Nuclear reactor safety system
[2] They are typically composed of actinides, lanthanides, transition metals, and boron,[3] in various alloys with structural backing such as steel.
In addition to being neutron absorbent, the alloys used also are required to have at least a low coefficient of thermal expansion so that they do not jam under high temperatures, and they have to be self-lubricating metal on metal, because at the temperatures experienced by nuclear reactor cores oil lubrication would foul too quickly.
On the other hand, the standby liquid control (SLC) system (SLCS) consists of a solution containing boric acid, which acts as a neutron poison and rapidly floods the core in case of problems with the stopping of the chain reaction.
PWRs also use boric acid to make fine adjustments to reactor power level, or reactivity, using their Chemical and Volume Control System (CVCS).
[10][11] Emergency core cooling systems (ECCS) are designed to safely shut down a nuclear reactor during accident conditions.
An LPCI is an emergency system which consists of a pump that injects a coolant into the reactor vessel once it has been depressurized.
This system consists of a series of pumps and spargers that spray coolant into the upper portion of the primary containment structure.
It is designed to condense the steam into liquid within the primary containment structure in order to prevent overpressure and overtemperature, which could lead to leakage, followed by involuntary depressurization.
To prevent damage, motor-generators can be tied to flywheels that can provide uninterrupted electrical power to equipment for a brief period.
It also serves to trap fission products, especially those that are gaseous at the reactor's operating temperature, such as krypton, xenon and iodine.
For this reason, materials such as magnesium and zirconium are used for their low neutron capture cross sections.
The primary containment system is designed to withstand strong internal pressures resulting from a leak or intentional depressurization of the reactor vessel.
This is very common in BWRs because most of the steam systems, including the turbine, contain radioactive materials.
In case of a full melt-down, the fuel would most likely end up on the concrete floor of the primary containment building.
The Chernobyl plant didn't have a containment building, but the core was eventually stopped by the concrete foundation.
[citation needed] The ABWR has a thick layer of basaltic concrete floor specifically designed to catch the core.
