One of the biggest problems with nuclear reactors is finding materials that can withstand the hostile environment of high temperatures, high pressures, and neutron bombardment. As neutrons penetrate the metal alloys that house the reactor core, they create dislocations in the crystalline lattice of the metal. These dislocations can migrate and collect, forming cavities that can combine into cracks. This makes the metal increasingly brittle and ultimately, the reactor must be retired because it is unsafe to continue to operate it. It has been difficult to measure the degradation in a particular reactor because of the radiation. Now, researchers have developed a new technique for measuring radiation damage to metal.
The current "gold standard" for measuring radiation damage is called transmission electron microscopy (TEM). It produces a great deal of information from a sample of the metal being tested but it does not reveal all of the changes that have an impact on the structural integrity of the metal.
Researchers at the MIT Department of Chemistry created a new technique which was applied by the MIT Mesoscale Nuclear Materials Laboratory. This new method of measuring damage may be able to provide a way of monitoring the integrity of metals in real time with no need to remove the components from their radioactive environment.
This new technique is called transient grating spectroscopy (TGS). In TGS, acoustic waves are induced in the surface of a sample to reveal thermal and elastic properties. Although the acoustic waves are traveling on the surface of the sample, defects below the surface of the sample can affect the waves.
The acoustic waves are created by aiming two lasers at the polished surface of the sample to create an interference pattern. The lasers shining on the metal surface heat it and cause a standing acoustic wave. This standing wave causes minute movements of the metal surface that can be read by another set of lasers. Rippling acoustic waves form and decay. Measuring their movement and decay rates can reveal the properties of the sample. Experiments at the lab have matched the theoretical models and simulations.
One of the tests conducted at the lab involved aluminum samples that were perfect single crystals that had different surface orientations. Although the samples looked identical to the human eye (even through a microscope), the new TGS system was able to easily distinguish between the different orientations.
The laboratory works mainly with aluminum samples because they can be very difficult to analyze and if the researchers can prove their method with aluminum, the new system should work with many other materials. The new technique has a sensitivity of one tenth of one percent. The new system can yield accurate results about radiation damage in a matter of seconds as opposed to the months or years that other techniques require.
The researchers have created simulations of molecules and their responses to the TGS that are highly accurate. Using this methodology, they are able to create models of different types of defects in metals and then predict what kind of signals will be generated by TGS. Experiments are then carried out to verify the modeling.
This new technique should speed up the development and testing of materials to be used in the construction of reactors and nuclear fuels. The researchers hope to be able to reduce their equipment from the current bulky machines in their laboratory to portable handheld devices.
