Gov’t Experts: Fukushima fuel rods melting 75 minutes after tsunami; Reactor core not covered with water 10 minutes after power blackout — Worker: Before explosion, Unit 1 “filled with what looked like steam or dust… a series of large bangs… noise never heard before”Kyodo News, Aug 14, 2014: Less than 12 hours after the quake-triggered tsunami [TEPCO] sought government approval for the unprecedented step of releasing radioactive steam [...] Prime Minister Naoto Kan, however, would soon become increasingly distrustful of TEPCO because of delays in starting the so-called venting operations, which were aimed at preventing damage to the reactor containment vessels [...] Kan arrived at the plant shortly after 7 a.m. [on March 12.] But he had no idea at the time that fuel was already melting in the No. 1 reactor. [Plant chief Masao Yoshida said] workers would quickly need to manually operate the valves [...] [Shift supervisor Hideyoshi Endo] hung a radiation meter from his neck capable of measuring up to 1,000 millisieverts per hour [...] Endo’s team was tasked [...] to open the valve [...] As he lit up the inside of the building with a flashlight, he saw it was filled with what looked like steam or dust. [They went] to a room housing a doughnut-shaped suppression chamber at the bottom of the reactor’s containment vessel. They heard a series of large bangs in the dark — a type of noise Endo had never heard before. [...] When Endo was some 30 meters away from the valve, however, he saw the radiation meter reading surpass 1,000 millisieverts [...] Endo no longer had a way of knowing how high the radiation level was. Endo only needed a few seconds to decide to return. [...] When the two returned to the main control room, all eyes turned to them. “It failed. The radiation meter scaled out,” reported Endo.
IRSN (France), International Workshop (pdf): Major events and their consequences — The core melt of Unit 1 started at 17h00 on March 11* [...] Most of the meteorological stations of the AMeDAS system and the SPEEDI, dose rate, measurement stations were out of use [...] The only station which was able to detect the radioactive plume from Unit 1, was the one located in Minami Soma approximately 25 kilometres north [...]
*M9.0 quake struck at 14h46; 14-meter tsunami inundated Fukushima Daiichi at 15h46 -Source
NKS Fukushima Seminar 2013 by Wiktor Frid, research leader in reactor technology at Sweden’s Royal Institute of Technology (pdf):
- The first release may come relatively early (the times stated below apply to Station Blackout scenario for Swedish reactors and are approximate)
- Core uncovery – 10 minutes
- Core melt begins – 40 minutes
- Reactor vessel melt-through – 4 to 5 hours [...]
- Early, unfiltered large release cannot be excluded, for example in connection with vessel melt-through [...]
- Severe accident early containment threats — Containment failure in the early phase of an accident, e.g. in connection with reactor vessel melt-through, could result in large radioactive releases to the environment.