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. Such a weapon is not known to have ever been built, tested, or used. Standard atomic mass: 74.92160(2) u.
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):
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.
The company says there is no ill effect on the environment.
The company thinks arsenic-75 present in the seawater turned radioactive inside the reactor.
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.
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.