Perhaps make UNUB stable until it is hit by a neutron, then have it perform the specified reaction? Also, another idea is to make it trigger a fission reaction, so that if it is hit by OXYG, it turns the OXYG into two CO2s. If CO2 hits it, it turns the CO2 into two NBLEs, and so forth. To add even more potential uses, perhaps if UNUB is sparked, it releases PROT in large quantities.
I was basing it on the fusion reaction that the game, which is HYGN -> NBLE -> CO2 -> OXYG.
UNUB would cause the reverse form of that reaction: OXYG -> 2 CO2 -> 2 NBLE -> 2 HYGN.