World - From squash court to submarine Nuclear reactors and their uses have not changed much over seven decades

THE NUCLEAR AGE began 70 years ago on a squash court in Chicago, under the watchful eye of a man with an axe. A team led by Enrico Fermi, Italy’s greatest physicist since Galileo, had been building a nuclear “pile” for weeks, slotting pellets of uranium and bricks of graphite into a carefully planned geometry through which ran various “control rods” of cadmium. The squash court was the only convenient large space available on the university campus. On December 2nd 1942 the pile had grown large enough to allow a nuclear reaction to take off when the control rods were drawn back from its heart.

The axeman was there in case the reaction went out of control. If it did he would chop through a rope, sending the main control rod crashing back into place, absorbing the neutrons driving the reaction and restoring stability. Like many of Fermi’s ideas, it had the charm of simplicity. Today every commercial power reactor has control rods poised to shut it down at a moment’s notice, a procedure called a scram—in honour, so it is said, of Chicago’s “safety control rod axeman”.

On that first occasion, nothing went wrong. As the pile’s other control rods were mechanically withdrawn, radiation counters ticked up. Once Fermi was satisfied that they were showing a true chain reaction, he had the rods reinserted. A celebratory bottle of Chianti was opened. A coded phone call informed the head of the National Defence Research Committee that “the Italian navigator has landed in the new world.” The axeman put down his axe.

The energy output of that first reactor was tiny: just half a watt. Today’s most powerful reactors produce ten billion times as much energy in the form of heat, about a third of which can be converted into electricity. Five gigawatts is an amount beyond easy comprehension, the daily equivalent of the energy given off by six bombs like the one that destroyed Hiroshima. Imagine that energy coursing through a few swimming pools-worth of water at three times the normal boiling point, trapped in a steel cylinder under pressures found a mile below the sea, and you have a sense of the hellish miracle that is a modern reactor.

Fermi and his nuclear family

If flights had lasted a billion times longer 70 years after the Wright brothers’ first one took off, they would have gone a thousand times round the world and taken centuries; a billion times faster, and they would have run up against the speed of light. Even at the heady rates of progress that Moore’s law ascribes to the computer industry (stating that the number of transistors on a chip doubles roughly every two years), things take 60 years to get a billion times better.

But such comparisons flatter nuclear technology. The Wright brothers wanted their first aircraft to fly as far and as fast as it could; the first reactor was designed to do things in as small and safe a way as possible. At the time Fermi demonstrated the first controlled chain reaction, the uncontrolled ones that would devastate Hiroshima and Nagasaki were already being planned. Reactors capable of generating hundreds of megawatts of heat were on his colleagues’ drawing boards. Nuclear power did not grow steadily over decades the way aircraft and computers have done. It blossomed fast, then held strangely steady.

Using reactors to generate electricity was not an early priority. The reactors of the Manhattan Project—the wartime nuclear programme begun in earnest shortly after Fermi’s success—were designed to further the project’s only goal: making bombs. A nuclear chain reaction, whether in a reactor or an exploding bomb, comes about when the splitting of a “fissile” nucleus by a neutron produces neutrons that will go on to split further fissile nuclei. The fissile nuclei in Fermi’s reactor were of a particular type or “isotope” of uranium, U-235. In natural uranium, only seven nuclei in every 1,000 are of this fissile sort. That was not a problem for Fermi’s reactor. The addition of graphite—a “moderator”—slowed down the neutrons that were being given off, which made them better at splitting other nuclei and enabled a chain reaction to take place even when fissile nuclei were scarce. Bombs are not the place for moderation: to make a uranium bomb you need a core that is almost entirely U-235. As separating out U-235 is extremely difficult, the Manhattan Project’s physicists were not sure they could provide it on the scale that bomb-makers would require.

That created a need for an alternative source of fissile material, and reactors provided it. When a neutron hits one of the non-fissile uranium nuclei— the vast majority—it can turn it into a new element: plutonium. Plutonium nuclei are fissile, and getting a bit of plutonium out of uranium that has been sitting in a reactor is far easier than separating uranium isotopes. Reactors could thus serve as plutonium factories, and the early ones were used exclusively for that purpose. By the mid-1950s some reactors in Britain and France were generating electricity as well; they needed to be cooled anyway, and using the gas that cooled them to drive steam turbines was good public relations. But their main purpose was still to provide fuel for bombs.

Rickover’s killer app

Almost all of today’s nuclear power plants have a different lineage. Hyman Rickover, a redoubtable American submariner, saw a niche for nuclear power plants in submarines. The diesel-powered sort needed to take on air through a snorkel; nuclear-powered ones would be able to stay submerged indefinitely. But a graphite-moderated reactor would never be compact enough for submarines. Rickover eventually settled on a reactor design that economised on space by using water as both moderator and coolant—a pressurised water reactor, or PWR. In many ways this is a poor compromise. Water cannot be heated much above 350°C, even under pressure, and still stay liquid. That limits the efficiency of energy conversion (the hotter the better in such matters). And normal water is not a very good moderator. So-called “heavy water”, which contains a different hydrogen isotope, is better, but a lot harder to come by than the “light” type.

Admirable admiral

To make up for poor moderation, a light-water reactor needs fuel enriched in U-235. It does not have to be enriched as much as uranium for bomb-making does, but the enrichment systems used to make fuel for such reactors can almost as easily be used to make the weapons-grade uranium that bomb-makers need. This is the technological basis of the stand-off with Iran, which has claimed unconvincingly that its enrichment facilities are just for reactor fuel. Other combinations of fuel and moderator would have allowed the use of non-enriched uranium, and could indeed have got by without producing plutonium as a waste product, thus establishing a much clearer dividing line between nuclear power and nuclear weapons. But they would not have powered the submarines of the 1960s.

In the early 1950s nuclear physicists were for the most part unexcited by the light-water reactor’s potential. By the time the nuclear age was just ten years old they already knew of many kinds of fuel, various moderators (as well as designs that needed no moderation) and many ways of getting heat out of reactors. They were excited by “breeder” reactors that both burned and created plutonium. America’s first was built in 1951, and the assumption was that any nuclear-energy economy worth the name would make use of the technology’s miraculous ability to produce its own fuel. The fascination continued for decades. Breeder reactors have been built in Russia, Britain, Germany, India, China, France and Japan as well as America. But they have not proved remotely attractive enough for commercial development in a world which seemed to have plenty of uranium.

By 1952 bomb-makers had also massively multiplied the power of their wares by adding nuclear fusion, in which energy is liberated by adding together small nuclei rather than splitting apart large ones. Schemes to make reactors on the same principle sprang up immediately. Sixty years on, this line of research continues, at great expense, without any prospect of commercial plausibility.

Make mine a PWR

The navy did not need exciting ideas; it needed submarine power plants that used available technologies. In Rickover it had an organisational genius capable of creating the industrial base needed to provide them, choosing and training the naval engineers needed to operate them, and instilling in that cadre the meticulous safety culture needed to stop the reactors from going haywire. Under Rickover’s tutelage American industry learned to make PWRs, which it went on to offer to electrical utilities. So PWRs became the mainstay of America’s nuclear-power industry as it grew up in the 1960s, with the boiling-water reactor (BWR)—a similar light-water design, less efficient and unseaworthy but in some ways simpler and possibly safer—providing an alternative. BWRs currently make up 21% of the world’s nuclear capacity, but that figure is set to diminish, not so much because the Fukushima reactors were of that kind (old PWRs in a similar setting would not necessarily have fared better) but because the few countries that ever went for the technology on any scale have little appetite for new plants (America) or none at all (Germany and Japan). The 68% of the world’s nuclear electricity from PWRs is thus set to increase, with other technologies trailing way behind.

Developing a nuclear reactor has never been a matter for barnstorming experimentation

Such homogeneity in a 70-year-old high-technology enterprise is remarkable. Seven decades after the Wright brothers’ first flight there were warplanes that could travel at three times the speed of sound, rockets that could send men to the moon, airliner fleets that carried hundreds of thousands of passengers a day, helicopters that could land on top of skyscrapers. Include unmanned spacecraft, and there really were flights a billion times as long as the Wright brothers’ first and lasting for years. But aircraft were capable of diversity and evolution and could be developed cheaply by small teams of engineers. It is estimated that during the 1920s and 1930s some 100,000 types of aircraft were tried out.

Explore our interactive guide to nuclear power around the world

Developing a nuclear reactor, on the other hand, has never been a matter for barnstorming experimentation, partly because of the risks and partly because of the links to the technologies of the bomb. And whereas there are lots of things you can do with aircraft, more or less the only practical things you can do with a reactor are to make plutonium for bombs, power submarines, produce isotopes used in medicine and generate heat and electricity. Only the last is big business, and it can easily be done by other means. So options have been closed down and eggs have been piled into single baskets. The world went with pretty much the first sort of reactor it saw deployed at scale, contenting itself with increasing its size and trying, over the years, to render it ever less in need of the attentions of the axeman.