PSI - Issue 28

Casper Versteylen et al. / Procedia Structural Integrity 28 (2020) 1918–1929 Versteylen/ Structural Integrity Procedia 00 (2020) 000–000

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5. Conclusions

An analysis is presented of a first estimation of the cumulative probability for cleavage fracture in a RPV under PTS. The cumulative probability was calculated using the master curve approach, under the assumption that the stress intensity factor is the only stochastic factor and the main parameter influencing the probability of cleavage fracture initiation. CFD analyses were performed on the thermal mixing of pressurized RPV during and emergency cooling procedure during a LOCA. The duration of the transient was chosen to be 1000 seconds, which appears to have captured the maximum stress intensity occurring during the transient. Postulated semi-elliptical cracks were simulated as prescribed by the ASME code. The crack size, position, and aspect ratio were varied for a common RPV steel; SA-508gr.3. The crack position is very important for the probability of cleavage fracture. This is an important fact in relation to the RPV inspection, since a crack at position 4 (lower in the vessel and not directly under the inlet) is approximately 2 order of magnitude less likely to fail than a crack at position 9 (directly under the inlet). It is possible to perform such analyses with a minimum detectable crack size for a specific RPV, in order to obtain realistic values for the cumulative probability of cleavage fracture. For SA 508gr.4N the probability of failure is significantly smaller due to the smaller chemical factor leading to less embrittlement during irradiation. More research is needed regarding the reference temperature and the chemical factor applied to SA 508gr.4N. Thermal strains are clearly a function of the temperature profile in the RPV. This temperature profile requires a full CFD analysis, since the thermal fluctuations have a large effect on the mechanical stresses. The highest J-integral values does not necessarily occur in the deepest point of a semi-elliptical crack, since the thermal gradient largely determines the stress state. The plastic deformation of the matrix around the crack tip affects the cleavage fracture initiation. The maximum principal strain and the size of the plastic zone are not directly related to the J-integral. It could be necessary to include the plastic strain and the plastic zone size in order to predict cleavage accurately. The method presented here provides reasonable results which are reproducible and shows logic trends. Choi, S., Surh, H. B., Kim, J. W., 2019. Effect of postulated crack location on the Pressure-Temperature limit curve of reactor pressure vessel. Nuclear Engineering and Technology 51, 1681-1688. Sharabi, M., González-Albuixech, V.F., Lafferty, N., Niceno, B., Niffenegger, M., Computational fluid dynamics study of pressurized thermal shock phenomena in the reactor pressure vessel. Nuclear Engineering and Design 299, 136-145. Jaros, M., Lafferty, N., Qian, G., Niceno, B., Niffenegger, M., 2017. Computational Fluid Dynamics Study of Pressurized Thermal Shock Transients in the Reactor Pressure Vessel. 26 th International Conference Nuclear Energy for New Europe. Bled, Slovenia, paper #201. Boyd, C., 2008. Pressurized Thermal Shock, PTS, in THICKET, Pisa, Italy, paper #29. Mora, D.F., Niffenegger, M., Qian, G., Jaros, M., Niceno, B., 2019. Modelling of reactor pressure vessel subjected to pressurized thermal shock using 3D-XFEM, Nuclear Engineering and Design 353, 110237 Uitslag-Doolaard, H.J., Stefanini, L., Shams, A., Blom, F.J., 2019. Numerical Prediction of a Single Phase PTS Scenario for the Crack Assessment in an RPV Wall, in International Congress on Advances in Nuclear Power Plants, Juan-les-pins, France. Beremin, F.M., 1983. A Local Criterion for Cleavage Fracture of a Nuclear Pressure Vessel Steel. Metallurgical Transactions A 14, 2277-2287. Ritchie, R.O., Knob, J.F., Rice, J. R., 1973. On the relationship between critical tensile stress and fracture toughness in mild steel. Journal of the Mechanics and physics of solids 21, 395-410. Wallin, K., 2002. Master curve analysis of the ''Euro'' fracture toughness dataset. Engineering fracture mechanics 69, 451-481. IAEA, 2005. Guidelines for Application of the Master Curve Approach to Reactor Pressure Vessel Integrity in Nuclear Power Plants, Vienna, Austria. Section XI non-mandatory appendix G BPVC.XI-2017, ASME Boiler and Pressure Vessel Code, New York ASME 2017. References