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Radiation probes indicate NO melt through at Fukushima Unit 1

Rod Adams's picture
President and CEO Adams Atomic Engines, Inc.
  • Member since 2006
  • 969 items added with 296,876 views
  • Oct 15, 2012

Tepco has recently released measurements that provide convincing evidence that virtually all of the corium in Fukushima Daiichi unit #1 remains safely stored inside an intact reactor pressure vessel. Despite all claims to the contrary, no substantial quantities of that material have melted through the pressure vessel to fall onto the concrete floor of the surrounding containment structure.

It has always seemed far fetched to me to think that material from a nuclear reactor that melted several hours after fission has stopped contains the power density necessary to melt through carbon steel pressure vessels that are 6-12 inches thick. My basis for making that statement comes from having spent several sleepless nights in a drydock watching people with specially designed torches cutting into submarine hulls to provide maintenance access. I also had the opportunity during at least one repair period to be the guy responsible for signing the requisition chits for the pallets full of gases used to power those torches.

Melting thick steel is not a job for a mass of metal that is only being heated by radioactive decay whose heat production is falling rapidly.

A couple of days ago, Tepco drilled a hole into the thick concrete foundation that supports the reactor pressure vessel. The hole was approximately 8.7 meters above the containment floor. The workers then inserted a radiation probe. Based on the readings from the probe, it is not possible to truthfully assert that the corium left the reactor pressure vessel and made it to the floor of the containment under the vessel.

The probe registered its highest reading (11 Sv/hr) at the level where it was inserted – 8.7 meters above the containment floor. I do not know the details of the apparatus, but it is reasonable to assume that the workers were using gravity to lower the probe and had no mechanism that would enable the probe to be raised above the entry point.

The measured radiation dose rate was lower at all points below the entry point until it reached a minimum at the surface of a pool of water at the bottom of the space. The other sensors inserted with the radiation measuring probe determined that the water was fresh – with a chloride concentration of about 19 parts per million – and that it was about 2.8 meters deep. The radiation reading at the surface of the pool of water was only 0.5 Sv/hr.

Those radiation readings indicate that the radiation source is at the top of the space, not the bottom. It is a little difficult to paint a complete picture with words, so I captured an image from a document posted on the US NRC web site titled Reactor Concepts Manual, Boiling Water Reactors.

The image is a BWR/6, which is not the same model as the Fukushima units, but it has roughly the same configuration of a reactor pressure vessel with control rods that are inserted from the bottom and is mounted in a concrete foundation with several meters of empty space between the bottom of the vessel and the floor of the containment.

The space under the reactor pressure vessel that I am describing is near the number 24 in the image below. As other points of reference, the normal location of the fuel in a BWR is at number 15, and the control rod drive mechanisms are at number 22.

Tepco’s recently measured information needs to be widely disseminated and discussed by nuclear professionals. Some of those professionals have spent decades attempting to model and predict the interaction of corium and concrete under the “worst case” assumption that reactors that melt due to insufficient cooling will also melt through their pressure vessels to fall onto the concrete.

Perhaps we can use real world experience from the unintended “theory to practice” events at Fukushima to show the modelers and the regulators that “worst case” analysis does not provide any realistic prediction of what will actually happen in the real world of carbon steel pressure vessels, latent heat of fusion, water, and measurably limited heat from radioactive decay.

Hat tip to Leslie Corrice who has been providing Fukushima Updates about three times per week for more than 18 months. The specific issue that provided this information is October 12, 2012.

If you want facts and data about Fukushima events and post accident efforts, Les’s Hiroshima Syndrome is a terrific place to start.

The post Radiation probes indicate NO melt through at Fukushima Unit 1 appeared first on Atomic Insights.

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Richard Bud's picture
Richard Bud on Oct 16, 2012

I often wondered how Jimmy Carter had the presumption to make decisions based on his Nuclear Navy experience?

Now I see the same arrogance instilled by the US Navy Nuclear Program in this 'analysis' of  radiation readings taken at Fukushima! This includes a complete lack of knowledge of decay heat and its profile.

After a Nuclear Reactor trips from power, the decay heat is about 15-20% of the maximum power of the reactor and even though the decay is exponential, the Fukushima cores were uncovered quickly and the energy available to melt the cores was immense and this melted mass had more than enough energy to melt through the reactor vessels despite what this author may think. Believe me, watching someone use a cutting torch to cut through material is NOT an experience to give anyone the ability to make pronoucements over what happens when a Nuclear Core melts.

My experience  comes from degrees in Nuclear Engineering and a 40 + year history in the Commercial Nuclear Power industry managing every aspect of Nuclear Power including Shift Manager as a licensed Senior Reactor Operator.

I have hired many individuals from the Nuclear Navy Program and they are good performers but sometimes possess an arrogance that what they were taught makes them capable of understanding the theoretical aspects of Nuclear Power. Nothing could be further from thet truth.

Nathan Wilson's picture
Nathan Wilson on Oct 18, 2012

The decay heat 1 second after reactor shutdown is under 7% of operating power, and it is under 1% within 2.4 hours, as described here: This 1%  value still amounts to 20 MWatts or so, but remember that in the high pressure environment of the reactor, even steam cools the rods 100 times better than air at normal pressure. 

Also, the cooling systems at Fukushima operated on diesel power for an hour after shutdown, then on battery power for many hours more.  In contrast, at Three Mile Island, the stuck-open valve drained enough water from the reactor to leave the core exposed only two hours after shutdown; still, there was no burn-through at TMI.

However, I agree that Rod seems to be putting more confidence in his gut-feel than the scientist who study this stuff for a living.  Perhaps recently he's spent more time reading worst-case scenarios crafted by anti-nuclear activists than reading real engineering studies.

Rod Adams's picture
Rod Adams on Oct 18, 2012


For someone with degrees in nuclear engineering, you have provided a rather creative number for the amount of power produced by decay heat. Here is a good refresher for you on the topic -

Please do not blame the US Navy Nuclear Program for my "arrogance"; though that program provided me with a solid base of understanding, I have spent the past 20 years outside of the program learning more about nuclear energy, materials, and modeling. I have also had the opportunity to read about the post accident analysis of real core melt events like the one that happened at Three Mile Island. There was a wonderful summary article published in Science that provided some pertinent information about just how far a melt will progress in a large, light water reactor that gets shutdown and subsequently loses cooling.

Finally, you might be interested in reading a more detailed analysis of the information that Tepco has recently released regarding the current conditions in the area under the Unit 1 Reactor Pressure Vessel.

Look for the October 17, 2012 installment of that running series of Fukushima Accident Updates.

Rod Adams

PS - Please do not disrespect my level of knowledge about nuclear energy by comparing it to that displayed by Jimmy Carter. He lasted just six months in the program, never existing the classroom phase. He was out of the Navy and farming peanuts in Georgia by October 1953; the USS Nautilus, the first nuclear powered submarine, did not get underway on nuclear power until January 17, 1955. That was 15 months AFTER Carter had already resigned his commission. In contrast, I served as the Engineer Officer of USS Von Steuben for about 40 months and 6 patrols. I freely admit that I have never managed any aspect of a large commercial nuclear power plant, but I have been in Maneuvering for more transients, startups and shutdowns than most commerical plants will see in 60 years of operation.

Richard Bud's picture
Richard Bud on Oct 18, 2012

"It has always seemed far fetched to me to think that material from a nuclear reactor that melted several hours after fission has stopped contains the power density necessary to melt through carbon steel pressure vessels that are 6-12 inches thick. "

Writing and publishing an article on the Internet based on ONE measurement conducted by TEPCO

and the above wishful thinking is unbelievable.

Anyone with a knowledge of scientific investigation would understand that there is NO way that one could reach the conclusion of "NO melt through at Fukushima".

I periodically search the Internet seeking to gather any and all information that I can about Fukushima. Frequently I come upon articles such as this, some with a political agenda, some with a misunderstanding about the physics of Nuclear Power and some just plain wrong. I think this article may fall into all of the above.

Unforteunatly, it will take a long time for the exact location of the Fufkushima cores to be located. The radiation from irradiated nuclear fuel is deadly for hudreads of thousand of years. TEPCO seems to think the cores can be removed but I don't believe so and I believe the final solution will interment such as Chernoble leaving some to wonder where the cores are and some to continue to pronounce that there was no core melt through at Fukushima.


Elidyl L's picture
Elidyl L on Oct 18, 2012

For a different view.

Unit 1, Where The Melted Fuel Is


Web site previously run by Rueters, and founded by Dean Wilkie (with 30 years experience in reactor operations and engineering at Department of Energy test reactor in Idaho).  Simply put, the numbers don't add up, and the contrasting data for Units 1 and 2 suggest we don't have clear answers, and further work is needed (and a great deal more time).  Conclusion is more consistent with TEPCO offical comments on the result: "it is difficult to identify where the source of the radiation is from the available data", Masayuki Ono, acting general manager of TEPCO's Nuclear Power and Plant Siting Division). 

Rod Adams's picture
Rod Adams on Oct 19, 2012


Thank you for the link to the Simplyinfo interpretation. It includes a rather fascinating leap of logic and guesswork that is not supported by the reported evidence.

The piece you pointed to makes the following scary claim:

"TEPCO found the radiation level in the bottom of the torus room under water to be 100,000 to 1,000,000 Sv/h * on the west side. "

Those are exceedingly high radiation readings. If true, they would convince me to eat crow and admit I was wrong and that a large portion of the reactor core has escaped from the reactor pressure vessel and is now in the torus room under water. So I followed the link ( to find that the linked article included a table in which the highest MEASURED radiation level was 10.3 Sv/hr with ASSUMED areas with doses that are many orders of magnitude higher because there was an ASSUMED radiation probe failure.

That assumption was apparently made because the radiation dose rates started dropping off after reaching a peak when someone (perhaps the people who operate "simplyinfo) expected the dose rate to keep going up.  

Here is another telling quote from the Simplyinfo article that describes the way the workers who were measuring the radiation levels in the torus room were dressed:

"The workers noted on the TEPCO documents had only rubber gloves.. it looks like they have shoes on under the coveralls (yellow coveralls)… no instrument held over the opening of the hole, no preparations for extremely contaminated cabling once they pull out… they should be in scott air packs for air… no sniffer hose down the hole which would go to a constant air monitor. Clearly a lack of radiological controls for protection of the workers." 

Perhaps the regular audience at Simplyinfo will buy the notion that radiation workers are stupid and careless with their own health. Since I have been working with trained nuclear workers for about 30 years, I have a much higher opinion of their level of knowledge and conservative decision making skills.

If the Tepco workers are dressed as described, it is because they know that the work they are doing does not require any additional protective measures. In other words, the highly trained radiation workers are pretty sure that they are not going to find a large quantity of radioactive material in the torus room.

The uncertainty expressed for media consumption by the acting general manager is also typical - many engineers and scientists in the nuclear industry have been inculcated with the notion that it is never okay to make bold statements that sound like you are not as conservative as possible.

Submariners are the only people who are both trained as nuclear workers and also trained that being correct is more important than being conservative. Most of the nuclear management level folks I know would never repeat a famous navy saying of "he who will not risk cannot win" because they do not want someone to think they are not appropriately "conservative".

Rod Adams's picture
Rod Adams on Oct 19, 2012


In your periodic searches for information about Fukushima, have you found and followed the three times per week updates at Hiroshima Syndrome?

I can assure you that my article was not based on a single reading and it is not a new idea - I have been skeptical about the containment breach scenario since day 1. If the reactors had been PWRs with no control rods in the bottom reactor pressure vessel head, I would have been even more bold and vocal about my certainty that the corium penetration in the specific case of a slow loss of cooling would not exceed an inch. (It was a measured 5/8 of an inch at TMI).

Since the geometry in a BWR is complicated by the existence of control rod drive penetrations, I have been a little less assertive, agreeing that there might have been some leakage at the penetrations, but I have been pretty sure even that was relatively minor. The radiation readings reported earlier this week simply confirm my long held hypothesis that is based on physical knowlege of the latent heat of fusion for the kind of steel used in the 6-8 inch thick reactor pressure vessels along with a reasonably accurate model of decay heat generation, the latent heat of vaporization of water, and the large volume of water that was in the core at the time that circulation was stopped.

Leslie Corrice's picture
Leslie Corrice on Oct 19, 2012

Hey, I started this, so I feel I should support Rod here. I'm the writer of the seminal "no-melt-through" posting in my Fukushima Updates ( I don't think it's Rod who's wrapped up in worst-case scenarios. Quite the seems to me that the Japanese nuclear community has a fixation on such assumptions. For more than a year, they have had the whole world thinking "melt-through", a worst case assumption if there ever was one. If they now say "Oops. We were wrong!" it would make it seem like a cover-up. They don't have the best PR reputation, you know. The point is, their first data from inside unit #1's PCV contradicts them. The numbers are all as backwards as it can get. Rod may not have commercial experience, but I spent 15 years working with a BWR (after 6 years nuke Navy). I was on the design team for it and saw it through the first 5 years of commercial operation. I have seen and appreciated the massivity of the technology. I've seen what the engineer sees...something the "scientist" rarely gets a chance to do. University and research reactors are VERY different from commercial ones. Regardless, the more I look into the issue and research responses to objections relative to my claim, the more I'm convinced Rod and I are correct.

Elidyl L's picture
Elidyl L on Oct 19, 2012

You can see how they are dressed here.


Workers were located on the First Floor of the Reactor building, and lowered a camera and dosimeter through piping to the Torus Room.  They do appear to be in regular work shoes, yellow coveralls, no air packs, and with no instruments held over piping.  A high reading of 10.3 Sv/h was recorded in Torus Room before dosimeter malfunction and very high radiation readings were reached.  


Unlike the west side of the reactor building and torus room, workers have still not been able to access or enter locations near East Torus Room of Unit #1 because radiation readings are too high.  

Elidyl L's picture
Elidyl L on Oct 19, 2012

Thanks Les.


Do you care to comment on the very different (by 9 orders of magnitude) radiation dose measurements taken at comparable locations in Units 1 and 2.  Unit 2 is larger with a larger containment, but have similar readings above the first floor grating (at 800 - 1000 mm from PCV wall).  


Unit 1 (Oct 10 at the first floor grating): 8.2 Sv/h (@1000 mm inside PCV).


Unit 2 (March 27 at the first floor grating): 72.9 Sv/h (@800 mm inside PCV).


Since you didn't compare Unit 1 dose measurements with anything else in your original post (such as comparable readings from the other Fukushima reactors), I would be interested in hearing your view.  I raised this issue on Rod's blog (under user EL), but only managed to generate more questions (and few specific answers on the difference in the measurements between the two reactors).   Do these results look consistent to you and others?  

Leslie Corrice's picture
Leslie Corrice on Oct 21, 2012

Unit 2 has a 65%  larger capacity, but the containment is essentially the same size as unit #1. Regardless, I think much of the unit #2 core did not melt and remains in the core barrel. The water level inside the RPV has not been posted for more than half a year, but the last postings show a level at or near the bottom of the core. The damaged fuel cells protruding above the water level are being cooled by the core spray inside the shroud. This means some of the damaged fuel cells are not under water and there is essentially no water shielding/attenuation of the gamma field being generated. Bigger core + less damage + less shielding = considerably higher rad readings inside the PCV. But this begs the question as to why the unit #1 under-the-PCV water is lower in chloride and radioactive Cesium than the water outside the PCV in the reactor building. Then there's the fact that the lowest rad levels are at the basement water surface and a much, much higher reading is at a height near the bottom of the vessel head. Both reasons continue to strongly suggest the unit #1 corium remains inside the unit #1's RPV bottom head.

Leslie Corrice's picture
Leslie Corrice on Oct 21, 2012

One more thing...the highest rad reading at the entry point of the detector in unit #1 is 11 sieverts/hr, and the unit #2 reading at roughly the same level is ~48 sieverts at the point of entry and 79 sieverts at the inner pedestal wall. The unit #1 detector could not be moved over to the pedestal wall, so the 79 sievert reading is not comparable to unit #1. Your innaccurate claim of a difference between 8 sieverts and 79 sieverts is not a nine order of magnitude change... it's one order of magnitude...almost. Nine orders of magnitude would be 8,000,000,000 sieverts.

Elidyl L's picture
Elidyl L on Oct 21, 2012

Quote: "Unit 2 has a 65% larger capacity"

The fuel inventory is only 37% larger in Unit 2 (548 assemblies v. 400 assemblies).  

Quote: "But this begs the question as to why the unit #1 under-the-PCV water is lower in chloride and radioactive Cesium than the water outside the PCV in the reactor building."

While lacking some detail, this issue was addressed by TEPCO in it's press conference on the new data: “TEPCO said radioactive materials may be flowing differently in the two reactors.”

Quote: "Then there's the fact that the lowest rad levels are at the basement water surface and a much, much higher reading is at a height near the bottom of the vessel head."

The "airborne" radiation dose results from the Torus Rooms of Units 1 and 2 are, indeed, very different: 

- Unit 1 (Torus Room Investigation on June 26, 2012): high of 10.3 Sv/h, via pipe penetration from above.

- Unit 2 (Torus Room Investigation on April 19, 2012): high of 110 mSv/h, via robot.

At the basement water level in Unit 1, doesn't the issue you raise about shielding apply here as well … with some 2.8 m of water filling PCV in Unit 1?

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