Lessons in Nuclear Safety Start With Axes: Peter Coy

April 13 (Bloomberg) -- Nuclear power plants will never be completely safe, but they can be made far safer than they are today.

The key is humility. The next generation of plants must be built to work with nature, and human nature, rather than against them, Bloomberg Businessweek reported in its March 28 edition. They must be safe by design, so that even if everything goes wrong, the outcome won’t be disaster.

In the language of the nuclear industry, they must be “walkaway safe,” meaning that even if all power is lost and the coolant leaks and the operators flee the scene, there will be no meltdown of the core, no fire in the spent fuel rods, and no bursts of radioactive steam into the atmosphere.

The beginnings are incorporated into nuclear power plants under construction in China and India and probably soon in the U.S.

The plants use “passive” safety features. That means the reactor’s safety doesn’t depend on active interventions, such as operators flipping the correct switches or sensors and actuators working properly. The safety depends, rather, on physics.

The new Westinghouse AP1000 (the AP stands for Advanced Passive), for example, has a huge emergency water reservoir above the reactor vessel that’s held back by valves.

If the cooling system fails, the valves open and a highly reliable force takes over: gravity. Water pours down to cool the outside of the containment vessel. Then another highly reliable force, convection, kicks in. As the water turns to steam, it rises. Then it cools under the roof, turns back into a liquid, and pours down again.

Three Days

Westinghouse estimates that the pool contains enough water to last three days, after which pumps operated by diesel generators are supposed to kick in and add water from an on-site lake.

Westinghouse, majority-owned by Toshiba Corp., is about halfway to completion of one of the plants at Sanmen in China’s Zhejiang province. It’s supposed to go online in 2013.

At the Vogtle plant in Georgia, excavation and non-safety-related construction has begun for two AP1000 reactors. Southern Co., the largest utility in the U.S. by market capitalization, has 1,400 workers on the site and is expecting a combined construction and operating license for them later this year.

France’s Areva SA and a venture between General Electric Co. and Hitachi Ltd. are among groups lined up behind Westinghouse in getting Nuclear Regulatory Commission certification for their own safer designs.

Ax Man

Critics continue to argue that there will never be a truly safe nuclear power plant, and it’s true: Any time you split atoms, there’s risk. But nuclear plants have one thing going for them that hasn’t changed since the leak at Fukushima. They generate badly needed electricity without creating greenhouse gases that cause global warming.

Nuclear power faces all sorts of challenges, including the multibillion-dollar construction cost of each new plant. Fukushima is a reminder that making the plants simpler and safer is the biggest challenge of all.

In late 1942, Enrico Fermi, the Italian physicist who was one of the key figures in the development of the atomic bomb, came up with nuclear power’s first safety mechanism: a man with an ax. In case of a runaway reaction, he decided, the ax man’s job was to cut a rope, dropping cadmium rods that would absorb neutrons and halt the reaction.

The ax man’s skills were never needed, but in the decades that followed, nuclear energy suffered a series of stigmatizing accidents.

Light Water

One reason for that is the almost total reliance on one particular design, the light water reactor, which has some inherent problems. The light water’s prominence dates to a fateful 1950s choice by Admiral Hyman Rickover, father of the nuclear Navy.

Rickover decided that the first nuclear submarine, the USS Nautilus, which was launched in 1955, would be powered by solid uranium oxide and would use water as both a coolant and a moderator. A coolant carries heat from the reactor to produce power; a moderator slows down the neutrons emitted by the fuel so they have a better chance of interacting with other fissile materials to keep the reaction going.

The design has some big drawbacks, as explained by physicist Robert Hargraves and nuclear engineer Ralph Moir in a 2010 article in the American Scientist.

Damaged Rods

The bundles of fuel rods are quickly damaged by heat and radiation. Short-lived byproducts such as xenon-135, which poisons the fission process, are tricky to manage -- and contributed to the instability that led to the Chernobyl explosion. Long-lived byproducts are abundant and highly toxic. The water is corrosive and radioactive. To raise its boiling point, it must be pressurized to 150 times atmospheric pressure. That necessitates a costly network of vessels, pipes, and valves -- and raises the risk of an explosive release of radioactive steam to the atmosphere. Overall, the design is the precise opposite of passive safety.

Rickover’s choice had lasting consequences. The first commercial nuclear power plant in Shippingport, Pennsylvania, used a design similar to that of the Nautilus, setting a pattern that endures today.

New reactors such as the Westinghouse AP1000 and Areva’s EPR don’t diverge from Rickover’s path. They too have the solid-fuel cores and high-pressure water cooling systems. But they are substantially safer than older light water reactors.


The Energy Department is also supporting development of small “plug-and-play” modular reactors such as the 125-megawatt mPower design from Babcock & Wilcox Co. and Bechtel Group.

If worse comes to worst, the safety systems that give the most confidence are the passive ones.

The International Atomic Energy Agency has created an elegant hierarchy of these passive systems. The ones at the highest level, Category A, require no signal inputs, no external power sources or forces, no moving mechanical parts, and no moving working fluid. Example: really thick concrete walls. Westinghouse’s reservoir above the AP1000’s containment vessel, clever as it is, doesn’t fit into Category A because it involves moving fluids and valves.

Generation IV reactors will be a much bigger departure. Many will do away with water, using elements such as helium or liquid sodium as a coolant. Most also get rid of solid uranium-235 as a fuel, relying instead on different uranium isotopes, or liquid uranium mixtures, or even thorium as the primary fuel.

Second Look

Some long-since-rejected concepts for nuclear energy are getting a second look as the faults of the incumbent technology become clearer.

The only requirement is that somehow a nuclear chain reaction must occur: A fissile material absorbs a neutron and splits, releasing energy and more neutrons that carry the process on. There are probably 1,000 conceivable combinations of the basic choices, an Oak Ridge official once calculated.

The new design that’s closest to commercial electricity generation is the pebble bed reactor, which has been under development for decades in Germany, then South Africa, and now China and the U.S.

Its uranium fuel is encased in more than 300,000 tennis-ball-sized “pebbles,” each one containing thousands of tiny graphite-coated fuel seeds, like a metal pomegranate. The radioactive fission products are absorbed in the coatings, and the fuel doesn’t get hot enough to melt down even if the plant loses all its cooling for days.

China Closest

China is closest to commercializing the pebble bed, pushing forward where the South Africans left off for financial reasons. Tsinghua University, working with Massachusetts Institute of Technology, already has a 10-megawatt experimental reactor in operation. It is building a 200-megawatt plant in Shidaowan in Shandong province with a unit of China Huaneng Group, the nation’s largest power group.

The Energy Department is leading a U.S. initiative, based at Idaho National Laboratory in Idaho Falls, called the Next Generation Nuclear Plant. It, too, may be a pebble bed design.

Energy Secretary Steven Chu has appointed a review committee to decide later this year whether to move on to the second phase of the project, which could lead to an operating pebble bed plant in the U.S. by around 2025, says David A. Petti, director of the Very High Temperature Reactor Technology Development Office at Idaho National Laboratory.

Scale Up

Pebble bed reactors don’t scale up well. Above 600 megawatts they lose their safety advantage over reactors with ordinary fuel rods, says Petti. At that scale the pebbles can get so hot that they can’t shed heat fast enough and can melt down just like any other uranium fuel, releasing radioactivity.

The sweet spot for pebble bed reactors is 250 to 600 megawatts, Petti says. He envisions them being located next to industrial plants, where their excess energy could be used to heat facilities and produce petrochemicals.

Liquid fuels are getting another look, too. As early as 1944, Fermi tested the world’s first liquid-fuel reactor in Chicago. It used uranium sulfate dissolved in water.

In 1965, Oak Ridge National Laboratory in Tennessee achieved criticality with a reactor using a more advanced liquid fuel: molten fluoride salt with uranium dissolved in it. The experiment ran for five years, then stopped when the money ran out. Now, scientists in India, France, and the U.S. are experimenting with another variety: liquid fluoride thorium reactors (known as “lifters”).

Perhaps the farthest-out design comes from a spinoff of Intellectual Ventures, a company headed by former Microsoft Corp. chief scientist Nathan Myhrvold and funded in part by Microsoft co-founder Bill Gates.

‘Massive Power’

TerraPower LLC, as the spinoff is known, used massive computing power to design a reactor that could run for decades on an isotope of uranium that is today considered waste.

The concept, first proposed in the 1950s, is to set up a slow-moving wave in which neutrons transmute inert, nonfissile fuel such as uranium-238 into fissile isotopes such as plutonium-239 that can split and throw off energy.

TerraPower says its spent fuel would not be useful for making weapons. The company, chaired by Gates, has been seeking a production partner and a host country. So far, no takers.

Technological breakthroughs such as the traveling wave reactor are part of the answer to nuclear safety, but not the answer in its entirety. Better management is crucial as well.

‘Clever Enough’

Often, says Gary Klein, a consultant to the Air Force Research Laboratory, planners underestimate dangers by assuming they will be clever enough to surmount any difficulty that arises. Klein, who is senior scientist of MacroCognition in Yellow Springs, Ohio, removes that overconfidence bias in sessions with clients by asking participants to assume that a terrible accident, however unlikely, has already occurred. The challenge is to explain how it happened.

“You show you’re smart by trying to identify things that are plausible and worrisome,” says Klein. “It almost gets into a competition.”

To say nuclear power can be made much safer is not to say it can be made safe enough. That’s a political judgment, not a question for nuclear engineers.

Fukushima could bring reactor development to a halt. Or it might stimulate the demand for safer designs. The future of atomic energy is at stake.

(Peter Coy’s column appeared in Bloomberg Businessweek’s March 28 edition. The opinions expressed are his own.)

For more news and information: For more news on Fukushima: {9501 JT <Equity> CN <GO>} For top commodities news: {CTOP <GO>} For more science news: {NI SCIENCE <GO>}

To contact the writer of this column: Peter Coy at

To contact the editor responsible for this column: Amit Prakash at

    Before it's here, it's on the Bloomberg Terminal. LEARN MORE