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Passively safe reactors rely
on nature to keep them cool

by David Baurac

Imagine a nuclear power plant so safe that even the worst emergencies would not damage the core or release radioactivity. And imagine that this is achieved not with specially engineered emergency systems, but through the laws of nature and behavior inherent in the reactor's materials and design. This goal, known in the nuclear industry as "passive safety," is pursued and even claimed by a number of reactor concepts.

Argonne researchers demonstrated the passive safety of a nuclear reactor design at the Argonne-West site in Idaho.

Argonne's advanced fast reactor(AFR) has demonstrated its passive safety conclusively on a working prototype. "Back in 1986, we actually gave a small prototype advanced fast reactor a couple of chances to melt down," says Argonne nuclear engineer Pete Planchon, who led the 1986 tests. "It politely refused both times."

He's joking, but only partly.

The reactor was Experimental Breeder Reactor-II (EBR-II), located at Argonne-West in Idaho. EBR-II was a small experimental facility, an AFR prototype with a 20-megawatt electrical output. Under Planchon's guidance, a series of experiments were conducted at EBR-II, starting at extremely low power and culminating in two landmark tests at full power that convincingly demonstrated the passive safety advantages of the AFR concept.

"We subjected the reactor," Planchon says, "to what are considered two of the most serious accident initiators for liquid-metal reactors: a loss of pumped coolant flow through the core and a loss of heat removal from the primary system. Both tests were performed at full reactor power with the automatic shutdown features intentionally disabled.

"Before the tests," he adds, "we had installed special systems to let us stop the reactor at any time. But they weren't needed, because the reactor performed exactly as we predicted."

In the first test, with the normal safety systems intentionally disabled and the reactor operating at full power, Planchon's team cut all electricity to the pumps that drive coolant through the core, the heart of the reactor where the nuclear chain reaction takes place. In the second test, they cut the power to the secondary coolant pump, so no heat was removed from the primary system.

Chart of reactor core temperature during 1986 loss-of-flow-without-scram test.

"In both tests," Planchon says, "the temperature went up briefly, then the passive safety mechanisms kicked in, and it began to cool naturally. Within ten minutes, the temperature had stabilized near normal operating levels, and the reactor had shut itself down without intervention by human operators or emergency safety systems."

The reactor was shut down permanently in 1994, having completed its research mission. But for 30 years, it operated safely and reliably while providing all the electricity for Argonne-West, the 900-acre site Argonne operates near Idaho Falls, Idaho. The reactor was a prototype AFR and demonstrated once and for all the technology's passive safety.

The basic purpose of reactor safety is to protect the public and plant workers from harmful radiation exposure.

Walt Deitrich, Argonne reactor safety expert, explains how modern reactor design approaches that task: "The goal of modern safety design is to provide this protection by relying on the laws of nature, rather than on engineered systems that require power to operate, equipment to function properly and operators to take correct actions in stressful emergency situations. We call this approach, which relies on the laws of nature, 'passive safety.'

"To achieve this," he continues, "you have to provide for passive performance of three basic safety functions: You have to maintain a proper balance between heat generation and heat removal, you have to remove decay heat, and you have to contain radioactive materials, primarily fuel and fission products." Decay heat comes from radioactive materials in the core, even when the reactor is shut off.

The AFR's passive safety is based on three key aspects of its materials and design: its liquid sodium coolant, its pool-type cooling system and its metal alloy fuel.