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To stop a nuclear reactor, an emergency operation, usually to prevent explosion and/or meltdown. May be effected by rapidly inserting control rods or dumping the moderator.

Also a has figurative sense of stopping something suddenly.

"Start Cutting Ropes Already, Man!" - there was a man with an axe, ready to cut ropes which would drop the cadmium rods into the reactor, and stop the reaction.

Slang: Get out of here. Go away, now! Beat it!
Origin:short for scramble.

One of the temp jobs I held was as a gopher at the Indian Point Nuclear Power Station on the Hudson River in New York State. I "aced" the intake interview with the plant shrink, and with the ink on the "SANE" stamp still moist on my forehead, eagerly made the dawn trek down Rt. 9, the artery of the beautiful Hudson Valley, to my first day at work. As it turned out, the actual work I did was completely boring, and required me to show up at 6am every morning, for a ten-plus hour shift, but I did learn some neat stuff there about nuclear plant lingo.

I second what the other noders here have written, the last resort procedure for shutting down the reactor does involve sending a person in there to manually cut whatever the control rods are suspended from (cables probably).

But I can't second Infinity's contribution, the way I heard it, the acronym S.C.R.A.M. stands for "Situation Critical, Reactor Axe Man". It is attributed to Enrico Fermi or one of his colleagues; their early experimental reactors used simple rope to suspect their control rods, which in a small, experimental reactor are much smaller and lighter than those in an industrial model. Fermi's crew would have used electric motors to raise and lower the rods, but since at the time people could only guess at what long-term exposure to radiation did to electric devices, they had a backup plan. "SCRAM" was the signal for the designated technician to drop whatever he was doing, grab the axe which had been issued to him, and cut the ropes.

SCRAM evolved from there to refer to any of the various emergency shutdown procedures, the vast majority of which happily don't require a man to go into an active core.

This came to me as a sort of oral history, a story handed down from engineer to engineer. I wonder if I had been able to glance at the plant's operating manual if it would have an authoritative answer for the actual meaning of SCRAM. There's more discussion under scram switch.

As far as anyone I talked to knew, no reactor in the West has ever had to attempt manual shutdown.

Another nugget from my time at the station, this time from a primary source i.e. someone who claims to have actually witnessed the phenomenon. When a reactor is shut down for maintenance, there are all sorts of brain-dead manual tasks that need to be done in the core, like unsticking a stuck bolt and so on. Even when shut down, there is still enough radiation in the core that spending a few minutes there gets you all the radiation you're entitled to for quite a while. So they hire unskilled labor to work on the bolt; in some cases, as little as 10 minutes work gets a week's pay, at well above minimum wage.

Suppress that reflexive cringe for a moment. These guys are not actually sacrificing their lives for a week's wages; the exposure rates are carefully monitored and are set quite low to avoid any short- or long-term health problems and the inevitable investigations and lawsuits. Also these aren't careers, more like seasonal labor, as reactor maintenance work can only happen during the few, hectic weeks when the reactor is taken offline.

Under union and other rules, these workers are not permitted to do other work away from radiation for the rest of the week. If they did, they'd earn a full two week's wages in a mere 40 hours, 10 minutes, which risks making them so prosperous, they'd no longer see a need to work at the plant OR be in a union!

Instead they recline at their barracks, go mildly stir crazy, and sometimes begin returning their wages at the "company store". Some take advantage of their forced idleness to smoke much more than they'd otherwise have time for, making it fun to guess how many cancer cases came from the plant and how many from the extra consumption of death sticks.

What does this have to do with SCRAM? The answer is, it's all about nuclear lingo, silly. A non-engineer who is sent into the core is known as a "Sponge".

Now you can cringe.

A 'manual scram' is a scram of a nuclear reactor initiated by the operator. The operator hits a switch, turns a crank, or otherwise manually operates a system which re-inserts the control rods back into the core of the reactor, thus stopping fission and dropping power levels below critical. Cutting the rope holding a safety rod out of the core with an axe is a good example of a manual scram.

An automatic scram, on the other hand, happens when an automatic system triggers a scram because an instrument reading is out of the boundaries considered safe. For example, when the Reed Reactor Facility power levels go beyond 110%, there is a legal obligation for the circuts that read reactor power to initate a scram.

A manual scram can happen for any number of reasons, from an operator suddenly noticing aberrant readings on the control panel to wanting to impress a tour group with the decrease in the blue glow of Cerenkov radiation. It is also a lazy way to end an irradiation.

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