Seismic buckling of vessels is one of the main concerns for the design of nuclear power plants in Japan. Rational design is important, especially for fast reactor plants. Although thicker walls are preferable in terms of prevention of seismic buckling, excessively thick walls cause unacceptable creep-fatigue interaction damage. In a previous study, we proposed an evaluation method for the seismic buckling probability of a reactor vessel considering seismic hazards and showed that among the random variables considered in the evaluation, seismic load had the most significant impact on buckling probability. This suggests that more rational vessel designs can be realized by taking appropriate account of seismic load variations. The load and resistance factor design (LRFD) method enables us to determine design factors corresponding to target reliability by considering the variations of random variables. Therefore, in this study, we used the LRFD method to develop a new design rule for the prevention of seismic buckling of vessels. The equation in the proposed rule is almost the same as that in the Japan Society of Mechanical Engineers fast reactor codes, but every random variable, seismic load and yield stress, has its own design factor. In addition, mean or median values are used in the evaluation instead of design values including conservativeness. The effectiveness of the new design rule was illustrated in comparison with the current provision.

We study the inflated phase of two-dimensional lattice polygons, both convex and column-convex, with fixed area A and variable perimeter, when a weight μ t exp[−Jb] is associated with a polygon with perimeter t and b bends. The mean perimeter is calculated as a function of the fugacity μ and the bending rigidity J. In the limit μ → 0, the mean perimeter has the asymptotic behaviour t /4 √ A 1 − K(J)/(ln μ) 2 + O(μ/ ln μ). The constant K(J) is found to be the same for both kinds of polygons, suggesting that self-avoiding polygons may also exhibit the same asymptotic behaviour.

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Seismic buckling of vessels is one of main concerns for the design of fast reactor plants in Japan. Rational design is important because of two conflicting requirements; thicker walls are preferable to prevent seismic buckling of vessels, while excessively thick walls introduce large thermal stress causing unacceptable creep-fatigue interaction damage. In previous studies, we discussed evaluation methods of seismic buckling probability of vessels by taking account of seismic hazards in order to rationalize seismic buckling evaluation, and proposed a rule for seismic buckling of vessels based on the load and resistant factor design method. The proposed rule is expected to widen design window regarding seismic buckling and contribute to more reasonable design of vessels of fast reactors. However, there is still a room for more rational design. The proposed method deals with only seismic load, but in actuality, dead weight and internal pressure also exist. The existence of these loads contributes to reducing the buckling probability because axial compressive load decreases. In this study, the rule was expanded so that dead weight and internal pressure can be taken into account. Furthermore, the influences of dead weight and internal pressure to seismic buckling evaluation were discussed. As result, it was shown that approximately 10 to 20% of further rationalization of allowable seismic load could be achieved by considering dead weight and internal pressure in the evaluation. In addition, it was found that the previously proposed design rule, not considering dead weight and internal pressure, includes approximately 2 to 10 times margins in terms of seismic buckling probability.

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