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| Terrorist Attack-World Trade Center |
But to create terror, the terrorist
doesn't necessarily have to kill or even injure his victims. What better
terrorist device is there, than one which simply disseminates terror?
A relatively small amount of fissile
material might be just what the terrorist needs for this purpose. A small
amount of plutonium (in a backpack or in a 'suicide' vest) that is made to
reach 'criticality' would have a dramatic effect, cause local injury and death
and 'terrorize' the targets who would not know how much real damage was
inflicted at the time nor what the future truly held for them.
The nuclear weapons and the nuclear
energy industries are subject to the normal range of accidents, from vehicle
wrecks to falls from a height but there is one type of accident which is unique
to these sectors, the criticality accident. This is an event where an
amount of fissile material accidentally comes together into a supercritical amount.
There is a sudden release of energy and deadly radiation.
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| The Atom |
It is all about the number of neutrons
which are released. By definition, criticality is
reached when conditions in a nuclear reactor are such that the
fissionable material can sustain a chain reaction by itself. In
effect, criticality is reached if each fission event causes, on average,
exactly one other, resulting in a self-sustaining fission chain reaction (see
post: Would You Prefer Plutonium or Uranium in Your Nuclear Weapon?).
When a uranium-235 atom undergoes fission, it releases 2-3 neutrons (average 2.4). Therefore, criticality is reached if every released neutron has a 1/2.4 = 0.42 = 42% probability of causing another fission event as opposed to either being absorbed by a non-fission capture event or escaping from the fissile core.
The criticality of a system can be calculated by comparing the rate at which neutrons are produced to the rate at which they are lost through absorption (boron-containing absorbing rods) and leakage out of the reactor core.
When a uranium-235 atom undergoes fission, it releases 2-3 neutrons (average 2.4). Therefore, criticality is reached if every released neutron has a 1/2.4 = 0.42 = 42% probability of causing another fission event as opposed to either being absorbed by a non-fission capture event or escaping from the fissile core.
The criticality of a system can be calculated by comparing the rate at which neutrons are produced to the rate at which they are lost through absorption (boron-containing absorbing rods) and leakage out of the reactor core.
A properly functioning nuclear reactor
is a system that controls this criticality or balance of neutrons. When a
reactor is said to have 'gone critical', it means it is in a stable configuration
producing a constant power.
'Subcritical' refers to a system where
the loss rate of neutrons is greater than the production rate of neutrons and
therefore the number of neutrons decreases as time goes on. During shutdown,
the nuclear reactor is placed in a 'subcritical' configuration so that the
neutron population and power decreases.
'Supercritical' refers to a system
where the production rate of neutrons is greater than the loss rate of neutrons
and therefore the neutron population increases.
It is when the neutron population
remains constant that there is a perfect balance between production rate and
loss rate, and the nuclear system is said to be 'critical'. The
criticality of a system can be calculated by comparing the rate at which
neutrons are produced, from fission and other sources, to the rate at which
they are lost through absorption and leakage out of the reactor core. A nuclear
reactor is a system that controls this criticality or balance of neutrons.
A criticality accident is an
unintentional increase of nuclear chain reactions (in enriched uranium or
plutonium), releasing a surge of neutron radiation into the
surroundings. Criticality accidents cannot reproduce the same conditions
of an atomic bomb, so a nuclear explosion does not occur. The nuclear
reaction however, does create heat which causes the fissile material to expand,
so that, within a few seconds, the nuclear reaction becomes subcritical again.
Since uses of atomic power were
developed, at least 60 criticality accidents have occurred between
1945 and 2011 in collections of fissile materials many in facilities outside
nuclear reactors. Many of these accidents related to nuclear weapons research
conducted after the bombings of Hiroshima (August 6, 1945) and Nagasaki (August
9, 1945). Some of these accidents have resulted in death through radiation
exposure of those close to the event. Other people further away from the source
of radiation have suffered long-term effects and development of cancer years
later.
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| Reactor-Idaho National Laboratory |
The SL-1 accident at the National
Reactor Testing Station near Idaho Falls, USA on January 3, 1961 was a
criticality accident that took place inside a military facility reactor (see
post: Man-Made and Natural Nuclear Accidents).
In the former Soviet Union, at the Mayak nuclear fuel
processing center in central Russia an experiment with plutonium purification
techniques was planned on December 10, 1968. Two operators apparently
placed plutonium organic solution into an 'improvised' container. After
most of the solution had been poured out, there was a flash of light and heat.
The operator panicked, dropped the bottle and ran from the room.
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| Mayak Nuclear Fuel Reprocessing Center |
Mayak is located in the closed
nuclear town of Ozersk, is still operational today and is Russia’s only
operational facility for reprocessing spent nuclear fuel from VVER-440 type
reactors and spent fuel from nuclear submarines, as well as fuel imported from
other countries. Decades of spilling radioactive poison into the nearby Techa
River Cascade and Lake Karachai have earned the site the moniker of being
the most radioactively contaminated place on earth.
On September 23, 1983, an operator
at the RA-2 research reactor in Centro Atomico Constituyentes, Buenos Aires,
Argentina died from a fatal radiation dose of 3700 rads (37 Gy) while
changing the fuel rod configuration with moderating water in the reactor. Two
other workers were injured.
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| Centro Atomico Constituyentes, Beunos Aires |
On September 30, 1999, at the Japanese uranium
reprocessing facility in Tokai, Ibaraki Prefecture, workers put a mixture
of uranyl nitrate solution into a precipitation tank which was not
designed to dissolve this type of solution and caused an eventual critical mass
to be formed, and resulted in two workers dying from radiation poisoning.
There have been many more criticality
accidents but the most infamous cases of criticality accidents occurred at
the Los Alamos Laboratory in New Mexico while scientists worked with
the 'demon core'. This type of investigation aimed at creating a
temporarily critical mass of fissionable material by sliding a slug of
uranium-235 hydride through a larger mass of the same substance. They called it
the 'Dragon Experiment'. These manual experiments continued with the
hemispheres of plutonium used in the Trinity bomb, the first test of an atomic
weapon (July 16, 1945), the Fatman, and the bombs used at Operation Crossroads
in 1946.
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| Los Alamos National Laboratory, New Mexico |
Along with the successful tests, there
were numerous criticality accidents with run-away output of energy and
radiation. Within a year of the Trinity test, accidents would claim the lives
of two members of the team that assembled the plutonium core at Trinity.
It became known as the demon core,
the 14 pound (6.2 kilogram) mass of plutonium which went
critical in two separate events at the Los Alamos laboratory, New Mexico,
the first in 1945 and the second in 1946. This same core was used in
an atomic bomb test in 1946, five weeks after the second fatal accident. The
explosion proved in practice to have a slightly increased yield over other
cores which had not been subjected to criticality excursions.
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| The Trinity (Test) Detonation |
Harry Daghlian was a
physicist who made a mistake while working alone performing neutron
reflection experiments on the (demon) core. On the night of August 21,
1945, Daghlian was working alone but accompanied by a guard sitting 12 feet
away making critical mass studies and measurements, stacking blocks of tamper
material (tungsten carbide bricks) around the mass of fissionable material. The
assembly was close to a critical configuration and Daghlian was lifting one
last heavy piece of tamper material. As this piece approached the set-up, instruments
warned that fission multiplication was imminent. He moved the block in hand to
set it further from the pile and accidently let it slip, dropping it directly
on top of the set-up.
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| Harry K. Daghlian |
There was a sudden 'blue glow' and
Daghlian quickly disassembled the critical material and its tamper. Harry
Daghlian absorbed huge doses of radiation, resulting in acute radiation
poisoning and died less than 28 days later. The guard, sitting 12
feet away, suffered no obvious injury at the time. His estimated exposure was
0.31 Gy. He died of acute myelogenous leukemia 33 years later, age
62.
Louis Slotin was a Canadian physicist from Manitoba working as a senior scientist at the Los Alamos Laboratory. On May 21, 1946, Slotin had already become the local expert, performing the same test almost a dozen separate times before, often in his trademark bluejeans and cowboy boots, in front of a roomful of observers.
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| Louis Slotin |
On this occasion, Slotin and seven other
scientists were conducting the experiment to verify the exact point at which a
subcritical mass of fissile material could be made critical by the positioning
of neutron reflectors. The test was risky and known as 'tickling the
dragon's tail'.
Enrico Fermi had told Slotin and others that they would be 'dead within a year' if they continued performing it.
Enrico Fermi had told Slotin and others that they would be 'dead within a year' if they continued performing it.
The technique involved bringing a hollow
hemisphere of beryllium (the neutron reflector) around a mass of fissionable
material which was resting in a similar lower hollow hemisphere. The
system was checked with two one-inch spacers between the upper hemisphere and
the lower shell which contained the fissionable material; the system was
subcritical at this time.
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| 'Tickling the Dragon's Tail' |
The experiment required the
operator to place the two half-spheres of beryllium around the core,
manually lowering the top reflector over the core via a thumb hole on the top.
As the reflectors were manually moved closer and farther away from each other,
counters would measure the relative activity from the core. Allowing them
to close completely would result in the instantaneous formation of a critical
mass and a lethal power excursion.
Spacers between the shells were removed
so that one edge of the upper hemisphere rested on the lower shell while the
other edge of the upper hemisphere was supported by a screwdriver.
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| Partially Reflected Plutonium Sphere |
Under Slotin's unapproved protocol, the
only thing preventing the energy surge was the blade of the standard flat-head
screwdriver. This upper shell edge was permitted to approach the lower
shell slowly. While one hand held the screwdriver, the other hand was holding
the upper shell with the thumb placed in the opening at the polar point.
Then the screwdriver slipped and the
upper shell fell into position around the fissionable material. Of the eight
people in the room, two were directly engaged in the work leading to this
incident. The blue glow filled the room, a heat wave felt by all.
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| Exposure Victims in Slotin Criticality Event |
Realizing what had happened, Slotin
knocked the two halves apart, stopping the chain reaction and likely saving the
lives of the other men in the laboratory. His body positioning over the
apparatus also shielded the others from much of the neutron radiation.
Slotin received a lethal dose in under a
second and died nine days later from acute radiation poisoning. The nearest
physicist to Slotin, Alvin Graves, was watching over Slotin's shoulder and
was thus partially shielded by him, receiving a high but non-lethal dose of
radiation.
Graves was hospitalized for several
weeks with severe radiation poisoning, developed chronic neurological and
vision problems as a result of the exposure, suffered a significant shortening
of his lifespan and died of a radiation-induced heart attack 20 years
later.
The other six people in the room were
far enough away from the assembly to avoid fatal injury, but they all suffered
other complications as a result of the accident. Two suffered severe shortening
of their lives and died years later from radiation-induced complications -
leukemia at age 42 for one observer (18 years after the accident) and the
second died from aplastic anemia.
The same demon core was later
put to use in an atomic explosion July 1, 1946, demonstrating that the
criticality experiments of Daghlian and Slotin had increased the efficiency of
the weapon.
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| Blue Glow of Criticality? |
Blue glow and heat are two
sensory effects of a criticality accident. Electric sparks in the
air (lightening is an example) will often appear blue. This effect occurs due
to the energy emission of excited molecules or atoms (mostly nitrogen and
oxygen) falling back to their unexcited state.
The cause of the heat effect experienced
in a criticality accident is uncertain. It is either due to the realization by
the observer of the terrible event which just took place (and its consequences)
or possibly due to non-thermal stimulation of heat-sensing skin nerves by the
energy of the radiation or by a true heat flux produced by the event. One other
possible cause of this heat sensation is non-thermal damage done to tissue
on the cellular level by the ionization and production of free radicals (see
post: Low Dose Radiation and Its Effects) caused by exposure to
intense radiation.
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| The Electromagnetic Spectrum |
There is a characteristic blue glow
associated with nuclear reactors called 'Cherenkov radiation'. This blue colour
is due to electromagnetic radiation which is emitted when an electron or
other charged particle passes through a medium such as water at a
speed greater than the velocity of light usually seen in that medium. The
charged particles polarize the molecules of the water which then turn back to
their ground state, emitting radiation (the blue glow) in the process.
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| Cherenkov Radiation |
Thirteen kilograms or 5 kilograms...it's
still not much to carry, under a vest or in a back pack. If there were a
technique, a mechanism whereby the demon core could be brought to the edge of
criticality, expose everyone within twenty feet to deadly doses of radiation,
then settle back down, a terrorizing effect would be realized.
The suicide wearer of the vest/back pack
could wander the crowded city of New York or London, triggering the plutonium
core towards criticality again and again until his exposure brings him to the
edge of death.
Exhausted, the terrorist could hand off
the demon core to an associate who could continue the process, exposing
hundreds, perhaps thousands to lethal radiation and maybe millions to the
longer term effects of radiation exposure (leukemia, heart disease, other
cancers).
The only clue that anything might be
amiss, would be the sensation of 'heat' pulses next to the terrorist and an
intermittent, pale, 'divine' blue glow (see post: The Colors of Faith), signs that death, an
unpleasant death, will soon upon you.
Click on the link below to watch a dramatization of a
fictionalized (but based on fact) Los Alamos criticality event.
*The risks of radiation: subject of
research for the novel Whip the Dogs -
Amazon Kindle
