I have received a few objections concerning the possibility that there have been no “melt-through” events with any of the reactor pressure vessels (RPV) at Fukushima Daiichi. The objections are based on my challenging Tepco’s official “party-line” on the issue. Tepco says that the melted fuel for unit #1 burned its way through the reactor’s bottom head and collected on the floor beneath and that the water discovered in the basement of the unit #1 PCV is keeping it cool enough to prevent the release of large amounts of radioactivity. However, their most recent chemical and radiation level study on unit #1 PCV does not support their position of RPV melt-through. My Friday commentary was literally “dashed off” in summary form in order to meet my self-imposed deadline for posting updates. Because of the subsequent objections, I feel it is important to provide a more detailed explication of my claim.
The water being decontaminated at F. Daiichi is coming from the turbine building basements. Some of the basement waters are transferred to waste treatment building before being run through the de-con system, and the rest directly through. After decontamination, the drinking-water-quality liquid is used to cool the decay heat-generating melted fuel in units #1 through 3. There’s no certainty as to where the water is going once it leaves the cooling-water pumps and enters the reactor containment. There’s little doubt that most is running through the RPVs because temperatures fluctuate with flow changes, but where it is going there-after has been a matter of debate for the last year-and-a half. The “official” RPV-to-PCV-to-reactor building basements-to-turbine basements flow-path assumption has considerable technical challenges.
The reactor building-to-turbine building part of the assumption at the end of the proposed flow-path seems reasonable, although no-one knows the precise point of influx into the T-Basements. However, the robust construction of RPVs makes the RPV-to-PCV portion of the flow scenario questionable. I personally witnessed the installation of a BWR RPV in the early 80s (1982, if memory serves). Because of this, I find the possibility of an integrity compromise through the massive bottom head questionable, and have since the issue was raised. The weakest points with respect to the bottom head’s construction are at the stand-pipes for the Control Rod Drive (CRDM) penetrations. These stand-pipes are integral to the 8-9 inch thick bottom head. It is possible that there may have been some “drip-through” of molten corium through the CRDM hydraulic connections to the bottom head’s installed stand-pipes, but this would not constitute the postulated full “melt through” proposed by Tepco.
Tepco’s recent inspections inside the unit #1 PCV indicate a high degree of physical integrity with respect to the PCV’s thick steel-reinforced concrete outer wall and steel liner. Thus, the postulated PCV-to-reactor building portion of the flow path must now be questioned. It might be the case that the water inside the PCV is stagnant, and has been there since seawater was introduced as a coolant to the RPV. Once the seawater impacted the molten mass inside the RPV (corium), it would have rapidly solidified (crusted) the corium and sealed the “drip-throughs”. Relatively little of the seawater itself would have leaked through the CRDM connections to the RPV stand-pipes before the corium solidified and sealed the leaks. This would account for why the inner-PCV basement waters are 10 times lower in chloride content than the waters outside the PCV walls.
Further, if the PCV basement water were in direct contact with a solidified corium mass, the radioactive content would be greater than analyzed. Corium is not “just” melted Uranium fuel. It also contains the materials from the fuel bundles inside all nuclear fuel cores, as well as the metals from the core support structures; an admixture of Uranium, Zirconium, steel, and control rod materials, if you will. Uranium is one of the densest materials found on earth. Even so, undamaged fuel pellets leach some fission products out of them: mostly inert gasses such as Xenon and Krypton, plus a few other radioactive elements (like Cesium) that are formed a few millimeters from the outer surface of the pellets. The Corium is less dense than Uranium and its leach rate of fission products should be higher than from undamaged uranium pellets. In other words, if the water in the PCV basement is actually covering the re-solidified mass of corium, the radioactive material concentration should be considerably higher than with the waters outside the PCV due to on-going fission product replenishment. It’s now known to be the other way around, which does not seem possible if there was a melt-through. While a cooled mass of corium will certainly leach fission products at a lower rate than hot corium, it will not eliminate it. Since it seems unlikely that the water being pumped into the RPV is finding its way into the PCV basement, and further since it seems unlikely that the basement water is finding its way out of the PCV and into the outer reactor building, the contamination level in the inner PCV water should be much, much higher than Tepco’s analysis shows…if the corium mass is actually there.
In addition, a related, albeit compelling bit of evidence further demonstrates that the vast majority (if not all) of the solidified corium from the unit #1 meltdown remains in the bottom head of the RPV – the radiation levels detected inside the PCV. At the surface of the basement’s water, the radiation field is 0.5 sieverts per hour. If the corium were in-fact beneath the water, the radiation level should have dropped as the detector was moved higher above the water’s surface. However, the opposite was the case! As the detector was moved up, the radiation field increased and peaked at more than 11 sieverts at the point of detector entry, about 8.6 meters above the basement floor. This would be at or near the same elevation as the bottom head of the RPV. This fact alone, independent of the water analysis, virtually verifies the claim that the corium is not under the water, but rather safely contained inside the RPV’s bottom head.
Thus, it seems there has been no catastrophic “melt-through” of the unit #1 RPV at Fukushima Daiichi. This, combined with my prior arguments of no melt-throughs for either unit #2 or #3, leads me to conclude that there have been no melt-throughs with any of the RPVs at Fukushima Daiichi.