What is Happening in the Reactor Cores of Fukushima Dai-ichi Nuclear Power Station?

Author:  Michio Ishikawa (Chief Adviser of Japan Nuclear Technology Institute (JANTI))

However, I’m now living in a disaster area of Hitachinaka City. I could not contact the outer world for three days until March 14 because of a power outage. As for news sources, I only could listen to a radio. I could eventually watch TV the night before last and knew what was happening in the world like Rip Van Winkle. So, I’m barren of specific values (information about the realities). As this report is the outline of a story reasoned from facts, I think that there are many particulars that are wrong.
As for the conditions of reactor cores, first of all, the behaviors above and under the water surface are largely different according to the findings from TMI accident. As the same may apply to Fukushima, I would like to discuss this point in detail.

First, the fuel rods under the water surface remain sound as they are cooled by water. It is needless to say.

As for the fuel rods exposed above the water surface, on the other hand, their heat can hardly be removed as they are surrounded by steam. Therefore, their temperature gradually increases due to decay heat. When their temperature reaches about 900℃, they react with surrounding steam and cladding tubes begin oxidizing. Because this reaction is a strong exothermic reaction, the temperature around such cladding tubes locally increases when oxidization begins. When the temperature reaches close to 1,300℃, the reaction accelerates and the increase in the temperature of cladding tubes cannot stop. As a result, the outer surface of cladding is covered by thin oxide films (zirconium dioxide), and the inner surface of cladding tubes also creates the same oxide films by taking away oxide from fuel pellets (uranium dioxide).

In other words, the inner and outer surfaces of cladding tubes are covered by thin oxide films, and zircaloy alloy as the material of cladding tube is sandwiched between them. It should be noted here that the melting point of oxide film is higher than the melting point of zircaloy, i.e., about 1,800℃, as the material of cladding tube. Therefore, the sandwiched zircaloy melts and flows down between films to form a puddle. On the other hand, the inner and outer oxide films stick to each other and are crimped by the reactor pressure onto fuel pellets. If it is likened, a fuel rod looks like a mass of pellets wrapped by polyethylene wrap. As the oxide films are strong at high temperatures, even if a fuel rod deforms a little, it keeps sealing radioactivity and standing upright above the water surface. This is the reason that radioactivity did not leak from fuel rods exposed above the water surface in the accident at the Fukushima power station. It is no wonder, and it is not an error of measurement, either.
This condition changes at the moment when water is added into a reactor core. The oxide film becomes brittle when the temperature decreases. Moreover, as the film cools down and contracts, a fuel rod is divided at the boundary of pellets, and drops to pieces (not melting) and such pieces accumulate in the water in a state like a toy box upturned. They can accumulate in the water because fuel rods under the water surface are sound. These are the conditions that happened in the reactor core at the time of TMI accident.

Let me get this straight now, the fuel rods that dropped into pieces remained cooled as long as they were soaked in water. This is because of the cooling effect (called communication path) of the water that flows and threads its way through divided pieces of fuel rods. Consequently, fuels did not melt and remained in a state of debris.

In conclusion, the upper part of the reactor core, which was exposed above the water surface, generated hydrogen and collapsed, but it was cooled without melting. As a result, the effect of pellets to retain radioactivity was maintained.

The problem is the fuels under the water surface. The water used to cool fuels becomes steam, but the flow of steam was blocked by debris above the steam, and the steam could not fully go up and the steam flowed in a lateral direction. In other words, a steam zone was formed directly under debris, and the same state of the fuels exposed above the water surface as mentioned earlier was created in the water. The conditions of exhausting heat from such zone are extremely worse than the conditions of the fuels above the water surface. As a result, the heat from oxidation of cladding tubes accumulated to melt fuel rods. A core meltdown occurred. However, the melting temperature is not the generally referred melting point of uranium dioxide, i.e., 2,800℃, but is said to be close to 2,300℃ that is the melting point of a ternary alloy of uranium, zirconium and oxygen. As concrete cannot be melted at this temperature, China Syndrome does not happen.

The undersurface of the molten core was in a state of hard crust like cast iron as it maintained contact with cooling water. However, above the undersurface of the core, melted fuels flowed in a lateral direction, contacted the core shroud that was made of thin stainless steel to make a hole. And, the melted fuels dropped from the hole hardened like a ball with a diameter of about 15 to 20 centimeters. There were many such balls found at the bottom of the core.

Those mentioned above are the core meltdown behaviors of TMI accident. The core behaviors in the accident of the Fukushima power station are similar to those of TMI accident. One of the similarities is that about two meters of upper part of the core was exposed above the water surface for many hours due to a lowered water level. Fission products such as cesium were created as a result of division of fuels by injecting seawater. As is known, generation of hydrogen caused explosion. The reactor core of TMI nuclear power plant was successfully cooled and stabilized one week after. So will Fukushima.

One of the differences between TMI and Fukushima is that there is a structure called a steam separator in the upper part of reactor core because the Fukushima power station has boiling water reactors (BWR). This structure works as resistance that hinders the steam in the core from going up through to the upper part of pressure vessel and keeps steam in the core, making it difficult for seawater to come into the core. Compared with TMI, it is difficult to cool a molten core of BWR.

Another difference is that fuels have channel boxes. As for this point, both advantage and disadvantage can be considered. However, considering that melting behaviors of the cores are relatively similar as mentioned above, it seems to be not a factor that causes a decisive impact. So, the balance of advantage and disadvantage is regarded as zero in this report.

Another major difference is that the stable cooling of the core of TMI was achieved by the actuation of a reactor coolant pump (equivalent to a recirculation pump of Fukushima). In the case of the pressurized water reactor (PWR) of TMI, as the primary coolant system was clearly separated and isolated from the turbine system, the reactor coolant pump could be actuated at ease because there was no need to worry about radioactive contamination of the turbine condenser that had high cooling ability due to the actuation of the reactor coolant pump. The molten core could be solidified by this forced cooling and then stabilized.

In the case of BWR, however, even though the recirculation pump is actuated, it merely stirs water in the core if the condenser is not used. It cannot be of any help to reduction in the temperature of the core. To use the condenser, however, it is necessary to run a risk to send highly contaminated reactor cooling water to the turbine building that only has light shielding equipment. Whether this decision can be made or not is a turning point to settle the accident as early as possible, i.e., achievement of a stably cooled state.

In short, the essence of nuclear safety has three points, i.e., shutdown, cooling and containment. This also shows the order of importance from a safety point of view. In the case of Fukushima, all reactors are shut down. Then, they have to be cooled. For this purpose, the power to send water is more necessary than anything else. The installation of temporary power sources is urgently necessary.

Next, as to the hydrogen explosion as the main cause that has worsened the accident. In TMI accident, hydrogen explosion was also caused. About ten hours after the outbreak of the accident, a huge explosion happened in the containment vessel. The amount of hydrogen exploded is said to be equivalent to oxidation of about half of fuel claddings. This amount agrees with the report that about half of the core was exposed above the water surface at Units 1 and 3 of Fukushima power station. In the case of TMI, the containment vessel was not damaged. In the case of Fukushima, the explosion happened outside the containment vessels and the reactor buildings were heavily destroyed.

In the case of TMI, about 1,000 residents around the power plant were exposed to radiation. The exposure dose was reportedly 100 millirems (1 millisievert) at the maximum and 1 millirem (0.01 millisievert) on average. The radiation level when vents were opened to reduce the pressure in the containment vessel was reportedly about 1.2 rems (12 millisieverts) high above the power plant site. The figure is close to the level at the time of opening vents at Fukushima. It was reported that radiation dose of 400 millisieverts was temporarily measured within the site. This is because the water in the spent fuel pool decreased. When water is successfully injected, the radiation level goes down. As is the case with TMI, it is also possible to keep radiation hazards as little as possible at Fukushima.

Let me talk about something slightly different from the subject of this report. Some people advertise that the accident at Fukushima is a second advent of the Chernobyl accident. But I cannot understand the point of such argument. As far as radiation hazards are concerned, there is no possibility at all for the accident at Fukushima to further worsen to the global contamination caused by the Chernobyl accident. The reason is that there is not any graphite fire that lifted radioactivity up to as high as the jet stream in the case of Chernobyl. Moreover, as the temperature of cooling water is low, only the radioactive materials with a low boiling point such as noble gases and iodine are released into the atmosphere. The accident at Fukushima is different at all from the Chernobyl accident.

Let me conclude my estimation of the accident conditions at Units 1, 2 and 3 of Fukushima Dai-ichi Nuclear Power Station. All power sources were lost by tsunami. I can merely bow to those who are fighting hard to settle the accident and mitigate the disaster under such conditions. I’m very sorry that explosions were caused and the buildings were destroyed. But the next job to stably cool the reactor cores is left. Though I’m sorry to have put them to so much trouble, I wish their strenuous efforts. The accident and disaster are changing every hour. I would like to cooperate by all means in spite of my old age.

Source: The Denki Shimbun