PLENTIFUL ENERGY

[15] Provisions are also made to collect leaking sodium in steel drip trays to avoid
reaction of sodium with structural concrete.
The main point to be made here is that in order to prevent the radioactive
primary from sodium reacting with water or air, a non-radioactive secondary
sodium system isolates the radioactive primary sodium from the steam system. As a
result, potential sodium/water reactions or sodium fires can occur only in the nonradioactive
secondary system. The primary system‘s integrity is not involved and
the prevention and mitigation systems are not safety-grade systems. Their role is to
protect the plant investment only.
There have been no sodium fires in primary sodium protected by inert
atmosphere. We suppose there may have been unreported sodium fires which
involved minute amounts of sodium in small pipes for sampling lines or purification
systems, with inconsequential impacts, but we are not aware of any.
Over the thirty-year life of EBR-II, there have been several small sodium fires in
the non-radioactive secondary system. They have been handled as routine matters.
Typically, white smoke coming off through the pipe insulation is detected, the
smoke is blanketed with MET-L-X, the sodium in the affected area is frozen and
the piping cut out, and a new section is welded in. In general, sodium draining has
not been required. In the most severe case, the freeze plug in the piping to permit
maintenance of the bellows seal valve was insufficiently cooled and it melted,
allowing sodium to flow from the open valve bonnet. The sodium ignited. The
operator drained the secondary sodium to the storage tank and fire retardant was
effective in putting it out.
By far the most severe sodium fire was in the (non-radioactive) secondary
system of the Japanese demonstration fast reactor, Monju. During the 40% power
tests in December 1995, an alarm sounded due to a high outlet temperature at the
intermediate heat exchanger and a fire alarm (smoke detector) sounded at the same
time, followed by a sodium leak alarm. The reactor power-down operations were
initiated. When the white fumes in the piping room increased, the reactor was
manually tripped after eighty minutes of initial alarms. The sodium drain started in
ninety-five minutes after the reactor trip and was completed in eighty minutes. The
cause of the leak was a thermocouple well that extended into the flowing sodium
which bent due to flow-induced vibrations and left a one-cm-diameter opening.
During the four-hour period before effective action was taken, a total of 640 kg of
sodium had leaked and burned. The piping was elevated, and below the leak
approximately one cubic meter of sodium oxide formed in a semicircular mound,
about three meters in diameter and thirty cm high, on the six-mm-thick steel floor
liner. The ventilation duct directly under the thermocouple well developed a hole
extending over half its perimeter with lumps of deposit around the opening. Sodium
aerosol was lightly diffused over the floor and walls of the piping room.
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