Using newest 1.13 build, set up array of breeding pools, wall in any yukkuri and apply Rescue OJ Pool. Put garbage chute all the way to the bottom to decant corpses as they pop out.
After a while, the mother yukkuri's speech will turn from "sweet sweet" chatter to chatter about "coming back gorgeously". This is because she is taking massive amounts of hunger and damage every tick. Nevertheless, Rescue OJ Pool will keep the central yukkuri alive unconditionally even with estimated over a million babies inside of her. In this state, her bottom pixel takes up 30% of the screen across the top. Her true size can only be estimated by the stress level. If the stress level is 41, it means 40 yukkuri are airborne between her mamu-mamu. SimYukkuri does model acceleration but probably not terminal velocity. If you assume one yukkuri is born every half a second, her progeny were falling from a height of around 2km and hitting the garbage chute at around 700km/h.
To initiate yu-fission, remove the OJ pool. Under the above conditions, this will freeze irrevocably. Whether the limitation is memory, processor or Java itself, in order to actually observe the results, a bomb with more modest yield with peak stress in the tens of thousands is necessary.
Screenshot above is T+1 minute or so after a 32ks-33ks (kilo-stress) explosion (the spike in the graph). Due to extreme stress, even those yukkuri who survive the explosion are instantly killed. This matches up with the damage numbers on the corpses which are generally less than 5000% or so and in many cases <1000%, indicating they died long before hitting the ground. The garbage chute I moved around and cleared out the initial wave of yukkuri that formed a square frame around the map.
Within seconds, yukkuri are only falling at the corners of the map and this persists. Note that stress at this point is still at over 15,000 and decreases only gradually even if I continually move the garbage chute over the corners with speed at Max. This means yukkuri are still falling and are likely to continue doing so for a long time. Whatever force the explosion had, it's propelled them far higher than normal birth height. Before the explosion they were hitting the ground within about four seconds - sustained stress before detonating to 32-33k was only 8. Note also the shadows only exist at the corners and at the top and bottom middle of the map. It seems that those two spots are where the final wave will fall.