Safety Concerns on Reactor Vessels of Nuclear Power Plants

Nuclear power is a proven technology for generating electricity all over the world. The increasing demand for nuclear energy is brought about by increase in population, concern over the greenhouse gas emissions produced by burning of fossil fuel to generate electricity and improved living standards that require more electricity. In nuclear power plants, the reactor vessels contain the nuclear fuel and they are made of thick steel plates that are welded together for the safety and protection of the workforce and the public.

Issues faced with reactor vessels of power plants

Embrittlement

One of the major issues faced by a power plant is embrittlement that occurs at the beltline which is the section of the reactor vessel that is closest to the nuclear fuel. Neutrons from the nuclear fuel irradiate the reactor vessel during operation. Embrittlement causes the steel from which the reactors vessels are made of to become less tough and less capable of withstanding flaws that may be present.

Pressurized water reactors are more prone to embrittlement than boiling water reactors since they normally experience less neuron irradiation. Utilities that make use of pressurized water reactors use core designs that reduce the number of neurons that will reach the vessel walls so as to reduce the potential for embrittlement. Steel with high proportion of copper and nickel are more susceptible to embrittlement than steel with a lower proportion of these components.

Another reason for the embrittlement of reactor vessels is pressurized thermal shock usually experienced by pressurized water reactors. Pressurized thermal shock occurs when cold water is accidentally introduced into the reactor vessel while it is still pressurized. The introduction of cold water will cause the vessel to cool rapidly resulting into large thermal stresses on the steel. These thermal stresses can cause the reactor vessels to crack and fail.

Primary water stress corrosion cracking

Primary water stress corrosion cracking is a safety concern because a nozzle with sufficient cracking could break off during operation. This would compromise the integrity of the reactor coolant system pressure boundary which is one of the primary barriers that protect the public from exposure to radiation. A break could also result into the ejection of the control rod which could damage nearby components.

The reactor vessel is the key component of a nuclear power plant which is considered as highest priority because it is irreplaceable which means that if their mechanical properties degrade, they can be the life-limiting feature with direct safety significance. Should a leak develop at or below the level of the core and the coolant leak path be greater than the maximum flow capable of being supplied by the emergency core cooling systems, then the reactor core could be uncovered and overheat. Massive failure of the reactor vessel could seriously damage the reactor core which can lead to safety concerns. Even if there is a low probability for these scenarios to happen, the main concern should always be on the strength and resistance of the reactor vessel. Structural integrity of the reactor vessels must be assured throughout its operating life particularly on the welds and heat affected zones with the highest probability for flaws.