Japan:All N-fuel may have fallen to outer vessel/TEPCO: Up to 68 tons likely melted...

First of all, when reading a document that covers all reactor and containment types you have to keep in mind that the Fukushima Units 1 to 4 use a Mark I containment. Unit 1 is different in the context of emergency core cooling systems from Units 2, 3, and 4; but for purposes of describing the core damage sequence starting with core re-location, the difference is unimportant. Section 4.7.1 of NUREG/CR-6042, Rev 2 provides an overview of a Mark I containment in the context of U.S. BWR plants. The reactor building of the U.S. and Japanese plants of this vintage are the same — driven by the license obtained by GE for that era’s BWRs.

On page 3.7-6 (first column) explains the term “downcomer” inside the reactor pressure vessel. It can be partially visualized by looking at Figure 3.7-18 (page 3.7-42). Page 4.1-2 (second column) explains the “downcomer” in the torus (suppression pool chamber). Also see Figure 4.1-7. Your confusing post earlier is based on Page 4.2-3 in the first column that describes an issue with a Mark II containment that is substantially different from the Mark I. See a Mark II containment illustration given by Figure 4.1-8 (page 4.1-18).

The core damage sequence starting with core re-loaction is described in Section 4.7.3 as I summarized in my previous post. An overview of the reactor vessel and Mark I containment is provided by Figure 4.1-7. The lower head region of a BWR reactor pressure vessel detail is illustrated on page 3.7-42. Details on the likely point of failure is shown on page 3.7-46. Section 4.7.3 describes the drywell sump immediately under the reactor vessel as I have described to you earlier. The Mark I “liner failure” mentioned in the NUREG you refer to is a hole through the “upside-down-light-bulb” steel structure in numerous illustrations of a Mark I containment.

Core concrete interaction in Section 4.4 is what was discussed at the NEI blog post linked in my original post for FR 2814972.

I hope this gives you an orientation into NUREG/CR-6042, Rev 2 that you need to understand on-going events at Fukushima. I expect the leftists at Daily Kos and Puffington Host to alarm readers with false information and hysteria. I do my best on FR to tamp down folks like you who take things out of context. Right-wing readers of FR ought to be the number one supporters of commercial nuclear power in its various forms. Criticism of TEPCo and other operating companies is out-of-context when you consider the millions of hours of successful operation of reactor plants around the world during the last ~60 years. It is a huge leap to think that being able to measure radiation, even at some distance, implies that there is an immediate health risk to the general public. As you probably know, Mother Nature bathes us in radiation constantly. The dose fraction contributed by man-made sources pales in the general population. Long-term exposure to fractional amounts of radiation has never been demonstrated to be a direct cause of disease in the general population. There is simply no way to control for all of the human variations of genetics, diet, and environment to attribute a particular incidence of disease to any single radiation-related cause. There is a substantial difference between hypothesis and demonstrated theory.

Addressing your technical response. Thanks for taking the time.

On page 3.7-6 (first column) explains the term “downcomer” inside the reactor pressure vessel.

You mean this blurb - It should be noted that some water is trapped in the downcomer region surrounding the jet pumps. This occurs because the initial temperature of the water in the jet pump region is less then the temperature of the water in the core region. Hence, a lower portion of the water in the downcomer region is flashed during the rapid vessel depressurization.

How does that explain the term downcomer in the RPV ? And why are you even talking about the RPV ? In unit #1, the core has left the RPV, completely. In unit #2 and #3 at least 1/2 of the cores have left the RPV. In total, 2/3 of the cores in these three reactors have left the RPV's. That is my only concern right now and what my original one line post was referring too. Corium on the concrete dry well floor. The RPV's are pretty much toast now.

It can be partially visualized by looking at Figure 3.7-18 (page 3.7-42).

A drawing of the bottom of the RPV again. Hate to break this to you, but the RPV's are fairly irrelevant now. Elvis has left the RPV's. And the RPV has nothing to do with my initial point. Here is my initial point that set you off.

All that don't matter one bit if some of the corium flowed down the drywell sumps and out into the downcomers. And Tepco sure wont tell you if that actually occurred.

The drywell sumps are not located anywhere near the RPV. They are in the dry well floor located way below the bottom of the RPV's. Obviously I was referring to the corium flowing into the drywell sumps.

Page 4.1-2 (second column) explains the “downcomer” in the torus (suppression pool chamber). Also see Figure 4.1-7.

Now we are talking about what I was referring too. Here is the relevant text. If a LOCA occurs, steam flows from the drywell through a set of vent lines and downcomers into the suppression pool, where the steam is condensed. LOCA stands for Loss of Cooling Accident, which is exactly what happened to these reactors.

Your confusing post earlier is based on Page 4.2-3 in the first column that describes an issue with a Mark II containment that is substantially different from the Mark I. See a Mark II containment illustration given by Figure 4.1-8 (page 4.1-18).

Now we get to the meat of the entire argument. Here is the text you are referring too. Two paragraphs. One about Mark I and one about Mark II.

A phenomenon of importance primarily for Mark I BWR's is shell (liner) meltthough. At vessel breach, the molten material may flow out of the pedestal region, across the drywell floor and then directly contact the steel liner, causing failure. The likelihood of this event and potential means for its mitigation are discussed in more detail in Section 4.7.

A phenomenon of importance for Mark II BWR's is downcomer failure. While Mark II designs vary significantly, there is often the potential for molten material to flow across the floor and into the downcomers. This molten material may directly fail the downcomer or, possibly, lead to a steam explosion that fails the downcomer. Downcomer failure does not lead to immediate containment failure, however the suppression pool is bypassed, thus negating its heat removal and fission product scrubbing capabilities.

If you take just these paragraphs, you are correct about the Mark II. However, note that the first paragraph states, discussed in more detail in section 4.7. Apparently the first Mark I design required an entire section to describe what happens in a liner fail by melt attack. So if we go to section 4.7 (BWR Mark I Liner Failure by Melt Attack). The melt through can attack the containment shell and is more appropriately called a shell failure not a liner failure. So your point is more or less immaterial, since once the shell has failed, containment is lost and that corium will go wherever it wants, including the wet wells via vent lines and downcomers. This paragraph below from section 4.7.6.1 Extension to other BWR Facilities also illustrates how shell failure can occur.

One of the most important geometric parameters with respect to the shell failure issue is the height of the vent line entrance above the drywell floor. This height determines the maximum depth of water over the floor and, should debris enter a vent pipe, local failure will be virtually certain. As discussed in Part 1 of Reference 3, the location of the vent line openings is plant specific, but in general the shorter heights are associated with the facilities that have the smaller cores and hence the smaller potential debris flows.

So in a nut shell, we are arguing about calling vent pipes, downcomers. My post was assuming they are essentially one and the same for this circumstance. Now every reactor is going to have a different specific configuration of sump volume and vent pipe height. If the corium melt overflows the sump, it will attack the containment shell and could rise high enough to flow down the vent pipes and downcomers into the suppression pool. So you are wrong.

And I just did some quick reading in section 5.1 and found this in section 5.1.4.5 Reactor Cavity Flooding.

In the Peach Bottom (Mark I) design, there is a maximum water depth of approximately 2 feet on the pedestal and drywell floor before water would overflow into the suppression chamber via the downcomer.

So your terminology problem is with the NRC, not me.

I see this was a complete waste of time for you. Hopefully other FR readers will benefit. You used the term downcomer incorrectly and imprecisely in your post -- you even admit as much. I have worked on severe accident studies for all reactor types during the years since 1991. In addition, I worked at one of the American BWRs with a Mark I containment during construction so I was able to walk the entire reactor building and inside the RPV itself unimpeded for months. Skimming one NUREG makes you some sort of armchair nuclear quarterback rather than a nuclear engineer with a specialty in risk assessment.

The point of the original post I shared from NEI is that the corium is contained within the concrete structure of the reactor building. Corium cannot "go wherever it wants" -- you are absolutely wrong. And like Chernobyl, the world is offered more empirical evidence via Fukushima that there is no credible means to create the hypothetical "China Syndrome".

You noted that "If a LOCA occurs, steam flows from the drywell through a set of vent lines and downcomers into the suppression pool, where the steam is condensed." That LOCA description is appropriate when there is a break in the pipe connected to the RPV. It is wrong to equate the Fukushima sequence to the design-basis-LOCA you cited. In the Fukushima sequence the long-term loss of AC-power led to a lack of water for injection and the water in the core barrel boiled away. The RPV and connected lines were intact. The steam lines are connected high on the RPV. Between the top-of-active-fuel there are large steel structures (steam separators, steam dryers). Corium forms because there is no water (LOCA) and the fuel rod cladding melts (like a candle burning down) letting the ceramic UO2 pellets (with a much higher melting temperature point) fall to the core support plate. In the Fukushima sequence, first water is driven out of the core barrel by boiling it away without replacement (LOCA). ADVs and SRVs dump the steam to the suppression pool -- a fairly routine occurrence. Then the corium forms. Corium will not flow up to the steam lines. Furthermore, the torus (steam suppression chamber) is considered part of containment in a Mark I (sometimes referred to as the wet-well). Thus your understanding of core melt progression, RPV design, and containment design (liner et al) are all seriously flawed.

Only pathetic anti-nukes who cannot tell reality from a Jane Fonda propaganda movie would believe "... containment is lost and that corium will go wherever it wants, including the wet wells via vent lines and downcomers." Even if 2 meters (about 6.5 feet) of that structure has been eroded, another 8.2 meters (almost 27 feet) of reinforced steel and concrete lies between the melted fuel and the external environment. It is sad to see that even FR has a virulent group of anti-nukes.

If I used the term wrong, then so did the NRC. See above post. The mistake you made was referring to the downcomer in the RPV. The RPV is immaterial to the corium melt on the drywell floor. At least you could have admitted your mistake, instead of mumbling on about the RPV in your previous post. All I did was the same thing the NRC did.

And once the corium breaks out of containment, it will go where ever it wants. Why else call it containment ?

I never once typed China Syndrome in this thread. Only you did.

Will check the rest of rant tomorrow. Will let you know your additional mistakes then. And note, only you went political. There is nothing political about making thousands of acres of land off limits to humans.

Hopefully other FR readers will benefit.

Well that should raise a bunch of red flags around here. If you are just worried about what other freepers think, it means your function here is to sway opinion. You are not worried about discovering what is really going on in Fukushima, where the Japanese themselves admit they cannot precisely verify the location of the corium.

You used the term downcomer incorrectly and imprecisely in your post -- you even admit as much.

Not really. Just used it the same way the NRC did. So your problem is with them. I do however admit that it was like throwing a bone to a dog. You did not have to bite.

Skimming one NUREG makes you some sort of armchair nuclear quarterback rather than a nuclear engineer with a specialty in risk assessment.

First of all, I applaud your work. It is obviously needed and lets all hope we never have another TMI in America. Now kindly point out where the heck I ever claimed to be a nuclear quarterback ? All I did was post one sentence to you. And that set you off. Your initial reply made no sense. I was thinking, why the heck is he even mentioning the RPV ? So I assumed you knew very little. That is why I asked you to go to the manual.

The point of the original post I shared from NEI is that the corium is contained within the concrete structure of the reactor building. Corium cannot "go wherever it wants" -- you are absolutely wrong.

That corium is not just traveling vertically. It is also traveling horizontally. And at drywell floor level it is right up against the steel containment wall with very little between the back side of the wall and the wet wells down below. You do realize how many vent lines and downcomers there are, right ?

You noted that "If a LOCA occurs, steam flows from the drywell through a set of vent lines and downcomers into the suppression pool, where the steam is condensed." That LOCA description is appropriate when there is a break in the pipe connected to the RPV.

That was a reference that you pointed out. I just pulled a sentence from the paragraph in the NRC manual that you pointed too. Those are not my words. There are the NRC's.

..Corium will not flow up to the steam lines. Furthermore, the torus (steam suppression chamber) is considered part of containment in a Mark I (sometimes referred to as the wet-well).

It is the splash affect when the corium comes pouring out of the concrete pedestal base and through the open single doorway in the pedestal base. That corium crashes into the wall opposite the doorway and rides up high with the splash. When you are talking tons of corium coming through a doorway, 2 foot splash is not out of the question when it crashes into the containment wall.

Thus your understanding of core melt progression, RPV design, and containment design (liner et al) are all seriously flawed.

It is all from the NRC manual.

Even if 2 meters (about 6.5 feet) of that structure has been eroded, another 8.2 meters (almost 27 feet) of reinforced steel and concrete lies between the melted fuel and the external environment.

The vertical profile of that concrete core catcher like assembly is not the only way it can leave. It can also travel horizontally and along cracks. And the depth on the concrete in the wet well, especially the horizontal depth, is much smaller.

I am done with this thread. Have fun. You can have the last word. At this point could care less. Your reaction to the bone I tossed, tells me a lot. Will continue trying to learn the truth, because I know you never get 100 % truth from government. And just in case you think the concrete has not been cracked.

you need to think about a new line of work

Everyone in the nuclear power industry needs to start thinking about that. Japan meltdowns + American public school system + Muslim immigrants + EEO laws = end of new nuclear power in America. Using Japanese workers is as good as it gets and even they couldn't do nuclear safely.

Right-wing readers of FR ought to be the number one supporters of commercial nuclear power in its various forms.

Because nuclear power means mostly high pay jobs for smart conservative engineer men? Conservatives are typically risk adverse, more so when the cost of failure is extremely high. Progressives are willing to experiment on a grand scale and don't mind if they destroy civilization in the process. The nuclear power industry cannot stand on its own in the free market, requires special liability limitation laws, tax breaks, and government funding. That's not conservative. Nuclear power has a lot in common with wind power, except the cost of failure can be astronomical. Despite the risk calculation promises there's a major accident about once every 10 years.