Fukushima Daiichi Nuclear Power Station Unit-5(BWR 784 MWe) is located in Futaba-machi area north to Okuma-machi area where Unit-1--4 are located. Construction of this unit was started on Dec. 1971 before Unit-4 construction. Commercial operation was started on April 4, 1978 and keeps operation with no trouble. With completion of the unit, capacity of Tokyo Elec. Power Co. four nuclear power plants become 2, 812MWe, which is about 10% of whole Tepco's electric power plant's capacity. Here are the summary of the plant facility and the operating experience.

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Two sets of bellows type expansion joints were tested using a sodium test apparatus. After this test being done, mechanical and metallurgical examinations were performed.
The bellows of these joints are of a single ply welded type. Each joint has two bellows, one of which contacts directly with sodium forms a pressure boundary, and the other is installed as a backup of the former one.
The test was conducted at 500°C and 2 atg of sodium temperature and pressure respectively, and 26, 600 cycles of expansion and contraction were continued alternatively for 700 hr in succession. During this test, neither leak nor deteriorated point were found.
After the sodium test, mechanical and metallurgical examination such as strength of welded parts, chemical composition, electro structure and X-ray analysis were conducted. In this examination, a slight carburization and decromium were found, however, we would not believe that it will lead to a definite fault.

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Sumikathene, Faifax and Marlex, which have different densities and crystallinities, were irradiated by a 1.8MeV electron beam, and the gel fraction for each material was measured. The radiation dosages used were 100Mrad and below.
The gel fraction increased sharply with dosage up to 20Mrad, then it reached a constant value. The ge1 fraction of Sumikathene only reached a saturated value of about 80%, but the fractions of the other two polyethylenes had not yet reached a constant at 100Mrad.
By introducing the factars of crosslinked polyeythylene (crosslinking coefficient α, decay coefficient β), which are functions of irradiation temperature, duration and atomosphere, density and crystallinity of polyethylene, and injected electron energy, a general formula was formed. The results obtained by calculation were very close to the experimental results. By calculation, the crosslinking ratios of Faifax & Marlex reached the values of 83% (at 180Mrad) and 87% (at 400Mrad), respectively. The gel fraction was used for the measurement of the crosslinking ratio.

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In determining the concentration of gaseous β-emitters in air with an ionization chamber, the ratio of the energy lost in the chamber as an ionization current to the initial energy of the β-particles must be known. This ratio is called ionization efficiency in this paper. This paper presents the calculation of ionization efficiency by the Monte Carlo method for two types of ionization chamber, parallel plate chamber and cylindrical chamber, filled with air and gaseous β-emitters such as ⁸⁵Kr, ⁴¹Ar, ¹³³Xe, ¹³¹I, ¹³N and ¹⁵O. The results of the calculations for the parallel plate chamber agreed well with those obtained by experiments. The results for the cylindrical one agreed with those by other workers.

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