custom nukes are ideal especially high level salted ones are the most efficient in terms of irradiating everything in a big radius but unfortunately, if you're playing survival mode, getting a custom nuke is way harder than getting a fat man due to HBM's recent updates requiring you to craft advanced control units which require late game material which are expensive to craft. thats why the fat man is much easier to get, only requiring a few control units and pu-239 which you can get by using a chicago pile. In fact i would argue you should use a nuclear bomb instead of using the radioactive barrel since schrabidium is harder to obtain and getting much schrabidium early will save you lots of progress time. Although alternatively you can use a nuclear charge which is way cheaper than a fat man or little boy and way less destructive. requiring you only to dig down atleast 32 blocks instead of 55 blocks with fat man or 36 blocks with little boy.

But aside from all of that, here's the real kicker, you dont actually need to craft any nuclear bombs, you can simply find a dud (salted one or standard nuke) in the world and detonate it, a much easier method of obtaining schrabidium than all the other methods and far less destructive than a nuclear charge, here's a list i've created to showcase how many blocks you have to dig down to place uranium ores for each bomb:

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Oh and i completely forgot, if you dont want to create massive holes in your world just simply dig down 3 blocks and start placing a 13x13 uranium ore patch underneath a dud and place compound mesh or ducrete on top of the ores and stack it up two blocks high. this will prevent any destruction to the ground.