Passively safe reactors rely
on nature to keep them cool
Continued ...
Cooling with a sodium pool
The sodium coolant is a highly efficient heat-transfer material and has the additional advantage of operating at normal atmospheric pressure. In the typical commercial reactor, the water coolant must be pressurized at 100-150 times normal to keep it from boiling away. But sodium can cool the core at normal pressure, because its boiling point is 300-400 degrees Celsius (575-750 degrees Fahrenheit) higher than the core's operating temperature.
"Basically," says Deitrich, "the sodium pool eliminates the possibility of the coolant boiling away during an accident and leaving the core uncovered, which is one of the more serious potential trouble spots in a light-water reactor. By submerging the core in thousands of gallons of liquid sodium, you provide the reactor with an immense heat sink that adds greatly to its safety. If the reactor starts to overheat, the pool can absorb vast amounts of heat and never approach its boiling point."
 Diagram of major components in an advanced fast reactor. (Click the diagram to see a larger image.) |
And the pool design, he adds, passively removes decay heat if the normal heat-removal systems fail. "When a reactor shuts down," he explains, "it continues to produce heat, because the core contains a large inventory of radioactive material that releases energy as it decays. But in our AFR concept, natural convection in the sodium pool can transport the decay heat to downstream systems. All of this can be done passively, without need for active systems or components."
Other benefits of sodium cooling
Sodium also increases the reliability and long life of components, partly because it does not corrode common structural materials, such as stainless steel. "Our experience in decommissioning EBR-II," says John Sackett, Argonne's deputy associate laboratory director for Argonne-West, "shows that materials and components in the core can operate in liquid sodium without significant damage or corrosion. We removed components from the sodium pool after 30 years and found them just as shiny as the day they went in. We saw original marks that welders and other craftsmen had made 30 years earlier when they created the component."
Other sodium properties also enhance reactor safety and reliability. For example, sodium is chemically compatible with the metal fuel. This makes small failures in the cladding, the stainless-steel tubes that encase the fuel, far less likely to grow. In addition, sodium tends to bind chemically with several important radioactive fission products, which reduces radioactive releases if fuel fails. Although sodium can be dangerous if allowed direct contact with air or water, with appropriate care, it makes a nearly ideal coolant. "Properly handled, as we did for 30 years at EBR-II, sodium offers significant advantages over water as a coolant," says Deitrich.
 Chuck Matthews (left) gives final instructions before the 1986 passive safety test at EBR-II. |
Advantages of metallic fuel
The third leg of AFR safety is its metallic fuel — an alloy of uranium and other metals. Metal fuel provides inherent, "reactivity feedback" mechanisms that alter a reactor's power when its core temperature changes.
The primary feedback in a metal-fuel reactor comes from thermal expansion of fuel, sodium and steel around the core. Simply put, when the core temperature increases, the fuel, sodium and the stainless steel components in the core expand, and that tends to shut down the reactor.
"When the fuel expands," Deitrich explains, "the distances between the fissile nuclei increase. This slows the chain reaction, because the neutrons necessary to drive it strike fewer fissile nuclei."
Radial expansion of the core also limits reactor power. "Normally," he says, "the sodium and steel around the core reflect neutrons back into the core to help maintain the chain reaction. But when sodium and steel expand, more neutrons escape from the core and are unavailable to drive the reaction."
The safety bottom line for the AFR is that all these natural feedback mechanisms tend to maintain coolant temperature near its normal 500 degrees C (930 degrees F) operating value — well below sodium's 900 C (1,650 F) boiling point — even when the reactor loses its engineered cooling systems. If an AFR started to overheat, the natural properties inherent in its materials and design would step in to shut it down without the intervention of human operators or specially engineered safety systems.
"When you put all these things together," Deitrich concludes, "you have a high level of passive safety. We've demonstrated all these effects in a working reactor. Each individual effect is predictable and so is their combination. Together, they provide a natural and reliable safety response based on features inherent in the advanced fast reactor concept."
For more information, please contact Dave Baurac (630/252-5584 or baurac@anl.gov) at Argonne.
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