Even a non-depleted uranium is not explosive until the pile reaches the critical mass.
In the laymen terms that reactor design would, under a very, very specific conditions, sharply increase the energy generation on the shutdown procedure (instead of decreasing), which would overheat the core, increase the fuel elements temperature and, consequently, increase the amount of steam from the water which is used as a coolant.
After the temperature and pressure rose too much the parts of the coolant and fuel systems started to break, which provided the reactor with a lot more fuel to work on, increasing the output 60 times (from 500MW to 30000MW), which insta-converted the all remaining coolant to the steam (and further raised the temperature and pressure in the reactor) and that steam pressure just ripped off the top cover of the reactor and send it flying.
There was another explosion (about two or three seconds after the first), but it's a mere consequence of the first one, happened in an already destroyed reactor and it is still not clear if that was just a plain chemical one (hydrogen) or the thermal one (ie what you would call a 'nuclear').
If anything, water is a neutron moderator ie it inhibits the reaction:
>> In the light-water-cooled, graphite-moderated RBMK, a reactor type originally envisioned to allow both production of weapons grade plutonium and large amounts of usable heat while using natural uranium and foregoing the use of heavy water, the light water coolant acts primarily as a neutron absorber and thus its removal in a loss-of-coolant accident or by conversion of water into steam will increase the amount of thermal neutrons available for fission
Depleted uranium is just a heavy metal, spent fuel is a heavy metal with a higher radioactivity (not suitable to handle with a bare hands) but not enough to go critical or even be close to go critical.
You can pour any amount of water on it, at best you would get a slightly radioactive water.
Just to be clear - you need something else, with a high yield, eg a chemical bomb with a high TNT equiv., to make a dirty bomb - still it wouldn't provide a self sufficient critical reaction.
Then, this is accurate, right?