Obninsk became the first in the world to feed electricity into an ordinary grid. Hold the reaction yourself, stop the pumps, hit the emergency stop and see what happens to the lights in the windows. Then repeat the first startup: load uranium one channel at a time until the reaction runs by itself.
A uranium nucleus catches a neutron and splits. Two or three new neutrons fly out and a lot of heat is released. If at least one of the new neutrons splits the next nucleus, the reaction keeps itself going. That’s a chain reaction.
Uranium is bad at catching fast neutrons, so they have to be slowed down. That’s why the fuel was surrounded by graphite: a neutron bumps into its atoms and loses speed. The uranium sat in 128 vertical channels in the graphite stack. Rods that swallow neutrons are lowered into the stack to damp the reaction and raised to speed it up.
Switch to “Manual” and try to hold the power steady. The reactor responds with a lag: some neutrons come out not instantly but seconds later. That’s the only reason a human can control it at all.
Water at 100 atmospheres flows through the channels. At that pressure it doesn’t boil even when very hot. In the steam generator this water gives its heat to a second loop of water, which turns into steam and spins the turbine. The turbine drives the generator, and the generator makes electricity.
The rest of the heat goes into the cooling water. And after shutdown the reactor keeps heating itself for a long time as uranium fragments decay — right after shutdown that’s a few percent of the former power. That’s why even a shut-down reactor needs its pumps.
Switch to “First startup”. There’s no uranium in the reactor yet; the counter only sees neutrons from the source. Load channels. Each channel picks up the source neutrons and multiplies them, so the count rises. The closer the reactor is to critical, the more each new channel raises it.
Physicists didn’t wait for the reaction to start: they plotted 1/M and extended the line down to zero. Where the line hits zero predicts how many channels are needed. Try it: the forecast on the plot comes true when the count starts rising without your help.
The higher the pressure, the hotter water must get before it boils. At 100 atmospheres that’s about 311 °C. A pressure cooker works the same way, only at about two atmospheres.
Natural uranium has only 0.7% of fissile uranium-235. For Obninsk it was enriched to 5%: otherwise a reactor this small couldn’t get enough fissions to sustain itself.
The Obninsk reactor is a distant ancestor of the RBMK: also graphite, water and channels. But it had 30 times less thermal power than one Chernobyl unit, and it ran for 48 years without a serious accident.
1. The Obninsk reactor produced 30 MW of heat, and the plant sent 5 MW to the grid. Find the efficiency. How much heat per hour went into the cooling water?
2. With 30 channels loaded the counter showed 200 counts per second, with 45 channels — 400. Assuming 1/M falls along a straight line, estimate how many channels make the reactor critical.