The Mainichi Shimbun has posted an editorial on the dangers of shut-down reactors. (#1, below) It is written in a Q and A format. I am not going to nit-pick the many exaggerations contained in the article because nuclear-related hyperbole is typical of the Mainichi. However, the Mainichi’s assessment of the current level of heat being generated from the damaged fuel of Fukushima Daiichi’s units #1 through 3, is false… fictitious… just plain WRONG! And, not by just a little bit, either. They should prominently post a correction.
First, what we are dealing with is called “decay heat”. When the U-235 and Pu-239 (near the end of core life) atoms split (fission), the two pieces of the original nuclei instantly become two other elements. These “new” elements are not unique to the universe, and can be found on the Periodic Chart (from Zinc to Dysprosium). These freshly-made elements are quite unstable and produce radiation. As the radiation is absorbed and/or dissipated inside the reactor core, heat is produced. Immediately after a rapid, full-core shutdown (SCRAM), decay heat can be op to 7% of full reactor power (after a power run of approx. 1 year). Fortunately, the vast majority of the “new” elements is highly radioactive and burn themselves out at a prodigious pace. Within an hour after SCRAM the decay heat rate is ~1.5%, and after a day is below 1%. The drop-off continues for a few centuries until it becomes less radioactive than natural Uranium itself.
Now, here’s the intolerable problem with the Mainichi editorial. It says, “Over a year has passed since the No. 1 through 3 reactors at the Fukushima No. 1 plant were stopped, and the heat has dropped to around 0.4 percent of what it was immediately after the shutdown.” This is way, way off. Actually, decay heat is well below 0.1%…in fact more like 0.05%. (#2, below) In other words, the Mainichi’s posted decay heat levels in the F. Daiichi cores are off by as much as a factor of 20! Instead of 5,000 liters of water possibly boiled off in an hour, it’s more like 250 liters. The fuel in the Spent Fuel Pools is even older and generates much less heat. This makes the speculations of apocalyptic consequences from SPFs going dry, little more than a scary fairy-tale.
We should demand the Mainichi to get their technical numbers right. Anything less would be worse than a mere oversight…it would be disrespectful to their readers!