Pyrex-style borosilicate glass has a CTE of 3.3e-6/C (referred to as 3.3 from now on). To make a feed through wire seal, you need a CTE that is more or less matching. Tungsten is usually used for this (CTE of ~4.5). Why didn't they make a nickel-iron alloy with 3.3 CTE to seal to boro? No idea. Why didn't they use a glass formulation with a ~4.5 CTE that they could easily use with tungsten? No idea. But neither of these things are what I am talking about.

Sealing thin tungsten wire directly into 3.3 CTE borosilicate usually works. But when you get to thicker wire or full-on rods, that isn't the case as much. So they make a graded seal. 3.3 CTE boro to ~4 CTE uranium-boro to ~4.5 CTE tungsten. It is a graded seal. That makes sense, that is just what you gotta do when there is a CTE mismatch. But why uranium glass? Adding uranium isn't the only way to raise glass CTE slightly. I just looked it up, Schott 8660 has 4.1 CTE, and  1250C working point. This would have worked fine, and without the uranium. I am sure there are many other options as well, this is just a quick example.

I can't imagine it was cheap to use uranium to make glass, why didn't they just use a different normal formulation to make the intermediate glass?

Obviously seeing uranium glass in power tubes and some lamps is a real treat, and I am glad they did that, it is very cool. And I also know it poses no threat to life or health from the miniscule amount of radiation released. But I would just like to know why they chose to use such an exotic solution for a very simple problem. Anyone know?