Wednesday, August 6, 2008

63rd Anniversary of Hiroshima

Today is the anniversary of the atomic bombing of Hiroshima. (One of these years we need to take a vacation across the Pacific that includes both Pearl Harbor and Hiroshima, to see both ends of that war.) The city that is now a thriving metropolis and home to the factory that built the car I drive was destroyed in a few seconds on 6 August 1945.

I knew one of the people in the photo recon plane that took the famous film of the mushroom cloud over Hiroshima, so that event means a bit more to me than it does to everyone in my generation who grew up under the threat of nuclear war.

One thing that many people don't know is that we NEVER TESTED the bomb we dropped on Hiroshima. (The test we performed in late July 1945 was for the bomb later dropped on Nagasaki.) This has important political /science consequences in today's world.

There is no need to test a uranium weapon.

The design of the "Little Boy" was simple and was invented independently by a Russian (Flerov) and the team (given their many nationalities, I hesitate to call it an "American" team) that put together our plans in the summer of 1942. The physics behind this is not trivial, but the calculations provided in the Los Alamos Primer show how easy it was to use the basic facts of nature and deduce that a gun (literally a gun, I was once told the exact model used) would provide enough velocity to push together the two sub-critical pieces of U-235 without the chain reaction starting prematurely. All we had to do was purify uranium, separating out the rare U-235 isotope from the U-238 isotope (which is called "depleted uranium" when it is used to make bullets and anti-tank weapons). Anyone who has pure U-235 can make a bomb.

That bears repeating. ANYONE who has enough pure U-235 can make an atomic bomb comparable to what destroyed Hiroshima. They don't have to test it. (The reputed fizzle of a North Korean weapon's test was almost certainly for a plutonium-based weapon, which is much trickier to design and does require testing.)

Eliminating excess U-235 that might fall into the hands of someone less responsible than the USSR or the USA is, therefore, a big deal. This is why the US has been burning Soviet U-235 in its reactors since a major initiative began in 1993. [It was important enough that it started under GHW Bush, was implemented by Clinton, and might have been the only major Clinton foreign policy program retained by GW Bush.] It is also why the enrichment of uranium by the Iranians is a big deal, and also is why obtaining "yellowcake" (which is not enriched) had squat to do with a WMD program in Iraq even if the report had not been a total fabrication.

However, the difference between taking natural uranium that is 0.7% U-235 and turning it into "enriched" reactor-grade uranium (around 3% pure, a factor of 5) or turning it into weapons-grade "highly enriched" uranium (probably around 99% pure, a factor of 140 or so) is significant. It takes many more resources (or a lot more time) to do the latter. The scale of such a facility (and its energy requirements) makes it easy to identify, but the only way to know its objectives is to inspect it. That is why the IAEA has always played such a major role in controlling the proliferation of nuclear weapons.

I highly recommend Richard Rhodes' book "The Making of the Atomic Bomb" for anyone interested in this subject. Not to name drop, but Robert Serber (who gave the lectures summarized in the Los Alamos Primer) told me it was an accurate description of what went on during the Manhattan Project.

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