All nuclear power plants operated in the Western world use a ‘defence-in-depth’ approach to safety and design. They require that systems are designed with the following features:
- High-quality design and construction
- The use of equipment to prevent or minimize the effect of operational disturbances or human errors.
- Comprehensive monitoring and testing to detect equipment and/or operator failures.
- The use of diverse and redundant systems to control damage to the fuel and to prevent significant radioactive releases.
- Provision to confine the effects of severe fuel damage to the plant.
These provisions can be summed up as: “Prevention, Monitoring and Action”. They are intended to mitigate the consequences of system failures.
The most basic safety functions in a nuclear reactor are to:
- Control reactivity,
- Cool the fuel and
- Contain radioactive substances.
There are several barriers between radioactive reactor cores and the environment.
Nuclear fuel is typically in the form of solid ceramic pellets. These pellets keep the radioactive fission products largely bound within them. Sealed zirconium tubes in the form of fuel rods house the pellets themselves.
A steel pressure chamber with walls up to 30cm thick confines these fuel rods. And then a reinforced concrete containment structure encloses all of this. The walls of this structure are at least one meter thick.
This system alone provides a highly effective multi-layered system of barriers. These barriers shield those working in the plant and the environment nearby from almost all the radiation.
But, just in case any one of them fails, workers constantly monitor them in real-time to immediately detect any failure.
Cool the fuel.
The workers monitor the barriers continually. They measure the amount of radioactivity in the cooling water to monitor the fuel cladding.
In addition, they monitor the high-pressure cooling system by measuring the leak rate of water. And, they monitor the containment structure by periodically measuring the leak rate of air at about five times the atmospheric pressure.
But the system also has inherent safety features. As the temperature of the reactor rises, the efficiency of nuclear reactions decreases. Some new designs use this to control power levels.
Furthermore, if steam forms in the cooling water, fewer neutrons can cause fission, fewer fission events take place and the reaction rate automatically decreases.
Control rods can be used to absorb neutrons and thus slow the reaction rate as required. Excess heat during normal operation can also be removed using emergency core-cooling systems which themselves have backups.
Active systems used to be the most common. These had many key elements which required an active electrical or mechanical operation. So that if the mechanical parts ceased to operate or if there was an electrical failure, those systems would cease to operate.
However, more modern designs employ passive systems that do not rely on the functioning of engineered systems. They rely on gravity, pressure or some other physical phenomenon and can thus activate when there is mechanical or electrical failure.
Such a system can operate when there is a total loss of electrical power. Such a cooling system which could activate despite a loss of power would have averted the Fukushima accident.
Contain radioactive substances.
We shall see that designers design power plants to withstand high-level natural disasters. They design them to properly contain any radioactive material from being released into the surrounding environment, in case of a natural disaster. This, of course, involves multiple robust and redundant systems.
Many people believe that terrorists can hijack nuclear power plants and use them for their nefarious purposes. While designers and workers cannot guarantee that nuclear power plants are impervious to terrorist invasion, they follow sufficient procedures to prevent all but the most extreme forms of terrorism.
Firstly, designers make nuclear power plants to be highly impervious to catastrophes. They house reactors in extremely robust containment buildings. In addition, they make the buildings with reinforced concrete containment structures and equip them with multiple and redundant plant shutdown systems.
They design systems to withstand the impact of hurricanes, tornadoes, floods and airborne objects of sufficient magnitude. Furthermore, they design facilities to withstand aircraft impacts.
Designers add several layers of protection surrounding the radioactive core. All of the layers must be breached for radiation to be released. Typically 1.2 (4 feet) meters of reinforced concrete with a lining of steel protects the containment structure alone. And steel that is typically 150mm (6 inches) in diameter makes up the reactor vessel.
Secondly, nuclear power plants employ armed security guards 24 hours of the day. They highly train their security guards and equip them with sophisticated electronic surveillance equipment which scans the area surrounding the plant.
The security guards engage in continual mock drills to prove that they can protect against paramilitary forces intent on sabotaging nuclear power plants.
Nuclear power plants have trained guards to deal with terrorists armed with automatic weapons and explosives, as well as terrorist forces armed with insider information that could aid them in their attack.