Replacement technology of in-core monitor (ICM) housing of BWR was developed and applied in the operating Plant. This technology replaces stress corrosion cracking (SCC) susceptible existing ICM housing by new housing composed of SCC resistant material. Major procedure steps are removal of the existing ICM housing, welding and trimming of a new weld buildup, welding of a new ICM housing, and joint of a new ICM guide tube with the ICM housing using shape memory alloy couplings. Each procedure step is performed remotely with automated tools, which are installed from the reactor refueling floor, 26m down to the Reactor Pressure Vessel bottom, through core plate etc. After several years of development, this technology was applied to an ICM housing of Fukushima Daiich Unit #4 where a crack caused by SCC was found on October 1997. and site work was successfully completed in the end of January 1998.

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About 50g of sodium was burnt on a 0.15m×0.20m stainless-steel pan sitting on an electric heater and measurement of sodium pool (or pan) temperature was made. The maximum temperature was 760°C on average in the open air. The pan was placed in an 80l combustion chamber with a lid of 0.1m×0.1m uncovered area (6.3% to the bottom) and mixture of oxygen and nitrogen gases was injected from the bottom at a flow rate of 80l/min. With decreasing the oxygen concentration, the sodium combustion was suppressed and the temperature went down. And the pure nitrogen gas injection worked effectively, leading to fire extinguishment after nearly the same volume of nitrogen as the chamber was fed. Injection of liquid nitrogen into 15l chamber was demonstrated to be clearly effective in ceasing fire, without causing vapor explosion.

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High performance control rods are required to improve the high temperature gas cooled reactor(HTGR) performance. In present study, a dummy control rod, with the same design parameter of HTTR (High Temperature enginnering Test Reactor), was fabricated with two dimensional reinforced C/C composites to testify productivity of the high performance control rod for HTGR with these composite materials and clarify its mechanical properties. Dummy control rod elements, that are, sheath elements consisted of outer tube(outer diameter×length×thickness=113×300×4mm) and inner tube (75×300×7.5mm), and joint elements of the screws(M16 and M8), were fabricated and tested on its mechanical properties.
From these results, the following conclusions were derived:
(1) High performance control rods for HTGR can be fabricated by the conventional manufacturing process of C/C composite materials.
(2) Mechanical strength of sheath elements reached to the design strength level of the materials, however, fiber enforcement to the radial direction of the sheath is to be required to improve tensile and buckling strength of the tubes, and high strength of the screw elements was obtained.

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