Nikkei Shinbun's Interview of Haruki Madarame (4/7): The Worst Case Would Be "High-Pressure Melt Through", Politicians Thought Recriticality Was Nuclear Explosion

(Continued from Part 1, Part 2 and Part 3, from Nikkei Shinbun: Testimony of Dr. Madarame, in the third year of the accident: "Worst case scenario was possible for Fukushima" by Junichi Taki, editorial board member)

■「食い違いがあることに気づかず議論をしていた」

"We were talking past each other, and we didn't even know it."


――12日午後に１号機で爆発が起きたとき、どう思ったのか。

--What did you think when an explosion happened in Reactor 1 in the afternoon of March 12, 2011?

「映像を見た瞬間に水素爆発だと思った。その時の記憶はあいまいだが、下村健一・内閣審議官（当時）の書いたものよると、私は『建屋に水素が漏れて、建屋には水素があるので爆発した』と淡々と説明したとされている。多分、事実だろう」

"I thought it was a hydrogen explosion the moment I saw the image. I don't remember clearly, but according to what Kenichi Shimomura, who was a cabinet counselor at that time, wrote, I calmly explained that "hydrogen leaked into the building, and since there is hydrogen (sic) [probably "oxygen"] in the building an explosion happened." I think it is probably true."

「この爆発を機に、首相は私の言うことを信用しなくなった。『安全委員会にはほかに専門家はいないのか』と問われたので、『久木田豊委員長代理（当時）も詳しい』と答えると、『すぐに呼べ』と言われたので、久木田さんに来てもらい、私はオフィスに戻った」

"From this explosion on, Prime Minister stopped believing me. He asked me if there were other experts at the Nuclear Safety Commission. I answered that Acting Chairman Yutaka Kukita was also knowledgeable. He told me to "call him here immediately". So I had Mr. Kukita come, and I returned to my office."


――１号機の爆発のあった時間帯は、海水注入の議論をしていたころでもある。

--Around the time of the Reactor 1 explosion, the debate was ongoing on seawater injection [into the reactors].

「（海水注入の議論の中で）『再臨界の可能性はあるか』と首相から問われたら、『可能性はある』と答えたとしてもおかしくない。私には尋ねられた記憶がない」

"In discussing the seawater injection, it would not be odd if I had been asked by Prime Minister whether there was a possibility of recriticality and I had answered there was a possibility. I have no memory of being asked."

「実は水素爆発の前の時点から、海江田万里・経産相（当時）が議長になって海水注入の問題点を総理応接室（官邸５階）で話し合っていた。塩が析出し腐食も問題になるので長期間は無理だが、いまは炉心を冷やすことを何より優先し海水を入れろと私は主張していた。首相が海水注入を止めるよう言うはずはないと思う。海水注入中断の問題は、国会事故調査委員会などが指摘するように東電の武黒一郎フェローの勝手な判断が介在していたように思う。いずれにしても、吉田昌郎所長（当時）の判断で注入の中断はなかった」

"Even before the hydrogen explosion, Minister of Economy Banri Kaieda was chairing a meeting in the PM Reception Room (on the 5th floor of the PM Official Residence), and we were talking about the potential problems of seawater injection. I insisted that seawater be injected as cooling the reactor core was the first priority, even though it was not a long-term solution because of salt deposition and corrosion problem. I don't think Prime Minister would order a halt in seawater injection. The issue of halting the seawater injection arose, as pointed out in the National Diet accident investigation commission, from an arbitrary decision by TEPCO's Fellow Ichiro Takekuro. In any way, there was no halt thanks to Plant Manager Masao Yoshida."

「後に福山哲郎・官房副長官（当時）はじめ、政治家の人たちの著書を読んで気がついたのだが、みなさん再臨界イコール核爆発だと思っていたらしい。再臨界が仮に起きても核爆発とは違うことは、ＪＣＯ事故などからも明らかだ。食い違いがあることに気づかず議論をしていた」

"I realized later when I read books by politicians including Mr. Tetsuro Fukuyama (then-Deputy Chief Cabinet Secretary) that everyone had thought recriticality meant nuclear explosion. It is obvious from accidents like the JCO accident that even if criticality happens that's different from nuclear explosion. But we were talking past each other, and we didn't even know it."

――その日は深夜に自宅に戻るが、すぐにまた官邸に呼ばれる。

--You went home late that night, but was called back soon to the PM Official Residence.

「ほとんど寝ていない。ただ13日になると、いろいろな専門家から見解を聞く余裕が出てきた。とくに久木田さんとの意見交換は貴重で、その時点で最も怖いのは高圧溶融物放出（ＨＭＴ=High-pressure Melt Through）という現象だと意見が一致していた。これは溶融燃料によって圧力容器の壁が溶けて薄くなった末、圧力容器内と格納容器の圧力差によって燃料が容器を突き破って外に飛び出す現象だ。格納容器の壁まで貫通してしまう恐れがある」

"I hardly slept. But on March 13, I began to have more time [or "peace of mind"] to listen to other experts. Discussion with Dr. Kukita was particularly valuable, and we both agreed that the most frightening possibility at that time was the phenomenon called "High-pressure Melt Through" (HMT). HMT happens when the melted fuel melts the Pressure Vessel wall thin, and the melted fuel gets ejected through the Pressure Vessel due to the difference in pressure between the Pressure Vessel and the Containment Vessel. It is possible that the melted fuel could pierce through the wall of the Containment Vessel."

「14日の３号機の水素爆発の後、２号機の逃がし安全弁を急いで開くように助言したのは、２号機でＨＭＴが起きるのを心配して、圧力容器と格納容器の圧力を均一化した方がよいと考えたからだ。吉田所長はまずベントの準備を整えてからと主張していた。安全弁を開くと圧力容器内の水が水蒸気となって格納容器に流れ出し、燃料が空だきになる恐れがあるので、注水の備えがないと安全弁を開けない。難しい判断だ」

"After the hydrogen explosion of Reactor 3 on March 14, I advised that the relief safety valve of Reactor 2 should be opened quickly. I was worried that HMT might happen in Reactor 2, so I thought it would be prudent to equalize the pressure in the Pressure Vessel and the Containment Vessel. Plant Manager Yoshida insisted that preparation for the vent be done first. If the relief safety valve is opened, the water inside the Pressure Vessel becomes water vapor and flows into the Containment Vessel, leaving the fuel heated without water. So you cannot open the relief safety valve without preparation to inject water. It is a difficult decision."

――そう考えると、原子炉の底部から溶融燃料が落ちたのは不幸中の幸いと言えるか。

--In that sense, it was lucky that the melted fuel dropped from the bottom of the reactor.

「そうとも言える」

"You could say so."

班目氏の行動（３月13日）
3:40ころ 自宅で原子力安全委事務局からの電話
5:00ころ 官邸へ（官邸到着前に安全委オフィスで他の安全委員らと意見交換）
この間、３号機の高圧注水系停止などの事態が進む
10:04 原子力災害対策本部の会議
13:55 安全委オフィスに戻り、官邸の状況を説明
14:35 官邸へ
この間、保安院の安井氏らも加わって、３号機の水素爆発の可能性を議論
15:30 官房長官記者会見に同席（これ以降、数回）
21:35 原子力災害対策本部の会議
久木田委員長代理と最悪のシナリオを議論。政治家にメルトスルー後のコンクリート反応を説明

Dr. Madarame on March 13, 2011:
3:40AM received call at his home from the secretariat of the Nuclear Safety Commission
5:00AM went to the PM Official Residence (after speaking with other commissioners at the Nuclear Safety Commission office)
The situation at the plant grew more serious as Reactor 3's high-pressure core injection system stopped.
10:04AM meeting of the Nuclear Disaster Response Headquarters
1:55PM went back to the Nuclear Safety Commission office, and explained the situation at the PM Official Residence
2:35PM went back to the PM Official Residence
Discussed the possibility of a hydrogen explosion in Reactor 3 with others including Mr. Yasui from NISA
3:30PM accompanied Chief Cabinet Secretary [Edano] in the press conference (would do a few more times later)
9:35PM meeting of the Nuclear Disaster Response Headquarters
Discussed the worst case scenario [high-pressure melt through] with Deputy Chairman Kukita of Nuclear Safety Commission. Explained the core-concrete reaction after the melt through to the politicians

It is interesting to note that, according to Dr. Madarame's March 13, 2011 schedule, he was debating with another nuclear expert (Kukita) about the worst case of "high-pressure melt through", in which the melted core (corium) will eject from the Pressure Vessel at a high speed, possibly piercing through the Containment Vessel, while telling the politicians a more "benign" core-concrete reaction scenario.

If HMT had happened in Reactor 2, as Dr. Madarame and Dr. Kukita feared, it may have been an immediate evacuation from the plant. The worst case that PM Kan says he received (but decided to sit on it for months and deny the existence of the report) may have come true then.

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