After WWII work continued on improving the technology behind the Gadget, and through a series of innovations the massive design was improved upon. Some were made more powerful, others were made smaller and eventually the first steps towards harnessing fusion were taken.
The B61 "Dial A Yield' image was from Martin Pfeiffer's blog:
https://deusexatomica.wordpress.com/
Background music is Tranquility Base by Kevin Macleod
http://incompetech.com/music/royalty-free/?keywords=tranquility+base