The difference in the background situations among leading LWR countries is pointed out first and their concepts to plan the extension of plant service term, therefore, differ each other. In Japan, there is no licensing term and the construction of the new LWR plant has been continuously starting every year. So there is much less needs to extend service term of aged plants mainly because they like to maintain the total output of nuclear power generation.
From the economic stand point, on the other hand, once-constructed plant is hoped to serve as long as possible before it is to be decomissioned. Fortunately, Japanese LWRs have been operating very well with better preventive maintenance and so, the age at which they retire can be estimated to be longer than 50 years or so.
Japanese PLEX Program started in 1985 based on those situation. It has been proceeded in 3 phases and the phases I and II have almost been completed. The results obtained up to March 1989 and our future work are summarized in the article.

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Details of the numerical modelling and method of the UEDA code which was developed for the analysis of the boundary plasma are described.
The code employs a two-dimensional dynamic fluid model for fuel and impurity ions. The transport of the neutral gas is solved with the Monte Carlo method and coupled to the fluid equations. Collision dominated Particle-in-Cell method is utilized for the solution of the fluid equations.
Representative numerical results of the edge/divertor plasma behaviour for the present day tokamak machine, and future reactor are shown.

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The overall temperature coefficient of reactivity of VHTRC-1 core was measured at the Very High Temperature Reactor Critical Assembly (VHTRC) to examine the calculation accuracy of temperature dependent neutronic characteristics of the High Temperature Engineering Test Reactor (HTTR).
The core of VHTRC consists of pin-in-block type fuel using coated particles of low-enriched UO₂. The whole assembly was heated by electric heaters and the reactivity change due to the temperature rise up to 200°C was measured by the pulsed neutron method to determine the overall temperature coefficient of reactivity in isothermal condition. The experimental results indicated that the temperature coefficient was -1.71×10^-4Δk/k/°C on the average in the measured range from 25 to 200°C and the absolute value was smaller by 20% near room temperature than at the higher temperature.
The calculation by the SRAC code system using the ENDF/B-IV nuclear data well predicted the experimental results.

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Small break loss-of-coolant accident (LOCA) tests simulating a pressurizer power operated relief valve (PORV) break LOCA were performed using the EOS (Emergency of System) test facility. The break sizes were 0.25 and 0.88% of a guillotine break of a primary piping. The following major conclusions were obtained and the useful data and information for the verification of a computer code were obtained:
(1) The pressurizer was almost full of water due to flooding limitation in the surge line of the pressurizer, when no water was in the hot leg piping.
(2) The core was kept completely covered with two-phase mixture during the small break LOCA.
(3) The core was sufficiently cooled down by reflux condensation in the steam generator even after the primary system natural circulation stopped.
(4) After the depressurization of the primary system was stopped or when the depressurization rate of the primary system was small, the operator action to bleed the steam in the secondary system could steadily depressurize the primary system.
(5) This operator action could resume the natural circulation in the primary system.

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A formula for prediction of the optimum operating pressure P_opt of the thermal diffusion columns at total reflux is derived based on the approximate formulae for the column constants which can be evaluated analytically. The formula is expressed explicitly in terms of (1) a parameter of cold wall radius r_c, and physical properties (evaluated at the reference mean temperature T) of gases to be separated, or μD'R/r³_cMg, where μ: viscosity, D': product of diffusion coefficient at a standard pressure and the standard pressure, R: gas constant, M: molecular weight and g: gravitational acceleration, (2) ratio of radii δ between hot wire r_h and r_c, and (3) the ratio of the temperature difference ΔT to the cold wall temperature T_c, in the form
The result is compared with experimental data ; (1) binary monatomic gas systems, (2) multicomponent monatomic gas systems, (3) isotopically substituted polyatomic systems, (4) systems of low atomic or molecular weight, and (5) mixtures of unlike gases ; mainly obtained by Rutherford & coworkers. Although the formula is based on the rather rough approximation for the column constants, the optimum pressures predicted by the present formula are in successfully good agreement with the experimental data even for the systems of low atomic or molecular weight and that of mixtures of unlike gases.

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Formation conditions, compositions and structures of precipitates or "the third phase" were systematically examined in the systems of Zr and radiation depleted products of TBP, such as HDBP, H₂MBP, H₃PO₄, by means of elemental analysis, X-ray diffraction, infrared spectra and ¹H-NMR. It was confirmed that one of the most important origins for the third phase is a complexation between Zr ion and the depleted products of TBP. Followings were also elucidated :
(1) When the depleted products co-exist with each other, the cooperative effects on the precipitate formation appear in low acid solutions.
(2) Precipitate formation depends on the mole ratio of HDBP/Zr. The amount of precipitate reaches the maximum at the mole ratio of approximately 2 and decreases with increasing concentration of HDBP and finally disappears at approximately 10.
(3) Precipitate formed at the mole ratio of approximately 2 has the chemical formula, Zr(NO₃)₂(HDBP)₂(OH)₂.
(4) Precipitates of the Zr-H₂MBP system begin to appear at the concentration of H₂MBP in one order of magnitude smaller than that of HDBP in Zr-HDBP system.
(5) Precipitates of Zr-H₂MBP system have no NO₃ ion and a basic structure of Zr (HMBP)₂(OH)₂ with an interlayer distance of 16 Å.

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Cesium-loaded zeolites of low Si/Al atomic ratios such as CsA, CsX and CsY were converted to pollucite ((Cs, Na)AlSi₂O₆) by hydrothermal treatment above 243°C, but Cs-mordenite of a high Si/Al ratio yielded no structural change owing to its high thermal stability. The structural degradation of zeolite and recrystallization to other minerals much depended on the treating temperature and time ; CsA was completely converted into pollucite at 300°C for 2h and at 243°C for 24 h. The hydrothermal treatment of zeolites A and X partially replaced Na with Cs resulted in the formation of solid solutions occluding pollucite in analcime (NaAlSi₂O₆•H₂O). As for Sr-loaded zeolites, Sr-feldspar (SrAl₂Si₂O₈) crystallized from SrA or SrX under mild conditions (150°C, 1 d) compared with those for Cs-loaded zeolites.

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Station blackout is reported to be a sequence that would likely be a significant contributor to the accident risk at a boiling water reactor (BWR). The occurrence frequency of station blackout is evaluated in probabilistic safety assessment (PSA) to be 6×10^-6 per reactor year at Limerick and less than 10^-7 per reactor year at BWR in Japan.
This report describes an analytical study of thermal-hydraulic and radionuclide behavior during a postulated severe accident of station blackout at a reference BWR plant. The analytical approach was shown in both of hand calculation and the THALES/ART code calculation to better understand wide physical and chemical phenomena in the processes of severe accidents.
We evaluated timing of key events, core cooling and core temperature, reactor vessel failure, debris temperature, containment pressure, and release and deposition of radionuclide in the containment. The THALES and CORCON models on the chemical reactions in the coreconcrete interaction lead to great differences in the increasing rate of containment pressure and the release rate of fission products from the core debris.

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