A nuclear reactorcoolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment. Frequently, a chain of two coolant loops are used because the primary coolant loop takes on short-term radioactivity from the reactor.
As the hydrogen atoms in water coolants are bombarded with neutrons, some absorb a neutron to become deuterium, and then some become radioactive tritium. Water contaminated with tritium sometimes leaks to groundwater by accident or by official approval
The fuel rods create high temperatures which boil water then turn water to steam. During a disaster, when a power outage happens and diesel power generators which provide emergency power to the water pump are damaged by a tsunami or an earthquake, if no fresh water is being pumped to cool the fuel rods then the fuel rods continue to heat up. Once the fuel rods reach more than 1200 degrees Celsius, the zirconium tubes that contain the nuclear fuel will interact with the steam and split the hydrogen from the water. That hydrogen can then be released from the reactor core and containment vessel. If that hydrogen accumulates in sufficient quantities-concentrations of 4 percent or more in the air, then that hydrogen can explode, as has apparently occurred at Fukushima Daiichi reactors No. 1, 3, 4 but reactor No. 2 opened its vent to let out radioactive hydrogen gas, decreasing the pressure of the hydrogen, but it contaminated the environment, so reactor No. 2 did not explode
Borated water
Borated water is used as a coolant during normal operation of pressurized water reactors as well as in Emergency Core Cooling Systems of both PWRs and boiling water reactors.
Advantages
, often in the form of boric acid or sodium borate, is combined with water — a cheap and plentiful resource — where it acts as a coolant to remove heat from the reactor core and transfers the heat to a secondary circuit. Part of the secondary circuit is the steam generator that is used to turn turbines and generate electricity. Borated water also provides the additional benefits of acting as a neutron poison due to its large neutron absorption cross-section, where it absorbs excess neutrons to help control the fission rate of the reactor. Thus, the reactivity of the nuclear reactor can be easily adjusted by changing the boron concentration in the coolant. That is, when the boron concentration is increased by dissolving more boric acid into the coolant, the reactivity of the reactor is decreased. Conversely, when the boron concentration is decreased by adding more water, the reactivity of the reactor is increased.
Disadvantages
Approximately 90% of the tritium in PWR coolants is produced by reactions of boron-10 with neutrons. Since tritium itself is a radioactive isotope of hydrogen, the coolant becomes contaminated with radioactive isotopes and must be kept from leaking into the environment. Additionally, this effect must be taken into account for longer cycles of nuclear reactor operation and thus requires higher initial concentration of boron in the coolant.
