APR: your source for nuclear news and analysis since April 16, 2010

Saturday, May 14, 2011

Fukushima Daiichi No. 1: Core melt / exit of pressure vessel speculated by TEPCO, NISA

There are a number of very serious reports on the various Japanese news media at this moment; we will report these and make commentary in a sober fashion.

NHK is carrying the report, from TEPCO apparently, that there is a hole in the bottom head of the pressure vessel at No. 1 plant. This appears to be in addition to the previously reported piping damage, although another report does indicate water leakage from piping connected to the primary plant occurring in the turbine building. JAIF's now-irregular status reports do now indicate probable damage and leakage in terms of the reactor pressure vessel and also the primary containment at the No. 1 reactor plant.

Link to the most recent JAIF status report.

Combining this estimation... the presence of a hole in the lower reactor pressure vessel head... with the previous information gleaned sporadically over time may in fact indicate a corium/concrete reaction has, or had, occurred. The source and routing of the large amount of hydrogen which destroyed the reactor building has not been adequately explained up until this point; partial corium exit from the reactor vessel into the dry well and the corium/concrete reaction that would result could help explain the violent destruction of the reactor building.

From ORNL "The Role of BWR Secondary Containments in Severe Accident Mitigation: Issue and Insights from Recent Analyses":

The reaction of the hot core debris with the primary containment concrete
would proceed in a very complex manner, releasing fission product vapors,
fission product-laden aerosols, additional hydrogen, carbon dioxide, carbon
monoxide, and water vapor. The exact mix of gases generated by the
core/concrete reaction is a function of the type of concrete employed, and the
amount of un-oxidized Zircaloy leaving the reactor vessel. It should be noted
that in an unmitigated BWR severe accident the entire Zircaloy inventory of
the reactor would eventually oxidize (either in the reactor vessel or on the
drywell floor), generating as much as 6000 lb (2722 kg) of hydrogen (plant
specific value).

I mentioned aerosolized fission product gases a long time ago on this blog.

Seven hundred pounds of hydrogen are thought to have burned inside the containment vessel at Three Mile Island No. 2 during that core damage accident, causing a pressure pulse of about 28 pounds. The effect of six thousand pounds, or even three quarters of that, might well explain the sheer devastation seen at the Fukushima Daiichi plant reactor buildings.

It would not be inconceivable that salt played a role in the failure, either; either by corrosion, or else by blockage of cooling water, or even of sheer mass inside the pressure vessel. These effects, or others due to salt, will remain only theories until the vessel is actually opened, which could take a decade.

All of the above hinges of course on the assumption that some or all of the core material exited the pressure vessel. As has been noted in a number of ORNL studies, and studies sponsored by others, it may be exceedingly difficult .. especially faced with the loss of indication attendant to a long-term station SBO casualty .. to determine whether or not the core actually has begun to fully melt down, to reach the bottom head, or to begin to exit the bottom head. (The early initiation of dry well flooding, which occurred very early in the accident timeline and which was then stopped, only to very recently be restarted, is a primary mitigation / control option to prevent a known melting core from exiting the pressure vessel as rapidly as it would without any direct water contact with the lower head.)

Some studies indicate that rad levels soon after the accident might help indicate vessel failure, as well as pressure vessel pressure. Very high rad levels were reported after the accidents, and we know when the pressure began to rise in the primary containments which in two cases shortly preceded massive hydrogen explosions which destroyed No. 1 and No. 3 reactor buildings. These events... the high rad levels, and the pressure pulses, may in fact have been indicative of the pressure vessel failure(s). It must be remembered, though, that the situation we are seeing at Fukushima Daiichi is one that has only ever been postulated, simulated, discussed, calculated and proceduralized to the highest point possible but is still, up until now perhaps, totally unseen in operating reactor plants and thus is still somewhat of a gray area.

TEPCO has reported over 3000 tons of highly contaminated water in No. 1 plant turbine building, with the amount increasing fairly rapidly. There is now absolutely no doubt in any quarters that TEPCO has lost containment on this plant.

The reports driving this post, at least in original detail, are lacking in many specifics which is why I've made this rather long post, given the facts, with informed speculation and commentary in order that we can all as either expert observers or as lay people begin to get our heads around the possibilities enumerated by the assumption of core melt and exit of the pressure vessel, at least in part.

11:10 PM Eastern Saturday 5/14


  1. Great summary, thank you. I apologise if this question is silly, but does that mean the worst in no 1 is already behind us or yet to come as a result? Or in risk speak, what is the potential impact of the situation you describe?

  2. Excellent round-up! If only the media and science "info" shows were so comprehensive and objective! The jittery Japanese public ought receive enlightening reports such as yours! Off-topic, prehaps as public acceptance assurance and safety reasons new reactors should be built underground to even better contain incidents as this.

  3. No more Fukushima updates? It appears you were quite correct - since June 6 TEPCO admitted that three cores have escaped containment vessel. I would be interested in an update on what you think is happening now, six months later. Are the coriums still reacting with the concrete?