Radioactivity levels around a crippled Japanese nuclear reactor
rose again on Tuesday, but U.S. experts said they don't yet know whether the
reactor or its containment building has been breached, despite a multitude of
media reports to the contrary.
Concerns rose yesterday after Japanese officials reported
that the Unit 2 reactor at the Fukushima Daiichi plant sustained damage to the
so-called suppression chamber, which holds water and steam released from the
reactor core. Damage to the suppression chamber could be a sign of a larger
problem, they said.
Still, if the plant's steel pressure vessels and its
reinforced concrete containment structures are intact, then the damage to the
surrounding area would be greatly reduced.
"Unless we breach the reactor pressure vessel, we shouldn't
worry too much," noted Professor Ahmad Hassanein, head of the Nuclear
Engineering Department at Purdue University. "This is not going to be a
Experts in the U.S. also cautioned that they don't know when
the extent of the damage will be fully understood. "Obviously, radiation levels
were way up and they had to evacuate the vast majority of people on site," said
Jeff Terry, an assistant professor of physics at Illinois Institute of
Technology (IIT). "But since then, the information has been pretty sketchy."
Secondary Buildings Damaged
The issue of a breach is critical because reactor pressure
vessels and containment buildings are designed to capture the by-products of
accidents. During a so-called meltdown, however, fuel rods can overheat and
molten uranium can burn through the nine-inch-thick steel vessel surrounding
the reactor core. In extreme cases, many observers worry that the overheated
fuel could potentially burn through the reinforced concrete floor slab beneath
the reactor, contaminating the ground below. Such a scenario is considered unlikely,
however, because the containment building's reinforced concrete floor and roof
slabs are at least four feet thick.
Although countless newspaper and television reports in the
past week have indicated that the containment buildings at Fukushima Daiichi's
Nos. 1, 2 and 3 reactors have exploded, experts in this country contend that
those explosions were confined to corrugated metal structures, which aren't
designed to capture a meltdown. They believe the containment buildings in
reactor Nos. 1 and 3 are undamaged, but aren't sure about reactor No. 2.
"The main containment buildings are still intact as far as
we know," said James F. Stubbins, a professor of nuclear, plasma and
radiological engineering at the University of Illinois. "The explosions
occurred in what they call the secondary containment,' which is a building
above and around the primary containment structure."
Engineering experts said that the buildings exploded when
hydrogen gas was released after built-in pressure release valves were
triggered. "As the water in the reactor core starts boiling away, the pressure
reaches such incredible levels that if you don't have a valve to release it,
the inner containment vessel would rupture," Terry said. "You'd rather release
radioactive nitrogen with a seven-second half-life than have the containment
vessel blow apart." Terry believes that the gases that built up inside reactor
buildings Nos. 1 and 3 were ignited by electrical sparks from nearby equipment.
High Radioactivity Readings
As a result of the explosions, radioactivity levels rose
sharply around the Fukushima Daiichi plant on Tuesday. The Nuclear
reported that a dose rate of 1.19 rems per hour was
observed at the site boundary and dose of 40 rems per hour was seen close to
the plant (a rem is a measure of biological damage to tissue). The 1.19 rem
reading dropped to about 0.06 rems per hour later in the day, however.
The readings are considered high, engineers said. On
average, individuals are subjected to about 0.6 rems per year. Nuclear power
plant workers are typically limited to about 5 rems per year. According the
1982 book, Nuclear Power: Both Sides
by Michio Kaku, 1,000 rems would kill a person a few days after exposure, 500
rems would kill half of the exposed population within a few weeks, 200-400 rems
would cause radiation sickness and hemorrhaging, and 50 rems would cause no
immediate visible effects, but could induce long-term damage.
The higher levels of radioactivity have caused countless
individuals around the world to purchase potassium iodide tablets, which are
said prevent accumulation of radioactive iodine in the thyroid. "There's a huge
amount of hysteria right now," Terry said. "As a result, you can't buy
potassium iodide in the U.S. at the moment. Every manufacturer has sold out."
Experts say that the only people in definite danger are
those who work at or near the plant. For that reason, many workers are being
shuttled in and out of the plant to shorten their exposure times.
Engineering professors contacted by Design News
said that the Japanese plants did extraordinarily well
in withstanding a mammoth earthquake, followed by a tsunami. "Let's not forget
this was an unprecedented disaster," said Hassanein of Purdue. "I'm impressed
that most of their 50 or so reactors are doing so well."
Still, the jury is out at Fukushima Daiichi reactor No. 2.
The Nuclear Energy Institute reported yesterday that the water level in the
reactor was approximately five-and-a-half feet below the tops of the 12-foot
fuel rods in the core. If that's true, it would mean that a great deal of
uranium is exposed to air -- typical reactors contain 40,000 or more rods,
bundled in hundreds of 8 x 8 assemblies.
"There's some melting of the fuel going on right now, but
it's not what many of use would characterize as a meltdown," said Stubbins of
the University of Illinois. "We're just keeping our fingers crossed that it
stays that way."