Air Lock and Brain Lock. The Three Mile Island Incident.
As the pressure in the primary system continued to decrease, reactor coolant continued to flow, but it was boiling inside the core. First, small bubbles of steam formed and immediately collapsed, known as nucleate boiling. As the system pressure decreased further, steam pockets began to form in the reactor coolant. This departure from nucleate boiling caused steam voids in coolant channels, blocking the flow of liquid coolant and greatly increasing the fuel plate temperature. The steam voids also took up more volume than liquid water, causing the pressurizer water level to rise even though coolant was being lost through the open PORV. Because of the lack of a dedicated instrument to measure the level of water in the core, operators judged the level of water in the core solely by the level in the pressurizer. Since it was high, they assumed that the core was properly covered with coolant, unaware that because of steam forming in the reactor vessel, the indicator provided false readings.[13] This was a key contributor to the initial failure to recognize the accident as a loss-of-coolant accident, and led operators to turn off the emergency core cooling pumps, which had automatically started after the initial pressure decrease, due to fears the system was being overfilled.[14]
With the PORV still open, the quench tank that collected the discharge from the PORV overfilled, causing the containment building sump to fill and sound an alarm at 4:11 a.m. This alarm, along with higher than normal temperatures on the PORV discharge line and unusually high containment building temperatures and pressures, were clear indications that there was an ongoing loss-of-coolant accident, but these indications were initially ignored by operators.[15] At 4:15, the quench tank relief diaphragm ruptured, and radioactive coolant began to leak out into the general containment building. This radioactive coolant was pumped from the containment building sump to an auxiliary building, outside the main containment, until the sump pumps were stopped at 4:39 a.m.[16]
After almost 80 minutes of slow temperature rise, the primary loop pumps began to cavitate as steam, rather than water, began to pass through them. The pumps were shut down, and it was believed that natural circulation would continue the water movement. Steam in the system prevented flow through the core, and as the water stopped circulating it was converted to steam in increasing amounts. About 130 minutes after the first malfunction, the top of the reactor core was exposed and the intense heat caused a reaction to occur between the steam forming in the reactor core and the Zircaloy nuclear fuel rod cladding, yielding zirconium dioxide, hydrogen, and additional heat. This fiery reaction burned off the nuclear fuel rod cladding, the hot plume of reacting steam and zirconium damaged the fuel pellets which released more radioactivity to the reactor coolant and produced hydrogen gas that is believed to have caused a small explosion in the containment building later that afternoon.
This is what happened at Three Mile Island. It sounds eerily similar to what is happening in Japan at Fukushima. So now we know where the hdyrogen comes from. Burning fuel rods. The top of the Fukushima reactor was exposed for some time and the failure to get water into the reactor to cover the core, which could have been exposed for a day or more, leads to one to believe there are a lot of broken down fuel rods and loose fuel in the cores at Fukushima. It only took one hour at Three Mile Island of lost water to do the damage. 8 minutes really. They have had a lot more time of no water at Fukushima to achieve the same effect. The hydrogen blasts tell us this has happened.
Based on this I believe melt down is a probably scenario. I would say this may not result in release of much radiation due to differences in design of the containment structures, but it will not be pretty. |
