Shown here is a map of the Japanese Nuclear Power plants, made by the U.S. Photo courtesy of U.S. government, image by Calmos.
In March 2011, Japan’s Fukushima Daiichi nuclear plant complex was hit by a tsunami triggered by a 9.0 magnitude earthquake. Three of the plant’s six reactors suffered a nuclear meltdown. Two of the reactors, 5 and 6, were in cold shutdown, while reactor 4 was de-fueled. All six are of boiling water reactor design.
Reactor 3’s building exploded on March 13, 2011, due to hydrogen buildup breaching the containment while the reactor continued its meltdown. Yet the story continues to linger, and the speculations have not stopped. What is happening in Fukushima now, and why is it happening?
With a high potential of fear-mongering, just the facts, please.
Reactor 3 Steam Cloud: July 2013
First noticed in July, 2013, the plant, Reactor 3, suddenly started releasing steam. It did not appear to be from the result of renewed nuclear reactions, a worst-case scenario that could potentially lead to a new release of radioactivity. How did they arrive at that conclusion arrived?
TEPCO stated its conclusions were based upon detecting no new chain reaction, fission byproducts, and the temperature remaining constant. At this time, officials did not know what was causing the steam release but hypothesized it was caused by rain waters seeping into the reactors damaged containment vessel which hit thermal hot spots and turned to vapor. The building itself is not accessible to view or entry due to the radiation levels.
New Cloud Of Steam From Reactor 3: December 2013
On December 19, 24, 25, and 27, Reactor 3 started releasing steam, again. The plume was captured on surveillance cameras and appeared to be coming from the 5th floor of the building housing the spent fuel pool and crippled reactor. Still, an unanswered question, this is a strong indicator of the fragile state the site is in and many unanswered issues still exist. Many events can cause steam to be released under these conditions, so let’s look at the three possibilities most discussed.
Could a Meltdown and Spontaneous Fission Be Taking Place in the Spent Fuel Storage Pool?
This is the most improbable of the possibilities, but let’s take a look. Reactor 3 spent fuel storage still houses 89 tonnes of spent fuel composed of 514 fuel rods. The concern is that the water in the spent fuel pool will boil off leaving the fuel rods to melt and release significant radioactivity in a meltdown scenario. In this scenario, the overheating water is the source of the steam plume.
To understand why this possibility is the most improbable we need to look at the spent fuel pool design.
A storage pool is typically 40 feet or more deep, with the bottom 14 feet equipped with storage racks designed to hold the spent fuel, including boron as a neutron absorber. The spent fuel only needs 8 feet of water to keep radiation levels below acceptable levels. If the water in the tank boils off, the low enrichment level coupled with a neutron absorber, boron, just doesn’t have enough material to self-sustain a fission reaction. If the water evaporates, there is no element to enable a chain reaction by moderating neutrons - so a fission process will not occur.
Has Corium Reached the Groundwater?
The core meltdown debris, a molten mixture, is called corium. Reactor 3 ran on a mix of uranium and uranium/plutonium mixed fuel unlike the other reactors. The core contained 566 fuel rods which experienced a complete meltdown. We don’t know for sure at this time, but the corium could have burned its way through the reactor vessel due to extreme heat, thus reaching the underlying ground water and producing steam.
Fukushima Plume Caused by Rainwater on Stray Fuel Elements?
Suggested as the reason for the original plumes in July, this would be the least worrisome possibility. Steam plumes could be caused by rain or groundwater coming into contact with fuel pellets and reactor rod fragments resulting from the initial hydrogen explosion.
Reactor 3: Is the World In Danger?
We don’t know what’s causing the steam plume at Fukushima, but we should know soon. Since entry into the Reactor 3 building is not possible due to radiation levels, the answer is in what the plume of steam is carrying. What fission byproducts exist in the steam and how much?
The isotopes TEPCO will be looking at are Xe-140 and Iodine-131. Both are volatile and have a very short half life. They both are produced from new fission. If the steam plume is the result of the corium reaching the groundwater, or rainwater hitting fuel pellets, they’ll see low Iodine and no Xenon. In the interim, we must wait and see the answer, which may surprise us.
Fukushima Steam Plume: Questions Remain
To date, there is no data to support the hypothesis that the observed steam plumes contain significant radioisotopes detrimental to human health, local or abroad. To suggest otherwise, at this point in time, with no data, is nothing more than fear mongering.
When should we expect an answer as to what the steam plume contains?
How does this compare to the release at Chernobyl?
TEPCO probably has the answer now but has not released the data. I would expect it shortly. Also, there is no comparison with Chernobly, different fuels, different plant designs, different accident.