Simple Calculation Of Cross-Pacific Dispersion

I was wondering what amount of dispersion could be expected from a plume of radioactive material crossing the Pacific from Japan, so I did a quick ISCST3 run.

ISCST3 is a comparatively-simple, formerly-popular EPA air quality dispersion model that has since been superseded by more sophisticated models, but I choose to use it here because of simplicity.

I use 1-hour average screening meteorology. I assume a ten meter tall stack, with a diameter of 1 meter, an exit velocity of 5 meters/second, and, for grins, a screening emission rate of 10,000 grams per second (which I make artificially-high in order that low concentrations far away still register in the model output).

Results follow:

Distance Downwind (km), Concentration (micrograms per cubic meter)
- - - - - - - - - - - - - - - - - - - - - - -
1.0, 592355
10.0, 172858
100.0, 15288
500.0, 2952
8,300.0, 213

Results suggest 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 (172858/213 approximately equals 1,000).

There are other factors, of course. Dispersion doesn't take into account either meander from synoptic-scale mixing, or for deposition of material to the ocean surface. So concentrations are likely to be even lower than 1,000 times less (maybe 2,000 or 3,000 times less).

Nevertheless, concentrations won't be zero, and depending how bad things get, concentrations may not be negligible either. We are downwind from all the troubles in Japan! If things fall apart there, we will all be affected.

Remember, in 1986, Chernobyl led to surprisingly-high radioactivity in places like Moldova and Sweden.

According to the news, nuclear specialists are watching:

As Japan races to avert multiple nuclear meltdowns, one expert warned Sunday that radiation could spread to the U.S.

Joe Cirincione, president of Ploughshares Fund, told Fox News' Chris Wallace Japan's nuclear crisis is unprecedented.

"One reactor has had half the core exposed already," he explained. "This is the one they're flooding with sea water in a desperate effort to prevent it from a complete meltdown. They lost control of a second reactor next to it, a partial meltdown, and there is actually a third reactor at a related site 20-kilometers away they have also lost control over. We have never had a situation like this before."

"The worst case scenario is that the fuel rods fuse together, the temperatures get so hot that they melt together in a radioactive molten mass that bursts through the containment mechmisms and is exposed to the outside. So they spew radioactivity in the ground, into the air, into the water. Some of the radioactivity could carry in the atmosphere to the West Coast of the United States."

"Really?" a surprised Wallace asked. "I mean, thousands of miles across the Pacific?"

"Oh, absolutely. Chernobyl, which happened about 25 years ago, the radioactivity spread around the entire northern hemisphere. It depends how many of these cores melt down and how successful they are on containing it once this disaster happens," Cirincione replied.