The Japanese bombsvwerent nothing to what came later. Testing was needed. Nowadays supercomputers do the 'blasts' so no real detonation. But it doesn't mean we still don't test the delivery system, as in ICBM"s. Look on YouTube for MMIII launch.
Actually the computers just record the data from the blast. They are using amdro a mixture of diesel and fertilizer like was used in the Kansas City bombing. So the test blast are no longer nuclear but they are still active test. The reason I know is because I worked at White Sands Missile Range.
We used a cement mixing plant to mix the diesel and fertilizer. The last test I was on used 4800 tons of mixture. We built a city complete with Airport and trees and had people dummies Vehicles everything was there and blew it to pieces.