Olin Lathrop wrote: > Bob Blick wrote: > > How much cooling do those reactors need if the fuel rods were OK and > > the control rods are fully inserted? More than passive water cooling > > with whatever flow can be achieved through convection? > > Apparently yes. The article said the pressure was building, and they did > several steam releases to cool it down. Eventually they ran out of water > such that the top of the rods became exposed. > > It seems to me that one good failsafe measure would be to ensure there is > enough of a water reservoir in the reactor chamber to allow such steam > releases for long enough to account for the residual radioactive decay he= at. > Perhaps a real nuclear engineer can explain why that's not feasible. The control rods in an enriched uranium plant is like having a throttle attached to an engine with elastic bands. The control rods when inserted absorb neutrons essential for a nuclear reaction but it takes some time to slow the reaction down to the point where a minimum amount of heat is produced. At the start of the shutdown even with the rods fully in a typical core produces a lot of heat similar to a fully operating reactor, 500Mw is a typical operating power for a single reactor. That much energy can boil a lot of water. Part of the problem when the core melts even partially is it changes dynamics of the problem. A melt can create a large mass of fissionable material in one place with nothing to absorb neutrons. It can also create other abnormalities like changing the fuel rod spacing again creating local hot spots. Canada had a partial melt down at a research reactor in the mid 50's. There was a big push after that in Canada to develop a fail safe reactor design. The Candu reactors were developed to use Uranium ore as fuel, controlled by heavy water. The D2O slows the naturally emitted neutrons so they can trigger a nuclear reaction. If for some reason the heavy water disappears there is a burst of high speed neutrons and the reaction essentially stops within a few minutes. Thermal decay in the core is the only heat generated leaving the core temperature between 100-200F. Regards, w.. -- Walter Banks Byte Craft Limited http://www.bytecraft.com .. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .