Generation IV reactor

About
A Generation IV reactor is a nuclear reactor design that is under research and has not been constructed yet. The designation originates from the United States Department of Energy as a classification of new reactor designs, and has since been taken up by the international community.

Briefly, the generations of nuclear reactor design are:
 * Generation I: Prototype fission reactors, such as Shippingport and Fermi 1
 * Generation II: Power reactors in current use, including LWRs, heavy water reactors like CANDUs, and advanced gas-cooled reactors
 * Generation III: Evolutionary improvements over Gen II designs
 * Generation IV: Reactor designs under current research

Six Gen IV reactor designs have been designated.

Thermal reactors

 * Very high-temperature reactor (VHTR): This is a solid fuel reactor using the uranium fuel cycle, but unlike LWRs, uses molten salts or helium as the core coolant. This enables a much higher heat output than LWR designs, theoretically up to 1000 degrees C.
 * Supercritical water reactor (SCWR): This is a light water reactor under additional pressure and operating at higher temperatures.
 * Molten salt reactor (MSR): This is a reactor where the primary coolant is a molten salt mixture. Both solid-fuel and liquid-fuel variants of the MSR have been proposed.

Fast reactors

 * Gas-cooled fast reactor (GFR): This is a fast spectrum reactor that uses the plutonium fuel cycle and helium as the core coolant.
 * Sodium-cooled fast reactor (SFR): This reactor is cooled by liquid sodium and uses a mixture of uranium and plutonium metals as fuel. While this design, like the MSR, operates at atmospheric pressure, care must be taken to isolate the sodium from moisture.
 * Lead-cooled fast reactor (LFR): This reactor is cooled by a eutectic mixture of lead and bismuth. It has excellent shielding and fuel breeding characteristics, but the use of toxic lead as the coolant is a large concern.