Inverse square law

I am looking at video’s of the helicopters dropping water on the stricken nuclear reactors in Japan. Rather than hovering and trying for a direct drop these vehicles seem to slowly keep moving, resulting in most of the water missing the target. From the height they are flying it seems they would have little fear of direct radiation, and that radioactive particles would likely be uniformly spread over the area. In short, what is the rush? I have no idea as to the amount of radioactivity a nuclear reactor would contain, and specifically what amount of radiation would still be present several hundred yards away. Anyone have any idea?

I think it’s more likely that no one wants to be hovering directly over the point of another possible explosion, whether or not there’s nuclear material involved.

the storage pools on top of the reactor have fuel rods which are shielded by water. these storage pools in reactors 3 and 4 seem to have gone dry. the fuel rods from reactor 4 are fresh and so are extremely radioactive.

The bigger worry is not direct radiation, but distribution of radioactive material. While this will still, on average, be governed by an inverse square law, the average isn’t very meaningful, since it’ll be dominated by winds, gravity, etc.

If there is Direct exposure to the rods - spent or otherwise - they could be emitting high Gamma radiation. If that is the case getting close enough to reliably hit the target could be very deadly. Unlike forest fire drops where dispersal is good, the prop wash is also working against them too.

Unless you KNOW lots of people will die unless you act… (chernobyl) most people are reluctant to sacrifice their life on a “lets try this” solution. The pilots are/were trying to buy time for the power crews to restore electricity to reactor pumps which might fix 90% of problems. – But since none of us know the skinny we’re all peeing in the wind - like the dumbass reporters who either ask stupid questions – or speak as if they are nuclear engineers with a direct line on the problem when they don’t know jack. My rant and 2cents.

Wouldn’t airplane water bombers be better at this? Wouldn’t something like a CL-215/415 have a more accurate drop without the effects of prop wash, not to mention the ability to perform several drops rapidly? Though I know that this isn’t even a guaranteed solution to the problem, so it might not be worth trying to bring some in. Japan doesn’t own any, anyways, so they’d have to be brought in from Thailand or Europe or Canada.

Might also be that hovering over the reactor is inadvisable in the event of an engine failure. When a helo hovers out of ground effect there’s really not anywhere to go but straight down. Forward momentum means that in the event of a power failure the aircraft has a chance at auto-rotating in a direction other than straight at the reactor.


Wildland fire suppression does not require pinpoint accuracy. Actually, fixed wing drops take advantage of the water dispersal and kill two legs of the fire triangle (oxygen and temperature) to suppress the fire. Also, a fixed wing water drop ends up dropping water where it’s not needed, perhaps on electrical systems the workers are attempting to fix.

In this case, you need pinpoint accuracy to drop as much water directly on the the site, one as a coolant, and two, as a moderator, to slow the chain reaction.