John Fleck, reporter for the Albuquerque Journal, notes that NNSA Administrator Linton Brooks clammed up when asked about the using uranium for the reliable replacement warhead:

My question to Brooks: Is uranium being considered?

I got the “is it classified?” pause, then “I don’t think we’re thinking of fundamental changes in the way we’ve looked at nuclear weapons design.” I immediately inferred that meant they’re not looking at uranium for RRW. But then Brooks suggested that he’d like to tell me more, but wasn’t sure where the classification boundary lies. He asked me to iterate with his staff after they’d had a chance to see what, exactly, they could say. I did. They concluded late this afternoon that they couldn’t say anything more.

That’s odd—recent reports don’t seem to indicate any interest in uranium.

One possibility is that NNSA has rejected the idea, but on technical grounds that would be classified.

It is worth noting that the government has fought tooth-and-nail to keep design information about thermonuclear weapons out of the public doman, from the court case against Howard Moreland to more recent silliness over aspherical primaries.

Despite such restrictions, one loyal reader suggests the B53 was enormous—Cochran, Arkin and Norris (Nuclear Weapons Databook Vol. I: U.S. Nuclear Forces and Capabilities, Cambruidge, MA, Ballinger 1984) estimate 3690 kg—and might represent the limit to miniaturizing nuclear weapons with uranium primaries. Explaining that objection would require addressing one of the topics in the Morland case.

There is clearly more work to be done on this question.

In case you don’t know about the Howard Morland case, he gave a lecture recently that is a really great read.