In a nuclear reactor, rods made of boron are used to slow down neutrons to control the nuclear
reactions (the more neutrons flying around, the more fission reactions). Boron has a light
nucleus. Why not use a massive nucleus like lead to slow the neutrons?
2 comments:
I think it's because there's a tipping point easily reached in terms of what's an acceptable limitation on the density and profusion of fission reactions.
Wikipedia tells me that boron actually has the downside of neutron absorption, but that it's often used anyway, because it's much cheaper than other "moderators" in the reaction process, such as heavy waters. Something about how the Germans in WWII messed up by using too much boron (components in their reactor had additional concentrations of boron which the scientists weren't taking into consideration).
Good heavy water moderators (even beryllium is used sometimes, but only "experimentally") permit reactions with unenriched uranium, but are almost always prohibitively expensive. Thus the common practice of uranium enrichment, during which "natural uranium" (U-238) is enriched to contain 3-4% "fissionable uranium" (U-235, typically present in natural uranium in 0.7% concentration) for commercial nuclear power, and up to 90% U-235 when enriching for military weapons.
Also, comparing the elemental properties of boron and lead, it seems to me that lead would be unsuitable as a replacement given that its melting point is only 600K, whereas boron's 2349K. Uranium's is 1405K, but to be honest, I don't really know whether melting points are applicable to the discussion at hand.
I'd further like to note that lead is way more dense than boron (though still only about half as dense as uranium). Maybe that's worth something. And while boron is classified as "nonmagnetic," lead is "diamagnetic" and uranium is "paramagnetic." Given that electromagnetic radiation is given off during nuclear fission, maybe using lead as a moderator would pose problems not encountered when using boron.
Now I'm going to go make scrambled eggs and pretend I'm a chef.
When a neutron collides with a nucleus, both momentum and energy are conserved. Lead nuclei are much more massive than a single neutron. When a neutron bounces off of a massive lead nucleus, it leaves the collision with nearly the same speed as it had initially. This is because the lead nucleus barely recoils. On the other hand, when an object collides with another object of equal mass, the two objects exchange velocities. The boron nucleus is not too much heavier than a neutron, so when a neutron hits it the boron nucleus recoils. The neutron leaves the collision with much less kinetic energy and therefore a lower speed. The boron nucleus recoils and eventually transfers most of its
energy to the boron lattice.
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