Though most of today’s nuclear reactors are cooled by water, we’ve long known that there are alternatives; in fact, the world’s first nuclear-powered electricity in 1951 came from a reactor cooled by sodium. Reactors cooled by liquid metals such as sodium or lead have a unique set of abilities that may again make them significant players in the nuclear industry.
At the U.S. Department of Energy’s (DOE) Argonne National Laboratory, a team led by senior nuclear engineer James Sienicki has designed a new small reactor cooled by lead—the Sustainable Proliferation-resistance Enhanced Refined Secure Transportable Autonomous Reactor, or SUPERSTAR for short.
Small modular reactors, or SMRs, are small-scale nuclear plants that are designed to be factory-manufactured and shipped as modules to be assembled at a site. They can be designed to operate without refueling for 15 to 30 years. The concept offers promising answers to many questions about nuclear power—including proliferation, waste, safety and start-up costs.
SUPERSTAR is an example of a so-called “fast reactor,” a type fundamentally different from the light-water reactors common today. Light-water reactors use water both as a coolant and as a moderator to slow down neutrons created in the fuel as it fissions. Instead, fast reactors use materials that don’t slow down neutrons—often a liquid metal, such as sodium or lead.
Like all new generations of reactors, SUPERSTAR has “passive” safety systems—backup safety measures that kick in automatically, without human intervention, in case of accidents. For example, all reactors have control rods incorporating substances that absorb neutrons and stop nuclear chain reactions. SUPERSTAR’s rods can be suspended above the reactor core held in place by electricity. If the plant loses power, the control rods will automatically drop into the core and stop the reaction.
In addition, SUPERSTAR’s lead coolant is circulated around the core by a process called natural circulation. While existing plants use electrically-driven pumps to keep the water moving, SUPERSTAR exploits a law of physics to move the coolant.
“In any closed loop, with heat at the bottom and cooling on top, a flow will develop, with the heated stream rising to the top and cooled stream going down,” explained Anton Moisseytsev, an Argonne nuclear engineer also working on the reactor design. “The SUPERSTAR design takes advantage of this feature—its lead coolant is circulated solely by natural circulation, with no pumps needed. And of course, having no pumps means no pump failures.”