In-vessel natural circulation alkali metal reactor system, purification system, and associated methods

A system, comprising:

• a nuclear reactor, comprising:
• a plurality of nuclear fuel elements;
• and
• a primary coolant loop configured to transport a liquid metal coolant to receive heat from the plurality of nuclear fuel elements;
• a cladding at least partially enclosing the nuclear fuel elements;
• and
• a nuclear reactor vessel system, comprising:
• an inner vessel that defines an inner volume sized to at least partially enclose the nuclear reactor;
• a pool of immersing fluid that occupies a volume inside the inner vessel;
• an outer vessel sized to wholly or substantially enclose the inner vessel;
• a core barrel positioned within the inner vessel;
• a riser positioned within the inner vessel;
• a barrier positioned within the inner vessel and enclosing the nuclear reactor, the core barrel, and the riser such that the barrier is submerged in the pool of immersing fluid;
• a downcomer positioned between the riser and an inner surface of the barrier;
• and
• a heat exchanger positioned in the downcomer and configured to transfer heat from the liquid metal coolant to an intermediate coolant or to a power conversion working fluid.