As a result of the recalibration and other maintenance work performed by plant staff on water level instruments for reactor pressure vessel (RPV) #1, it appears that water level has been much, much lower than anyone had previously speculated. In fact, TEPCO and NISA now believe that the fuel cell was completely uncovered for at least a half day between the loss of power on March 11, and seawater injections on March 12…perhaps even longer. We can now re-examine RPV #1 events the first few days of the emergency. The following has been deduced by this writer resulting from more than two months of constant study on the Fukushima situation, and greatly influenced by the current understanding of water-levels in the core March 11-12…
- About 2 hours after the earthquake, cooling water flow into the reactor was no longer possible due to the complete loss of power. Decay heat, at this point, was in the 4 megawatt range, which heated up the water in the RPV.
- The morning of March 12, RPV pressure indication was increasing rapidly, which prompted planning for the operators to vent steam pressure out of the RPV. Early that morning, Prime Minister Kan arrived at the Emergency Facility 5 km from the power plant complex, and demanded venting begin as soon as possible. Venting occurred a few hours after Kan left. The hydrogen gas which built up in the upper refueling deck and exploded had to have come from the core, so it seems the operators were ordered to vent internally, and not through the tall stack external to the reactor building. The external release would have been into the atmosphere, which Kan was probably trying to avoid. A stack vent may have avoided the hydrogen explosion, in hindsight.
- Five hours after venting began, at ~3:30 pm, the top of the reactor building for unit #1 was decimated by a hydrogen explosion.
- When venting began at just after 10 am, it is probable that the decreasing water level inside RPV #1 accelerated. By the time of the refueling deck explosion, it is quite possible the entire fuel core of reactor #1 was no longer covered with water. However, the environment within the vessel was not completely dry. Water was still in the bottom of the vessel, producing steam. Steam is about 20 times less effective than hot water as a cooling medium, so while some decay heat was being carried away by steam, most of it stayed within the fuel bundles. Eventually, the bundles over-heated, with the first fuel damage being embrittlement of the zirconium cladding around the fuel pins. Copious amounts of hydrogen were rapidly generated as the hot zirconium stripped the oxygen from the water molecules in the steam. The hydrogen production probably started after the venting began. As venting continued, the hydrogen got out. The core continued to heat up faster than the residual steam could cool it. Fuel melting eventually began.
- Using the TMI studies of the melted fuel taken from it’s core, we might assume similar processes occurred in RPV#1. The uranium fuel, zirconium, boron-carbide control rods, and steel supports that held the fuel bundles in place were liquified into a composite metallic substance called “corium”. As the degree of meltage progressed in, through, and/or around the core, the liquid corium ran down and/or through the unmelted fuel bundles, with much of it going into the bottom RPV head. It’s possible the super-hot corium may have boiled off the residual water in the RPV bottom. As with all liquids, the corium spread out along the bottom head’s inner surface as it accumulated. Some of the corium probably worked it’s way into the instrumentation penetrations that go through the lower RPV and compromised their physical integrity. Plus the extreme heat, over 2000 oC, probably cracked some of the other instrumentation penetrations in the sides of the vessel. But one thing is sure…the corium did not burn its way through the thick, cast-steel bottom head. This is because the temperature instrument on the very bottom of the head is still working, which would not be the case if corium had burned its way through.
- Corium production continued as time passed. At ~10:20 pm, plant operators began injecting seawater into the RPV. While it is somewhat unclear as to how they did it, it seems likely they used a steam-powered emergency cooling (RHR) pump. The water flow was very low at the start, but the cooler water immediately began knocking down the high pressure inside then RPV. As the pressure dropped, the pump was injecting water at a faster rate because it was working against decreasing resistance. Soon, the corium inside the bottom head was covered with water, cooled and solidified. The water level inside the RPV rose as pressure dropped and water injection increased. When steam pressure inside the RPV dropped too low to keep the pump operating, the pump was shut down. When decay heat raised the steam pressure sufficiently, flow was resumed. With water injection occurring in an “on and off”fashion, it’s probable that the fuel core’s temperature and water level fluctuated up and down with it. Regardless, somewhere after midnight of March 12, the meltdown was essentially over. The damage had been done. Where the Three Mile Island core was uncovered for maybe an hour or two, it now seems the Fukushima #1 core was uncovered for many hours. How RPV #1 meltage compares to TMI will not be known until the vessel head is removed sometime in the future.
- It would be more than 4 days before fire trucks and their pumps would assist in the water injections, and fresh water would be used instead of seawater. Until fresh water was used, the seawater in the core was subject to neutron irradiation. All nuclear fuel, once it has been in an operating core for a brief period, naturally releases neutrons out of its fission products. The longer the fuel is in the core, the more intense the emitted neutron field becomes. Most of the fuel in RPV #1 was in the core more than 2 years before March 11, thus the neutron field in and around the fuel was considerable…and this included after the reactor SCRAMed (auto shutdown acronym), and after the fuel melted. The neutron field continues until this very moment, and will continue until the neutron-emitting fission products have radioactively burned out. The neutron field unquestionably caused most (if not all) of the Chlorine-38 production, which many “experts” and prophets of nuclear energy doom said was an indication of “re-fissioning” in mid-March. There was never even the remotest possibility of re-fissioning! The control rods, melted into the corium and uniformly mixed with the fuel itself, made re-fissioning impossible. The Cl-38 was produced by the neutron field generated by the “waste” atoms in the fuel.
- TEPCO and NISA have both maintained the core was partially uncovered for the past two months. This writer found the concept of certified reactor operators allowing this to be something unthinkable…unless they had no other choice. As it turns out, this may have been the case all along, and the reasons also answers the question of why a constant injection of water over the past two months hasn’t raised indicated water levels in the RPV. First, we now know that the extreme heat generated by the un-cooled core caused the water level instruments to malfunction. Until yesterday, nobody knew what the level of water in the core was. Also, it now seems that much of the upper fuel cell has melted and slumped/dripped into the lower core area and the bottom head, thus lowering the actual top of the fuel several feet. TEPCO and NISA indicate that the actual water level in the fuel core area, after yesterdays recalibration, is about half-way up the damaged cell. In other words, the remaining fuel inside the cell after the meltdown has probably been adequately covered and cooled for more than a month….maybe since March 13.Why did the operators not fill the RPV any higher? First, they didn’t know the actual water level. Second, they were limited in how fast they could inject water into the core (reasons are currently not available). Finally, there were slow leaks out of the reactor vessel through the heat-damaged instrument penetrations on the RPV itself, as well as weeping, seeping leaks through cracks in the overheated piping attached to the RPV. Even if the operators knew the core was actually uncovered, they were severely limited as to what they could do about it. TEPCO reports that 10,000 tonnes of water have been injected into RPV #1, and more than 8,000 tons have leaked out, into the surrounding primary containment. It’s still there, it seems. Primary Containment #1 shows no evidence of physical compromise.
Now for today’s abbreviated updates…
- TEPCO and NISA report that the increased water flow into RPV #3 has begun to drop temperatures inside. In fact NISA has posted a rather comprehensive chart of all operating temperature instruments on RPV #3, though they now note that these may be reading incorrectly due to heat damage.http://www.nisa.meti.go.jp/english/files/en20110513-1-6.pdf
- NISA speculates that since the water injections seem to have curtailed further RPV #1 fuel damage since March 13, and current injection rates seem to be keeping the remains inside the reactor adequately covered and cooled, there may be no need to completely fill the vessel in order to achieve cold shutdown. Further, filling the primary containment to a level equivalent to the top of the fuel in the reactor may also be all that is needed.
- While pumping water out of Turbine #3 basement, a leak developed in one of the tunnel accesses outside the building. The leak was stopped by plugging it with “cloths and concrete.” Regardless, some radioactive water did find its way into the enclosed sea port area next to the plant, inside both the silt dams and break-wall. None of the contaminated water seems to have reached the open sea.
- NISA also reports that two-thirds of Japan’s nuclear power stations are currently shut down, some due to automatic earthquake functions (and required follow-up safety inspections), others due to regularly scheduled outages, and (of course) the three good units at Hamaoka. By the end of May, another five units will be going into scheduled refueling outages, resulting in 75% of Japan’s nuclear plants shut down. NISA is concerned that the utilities will be too intimidated by government actions at Hamaoka to restart the units when they are ready to go. This will probably lead to summer power shortages.