The FLASH in the secondary blast a few seconds after impact of a “depleted” uranium shell is a clear proof of a criticality event (first link : soldier report, second link : National Geographic documentary on the Gulf War, third link : video of a flash recorded after the impact). This works also with missiles on the instant of the blast (undelayed), see for instance here for the Brimstone.
Here is another example : this video of the plume of “plasma” after a Saudi bomb smoked a bunker in Yemen. There is a first explosion at 0:16 at the impact of a very fast missile. Then at 0:21 there is an explosion INSIDE the plume, which spreads lots of sparkles and creates an almost blinding light.
No fuel explosion can create a flash like that, it’s too slow to compress air particles. Have you ever seen a car explosion generating a flash of light ? The use of uranium + beryllium / heavy water + sodium 24 / thallium 208 / polonium 210 – or a uranium – (palladium) – deuterium – tritium mix in DU shells generates fast neutrons (from (a, n) or (gamma,n) reactions or a low energy nuclear reaction in the case of the tritium mix, see again links just above). These neutrons are slowed down because they have to cross the beryllium tamper. The fission of 235U from slowed down neutrons generates at the impact a first fission shot (criticality event). As you know there is always some 235U in depleted uranium (0.2%, and frequently natural concentrations of uranium 235 are used) and it will be a much, much better recipient of neutrons than 238U once these neutrons have slowed down (because most collisions w/ U238 do not result in fission or capture), especially as the uranium 235 is concentrated on the tip of the missile or in the center of the cone of the shaped charge (so that the entire uranium mass is 0.72% of 235U but the core is 95% 235U). For the Brimstone missile this happens directly at the first blast, because there is fission, but then the matter is spread around (because there is no receptacle like a tank) so it cannot react again.
In the case of tanks and of the Saudi missile the delay comes from the delayed neutrons from the first criticality event. Even 1% of fission neutrons from a criticality event (that is the percent of neutrons that are delayed) means a HUGE amount of neutrons. Furthermore let’s underline that in average delayed neutrons are much slower than prompt neutrons, in average 405 keV instead of 2 MeV, which means they will need much less moderation for starting up fission. The mix has oxydated, helping also slowing down these neutrons. The delayed neutrons will come progressively (the average delay in a nuclear reactor is about 8 seconds according to Charpak, Journé and Garwin in their book De Tchernobyl en tchernobyls, there are several waves with different half lives but some waves will produce neutrons that are slower than others) and this delay matches pretty well the observated timeframe between impact and flash.
In the tank, the magma is full of neutrons from the fissions, the (a,n) and (n,2n) and the cavity it has dug into the bottom of the tank allows for compaction of the magma. The tank is an almost closed “cave” (there’s just the hole of the shell on a side), the bodies of the crew have burned, thus there is vapor in the tank (the human body is mostly made out of water), neutrons spraying out of the molten uranium onto the top of the “cave” and reflected into the core are slowed down by the vapor. The U235 from the tip is obviously now in the state of a gas, partly mixed with vapor, the atoms of the U235 gas move VERY fast, improving sharply the probability of a collision with a neutron. The tank behaves exactly as a nuclear reactor with a positive void coefficient and the core explodes. The flashs are clearly a proof of the arrival of the delayed neutrons. The amounts of energy released cannot of course be compared to a kiloton-range nuclear bomb, yet this is a nuclear explosion.
Same for the much bigger Yemen bomb : the only sensible explanation for that second explosion is exactly the same as above, the U235 of the tip of the weapon, oxidized, is vaporised into the air by the first explosion and is into the plume, together with the hydrogen that is used in the fuel of the missile and may be used as a cryogenic coolant for a highly radioactive neutron source in the warhead (it may have also been present in big quantities in the ammunition warehouse that was targeted, as fuel for rockets for instance). Furthermore U235 in the plume is HIGHLY AGITATED because it’s so hot, and this increases the probability of a neutron crossing the path of an U235 atom. Hydrogen is also an excellent neutron moderator. What is the ONLY hypothesis for a delayed explosion, at 0:21, that creates so much BLINDING LIGHT and that actually happens inside a PLUME is CLEARLY and DEFINITIVELY a phase of supercriticity of the oxidised U235 inside the plume, above the ground, thanks to an excess of delayed neutrons from the first impact. End.