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| Fig. 1: Typical Boiling Water Reactor (Source: Wikimedia Commons) |
Safety systems for nuclear power plants are designed to protect against accidents which can be caused by situations inside and outside the plant. Some outside causes of accidents could include natural disasters such as earthquakes, tornados, floods, and tsunamis whereas inside accidental situations are also possible due to system malfunctions and even human error. The design of safety systems to handle these types of situations are called design basis accidents (DBAs). [1] In the United States of America there are two main types of nuclear power plants: boiling water reactors (BWRs) and pressurized water reactors (PWRs). [1] In 2012 there were a total of 104 nuclear power reactors in the U.S. of which 69 were pressurized water reactors and 35 were boiling water reactors. [1] Both types of reactors have specialized safety systems to deal with situations unique to the reactor type.
Boiling water reactor systems (see Fig. 1) have a relatively simpler design when compared to other light water reactors. [1] A boiling water reactor system creates steam as water flows through the reactor core and, in turn, the water also cools the reactor core itself. [1] A very important issue to keep in mind for boiling water reactors is the lack of coolant. In this event, also known as a loss of coolant accident (LOCA), the reactors core will continue to heat up and ultimately could lead to a meltdown. [1] To protect from this, the reactor has an emergency core cooling system (ECCS) which quickly cools down the reactor core using water. [1] The emergency core cooling system is independent and does not need its own auxiliary power for electricity. [1]
Unlike boiling water reactor systems, a pressurized water reactor system creates steam on the outside of the reactor (see Fig. 2). [1] This difference means that one mechanism transports water that has been heated from the reactor core to an external steam generator. [1] Afterwards the steam is transported to a turbine generator which is powered to generate electricity. [2] Pressurized water reactors generate about 65,100 net megawatts of electricity. [2] The emergency core cooling system in pressurized water reactor systems is different from the emergency core cooling system in boiling water reactor systems, in fact the PWR ECCS utilizes four different components. [1] Ultimately these different components pump water to the reactor core in order to cool the system but must do so while accommodating different pressurized environments. [1]
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| Fig. 2: Typical Pressurized Water Reactor (Source: Wikimedia Commons) |
As discussed earlier, seismic activity can be very harmful to nuclear reactors. Reactors are therefore designed to have a series of requirements such as the ability to maintain the reactor pressure system, the ability to shut down in a safe way, and the ability to prevent external exposures by containing possible accidents. [1] On March 11, 2011 the Fukushima Daiichi nuclear accident occured which was caused by the Great East Japan Earthquake and the following tsunami. [3] As a consequence, the U.S. saw a series of hearings, bills, and new safety standards concerning power plant safety and earthquake safety that were introduced in the U.S. [1] Some examples include the Natural Hazards Risk Reduction Act of 2011, which amended the Earthquake Hazards Reduction Act of 1977, and the Nuclear Power Licensing Reform Act of 2011. [1]
There is a lot to consider when designing safety systems for nuclear power reactors. With so many types of designs and so many hazardous factors to consider it is an ongoing challenge to make sure energy production is done as safely as possible.
© Leon Bi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. P. Argyriou, Nuclear Power Plants: Design and Safety Considerations (Nova Science, 2012).
[2] S. Shaw, "Advantages of Pressurized Water Reactors (PWR)," Physics 241, Stanford University, Winter 2017.
[3] Lessons Learned From the Fukushima Nuclear Accident for Improving Safety of U.S. Nuclear Plants (National Academies Press, 2014).
