Abstract: A nuclear reactor start-up method of starting up a boiling water reactor which is in the cold shut-down state. The method includessetting a subcooled temperature of a coolant in a reactor core inlet to a range smaller than a critical subcooled temperature which is determined by the condition whereby the coolant starts boiling in the reactor core and the condition whereby the instability due to a periodic variation of buoyancy begins,thereafter starting an increase of power of the reactor.

Abstract: An apparatus for purifying sodium by electrically removing oxygen combined with liquid sodium from the liquid sodium, which comprises a liquid sodium to be purified and a reducing agent, the liquid sodium and the reducing agent being partitioned by a wall of solid electrolyte, a negative electric potential and a positive electric potential being applied to the liquid sodium side to be purified and the reducing agent side, respectively, whereby the combined oxygen is passed through the wall of solid electrolyte in a form of oxygen ion, O.sup.-.sup.-, and the liquid sodium is purified.