July 17, 2011

Carnival of Nuclear Energy 61

The 61st Carnival of Nuclear Energy is up at ANS Nuclear Cafe

Neutron Economy covers an IAEA review of Fukushima. The notes were prepared by Steve Skutnik about a recently attended talk by Randy Beatty of the IAEA, who gave the Agency’s perspective on the Fukushima disaster.

-The 50-mile evacuation order by Jaczko was wrong. The presentation had some numbers to prove this. A 20-30 km radius implemented by the Japanese was completely appropriate.

Air measurements conducted around the area of the plants indicate that much of the plume containing radioactive materials traveled northwest (i.e., due to wind); doses outside the main plume spread to the northwest were found to be relatively negligible. Likewise, with the exception of the plume "tail" in the northwest, nearly all of the elevated dose rates in air were concentrated within a radius of 30 km from the plant, calling into question the logic of NRC Chairman Jazcko's order for Americans within 50 miles (around 80 km) to evacuate.

Map of air dose plume release from Fukushima. Red: 9-91 microSv/hr, Orange: 9.5-19 microSv/hr, Yellow: 3.8-9.5 microSv/hr, Green: 1.9-3.8 microSv/hr, Blue: 1.0-1.9 microSv/hr, Indigo: < 1.0 microSv/hr (Image credit: IAEA)

As is clear from the evacuation zone map, the Japanese evacuation order of 20 km with a deliberate evacuation of Iidate and Katsurao prefectures appears to be based upon a sound evaluation of the measured exposure rates. Additional precautions in place (e.g., staying indoors and prophylactic measures such as distributing potassium iodine tablets) were implemented in areas within the 30 km radius.

Overly conservative evacuations based upon marginal risks (i.e., outside 20-30 km) place an additional burden upon already strained emergency resources (shelters, medical facilities, etc.) while giving little additional benefit. Even under the best of circumstances (which clearly was far from the case here), broad evacuation orders can result in tremendous hardship; the gains from evacuation must therefore be balanced against the additional burdens imposed.

- The IAEA believes the hydrogen explosion in Unit 4 may have been due to a joint exhaust from Unit 3

- No evidence of fuel uncovering was found in physical examinations or the water chemistry of Unit 4

During the crisis, some speculated based upon limited data (little to no actual instrument data from the pool was available due to the lack of power) that the water levels have precipitously dropped; NRC Chairman Gregory Jaczko even (now infamously) speculated that the pools had gone "completely dry." Jaczko was wrong again.

However, recent images taken of the spent fuel in Unit 4 by the IAEA indicate that the fuel appears to be intact and undamaged; thus an uncovering of the fuel in Unit 4 appears to be extremely unlikely.

- Estimates for the total time the fuel was uncovered was generally only a few hours – i.e., we had gaps in cooling of anywhere between 6-12 hours, but fuel uncovering only seems to have occurred for a few of those. Still, pretty remarkable at how fast things progressed once this happened

- While TRU was reported being found, evidence has been scarce and inconsistent.

Radiation Estimate

Estimates as to the total activity released to the atmosphere put the activity of I-131 released around 1.5 to 1.6x10^17 Bq (one becquerel is one atomic decay per second), and around 1.2 to 1.7x10^16 Bq of Cs-137. (Note that not all of this activity was spread inward; large amounts of this release were likely dispersed over the ocean, where they are greatly diluted.)

Worker and resident doses received

Estimating doses for workers was found to be extremely challenging, due both to the lack of instruments at the facility (from the power outage) as well as the flooding, which either swept away or ruined many of the personal dosimetry meters worn by workers or stored at the plants. Of 7800 workers measured, the average worker dose was about 7.7 microSv (well short of the yearly dose limit); 30 were found to have doses greater than 100 milliSv, while 2 workers were found to have equivalent doses to the skin of about 2-3 Sv (due to contact with radioactive water in flooded buildings, causing minor skin burns similar to a sunburn).

Estimation of doses received by the public was measured primarily by the level of surface contamination (i.e., number of radioactive counts per minute). Most of the 195,354 individuals screened were found to be below the 100,000 counts per minute limit; of those who were found to be above this limit, most of these individuals had residual contamination on clothing only and were cleared after changing clothes and vigorously washing their skin.

Gail Marcus at Nuke Power Talk looks at two Japan Times articles about the history of the Fukushima nuclear plant construction.

Japan Times article points to the huge irony that Fukushima Dai-ichi was built on a hill that had been cut down in an effort to make the plant more secure against earthquakes! It is a well accepted practice in earthquake areas to anchor building structures in bedrock in order to increase the stability of the building in case of a severe earthquake. Coupled with the facts that have already come out about how the potential magnitude of the maximum possible tsunami was grossly underestimated, it probably seemed like a reasonable engineering tradeoff at the time. And the subsequent cost tradeoffs also seem rational in that light. Although we cannot know for certain today what damage the plant might have suffered from the earthquake had it not been built in the bedrock, clearly, the decision to level the hill exacerbated the effects of the tsunami.

A second Japan Times article indicates that the decision on where to place the diesel generators was made by GE, and that some TEPCO engineers are now saying that they had concerns about this decision but that it was the long-standing policy at Japanese utilities (pre-dating nuclear power) not to make alterations to imported designs. What is not clear yet is if anyone at TEPCO or the regulatory authority at the time voiced these concerns

Russia is preparing to start the one gigawatt Kalinan 4 reactor in Sept, 2011

China has built the foundation of the Pebble bed reactor and ordered over 90% of the components. China is planning a 6 module Pebble bed follow up.

* A 600 MWe Multi-Module HTR-PM Super- critical Steam Turbine Plant, (Six 250 MWt
HTR-PM module and one 660 MWe steam turbine)
* standardize the reactor module,
* is inherently safe and competitive,
* and usable for co-generation.

Kirk Sorenson is interviewed by Sander Olson for Nextbigfuture about Thorium reactors and Kirk's company Flibe Energy. Kirk Sorensen indicates that he is confident to have sufficient funding to have a prototype thorium by 2016.

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