In the movie *Shin Godzilla*, the humans came up with a blood coagulant and a plan to inject it into Godzilla in order to freeze him. It’s mentioned in the film that doing so would force him to perform a nuclear SCRAM since he’s fueled by an internal nuclear reactor. The coagulant works, shutting down his circulation (which is his cooling system), and causes him to freeze.
But what IS a nuclear SCRAM? i’ve looked it up before, but I just can’t seem to make heads or tails of it.
In: Physics
SCRAM in its original (and I do mean original) context meant **S**afety **C**ontrol **R**od **A**xe **M**an. In the opening years of nuclear technology, the first true nuclear reactor, the Chicago Pile, needed a way to shut it down should the attempt at achieving criticality go wrong. On top of the reactor there was an extracted control rod held up with a rope, and a man standing there ready to cut the rope with an axe. If cut, the rod would drop into the reactor and shut it down. The criticality attempt went well, the chain reaction started, and after letting it run at a very low power level (IIRC it was like 2 watts of power) for a bit, the man on top was told to cut the rope, and the reactor was shut down.
In modern current day context, SCRAM means “**Shut the reactor down as fast as possible**”. In every modern reactor (yes even Chernobyl #4) there is a command or action that would insert every control rod into the core to terminate the reaction as fast as possible. For some reactors, they have additional safety systems and control rods that are “fail-safe”; if the conditions in the reactor or supporting systems fall out of spec, no human interaction is needed to get them to insert into the core to stop the chain reaction. For example, in some western reactor designs the control rods are held to their actuators above the core via electromagnets. If there was an abrupt power loss (intentional or otherwise) in the reactor building, those electromagnets would lose power, and the control rods would drop in under the force of gravity (often assisted by pressurized gas and/or springs). In this manner, a modern reactor can be shut down within seconds of issuing the command or something going wrong.
The difficulty lies in the aftermath of a SCRAM. Nuclear reactors dont like having their power output changed much, and going from full or near-full power to hard shutdown in seconds almost always becomes “Reactor is in hot-shutdown for the next 3 days”. The reason for this is that even though the primary reaction has stopped, there’s still a large number of decay events going on. One of these involves Iodine-135 and its immediate decay product, Xenon-135. Iodine-135 is a small but critical part of the reactor environment, because while it itself isnt much of a neutron absorber (and thus has minimal effect on the reaction chain), once it decays to Xenon-135 is when the trouble begins, as it is the strongest known neutron absorber. If you’re trying to restart a reactor after a SCRAM, you have to do it immediately before the reactor falls too far into what is known as the “*Iodine Pit*”, or wait for the Xenon-135 in the core to decay to Cesium-135 before attempting restart.
In old and current context: “Shut the reactor down now!”
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