Increase the mass of the bomb. Thats it.I thought it needs like 85 or 90% refined. Perhaps, 60% doesn't give off that same massive energy level as a 90% or higher.
And enrich uranium from 60% to 90% is a simple and small scale job.
you have to run throught the plant 400 kg U equivalent of gas to get 267kg.
To get 400 kg 60% you need to run thorught the machine 48000 kg of natural uranium .