A loss-of-coolant accident (LOCA) is a mode
of failure for a nuclear reactor; if not managed effectively, the results of a
LOCA could result in reactor core damage. Each nuclear plant's Emergency Core
Cooling System (ECCS) exists specifically to deal with a LOCA.
Nuclear reactors generate heat internally; to remove this
heat and convert it into useful electrical power, a coolant system is used. If
this coolant flow is reduced, or lost altogether, the nuclear reactor's
emergency shutdown system is designed to stop the fission chain reaction.
However, due to radioactive decay the nuclear fuel will continue to generate a
significant amount of heat. The decay heat produced by a reactor shutdown from
full power is initially equivalent to about 5 to 6% of the thermal rating of
the reactor. If all of the independent cooling trains of the ECCS fail to
operate as designed, this heat can increase the fuel temperature to the point
of damaging the reactor.
● If water is present, it may boil,
bursting out of its pipes. (For this reason, nuclear power plants are equipped
with pressure-operated relief valves and backup supplies of cooling water.)
● If graphite and air are present, the
graphite may catch fire, spreading radioactive contamination. This situation
exists only in AGRs, RBMKs, Magnox and weapons-production reactors, which use
graphite as a neutron moderator. (see Chernobyl disaster.)
● The fuel and reactor internals may melt;
if the melted configuration remains critical, the molten mass will continue to
generate heat, possibly melting its way down through the bottom of the reactor.
Such an event is called a nuclear meltdown and can have severe consequences.
The so-called "China syndrome" would be this process taken to an
extreme: the molten mass working its way down through the soil to the water
table (and below) - however, current understanding and experience of nuclear fission
reactions suggests that the molten mass would become too disrupted to carry on
heat generation before descending very far; for example, in the Chernobyl
accident the reactor core melted and core material was found in the basement,
too widely dispersed to carry on a chain reaction (but still dangerously
radioactive).
● Some reactor designs have passive safety
features that prevent meltdowns from occurring in these extreme circumstances.
The Pebble Bed Reactor, for instance, can withstand extreme temperature
transients in its fuel. Another example is the CANDU reactor, which has two
large masses of relatively cool, low-pressure water (first is the heavy-water
moderator; second is the light-water-filled shield tank) that act as heat sinks.
Under operating conditions, a reactor may passively (that is,
in the absence of any control systems) increase or decrease its power output in
the event of a LOCA or of voids appearing in its coolant system (by water
boiling, for example). This is measured by the coolant void coefficient. Most
modern nuclear power plants have a negative void coefficient, indicating that
as water turns to steam, power instantly decreases. Two exceptions are the
Russian RBMK and the Canadian CANDU (in the latter case, for reasons outlined
at the site Nuclearfaq, which also describes the safety systems designed to
reliably handle this feature of the design). Boiling water reactors, on the
other hand, are designed to have steam voids inside the reactor vessel.
Modern reactors are designed to prevent and withstand loss of
coolant, regardless of their void coefficient, using various techniques. Some,
such as the pebble bed reactor, passively slow down the chain reaction when
coolant is lost; others have extensive safety systems to rapidly shut down the
chain reaction, and may have extensive passive safety systems (such as a large
thermal heat sink around the reactor core, passively-activated backup
cooling/condensing systems, or a passively cooled containment structure) that
mitigate the risk of further damage.
A critical accident (also sometimes referred to as an
"excursion" or "power excursion") occurs when a nuclear
chain reaction is accidentally allowed to occur in fissile material, such as
enriched uranium or plutonium. The Chernobyl accident is an example of a
criticality accident. This accident destroyed a reactor at the plant and left a
large geographic area uninhabitable. In a smaller scale accident at Sarov a
technician working with highly enriched uranium was irradiated while preparing
an experiment involving a sphere of fissile material. The Sarov accident is
interesting because the system remained critical for many days before it could
be stopped, though safely located in a shielded experimental hall. This is an
example of a limited scope accident where only a few people can be harmed,
while no release of radioactivity into the environment occurred. A criticality
accident with limited off site release of both radiation (gamma and neutron)
and a very small release of radioactivity occurred at Tokaimura in 1999 during
the production of enriched uranium fuel. Two workers died, a third was
permanently injured, and 350 citizens were exposed to radiation.
Decay heat accidents are where the heat generated by the
radioactive decay causes harm. In a large nuclear reactor, a loss of coolant
accident can damage the core: for example, at Three Mile Island a recently
shutdown (SCRAMed) PWR reactor was left for a length of time without cooling
water. As a result the nuclear fuel was damaged, and the core partially melted.
The removal of the decay heat is a significant reactor safety concern,
especially shortly after shutdown. Failure to remove decay heat may cause the
reactor core temperature to rise to dangerous levels and has caused nuclear
accidents. The heat removal is usually achieved through several redundant and
diverse systems, and the heat is often dissipated to an 'ultimate heat sink'
which has a large capacity and requires no active power, though this method is
typically used after decay heat has reduced to a very small value. However, the
main cause of release of radioactivity in the Three Mile Island accident was a
pilot-operated relief valve on the primary loop which stuck in the open
position. This caused the overflow tank into which it drained to rupture and
release large amounts of radioactive cooling water into the containment
building.
Transport accidents can cause a release of radioactivity
resulting in contamination or shielding to be damaged resulting in direct
irradiation. In Cochabamba a defective gamma radiography set was transported in
a passenger bus as cargo. The gamma source was outside the shielding, and it
irradiated some bus passengers.
In the United Kingdom, it was revealed in a court case that
in March 2002 a radiotherapy source was transported from Leeds to Sellafield
with defective shielding. The shielding had a gap on the underside. It is
thought that no human has been seriously harmed by the escaping radiation.
Equipment failure is one possible type of accident, recently
at Białystok in Poland the electronics associated with a particle
accelerator used for the treatment of cancer suffered a malfunction. This then
led to the overexposure of at least one patient. While the initial failure was
the simple failure of a semiconductor diode, it set in motion a series of
events which led to a radiation injury.
A related cause of accidents is failure of control software,
as in the cases involving the Therac-25 medical radiotherapy equipment: the
elimination of a hardware safety interlock in a new design model exposed a
previously undetected bug in the control software, which could lead to patients
receiving massive overdoses under a specific set of conditions.
An assessment conducted by the Commissariat à l’Énergie
Atomique (CEA) in France concluded that no amount of technical innovation can
eliminate the risk of human-induced errors associated with the operation of
nuclear power plants. Two types of mistakes were deemed most serious: errors
committed during field operations, such as maintenance and testing, that can
cause an accident; and human errors made during small accidents that cascade to
complete failure.
In 1946 Canadian Manhattan Project physicist Louis Slotin
performed a risky experiment known as "tickling the dragon's tail"
which involved two hemispheres of neutron-reflective beryllium being brought
together around a plutonium core to bring it to criticality. Against operating
procedures, the hemispheres were separated only by a screwdriver. The
screwdriver slipped and set off a chain reaction criticality accident filling
the room with harmful radiation and a flash of blue light (caused by excited,
ionized air particles returning to their unexcited states). Slotin reflexively
separated the hemispheres in reaction to the heat flash and blue light,
preventing further irradiation of several co-workers present in the room.
However Slotin absorbed a lethal dose of the radiation and died nine days
afterwards.
Lost source accidents, also referred to as an orphan source
are incidents in which a radioactive source is lost, stolen or abandoned. The
source then might cause harm to humans. For example, in 1996 sources were left
behind by the Soviet army in Lilo, Georgia. Another case occurred at Yanango
where a radiography source was lost, also at Samut Prakarn a cobalt-60
teletherapy source was lost and at Gilan in Iran a radiography source harmed a
welder. The best known example of this type of event is the Goiânia accident
which occurred in Brazil.
The International Atomic Energy Agency has provided guides
for scrap metal collectors on what a sealed source might look like. The scrap
metal industry is the one where lost sources are most likely to be found.
Some accidents defy classification. These accidents happen
when the unexpected occurs with a radioactive source. For instance if a bird
were to grab a radioactive source containing radium from a window sill and then
fly away with it, return to its nest and then die shortly afterwards from
direct irradiation then a minor radiation accident would have occurred. As the
hypothetical act of placing the source on a window sill by a human permitted
the bird access to the source, it is unclear how such an event should be
classified, as a lost source event or a something else.Radium lost and
found describes a tale of a pig walking about with a radium source
inside; this was a radium source lost from a hospital. There are also accidents
which are "normal" industrial accidents that involve radioactive
material. For instance a runaway reaction at Tomsk involving red oil caused
radioactive material to be spread around the site.