Hacker News Reader

Comment by th0ma5 | Hacker News Reader

What about the combined fusion/fission options? I mean that's what the h-bomb and the sun do right?

I think combined fusion/fission options are the clear choice for the future. (Also, yes, that is what the H-bomb does, and no, that is not what the sun does.)

Here's how combined fusion/fission plants would theoretically work:

A fusion reaction is used to create a large quantity of high-energy neutrons. These high-energy neutrons are then used to cause fission in an otherwise subcritical fission assembly with extremely low enrichment fuel. 'Natural' uranium (U-238) is fissionable, which means that it can undergo fission in the presence of /high energy/ neutrons.

The H-bomb uses a fission A-bomb to get enough temperature and pressure to produce the fusion reactions.

The sun is a 100% fusion reactor.

If you mix a fusion and a fission reactor, probably you will have most of the disadvantages of the fusion reactors; in particular you will have a lot of long-living radioactive byproduct of the fission reactions.

In fusion-based nuclear weapons, a fission bomb is used to create the temperature and pressure needed to kick off nuclear fusion. Calling that "combined fusion/fission" is like calling my car an electric hybrid because it uses an electric starter.

Detonating a nuclear weapon inside of a power plant isn't really practical. :-) Existing nuclear power plants don't get anywhere near the temperature and pressure needed to start nuclear fusion.

So, now we have the engineers trying all sorts of ways to get to the needed temperature and pressure without turning the plant into a smoking crater. That's where all the fancy lasers and magnetic pulses described in the article come in: They're trying to aim the heat source so they can superheat the fuel without melting the containment vessel.

That's a popular misconception, IIRC. Actually, in thermonuclear weapon the bulk of energy comes from fission. [1] Fusion is technically only used to get the high energy neutrons to get the bulk of fissionable material to undergo fission. So, ironically, calling it a "fusion bomb" is very much like calling your car an electric hybrid.

1. http://en.wikipedia.org/wiki/Thermonuclear_weapon

Where those weapons get the most energy from depends heavily on the design. It's true that multi-stage thermonuclear devices often have a final fission stage where huge amounts of energy are released, however if one were to purposefully maximize energy release while minimizing fallout there are some relatively clean fusion/fission ratios achievable.

For instance, the test version of the Tsar bomb had its last fission stage removed, reportedly resulting in a 97% fusion detonation. Of course, those still cause a huge lethal neutron flux, but at least the amount of fission material released is pretty small in relation to the overall power of the device.

Nope, both are fusion reactions. You fuse two particles together below 56 on the periodic table and achieve energy. After 56 and up, you have to use fission, where you split atoms apart, to achieve energy.

Reply on news.ycombinator.com