The National Ignition Facility (NIF) has a new concept that uses lasers to trigger a fusion that feeds neutrons out to drive a fission reaction. The idea has been known since 1951 when Andrei Sakharov, Hans Bethe and other scientists explored using high-energy neutrons from fusion reactions to transmute, or burn, fissile material. Its not a new concept, but hasn’t been tried yet at any scale.

Called fusion-fission engines the idea for power generation centers on using laser-driven fusion targets to drive a fission blanket into reaction for generating heat. Part of the concept is transmuting nuclear waste, eliminating uranium enrichment upfront and waste reprocessing out the back.

Today the concept is called “LIFE” from Laser Inertial Fusion-Fission Energy. Developed at the Lawrence Livermore National Laboratory its being set for testing at the National Ignition Facility. The ability of lasers to create the conditions required for ignition and a thermonuclear fusion burn in the laboratory with inertial confinement fusion is planned for demonstration on the NIF within the next two to three years. The NIF suggests that with a funded research, development and engineering program, LIFE engines could begin to provide electricity to U.S. consumers within 20 years.

LIFE Energy Flow.  Click image for more.

LIFE Energy Flow. Click image for more.

The LIFE system is designed to operate with fusion energy gains of about 25 to 30 and fusion yields of about 35 to 50 MJ that provides about 500 megawatts of fusion power, of which about 80 percent comes in the form of 14.1 million electron-volt neutrons with the rest of the energy in X-rays and ions. That would yield approximately 1019 14.1-MeV neutrons per ‘shot’ (about 1020 neutrons every second). The neutrons then exit to the fission blanket where the neutron driven reaction generates an additional energy gain of four to ten depending upon the details of the fission blanket, That totals up an overall LIFE system energy gains of 100 to 300 compared to the energy used to drive the lasers that trigger the sequence.

LIFE Target Diagram. Click image for more info.

LIFE Target Diagram. Click image for more info.

The fission fuel blanket would be about 40 tons of depleted uranium, un-reprocessed spent nuclear fuel, natural uranium or natural thorium or a few MT of the plutonium-239, the minor actinides such as neptunium and americium, and the fission products separated from reprocessed spent nuclear fuel. It looks like an effective way to get the power out of those fuels and greatly reduce the waste stream.

Once the lasers fire at the pea sized deuterium-tritium pellet of fusion fuel the neutrons pass first through a structural steel wall, then a first-wall coolant and on to a layer of beryllium pebbles, which generate 1.8 neutrons for every neutron they absorb. The newly generated neutrons have a significantly lower energy spectrum that is ideal for fission energy generation. Those neutrons strike the one-meter-thick, subcritical fission blanket containing 40 MT of fission fuel pellets. The neutrons absorbed by the fuel pellets drive neutron capture and fission reactions, releasing tremendous amounts of heat to drive turbines. There you have a heat source to generate power.

To get the heat out the pellets are immersed in a molten salt called flibe (2LiF + BeF2 ) that carries away heat and also produces tritium that can be harvested to manufacture new deuterium-tritium fusion targets.

All simple so far in theory. The Livermore Lab physicists and engineers conceive the LIFE engine’s fusion targets to be about one centimeter long and one-half centimeter in diameter. They would be injected at 10 to 15 times a second into the precise center of the fusion chamber. That’s pretty quick even for a robot.

The lasers themselves have to fire exactly simultaneously or timed to have the protons arrive at the same instant. The aim of the lasers must be precise as well.

The interesting thing is the fuel pellet is built from solid density gold with the fuel inside. The pellet is called a “hohlraum” (where do they get these names?). When the lasers irradiate the hohlraum it emits a “bath” of X-rays that heat and vaporize the outer layer of the BB-sized target capsule, causing it to rapidly implode. The resulting temperature and pressure forces the hydrogen nuclei to fuse and ignite in a controlled fusion reaction.

Theorist being theorists, suggest in “fast ignition” the targets are first compressed by one laser and then ignited to fusion conditions by a second.

OK. I expect this can be made to work. It’s really quickly going to go through the fusion fuel of deuterium-tritium and gold. An on site fabrication plant would need built for each system, with fuel transport set up to feed the reactor. All of this would depend on a large supply of current to drive the lasers. Might want to keep other options open.

On the plus side the design is thought to get out 99% of the fuel’s energy resulting in greatly enhanced energy generation per metric ton of nuclear fuel. That would mean a huge drop in waste material production. What’s left when the whole 40 tons of fuel is spent is 39.6 tons of fission products. This remaining waste has such a low actinide content that it falls into DOE’s lowest attractiveness category for nuclear proliferation. That is not very attractive at all, which over time becomes barely dirty bomb level stuff.

On the downside beyond sheer complexity is, because of the very high fission product content, the waste is self-protecting for decades: its radiation flux is so great that any attempt at stealing it would be suicidal. I don’t find that comforting at all. Tolerable due to the huge reduction of net waste, but I wish they’d find a better way to point out that the concentration of radiation is very dense with terms other than ‘suicidal.’

All said, this technology should proceed, as it is one likely possibility of the list of possible means to generate power as well as use up the enormous fuel store on hand from current reactors. The NIF has cool stuff to make their case, graphics and even a video. With the demand for electricity growing, and the growing interest in nuclear power across the full range of governments alternatives both in investment, reliability and fuel use is getting more important by the day.

Its not a perfect solution – one may never come – but on the scale of price per kilowatt for business and homes, the risks to society and nature this is one technology that needs attention as well as others. One just hopes that more information is forthcoming about the results from the various possible fuel mixes in cost and waste stream risk.


9 Comments so far

  1. Matt on April 14, 2009 6:37 AM

    Here’s an idea. If you can’t get fusion to work – let’s double the complexity and surround it with a fission reactor. And, even better, let’s vaporize real gold pellets.

    This is like something out of a high tech Monty Python sketch.

    Yeah, it should work. The only limiting factors are time, money and engineering personnel.

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