Thanks to a tip from a friend, I spent Friday morning on a tour of the MIT Nuclear Reactor, which has intrigued me every time I walked by it on Massachusetts Avenue for almost 10 years. No cameras are permitted in the secure zone around the reactor building, so I can’t show you what’s inside the mysterious robin’s egg blue dome. It’s just as well, as the reactor core (less than a cubic meter) is utterly dwarfed by the shielding that surrounds it. Of course, I didn’t expect to get a direct look into the nuclear fire, and I was pretty well satisfied with the photograph in the very nice opening presentation, showing the 400 core plates lit by their own Čerenkov glow (a light, as the speaker carefully noted, produced by the beta emissions, as gamma and neutrons are not charged).
The reactor was built in 1958, and has been upgraded frequently since then, leading to a facility that is virtually a study in contrasts. The reactor building is built tough the old fashioned way, with two battleship steel shells and 4 meters of concrete between them. The only way in or out is through two manually operated airlocks. The catwalks and “polar crane” overhead all look far from modern. In the control room, about half the controls look like they could be six decades old.
The other half have been transferred to flat-panel displays shoehorned into the old switchboard racks. They track neutron flux and core temperature, as well as the flow of ultra-high-purity silicon ingots along a conveyor belt near the core for Neutron Transmutation Doping, which provides some of the income to support the reactor.
The MIT reactor is not widely known in Cambridge and Boston, and only makes the news when someone declares that they find it a worrying security threat. In fact, it’s a Highly Enriched Uranium reactor, meaning it contains the right kind of fuel to build an atomic bomb. In fact, this threat is very remote. Once a fuel rod has spent some time in the core, it contains an increasing level of fission byproducts that interfere with the chain reaction. These are called nuclear poisons, because they poison the chain reaction required for a nuclear explosion, rendering the fuel useless for bombs. The reactor only replaces a few fuel rods at a time, so there is never more than a very small amount of pure “HEU” on site, a tiny fraction of what would be needed for a bomb. Nonetheless, the reactor is switching to Low Enriched Uranium next year.
A greater worry is the spent fuel, after it is removed from the reactor. Uranium 235 has a half-life of 700 million years, and such great stability produces very little radiation. The fission byproducts have half-lives spanning days, months and years, so they are too radioactive to handle. Some people worry that such spent fuel might be used in a “dirty bomb”, dispersed as carcinogenic shrapnel by a conventional explosive. The reactor team is very cautious about spent fuel, to the point that our tour guide declined to tell us where it is stored.
I was very impressed to learn that the reactor can be staffed by a total of two people … and those people are often both MIT undergraduates. Talk about empowering students!
EDIT: Argh. Looks like WordPress can’t handle Unicode, and instead decided to truncate my post. Had to recreate it from memory.