PUTTING NUCLEAR WASTE TO WORK
A humble lawnmower engine–and a junked one at that–has pointed the way toward a novel solution for disposing of nuclear waste. If it works as well as its developer expects, it might even turn nuclear power into an energy source an environmentalist could love. Okay, maybe not love, but at least learn to live with.The idea revolves–literally–around a new type of reactor. Called a Nuclear Powered Turbo-Reciprocating Engine (NPTRE), it runs on a mix of "fresh" and "spent" nuclear fuel.
The NPTRE is the brainchild of Claudio Filippone, an electrical engineer who, after working with leading automakers, started on a new tack by enrolling in the University of Maryland's graduate program in nuclear engineering. Before long, he decided that the familiar piston engine just might hold the key to safely disposing of the world's growing stockpiles of radioactive waste.
There are several types of radioactive waste, ranging from gloves worn by nuclear medicine technicians to underground tanks bubbling with millions of gallons of lethal leftovers from the Manhattan Project and Cold War bomb-building. But the big problem, both in terms of waste volume and radioactive content, is created by the fuel removed from commercial power plants when they are shut down for refueling once every 18 to 24 months (the refueling cycle for nuclear submarines is more frequent). Each time this is done, a portion of the nuclear fuel in the core of the reactor is removed and placed in a "spent fuel pool" near the reactor.
Fresh reactor fuel contains mostly natural uranium (U-238), enriched with between 2% and 4% neutron-emitting U-235 uranium isotope. "The splitting of the U-235 and U-238 produces fission fragments which will transform their kinetic energy into heat and continue decaying through radioactive processes," explains Filippone.
Depending upon the power-plant design, the heat created in the fissioning chain reaction produces steam or boiling water, which in turn drives a turbine connected to electric generators. The fission fragments, although radioactive, produce too few fast-moving neutrons to continue to support fission. When this happens, the fuel is considered spent–even though it still contains a large amount of U-238.
Because some of the material in spent fuel remains radioactive for thousands of years and can also be used in making nuclear weapons, the law requires that spent fuel be stored in a permanent repository. By 2020, the Department of Energy estimates, 85,000 tons of spent fuel will have accumulated. A repository to hold it still hasn't opened, so it is backing up in the local pools.
This is where the NPTRE comes in. It would allow fuel to remain in nuclear plants, where the radiation it releases can be put to work. The NPTRE's basic mechanical operation would be familiar to anyone who has changed a lawnmower spark plug. In the NPTRE, the piston is pushed by a small volume of liquid water that is quickly converted–flashed–into a large amount of superheated steam. This phase change occurs when the piston is at top dead center (TDC) and immediately after liquid water has been squirted into a specially shaped heat cavity. The steam, which now occupies more volume due to its expansion, drives the piston down.
Heat to flash the water into steam is produced by a nuclear reaction that begins when a small amount of U-235 embedded in the piston enters a section of the reactor surrounding the cylinder head.
The NPTRE actually is made of two reactors placed one on top of the other. The one that influences the piston when it is at TDC creates a chain reaction that takes place in "new"–U-235-enriched–fuel surrounded by a water-moderated reactor, the top reactor. The moderator slows the neutrons coming off the piston and the surrounding cylinder, so they can be captured, and absorbed, by uranium atoms, which then split apart to sustain the chain reaction.
As the piston travels down the cylinder, it exits the water-moderated reactor and enters a second reactor. This one is filled with spent–U-235-depleted–fuel moderated by graphite. Graphite has special neutron-scattering characteristics that make a sustained nuclear chain reaction almost possible in the spent fuel. "However, by itself, the spent fuel and graphite combination cannot sustain a usable fission reaction," explains Filippone. "They need a little something extra."
That something extra comes in the form of neutrons emitted from the radioactive piston. As it approaches bottom dead center (BDC), it adds enough neutrons to support a pulsed chain reaction in the lower reactor. It produces a small amount of additional heat, which can be circulated through a heat exchanger or directly into the top reactor, and later used to spin a turbine.
Nothing lasts forever. Eventually, the amount of U-235 in the piston decreases to the point where it produces an insufficient number of neutrons to continue the chain reaction. "However, we're talking about extending the lifetime of the fuel and its permanence in the reactor-shielded environment, perhaps as many as four to seven times longer than the current utilization," says Filippone.
And that's not all. When all of the heat and motion is accounted for, the NPTRE will achieve a thermal efficiency of 56%. By comparison, a conventional reactor operates with a thermal efficiency of 30% to 33%.
Filippone is confident about the system's high efficiency because in order to convince his Ph.D. committee that his idea would work he built a prototype. The piston and cylinder were scavenged from a junked lawnmower engine, and the high-pressure water injector is a modified 8-cylinder Oldsmobile diesel pump.
To simulate the heat released when the piston reached TDC, he used a heating element and a fast-switching electric power supply. The prototype worked and he received his doctorate. Looking at Filippone's handiwork, a member of his dissertation committee remarked that the NPTRE looked like something out of the pages of Popular Mechanics–which of course it now is.
Although the Department of Energy has expressed interest in funding more research, Filippone is realistic about NPTRE's prospects. However, he believes that even if no NPTRE is ever built, the research that went into the project will produce dividends. The heart of the system–the intricate heat cavity that flashes water into steam–can coax higher efficiency from any type of heat engine. Including those that just putter along, cutting grass.