The explosions of reactors at Japan's Fukushima Daiichi nuclear power station following the 9.0-magnitude earthquake and tsunami on March 11 have caused countries around the world to question the safety of their own nuclear plants.

The graphics of a nuclear plant [45nuclearplants.com]

According to BBC, the function of a nuclear power plant is to make electricity. It does this the same way as coal, oil, and natural gas power plants do; namely, it heats water into steam in order to turn a generator. The difference is the way that it heats water.

Instead of burning fossil fuel, a reactor will heat water by bringing pellets of uranium close together until they form a critical mass. When uranium decays, neutrons and heat are released. If enough uranium is near by, then the neutrons will run into other uranium atoms, causing them to split apart, or fission, and release more neutrons and more heat. Nuclear reactors submerge the fissioning uranium in water. The fuel heats up the water, and the water boils and turns to high pressure steam, which turns the turbine.

There are two main types of reactors in use today: Boiling Water Reactors (BWRs) and Pressurized Water Reactors (PWRs).

In PWRs, the reactor and associated pipes and pumps are called the Reactor Coolant System (RCS), or the 'Primary Loop'. The RCS is a closed system in that nothing in the system is allowed to escape during normal operation, as it would be radioactive. Water in the RCS is not allowed to boil. Instead, the water is pressurized to prevent it from boiling. While pressurized, the water can now be heated to a temperature of several hundred degrees, and is sent to a steam generator.

The steam generator has two types of water in it: primary loop water, and secondary loop water. The two types of water are never allowed to mix. The primary water enters the steam generator through hundreds of small tubes. The tubes heat up and causes the secondary water to boil and turn to high pressure steam, which is used to turn the turbine and create electricity. The advantage of PWRs is that the RCS is the only part of the plant that is radioactive.

In BWRs, water in the RCS is boiled directly. The RCS water itself is turned to steam and is used to turn the turbine. The advantage of this system is that there are no steam generators involved, which means that there is less complexity, fewer components, and less maintenance to perform. The disadvantage is that the turbine is now contaminated with radioactivity.

The safety and security of the nuclear plants:

Each nuclear plant design features reliable and diverse safety systems and strong physical barriers to prevent incidents that could pose a threat to public health and safety. The same features that safeguard the public and the environment from a radiation release also defend the reactor from outside interference.

The reactor is typically protected by about four feet of steel-reinforced concrete with a thick steel liner, and the reactor vessel is made of steel about 6 inches thick. Steel-reinforced concrete containment structures are designed to withstand the impact of many natural disasters, including hurricanes, tornadoes, earthquakes and floods, as well as airborne objects with a substantial force.

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