Tragedy strikes, cont.
Sandia has studied lightning effects for
decades, primarily to understand how it
might affect critical nuclear weapons facilities
at U.S. Department of Energy facilities such as
the Pantex Plant located outside Amarillo and
underground facilities at the Nevada Test Site.
“Accident investigators had been suspicious
all along that lightning was the cause of the
explosion, but there had been no definitive
proof one way or the other,” says Sandia
manager Michele Caldwell.
Two scenarios
Sandia researcher Dawna Charley,
left, and a miner at the Sago Mine’s
entrance during experiments last
November.
The Sandia team investigated two modes of
transmitting lightning energy into the mine,
Caldwell says. The first mode was direct
attachment onto metallic penetrations — such
as conveyers used to extract the coal, rails
used for transporting people and equipment,
or power and communication lines — from
the entrance to deep inside the mine. The
second mode was energy propagation through
the earth’s surface from the point of a surface
lightning strike or overhead arc channel.
For the metallic penetrations, a small, continuous
electrical drive signal was applied at
the entrance to the mine, and signals were
measured with current and voltage probes at
various points in the mine as far as two miles
in. The goal was to see how much the signals
decreased as a function of the distance from
the entrance to the mine.
Matt Higgins conducts an experiment at the Sago Mine.
For measuring propagation of lightning energy
from the surface to the mine cavern 300 feet
below, the drive signal was applied to a long
wire stretched on the surface. Directly below,
inside the mine, an antenna was set up to pick
up the transmitted signals. Multiple antenna
measurements were made inside the mine.
The measurements were compared to analytical
models simulating lightning field propagation
through the earth.
The results were combined with a theoretical
lightning strike waveform to determine if voltages
could get high enough inside the mine to
be of concern.
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