|Fig. 1: The reactor chamber of a Pressurized Water Reactor (Courtesy of the NRC)|
In the wake of incidents like Three Mile Island, Chernobyl, and Fukushima, the safety of nuclear reactors is always scrutinized. As with any complex and potentially dangerous technology, misinformation abounds in relation to the safety of nuclear reactors. This paper aims to study the most common type of nuclear reactor in the United States, the "Pressurized Water Reactor", and explain the safety systems that prevent modern power plants of this type in the United States from experiencing critical failures.
America is home to approximately 99 operational nuclear reactors as of 2015, and of these, about two thirds are designated as "Pressurized Water Reactors."  The Pressurized Water Reactor is the most common type of nuclear reactor globally, and is named for the cooling water, which flows through the reactor's core at very high pressures.  Pressurized Water Reactors are the most common type of nuclear power plant across the world, in no small part because they use a number of safety systems to maintain stable and safe operation.
High profile nuclear incidents have illustrated engineering deficiencies in reactor designs, meaning that modern nuclear reactors are safer than those that preceded them. For example, in the year 2012, the U.S. Nuclear Regulatory Commission voted to approve construction of two Westinghouse AP1000 pressurized water reactors.  These reactors " employ no pumps, fans, or other rotating machinery and do not require any AC power. Any valves in these systems require power to stay closed. Should power be interrupted or otherwise fail, safety valves open automatically using mechanical force."  Additionally, the plant's cooling water sits above the reactor core, and would "in a case like the Fukushima meltdown" automatically flow into the core for cooling, enabled by a heat sensitive valve that requires no operator input.  Westinghouse claims that their AP1000 reactor has enough water cooling capacity to survive for up to three days without power or human intervention. 
Additional safety measures come from the basic design principle of the pressurized water reactor. These reactors use negative feedback effects involving steam to automatically regulate temperatures. If reactor coolant system temperature starts to increase, the density of the reactor coolant will decrease, and the water will take up more space. Since the pressurizer is connected to the reactor coolant system via the surge line, the water will expand up into the pressurizer. This will cause the steam in the top of the pressurizer to be compressed, and therefore, the pressure to increase. A Congressional Research Service report for members of Congress expands on this effect: Water in the reactor core reaches about 325°C, hence it must be kept under about 150 times atmospheric pressure to prevent it boiling.  Pressure is maintained by steam in a pressurizer (see diagram). In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type.
In addition to the negative feedback properties of the cooling and moderation systems, an emergency measure that can be used in Pressurized Water Reactors is the release of large quantities of borated water.  This water can be dispensed by a number of different systems, some of which operate under electric power, which can be replaced by diesel generators if the plant looses power. Other systems for dispensing borated water do not rely on any power. These include the storage of the water in tanks above the reactor core, as discussed above, in which valves keeping the water in the tank are powered closed, and will mechanically open in the event of a power loss. An additional method of power-free borated water dispersion comes in the form of large tanks containing the water and a nitrogen gas bubble at the top.  If the pressure of the cooling system drops too much, the pressure of the gas will force water out of the tanks and cool the reactor.
As nuclear power transitions into the 21st century, it takes with it the stigma of incidents that bear clearly in the memories of the public. However, using systems that are designed to protect a power plant without power and without operator intervention means that these incidents will stay in the past. Pressurized water reactors use more than 100 support systems, active and passive, to ensure the safe and reliable generation of power.  Although not every situation can be accounted for, reactors can be designed to maximize their ability to react to new and unexpected scenarios.
© Reed Kraus. 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.
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