ISCST3 is a comparatively-simple, formerly-popular EPA air quality dispersion model that has since been superseded by more sophisticated models, but I chose to use it because of simplicity.
Results suggested that concentrations would drop by about a factor of 1,000 from a point 10 kilometers downwind to a point 8,300 kilometers downwind, which is about the distance between San Francisco and Tokyo.
It is hard to exaggerate just how primitive a dispersion tool ISCST3 is for these long-distance calculations, but I suspected the calculations might be robust and more-or-less accurate, because long-range dispersion tends to be simple and predictable. It would hardly matter what calculation tool you used. Even back-of-the-envelope calculations might suffice, and ISCST3 used with screening meteorology is just one step more sophisticated than back-of-the-envelope calculations.
In this morning's Sacramento Bee, there is a story that summarizes much more sophisticated calculations about cross-Pacific dispersion:
But as Bandrowski noted, the map does not show radiation levels. Rather, it appears to forecast how emissions from Japan might disperse based on wind across the Pacific Ocean. It indicates that whatever reaches California would be diluted by a factor of 1,000 compared with emissions at the reactor site.How about that! We are in the same ballpark, more-or-less!
Because sophisticated model simulations take skill and time to set up properly, their results often enter late into public discussions. In a crisis, quick results are the best. Sometimes, they don't even have to be accurate to be useful. As noted in the news story:
Some observers blamed the public confusion on the government's lack of clarity amid the crisis.The generation of American scientists that were active in the years immediately following World War II were well-trained in the art of back-of-the-envelope calculations. They became well-versed in the art of quick calculation, usually out of necessity in the topsy-turvy world of a world at war. The proliferation of cheap and easy computing power in the last generation has been a tremendous boon to science, but it has robbed the current generation of scientists of the skill and confidence that the art of the quick calculation allows. Ever wonder why scientists and government officials today seem to speak with less confidence than they did in the 1950's? We know more today, but without that quick facility with numbers, we act with less confidence!
"Without any information, people are just clueless, and it leads to speculation and fear," said Jim Metropulos, a Sacramento lobbyist for the Sierra Club, which has long pressed for stricter regulation of the nuclear industry.