Research topic　－　Safety and reliability of elevated temperature systems

The structural design of energy plant is based on load evaluation techniques and strength analysis. Our research aim to realized this technology integration in structural design to achieve excellent safety and reliability of plants design.
Since it is necessary to take into account the engineering and sociological point in the piratical application of our research, human resources and knowledge integration are also part of our research activities.
Theme_1_01
Light water reactor and Fast breeder reactor plant
The comparison of light water reactor (LWR) and fast breeder reactor (FBR) designs is shown below. In FBR the coolant temperature range is higher but the working pressure is lower allowing thinner vessel design. However, given this conditions, stress caused by thermal transients becomes the dominant load。 Furthermore, due to high temperature working conditions, cycling temperature transients such as ratcheting induces creep deformation and elastic-plastic behaviour of the materials leading to important fatigue damage.
Theme_1_02
Framework for "Design by Analysis" and structural design criteria

Design by Analysis

ASME Boiler and Pressure Vessel Code Sec.III, (1963)

「The goal is to provide design criterion taking into account every possible damage types corresponding to every possible failure modes by achieving a detailed structural analysis of the design.」
１．Failure modes assumption
　　
Clarification of failure mode should be considered in the design phase. In the current design standards for LWR, it is assumed that the temperature range of variation never induced creep damage, for high temperature component the temperature range of variation can induces creep damage and has to be taken into account.
２．Damage mechanism clarification　
　　　
For each failure modes, governing factors of damage mechanism are identified(stress, strain, etc..) and damage limitation is formulated based on the strength theory.
３．Stress/strain calculated by elastic analysis
　　　
Stress and strain analysis specifies how to identify the governing factors of damage mechanism essentially by elastic analysis.
４．Setting stress/strain limits
　　　
Finally a design factors is created combining the material strength from material data and the incidence and severity of the loads. This factor is used to set stress/strain limit.
Extension proposition of load and structural response evaluation for design.
１．Failure modes assumption：
　Asumption of failure mode combined with the heat load.

(Example)
  • Ratchet deformation due to thermal stratification interface movement and creep-fatigue
  • Thermal fatigue induced by high-cycle thermal striping
  • Ratchet thermal transient deformation inducing creep fatigue
２．Failure mode damage mechanism formulation：　
　Governing mechanism of damage mechanism do not only arise from structural aspects (stress/strain) , but also component and system characteristics such as pump control system characteristics or thermal hydraulic factors such as frequency of temperature change and amplitude. Thus, the damage factor has to be also based on these parameters.
３．Damage factor calculation method：
　It is not easy to predict creep deformation or heat flow. Thus complicated non-linear elastic-plastic analysis are required。To mitigate the burden of complicated analysis we develop damage factor methods that does not depend on complicated analysis but do not also underestimate the damage mechanism and thus make a safe damage prediction. Example : As structure does not respond to high frequency temperature fluctuations, it is not required to predict them.
Development of analysis technology to concisely evaluate complex coupled phenomena such as elastic-plastic behavior due to creep deformation combined to non-linear heat flow.
  • To clarify the occurrence of failure modes due to high temperature transient phenomena by coupling heat flow and elastic-plastic creep response of the structure.
  • Identify the main factors and criteria that govern the structural integrity.
  • To develop technologies that focus on the element above to evaluate the structure response and strength to each loads.
  • To include these technologies in the development of consistent strength response evaluation models.
  • Acceptable damage limit is set based on the understanding of the major factor limit.
  • The design factor is then set on risk analysis based on uncertainty of governing factors and frequency occurrence of loads.
 theme_1_03
Understanding of damage mechanism from loads generated on FBR vessels
Understanding of damage mechanism due to complex fluid-structure interaction knowing the range of uncertainty for each factors.
Theme_1_04
Development of consistent evaluation method from load generation to failure for FBR vessel.
Theme_1_05
Improvement of degradation prediction method accuracy using knowledge based simulation and monitoring inspection
Theme_1_06
Degradation prediction and defect diagnosis of piping using PCA (principal component analysis) analysis.
  • Thermal fatigue monitoring by installing various sensors on pipes for data.
  • Comparison of thermal fatigue data from monitoring and simulations.
  • PCA analysis of monitoring signal⇒defect diagnosis、(progress estimation)
Theme_1_07