Japanese Engineer: "There Was a Nuclear Explosion in Reactor 3 in Addition to a Hydrogen Explosion"

There are foreign nuclear experts who have said the explosion in Reactor 3 on March 14 at Fukushima I Nuclear Power Plant was a nuclear explosion. But this Japanese engineer and whistleblower at JNES (Japan Nuclear Energy Safety Organization) Setsuo Fujiwara says there were two explosions at Reactor 3: a hydrogen explosion, and a nuclear explosion at the Spent Fuel Pool.

The following is my best-effort translation of the interview Fujiwara did with the SPA magazine, without detailed technical knowledge of nuclear physics, subject to revision.

From Zakzak (12/13/2011):

「福島第一原発３号機で３月１４日に起きた爆発はピカドン（核爆発）だ！！」

"The explosion in Reactor 3 at Fukushima I Nuke Plant on March 14 was nuclear!"

　そう語るのは、’１０年の春まで日本原子力安全基盤機構（ＪＮＥＳ）で原発検査員を務めていた藤原節男氏。原発の施設と運用について隅々まで知る専門家の一人だ。

So says Mr. Setsuo Fujiwara, who worked at Japan Nuclear Energy Safety Organization (JNES) until the spring of 2010 as nuclear plant inspector. He is one of those experts who know the nuclear power plant facilities and operations in great details.

　「３号機の爆発では、一度ピカっと炎が出た後、ドーンと黒煙がまっすぐ建屋上方へと立ち上っています。水素爆発であんな黒い煙は出ません。キノコ雲の形状といい、核爆発の現象に酷似している」

"In the Reactor 3 explosion, there was a flicker of fire, then a vertical, black smoke up the reactor building. A hydrogen explosion does not produce such a black smoke. And the mushroom cloud. It resembles a nuclear explosion."

　しかし、政府、東電の発表では、原子炉内部は安定を取り戻してきているはずだが？

But according to the government and TEPCO, the nuclear reactors are getting more stable, aren't they?

　「重要な放射能飛散原因は、使用済み燃料プールです」

"A more important source of radioactive materials dispersed is the Spent Fuel Pools."

　彼は一原発を陸側から写した航空写真を取り出した。

Fujiwara takes out an aerial photograph of the plant shot from the land side.

　「建屋上部フレームは、使用済み燃料プールの場所が吹っ飛んでいます。プール内で爆発が起こり、そこにあった燃料棒は飛び散ってしまったと思われます」

"The upper frames of the reactor building are blown off at the location of the Spent Fuel Pool. I believe there was an explosion inside the SFP, and the fuel rods inside were blown out."

　だが、たとえ使用済み燃料が溶融して下に溜まっても、果たしてそれで核爆発は起きるのだろうか。

If the spent fuel had melted and sank to the bottom of the pool, would that cause a nuclear explosion?

　「３号機の燃料プール内では、爆発が生じるまでに冷却水が少なくなり、ジルカロイ・水反応で水素が発生。上方の燃料被覆管が溶けて、中のペレットはブロック崩し状態。プール内が原子炉さながら、小出力で臨界状態となって水が沸騰したと思われます。そして、プール水面上方で水素爆発。その圧力で沸騰水中のボイド（水蒸気）が圧縮。ボイド反応度係数はマイナスなので、一気に核分裂の反応度が高まり、即発臨界の核爆発が起きた。３号機爆発のスローモーションビデオを観ると、爆発音が３回聞こえる。これが、水素爆発の後に核爆発が生じた証拠です」

"The amount of cooling water decreased in the Reactor 3 SFP prior to the explosion, and hydrogen was generated from the zircaloy-water reaction. The upper part of the cladding melted, and the pellets fell out and piled [at the bottom of the pool?]. Inside the SFP, it was like a nuclear reactor becoming critical, and the water boiled. Then there was a hydrogen explosion above the surface of the water in the SFP, and due to the pressure from the explosion, voids (steam bubbles) in the boiling water were compressed. The void coefficient was negative, so the reactivity of nuclear fission was suddenly heightened, resulting in a nuclear explosion from the prompt criticality. When you see the slow-motion video of Reactor 3's explosion, you hear three explosive sounds. It is the evidence that the nuclear explosion occurred after the hydrogen explosion."

　続いて彼が指差したのは、排気筒と３号機を結ぶ配管部分だ。太いパイプはそこで断裂し、短い管が口を空けて転がっている。

Next, he points to the pipe that connected the exhaust stack and Reactor 3. The big pipe is broken, and the short segment of the pipe is lying on the ground.

　「東電は、定期点検中の４号機で水素爆発が起きたのは、３号機で発生した水素がこの配管を通って、４号機建屋に入ったためだと説明しました。しかし写真を見ると、このとおり配管は繋がっていない。４号機でも使用済み燃料プール内で水素が発生して、爆発したと言える。３、４号機爆発とも、使用済み燃料プールの水素なら、１号機も使用済み燃料プールの水素による爆発ではないか。これら重要な事故シナリオについて、誰もダメ出しをしていない」

"TEPCO explained that the hydrogen gas generated in Reactor 3 passed through this pipe and entered the reactor building of Reactor 4, causing the hydrogen explosion in Reactor 4 which was in regular maintenance at that time. However, if you look at the photo, the pipe is broken. I think it was a hydrogen explosion in Reactor 4 also, caused by hydrogen generated inside the Spent Fuel Pool. If Reactors 3 and 4's hydrogen came from the Spent Fuel Pools, is it possible that the explosion of Reactor 1 was also caused by hydrogen from the Spent Fuel Pool? But no one is questioning [TEPCO] hard on these important points in reconstructing the accident."

　彼は、脱原発の技術者たちにもこれら事故シナリオ内容を投げかけたが、コメントを控えたという。「日本の技術者は、自分の専門領域以外のことにはなかなか発言しようとしない」と藤原氏は苛立つ。

Fujiwara says he tried to run his scenario of the accident with the engineers who are anti-nuclear, but that they withheld comments. "Japanese engineers are too reluctant to comment on things outside their specialties", says Fujiwara, irritated.

More on "void coefficient" here (wiki).

People like Arnie Gundersen talked about prompt criticality as a shockwave from a hydrogen explosion "moved and distorted" the fuel rods. I didn't quite understand the mechanism of distorted fuel rods resulting in prompt criticality. Fujiwara's explanation makes the mechanism much clearer for me, but it is also possible that I (non-expert) am imagining that I understand.

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