- Last Updated: 29 January 2016 29 January 2016
Narrative Part 4: Hydrogen
The presence of hydrogen in reactor coolant is not unusual. A small amount of hydrogen, in fact, is added to the cooling water intentionally. This hydrogen causes no problems, and is there to combine with any excess oxygen that might form in the system, thus to inhibit oxidation of the metal components.
When Zirconium, the metal used in the fuel rod cladding, is exposed to high heat in the presence of steam, a chemical reaction known as hydration takes place. This reaction destroys the metal and releases hydrogen gas. For hours, this additional hydrogen had been collecting in the reactor vessel, mixed into the steam bubble lurking just under the vessel's head seals. The gas was also present in large quantities in the containment building atmosphere; it came fizzing out of solution as the primary water spilled onto the containment floor.
The first warning of the presence of hydrogen in the system was quite violent, but thanks to the heavily overengineered containment structure, it was almost anticlimactic save for its implications. A poorly shielded relay sparked, detonating the hydrogen in the containment. The instrument measuring containment pressure zoomed to a frightening 28 pounds per square inch before starting down again. Later analysis showed that this instrument's response was quite slow, and the real peak pressure was probably closer to 80 PSI! The force shook the control room floor noticeably, and energy released was thought to be equivalent to the explosion of several modern 1,000 pound bombs. Despite the violence of the explosion, permanently installed strain gauges in the containment structure showed that it caused no undue stress.
While there was no significant damage from this detonation, quaintly termed a "bump", it was an ominous sign. While a steam bubble could be collapsed with little more than changes in pressure or temperature, a hydrogen bubble was there to stay until it was either vented or removed chemically. Hydrogen, the least massive of all gases, had even less potential for cooling the core than the steam bubble itself. Operators, still confused, thought initially that the shock they felt was a ventilation damper closing, and only during later analysis did the true cause become known.
Of course, hydrogen can't burn without oxygen, and since it was fairly certain that only steam and hydrogen comprised the bubble, there was little concern about a hydrogen explosion inside the reactor vessel. No one even wanted to consider the possibility, in fact. The chances of the reactor vessel and primary loop piping surviving that kind of shock ranged from slim to none.
Ironically, it was an engineer-turned-bureaucrat named Joseph Hendrie who raised the issue. As chairman of the NRC, Hendrie was getting disturbing information from his man on-site, Harold Denton. Half the core uncovered. Large amounts of hydrogen, and an eight-second, 28 PSI pressure spike in the containment that sounded an awful lot like a hydrogen burn. Gamma and neutron flux higher than anything ever seen before except in nuclear weapons tests. Hendrie knew that sufficient amounts of radiation could cause a little-known phenomenon called radiolysis. This would break the chemical bonds that held water together, putting oxygen and even more hydrogen into the system -- and gas samples at the auxiliary building vent header indeed had begun to show higher and higher levels of oxygen.
Calculations seemed to indicate that a 5% partial pressure of oxygen in the hydrogen bubble would be the flammability threshold. At or above that level, any source of ignition inside the vessel would cause the mixture to explode. At 11% partial pressure, with the temperatures involved, the mixture would self-ignite, needing no additional source of ignition at all.
On March 30 (day three of the accident), Hendrie presented his case at a White House briefing, stressing the need for immediate action. Fortunately, he wasn't ignored. The NRC formed a special unit known as the "Bubble Squad", who managed to repair, reconfigure, and press into service one of the hydrogen recombiners in the auxiliary building. It took 150,000 pounds of lead brick, flown in for the purpose, just to satisfactorily shield the machine.
Meanwhile, efforts to re-cover the core with water and restore normal core cooling continued in earnest. Pressure was dumped in order to allow low-pressure pumps to deliver additional coolant, then the system was repressurized yet again. Results were mixed, but generally positive.
It was vitally important to get at least one of the huge main coolant pumps restarted, to aid cooling of the reactor core. Predictably, this wasn't going to be easy. An oil-lift pump used to lubricate the bearings of the gigantic motor ... wouldn't start. An operator was dispatched to repair it. Safety systems, noting the unusual conditions surrounding the steam bubble in the reactor vessel, refused to allow the pump to start. With some difficulty, these were bypassed. Finally, fifteen hours after the onset of the accident, the pump was cautiously tested, then started and allowed to run. The reactor was once again being cooled.
Over the next several days, the recombiner along with other chemical and physical processes were used to remove the steam and hydrogen from the reactor. The simplest and most dangerous method of removing this gas was to vent it into the atmosphere. Engineers knew this would be dangerous to the public. After all, there was no way to release the hydrogen without releasing other, radioactive gases with it. They were forced, however, to weigh this danger against the danger of a hydrogen explosion in the reactor vessel, which would most likely cause the release of radioactive water, steam, and deadly fission products on a much larger, and totally uncontrolled basis. Some gas was released, and carefully monitored.
When the bubble was finally collapsed and something resembling a normal coolant flow was restored, engineers took stock of the situation. Present-day thinking backs up their belief that the reactor vessel bubble contained at least 5% oxygen for at least 24 hours.
The story was far from over, however. Thousands of Curies of radioactive noble gases had been released into the air at various points during the accident, and these releases would have to be accounted for. Radioactive water had been released into the Susquehanna river to prevent the reactor building sumps from overflowing, and this too had to be tracked and monitored. Every single one of the 36,000 fuel rods in the reactor had ruptured, releasing radioactive gases and fission products into the coolant. More than seventy percent of the core had been out of the water for as much as two hours. The core was a mass of rubble, and parts of it had reached temperatures approaching 4,300 degrees. Uranium oxide fuel melts to a liquid state at 5,000 degrees.
The last major venting of noble gases from unit 2 occurred in 1981, and cleanup efforts have progressed well since then. All of the contaminated water has been cleaned up and evaporated, and all but a few kilograms of fuel has been removed. While radiation inside the containment is still above normal, manned entries have been made into the area, and radiation levels on the refueling floor were low enough to permit several hours of exposure without significant danger to personnel. The plant is in what is known as a "safe storage" condition, and will not be decommissioned completely until 2005, to allow the utility to decommission both units at once.
TMI Unit 1, which had just been refueled at the time of the accident and was preparing to go back online, had its license pulled a short time after the accident. The Nuclear Regulatory Commission was in a state of panic; TMI-1 had the best operating record in the industry at the time of the accident. Unit 1 stayed down for nearly 5 years. Part of this was due to modifications required by new NRC regulations, and part of it was politics. The plant has now resumed operation.