Boron Carbide.
Not a nuclear reactor!
This is used in the nuclear reactor that is known as Boiling Water Reactor (BWR) in which heat produced by the nuclear fission in the nuclear fuel allows the light water reactor coolant to boil. Then, the nuclear reactor moisture separator is used to increase the dryness of the produced steam before it goes to the reactor steam turbines.
Typically, Uranium-235 is used as fuel in nuclear reactors.
uranium
In most reactors in the world, water is used to absorb heat released from the reactor.
cadmium
cadmium
The term reactor refers to a structure in which material can be made to undergo a controlled nuclear reaction. They are used widely in industrial plants.
Not a nuclear reactor!
Boron is a very good absorber of neutrons, and it is used in control rods and as a soluble material sometimes, to adjust the reactivity state of the reactor.
Fuel pellets are typically clad in Zircalloy, if that was your question. Especially in water cooled reactors.
This is used in the nuclear reactor that is known as Boiling Water Reactor (BWR) in which heat produced by the nuclear fission in the nuclear fuel allows the light water reactor coolant to boil. Then, the nuclear reactor moisture separator is used to increase the dryness of the produced steam before it goes to the reactor steam turbines.
If you are talking about the Nuclear Reactors in Japan, they were damaged because when they lost power, the water pumps used to cool them stopped, and all of the nuclear material overheated.
Never a nuclear reactor is used in airplanes. However, it is used in submarines.
Boron is used inside a nuclear reactor inside a control rod which is used to 'soak' up the neutrons inside the nuclear reactor, a control rod can be used to control the rate of fission inside a nuclear reactor.
The pressurised water reactor (PWR)
fuel