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Estimation of "Status of reactor cores at Fukushima"

Published on Mar.18.2011 : The Denki shinbun(The Electric Daily News)

Dr. Michio Ishikawa
Chief Adviser(Former President & CEO)
Japan Nuclear Technology Institute(JANTI)


This is my emergency estimation about what could be happening at reactor cores at Fukushima, what could happen next and what actions can be taken, based on the facts and developments at the time of the Three Mile Island nuclear accident.

Please bear in mind that I live in the quake-affected city of Hitachinaka. Three whole days of power blackouts up until March 14 left me incommunicado with the outside world. The only source of information was radio broadcast. I had no idea what was happening in the world until the television came on finally in the evening of March 14. Hence, I am a little short on facts and figures. This article describes a scenario that I have put together based on limited facts. Please excuse me for any minor mistakes.

First of all, the state of reactor cores. Knowledge from the TMI accident indicates that reactor cores behave very differently depending on whether they are under or above the water level. This is a relevant point for Fukushima, so let me go into more detail.

The submerged part of the fuel rods is cooled with water, and can maintain a sound state. There is no argument on this point.

On the other hand, the exposed part of the fuel rods is surrounded by steam, and in a poor condition for heat removal. With the temperature increasing gradually with decay heat, the fuel rods begin reacting with steam at around 900 degrees Celsius, oxidizing claddings. This reaction generates strong heat, causing a localized increase of temperature in the immediate area. At around 1300 degrees Celsius, the reaction becomes more active, and the temperature rise on the claddings becomes unstoppable. The claddings become coated with thin oxide film (zirconium dioxide) on the outside, as well as on the inside due to oxygen removal from fuel pellets (uranium dioxide).

In other words, thin oxide film coats the claddings, made of zircaloy, both inside and out. It should be noted that the oxide film has a higher fusing point than the cladding material, zircaloy, whose fusing point is approx. 1800 degrees Celsius. When zircaloy melts, it drips down between the films to form a puddle. The oxide films on both sides become fused together and pressed onto fuel pellets with the pressure of the reactor. At this stage, a fuel rod can be likened to fuel pellets wrapped in cling wrap. Oxide film is resilient at high temperature, and seals in radiation even with some disfigurement to the fuel rods, keeping them upright in water. This is why no radiation was released from exposed fuel rods at Fukushima. It was no case of measurement error.

This condition changes at the moment when water is added to the core. Oxide film becomes weaker as the temperature drops, and shrinks when cooled down. Fuel rods disintegrate into individual fuel pellets and collapse (not melting), scattering in the reactor water as if a toy box is tipped over. They can stay scattered in water because the submerged part of fuel rods is still sound. This is what happened at the reactor core in the TMI accident.

Collapsed fuel rods are cooled as long as they are submerged in water, thanks to the cooling effect of water flowing through the debris (communication path). Consequently, fuel pellets stay in the state of debris without melting.

Summing up, the exposed top part of fuel rods generated hydrogen and collapsed, but the debris was kept cool, retaining the pellets' radiation containing effect.

The problem lied with the submerged part of the fuel. Water turns into steam as it cools fuel rods. However, in this case, the flow of steam was blocked with the debris, and could not escape, forming a steam zone immediately below the debris. This created a condition similar to the exposed fuel above water. Under water, heat dissipation performance was substantially worse. Heat from cladding oxidization built up and melted fuel rods, initiating meltdown. However, the meltdown temperature was believed to be around 2300 degrees Celsius, which was the fusing point of the ternary alloy of uranium, zirconium and oxygen, rather than the uranium dioxide's fusing point of 2800 degrees Celsius. This meltdown temperature was not high enough to melt concrete, and therefore could not cause a "China Syndrome" scenario.

Since the underside of the meltdown was touching cooling water, it turned into a hard crust state, much like cast iron. Yet, immediately above that, melted fuel flowed in the side direction, came in contact with the core shroud, made of thin stainless steel, and put holes through it. Fuel that dripped from the holes formed balls measuring 15–20 centimeters in diameter, which were later found at the bottom of the reactor core.

This is how the core meltdown occurred at TMI. The Fukushima plants are showing similar core behaviors. One of the similarities is the fact that the top 2 meters of fuel rods have become exposed above the reactor water for an extended period of time. Cesium and other fission products were released as a result of fuel rods disintegrating upon the injection of seawater. The formation of hydrogen led to explosions, as has widely been reported. The reactor core at TMI was cooled and stabilized after one week. Fukushima will also be successfully brought under control.

The difference between TMI and Fukushima is the existence of a steam-water separator at the top part of the reactor core, because Fukushima uses the BWR system. This structure serves as resistance to releasing steam from the core to the top part of the pressure vessel. It therefore keeps steam in the core, undermining the injection of seawater. Compared to the example of TMI, BWR has a design that may make it difficult to cool the molten core.

Another difference is the use of channel box in nuclear fuel. This could turn out to be a positive or a negative. Yet, it is not a deciding factor, considering that the core has a similar meltdown behavior. In this article, I assume that the positive offsets the negative.

One more major difference is the fact that TMI's reactor core was stabilized with the use of the primary coolant pump (equivalent to the recirculation pump at Fukushima). With PWR, the primary cooling system is clearly separated and insulated from the turbine system. A turbine condenser, which has a high cooling capacity, would never suffer radiation contamination with the activation of a primary coolant pump. This powerful cooling ability successfully halted the meltdown and stabilized the core.

However, with BWR, simply activating a recirculation pump would do no more than agitating the reactor water unless a condenser is also used. The pump alone does not contribute to lowering the core temperature. However, using the condenser runs the risk of sending highly contaminated reactor cooling water to the turbine building, which has only limited shielding facilities. Whether the authorities can make this decision marks a turning point in the on-going efforts to bring the reactors to stability.

The three functions of nuclear safety are to "shut down", "cool down" and "contain". This also represents the order of importance in these safety actions. At Fukushima, all reactors shut down. The next step is to cool them down. For this purpose, motor power to send water is needed more than anything else. The installation of temporary power source is the task of utmost urgency.

Let me move on to the issue of hydrogen explosions. Such an explosion also rocked the TMI accident. A massive explosion occurred in the containment vessel some ten hours after the accident started. The amount of hydrogen involved in the explosion, according to the post-accident calculation, was equivalent to the amount generated if about half of the fuel claddings became oxidized. This corresponds to the case at Fukushima Daiichi Unit-1 and Unit-3, where fuel rod exposure was reported to be about 50%. In the case of TMI, there was no damage to the containment vessel. In Fukushima, explosions occurred outside the containment vessels, destroying reactor buildings.

In the TMI accident, approx. 1,000 area residents became exposed to radiation at the rate of up to 100 mrem (1 mSv), and 1 mrem (0.01 mSv) on average. The level of radiation when the ventilation operation was conducted to depressurize the containment vessel, was reported to be approx. 1.2 rem (12 mSv) in the skies above the station site, which is similar to the level recorded at the time of ventilation at Fukushima. The radiation dose recorded in the skies above Fukushima Daiichi Unit-4, is said to be 400 mSv. This is because of the loss of water in the spent fuel storage pool, and is set to decrease once the water level is restored. It is still not impossible to keep radiation leak to a minimum in Fukushima, just as in the case with TMI.

Slightly off the topic, there are some people who call the Fukushima case as another Chernobyl. It is unclear what their arguments are. As far as radiation emergency is concerned, there is no possibility that the Fukushima case could cause contamination of the international scale experienced at Chernobyl. This is because of the absence of a graphite fire, which sent radiation high into the air to reach the jet stream. In addition, the low temperature of cooling water means only the radioactive materials with a low boiling point, such as noble gas and iodine, could be released into the atmosphere. The situation is nothing like what happened at Chernobyl.

This summarizes my estimation of the state of accident at Fukushima Daiichi Nuclear Power Station's Units-1–3. I have nothing but respect for all the personnel who continue to fight the desperate fight to bring the plant under control and alleviate the extent of the emergency under the current condition with all power sources swept away in the Tsunami. It is regrettable that the situation has escalated to explosions and damage of reactor buildings. Another task still remains to stabilize the reactors. I wish to send my support for those people on the frontline. Situations change in emergencies like this every minute. I am prepared to provide as much cooperation as possible despite my old age.

  (wrote on Mar.15.2011)


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