Urea is relatively basic compound. So if there is free acid, it eventually do form to some extent. As always with weak bases or acids, reactions are reversible.
Diluted urea nitrate in solution isn´t that extremely bad. Also urea nitrate in it´s pure form, but wet, is somewhat hard to explode. But it certainly can be detonated - with nicely big primer charge. From scraping the edges of the bucket - I do not think that this can be the trigger.
Additionally, with base treatment, nitric acid would be slowly bound to more basic compound - thus eg addition of NaOH would create sodium nitrate and free urea - which will start to decompose in alkaline conditions and liberate ammonia.
Me personally - I don´t use urea in anything regarding refining. I hate the smell of ammonia during workup - and cannot conveniently place drums of waste into the hood, so just sitting in the lab - no way. Also imperfect de-noxing, actual troubles with other things...
When I am doing first drop of gold from eg pins in straight AR, I carefully watch nitric dosing beforehand. Not to have unhealthy excess of nitric. Slurry the SMB in some water to make fluid paste, keep the AR solution hot and with good stirring slowly denox+drop with SMB... If it is first dirty drop, even this is sufficient. Sulfamic is expensive, but better. SMB is dirt cheap, not that nice de-noxer, but it does the job quickly, to acceptable outcome. Waste processing is then more pleasant contrary to urea - smell of SO2 is gone after few additions of slaked lime, as it is the second thing to be neutralized, after excess HCL. From then, just add more to the nicely basic pH, and no acid fumes or other corrosive ballast goes through the pump - so you can use dirt cheap membrane pumps for filtering this, and they last long.
