It will be good if we add solid plutonium and solid uranium, because I saw on Wikipedia and some images that uranium and plutonium are solid, heavy metals. And it will be very helpful.
While I agree that it would be useful to have solid radioactive elements, the devs aren't going to modify existing elements: because URAN and PLUT are so widely used, a shitton of saves would break irreparably. This should definitely be a new element though, and while they're at it, make it ignore pressure for its fission mechanics.
At one point when Skylark was visiting, he made a made a mod with Solid Plutonium and Uranium for other purposes. I'm not allowed to say anything else because that's the agreement of authorship.