Abstract: An apparatus for cleaning up reactor coolant includes a piping for taking out water in a reactor core, and this piping is provided separately from a primary loop recirculation piping and has its open end located at the bottom of a reactor pressure vessel. The taking-out piping extends upward from the open end within the reactor pressure vessel and extends through and outside of the side wall of the reactor pressure vessel. In a preferred form, the taking-out piping is provided near the highest position of its portion outside the reactor pressure vessel with a siphon brake valve.

Abstract: A natural circulation type boiling light-water reactor comprises a pressure vessel, a reactor core disposed within said pressure vessel to heat light water so as to generate main steam, a steam outlet provided in a wall of said pressure vessel for drawing out the main steam therethrough to supply the same to a turbine system, and a steam path along which the main steam flows from the reactor core to the steam outlet. The reactor further comprises a steam guide for prolonging the steam path and/or a radially increased chimney for reducing the velocity of the main steam flowing through the steam path. These causes radioactive isotope .sup.16 N contained in the main steam to flow within the pressure vessel for a time period exceeding its own half-life, so that the inventory of .sup.16 N in the main steam directed towards the turbine system is reduced remarkably. In consequence, shield structures for piping and turbine systems can be reduced in size and weight.

Abstract: The nuclear reactor container of a boiling water reactor has a dry well and a pressure suppression chamber. The level of the radioactivity in the cooling water filling the pressure suppression chamber is measured by a liquid radiation monitor. An iodine monitor measures a level of the radioactivity of the iodine in the space above the surface of cooling water in the pressure suppression chamber, while a noble gas monitor measures a level of the radioactivity of noble gas in the same space. Outputs from the liquid radiation monitor, the iodine monitor and the noble gas monitor are delivered to an accident judging device which makes judgement as to occurrence of perforation of fuel rods and melt down of fuel rods in the event of a Loss Of Coolant Accident (LOCA), the result of which is displayed at a display device.