A reactor vessel in a boiling water reactor is approximately 300 tons.

'Stirred vessel'? Not an expression I know.

for pressurized light water reactor type, as an example, the nuclear reactor components are

* Reactor vessel (that contains the nuclear fuel and surrounded with water and contains control rod for power control and for safety) * reactor coolant pump * steam generator * reactor pressurizer

* piping out of the vessel to the pressurizer, from pressurizer to steam generator, from steam generator to reactor coolant pump, and from pump back to the reactor vessel.

Nuclear fission takes place in the nuclear fuel rods that are placed in the reactor core that is situated in the reactor pressure vessel. The reactor pressure vessel is usually situated inside the reactor containment.

Uranium-235