IEEE Spectrum - North American - March 2016 - 40

MaR 2016

nORTh aMERICan

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To explain the principles at work, let's start with high school physports and borders that need to screen for nuclear
ics. Recall that an atom is mostly empty space, with a hefty nucleus
materials, extolling the system's superiority to
surrounded by a cloud of diminutive electrons. Most of the time, a
existing technologies that use X-rays or gamma
passing muon streaks through the atom's space unimpeded, but occarays: Muon-imaging systems can look through thick
sionally it interacts with the atom's constituent particles. In one kind
layers of lead and steel, and they don't make use
of interaction, the muon, which carries either a positive or negative
of a dangerous radiation source.
electric charge, either pulls or pushes a negatively charged electron
Then, in March 2011, Fukushima Daiichi melted
out of its normal orbit. That process causes the muon to lose a few
down.
electron volts of energy and slow down.
Some decades back, physicists came up with a basic imaging method
aFtEr bringing a MEaSurE of stability to the
to take advantage of these electron interactions, focusing on lowcrippled reactors, TEPCO has been striving to charenergy muons that lose enough energy to halt them in their tracks.
acterize the mess they have become. Roboticists at
This technique got an exotic tryout in 1968, when a team of physicists
Japan's major tech companies are developing unique
installed a detector inside the base of an Egyptian pyramid. The detecshape-shifting bots that can survive the site's lethal
tor counted incoming muons and traced their paths back through the
radiation and navigate the reactor buildings' compyramid to determine whether they'd traveled through solid limestone,
plex layouts. Last year TEPCO sent the first robots
where densely packed atoms would cause more low-energy muons to
into the containment vessels, the concrete and
stop, or the air of a secret chamber, which would allow more to whiz
steel structures that surround the reactor vessels.
through. After several months of analyzing the data, the physicists
TEPCO has deemed these robot excursions a sucgot their answer: no hidden chamber (and hence no hidden treasure).
cess (including one in which a bot got stuck inside),
But there's another and more subtle signal that can be teased out
and plans to continue them. But no robot has yet
of the interplay of particles. If a speeding muon passes close enough
approached the centers of the containment vessels,
to the nucleus, which carries a positive charge, the muon is either
where thick concrete pads sit directly under the
repelled or attracted. That interaction
reactor vessels. If nuclear fuel melted
alters the direction of the muon's flight.
through the reactor vessels (a near cerYou may remember that each element in
tainty in reactors 1 and 3), that's where
the periodic table is defined by the numit landed.
ber of protons in its nucleus; helium has
Barrett, TEPCO's decommissioning
2 protons, for example, while uranium
consultant, says critical questions must
has 92. Now imagine two muons, one
be answered about the breach of the
flying through a helium-filled balloon
reactor vessels: "Was it a slower flow,
and another zipping through a chunk
or a catastrophic blowout? How much
of uranium ore. The muon passing
water was on the floor of the containthrough uranium experiences a stronment vessel when it breached through?
ger repulsion or attraction from the hefty
Did it flow horizontally, like the lava flow
nucleus, and is pushed or pulled farther
from volcanoes?" What's more, in the
off course.
superheated conditions during the meltBorozdin's Los Alamos group saw
down, the nuclear fuel must have mixed
potential here. By analyzing how muons
with other disintegrating reactor comscattered after passing through a mateponents to form an unpredictable subrial, they could determine what that
stance that nuclear specialists wryly
material was. In a breakthrough 2003
refer to as corium, he says. "Uranium,
report, published in Nature, they sugsteel, lead, debris, concrete, handrails,
gested that detectors based on muon
electric cables, aluminum-it's going to
scattering could, for example, distinbe a real agglomeration."
guish "a block of uranium concealed
Muon imaging may be the best way to
inside a truck full of sheep."
look inside Fukushima's deadliest spaces
Decision Sciences was soon formed to
and find answers. In 2011, two teams of
turn the concept into a commercial techphysicists came to this conclusion and
nology. Borozdin helped the company
started building muon detectors specifdevelop its scanning system, and he offiically for Fukushima Daiichi. Japanese
cially joined the staff in 2014. The sysresearchers at the High Energy Acceltem uses two detectors: one above the
erator Research Organization (known
FirSt tryout: the muon transmission
object of interest to register the incomas KEK) constructed a system that uses
detectors installed next to reactor 1
[top photo] produced shadowy images
ing muons' trajectory, and one below to
muon transmission imaging, the same
of the building's interior after collecting
record the scattering. Decision Sciences
type of system used in the Egyptian
data for one month and three months
began marketing its scanners to cargo
pyramid. In a separate effort, Toshiba,
[bottom two images].

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