Dr Carle Gibbons,
Nuclear Safety Specialist
On the 29 March 2011 Tokyo Electric Power Company
(TEPCO) reported that it was not clear where plutonium 238, 239 and 240
had originated but that the levels were not in concentrations that are
dangerous to human health.
Taking into account the series of events since the
start of the problems at the Fukushima Dai-ichi nuclear plant one may
question why plutonium hadn’t been detected earlier. Not having access
to all data and information regarding the measures taken since the start
of the accident puts those with interest, in establishing what is going
on, at a considerable disadvantage. However, now with recent unrolling
events and information, questions arise regarding the choices made by
TEPCO in their response:
* Did TEPCO consider all appropriate options in
dealing with the accident, including those which offer much greater
containment, being dry options (encapsulation using sand/boron/cement)?
* In their selection of using water to cool fuel did
they identify all hazards and assess risks associated with this option,
including the risk of producing more hydrogen and further explosions?
* Also, in using sea-water to cool the fuel, did they
assess metallurgical damage of containments and chemical interaction
with the fuel?
* Did they assess the increased potential for
migration of fission products in clouds of steam and, more important,
the mobility of dissolved radioactive salts and particulates in the
sea-water escaping through breaches in the plant?
* An assessment should have shown a significant risk
of contaminated water escaping from a damaged containment. Has TEPCO
also considered the risk of escaped radioactive material returning to
land as a result of the high spring tide or another tsunami?
* Knowing that fuel ponds had become dry and that the
fuel cladding was not intact, did TEPCO consider that their initial
response of dumping small amounts of water from a helicopter (futile in
terms of cooling effect) could make matters worse by allowing fuel
containments to further break down in generating steam?
* Again, considering their indiscriminate water
spraying from fire trucks, did TEPCO assess any associated risk
regarding this action?
Units 1, 2, 3 and 4 are of major concern at the at
the Fukushima Dai-ichi site. Mixed oxide fuel (MOX) is used to fuel Unit
3 at Fukushima. Plutonium oxide, recovered from spent fuel, is mixed
with depleted uranium oxide (8% Pu-239 mixed with U-238).
The fuel used by the other reactors at Fukushima is
enriched with U-235 (5% U-235 enrichment is the equivalent of 8%
Pu-239). During fission some of the remainder the fuel, U-238, partially
converts to plutonium isotopes. It is the plutonium, other fission
products and radioactive material escaping that are now of major
concern. Some have relatively short half-lives but others, like
plutonium, remain a hazard and a source of radiologically-induced cancer
for many thousands of years.
Understanding
Plutonium
Plutonium is a radioactive metal with Atomic Number
94 and is considered a man-made element. The most common radioisotopes
of plutonium are plutonium-238 (half-life of 87.7 years), plutonium-239
(half-life is 24,100 years) and plutonium-240 (half-life of 6,560
years). One kilogram of Pu-239 can produce sufficient heat to generate
nearly 10 million kilowatt-hours of electricity. All its isotopes can be
found in spent fuel that has been used in nuclear reactors.
External exposure to plutonium poses very little
health risk, since plutonium isotopes emit alpha radiation (which cannot
penetrate even the top-most layer of human skin), and almost no
penetrating beta or gamma radiation. In contrast, internal exposure to
plutonium is an extremely serious health hazard. It generally stays in
the body for decades, exposing organs and tissues to radiation, and
increasing the risk of cancer. Plutonium is also a toxic metal, and may
cause damage to the kidneys. Tolerance levels to exposure of plutonium
are difficult to pinpoint, only to say that they are very low. Limits
have been put as 5 micrograms based on studies of workers who made the
first atomic bomb. It should be considered as being extremely dangerous
to exceed 100 micrograms. (See accompanying chart.)
Future Risks at
Fukushima
The handling of the accident by TEPCO at the
Fukushima Dai-ichi nuclear plant has been questionable. Fuel
containments remain open for further migration of radioactive material
and, with significant danger, plutonium.
Reports of meltdown and breach of containment become
even more worrying. It is still not too late for other safety measures
to be considered. For example boron can be used to reduce the risk of
further fission occurring. Mixed with sand there is less likelihood that
sand will be able to displace loose fission materials to form a critical
mass and start a chain reaction. If a critical mass was to develop,
considerable heat would be generated, resulting in high levels of
radiation.
Regarding the current situation it is possible that
fissionable materials could move in the reactor vessel and form a
critical mass. This could be mitigated (to lessen the chance of
criticality occurring) by adding boric acid to sea-water (but did TEPCO
do this?). Should criticality occur, the core would increase to a high
temperature. Subsequently large volumes of gases generated would be
likely to result in an explosion occurring inside the containment
vessel. Such a breach would produce Chernobyl-like airborne
contamination (although Chernobyl was primarily a steam explosion) and a
widespread radiation risk to countries outside Japan.
My original assessment was made at a time when the
Japanese had declared the accident as an INES Level 4 (see table). Also,
at that time, the action by TEPCO was considered to be a well prepared
response to bring the situation under control. However, this is no
longer the case. Further analysis of the situation is that there is a
considerable risk of a disaster that is at least INES 5 and could
escalate to a Level 7 accident.
My assessment at 29 March 2011 is that the
probability of an event leading to criticality remains low. However,
breaches of containment have occurred and further breaches remain a high
risk. I fear that the situation will worsen from this moment in time as
it appears that these four reactors (1, 2, 3 and 4) are not yet under
control; hazards have not been identified; and therefore risks cannot be
assessed.
The
International Nuclear and Radiological Event Scale (INES)
Level Description
7 Major release of radioactive material with
widespread health and environmental
effects requiring implementation of planned and extended
countermeasures.
6 Significant release of radioactive material likely
to require implementation of
planned countermeasures
5 Limited release of radioactive material likely to
require implementation of some
planned countermeasures.Several deaths from radiation.
4 Minor release of radioactive material unlikely to
result in implementation of
planned countermeasures other thanlocal food controls.At least one death
from radiation.
3 Exposure in excess of ten times the statutory
annual limit for workers.Non-
lethal deterministic health effect (e.g., burns) from radiation.
2 Exposure of a member of the public in excess of 10
mSv.Exposure of a worker in
excess of the statutory annual limits.
1 Anomaly
