Reactor itself is fairly well protected against 155 mm shells or HIMARS rockets. But there are two ways things may go south. First - if spent fuel storage is hit. In ZPP spent fuel stored first in special cooling pool and when most active isotopes decayed and heat generation in spent fuel dropped by few orders of magnitude spent fuel is placed in concrete containers which are stored in open air. Those containers are not as sturdy as reactor itself and can be breached by direct hit causing pollution.
Second way is to destroy reserve power (reserve power line and diesel generators) then trigger reactor stop by destroying outgoing power lines. This would cause Fukushima type event when lack of reserve power (both power lines and diesel generators were destroyed by tsunami) caused overheat of stopped reactor core (which continues to generate heat due to decay of radioactive fission products) as there is no external cooling.
The core would melt and in the end will breach the containment, causing leak of radioactive isotopes.
Fukushima was way worse than Chernobyl in terms of amount of leaked radioactive isotopes but most of those went into Pacific Ocean and were diluted so damage was limited. In case of Zaporozhye gaseous isotopes and some volatile substances will go to Europe with wind, while the rest (major part) contaminate surroundings and Dnieper river - and then Black Sea (say bye-bye to cheap Turkey resorts, though same goes for Russian, Romanian and Bulgarian resorts as well as remaining Ukrainian ones).
Triggering first event is easy - but damage will vary depending on how much containers is breached (or how much fuel rods is breached in cooling pool). It may be achieved with just few shells. Second event requires constant attacks for a prolonged time with constant target correction to destroy extra diesel generators which undoubtedly will be rushed in to keep the cooling up.