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It's all scenario dependant. Commercial and military reactors will SCRAM on their own by design, but that doesn't mean there can't be an accident or failure. Fukushima utilized Second Generation GE and Westinghouse Pressure Boil Reactors like most commercial reactors in the US.
Chernobyl was an experimental Graphite reactor the Soviets pressed into commercial use. During an experiment a series of operator failures and system failures resulted in an automatic SCRAM (SCRAM is a term for immediate shut down and storing of rods in their container to reduce the ongoing reaction). The rods at the Chernobyl plant where SCRAM'd, but were still reacting and went critical, melting through their containment vessel, through the exterior cooling lines causing two massive steam explosion, and into a concrete catch pool. Then proceeded to melt through the concrete of the catch pool and onto the ground floor of the reactor housing. To this day, the radioactive slurry of boron, salts, and radioactive fuel sit red hot at the bottom of rector.
The explosions resulted in the release of radioactive material that affected most of Western Europe in some way. In the immediate area, Ukraine, Belarus and parts of Russia. However, the Soviets had kept the disaster under their hats. It wasn't until a nuclear facility in Sweden detected extremely high amounts of contamination during routine monitoring. Once it was determined it wasn't a leak at the facility or any Swedish facilities; NATO, NRC, and IAEA were informed and monitoring of Europe was begun.
In the immediate area of the facility, you're going to have thousands of radioisotopes present, some only lasting for seconds others for minutes and still more for days. The worst last for months or years, Strontium-90, Ceasium-137, ect… the explosion at Chernobyl caused a radioactive plume to be thrust into the atmosphere and caused radioactive material to be carried across Europe by wind and it contaminated crops, food, water, livestock, ect…
Fukashima had a similar scenario play out, but the cause wasn't operator failure and poor design. The flooding of the facility caused failure of reserve water pumps for cooling systems and the storing of spent fuel on site caused further release of radioactive materials. The rods were SCRAM'd, but failed to stop the reaction, causing a meltdown and breach of the containment vessel. However, the reactor housing contained much of the resulting melted fuel. The release of hydrogen into the reactor housing caused a violent reaction with oxygen, thus resulting in an enormous explosion, releasing large amounts of radioactive material, damaging the spent fuel pond, throwing fractured fuel rods all over the place. The resulting fractured rods allowed the release of fuel pellets (MOX fuel pellets, a Uranium-Weapons Grade Plutonium Fuel Mixture) to be scattered over the surround area and were buried in the local area.
The affect and impact Fukushima had, not only on Japan, but the world was very damaging. In the US alone, background radiation rose, the West coast saw an major increased in post-rain monitoring levels, the fishing was heavily affected because of the contamination of ocean waters. Alaska has been continuously monitoring its bays and harbours seeing increased levels of radioactivity in fish, sea life, driftwood, ect…
Both Chernobyl and Fukushima are surrounded by expansive "Exclusion Zones". The zones are rout with lethal pockets of radiation, mutated wildlife and plant life. For example, around Chernobyl some of the local conifer (pine trees) glow red at night, military patrols have reported seeing mutated wild horses and dogs. Feral animals in the zone are rampant. Nothing growing in the zone is edible, the radiation levels are too high, same with water.