Faulty Thermometer Likely Cause of Fukushima Temperature Rise

Last week, we alerted our readers to reports out of Japan that the temperatures inside Unit #2 at Fukushima Daiichi were rising. At the time, we noted that some of the reports of the news were, "rather breathless." That judgment has been borne out, as we received the following welcome news from Japan overnight:

A faulty thermometer is likely to blame for rising temperatures inside a stricken nuclear reactor at the Fukushima-Daiichi plant, authorities said Monday, as Japan prepares to mark one year since a devastating earthquake and tsunami triggered a nuclear meltdown.

[...]

A nuclear expert agreed that a faulty temperature gauge inside the Unit 2 reactor is the most likely cause for the higher heat reading.

Tokyo mega-quake prediction Inside the Japan nuclear exclusion zone Japan considers restarting two reactors Japan exclusion zone's lone resident
Michael Friedlander, a former senior operator at U.S. nuclear power plants, told CNN that the prospect of another catastrophic explosion at the Fukushima-Daiichi is "virtually zero."

"If the reactor was going to become critical it would have become critical in March of last year, not now," he said.

We'd be remiss if we didn't not that the first person who raised the possibility of the faulty thermometer was none other than Will Davis at Atomic Power Review.

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.

Industry Presents New Strategy to Increase Safety, Address NRC’s Post-Fukushima Recommendations

The industry will present a strategy to the Nuclear Regulatory Commission today on how it plans to enhance safety at the nation’s 67 plant sites to better equip them for unexpected events. The strategy—known as the “diverse and flexible mitigation capability,” or FLEX—addresses many of the recommendations set forth by the NRC’s Fukushima task force and takes into account some of the early lessons from the Fukushima accident on the need to maintain key safety functions amid conditions where electricity may be lost, back-up equipment could be damaged, and several reactors may be involved.

NEI’s Adrian Heymer, executive director for Fukushima regulatory response, held a media briefing Wednesday to explain the FLEX approach:

FLEX is a set of portable equipment that is located in diverse locations around the plant. We think there needs to be more than one set of equipment at diverse locations that can be quickly deployed and connected to provide injection and power supplies for instrumentation. What you want to do is inject water so that you keep the reactor [and spent fuel pools] cool. At the same time you want to know what is going on in the reactor—so it’s instrumentation for monitoring, for which you need power supplies.

FLEX will include equipment such as additional pumps, generators, batteries and chargers that will be located in diverse locations—for instance, on the east and west sides of the plant site. The equipment will be commercial-grade, but with program controls—which are still being defined—so that the equipment will be tested with results being subject to NRC oversight.


The strategy is “flexible” in that it does not dictate that permanent equipment be installed, but rather that the plant sites prepare portable equipment that could be used for any catastrophic event. The New York Times’ Matthew Wald explains:

A clear problem at Fukushima, he [Heymer] said, was that the tsunami was bigger than what the plant was designed for. If the operators had taken an approach based on specific hazards, he said, “instead of having a meter high barrier, they might have had a 10-meter high barrier,” although the actual tsunami was 14 to 15 meters high. The institute’s approach would be to take some general precautions rather than depend on the commission’s regular approach of determining probability before deciding what steps are needed.

Thus, the FLEX approach allows the industry to more quickly address high-priority safety concerns ahead of NRC regulations, which Heymer said could take time to implement due to the administrative analyses and technical reviews that would be involved:

Eventually there would be, we think, a rulemaking that would go in parallel. But this is a way of installing and achieving additional mitigation contingency in a shorter period of time. So, you get the same benefit, but rather than going through the normal process we try to expedite it by just getting on and installing the equipment and having a rulemaking to go in parallel.

In a blog post yesterday, the NRC acknowledged the industry’s FLEX plan as a step in the right direction:

The NRC staff believes this approach is a reasonable starting point, although more work is needed on defining these strategies. We also must ensure the NRC can inspect how plants put the strategies in place and that we can hold plants accountable for keeping those strategies ready and available.

The bottom line is that we believe these combined developments may enhance the agency’s approach to implementing the recommendations.

The FLEX approach is just one part of a larger industry response to the events at Fukushima. Heymer said that the FLEX strategy would allow for at least three days of keeping the nuclear fuel cool, and that regional response centers are also being pursued as yet another line of defense against a catastrophic event. As the various levels of safety enhancements are added, the industry plans to train and test its plant workers regularly so that they are well-equipped for emergency situations.

The FLEX concept is based on how the industry responded to the events of 9/11, in which additional security precautions—such as portable generators, water pumps, hoses and batteries—were put in place to mitigate against “beyond design-basis events,” or unlikely events that are considered outside the scope of what a plant should be designed or regulated to withstand.

Please note, this story was also cross-posted at NEI's Safety First microsite.