The Level of the Fukushima-Contamination in Tokyo is decreasing!

In Daiichi we saw a more or less controlled Melt-Down
and there was not a huge Release of the heavy and deadly
Isotopes, even in the Hot Zone we cant find "much" of
Plutonium, Strontium or other ones!

There seems to be some confusion as to how the containment cap bolts could have stretched allowing the containment cap to lift and allow gases to escape.
The primary containment vessel is supposed to vent high pressure into the wetwell[torus]. The torus operates at a negative pressure to the vessel and with a checkvalve. They are investigating whether the system failed or if venting was blocked.

Here are a couple of earlier reports concerning this issue...

from mar 2011.....

[link to www.nytimes.com]

[snip]

David Lochbaum, chief nuclear safety official of the Union of Concerned Scientists, has another possible explanation. As steam and hydrogen built up within the primary containment shell, high pressure may have forced an opening between the primary containment shell, called the drywell, and the metal cap that is bolted onto the shell. A pressure test decades ago at the Brunswick Nuclear Plant in North Carolina demonstrated that such a high-pressure leak could occur in Mark I reactors built at the Fukushima plant, Lochbaum said.

"This tragedy will be closely examined for its causes, what happened and why," Lochbaum said. "That scrutiny must determine how hydrogen got into the reactor buildings to cause the catastrophic explosions. The drywell head pathway may be that answer. We need to stress that we're not putting this forward as the only answer for this question, but it's the most plausible explanation that we've heard to date."

and


from sept 2011.....


[link to www.simplyinfo.org]
[snip]

Mark 1 containments were not specifically designed to handle these situations:
Reactor Vessel Rupture
Steam Generator Rupture
Large break (LOCA) Loss Of Coolant Accidents

These kinds of accidents produce higher containment pressure and more fission products. High heat caused by these kinds of accidents can also lead to failing containment by way of cracks or ruptures.

Statistics for Peach Bottom are likely similar to units at Fukushima as they have similar designs.
Peach Bottom BWR Mark 1 Peak Design Basis Accident Pressure PSIG 49.1 (KPA 339)
Peach bottom design: 56psig, ultimate failure pressure 148 psig

Containment Cap:

The yellow containment cap bolt style used at Peach Bottom nuclear plant is the style used at Fukushima and is prone to bolt elongation (stretching) and flange separation (opening) under high temperatures. Accidents where high containment (drywell) temperatures occur are more likely to have the containment cap bolts stretch.

The silicon seal of the containment cap can also fail, it degrades significantly at temperatures over about 600 K (620 ‘F). These two flaws can create an outlet for containment in situations where the pressure and temperatures are both high.

**This is certainly a potential scenario as part of the unit 3 explosion. Considerable steam and debris was ejected high in the air but the containment cap or parts of it have not been found in the debris.





This is not a new revelation...

waterbug

so calling what is going as "controlled Meltdown" is a PR stunt.