The pumps in the secondary loop shut down automatically,
due to a slight malfunction.|
|1 sec ||
Alarms sound within the control room.|
|2 sec ||
Because heat is no longer being transferred to the secondary loop,
water pressure and water temperature in the primary loop rise. This
is normal and no cause for concern.|
|3 sec ||
The PORV (pressure relief valve) opens automatically, releasing
steam into a holding tank.|
|4 sec ||
Backup pumps within the secondary loop automatically turn on.
However, these pumps are disconnected from the system by cutoff valves.
Operators assume that the pumps are doing what they're supposed to do. |
|9 sec ||
The control rods , made of boron and silver, are lowered, which slows down the nuclear chain reaction within the core. Heat
is still generated, though. In this state, the core produces enough energy to
light 18,000 homes.|
The PORV light goes out, indicating that the valve is now closed. The valve is
actually still open.
Steam and water continues to be released through the PORV, creating a LOCA
(loss of coolant accident).
|2 min ||
The EIW (emergency injection water) is activated, and water flows
into the primary loop. This is a safety device, designed to keep the water at a
safe level in the event of a LOCA. Operators are not too concerned when this
happens, though -- the EIW has turned itself on many times in the past when
there has been no leak.|
|4 min 30 sec||
Operators observe that the water level in the primary system is rising and
that the pressure is decreasing. They turn off the EIW . The water level
still appears to be rising. The level is actually dropping. The water,
along with the steam, is now released through the PORV.|
|8 min ||
An operator notices that the valves for the backup pumps in the secondary
loop are off. He opens the valves. The secondary side now operating normally.|
|15 min ||
By this time approximately 3,000 gallons have escaped from the primary
loop. The instrument that checks radioactivity levels does not trigger an
alarm, so operators still have no reason to suspect a LOCA.|
|45 min ||
The water level in the primary loop continues to drop. Gauges in the
control room erroneously indicate that the water level is up.|
|1 hr 20 min ||
The pumps that push water through the primary loop begin to shake
violently. This is caused by the steam passing through the pumps. Two of the
four pumps are turned off.|
|1 hr 40 min ||
The other two pumps turn off. Steam within the primary loop, now no longer
circulating with the water, rises. |
Also, because the water isn't circulating, the core heats up even more,
converting more of the water into steam.
|2 hr 15 min ||
Water no longer covers the top of the core. The heat from the exposed core
soon turns the steam into superheated steam. The control rods, in turn, react
to the superheated steam and begin to release hydrogen and radioactive gases.
These are also released through the PORV.|
|2 hr 20 min ||
An operator from the next shift comes on duty and notices that the PORV
discharge temperature is abnormally high. He stops the leak by shutting the
PORV's backup valve. More than a quarter of a million gallons of radioactive
cooling water has been discharged since the PORV first opened.|
Operators still don't realize that the water level in the primary loop is low.
The water within the loop continues to boil away, which causes more damage to
the core, more heat, and more radioactivity.
|2 hr 30 min ||
The operators receive the first indication that radiation levels
are going up.|
Radiation alarms sound. A site emergency is declared.|
Half of the core is now uncovered, and the radioactivity of the water in the
primary loop is 350 times its normal level.
|3 hr ||
Even higher radiation levels prompt the declaration of a general
High temperatures in the core lead some to believe that the core is uncovered;
others do not trust the temperature readings.
|7 hr 30 min ||
Even after operators pump water into the primary loop, pressure is
still high. The PORV' backup valve is opened to lower the pressure.|
|9 hr ||
Hydrogen explodes within the containment structure, causing a
pressure spike on the control room gauges and an audible thud. The spikes are
believed to be caused by an electrical malfunction; the thud is thought, at
least by some, to be just a ventilator damper.|
|15 hr 50 min ||
The primary loop's pumps are turned on, which circulate water around the
core. The core's temperature is finally under control, although half of it is
melted and part of it has disintegrated. Also, there's still hydrogen in the
primary loop. |
Now that the path toward meltdown has been averted, the significant problem of
leaking radiation must be dealt with.