Inside MIT’s nuclear reactor laboratory

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Tucked away behind a brick building on MIT’s campus sits a nuclear reactor. I’ve been hearing about this facility for over a decade, and it’s taken on a somewhat mythic quality in my mind. So I was excited to finally get to see it for myself last week. 

MIT’s research reactor was built in the 1950s, and its purpose has shifted over the decades. At various points, it’s been used to study everything from nuclear physics to medical therapies, alongside its consistent use for teaching the next generation of nuclear scientists. 

But I was most excited to hear about an energy-focused project, which is aimed at bringing novel reactor technology to reality. 

While virtually all commercial nuclear reactors today are cooled using water, a growing number of startups are looking to molten salt as an alternative. MIT’s nuclear reactor lab is working on a new research space that could help illuminate how well alternative technologies withstand the intense conditions inside a nuclear reactor. 

So for the newsletter this week, come along on my tour of MIT’s nuclear reactor lab. On the way, we can get into what all the buzz is all about with molten-salt reactors. 

Hot topics

The first stop on my tour was the front desk, where I and each member of my group picked up a personal dosimeter to track any potential radiation exposure. We then handed over our bags and phones and got stern warnings not to touch anything or wander out of our tour guide’s sight. 

Finally, we filed through a set of reinforced metal doors and into the lab. We passed rows of yellow lab coats as David Carpenter, the head of reactor experiments and our tour guide, walked us through some history and basic facts.  

This is the second-largest university research reactor running in the US today, producing about six megawatts of thermal power. Commercial reactors tend to have capacities hundreds of times greater than that—around 3,000 megawatts (or three gigawatts) of thermal power. 

(Speedy nuclear basics: nuclear reactors are powered by fission reactions, where uranium atoms break apart. These reactions produce neutrons, which are a type of ionizing radiation, as well as heat that can be harnessed and transformed into electricity.)

Reactors used on the power grid generate heat in the form of steam and turn it into electricity. But for this research reactor, the heat is basically a by-product, and the focus is all on the neutrons. 

MIT’s reactor is better than most other university research facilities at mimicking radiation conditions in larger commercial reactors. For that reason, the facility is used today for a lot of engineering research and development, Carpenter says. Before companies use new materials or sensors inside or near nuclear reactors, they can test them at similar radiation, temperature, and pressure conditions in a controlled environment in the research reactor. Samples can either be put directly into the core or subjected to radiation that’s allowed out in controlled corridors called beam lines. 

Chain reactions

I had the distinct feeling that I was about to get launched into space as we approached the entrance to the reactor room, though the doors were painted a surprisingly whimsical robin’s egg blue. After Carpenter went through a round of security checks, the first door swung open, revealing a small airlock chamber and a duplicate blue door. 

After we waited a few seconds inside the airlock, the second door swung open and we were suddenly faced with the reactor. While the core where the fuel is contained is only about two feet tall (less than a meter), the whole setup is several stories high.  

Carpenter walked us around the reactor, pointing out a chamber that used to be dedicated to medical neutron therapy in the early 2000s. That research has fizzled out, so now the space is getting a makeover. Its new purpose will be to test out aspects of molten-salt-cooled reactors. 

Molten salt was a candidate for cooling reactors as early as the 1950s. Interest slowed as water-cooled reactors started entering commercial operation, but in the early 2000s, scientists—including some at MIT—revived the work. 

Several startups, including Kairos Power and TerraPower, are working to bring molten-salt reactors into commercial operation. These companies are building demonstration systems of their cooling setups and seeking licenses for test reactors. 

MIT’s lab won’t be operating a molten-salt reactor. Instead, it will help gather more data on how the technology will work in the real world. The space will allow companies and academic researchers to test not only small pieces of material used to build reactors and individual sensors, but a whole operating set of pumps and pipes to move hot salt around in a circuit and see how everything reacts to radiation. “Given where the new molten-salt reactor industry is today, we still need to investigate more basic functions,” Carpenter told me in an email after our tour. 

Data from MIT and other research facilities could help determine how molten-salt setups will handle what it’s really like inside a nuclear reactor. The facility should be fully up and running in 2024. 

Related reading

My colleague James Temple visited the MIT Nuclear Reactor Lab in 2017. The plan for the molten-salt work has shifted a bit since then, but take a look at his story for more on the facility. 

Advanced nuclear reactors were on our 2019 list of breakthrough technologies—read more in this feature about the potential for progress in nuclear power. 

While nuclear reactors can provide stable, low-carbon power for the grid, Germany shut down the last of its nuclear power plants earlier this year. 

In addition to changing up cooling approaches, some companies are looking to change nuclear technology by shrinking it. Read more about small modular reactors in my story from February.

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