In reporting on the nuclear situation in Japan, terms describing the
possible state of the reactors have been used by Japanese officials and
international experts. Unfortunately, these terms are not always used
consistently. We discuss some of them below.
The reactor core containing nuclear fuel is the “engine” of
a nuclear power reactor. The reactor core resides in the
lower region of a metal “pot” called the reactor vessel. The
reactor vessel is enclosed within the primary containment
structure, consisting of the drywell and wetwell (see
diagram), which is designed to contain radioactive
materials released during a reactor accident. The reactor
building provides the secondary containment, which is
intended to prevent any leaks from the primary
containment from escaping to the environment. The air in
the reactor building is sent through filters to remove any
radiation before being released to the outside.
The fuel consists of uranium or uranium/plutonium
pellets stacked inside long tubes made from zirconium
metal. When a reactor is operating, the fuel gets very hot.
The fuel is immersed in water, and the heat produces
steam, which is used to drive a turbine to produce
electricity. The water also keeps the fuel from overheating,
and is continuously circulated through the reactor core to
carry away excess heat. Even f the reactor shuts down, the
fuel will remain hot for a long time, so must still be cooled.
Fuel Damage
If the pumps that circulate the cooling water are not
operating, the water in the reactor vessel will heat up and
evaporate, and the fuel can become uncovered inside the
reactor vessel. This situation also occurs if something like a
pipe rupture causes the water to drain out of the reactor
vessel. At this point, the zirconium cladding on the fuel
rods will start to heat up, blister, and then rupture.
Radioactive material that normally collects in the gap
between the fuel pellets and the cladding is then released
into the reactor vessel.
Radioactivity Release Pathways
The radioactive material released from damaged fuel into
the reactor vessel can get into the primary containment. To
protect the reactor vessel and attached piping from
rupturing due to high pressure, relief valves automatically
open to discharge steam—and the radioactive material
along with it—into the primary containment structure.
Workers may also manually open the relief valves to
prevent high pressure in the reactor vessel from impeding
the flow of makeup water, such as the sea water that has
reportedly been injected into some of the Japanese
reactors.
Nuclear Accident ABCs
F A C T S H E E T
MARCH 2011
Mark I containment system. Figure adapted
from www.nucleartourist.com
© UCS March 2011
In addition, a steam-driven emergency system called the
reactor core isolation cooling (RCIC) system uses steam
from the reactor vessel to spin a small turbine connected
to a pump that transfers makeup water to the reactor
vessel. After the steam is used for this purpose, the
steam—along with the radioactive material—is deposited
into the primary containment.
Partial Meltdown
If the fuel is uncovered by water and exposed for a period
of hours, it will start to melt. This makes cooling more
difficult as the melted fuel clogs the spaces between the
fuel rods. The melted fuel will start to collect on the
bottom of the steel reactor vessel (sometimes called the
lower head). The molten fuel will begin to burn its way
through the reactor vessel.
Complete Meltdown
If the water level is low enough, essentially all the fuel in
the core can melt and will fall to the bottom of the reactor
vessel. It will be a matter of hours before the fuel melts
through the steel reactor vessel and onto the concrete
floor of the primary containment. The containment is
designed to contain the melted fuel and its radioactive
emissions, but there are ways in which the primary
containment can fail in the event of a meltdown; this has
been a concern with the Mark I containment, which the
affected Japanese reactors use. The emissions will generate
increasing pressure over a period of days and weeks, which
can lead to collapse of the primary containment if it is not
relieved.
The primary containment has a typical leak rate of roughly
1% of its volume per day, so some of the radioactive
material will leak into the secondary containment (the
reactor building). The reactor building is normally kept at a
reduced pressure relative to the outside so any radioactive
emissions will stay inside the building. However, this
requires the building itself to be intact.
A meltdown does not necessarily mean that there will be a
large release of radioactivity. This will depend on the
integrity of the primary and secondary containments.
Radioactive Isotopes
Radioactive materials decay, releasing particles that can
damage living tissue and lead to cancer. Some elements
have different forms, called isotopes, that differ in the
number of neutrons in the nucleus. The radioactive
isotopes of greatest concern in a nuclear power accident
are iodine-131 and cesium-137.
Iodine-131 has a half-life of 8 days, meaning half of it will
have decayed after 8 days, and half of that in another 8
days, etc. Therefore, it is of greatest concern in the days
and weeks following an accident. It is also volatile so will
spread easily. In the human body, iodine is taken up by the
thyroid, and becomes concentrated there, where it can lead
to thyroid cancer in later life. Children who are exposed to
iodine-131 are more likely than adults to get cancer later in
life. To guard against the absorption of iodione-131,
people can proactively take potassium iodine pills so the
thyroid becomes saturated with non-radioactive iodine and
is not able to absorb any iodine-131
Cesium-137 has a half-life of about 30 years, so will take
more than a century to decay by a significant amount.
Living organisms treat cesium-137 as if it was potassium,
and it becomes part of the fluid electrolytes and is
eventually excreted. Cesium-137 is passed up the food
chain. It can cause many different types of cancer.