Arsenic-76 Radioisotope from #Hamaoka Nuke Plant Reactor 5 Exhaust Duct

Hamaoka Nuclear Power Plant in Shizuoka Prefecture, where the sea water got mixed up in the Reactor 5's RPV (reactor pressure vessel) as the reactor was being shut down, had radioactive arsenic (As-76) released from the exhaust duct of the Reactor 5 ancillary building.

Checking wiki on arsenic isotopes:

Although arsenic (As) has multiple isotopes, only one of these isotopes, ⁷⁵As, is stable; as such, it is considered a monoisotopic element. Arsenic has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of ⁷⁵As, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope ⁷⁶As with a half-life of 1.0778 days and produce approximately 1.13 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several hours.^{[citation needed]} Such a weapon is not known to have ever been built, tested, or used. Standard atomic mass: 74.92160(2) u.

That's cheerful...

There are so much information of the "incidents" that have occurred at this particular plant I don't know where to start. For now, suffice to say that this was the plant that Godzilla attacked in the 1984 movie ("Godzilla", series No.16); Godzilla must have had a very good reason.

From Yomiuri Shinbun (9:14PM JST 5/18/2011):

中部電力は１８日、運転停止作業中に冷却水に海水が混入するトラブルが起きた浜岡原子力発電所（静岡県御前崎市）５号機に隣接した補助建屋の排気ダクトの出口で、ごく微量の放射性核種「ヒ素７６」を検出したことを明らかにした。

Chubu Electric announced on May 18 that a minute amount of arsenic-76, radionuclide, was detected at the exhaust duct of the ancillary building to the Reactor 5 reactor building at Hamaoka Nuclear Power Plant, where the sea water got mixed up in the reactor coolant while the reactor was being shut down.

　周辺環境への影響はないという。

The company says there is no ill effect on the environment.

　同社は、トラブルの影響で、海水中に存在する「ヒ素７５」が原子炉内で放射化したものとみている。

The company thinks arsenic-75 present in the seawater turned radioactive inside the reactor.

　また、同社は同日、トラブルの原因調査に向けた点検作業に着手した。混入が確認された「主復水器」内部の水抜きを行った後、混入経路や原因を調べるという。

The company started the investigation as to why the sea water entered the reactor. They will drain the water from the main condenser to find out where the sea water came from and why.

　同社によると、１４日午後、原子炉を冷やす「冷温停止」状態への移行作業中に海水約４００トンが混入するトラブルが発生。放射性物質の外部への漏えいはなかった。

On the afternoon of May 14, as the Reactor 5 was being shut down, about 400 tons of sea water entered the reactor [other news says it was 500 tons]. There was no release of radioactive materials in the environment from the incident.

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