Saturday, March 26, 2011
Nuclear safety: Reactors that can't melt down ... Kelvin Kemm
Nuclear safety: Reactors that can't melt down Kelvin Kemm
The recent tragic events in Japan have brought the issue of nuclear energy to the forefront of public discussion. While some have exploited the tragedy to advance anti-nuclear policies, others have tried to defend this important energy source on the grounds of its importance to our economy and standard of living.
Missing in the discussion are several important facts. First, the 9.0 earthquake and 30- to 40-foot-high tsunami was a disaster of unprecedented proportion. It killed at least 10,000 people and possibly as many as 18,000.
Second, the Fukushima Daiichi 1 nuclear power plant withstood the quake, which released 32 times more energy than the plant was designed to absorb. But the tsunami came over the 25-foot-high seawall, carried off fuel supplies for the plant’s backup generators, shorted their circuitry and caused other damage, while also knocking out all remaining primary electrical power for dozens of miles. Even so, plant workers and other emergency crews avoided the kind of nuclear disaster many initially feared would occur.
Third, the Fukushima plant had many upgrades since it was first constructed. Numerous enhancements have been added to dozens of other nuclear reactors built since then, under that original design and newer designs.
Fourth, and equally important, significant breakthroughs in nuclear engineering continue to be made. They should now be vetted properly – as they could further reduce or even eliminate the threat of nuclear meltdown. To grasp the significance of these breakthroughs, informed citizens should have a basic understanding of how nuclear power technology developed over the years. The world’s first nuclear power plants began operating fifty years ago. Since then, nuclear power has advanced considerably, to the point that today some 16% of the world’s electricity is produced by nuclear power. France is the world leader, producing nearly 80% of its electricity from nuclear – and exporting a substantial amount of nuclear-generated electricity to countries like Italy and the UK.
As one might expect, nuclear power technology has improved dramatically over the last half century. In line with any technology development, various routes and options were examined, and rejected or implemented.
Because nuclear plant technology evolved from systems designed for nuclear submarines, early nuclear plants for generating electricity were engineered to be cooled by water. As a result, most of the world’s large nuclear power plants are situated on ocean coastlines or the banks of large inland rivers and lakes.
Basic nuclear power production physics involves a nuclear reaction that produces heat, which then converts water to steam. Most nuclear reactors use uranium as fuel. Pellets containing uranium are placed into tubes grouped in clusters, known as fuel elements. A number of fuel elements stand vertically in the core of the reactor, where they are covered by water. As uranium atoms are split via nuclear fission, the heat this reaction generates is extracted to convert water into steam.
The steam drives a turbine, which in turn drives an electrical generator. Water pumped from the ocean, river or lake cools the steam after it has passed through the turbine, condensing the steam back to water, so that it can be returned to the reactor heat source and reheated. From the early beginnings of reactor development, two branches of water reactor evolved.
In the Boiling Water Reactor (BWR), the water around and in contact with the fuel elements boils to produce steam, which then passes directly to the turbines.
In the Pressurized Water Reactor (PWR), water around the fuel elements becomes very hot but does not boil, because it is under pressure. This water then flows to a heat exchanger, which passes the heat to another water circuit that converts the second volume of water to steam. A PWR thus has two independent water circuits, and coolant water and steam never come in contact with the fuel elements.
Over the years the PWR has emerged as the preferred technology, and all modern water-cooled nuclear plants operate as PWRs. However, Japan’s 40-year old Fukushima Daiichi nuclear plant was a BWR design, and was approaching retirement. Sadly, before that happened, Japan suffered the worst earthquake and resultant tsunami in its recorded history.
The severe earth movement caused eleven Japanese nuclear power stations to shut down, as their design intended. However, the Daiichi plant was then hit by the tsunami’s massive wall of water. Together they destroyed electricity supply lines to the power station’s primary cooling pumps, while the tsunami knocked out the diesel powered backup systems. Batteries took over, but had a life of only eight hours.
As a result, although the reactors had been shut down successfully, their residual or “decay” heat was still enough to boil water to excessive pressure inside the reactors, in the absence of a functioning cooling system.
Reactor staff then had to release some of the steam to the atmosphere. With it went hydrogen gas, which mixed with air to produce an explosive mixture. That detonated in the outer building structures, blowing them open. The TV images were dramatic, even though the plant’s actual containment structures remained intact.
The reactor operators then had to resort to pumping sea water directly into the reactors to cool them, as their decay heat died away. Their actions appear to have worked, averting a serious nuclear disaster, even though some radiation was released on several occasions. Thus even this very old plant avoided a disaster.
However, over recent years, engineers have developed an innovative alternative nuclear reactor design, known as High Temperature Gas Reactors. Instead of water, they employ helium gas as a coolant. In South Africa, a similar reactor design was developed: the Pebble Bed Modular Reactor (PBMR). Its fuel is small tennis-ball-sized graphite balls containing granules of uranium, rather than large metal fuel elements. The balls cannot melt.
The PBMR design was developed to be “walk away safe,” which means that the nuclear reactor and its cooling system can be stopped dead in their tracks. The reactor cannot overheat, but will just cool down by itself.
A real-world trial of the reactor system was carried out in Germany, and the reactor cooled just as designed. The operating team really can walk away to have lunch, and the reactor will take care of itself in the event of an emergency shutdown.
As time passes, one would expect that BWR-type reactors will pass into the pages of history, as gas-cooled reactors and other more modern designs move to centre stage. In the meantime, though, steady improvements in nuclear power plant design and safety features have been implemented worldwide.
Nuclear power will likely be the world’s future power source, as nuclear technology continues to evolve. Many great minds have trodden the path of nuclear development over the last half century, and many more are following.
From the dry, dusty plains of Africa, a great contribution has been made toward even safer, more dependable nuclear power, with the development of a reactor type that does not have to rely on large volumes of water.
As Pliny the Elder said almost 2,000 years ago: “There is always something new out of Africa.” Nuclear power will one day power Africa, and the world – helping to lift billions out of poverty and ensuring that billions more continue to enjoy living standards that poor nations also deserve to have. __________ Dr. Kelvin Kemm holds a PhD in nuclear physics, is currently CEO of Stratek and lives in Pretoria, South Africa. He also serves as a scientific advisor to the Committee For A Constructive Tomorrow.