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On the Temperature Increase at Fukushima Daiichi Unit #2

Over the past 24 hours we've seen a number of account concerning rising temperatures inside reactor #2 at Fukushima Daiichi. While we noted this item over at SafetyFirst.nei.org yesterday morning, some accounts of the news have been rather breathless.

If you'd like a sober account of what's actually happening there right now, I'd suggest reading the following account from World Nuclear News. Here's the relevant passage:
This stability of unit 2 was disturbed for a few days, however, when Tepco tried to improve cooling further by tuning the rates of water injection.

[...]

After making this change, Tepco noted a tendency for increasing temperature at the bottom of the reactor vessel. Within a matter of hours the company decided to reverse the change and restore the previous injection rates, but the temperature continued to slowly rise.

Two of the three temperature sensors at the bottom of the reactor vessel edged up by about 2 degrees C. The third, however, rose by around 20 degrees C to hit 72.2 degrees at 5.00am today. Tepco acted to stem this increase by injecting an extra cubic metre of water per hour through the feedwater line, and this stabilised the sensor at about 70 degrees C. It has since decreased to 68.5 degrees C, while the other two sensors were at a new low of around 41 degrees C.

Tepco was able to discount recriticality as a potential cause of the temperature rise after conducting an analysis of charcoal filters in the containment gas control system. These showed very low traces of fission products that were below the threshold that would indicate criticality. Nevertheless Tepco this morning injected boric acid into the reactor vessel as a precaution and increased the core spray injection rate by three cubic metres per hour.
The bottom line here: at no time did the temperature of the bottom of the reactor vessel exceed, or even approach, 100 degrees C, which was one of the important conditions TEPCO has to maintain for cold shutdown conditions.

We're keeping an eye on this and will report back if events warrant.

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