Development of failure evaluation method for BWR Lower head in severe accident; high temperature creep test and creep damage model

Abstract

After the Fukushima Daiichi nuclear power plant accident due to the pacific coast of Tohoku earthquake and Tsunami, we have been developing an analysis method considering creep damage mechanisms based on threedimensional analysis in order to estimate condition of the fuel debris and failure behavior of the reactor due to severe accidents for the early completion of the decommissioning of nuclear power plants. We have also been obtaining material properties that are not provided in existing databases or literature for the analysis. In this study, we measure the tensile and creep properties of low alloy steel, Ni-based alloy, and stainless steel at high temperatures near the melting points. Using experimental data, some parameters for the creep constitutive law and creep failure evaluation method are determined. It is confirmed that the estimated rupture time by the failure evaluation method agree well with the experimental one.

Expansion of high temperature creep test data for failure evaluation of BWR lower head in severe accident

“…Therefore, to simulate creep behavior during the tertiary creep stage, we selected the K-R model, which was developed on the basis of a creep damage mechanism based on the growth and coalescence of voids and necking generated by plastic deformation. The validity of this model has been confirmed in our previous study (Yamaguchi, 2017). The strain rate based on the K-R model is expressed as follows: where t r and  r are the rupture time and the damage parameter at the rupture time, respectively.…”

“…As shown in the figure, we could confirm that the equipment can measure stress and strain values with sufficient accuracy for obtaining the yield stress and ultimate tensile strength based on JIS G0567 (2012). The values of yield stress and ultimate tensile strength at high temperature are listed in Table 2 along with the test results from our previous study (Yamaguchi 2017). The bold-italic characters in Table 2 are the new results obtained in the present study.…”

“…The increase in the creep rate during the tertiary creep stage is due to the growth and coalescence of voids and necking generated by plastic deformation. The results of the creep tests are summarized in Table 3 along with the test results reported in our previous study (Yamaguchi, 2017). Here, the digits of each value were determined considering the accuracy of load-cell, displacement sensor, etc.…”

“…Figure 7 shows the relationship between the LMP and stress for each material. The creep data for SA533B, Type 316SS, and Alloy 600 are compared with the creep data published in our previous study (Yamaguchi, 2017). Linear equations for each material were determined on the basis of a best fitting of all creep data using Eq.…”

“…For the FEA, we have also been obtaining material properties not provided in existing databases or in the literature (Yamaguchi, 2017). In particular, we have obtained the uniaxial tensile and creep properties at high temperatures near the melting point for low-alloy steel, nickel-based alloy, and stainless steel, which are materials constituting the lower head of an RPV.…”