Is it possible to greatly speed up the fission of uranium with excess neutrons? If so would beryllium make a good neutron source? Uranium produces alpha radiation which is absorbed by beryllium which intern releases neutrons. It seams like you could ramp up the power of a nuclear reactor by controlling the interaction between beryllium and uranium.


3 Answers 3


Yes it is possible to speed up the fission of uranium with excess neutrons. The result is known as an atomic bomb.

Ramping up the power of a nuclear power station is not the problem. The problem is stopping the nuclear power station from going super-critical and producing another Three Mile Island or Chernobyl experience. This is done by absorbing the neutrons produced by the fission, typically by using control rods. Adding extra neutrons is not a good thing.

  • $\begingroup$ What if you just used U-238 or something else not fissile? It wouldn't explode right? $\endgroup$
    – Seth A
    Feb 15, 2017 at 16:28
  • $\begingroup$ U-235 is fissile, meaning it wants to fission without much provocation. U-238 is fissionable, meaning it can fission but requires a little energy to do so. A nuclear bomb is almost entirely U-235. A nuclear power plant is only about 20% U-235 And the rest is U-238. It's a self sustaining chain reaction. So the point of nuclear power is not to add more to get more power. The point is to reduce neutrons to keep the reaction under control. This is done by inserting neutron absorbing control rods, made of boron or cadmium. $\endgroup$ Jan 1, 2018 at 23:05

Beryllium mixed with an alpha emitter is a traditional method of making a neutron source--called an alpha-neutron source. I have used the unfortunately named PuBe source, which came in a terrifying 55-gallon drum (full of paraffin shielding), that looked like it belonged in a zombie movie. Both $^{238}$Pu and $^{239}$Pu are used.

Other $\alpha$-emitters used are $^{210}$Po, $^{226}$Ra, $^{227}$Ac, $^{228}$Th, $^{241}$Am, $^{242, \ 244}$Cm, while $^9$Be can be replaced with $^{10}$B, $^7$Li, $^{19}$F, $^{13}$C, and $^{18}$O.

As far as sticking them in nuclear reactors--@John Rennie covered that. Note that the general principle of nuclear reactors is to keep prompt neutrons subcritical (since the control hardware can't responds with a microsecond timescale), and use delayed neutrons to push $k$ past 1.

That being said, it appears PuBe is used to kick start subcritical reactors (which are obviously not for power): Vega-Carrillo HR. TEORÍA DE REACTORES NUCLEARES: REACTOR NUCLEAR SUBCRÍTICO. Saarbrücken, Germany. Editorial Académica Española. (2012).

  • $\begingroup$ I too used a PuBe source in the 55 gallon drum, with little wheels on the bottom to allow you to move it. It was scary looking! $\endgroup$
    – Natsfan
    Jan 2, 2018 at 0:38
  • $\begingroup$ So why is a 55 gallon drum scary looking? $\endgroup$
    – mmesser314
    Jan 2, 2018 at 2:18
  • $\begingroup$ @mmesser314, Any non expert would fear said drum, covered in yellow trefoils: it is the epitome of unseen nuclear death. For the rad-trained: most sources are tiny things in little Pb, Al, or plastic shielding. When the drum is wheeled in, it gets your attention. Plus, anyone who isn't a bit concerned with their first contact with plutonium needs a new line of work. $\endgroup$
    – JEB
    Jan 3, 2018 at 14:30

Putting beryllium in a reactor would do more harm than good. When a uranium atom absorbs a neutron and fissions it puts out 2 to 3 neutrons. That's how the reaction grows on it's own. If you put beryllium inside, it may absorb an alpha and put out a neutron. But some of it will absorb neutrons turning into boron which is a sure fire way to kill the nuclear reaction, which is why control rods are made of boron.

However a beryllium americium source is used to start the reaction.


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