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Can someone explain why the scram rods are not made to automatically fall into the reactor when power is cut? I would image power would keep them out of the re

Can someone explain why the scram rods are not made to automatically fall into the reactor when power is cut? I would image power would keep them out of the reactor and when power is cut, they drop in. My ill informed understand of the Chernobyl accident is that specific safety measures had been deliberately disabled as part of testing. Does anyone know why the Fukushima reactors weren't scrammed? Also, can you scram the waste pools?

ScotterC 16y ago
They were scrammed. Immediately. What they're dealing with is called residual heat removal. Even though the reactor was turned off within seconds (same as three mile island), the fuel is extremely hot and takes about a week to cool it to a normal level. Control rods are automatically scrammed by gravity when the power is cut. They are held by electro magnets on most designs. However, I admit that I'm not too familiar with GE's BWR.
- wnewman 16y ago
  When you write "the fuel is extremely hot and takes about a week to cool it to a normal level," perhaps what you mean is not that the fuel is literally physically hot, but that the fuel is so radioactive that it continues releasing a significant amount of energy from spontaneous decay even in the absence of significant neutron flux to trigger further U or Pu fission. When a U or Pu nucleus fissions there are several possible outcomes. Some of those outcomes yield radioisotopes whose half-life is on the order of a week. Thus when you suddenly interrupt a running nuclear reactor, you tend to still have a some energy output queued up to happen spontaneously regardless of any further fission/neutron chain reaction. IIRC, your "week" figure is roughly correct, but not because of ordinary heat capacity, but because of this continuing radioactive decay: the energy output typically continues for days to be several percent of steady state power output preceding the shutdown. (Which is enormously more energy than could be stored as heat in the mass of the reactor at any earthly temperature.)
  - ScotterC 16y ago
    Yup. That's what I meant :)
brazzy 16y ago
The Fukushima reactors WERE scrammed. The problem is that this ends only the Uranium fission chain reaction, but still leaves you with a lot of fission products that are themselves unstable with various half-lives. These keep producing a gradually diminishing amount of heat for weeks and months. Which is why you have the waste pools. And no, you CAN'T scram those because it's not a chain reaction going on but simply radioactive decay - there is no known force in the universe which can stop an unstable atom core from decaying.

ScotterC 16y ago

They were scrammed. Immediately. What they're dealing with is called residual heat removal. Even though the reactor was turned off within seconds (same as three mile island), the fuel is extremely hot and takes about a week to cool it to a normal level. Control rods are automatically scrammed by gravity when the power is cut. They are held by electro magnets on most designs. However, I admit that I'm not too familiar with GE's BWR.

wnewman 16y ago
When you write "the fuel is extremely hot and takes about a week to cool it to a normal level," perhaps what you mean is not that the fuel is literally physically hot, but that the fuel is so radioactive that it continues releasing a significant amount of energy from spontaneous decay even in the absence of significant neutron flux to trigger further U or Pu fission. When a U or Pu nucleus fissions there are several possible outcomes. Some of those outcomes yield radioisotopes whose half-life is on the order of a week. Thus when you suddenly interrupt a running nuclear reactor, you tend to still have a some energy output queued up to happen spontaneously regardless of any further fission/neutron chain reaction. IIRC, your "week" figure is roughly correct, but not because of ordinary heat capacity, but because of this continuing radioactive decay: the energy output typically continues for days to be several percent of steady state power output preceding the shutdown. (Which is enormously more energy than could be stored as heat in the mass of the reactor at any earthly temperature.)
- ScotterC 16y ago
  Yup. That's what I meant :)

brazzy 16y ago

The Fukushima reactors WERE scrammed. The problem is that this ends only the Uranium fission chain reaction, but still leaves you with a lot of fission products that are themselves unstable with various half-lives. These keep producing a gradually diminishing amount of heat for weeks and months. Which is why you have the waste pools. And no, you CAN'T scram those because it's not a chain reaction going on but simply radioactive decay - there is no known force in the universe which can stop an unstable atom core from decaying.