Research and Development of Advanced Nuclear System

1-2

Evaluation of the Structural Integrity of Reactor Vessels–Measurement of the Thermal Deformation near the Fluid Surface
Using the Laser Speckle Method–

Coil for induction
heatingSpecimen
Movement

Laser (green)

Temperature (°C)

700

① Temperature rising
600 ② Movemet③ Temperature fall
500
400300200
1000Laser (red)050100150200250300

Axial coordinte Z (mm)Observed speckle patternFig.1-5 Laser speckle system and its measurement ofa specimen

Axial component of elastoplastic strain (%)

Fig.1-6 Time history of temperature distribution along
the axis of the specimen

CCD camera

2.0%
Experimental Result
Simple Inelastic Model
Detailed Inelastic Model

1.5%

Elastic Model
Reference:
Detailed Inelastic Model(best -fit analysis)

1.0%

All of these Inelastic model
are not standardized now

0.5%

0.0%
05

10

15

20

Number of the temperature cycles

Fig.1-7 Comparison of the strain found in experiments
and analyses (Z=150mm in Fig.1-6)

A main vessel in the FBR is subjected to a severe cyclic
thermal load due to cyclic movement of longitudinaltemperature distribution with a large gradient. This is generated
by the combination of heating of the lower part by hot liquidsodium and cooling of upper part by ambient gas during startup and shut down of the system. As a result, inelastic strain
accumulates near the sodium liquid surface. Since excessive
inelastic strain breaks down the structural integrity, precise
prediction of this strain is important for structural design ofthe reactor vessels. To understand the actual behavior of theinelastic strain and so evaluate an inelastic strain analysis, testsof the strain on a reactor vessel model were carried out.
Fig.1-5 shows the tested cylindrical specimen (material:316FR steel, height: 300mm, outer diameter: 55mm, thickness
3.5mm). This specimen was given the temperature distribution
shown in Fig.1-6, which imitates an actual reactor vessel. In
this experiment, the following factors were measured: (1)temperature, (2) radial deformation, and (3) axial strain. In
the past it has been difficult to measure local strain on a hightemperature structure with high precision. For the measurementof this strain, we developed a non-contact measurement system

based on the laser speckle method. This system measured thespeckle pattern (Fig.1-5 lower right) of diffuse reflection ofa laser. Through the image processing of the change of thispattern, the strain on the structure surface can be estimated.
The experiments showed that movements of the longitudinaltemperature gradient caused axial strain, especially in theearly stage. With succeeding repetitions of the temperature
movement, less incremental axial strain is caused. As shown in
Fig.1-7, upon 3 or more cycles of the temperature movement,
the traditional elastic analysis overestimates the axial strain.
Although the elastic analysis is on the safe side, the strain
prediction should be based on inelastic analysis to rationalize
the reactor vessel design.
The strain measured here is important and useful for
improvement of inelastic analysis needed to make the structuralmodel. In the future, using this data we will develop a moredetailed model to explain actual behavior such as temperaturedependency, non-linear hardening, and cyclic hardening. Thismodel will improve the prediction of the inelastic strain, andenable design of compact reactor vessels.

Reference
Watanabe, D., Chuman, Y., Otani, T., Shibamoto, H., Inoue, K., Kasahara, N., An Experimental Validation of the Guideline for Inelastic Design Analysis
through Structural Model Tests, Nuclear Engineering and Design, vol.238, issue 2, 2008, p.389-398.
JAEA R&D Review 200811