- Last Updated: 28 March 2016 28 March 2016
The stuck relief valve still went unnoticed. Although the water level in the primary system was now under control, pressure was dangerously low. Soon, the pressure and temperature conspired to cross the magic line on the engineer's steam table, and the coolant began to boil. Steam bubbles flowed through the loop and reached the main coolant pumps. The immense machines, as large as a cement truck and twenty times as powerful, began to vibrate dangerously, and their motors strained as they struggled to pump the frothy mixture of steam and water. Flowrate dropped, temperature increased, and things began to look very bad. Vibration like this could blow the seals on the pump rotors, spilling primary coolant and rendering the pumps unusable.
Knowing that he had no choice, Zewe ordered the pumps shut down before they destroyed themselves and the pipes to which they connected. Now only natural convective circulation remained to move water through the core. What the operators didn't know is that because of the boiling, parts of the primary loop were now blocked by steam, so that water could not circulate by convection alone. A huge steam bubble, or "void", developed in the upper part of the reactor vessel, and grew quickly. Soon, the upper part of the reactor core was uncovered and beginning to overheat.
A frustrating series of human errors followed, complicating the operators' attempts to diagnose the problems with the plant. Zewe, in a sudden burst of insight, suspected that the relief valve might actually be stuck open. He asked a technician for a temperature reading at the valve outlet. A high reading would indicate that the valve was venting steam, but the technician mistakenly read him the temperature of another valve outlet instead -- which was low and normal.
Meanwhile, steam flowing at over a thousand pounds per minute finally overflowed the quench tank to which it was routed, and ruptured the safety disk. Soon, the containment building was flooded with radioactive water, some of which made its way through floor drains to sumps elsewhere in the plant. Frederick thought of checking the tank level (on an instrument behind the main panels and out of sight), but only after the rupture, and by that time the tank had drained through its safety disk and its level was normal. Radiation alarms began to sound, and containment building pressure began to rise.
Fresh eyes can sometimes see things that others are looking too hard to see. Around 6:00 AM, the day shift staff started to arrive. The day shift engineer, a man named Ivan Porter, looked around, saw that primary pressure was low and containment pressure was high, and made the connection. He suggested closing a block valve in the steam line, immediately past the stuck PORV. As soon as the switch was thrown, RC pressure began to rise again.
Just as Ivan Porter arrived in the control room, water from the containment building, now contaminated with radioactive fission products, made its appearance in the plant's auxiliary building via the floor drains. Radiation monitors' needles pinned at the upper end of their scales, and Porter later said that when the entire radiation monitoring panel lit up simultaneously in alarm, it was the closest thing he could imagine to having a heart attack.
Workers hastily vacated the building and sealed it. Radiation alarms went off all over the plant. A site emergency was declared, and an evacuation of nearby areas of Pennsylvania began. Most thought it was just another drill.
The first thought on the minds of the operators and plant staff at this point was to determine the status of the core. It's impossible to see the inside of a reactor vessel, and no water level measuring instruments were provided because core exposure was thought to be an "incredible" scenario. It is doubtful that a water level instrument could survive the strong radiation and high temperatures in the reactor vessel anyway.
Knowing the temperature in the core might be useful, though. There were several computer-monitored temperature instruments in the core, but the computer had only been calibrated to read temperatures below 700 degrees. Above this, the computer would only print question marks. The software designers quite understandably never expected a temperature higher than this to ever develop.
To circumvent the computer, whose printer was hours behind by now anyway due to the thousands of alarms that had been registered, a plan was developed. A crew was dispatched to the cable fanout room beneath the control room, and Porter used a multimeter to read the thermocouples directly. The readings he saw corresponded to temperatures of some 10,000 degrees on several core thermocouples. The technicians simply could not believe their eyes. Porter himself nearly dismissed the readings as defective thermocouples, but then noticed a gradient -- temperatures near the center were higher than those at the edge. Drenched in sweat, Porter at that moment knew, as few others could, the gravity of the situation. The core was severely damaged.
One man braved the high radiation levels in the monitoring lab to obtain a primary cooling water sample. Wearing a heavily shielded suit and working with tongs, he drew a small sample of water from a tap connected to the primary loop, and jumped back in horror. The water was frothy, fizzing like a carbonated soda, and darkly colored with radioactive contaminants. He retreated, hastily. It was at this point that all concerned finally grasped the true gravity of their situation. This plant wasn't going back online in a week, or a year. It was going to be lucky to make it through the day!