<p>One
of the major degradation mechanisms in a nuclear power plant structural or
mechanical component is the neutron embrittlement of the irradiated steel
component. High energy neutrons change the microstructure of the steel, so the
steel loses its fracture toughness. This neutron embrittlement increases the
risk of the brittle fracture. Meanwhile, the reactor pressure vessel support is
exposed in low temperature with high neutron irradiation environment which is
an unfavorable condition for the fracture failure. In this study, the failure
assessment of a reactor pressure vessel support was conducted using the
fitness-for-service failure assessment diagram of API 579-1/ASME FFS-1(2016, API)
with quantifying the structural margin under the maximum irradiation and
extreme load events. </p>
<p>Two
interrelated studies were conducted. For the first investigation, the current
analytical methods were reviewed to estimate the embrittled properties, such as
fracture toughness and the yield strength incorporates the low irradiation
temperature. The analytical results indicated that the reactor pressure vessel
support may experience substantial fracture toughness decrease during the
operation near the lower bound of the fracture toughness. A three-dimensional
(3D) solid element finite element model was built for the linear stress
analysis. Postulated cracks were located in the maximum stress region to
compute the stress intensity and the reference stress ratio. Based on the
stress result and the estimated physical properties, the structural margin of
the reactor pressure vessel support was analyzed in the failure assessment
diagram with respect to the types of the cracks, level of the applied load and
the level of the neutron influence. </p>
<p>The second study explored the structural stress analysis
approaches at hot-spot which was found to be key parameter in failure analysis.
Depending on the methods to remove the non-linear peak stress and the stress
singularities, the accuracy of the failure assessment result varies. As an
alternative proposal to evaluate the structural stress in 3D finite element
analysis (FEA), the 3D model was divided into two-dimensional (2D) plane models.
Five structural stress determination approaches were applied in 2D FEA for a comparison
study, the stress linearization, the single point away approach, the stress
extrapolation and the stress equilibrium method and the nodal force method. Reconstructing
the structural stress in 3D was carried by the 3x3 stress matrix and compared
to the 3D FEA results. The difference in 2D FEA structural stress results were
eliminated by the constructing the stress in 3D. </p>
<p>This
study provides the failure assessment analysis of irradiated steel with
prediction of the failure modes and safety margin. Through the failure
assessment diagram, we could understand the effects of different levels of
irradiation and loadings. Also, this study provides an alternative structural
stress determination method, dividing the 3D solid element model into two 2D
models, using the finite element analysis. </p><br>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7413614 |
| Date | 16 January 2020 |
| Creators | Go-Eun Han (5930657) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/A_STUDY_ON_THE_FAILURE_ANALYSIS_OF_THE_NEUTRON_EMBRITTLED_REACTOR_PRESSURE_VESSEL_SUPPORT_USING_FINITE_ELEMENT_ANALYSIS/7413614 |
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