Passive Safety Nuclear Reactors
In a normal nuclear reactor, the chain reaction can be
shut down quickly with control rods. However, the fission
products (the smaller atoms created with the big atoms
split) are highly radioactive and give off a tremendous
amount of heat for a few days, enough to melt the
reactor core down.
What happened at Fukushima was that a tsunami destroyed
the generators that were needed to pump coolant through
the reactor to keep the temperature under control. Without
the coolant, heat from the fission products caused the
plant to overheat so badly that a meltdown occurred,
and the hot core melted through the floor of the plant.
In newer designs, the goal is to design a plant so that
if every pump in the plant fails and the
whole Balance of Plant (the steam turbines and generators
and cooling towers) is knocked out, the plant will still
be kept cool enough not to melt for the few days until
the fission products have decayed and the threat of
meltdown has passed.
MARK
AP1000 Passive Safety Light Water Reactor

The AP1000 design is a traditional light-water design
(most nuclear power plants that have been built, both
on shore and in the Navy, were
light-water reactors) with features added for passive
safety.
The AP1000 was built in such a way that if
all the pumps in the plant failed, natural
convection combined with room-temperature water from the
PCCS Gravity Drain Water Tank (see diagram above) fed by
gravity would be enough to cool the reactor for several
days until fission products have decayed and the threat
of meltdown is averted.
The AP1000 design is being built in many places.
A couple are operating in China. Some
reactors were built at the Vogtle plant in Georgia. This
was controversial because the plant was over budget,
behind schedule, and resulted in increased electric rates
for Georgia residents. The following points need to be
made:
- Electricity in Georgia with their nuclear plant is
still half as expensive as electricity in California,
with all their wind and solar.
- Vogtle was the first nuclear plant built in the US
in a generation. The experienced nuclear engineers
had mostly retired and new ones had to be trained
from scratch.
- The first plant of a given design tends to have
a lot of growing pains. The United States has
generally made the mistake of making only a few of
each design. Many other countries have had much
better success building large numbers of a few
designs
Passive safety light water reactors are advantageous
because the human race has so much experience with
light water reactors that the learning curve is smaller
than with next-gen designs.
Passive Safety Molten Salt Reactors
Some reactors that are planned dissolve the fuel in
molten salt. At the bottom of the container of the
molten salt is a drain with a "Freeze Plug" in it.
In the event of any overheating within the reactor,
the "Freeze Plug" located in the drain will melt,
allowing the fuel/salt mixture to flow out into a
series of small "Emergency dump tanks" positioned at
intervals.
Due to the tanks' limited size and significant
spacing, coupled with the absence of a moderator
between them, the fuel/salt no longer forms a
critical mass, causing the fission reaction to cease
and facilitating the cooling process.
The reactor's meltdown prevention relies on two
crucial assumptions:
- The "Freeze Plug" will melt at the expected
temperature.
- Gravity operates as anticipated.
These are extremely reliable assumptions.
The United States government successfully conducted
tests on a prototype of a meltdown-proof reactor in
Idaho during the early 1990s. Despite subjecting the
reactor to various attempts to induce a meltdown,
including disabling the cooling system (similar to
the events at Fukushima), it consistently shut down
without issues or any leakage of radiation.
However, following this achievement, federal
funding for nuclear research was discontinued due to
pressure from the anti-nuclear movement.
The Chinese has surpassed the
United States and installed
a meltdown-proof plant supplying power to their grid
at Shidao Bay, Shandong Provice in 2024.
Meltdown-proof nuclear plant currently
providing power to the
Chinese power grid.
An important aspect of a meltdown-proof reactor is
that it eliminates the need for every component in
the plant to adhere to a safety standard of
99.9999%, which is currently necessary in existing
plants with the potential for meltdown, results in exorbitant
costs, as one observer put it,
"the toilet paper in a nuclear power plant has to be
gold-plated>". With a meltdown-proof reactor
core, these stringent standards can be significantly
relaxed for the remaining plant infrastructure,
resulting in substantial cost savings.
It is worth noting that most conventional reactors
in the United States utilize water as a neutron
moderator (Chernobyl used graphite). Without water,
the neutrons become too fast for sustained fission,
effectively halting the chain reaction. However, the
immediate byproducts of uranium fission are highly
radioactive and generate considerable heat during
their decay process, potentially leading to a
meltdown. Therefore, to ensure a meltdown-proof
reactor, the tanks into which the liquid fuel drains
must be surrounded by sufficient thermal mass
capable of absorbing this heat even without an
active cooling system.