Nuclear Power Plant Radiation Safety Systems Designed to Prevent Catastrophes

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Researched by Industrial Info Resources (Sugar Land, Texas)--A nuclear power plant safety system is designed with two primary safety functions in mind: to contain radioactive substances and to cool the nuclear fuel. The safety systems comprise a sequence of barriers between the radioactive core and the outside environment. Each system has a backup plan and human-error defenses. Nuclear power plant safety systems account for approximately 25% of the cost of a nuclear power plant reactor.

There are three layers of defense to prevent the release of hazardous materials into the environment. The likelihood of one line of defense failing is slim, and the chance of all three failing is very remote.

The first line of defense is the fuel cladding, which is the outer layer of the fuel rod that contains the fuel pellets. During normal online nuclear operating activity, about 99% of radioactive fission products remain inside the uranium pellets as the fuel is burned. Fuel pellets are packed within the fuel rod and encased within cladding that is designed to contain the remaining 1% of radioactive material. These fuel rods, or collectively "the core," are contained within a steel vessel with walls that are up to ten inches thick. This vessel must be covered with enough water to provide cooling for the reactor; otherwise a breakdown could occur and subsequently cause a release of nuclear products. If the vessel were to overheat, 99% of nuclear product within the fuel pellets would be released. Extreme overheating would cause the fuel to essentially melt, hence the term "nuclear meltdown."

The second layer, or primary cooling system, is in place to ensure that enough cooling water is available in even the most extreme accidental situations should nuclear fission products be released from the fuel rods. If cooling water is insufficient for the active nuclear activity, control rods are inserted into the core to stop the nuclear fission process, this is known as a "scram." Although there is no new radiological activity, "decay heat" is being released from the remaining nuclear product. Decay heat must continually be removed to prevent damage to the reactor core. Safety systems are put into action with emergency backup systems on standby to make certain the task can be performed, even after the reactor is shut down and the nuclear process has stopped. If these two barrier systems should fail there would be a release of radioactive material into the third barrier, known as the containment.

The containment building is the large-domed structure seen at nuclear power plants and is made from high-density, reinforced concrete that is up to six feet thick. The structure is engineered to withstand various natural and manmade disasters and is designed to be the last defense should nuclear radiation escape the core and the primary cooling system. All three barriers are inspected frequently. The core is monitored by measuring the amount of radioactivity in the primary cooling water. The cooling water system is monitored by the measurement of water loss and leakage, and the containment building is checked for radiation leakage, concrete fracturing or loss of density.

Nuclear power is a clean, safe power source with a low probability of accidental radiation exposure. There have been three major nuclear power plant accidents: Three Mile Island, which was contained and believed to have not caused harm to anyone; Chernobyl, which involved an intense fire and loss of radioactive material containment; and Fukushima Daiichi, which severely tested the integrity of the containment structure and allowed a small release of radioactive material. These three accidents are the only major incidents to occur in more than 14,000 cumulative years of commercial nuclear operation in 32 countries.

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