Fukushima “Melt Throughs”: Fact or Fiction?

February, 2012

Fukushima “Melt Throughs”: Fact or Fiction?

(A summary of the Fukushima Commentary postings about the degree of meltdown for Fukushima Daiichi units #1, #2, and #3. Did any of the material melt through the Reactor Vessel and burn through the bottom of the Containment? The opinions posted here are mine and not those of any “official” organization inside or outside Japan.)

a. How many full meltdowns happened at Fukushima?

Early 2012 postings by Japan’s Atomic Industrial Forum of pressures and temperatures inside the three damaged Reactor Pressure Vessels (RPV) at Fukushima Daiichi suggested that only unit #1 experienced a complete core-relocating meltdown. Unit #2 and #3 RPV temperatures during the accident indicated severe melting of core materials, but not full meltdowns.
Unit #1 had 40% less fuel in its core than either unit #2 or #3, thus it had less decay heat being produced at all times during the station blackout. Decay heat comes from the radioactive decay of the pairs of new atoms caused by the splitting (fission) of Uranium-235. At immediate shutdown following extended full power operation, the heat of radioactive decay can be as high as 7% of full power output. Most of the decay products have very short half-lives and burn themselves out rapidly. Because of this, decay heat output drops to about 1.5% an hour after shutdown and below 1% in less than a day. This is still enough to cause severe fuel damage, and possible meltdown, if cooling water flow cannot be maintained through the core. Unit #1 lost all emergency cooling pumping systems, except one, with the power blackout on 3/11/11. The remaining system, known as RCIC, remained operating for at least another 8 hours after the blackout began because the pump was driven by steam from the RPV. However, the water flowing through RCIC could not be cooled very well, so reactor pressure and temperature rose steadily. Operator records indicate that melting of the core probably began at some point soon after midnight on March 12. Fuel damage progressed for more than nine hours before the plant was depressurized and low-pressure fire pumps could inject cooling water. If any of the three units at F. Daiichi melted through the RPV’s bottom head, it was unit #1.
The time frames relative to unit #2 and #3 were much longer before core damage began because they had more steam-powered emergency cooling systems available. Decay heat levels in both cases were much less than with unit #1 when cooling to their fuel cores was lost, and the duration of lost cooling capability was less than unit #1 in both cases. Thus, units #2 and #3 should have experienced less fuel damage than unit #1. And, there was more evidence of this in the JAIF posting.
The faster water flows through any heat-producing system, the lesser the amount of heat the water removes as it passes through. The slower the flow, the greater the amount of heat absorbed as it passes through. With the entire fuel cell of unit #1 completely melted and relocated, there was literally nothing to obstruct the flow of water through the core barrel. The water flowed through unit #1 rather rapidly and picked up minimal heat from the corium (mixture of uranium and other core metals) relative to the other two damaged units. All three units had essentially the same amount of water being pumped to each RPV. Unit #1 RPV temperature in early 2012 was at 26oC. On the other hand, unit #2 temperature was at 52oC and unit #3 temperature between 47-55oC. The higher temperatures strongly indicated there were some pretty severe flow restrictions in and around the core barrel of units #2 and #3. This suggested that for units #2 and #3 some or most of the damaged fuel had not relocated and remained inside the core barrel! The un-melted portion in units #2 and #3 were unquestionably deformed, restricting the flow pathways. This allowed the water itself to pick up more heat than would otherwise have been the case.
At this point, it seemed safe to assume that government, Tepco, and news media reports of full, core-relocating meltdowns for all three reactors were probably incorrect. With unit #1, the assumption was probably correct! With units #2 and #3, probably not.

b. Did Unit #2 have a “phantom melt-through”?

In mid-January, Tepco made a brief visual inspection inside the primary containment (PCV) structure surrounding the unit #2 RPV with an endoscope. What seemed surprising to Tepco and the Japanese Press was no indication of melted fuel outside the RPV. Tepco spokesperson Junichi Matsumoto told Japan Times, “We could not spot any signs of fuel, unfortunately.” Clearly, Tepco wanted to find signs of melt-through, but was disappointed. Matsumoto added that the endoscopic device used for the inspection only looked at a small portion of the interior, so a better inspection technology and longer visual examinations could find evidence of melted fuel having leaked from the RPV. Thus, they maintained their melt-through assumption with unit #2.
I had maintained since the Fukushima Daiichi control room records were released in July, 2011, that unit #2 probably did not experience a full, core-relocating meltdown. Severe fuel damage inside the fuel cell was a given. From the operator records, it seemed the fuel cell had no cooling water flow for up to 5 hours. With unit #2 generating a very low decay heat-rate after four days, 5 hours probably wasn’t enough time for a total catastrophic meltdown! Some melting was probable in the upper, central portions of the core. It seemed that it might have sustained the degree of melting found at Three Mile Island in 1984, but not the total core-relocating event postulated by Tepco. In other words, I posted that Tepco was chasing a phantom. A few corium drips solidified on the vessel-penetrating control rod mechanisms? Maybe. But a full core melt-through, dropping to the concrete floor beneath the RPV, just didn’t seem likely, and the preliminary endoscopic images taken on January 19th only added fuel to my assumption.
My blog was probably the singular place on the internet you would have found this speculation in January, 2012. I looked diligently for months and found this assumption nowhere else. I wrote that if I was wrong, I would be the first to admit it and ask that my crow be served medium rare with fries and plenty of ketchup. But, that’s one meal I don’t think I’ll have to eat. As it turned out, later evidence from F. Daiichi made that meal even less-likely.

c. No “Melt-throughs” at Fukushima Daiichi

On October 12, 2012, Tepco posted the results of the first water sample taken inside the unit #1 Primary Containment Vessel (PCV). I found the results to be more than a bit of a surprise. The interior water’s results were compared to analyses of the water’s outside the PCV taken in late September from the basement of the unit #1 reactor building, and the results were not as anticipated. First, the Cesium contamination level inside the PCV was half of the concentration found outside - 35,000 Becquerels/milliliter inside vs. 74,000 Bq/ml outside. Second, the chloride level inside was ten times less than outside (19 parts per million vs. 200ppm). This means that the salt concentration inside the PCV from the seawater, used to cool unit #1 beginning at 8pm on March 12, 2011, was significantly lower than outside. (Tepco Press Release; Kyodo News) This strongly suggested that very little of the seawater used to cool the damaged fuel had flowed out of the bottom of the RPV.
This strongly suggested several possibilities. To begin, Tepco assumed water inside the PCV was being recirculated more efficiently than that outside the robust containment walls. In other words, the recycled fresh water being injected into unit #1 was diluting the interior waters better than the exterior. But, if this was the case, then where was the interior water going? If it was leaking out, it should have been mixing with the building’s basement waters and diluting them as well. There could have been be no mixing of interior and exterior waters or the two sets of analyses would have had greater similarity between the saline levels. While the salt differences gave us a better picture of the PCV’s interior environment, it raised a whole new set of questions as to what the actual water flow-path(s) from the RPV to the outer building might be.
Finally, the interior water being lower in Cesium content than the exterior implied that the melted-then-re-solidified corium had not melted through the many inches thick, steel bottom head of the RPV. If the corium was mostly outside the RPV and heaped on the base-mat of the PCV - melted completely through the Reactor Pressure Vessel (RPV), as surmised by just about everyone in Japan - the interior waters should have been much higher in Cesium content than the exterior. But, the data indicated the opposite.
In addition, a related, albeit compelling bit of evidence further demonstrated that the vast majority (if not all) of the solidified corium from the unit #1 meltdown remained in the bottom head of the RPV, and had not melted through. Specifically, the radiation levels detected inside the PCV on October 12. At the surface of the inner PCV’s water, the radiation field was 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 when it reached the point of detector entry, about 8.6 meters above the basement floor and several meters above the waters below. This was at or near the same elevation as the bottom head of the RPV. This fact alone, independent of the water analysis, virtually verified that the corium was not under the water, but rather remained safely contained inside the RPV’s bottom head!
I had previously said that it was possible that some of the unit #1 corium had melted through the bottom head, but the October 12 analyses quashed that speculation. I immediately updated by speculations for meltdowns and RPV melt-throughs at F. Daiichi as follows... First, there was no melt-through of the unit #2 RPV. Unit #2 seemed to have experienced core damage in the range of Three Mile Island in 1979, with most of the fuel cell melting and relocating to the RPV’s bottom head – but no melt-through. Unit #3 may well have experienced a total core meltdown with full-core-relocation to the bottom head of the RPV, but once again, no melt-through. Given the differences with respect to the chemical and radiological make-up of the interior and exterior waters relative to the unit #1 PCV, and the fact that the highest radiation level inside the PCV is essentially parallel to the bottom head of the RPV, I suddenly came to understand that the worst-case scenario for unit #1 was incorrect. I came to the startling conclusion that none of the reactors at Fukushima Daiichi experienced catastrophic RPV melt-through.
After posting this opinion, I received a few objections. The objections were based on my challenging Tepco’s official “party-line” on the issue. Tepco said that the melted fuel for unit #1 burned its way through the reactor’s bottom head and collected on the floor beneath. They felt that the water discovered in the basement of the unit #1 PCV was keeping it cool enough to prevent the release of large amounts of radioactivity. However, their 10/12/12 chemical and radiation data inside and outside unit #1 PCV did not support their position of RPV melt-through. Because of the objections, I felt it was important to provide a more detailed explanation.
The water being decontaminated (Cesium-stripped) at F. Daiichi was coming from the turbine building basements. Some of the basement waters were transferred to the waste treatment building before being run through the Cesium removal system, and the rest directly through the absorbers. After decontamination, the Cesium-stripped liquid was used to cool the decay heat-generating melted fuel in units #1 through 3. There was no certainty as to where the water was going once it left the cooling-water pumps and entered the RPV. However, there was little doubt that most was running through the RPVs because its temperatures inversely fluctuated with flow changes, but where it was going there-after had been a matter of debate for the previous year-and-a half. The “official” RPV-to-PCV-to-reactor building basements-to-turbine basements flow-path assumption had considerable technical challenges, and the 10/12/12 analyses only added to tits problems.
The reactor building-to-turbine building part of the assumption, at the end of the “official” flow-path, seemed reasonable although no-one knew the precise point of influx into the T-Basements. However, the robust construction of RPV made 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 found the possibility of an integrity compromise through the massive bottom head questionable from the first day it was officially postulated. The weakest points with respect to the bottom head’s construction are at the “stub tubes” for the Control Rod Drive (CRDM) penetrations. These stub tubes are integral to the 8-9 inch thick bottom head, and made of the same high-carbon steel. It is possible that there may have been some "drip-through" of molten corium through the CRDM hydraulic technology inside the bottom head's installed stub tubes, but this would not constitute the postulated full "melt through" proposed by Tepco.
Tepco's early-2012 inspections inside the unit #1 PCV indicated 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 could be safely questioned. 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 technology inside the RPV stub tubes 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 should have been greater than analyzed. Corium is not “just” melted Uranium fuel. It also contains the structural materials from the fuel bundles inside the nuclear fuel core, as well as the metals from the core support assemblies; 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 a bit less dense than Uranium alone 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 was actually covering the re-solidified mass of corium, the radioactive material concentration should have been considerably higher than with the waters outside the PCV due to on-going fission product replenishment. But it was the other way around. Since it seemed unlikely that the water being pumped into the RPV was finding its way into the PCV compartment below the RPV, and further since it seemed unlikely that the basement water was finding its way out of the PCV and into the outer reactor building, the contamination level in the inner PCV water should have been much, much higher than Tepco’s analysis showed…if the corium mass for unit #1 was actually there.
Speculations on melt-throughs had a few other problems, as well.  First, the corium would have to have been molten long enough to burn through 8 inches of solid, cast, high-carbon steel. For this to happen, the corium in the bottom head would have to have been virtually dessicated for many hours. Any water in or surrounding the corium would be rapidly boiling away during zero coolant flow periods, and taking considerable heat away with it. However, even at the very low level of sporadic injection flows into the RPVs between March 11-15, it is highly unlikely that the corium was completely molten long enough for such a severe burn-through.
Second, if a burn-through of this magnitude did happen, the cores would have rapidly depressurized...rapidly. And, any level of re-pressurization would have been highly unlikely. Full melt-troughs would produce the equivalent of a very big hole. However, instruments showed that while pressures dropped dramatically from time to time, there was never a complete depressurization of any of the vessels. TEPCO assumes the pressure drops indicated several moments of core burn-through for each of the RPVs. However, each of the suspect pressure drops come just after water injections began. Any physics teacher knows that introducing room-temperature water into a closed, super-hot container will cause a rapid, significant internal pressure drop due to quenching.
Considering all of the above, it appears there was no catastrophic “melt-through” of the unit #1 RPV at Fukushima Daiichi, no melting through the many-feet-thick steel and concrete base-mat beneath the RPV, and no escape of the corium into the geological environment beneath the structure. Plus, it seems there were no melt-throughs for either unit #2 or #3.
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On January 23, 2014, A Japanese research group proved that nuclear fuel can be detected using cosmic rays. A team from Tokyo University’s High Energy Accelerator Research Organization (KEK) used muon detection to correctly pinpoint the location of the core and spent fuel at Tokai unit #2 from outside its reactor building. Muons are a major part of naturally-occurring cosmic rays that constantly strike Earth from outer space. Muons generally pass through most everything on the Earth’s surface and are attenuated (stopped) by the dense material inside our planet’s crust. A very dense material like Uranium (and/or corium) will attenuate more muons than the surrounding materials inside a reactor building. The detectors identified these changes at Tokai and correctly mapped out the location and rough geometry of the core. KEK would now like to use their system at Fukushima Daiichi to see where the cores of units #1 through #3 are located. This could possibly answer the question of whether or not any of the three actually melted through the many-inches-thick steel bottom of the Reactor Pressure Vessel. It could also indicate the degree of core damage inside each of the units. I fully encourage Tepco to let KEK run their muon detection system on all three damaged units at F. Daiichi.
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In Mid-March of 2015, the first image of the interior of Fukushima Daiichi unit #1 reactor vessel was posted. Muon tomography uses sub-atomic particles naturally-produced in our atmosphere to slowly create shadow-like images inside objects. For example, it has been successfully used to scan inside Egyptian pyramids. Early in 2015, one of the devices was set up to scan inside the reactor pressure vessel of unit #1. It took more than two months to create a clear image. The image was compared to an earlier taken at Fukushima Daiichi unit #5, showing its still-intact fuel core. The unit #1 image showed that all of its core had malted, along with all internal structures around and below the fuel assemblies, had melted away. Unfortunately, geometric limitations associated with unit #1 and the best-possible location for the Muon imaging technology would not allow researchers to “see” below the lower core-support plenum, leaving the bottom head of the RPV completely out of the picture. We could see that the core had completely re-located, but where it has finally come to rest remained unknowable.  

On June 30, 2016, NHK World reported that the corium (formerly molten and re-solidified fuel core) for unit #2 is in the reactor’s (RPV) bottom head. This was subsequently re-posted by Kyodo News the following week. High-tech muon imaging for unit #2 included the bottom head, which was not possible with the earlier imaging for unit #1. NHK reported the still-in-process image now shows a “large, black shadow” inside the 8-inch thick steel bottom head of unit #2, strongly indicating that the corium was contained. No melt-through, if you will. Finding the re-solidified mass in the bottom head of unit #2 literally dashed the “nobody knows” speculations to ashes. We could be assured that we know where the unit #2 fuel core ended up, at the very least. Further, the unit #2 discovery suggests that unit #3’s corium is also cooled and pooled inside its RPV bottom head.

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On July 28, 2016, Unit #2’s re-solidified fuel was confirmed to be inside the reactor vessel (RPV). Tepco has posted a detailed Press handout concerning the muon scanning results for unit #2 (link below). On page four of the handout, we can see that the corium remains in the bottom head of the RPV. The image also indicates that some of the damage fuel is still in the core area, where it was located before the March, 2011, tsunami-spawned nuclear calamity. Page six of the handout shows that of the 210 tons of fuel and support structures that originally comprised the undamaged core, 20-50 tons remain in the core barrel and “about 160 tons” is collected in the RPV’s bottom head The inherently limited resolution with muon imaging compels approximation of the relative masses, which leaves the speculative door open for believing that as much as 14% of the core might possibly have worked its way through some of the bottom head and re-solidified on the base-mat beneath the RPV. 

(unit #2 imaging) http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2016/images/handouts_160728_01-e.pdf 

(unit #1 imaging, for comparison)  https://www4.tepco.co.jp/en/nu/fukushima-np/handouts/2015/images/handouts_150319_01-e.pdf