March 21, 2013
The power loss to the F. Daiichi spent fuel pools (SFP) continues to dominate the Japanese Press. Full power restoration occurred at 12:02 am, Wednesday morning (Japan time). Most Japanese newspapers have dutifully reported the good news. Some have not. It seems a rat was electrocuted on one of the three temporary power-supply switchboards for the SFPs which (because of interconnections) knocked out all three. (Kyodo News; Sankei Shimbun; Japan Today) The Press articles strongly imply that the event was the result of the temporary switch boards being “makeshift” – literally slapped together in a chaotic scramble to get SFP cooling restarted before the nigh-apocalyptic worst-case scenario happened – which allowed the culprit rat to get into one of them. The term “makeshift” is misleading and implicitly inappropriate. Further, we have the greater issue continuing – an incessant focus on a “worst-case scenario” based on embellishment and impossibility.
First, the switchboards are incorrectly, albeit misleadingly dubbed “makeshift” by all but one of Japan’s major news outlets, the lone exception being the Yomiuri Shimbun. Makeshift is defined as a crude and temporary expedient used as a substitute. (Meriam Webster) However, the Press uses the term in the context of synonyms such as “slapdash, hit-or-miss, primitive and/or amateurish”. Nothing could be further from the truth. Yes, the installed switchboards are temporary until the in-plant panels can be re-powered…but that will not happen until the radiation levels are severely lowered inside the reactor buildings. To be precise, the temporary technology has functioned without a hitch for nearly two years. Such a track record hardly qualifies as primitive or amateurish. Two of the boards were erected inside a nearby building to keep environmental effects from causing a problem to them, and they were not invaded by the rat. The third is located on a truck, the bed of which is enclosed. It was the one on the truck that was violated by the now-deceased rat. The technology-itself did not fail because it was crude or slapdash. Regardless, the term “makeshift” is misleading to the point of being incorrect, what other term should the Press be using? “Temporary” is OK. “Substitute” is better. “Surrogate” is probably the best.
Second, the ubiquitous SFP worst-case scenarios contain exaggerations that border on the ridiculous; embellishments steeped in exaggeration. The common worst-case scenario speculates that if power is not restored for a number of days, the decay heat will build up enough to boil off all the water, dry out the fuel bundles and temperatures will necessarily soar. To begin, we have real-world, Fukushima-specific evidence to show that the pools will not get hot enough to boil. Please recall that main power to all SFP cooling systems was lost on 3/11/11. Recovering power to any SFP cooling system at the station was not possible until the temporary power cable from the 1km-distant transmission system was spliced together and energized on 3/17/11. Days of inefficient water drops from helicopters provided a little help with units 1, 2 and 4 SFPs, but it was but a a band-aid on an open gash. . For all intents and purposes, SFP cooling was lost for six days! The level of heat generation in all the pools was at their peak. But, none of the pools boiled. The waters in the unit #3 and 4 pools were heated to about 90 OC and evaporating rapidly enough to produce the “white smoke” wafting above their destroyed refueling decks, but there was no boiling. This site, AREVA and MIT pointed out that it would have taken at least ten days before the tops of the fuel bundles in any of the pools would have become uncovered. It would have taken 5-7 more days for any of the pools to have evaporated to dryness. Now, here’s the rub… decay heat production has dropped constantly over the last two years, so that the current rate of heat-up is but a minor fraction of the original case. It would take three or more weeks, not many days, for the pools to evaporate enough to “dry out”. And this would occur if-and-only-if no-one did anything to mitigate the problem, which simply will not happen in a situation where the whole world is watching.
But, is it possible that dried out fuel bundles could get hot enough to cause a massive release of radioactivity? Possible does not mean inevitable. Possible means conceivable or hypothetical, but not inescapable. Regardless, the dried out fuel bundles would have to heat up beyond 900 oC for sufficient deterioration of the outer Zirconium tubes in the bundles to allow a radiological release. Conceivable? Yes. Probable? No! As the two-year-reduced heat production warms up the bundles, the heat will be naturally lost to the surrounding air. As the bundle temperatures increase, the rate of heat loss necessarily increases with it. At what point will the bundles temperatures peak? That’s a matter of assumption, but it is certainly considerably less than 900oC. Further, it’s absurd to believe all the stored fuel bundles will hot enough for an SPF “meltdown. Meltdown of Uranium (that’s what we’re talking about) begins at ~2,500oC, making it realistically out of the question for the hypothetically dry SFP environment.
Lastly, many news outlets continue to include something impossible as the scariest of all “facts” about an SFP worst-case scenario. Several news outlets say the worst of the worst will occur “…if its [Uranium fuel] temperature is allowed to rise uncontrollably to the point where a self-sustaining critical reaction begins, causing a meltdown.” (Japan Today; Japan Times; Jiji Press; Kyodo News, AFP News) As explained in Tuesday’s commentary, dried-out reactor fuel bundles cannot achieve a chain reaction. The fuel is so dilute in U-235 and Pu-239 isotopes (the only ones that undergo the chain reaction) that it is imperative for the bundles to be immersed in water for criticality to occur. The neutrons released from fission are about a million times too energetic for a chain reaction. After a dozen or so collisions with the Hydrogen atoms in water flowing through the bundles, the neutrons are at a low enough energy level for a chain reactor. No water = no chain reaction. Period!