[DuncanWaring]

Now they must design a reliable trigger system, do many tests to perfect it.

I think you're referring to a plutonium-based "Fat Man" design.

"Our" first uranium-based bomb, "Little Boy", did not, to the best of my knowlege, have a particularly sophisticated trigger, and was untested prior to first use.

[Marine_Uncle]

"I think you're referring to a plutonium-based "Fat Man" design. "Our" first uranium-based bomb, "Little Boy", did not, to the best of my knowlege, have a particularly sophisticated trigger, and was untested prior to first use."
One can only go by what we see in diagrams. I believe with the uranium bomb it was a proximity fuse that set off a explosive charge that drove the lesser of two fission masses into the larger one (down a tube). I believe that essentially a similar mechanism was used for the plutonium bomb in the context both used a proximity fuse that then set off a explosion moving the sub critical masses together in of course different geometric configurations. The bombs guts as you well know looked a lot different in how things where arranged.
So perhaps you are on to a very good point. In neither case would it be very difficult to design a functional "trigger mechanism". As we view the various diagrams it is apparent the plutonium bomb used a somewhat more sophisticated design both as how the sub critical masses where segregated as well as the means of triggering. For all we know they have various designs in place perhaps from the Paki.
I hope I am not coming off like they cannot eventually design a functional atom bomb. For they can. I mostly was trying to appeal to folks, that it may be a lot longer then some reports have suggested based on the idea if they are soley depending on using gas centifuge techniques, they have a long way to go before they will obtain adequate quantities of weapon grade uranium.