#Fukushima I Nuke Plant Reactor 1: TEPCO and NHK's Obfuscation on Corium in the Concrete

In the first post on the subject, I translated what NHK reported:

東京電力福島第一原子力発電所の事故で、メルトダウンが起きた１号機の燃料は、鋼鉄の原子炉の底を突き破って相当の量が格納容器に落下し、容器の底のコンクリートを溶かして最大で６５センチ浸食していると推定されることが、東京電力の解析結果から分かりました。

It has been discovered by TEPCO's analysis that the significant amount of Reactor 1's melted fuel pierced through the steel Reactor Pressure Vessel and dropped onto the Containment Vessel, then melted the concrete at the bottom of the CV. It is estimated that the melted fuel may have eaten into the concrete to maximum 65 centimeters deep.

Maximum 65 centimeters deep from the bottom of the concrete floor, right?

Well no. It's 65 centimeters from the bottom of the deep groove on the concrete floor.

And neither NHK nor TEPCO would bother to tell you how deep the groove is.

At least, NHK Kabun (NHK's last remaining conscience, as far as I'm concerned) tweeted and gave the link to its blog post, where NHK's analysis of the concrete-eating corium is shown with the screenshots from the program:

The text below the screen says: "In the worst case, it [the corium] may have reached 65 centimeters from the surface of the concrete. Where the concrete is the thinnest, it may have reached within 37 centimeters from the steel plate [of the Containment Vessel].

The "surface of the concrete" turns out to be the surface of the bottom of the groove that is X centimeter deep.

It's one thing for TEPCO's Matsumoto to say "surface of the concrete" (as we've gotten used to hearing technically correct explanations from him), but for NHK to say so while showing the graphics indicating it is the surface of the bottom of the groove is at once deceptive and covering its behind. NHK can now say "We showed it in the graphics that it was 65 centimeters from the bottom of the groove, and people should have paid attention."

(I'm asking NHK Kabun if they know the depth of the groove, but does anyone know or have access to a schematic diagram with measurement?)

Well, it does look like the Institute of Applied Energy is more right than TEPCO in saying the corium has eaten 2 meters into the concrete. And perhaps people like Hiroaki Koide of Kyoto University and the engineers who designed the reactors at Fukushima I Nuke Plant may be more right than the Institute of Applied Energy, and the corium has indeed escaped the Containment Vessel long time ago.

As the wiki entry on "corium" states, the corium can eat into the concrete 1 meters in the first one hour, and several centimeters per hour afterwards:

The fast erosion phase of the concrete basemat lasts for about an hour and progresses into about one meter depth, then slows to several centimeters per hour, and stops completely when the melt cools below the decomposition temperature of concrete (about 1100 °C). Complete melt-through can occur in several days even through several meters of concrete; the corium then penetrates several meters into the underlying soil, spreads around, cools and solidifies.[3] During the interaction between corium and concrete, very high temperatures can be achieved.

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Institute of Applied Energy: Corium Could Be 2 Meters Deep into Concrete

TEPCO's worst-case scenario (here and here) pales in comparison with the analysis by the Institute of Applied Energy, also presented on November 30 at the workshop held by the Nuclear and Industrial Safety Agency.

From what Yomiuri Shinbun reported (01:01AM JST 12/1/2011):

国の委託を受けて実施したエネルギー総研の解析では、１号機は燃料の８５％、２、３号機は７０％が格納容器に落下。炉心を取り囲むステンレス製の大型構造物「シュラウド」が損傷したり、格納容器の床のコンクリートも最大２メートル侵食したりしていると指摘した。そのため、コンクリートに支えられた圧力容器が傾いている可能性もあるとした。

The analysis done by the Institute of Applied Energy commissioned by the national government, 85% of fuel dropped to the Containment Vessel in Reactor 1, and 70% of fuel dropped to the Containment Vessels in Reactors 2 and 3. The researchers at the Institute pointed out the possibility of the damage to the stainless-steel shroud that surrounds the fuel core, and of the corium having eaten away the concrete floor of the Containment Vessel up to 2 meters deep. Because of that, they also said it was possible that the RPV got tilted.

Yomiuri doesn't specify which Reactor the Institute of Applied Energy was talking about, but my guess is Reactor 1.

Some nuclear experts have suggested that if the corium had escaped from the RPV it would spread out flat and evenly on the pedestal and be easily cooled by water. Well, even TEPCO admits that may not the case as far as the shape and the location of the corium is concerned (they do say the corium is cooled), and the Institute of Applied Energy says the corium could be 2 meters deep into the concrete.

My totally amateur 2 cents are that the concrete foundation may have cracked in the earthquake, and that it is possible that the crack or cracks are there in the pedestal. So, even if the corium wanted to spread out thin and flat, it would find those cracks and go there. Once the core-concrete reaction starts, it would be a positive feedback loop; the temperature gets higher not just from the contact with the corium but from the core-concrete reaction, the hole in the concrete would get bigger, and more corium would go into the bigger hole.

(From my other post on the topic, TEPCO's drawings indicating the thickness of the CV concrete)

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TEPCO Says Corium Would Stop at 70 Centimeters into CV Pedestal, So No Worry

(Update: Institute of Applied Energy says it could be 2 meters deep. See new post.)


Update to my previous post on the corium eating into the CV pedestal in Reactor 1 of Fuku I:

TEPCO says they are pretty sure that the corium would stop at 70 centimeters deep, and the core concrete reaction has stopped.

The company is pretty confident that the corium escaped from the RPVs of Reactors 2 and 3, if any, remains on the pedestals of the CVs and not eating into the concrete.

From their presentation on November 30, 2011, Reactor 1:

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"Now They Tell Us" Series: TEPCO Admits Reactor 1 Corium May Be 65 Centimeters into the Concrete Pedestal of Containment Vessel

(UPDATE: See TEPCO's drawing of Reactor 1 in my next post.)
(UPDATE 2: Government-commissioned research institute says "corium 2 meters into the concrete". See my post.)

There you go! It took TEPCO only 8 and a half months to say what many people have been saying at least for 8 months.

The corium has long escaped the Reactor Pressure Vessel as admitted by TEPCO and the government. There are experts (like Hiroaki Koide) who have suggested the corium may have already left the Containment Vessel of Reactor 1, and TEPCO itself said back in May that Reactor 1 CV may have a 7-centimeter hole. Now the company says the corium may have eaten into the concrete floor of the CV to about 65-centimeter deep.

For Reactors 2 and 3, TEPCO thinks (hopes, wishes...) that a good chunk of the corium dropped from the RPV onto the CV. No mention whether the corium there is eating into the concrete or not.

From NHK News (11/30/2011; quick translation, subject to revision):

溶融燃料 相当量が格納容器に（11月30日 19:00更新）

Significant amount of melted fuel in the Containment Vessel

東京電力福島第一原子力発電所の事故で、メルトダウンが起きた１号機の燃料は、鋼鉄の原子炉の底を突き破って相当の量が格納容器に落下し、容器の底のコンクリートを溶かして最大で６５センチ浸食していると推定されることが、東京電力の解析結果から分かりました。

It has been discovered by TEPCO's analysis that the significant amount of Reactor 1's melted fuel pierced through the steel Reactor Pressure Vessel and dropped onto the Containment Vessel, then melted the concrete at the bottom of the CV. It is estimated that the melted fuel may have eaten into the concrete to maximum 65 centimeters deep.

２号機と３号機についても一部の燃料は格納容器に落下していると推定しており、改めて事故の深刻さが浮き彫りになっています。

For Reactors 2 and 3, TEPCO also estimates that part of the fuel has dropped to the Containment Vessels, showing how severe the accident has been.

福島第一原発の１号機から３号機については、核燃料が溶け落ちるメルトダウンが起き、一部の溶けた燃料が原子炉から格納容器に落下したとみられていますが、事故から８か月以上がたっても、詳しい状況は分かっていません。

In Reactors 1 thorugh 3 of Fukushima I Nuclear Power Plant, core meltdowns have occurred, and it is considered that part of the melted fuel has dropped from the RPVs to the CVs. However, the details are not yet known even after more than 8 months since the accident started.

これについて東京電力や国内の複数の研究機関が、これまで得られた原子炉の温度や注水状況などから溶けた燃料の状態を異なる方法で解析し、３０日、国が開いた研究会で結果を発表しました。

Using different methods, TEPCO and various other research institutions have been analyzing the state of the melted fuel based on the reactor temperatures and the amount of water being poured into the reactors, and the results were announced on November 30 at a workshop held by the national government.

このうち東京電力の解析では、最も厳しい評価をした場合、１号機については、すべての燃料が溶け落ち、原子炉の底を突き破って相当の量が格納容器に落下したと推定しています。

TEPCO's result shows that, in the most severe case, all of the fuel would have melted, of which a significant portion pierced through the bottom of the Reactor Pressure Vessel and dropped onto the Containment Vessel.

格納容器の底にはコンクリートがあり、さらに鋼鉄の板で覆われています。

There is a concrete platform [pedestal] at the bottom of the Containment Vessel, which is then covered with steel plates.

燃料が格納容器の底に落ちると、高熱で反応してこのコンクリートを溶かして浸食するということで、最悪の場合、１号機で６５センチの深さまで達すると推定しています。

When the melted fuel drops to the bottom of the Containment Vessel, a core-concrete reaction takes place at a high temperature, melting the concrete. In the worst case, in Reactor 1, the melted fuel could reach 65 centimeters deep into the concrete.

最もコンクリートの薄いところでは、格納容器の鋼板まで３７センチしかないということで、改めて事故の深刻さが浮き彫りになっています。

At the thinnest part of the concrete, it is only 37 centimeters to the outer steel plate of the Containment Vessels. This is a very severe accident.

また、２号機と３号機についても、最悪の場合、それぞれ５７％と６３％の燃料が溶け落ちて、その一部が格納容器に落下したと推定しています。

TEPCO also estimates that in the worst cases for Reactors 2 and 3, 57% and 63% of the fuel have melted, respectively, and part of the fuel dropped onto the Containment Vessels.

東京電力によりますと、原子炉と格納容器の温度は、２１日現在で、いずれも１００度以下になっていて、溶けた燃料は水で冷却されており、コンクリートの浸食は止まっていると評価しています。

According to TEPCO, the temperatures of the RPVs and CVs as of November 21 are all below 100 degrees Celsius, the melted fuel is cooled by the water, and the erosion into the concrete has stopped.

研究会では、このほかの研究機関の解析結果も発表され、複数の結果を基に原子炉や燃料の状態について議論されました。
東京電力や国は、今回の解析結果をさらに詳しく分析し、今後の廃炉に向けて核燃料をどのように取り出すかなどについて検討することにしています。

In the workshop, the results from other research institutions were announced, and the experts discussed the conditions of the reactors and fuel based on those results. TEPCO and the national government plans to further analyze the result of the study, and determine how to remove the fuel for decommissioning the reactors.

原子力安全基盤機構、技術参与の阿部清治さんは、東京電力の解析結果について「間違っているとは思わないが、まだ第一歩だと受け止めている。解析結果は一つだけでは答えを導き出すことができないからだ。今後はいろいろな解析結果を積み重ねて、事故の実態を分析していく必要がある」と話しています。

Seiji Abe, technical advisor to the Japan Nuclear Energy Safety Organization (JNES) says about TEPCO's analysis, "I don't think it is wrong, but it is only the first step. You can't get answers from only one analysis. We will need to understand the situation from various analyses."

TEPCO's presentation is posted on their site, but only in Japanese:

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_07-j.pdf

Here's the list of documents that were presented in the workshop on November 30:

Status of the fuel core in Reactors 1 through 3:
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_09-j.pdf

Plant parameters after the accident (including data that hasn't been disclosed so far)
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_03-j.pdf

Modification of JAEA model
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_04-j.pdf

Various approaches taken to estimate the condition of the fuel
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_05-j.pdf

MAAP analysis and core-concrete reaction
http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111130_06-j.pdf

(You can still flip through the pages to see the charts and graphs.)

TEPCO's drawing that indicates the thickness of the CV concrete:

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