Nearly 40 years after the worst nuclear power
plant fire in the United States, the circumstances
still seem a bit incredible.
It happened on March 22, 1975, at the Browns
Ferry Nuclear Power
Plant, near Decatur, Alabama.
Plant workers were using a candle to test
seals for air that could be
leaking into the reactor building.
But the polyurethane foam seals they were
testing were not
made to resist flames, and the candle ignited
one seal and the electrical cables that passed
through it.
The fire burned for almost seven hours – affecting
more than 1,600 electrical cables – about
a
third of which were important to plant safety.
Emergency repairs were needed even just to
allow
the reactor to shut down safely.
Subsequent investigations revealed flaws in
the design of fire protection features at
nuclear
power plants, and in procedures for responding
to a fire.
As a result, the NRC substantially
revised its fire protection regulations to
reduce the chances of a fire starting, to
quickly detect
control and put out any fires that do occur.
This approach, called deterministic fire protection,
ensures plant operators can shut down the
reactor safely despite a fire.
These deterministic requirements are based
on a set of postulated serious fires, including
computer simulations of how fires spread.
The NRC requires that nuclear power plants
use
multiple layers of fire protection, including
fire barriers such as insulation, systems
to detect fires,
and systems, such as sprinklers, to suppress
fires.
If for any reason, a plant does not have one
of those elements, they must compensate by,
for
example, putting dedicated personnel on a
continuous fire watch.
Since 2004, the NRC has offered plants another
approach, called risk-informed fire protection.
These requirements use risk assessments to
establish fire protection requirements focused
on
issues according to their importance to public
health and safety.
These regulations rely on what
we want as the outcome rather than specifically
how that outcome should be achieved.
This gives
the plants flexibility in how they meet our
standards.
The NRC oversees fire protection at nuclear
power plants through inspection and oversight.
In
2000, we implemented the Reactor Oversight
Process, which includes quarterly, annual,
and
triennial fire protection inspections.
In addition, we work with international codes
and standards
organizations, nuclear industry representatives,
professional societies and research organizations
to address ongoing fire protection activities.
In 2006, we issued a letter related to a problem
we found with some of the fire resistant materials
used to protect cables.
Our tests had found that some of these materials
did not perform as
designed.
All licensees had to give us information on
how they addressed this issue.
In 2009, we issued a regulatory guide revision
related to how we require plants to analyze
electrical circuits needed to shut down a
reactor after a fire.
In 2010, two plants, Shearon Harris and Oconee,
took the lead in completing their transition
to
the risk-informed fire protection program.
Since then, several dozen plants have told
the NRC
they’ll also move to the newer approach,
and we’ve got specialists devoted to reviewing
those
plants’ plans to make sure they’re appropriate.
Regardless of which fire protection approach
a plant takes, the NRC’s inspections and
guidance
will ensure every plant can safely shut down
and protect the public if a fire breaks out.
