M. Mitchell Waldrop offers a marvelous, compact and easily read overview of where the fission nuclear energy field is today. With a history and review of the options, the article “Nuclear Energy: Radical Reactors” has been published by Nature. It’s a medium length article well worth the time. Meanwhile, here’s a quick overview for the time pressured.
Today’s Light Water Reactors (LWR) are the predominating technology simply because – the technology was first. LWR were originally developed in the late 1940s as a compact power source for nuclear ships and submarines. Then the United States sought to put a peaceful face on atomic energy by creating a commercial nuclear-power industry using the light-water design adapted and scaled up during the 1950s. A LWR uses ordinary H2O, plain water, which flows through the reactor core absorbing the heat and circulating with steam to a conventional steam turbine that turns the heat into electricity. The system worked, and worked well with the materials technology at hand more than 50 years ago.
Originally LWR was meant to be part of a larger system that would make up for a basic inefficiency because when left alone a LWR nuclear reactor will quickly poison its own fuel. As the chain reaction proceeds, the fuel accumulates more and more of the fragments left over after the uranium atoms split, which in turn absorb more and more of the neutrons required to keep the reaction going. After perhaps 18 months, the fuel is ‘spent’ and has to be removed – even though it still contains better than 90% of its original energy.
Nuclear Plant Block Diagram.
A network of reprocessing plants was to take the spent fuel, chemically extract the still-usable components – mostly uranium-235, plus the fissionable plutonium-239 formed when neutrons are captured by non-fissile uranium-238 – and then turn them into fresh reactor fuel. Ultimately, the plan was to transition to a new generation of ‘breeder’ reactors designed to maximize plutonium production. The only waste was to be a comparatively small residue of intensely radioactive fission products that would decay within a few centuries, and could be disposed of in, say, a well-designed concrete bunker.
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That’s the original plan – but in May 1974 India set off a nuclear bomb made with plutonium extracted from reactor fuel. Rampant nuclear-weapons proliferation suddenly became a very real concern. In 1977 Jimmy Carter banned commercial reprocessing. Even though President Ronald Reagan lifted that ban a few years later the costs had skyrocketed out of reach. Only France has completed two reprocessing plants.
Events and the reactions have left the complicated disposal problem of isolating tens of thousands of ton of spent fuel for hundreds of centuries, thanks to the 24,100-year half-life of plutonium-239.
But the U.S. had continued with other technologies from the early 1950s until the mid 1970s. The fascinating molten-salt technology had been demonstrated and the fluid uranium- or thorium-containing fuel offered major advantages. Molten-salt reactors would be impervious to catastrophic meltdown, for example, and instead of producing nuclear waste laced with plutonium and other long-lived radioisotopes, they could destroy those isotopes almost completely.
Other technologies include ‘fast’ reactors that would also burn up nuclear waste, and high-temperature reactors that could take a huge bite out of greenhouse-gas emissions by generating zero-carbon heat for industry. Taken together, these alternative technologies could eliminate most or all of nuclear energy’s drawbacks. But they have received only fitful attention from researchers over the decades, thanks to constantly shifting and competing special interest agendas plus volatile and unreliable funding levels.
Bringing these technologies up to today’s needs won’t be easy or quick. Even though the basic designs were worked out decades ago, today’s engineers hoping to put them into practice will have to develop modern material based solutions for radiation-resistance, more-efficient heat exchangers and improved safety systems and prove to regulators that all these systems will work.
Tiny, but big dollar steps have been made. In 2002 the Department of Energy started a cost-sharing plan designed to help manufacturers to develop and license light-water reactors with advanced safety features. Last year the DOE launched a cost-sharing program for Small Modular Reactor (SMR) development.
The main interest in the SMR are some designs called high temperature reactors that generate steam at up to 1,000 °C, much hotter than the roughly 300 °C available from light-water reactors. Such reactors cannot melt down: the fuel is stable up to 1,600 °C, hundreds of degrees hotter than the core would become even if all power and coolant were lost. The high temperatures would make the reactors more efficient at producing electricity.
Not funded but looking attractive are fast reactors, which tackle a problem of spent nuclear fuel. Fast reactors could consume spent fuel, turning today’s dangerous waste into energy and easing the disposal problem. This technology will be difficult to engineer and need quite exotic materials for future installations, but some 20 fast reactors have been operated over the years – many of them following the 1970s breeder design that was built to maximize plutonium production instead of consuming it – and at least four manufacturers are developing small fast reactors for spent-fuel consumption.
For many the Molten Salt Reactor (MSR) is the next technological goal. The MSR offers extreme safety – after all the meltdown is part of the design – with fuel that operates in a melted state. The MSR fuel can be uranium, spent fuel, plutonium and thorium mixed with lithium fluoride and beryllium fluoride called FLiBe. However the MSR is a self-poisoning system, so the working fuel is circulated through an external recycling unit that extracts the fission products continuously, keeping the fuel active. If the recycling stops the reaction soon stops. Loss of control, or more appropriately loss of management to keep it going, the fuel would melt out a cooled solid fuel plug at the bottom of the reactor draining the fuel out of the reaction. The problem is the information needed is now over 40 years back in history.
Back in 2011 the DOE funded what could be called a hybrid technology, “a salt-cooled solid-fuel reactor”. A high temperature design, the reactor core could be four to five times smaller than those in other designs and, because of the stability of the molten salt, again a mixture of lithium fluoride and beryllium fluoride that serves as a coolant, it would always be hundreds of degrees below the failure limits.
There are several other ideas in research as well.
Waldrop winds up with Paul Genoa, director of policy development for the Nuclear Energy Institute trade group in Washington DC, expressing the long view. “We did the light-water reactors first, to get going,” he says. Next, in the 2020s, will come advanced light-water reactors for increased safety, followed closely by high-temperature reactors that expand the attack on carbon emissions. “And then we build fast reactors to consume the waste.”
Molten-salt reactors are something of a wild card, says Genoa, but are worth developing. Some even wilder cards are under investigation: one notable example is the accelerator-driven reactor, which would drive fission reactions using neutrons from a high-energy particle accelerator. It could be fuelled with thorium, and shut down instantly by switching off the accelerator.
Fission power is a sure thing. The political arena and funding are the problems. The risk lies in success coming from various forms of fusion technology. The world’s population is going to need the power from nuclear that either fission or fusion can offer.
That’s the short gist of Waldrop’s terrific article. Waldrop’s Nature article is far more complete and truly deserves a few minutes of your time when it’s available. But for now, the barriers are the politics and money – items that will always stand in the way of progress until the market comes-a-callin’.
By. Brian Westenhaus