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[cdn-nucl-l] Fail-Safe Fission
Posted on the February issue of Popular Mechanics web page (maybe the
printed too?) at:
Something we might be seeing more of.
BY JIM WILSON
Illustration by Patrick Gnan/Paul DiMare
If the reactor overheats, the Li-6 (blue) expands, absorbing more
The alarm in the control room of the Prairie Island 1 reactor, 28 miles
southeast of Minneapolis, sounded at 7 am. Inside the adjacent
reinforced concrete containment structure, a sensor had detected a
sudden drop in the production of neutrons in the steel reactor pressure
vessel. Power was automatically cut to the latching mechanism that held
fission-stopping control rods poised above the reactor. This is the
nuclear equivalent of standing on the brakes. Only it didn't work this
time. Gauges in the control room showed that six of the 29 control rods
were stuck part of the way down. To stop the nuclear chain reaction,
operators flooded the core with 1000 gal. of water mixed with
neutron-absorbing boron. The next day, Aug. 3, as operators attempted to
restart the fission reactor, alarms sounded again. A fire broke out in a
different electric panel and was extinguished in 10 minutes. "Most
likely it was caused by improper maintenance," a spokesman for the
regional office of the Nuclear Regulatory Commission (NRC) told POPULAR
MECHANICS. Two members of the plant's fire brigade were treated for heat
exhaustion. The reactor was kept off line for maintenance until
mid-September and its owner, Xcel Energy of Minneapolis, was forced to
ask its customers to curtail their household energy use.
The control-rod problem at Prairie Island 1 was hardly unique. Control
rods that get stuck, give false readings or shoot out of reactors--in
one case killing a technician by pinning him to the ceiling--have beset
the industry since its infancy. On the very same day that the fire
brigade was called in to battle the panel fire at Prairie Island 1, the
NRC issued warnings about control rods at 69 other reactors. Maintenance
inspections at several of these reactors revealed "cracked and leaking
penetration nozzles in the top of reactor pressure vessels."
Penetration nozzles are the holes through which control rods are
inserted. "One function of the nozzles is to maintain the reactor
coolant-system pressure boundary," an NRC spokesman explains.
"Control-rod driveshafts pass through penetration nozzles, which sit at
the top of a reactor vessel head. Cracking represents a degradation of
the primary reactor coolant-system boundary. Hence, it is potentially
Owners of scores of nuclear power plants are now or will soon seek to
have their original reactor licenses extended for as long as 40 years.
The NRC, while generally supportive of extensions, has opened a
technical debate on safety issues. Until recently, the two sides of the
relicensing debate divided themselves as they always had, between pro-
and antinuclear lobbies. The control-rod problem, however, has caused a
debate within the nuclear industry. Engineers who work for companies
that operate reactors believe the problems can be solved with technical
fixes and additional inspections. Nuclear experts employed outside the
power industry, at NASA and major universities, believe it is time to
retire reactors that were designed back when Detroit was still putting
tail fins on cars, and come up with fresh ideas.
Because university-based nuclear-engineering programs in the United
States draw about as much interest as courses in pre-Columbian pottery,
some of the most forward-looking ideas for 21st century reactor design
are coming from abroad.
Some engineers believe the best solution to the control-rod problem is
to abandon them entirely, and switch to self-regulating fission reactors
that work without these trouble-prone control devices. One of the more
promising of these innovations is the Refueling by All Pins Integrated
Design (RAPID) created by a team led by Mitsuru Kambe, a research fellow
at the Central Research Institute of Electric Power Industry in Tokyo.
Kambe proposes replacing complex, electromechanical control rods with a
thermometer-like device that he calls a lithium expansion module (LEM).
Kambe's LEM relies on basic physics: Hot liquid expands. "The LEM is
composed of a reservoir and an envelope in which 95 percent enriched
lithium-6 (Li-6), a neutron absorber, is enclosed. The lower part of the
envelope also contains the gas argon," Kambe says. "Li-6 melts at 358°F,
so it is liquid during reactor operation."
In conventional nuclear reactors, control rods are solid assemblies.
Damage to the channels that guide these rods can prevent them from fully
engaging. Kambe says that in the LEM, the Li-6 is supported in the upper
part of the envelope by the surface tension exerted on the gas-liquid
interface. "This is the same principle as a thermometer," he says. "If
the core temperature increases to an unacceptable level [see 1], the
gas-liquid interface goes down [see 2], and Li-6 in the core region
absorbs the neutrons. The reactivity of the core decreases automatically
The simplicity and reliability of the design reflect the original plan
for the reactor. Kambe had first thought of the idea as a power plant
for a base on the moon. "RAPID-A is the terrestrial variation of
RAPID-L," he says. The "L" stands for Lunar Base. Its development is
funded by Japan's Atomic Energy Research Institute. "The design criteria
was to assure maintenance-free and reliable performance without any
inspection for 10 years. It would be quite easy to operate similar
RAPID-A reactors on the Earth."
Introducing a new reactor design won't be as easy as redesigning a car
for a new model year. Kambe, however, remains optimistic. "While we have
not conducted a reactor test, RAPID designs are based on the proven
fast-breeder reactor technology," he says, referring to the Phenix and
Super Phenix reactors in France and the Joyo reactor in Japan. A
considerable investment was made in this technology because fast
breeders, which produce more fuel than they consume, were envisioned as
the long-term favorite. In the United States, the technology was
abandoned after Three Mile Island sent the nuclear industry into an
economic tailspin. "The only new technologies introduced in RAPID
designs are the innovative reactor control systems. We have made many
test specimens and tested them, so their function is already verified."