Accident to the Loss of Coolant
First of all, here is my conclusion and assumption. I think there was an accident to the loss of coolant immediately after the earthquake in the Fukushima Daiichi NPP Unit 1, shakes at the time of the earthquake (seismic motion) caused medium or large scale damages to certain pipelines. These damages triggered loss of coolant. This loss of coolant is most feared in nuclear power plants’ accidents but was never assumed from technical point of view and therefore was labeled “hypothetical accident”. My assumption is based on limited available data. These data strongly suggest that an accident to the loss of coolant occurred in Unit 1.
An accident to the loss of coolant while nuclear reactor was in operation also occurred in the Three Mile Islands Nuclear Power Plant’s accident in the United States in 1979. This accident to the loss of coolant was a result of several consecutive human errors. Suppose an accident to the loss of coolant occurred in the Fukushima nuclear power plants this time, it means a very serious accident to the loss of coolant in the sense that what was never assumed from earthquake resistance point of view actually occurred. If this is the case, we cannot conclude that Fukushima Daiichi accident was due to natural phenomenon, namely big tsunami “beyond assumption”. And it will affect question on safety with regard to earthquake-resistance in all the nuclear power plants in Japan.
The only available data, at the time of writing, which common people should depend on to learn series of events in Unit 1 right after the earthquake and which are consistent to some extent is in the “Report on the Accident dated 27th March”. [This report was released in a homepage by the Prime Minister’s Official Residence on 27th March: “ On the Heisei 23(2011) Accident of Fukushima Daiichi and Daini NPP (as of 23:00, 26th March)” and was later deleted from the homepage by the Prime Minister’s Official Residence.] This report includes a table of consecutive changes with regard to “nuclear reactor water level”, “nuclear reactor pressure”, “dry well pressure” (pressure in a containment of nuclear reactor pressure vessel) of all the six Units (Unit 1--6) of the Fukushima Daiichi NPP. However the most important data on the day of the earthquake (11th March) are not disclosed (I already requested its disclosure). Was there any trouble in operating Unit 1-- 3 after they stopped automatically? Did safety valves get stuck and were left open? I am concerned with such issues. If these problems did not take place, then drastic changes in nuclear reactor water level, nuclear reactor pressure, and dry well pressure in the Unit 1 before the hydrogen explosion in the afternoon of the 12th seem to be a typical “accident to the loss of coolant”.
Plummeted Nuclear Reactor Pressure and Skyrocketed Dry Well Pressure
On that day three nuclear reactors 1-3 of the Fukushima Daiichi NPP in operation stopped immediately and automatically right after the earthquake (Unit4-6 were undergoing inspection, hence not in operation). That is to say, nuclear fission reaction stopped after control rods were inserted automatically to reactor cores because seismometer recorded earthquake shakes beyond a certain level. I would like to know in details how pressure and water level changed in nuclear reactor pressure vessels right after automatic stop. But as I mentioned before, data of 11th March are not shown in the “Report on the Accident dated 27th March” for some reason. In the same report, the recorded time of pressure and water level of the troubled Unit 1 reactor core first appears at 2:45 a.m. on 12th March, about 12 hours after the earthquake. What is surprising is that the pressure of the reactor core was about 0.800Mpa (about 8 atmospheric pressure), which is abnormally low. Pressure just before automatic stop must have been around 70 atmospheric pressure which is usual pressure in operation. This means the pressure plummeted from 70 atmospheric pressure to 8 atmospheric pressure in 12 hours! Such a tendency is not seen at all in Unit 2 and 3.
Then how about water level? At 5:20 a.m. on 12th March the water level of the Unit 1 was 1300mm above the top of fuel rods. The whole fuel was under the water. But at 8:49 a.m. on the same day, the water level was below 400mm from the top of fuel rods. Alas, fuel rods were 40cm above the water level.
Now I want to focus on the most important and suggestive data, namely dry well pressure. Nuclear reactor pressure vessel is in structure called containment which consists of 2 structures. One is a flask-shape dry well to store nuclear reactor pressure vessel. Another is suppression pool (or wet well). <ellipsis> These two structures －dry well and suppression pool － are connected with 8 vent pipes.
There is only one reason for this gigantic containment to exist. The containment is there to prevent coolant, which contains radioactive material, to spout out all at once from any damaged or fractured pipe connected to nuclear reactor vessel. In short, it is a gigantic protective wall for imaginary accident, i.e. accident to the loss of coolant. Nitrogen gas is sealed into containment to prevent explosion even if hydrogen gas gets inside, while nuclear reactor is in operation. Moreover pressure is set a little below atmospheric pressure (about 1 atmospheric pressure).
Furthermore, design pressure and design temperature are set for containment. These are estimated pressure and estimated temperature in containment if/when coolant spout all at once inside containment due to fractured thick pipe in recirculation line. The pressure is about 4 atmospheric pressure and the dry well temperature is around 170℃. Containment is designed structurally to hold against these pressure and temperature.
According to the “Report on the Accident dated 27th March”, at 2:45 a.m. on the 12th, dry well pressure of the Unit 1 was absolute pressure, including atmospheric pressure, which was 0.941Mpa(around 9.4 atmospheric pressure). Normally gauge pressure (after deducting atmospheric pressure, i.e. 1 atmospheric pressure) is applied in structure design. So if we deduct 1 to get gauge pressure, the answer is about 8.4 atmospheric pressure. This is indeed twice the structure pressure (about 4 atmospheric pressure), which I have just explained above! This is a very high pressure which can undoubtedly damage any gigantic containment anytime. Why did such a high pressure act upon the Unit 1 containment only after half a day since the earthquake occurred?
It is not so difficult to get a consistent answer logically. The answer is the following: Because a strong earthquake hit, some pipe connected to nuclear reactor pressure vessel was damaged (or ruptured). For example, this could be a pipe in recirculatation line of which soundness has been questioned at the time of earthquake. Then an accident to the loss of coolant occurred after a huge amount of coolant spout out continuously. Henceforth, pressure in containment shot up fast and reached twice the design pressure.
If pipes get damaged, pressure in nuclear reactor pressure vessel should drop in a short time. And if coolant spout out in bulk from those damaged pipes, the water level in the nuclear reactor pressure vessel should drop gradually. In fact, the data show accordingly. As I mentioned before, pressure in nuclear reactor pressure vessel dropped only to about 8 atmospheric pressure by 2:45a.m.on the 12th. The water level of coolant dropped to 400mm below the top of the fuel rods by 8:49 a.m. on the 12th and reached 1700mm(!!) below by 13:38 the same day. More than 40% of about 4 meter-long fuel rods were above this water level.
Worst of all, almost 2 hours later at 15:36, a big scale hydrogen explosion occurred on the top floor (usually called operation floor) of the Unit 1! This is an inevitable result.
If I may repeat, in the process of the foregoing reasoning, neither big tsunami nor loss of all the alternating current electric sources was mentioned. If big tsunami did not come, all the alternating current electric sources might not have been lost. Then extremely dangerous situation, i.e. hydrogen explosion, perhaps could have been avoided. But this does not mean that the accident itself to the loss of coolant could be avoided.
Was there not a Problem Regarding Earthquake Resistance in Suppression Pool
It is perhaps an important question why the explosion in the Unit 2 occurred near the suppression pool. After all, it may be hydrogen which exploded. It was described as “there was a strange sound” in a press conference. It must have been a fuzzy and indistinct sound because the explosion was in the basement of the nuclear reactor building.
Hydrogen is naturally produced inside nuclear reactor pressure vessel. There, high temperature zircaloy (zirconium alloy) coated pipes reacted with steam to form hydrogen. Since hydrogen is light, it cannot go down to the basement by itself and stagnate there. But there is one route for hydrogen to go down to the basement. This is the route to let hydrogen leak from nuclear reactor pressure vessel via safety valves and pipes into suppression pool. If the suppression pool functions properly, hydrogen does not explode there because nitrogen is sealed in the suppression pool. Then, under what circumstances does hydrogen explode? One possibility I can think of is that hydrogen leaked from raptures and exploded. These raptures were in the structurally vulnerable parts in the suppression pool (e.g. expansion bellows, welded part of curved pipes) and were caused by the earthquake shocks.