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Nuclear
Meltdown
The term - Meltdown -
refers to melting of the fuel in the reactor. Unfortunately, the term has
been loosely applied to refer to any case of fuel melting, however minor.
Only in several events - Three Mile Island 2 and Chernobyl - has there
been significant fuel melting and only in the case of Chernobyl were there
significant offsite releases.
Overheating of the fuel
typically can be caused only if there is an inability to remove heat from
the fuel. Two situations are the only likely causes:
Loss of coolant in the
reactor cooling system followed by a failure of the emergency core cooling
systems to operate
Failure of the reactor protection system to shutdown the reactor down when
required for a major fault
Such conditions are considered to be outside the design basis for nuclear
plants and are referred to as Class 9 accidents. The design of the plants
is intended to assure that such conditions do not occur - due to the
redundancy and diversity of the reactor protection, emergency core
cooling, and containment isolation systems, as well as the containment
structure itself. In spite of this, the US Nuclear Regulatory Commission
is requiring licensees to develop procedures for such cases. The
procedures are referred to as Severe Accident Guidelines.
If a meltdown does occur, a
release of radioactive materials to the environment can occur ONLY IF
there is also a major failure of the containment structure. For this to
occur, the following would also have to happen:
Overpressure of the
containment
Failure of the containment isolation systems, lines, and valves to close.
Potential causes of containment overpressure are:
Steam explosion in the
reactor vessel or a dropping of at least 20% of the fuel mass of a molten
core
Generation of gases either due to hydrogen generated from a chemical
reaction between Zircaloy (used in the fuel cladding) and steam at
temperatures above 3400F or due to carbon dioxide generated from
interaction of molten core material with the concrete structures under the
reactor.
Heating of the containment atmosphere due to a failure of the containment
cooling and spray systems.
For there to be a meltdown with releases offsite, the following sequence
would have to occur:
Failure of the reactor to
shutdown when required such that it continues to produce heat at a high
rate OR a major amount of coolant is lost from the reactor cooling system,
Diverse and redundant high and low pressure emergency cooling systems are
unable to provide cooling to the reactor cooling system,
Fuel melting starts and blockage of flow channels occurs in the reactor
such that cooling cannot be provided,
Diverse and redundant containment cooling and spray systems are unable to
provide cooling to the containment atmosphere,
Redundant Hydrogen recombiners will not operate,
Containment isolation system and associated valves do not close as
required,
Specialized high efficiency particulate, absolute, and charcoal filters do
not function as required.
A meltdown occurs when fuel
has overheated, melted, and flowed to the bottom of the reactor vessel,
where it will burn its way through the steel and then collect on the floor
of the primary containment structure. It is possible to have a meltdown
without a loss of primary containment; the containment is designed to hold
the melted fuel and its radioactive emissions. A loss of primary
containment occurs when the integrity of the containment structure is
compromised, allowing the melted fuel and/or radioactive isotopes to leak
into the secondary containment. The loss of secondary containment would
allow the melted fuel and/or radioactive isotopes to escape to the outside
environment.
“Meltdown” refers to
damage to fuel rods due to excessive heating when the reactor’s cooling
systems fail. Because of their high level of radioactivity, fuel rods in a
reactor core or a spent fuel pool generate a lot of heat even if the
reactor is not operating. So they must be surrounded by water that is
circulated and cooled to carry heat away from the rods. If something
disrupts this cooling, the fuel rods will heat up the water and eventually
cause it to boil off.
If the water drops low
enough to expose a significant length of a fuel rod, it will get hot
enough that the zirconium cladding of the rod will start to oxidize (i.e,
burn). This damage to the cladding will begin to allow the release of
radioactive elements in the rod. If heating continues, the fuel pellets in
the rod will start to release much larger amounts of radioactive gases.
Eventually, the heat can get high enough that the fuel pellets will begin
to melt. If only a fraction of the fuel pellets melt, that is called a “partial
meltdown.”
A partial meltdown will
release large amounts of radioactivity. In general, that radioactivity and
the damaged fuel will be contained in the steel reactor vessel, which is
itself in the reactor’s “primary containment” structure to isolate
it from the environment. That means that even if a partial meltdown
occurs, it may not lead to a large release of radioactivity into the
atmosphere since it will be confined inside the reactor. That is the
situation that occurred during the Three Mile Island nuclear accident in
1979.
However, if a partial
meltdown occurs in spent fuel that is not in the reactor core but has been
moved to the spent fuel pool, the radiation released is much more likely
to get into the atmosphere. The pool is not surrounded by the same layers
of confinement as the reactor. In the case of the Japanese reactors,
explosions have damaged the reactor buildings and that would allow
radioactive gases from the spent fuel pool to travel directly into the
atmosphere.
A “complete meltdown”
can occur when cooling water drops enough that the nuclear fuel in the
reactor core is entirely uncovered. If a large quantity of fuel melts, the
molten mass can run to the bottom of the metal reactor vessel, and may
remain hot enough to burn through the vessel floor. The mass would then
drop onto the concrete floor of the primary containment. In the case of
the Mark I containment used in the Japanese reactors, if the mass is large
enough it can spread to the metal containment wall and burn through it. If
so, the containment can be breached and radioactivity will escape.
The concern about a full
meltdown in the reactor is that it will potentially breach the primary
containment structure, which would greatly increase the probability that
it would escape into the atmosphere.
This is different for spent
fuel pools. Since the pools are already outside primary containment, a
complete meltdown would not necessarily be significantly worse than a
partial meltdown, although the total amount of radioactive gases released
would likely be largerin the former case.
Glossary
Auxiliary feedwater - (see emergency feedwater)
Background radiation - The
radiation in the natural environment, including cosmic rays and radiation
from the naturally radioactive elements, both outside and inside the
bodies of humans and animals. The usually quoted average individual
exposure from background radiation is 300 millirem per year.
Cladding - The thin-walled
metal tube that forms the outer jacket of a nuclear fuel rod. It prevents
the corrosion of the fuel by the coolant and the release of fission
products in the coolants. Aluminum, stainless steel and zirconium alloys
are common cladding materials.
Emergency feedwater system
- Backup feedwater supply used during nuclear plant startup and shutdown;
also known as auxiliary feedwater.
Fuel rod - A long, slender
tube that holds fuel (fissionable material) for nuclear reactor use. Fuel
rods are assembled into bundles called fuel elements or fuel assemblies,
which are loaded individually into the reactor core.
Containment - The gas-tight
shell or other enclosure around a reactor to confine fission products that
otherwise might be released to the atmosphere in the event of an accident.
Coolant - A substance
circulated through a nuclear reactor to remove or transfer heat. The most
commonly used coolant in the U.S. is water. Other coolants include air,
carbon dioxide, and helium.
Core - The central portion
of a nuclear reactor containing the fuel elements, and control rods.
Decay heat - The heat
produced by the decay of radioactive fission products after the reactor
has been shut down.
Decontamination - The
reduction or removal of contaminating radioactive material from a
structure, area, object, or person. Decontamination may be accomplished by
(1) treating the surface to remove or decrease the contamination; (2)
letting the material stand so that the radioactivity is decreased by
natural decay; and (3) covering the contamination to shield the radiation
emitted.
Feedwater - Water supplied
to the steam generator that removes heat from the fuel rods by boiling and
becoming steam. The steam then becomes the driving force for the turbine
generator.
Nuclear Reactor - A device
in which nuclear fission may be sustained and controlled in a
self-supporting nuclear reaction. There are several varieties, but all
incorporate certain features, such as fissionable material or fuel, a
moderating material (to control the reaction), a reflector to conserve
escaping neutrons, provisions for removal of heat, measuring and
controlling instruments, and protective devices.
Pressure Vessel - A
strong-walled container housing the core of most types of power reactors.
Pressurizer - A tank or
vessel that controls the pressure in a certain type of nuclear reactor.
Primary System - The
cooling system used to remove energy from the reactor core and transfer
that energy either directly or indirectly to the steam turbine.
Radiation - Particles
(alpha, beta, neutrons) or photons (gamma) emitted from the nucleus of an
unstable atom as a result of radioactive decay.
Reactor Coolant System -
(see primary system)
Secondary System - The
steam generator tubes, steam turbine, condenser and associated pipes,
pumps, and heaters used to convert the heat energy of the reactor coolant
system into mechanical energy for electrical generation.
Steam Generator - The heat
exchanger used in some reactor designs to transfer heat from the primary
(reactor coolant) system to the secondary (steam) system. This design
permits heat exchange with little or no contamination of the secondary
system equipment.
Turbine - A rotary engine
made with a series of curved vanes on a rotating shaft. Usually turned by
water or steam. Turbines are considered to be the most economical means to
turn large electrical generators.
Credit:
U.S. Department of Energy, International Atomic Energy Agency, U.S. Nuclear
Regulatory Commission, Nuclear Science Division ---- Lawrence Berkeley National
Laboratory, Penn State Radiation Science and Engineering Center, American
Nuclear Society, European Commision, The Union of Concerned Scientists
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