Time: 8:00 a.m. 28 March 1979
Place: Three Mile Island Nuclear Power Plant near Middletown, Pennsylvania
A series of human and mechanical failures have nearly
triggered a nuclear disaster. In the control room, you face a confusing
situation. The control
rods have been successfully inserted into the reactor
core, theoretically stopping the chain
reaction that "burns" uranium nuclei to an "ash" of (for
example) xenon and zirconium. As additional safety measure you release 2 kg of
boron into the reactor water: boron acts like a control rod, stopping the
uranium reaction (as long as the water continues to circulate through the
reactor core). The boron will be thoroughly mixed in the turbulent reactor water
system. But the ash recently produced in the core is itself radioactively
"hot" and as it decays additional nuclear energy is released -- even
though the main uranium reaction has stopped.
Thus heat must be removed from the reactor core, or the
ash will heat the core to the melting point of uranium. The resulting liquid
uranium will collect at the bottom of the reactor, and then -- lacking the
damping of the control rods or boron -- the nuclear chain reaction will restart
totally outside of human control.
Cooling water, circulating through the core, is required
for a "safe" end to this "event". But how much water
remains? An unknown amount was lost (4 hours ago) when the problem started. The
water-level sensors currently give erratic readings (due to the build up of a
hydrogen bubble in the core, as it turns out). The most recent chemical analysis
[click
here to read analysis] of a sample of the core water shows that the core has
not yet melted (otherwise it would show nuclear ash dissolved in the water).
Instead all that it shows is the boron you released and the corrosion inhibitor
that is always in the core water. (The main reason for frequent core-water
sampling under normal operating conditions is to check for corrosion.)
If there is more than 1,000,000 L of water in the core,
the core is covered and the reactor is in a safe mode. If there is less than
500,000 L of water, the core is totally uncovered and the reactor should be
heating up towards a disaster of major proportions: lives (including your's)
would be at risk. With a partially covered reactor, nobody is quite sure what
will happen.