The service life of nuclear power plants has been extended. On the other hand, there has not been enough test pieces (for the surveillance test) inserted in the reactor to monitor the integrity of the reactor pressure vessel. Therefore, a novel method should be established in a hurry for evaluating nondestructively and rationally the embrittlement due to the neutron irradiation of pressure vessel steel. If a nondestructive method can be used for evaluating the amount of irradiation embrittlement, specimens for surveillance tests are not only effectively used, but also this is useful to decrease radioactive disposal. Here, we have developed a nondestructive method to investigate the change in ultrasound velocity, which propagates in a specimen that has been irradiated by neutrons.
Figure 10-4 shows the outline of a remote-operation type measurement system after irradiation, set up at the hot laboratory in the Oarai Establishment. We connect the sound probe, which sends and receives ultrasound waves by remote operation using a manipulator outside the hot cell, and measure the transmission time of ultrasound. Furthermore, we obtain the ultrasound velocity propagating in a specimen from the measurement results. We have performed the test for the pre-irradiated materials, irradiated ones that are embrittled by neutron irradiation and the post irradiation annealed ones at 673K after irradiation in which a part of the embrittlement is recovered. As a result, it turned out that the ultrasound velocity propagating in a specimen decreased in the course of embrittlement due to neutron irradiation as shown in Fig. 10-5. The results indicate that we can evaluate the amount of embrittlement for pressure vessel steel that was irradiated by neutrons, by measuring the ultrasound velocity. |
