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So most nuclear plants are large in scale, producing ~1000 megawatts. I've learned about the rating systems for meltdown management, which involve site radius and wind direction to estimate dust dispersion and contamination risk for surrounding areas. I don't adequately understand how radioactive dust travels. Does a small reactor, say one that produces only 5 megawatts, have a proportionally smaller risk radii? Or is it a matter of concentration of particles? Or something else?

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First some background: a nuclear reactor continues to produce some heat for hours after the reaction is stopped, due to the radioactive decay of short-lived fission products. On many large, older style reactor designs, if there's a total loss of power to all coolant pumps for a long time (e.g. hours) this "decay heat" can build up in the core enough to melt the fuel rods (which can then lead to further problems if conditions are wrong). So the safety posture for these reactors is based on having many redundant sources of backup power and alternate pumps to avoid that situation.

One of the key safety features of new small modular reactor designs is that, because they're small, the decay heat can passively conduct away into the surrounding environment (e.g. a pool of water under the reactor, or the earth) even without coolant pumps. That's sometimes described as "walk-away safe". I think that's probably the safety feature you've heard about.


None of this is anything to do with any sort of radius for dust dispersion. To answer your question on that: basically if you have an accident like Fukushima such that you're burning radioactive material, you'll get smoke with radioactive dust particles in it. Which then moves with the wind until it settles out. (And if it settles in high enough concentrations, there's risk from people/animals eating it, until the particles radioactively decay away or get sufficiently diluted by rains/oceans.) I suppose a small reactor would produce less total smoke if you incinerated it, but that's not what nuclear safety analysis is really focused on; the focus is on avoiding situations that would release radioactivity. So the "walk-away safety" is the important feature.

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