Nuclear safety: Reactors that can’t meltdown

The recent tragic events in Japan have brought the issue of nuclear energy to the forefront of public discussion. While radical environmentalists have exploited the issue 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 is the fact that important breakthroughs in nuclear engineering should now be given proper vetting – developments that could reduce or eliminate the threat of nuclear meltdown.

In order to grasp the significance of these breakthroughs, a basic understanding of the development of nuclear power technology is in order.

The first nuclear power plants in the world started operating fifty years ago. Since then nuclear power has advanced considerably to the point at which today some 16% of the world’s electricity is produced by nuclear power. In this respect France is the world leader producing nearly 80% of its electricity from nuclear. In fact France exports a substantial amount of nuclear-generated electricity to countries such as Italy and the UK.

Over the last half century the technology of nuclear power has improved dramatically, as would be expected of any technology that evolves over a fifty year period.

As with any technology development, various routes and options were examined and implemented. In the case of nuclear power reactors their technology essentially developed from the early power packs designed for nuclear submarines. This led to reactors that are cooled by water.

As a result, most of the world’s large nuclear power plants are situated on a coastline or on the banks of large inland lakes.

The basic physics of nuclear power production is that a nuclear reaction produces heat which then converts water to steam to drive a turbine, which in turn drives an electrical generator. Cool seawater is then pumped from the sea, to cool the steam after it has passed through the turbine. The cool water condenses the steam back to water, and this fresh water is then returned to the reactor heat source to be heated again.

Most nuclear reactors use uranium as fuel. Uranium atoms are split by a process called nuclear fission which gives off heat in the process. It is this heat that is extracted to convert water into steam.

Pellets containing uranium are placed into tubes which are grouped into clusters, known as fuel elements. A number of fuel elements stand vertically in the core of the reactor, where they are covered by water. The water removes the heat produced by the fuel elements.

From the early beginnings of reactor development two branches of water reactor evolved, the Boiling Water Reactor (BWR) and the Pressurised Water Reactor (PWR). In the BWR the water around the fuel elements boils to produce steam. This steam then passes directly to the turbines.

In the PWR the water around the fuel elements does not boil, it becomes hot, but does not boil because it is under pressure. This water then flows to a heat exchanger where the heat passes to another water circuit which converts the second volume of water to steam. Thus a PWR has two independent water circuits.

Over the years the PWR has emerged as the better technology, and all modern water-cooled nuclear plants operate as PWR’s. The Fukushima Daiichi nuclear plant, in Japan, was a BWR plant, with only one water circuit. This nuclear power plant was some forty years old, and was approaching retirement.

Sadly for Japan, the country suffered the worst earthquake and resultant tsunami of its recorded history. The earth movement caused eleven Japanese nuclear power stations to shut down, as their design intended, but in the case of Fukushima it was the tsunami’s wall of water that did the real damage. The earthquake destroyed the electricity supply lines to the power station’s primary cooling pumps. Diesel pumps then kicked in, only to be knocked out as the tsunami water washed away their fuel tanks. Batteries took over, but the batteries only had a life of eight hours.

The result was that although the reactors had been shut down successfully the residual heat, known as decay heat, was still enough to boil water to excessive pressure inside the reactors.

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, which detonated in the outer building structures, blowing the buildings open. The TV images were dramatic.

The reactor operators then had to resort to pumping sea water directly into the reactors to cool them, as their decay heat died away.

But over recent years an innovative alternative nuclear reactor design has evolved. This family of reactors is known as High Temperature Gas Reactors. They do not use water as a coolant, but use helium gas. In South Africa such a reactor design was developed, known as the Pebble Bed Modular Reactor (PBMR). Its fuel is small tennis ball size graphite balls containing granules of uranium, and not large metal fuel elements. The balls cannot melt.

The PBMR design was developed to be ‘walk away safe’ which means that the nuclear reactor plus its cooling system, can be stopped dead in its tracks and the reactor cannot overheat, it will just cool down by itself. A real 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.

So as time passes one would expect that BWR-type reactors will pass into the pages of history, as more modern designs such as gas-cooled reactors move to centre stage.

Nuclear power will be the world’s power source of the future, as the evolution of nuclear technology continues. 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 towards nuclear power development, with the development of a reactor type that does not have to rely on great volumes of water. As Pliny the Elder said “There is always something new out of Africa.” Nuclear power will be the power source to power Africa, and the world.

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About the Author: Kelvin Kemm

Kelvin Kemm

Dr Kelvin Kemm is the CEO of Nuclear Africa, a nuclear project management company based in Pretoria, South Africa. He is a member of the International Board of Advisors of CFACT. Dr. Kemm received the prestigious Lifetime Achievers Award of the National Science and Technology Forum of South Africa.