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
It’s an emergency shutdown of a reactor.
In normal circumstance, control rods are inserted into the reactor core somewhat slowly. This allows the power level to come down, heat to dissipate, and so on.
In a scram, the rods are basically slammed into the core as quickly as they can be to immediately stop the reaction. The important part is heat – the rods (and nuclear fuel) may be damaged (melted) in a scram, and you may not be able to restart the reactor later. That doesn’t matter though, because you’ve avoided the crisis that caused you to scram in the first place.
It’s kinda like the difference between shutting down your laptop via the start menu vs popping the battery out. The second method will shut it down very, very quickly, but not necessarily in a way that’s good for the laptop.
Edit: this is ELI5. I’ll concede that in a normally functioning, controlled and stable reactor, a scram does not guarantee damage. However, in a normally functioning, controlled and stable reactor, a scram is not the usual shutdown method. Scramming a reactor is likely to cause damage to the reactor, precisely because it is not likely to be normally functioning, controlled and stable.
A SCRAM is an emergency shutdown, as opposed to a planned shutdown.
In a SCRAM, the reactor trip breakers are opened which causes the control rods to drop into the fuel assemblies for a PWR, and rise into the fuel assemblies for a BWR. The result is the same: the reactor is for all intents and purposes not fissioning anymore.
A reactor scram (sometimes placed in all caps but not an acronym) is a term for an emergency shutdown of a nuclear reactor to prevent a catastrophic emergency, for more detailed information checkout r/NuclearPower or perhaps r/NavyNukes.
Most of these answers are wrong. The 5 year old answer is that a SCRAM is a shutdown of a nuclear reactor.
TL;DR: SCRAMs are a nornal part of reactor operation and don’t threaten the safety of the reactor or the fuel container inside it.
The more in depth answer is that SCRAM is an acronym that a stands for Safety Control Reactor Axe Man. It’s from the very very early days of reactor experimentation when they pulled control rods out using literal ropes and if there was an emergency the “axe man” would quite literally chop the rope allowing the control rods to fall into the core stopping the reaction. SCRAM, as a term is still used, today but signifies a shutdown. Modern reactors are much safer and despite comments by several others, a SCRAM isn’t something that can potentially damage the fuel or reactor components. Typical modern reactors control their shutdowns by lowering power slowly and then they initiate a manual SCRAM from a lower power level. That has nothing to do with the fuel but instead is for the non-nuclear component life like the main electrical turbine and sensitive control valves. The reactor is made to withstand any stress from a SCRAM unlimited times. When anything abnormal is experienced they’ll SCRAM from full power, either manually or automatically based on many monitor ed patameters. From the nuclear fuel side it’s exactly the same as a normal shutdown. The steam plant experiences higher stress but everything that protects nuclear fuel is unaffected. In the US, every nuclear plant is required to report every SCRAM, whether planned or I planned and it’s posted publicly along with any other events of note. You can find them here:
https://www.nrc.gov/reading-rm/doc-collections/event-status/event/2024/index.html
Source: was a licensed nuclear operator for many many years.
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!”
The word itself has ambiguous origins, and has no real meaning outside of a nuclear power plant.
What the button does, in a nuclear power plant. Is IMMEDIATELY lower ALL control rods, into the reactor, which absorb fissible materials. Halting nuclear fission in an emergency.
Its basically a way to stop the reactor. A reactor works based on neutron economy, a atom of fuel is split by a neutron, which releases more neutrons, some neutrons are lost, others go on to split other atoms of fuel, thus sustaining the reaction. The amount of neutrons in the system has to be stable for stable operation, if the quantity goes up, so does the power, if the quantity goes down, so does the power. Various things can be changed in the system, materials can be added or removed to the reactor which influence this neutron economy. SCRAM is a emergency procedure to stop the reactor this way. In the very first reactor in Chicago this was implemented by a man with an axe standing by the ropes that were pulling cadmium control rods out of the reactor, if there was need for emergency stop, he would have cut the ropes, the rods would have fallen in and absorbed neutrons fast, thus stopping the reactor.
SCRAM stands for **S**afety **C**ontrol **R**od **A**xe **M**an. It’s an emergency shut down of the reactor.
In a nuclear reactor, you need free, slow-moving neutrons bouncing around inside the reactor core in order to split uranium atoms in the fuel. That’s where all the heat and energy comes from. Control rods absorb those neutrons, preventing them from doing their thing.
You can carefully control the amount of energy you’re producing by sliding the control rods in and out of the reactor. All the way out, and the reactor is at max capacity. All the way in, and the reactor stops.
When a SCRAM happens, ALL control rods (there are many) are immediately and forcefully inserted into the reactor to kill it as fast as possible. There are even some extra rods that never get used except in a SCRAM situation just to make sure the reactor is dead.
The very first reactor pile at the University of Chicago had a single control rod attached to a rope hanging from a pulley. A man with an axe was in position to cut the rope and drop the rod just in case the shit hit the fan. Fortunately, it never did, but this is where the “Axe Man” part of the acronym comes from.
A scram is an emergency shutdown of a nuclear reactor. This can be caused automatically by reactor protection circuitry or can be manually initiated by an operator. When proper maintenance and procedures are followed, this will not cause damage to the reactor but instead will prevent damage from occurring.
A reactor works by using fission to produce heat. Fission is when a neutron collides with an atom of fuel, resulting in the atom splitting apart. This process generates heat, plus more neutrons that can go on to cause even more fission. Control rods are made of a material that is really good at absorbing neutrons. When control rods are inserted, most of the neutrons get absorbed by the control rods instead of causing fission. When control rods are withdrawn, some neutrons still get absorbed by the rods (and other things) but enough go on to cause fission to create enough heat to be useful.
So a scram is when control rods are inserted far enough into the core to make sure the reactor is shut down. Since it’s an emergency, this happens very quickly depending on how the reactor is designed. Think of a spring slamming the rods as far into the core as possible.
Some others have incorrectly stated that a scram would cause damage. This is not true. A scram in itself will not cause damage, but is designed to prevent damage from occurring.
Source: I’ve scrammed a reactor or two in my day.
I remember my first deployment on a nuclear aircraft carrier, the U.S.S. Carl Vinson, where they were conducting reactor safety drills, shutting down one reactor as the result of a simulated malfunction, and hearing the voice come over the 1MC (ship-wide comms) saying something to the effect of “Number one reactor failure. SCRAM number one reactor, SCRAM number one reactor!”.
Having spent several deployments aboard a conventional aircraft carrier, that’s when it hit me that I was aboard a different kind of beast, but knowing it was just a drill, it did sound pretty cool at the time.
As others have said, it originally stood for Safety Cut Rope Axe Man, which basically means an emergency drop of the control rods to block neutron transfer between the uranium fuel rods, which slows down, if not stops, the nuclear reaction in a reactor core.
Latest Answers