(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)
"We were talking past each other, and we didn't even know it."
--What did you think when an explosion happened in Reactor 1 in the afternoon of March 12, 2011?
"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."
"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."
--Around the time of the Reactor 1 explosion, the debate was ongoing on seawater injection [into the reactors].
"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."
"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."
"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."
--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."
"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."
--In that sense, it was lucky that the melted fuel dropped from the bottom of the reactor.
"You could say so."
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.