Study on Minimum Wall Thickness Requirement for Seismic Buckling of Reactor Vessel Based on System Based Code Concept

Abstract

The minimum wall thickness required to prevent seismic buckling of a reactor vessel (RV) in a fast reactor is derived using the system based code (SBC) concept. One of the key features of SBC concept is margin optimization; to implement this concept, the reliability design method is employed, and the target reliability for seismic buckling of the RV is derived from nuclear plant safety goals. Input data for reliability evaluation, such as distribution type, mean value, and standard deviation of random variables, are also prepared. Seismic hazard is considered to evaluate uncertainty of seismic load. Minimum wall thickness required to achieve the target reliability is evaluated, and is found to be less than that determined from a conventional deterministic design method. Furthermore, the influence of each random variable on the evaluation is investigated, and it is found that the seismic load has a significant impact.

Influence of dead weight and internal pressure to seismic buckling probability of fast reactor vessels

“…Both of dead weight and internal pressure contribute as negative axial compressive load. Therefore, the limit state function, g2, is revised as follows; (19) According to previous studies (Takaya et al, 2015a;Yokoi et al, 2016), the probability distributions of sW and sp are assumed as follows;…”

“…Three combinations of probabilistic distributions shown in Table 2 were considered for sw and sp, according to a previous study (Takaya et al, 2015a): "High", "Low" and "N/A (Not Applied)". Deference in influence of sw and sp by site was also investigated by considering three sites in Table 3: Sites A, B and C. The same seismic conditions were assumed for these sites as in a previous study (Takaya et al, 2017).…”

“…Implementation of rational design is very important because there are two conflicting requests to wall thickness; thicker walls are preferable in terms of prevention of seismic buckling, while excessively thick walls introduce large thermal stress causing unacceptable creep-fatigue interaction damage. In a previous study (Takaya et al, 2015a), we evaluated seismic buckling probabilities of fast reactor vessels by considering seismic hazards, and showed that seismic load gave a significant impact on buckling probability. In a subsequent study (Takaya et al, 2017), we proposed a new alternative design rule for seismic buckling of vessels to the conventional deterministic design rule in the Japan Society of Mechanical Engineers (JSME) fast reactor codes (The Japan Society of Mechanical Engineers, 2016), using the load and resistance factor design (LRFD) method.…”

Load and resistance factor design approach for seismic buckling of fast reactor vessels

“…According to our previous work (Takaya et al, 2015a), σac and σb are evaluated by the following equations with axial compressive stress due to a design basis earthquake, σac Ss , bending stress due to a design basis earthquake, σb Ss , and the annual maximum earthquake normalized by a design basis earthquake, X;…”

“…Although thicker walls are preferable in terms of prevention of seismic buckling, excessively thick walls introduce large thermal stress causing unacceptable creep-fatigue interaction damage. In a previous study (Takaya et al, 2015a), we proposed an evaluation method for the seismic buckling probability of a reactor vessel considering seismic hazards. It was shown that among the random variables in the evaluation, seismic load had the most significant impact on buckling probability.…”