Small Specimen Test Technique
Due to the limited irradiation volume available in the test modules, small specimen test technique (SSTT) is being developed under IFMIF/EVEDA frame by various Japanese universities (College of Hachinohe, Kyoto University and Tohoku University) and institutions (NIFS) under the coordination of JAEA. The understanding fracture mechanics of irradiated specimens, as well as fatigue behavior in the irradiated materials of the first wall in the fusion reactor vessel, is essential.
Tests have been carried out with specimens fabricated with the RAFM F82H. The reduction in size from standard size specimens has been demonstrated achievable. Two types of fatigue specimens were tested with a perfect match for the round bar type in the range of interest. Further tests are needed to understand a slight deviation from the expected hysteresis behaviour.
Transition fracture toughness has been measured with ¼CT-specimens aiming at the definition of Master Curve (MC) following ASTM E1921 ‘Determination of Reference Temperature, To, for Ferritic Steels in the Transition Range’. The results indicate that the correlation developed for ferritic steels cannot be exactly applied. A new correlation has been proposed, but further tests are needed to statistically secure it. The impact of cold work on toughness to simulate irradiation effects has been investigated for F82H. Whereas standard Charpy specimens correlate well, it has been shown that Compact Test (CT)-specimens do not follow that trend and it cannot replace the impact of irradiation at the spectrum and flux to be expected in a fusion power plant. It is to be noted that Charpy tests for minituarized specimens is already covered by ASTM E1253 ‘Reconstitution of Irradiated Charpy-Sized Specimens’.
The crack growth rate through environmental assisted cracking and stress corrosion cracking, well known effects in austenitic steels for fission reactors, has been assessed for RAFM by Hachinohe College with no significant difference observed with the data accumulated with austenitic steels.
Fission chambers (FC) will give information (on-line) on the Test Cell irradiation conditions. Although these detectors are commonly used in power plants, the detector functioning in the harsh environment of the Test Cell must be proved. Therefore, validation experiments had to be performed under conditions as close as possible to the IFMIF ones.
During 2008 two prototypes (one FC and one ionization chamber) were acquired. Aiming to studying the FC behavior in an environment free of neutrons, they were tested in an x-rays and in 60Co γ-rays irradiation facilities at CIEMAT. After these preliminary tests the detectors were sent to the BR2 reactor (SCK·CEN) for monitoring fast neutron fluxes. The integrated γ-rays absorbed dose was about 4 x 1010 Gy and the fast neutron fluence (E > 0.1 MeV) 4×1020 n/cm2. Figure 32 shows the comparison between calculated values and experimental data in three BR2 different channels. Fission rates were calculated with ACAB code, while the final FC current was calculated through a numerical code developed at CIEMAT.