The perenial "Nuclear Power Plant" example
Posted Oct 12, 2004 19:37 UTC (Tue) by arget
In reply to: The perenial "Nuclear Power Plant" example
Parent article: Approaches to realtime Linux
A nuclear power plant operates with that wonderful oxymoron, a controlled fission chain reaction. A highly energetic neutron hits a Uranium (or Plutonium) atom, and splits it into two smaller atoms, with some heat energy and a neutron or two left over, that in turn can go on to split more Uranium atoms. It's a balancing act, too many neutrons, and the reaction goes "super-critical" and releases exponentially more energy, potentially doubling in sub-second time frames (periods). A bomb is designed to go super-critical very, very quickly. A normally functioning reactor will operate in "critical" with a period of infinity, right on the razor's edge between super-critical and sub-critical (where there are not enough neutrons to sustain a chain reaction). Now, because of some inherent randomness, the reactor is generally a hair one side or the other of critical. Modern reactors are designed so that the geometry is such that things don't get too "hot" (or too "cold") too quickly, and you have some time to adjust as your period drops into positive or negative numbers from infinity. The razor's edge is more like a broad ridge. Even so, you want to be able to respond quickly. You can't wait for a computer to reboot. Is it ever on the order of micro or even milliseconds in a (modern, Western) reactor? Nah, but it could get within minutes, or tens of seconds. Really, space travel is probably a better example of something that needs to be controlled within microseconds.
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