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Do We Really Need Nuclear Fusion for Power?

Feb 13 2014, 2:21am CST | by

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Do We Really Need Nuclear Fusion for Power?
 
 

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Do We Really Need Nuclear Fusion for Power?

The science of nuclear fusion took a major step forward today. The National Ignition Facility (NIF ) at the U.S. Department of Energy’s Lawrence Livermore National Laboratory blasted a small amount of deuterium and tritium (two isotopes of hydrogen in a hohlraum target) with lasers fed with 500 megajoules of electricity over a hundred millionth of a second, about the entire electrical output of America over that short time period. The target then produced more energy than it absorbed (USA Today ).

But before you get your Star Trek boots on, the actual breakthrough was more of a baby step:

“A key step on the way to ignition [of nuclear fusion] is to have the energy generated in an inertially-confined fusion plasma exceed the amount of energy deposited into the deuterium–tritium fusion fuel and hotspot during the implosion process, resulting in a fuel gain greater than unity.” NIF achieved this using “…a manipulation of the laser pulse shape in a way that reduces instability in the implosion. These experiments show an order-of-magnitude improvement in yield performance over past deuterium–tritium implosion experiments. We also see a significant contribution to the yield from α-particle self-heating and evidence for the ‘bootstrapping’ required to accelerate the deuterium–tritium fusion burn to eventually ‘run away’ and ignite” (Nature ; On the Path to Ignition ).

“These results are still a long way from ignition, but they represent a significant step forward in fusion research,” says Mark Herrmann of the Sandia National Laboratories’ Pulsed Power Sciences Center in a related article.

Wow, good stuff. And great science, but hardly the breakthrough to a Green Energy future we’ve all envisioned with fusion since the hydrogen bomb. But it is really cool that we may have a handle on the energy gain issue and that we could eventually have fusion power (WNA ).

But as a nuclear scientist, I will commit heresy and state that we don’t need nuclear fusion as a general energy source. Having said that, I really want to push for continued research to get that capability since, like the space program, the technological benefits that will come out of it are worth the investment, and are usually unknown before you go in.

Who’d have thought getting to the Moon would give us hand calculators, solar cells, scratch-resistant lenses, Teflon-coated fabric or medical breakthroughs for autoimmune syndromes? (Speaking of the Moon, a fusion power source would be fantastic for space colonies – AAPG Memoir 101 )

But we don’t need fusion for general energy on Earth. Yes, the waste issue is different than for our commercial fleet of light water reactors, you don’t generate actinide elements and fission products with fusion, and the activated core of the fusion reactor remains radioactive for only a hundred years or so.  By the same token, the amount of spent fuel we have from our reactors now is not very much either and, if burned in the new generation fast reactors, is only radioactive for 200 years or so.

But just think about the temperatures and pressures involved in this fusion reaction since, unlike fission, we‘re literally working with the-surface-of-the-Sun-type conditions. Sure, we can deal with it, just like we can deal with any engineering issue we put our minds to. Besides, creating a magnetic plasma containment field to hold a small sun is a piece of cake. I mean, we did it for dilithium crystals in the Selcundi Drema sector.

But talk about bombs! The NIF was created by the fusion-bomb weapons guys and the idea to use inertial confinement fusion as an energy source was a brilliant way to fund this micro-fusion-bomb testing device. But to use it for energy generation is a technonightmare at best.  Imagine the number of hohlraum targets per second needed, each whisked from a cryogenic BB-making machine, zapped into the middle of a huge chamber to absorb the ensuing micro-sun energy as heat for…

…generating steam to turn a turbine.

Heat exchange between the media used to absorb the energy from the decelerating protons, neutrons and gamma rays is going to be difficult, too.  You want the chamber to be transparent so as to not absorb the laser ignition energy as the pulses go through the medium.  But you also need lots of matter to absorb the energy when the hohlraum targets ignite.  It doesn’t do any good to let the gamma rays hit the walls of the chamber because that energy is just lost.

You can’t have it both ways – empty to transmit the laser, but full to absorb photons from the explosion.

But the biggest irony of all is that this is workable only for a deuterium/tritium target. That means you need a source of tritium.  Where are you going to get that?  Unless you’re on the Moon, it will be a fission reactor.  Why build a fission reactor to make tritium via neutron capture on deuterium to make the fuel for a fusion reactor, when you could just use the fission reactor to make the energy in the first place?

Essentially the biggest selling points for fusion are, “A fusion power plant would produce no greenhouse gas or other noxious emissions, operate continuously to meet demand, and would not require geological disposal of radioactive waste. A fusion power plant would also present no danger of a meltdown and provide virtually unlimited safe and environmentally benign energy” (The Guardian ).

Wait a second. These are the same selling points as for a fast fission reactor (Argonne National Lab ). When an atom in a nuclear reactor fissions, neutrons are released at high energies (fast speeds). In thermal reactors, like all of our commercial light water reactors in the U.S. and most of the world, the fission neutrons are slowed down (moderated) to low (thermal) energies (speeds) by collisions with hydrogen in the water. At these speeds only really fissile atoms like U-235 and Pu-239 split. U-238 and the other (fertile) heavy atoms just capture the neutrons and slowly convert to heavier atoms, which are the actinides that make nuclear waste long-lived.

However, in a fast reactor the fission neutrons are not slowed down and instead cause fissions by colliding with all the heavies, even U-238 and any actinides, at high enough energies to split them as well. This is important because more neutrons are released from fissions caused by high-energy neutrons than from fissions caused by thermal neutrons. So the entire fuel gets burned up into fission products. Fission products only have half lives less than 30 years, making the waste radioactive for a little over 200 years. The result is ten times more energy from the U with waste that only has to be managed for 200 years.

So we should just go to fast fission reactors and we’ll be good from both the energy density and nuclear waste perspective. We have enough uranium (and thorium) for well over 30,000 years of energy for 10 billion people.

But fission has to evolve to this next generation, which are the fast reactors like Bill Gates, General Atomics and others have designed (TerraPower ; EM2 ). Since fast reactors can burn the existing waste we have left over from our present and past light water reactors, the million-year nuclear waste problem is gone. Fast reactors get ten times the energy out of the fuel compared to ordinary reactors because they burn the U-238 and any generated actinides which make up most of the fuel.

You can even burn old Iraqi tank armor in a fast reactor.

Fast reactors can’t melt down or blow up, and don’t use water to cool.  We have those designs now, and have built many that work fine. Some submarines are even powered with fast reactors. We just need to start building them on a commercial scale for energy.  Making them as small modular reactors would be an ideal way to start.

However, the present energy markets, with natural gas so cheap and plentiful, do not encourage any new technologies at all, certainly not fusion. However, that will change once we get our gas exports up and re-connect to the global market, pretty much a given now that Senator Landrieu chairs the Energy Committee (Washington Post ). Also, if we ever get a carbon tax or credit for low-carbon energy, that would definitely spur nuclear development as well as any other low-carbon generation, like solar.

In the end, nuclear energy, either fusion or fission, provides virtually limitless power. We know how to do fission right now. We aren’t yet there with fusion.

Source: Forbes

 

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