Trying to understand what is going on at Fukushima has not been an easy task. Even after the first weeks, information out of Japan was hard to parse. For example, on April 4, the Capacity Factor blog attempted to understand inconsistent radiation readings in Japan, while on March 27, Cheryl Rofer blogged about the ways radiation measurements could go wrong.
Then there was the entire "tellurium shows there is re-criticality" business.
First, the Japanese reported the presence of tellurium in a water sample.
Second, Arnie Gundersen and others were quick to claim that this proved the reactors had "gone critical" again with a fission chain-reaction.
Third, the Japanese admitted that their measurement was wrong: there was no tellurium present.
The measurement was wrong and Gundersen's re-criticality theory was wrong, too.
In other words, we had a complete lack of clarity.
We Move to Oversimplification
There's a scale, the INES scale, for nuclear accidents. On this scale, Three Mile Island was a 5, and Chernobyl was a 7. Seven is the highest level on the scale. Until April 11, the Fukushima accident level was assigned as a level 5, the same as Three Mile Island (TMI). That was obviously too low an assignment: nobody was evacuated or burned by radioactive water at TMI. Then the Fukushima level was raised to a 7, the highest level possible. Fukushima has only emitted 1/10th the radiation that Chernobyl did. (Also, unlike Chernobyl, nobody has been killed by radiation at Fukushima.) Capacity Factor has an excellent post comparing Fukushima level 7 with Chernobyl level 7.
So, the INES scale has the same rank (7) for two accidents that are an order of magnitude apart in terms of radiation release. However, what about a bigger-than-Chernobyl accident? A two-reactor Chernobyl? That would be an INES 7 also.
Real Simplification, Also Known As Clarity
Luckily, in recent days, some clarity has come to general understanding of the Japanese situation. We first moved from complexity to oversimplification, and now we are approaching knowledge.
A wonderful interactive graphic from the New York Times puts many types of earthquake damage on the same graphic (finally!) in a move that Edward Tufte would appreciate.
For our purposes, though, we should look at the "radiation" levels tab of the graph. If you look at a given circle on the map, the color indicates the radiation dosage at that point. Each circle also contains information on the dosage rate, and how long it would take (at that dosage rate) to receive an exposure that:
- equals a chest xray,
- equals the allowed dosage of a United States nuclear worker,
- becomes a dosage at which long-term health effects become more likely.
Furthermore, in the Radiation tab of the Times graphic, there is a slider under View Readings Each Day. If you move the slider to March 17 and then move it forward in time, you can see the overall radiation levels decrease with time. The colors of the dots become less intense. It's a great visual and shows that the radiation is getting under control. Airborne radiation levels are diminishing.
This makes sense, because most of the radiation was released in the early days of the accident.
There's another good source if you want to see the time evolution more directly. The NEI (Nuclear Energy Institute) blog linked to a new graphical blog based on Japanese data sources. This blog has graphs of the main parameters of the reactors over time on one set of charts. For our purposes, another set of charts is more interesting: these charts map the evolution of the radiation levels in Ibaraki prefecture and Tokyo, with a comparison to European background levels.
Once again, the charts show the same clear story of steadily decreasing radiation levels. Radiation spikes at the beginning of the accident, then steady decreases.I have grabbed one of these charts below. As usual, double-click to enlarge it.
Final clarity is not available yet. Power has been restored to the Fukushima site, radiation levels off-site are decreasing, the major radiation outflow from the plants now is water, which is intrinsically easier to control than air-borne emissions. But nobody would say that the problems are over.
Fukushima isn't Chernobyl, and it isn't Chernobyl on steroids (as Gundersen predicted) but it is serious. This Fox news clip is not particularly upbeat, but I think it is a good summary of the situation to date.
The graphic at the top of this post is a picture of a snubber at Brown's Ferry. Brown's Ferry is a sister plant of Fukushima and Vermont Yankee. The snubber is isolates vibrations for Unit 1 and is part of the seismic activity safety equipment for the plant. Photo used with permission of TVA.