Towards the future of nuclear energy: materials
The MIT Mesoscale Nuclear Materials Group aims to understand material performance for safe use of nuclear energy
MIAOMIAO JIN
Clean energy is the key to sustainability for present and future generations. One form of clean energy, nuclear energy, comes from the splitting of atoms, during which a huge amount of energy can be harvested to generate electricity. In the U.S., nearly 20 percent of electricity is generated through nuclear power, and it has been more 60 years since it became a part of the energy portfolio. To maintain sustainability, reduce carbon emissions, and address the concern of global climate change, nuclear energy has played and will continue to play a significant role in the future. For nuclear energy to make further improvements on economics and safety, a new generation of reactor designs (Gen-IV) have been pursued.
Materials have always been a key aspect in developing the advanced reactor systems where safety and durability are of top priority. Nuclear reactors present an exceptionally harsh environment for materials due to a combination of high temperature, serious corrosion, and intensive radiation. The Mesoscale Nuclear Material (MNM) Group, built in 2013 and led by Michael P. Short, aims to address problems of material performance by reinventing our understanding and measurement techniques of nuclear materials degradation. The MNM lab focuses on developing new methods and experimental equipment to simulate material behavior in real systems that will push forward the frontier of material understanding and design towards clean, carbon-free, and affordable nuclear energy resources for the future.
Weiyue Zhou G, a fifth-year PhD student in the group, is determined to take the initiative of resolving the unknowns in a Gen-IV reactor design known as the molten salt reactor (MSR), which is designed with ultra-reliable safety features such as non-proliferation and accident tolerance. He said, “As MIT graduate students, we are always aiming at the future, and I want to do something that will benefit society.” Since the study is first of its kind, and there are a lot of challenges resulting from the extreme environment (high temperature and intense radiation), he decided to build an experimental facility from scratch. Zhou wanted to uncover the mystery of structural material performance, specifically the material corrosion process in molten salt with intense radiation. Such understanding is of utmost importance in assessing material reliability during reactor operations. To investigate this, Zhou designed a detailed blueprint of the experimental facility, with concurrent corrosion and irradiation to the material samples at 650 ºC, which approximates the operating temperature in MSR. The neutron radiation field is mimicked using a proton beam. For the safety of the experimentalists, not only do they need to consider the high temperature, but also the radiation issue to human health. The harsh conditions require additional shielding so that the gamma rays produced during the experiment will not pose a health concern. With three years of persisting efforts devoted to optimizing the design and resolving technical problems based on various failed practices, the facility finally made its debut with a successful test on alloys.
The experiment uses a particle accelerator to generate high energy protons and pass them through a metallic sample foil. Protons penetrating through the sample introduce radiation damage to the sample. At the other side of the sample foil, a reservoir holds molten fluoride salt at 650 ºC. Overall, the molten salt corrodes the materials, under the influence of proton irradiation damage. Zhou performed multiple experiments to elucidate temperature-influenced corrosion rates and the mechanisms of irradiation-affected corrosion in molten salt.
From the preliminary results, the group found that proton irradiation decelerates the corrosion of model alloy samples in molten salt. It is highly unexpected as radiation normally exacerbates material performance significantly. Zhou noted that if further experiments can confirm that radiation slows down structural material corrosion in MSR, then one could conduct testing with just a corrosive experimental condition to evaluate material corrosion resistance as the worst scenario. Using a single condition would significantly speed up the material development cycle for MSR, as radiation experiments require more demanding treatment due to reactivity. This facility developed by MNM readily allows for a rapid, inexpensive, and safe testing for candidate structural alloys for MSRs. Zhou emphasized, “Our work effectively pushes forward the frontier of material understandings in this field amid the increasing popularity in MSR nowadays.”