[en] 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

[en] The No. 1 and No. 4 Units of Fukushima-Daiichi Nuclear Power Station (1F-1 460MWe BWR-3; and F1-4 784MWe BWR-4) started commercial operation in 1970 and 1978, respectively, and are now operating the 12th and 9th fuel cycles, respectively. A Radiation reduction program for 1F-1 and 1F-4 has been implemented since 1979. This report describes the effectiveness of radiation level improvement that has been executing for 1F-1 and 1F-4. The following improvements have been made to reduce the radiation level: (1) increase in frequency of condensate demineralizer backwash, (2) low liner velocity of condensate demineralizer (only in 1F-1), (3) replacement of condensate demineralizer (only for anion resin in 1F-1, cation resin used continuously), (4) installation of hallow fiberfilter (50 % of condensate flow rate in 1F-1, 33 % of condensate flow rate in 1F-4), (5) improvement of lay-up during plant shutdown, and (6) replaced low cobalt materials. As a result, iron concentration in feedwater, radioactive crud concentration in reactor water and hot spots of radiation level have been reduced. But the level of 1F-1 and 1F-4 are still higher than that of new plants. It is necessary to investigate the effective radiation level improvement further. (Nogami, K.)

[en] Persistent Photoconductivity (PPC) in hydorogen-ion implanted (001) oriented KNbO₃ bulk single crystals (perovskite structure at room temperature; ferroelectric with a band gap of 3.16 eV) is studied in air at room temperature to prevent the crystallinity degradation caused by the phase transition. Hydrogen is implanted into KNbO₃ bulk single crystals using the energy (the peak ion fluence) of 500 keV (5.0 × 10¹⁵ cm^-2). The resistivity varies from ∼10⁸ Ω/□ for an un-implanted KNbO₃ sample to 2.3 × 10⁵ Ω/□ for as-implanted one. suggesting the presence of donors consisting of hydrogen interstitial and oxygen vacancy. The PPC is clearly observed with ultraviolet and blue LEDs illumination rather than green and infrared, suggesting the release of electrons from the metastable conductive state below the conduction band relating to the charge states of the oxygen vacancy as observed in electron irradiated ZnO. (paper)