What kind of nuclear accident is the most common?

There were 2 major power plant meltdowns the first one occurred in
Ukraine a power plant named Chernobyl this happened 26 years ago 1986 April 26th Saturday at 1:23:58am the reason of the explosion was a mislead by a Cheif Named Anatole Diatlov reactor 4 had a test on it to see how it co-ops with low output power the chief ordered the amount of megawatts to be in 200mw but he didn't know that it would cause a catastrophic disaster if placed test on a power plant it should be to 700mw till 1000mw not only was the rule broken there was a warning that a bubble built in the reactor was present which was a heated area producing more and more heat.although dangerous but test was resumed as scheduled turbine 8 was shut down suddenly that caused more risk all turbines were shut down and the bubble begun producing more heat with less water soon steam started lifting the 350kg cover of were the heat comes out to blocks in within 3seconds one of the engineers had pressed the A3-5 button that was able to shut down the reactor fully but that cause more damage for the bubble in the reactor later in seconds the first explosion occured destroying 50 uranium fuel rods and leading for the second explosion to give more damage the fuel rods were melted and had the temperature of 12,000c this lead to flipping the 1000tonne roof of the reactor 4 to its side immediately radiation and dst started to escape the whole reactor 4 was damaged the day after the aftermath workers found another situation in the reactor that made it even worse to handle.
after 26 years radiation is still present in the Chernobyl plant the whole 162km radius had to be evacuated from the area of Pripyat.notice the power plant is not shut down its still working and has doses of radiation

the second is Japan in the coast of Honshu power plant named Fukushima Daiichi.On march 11th Friday 04:46:24pm GMT the result of the meltdown was caused by 2 natural disasters at first an earthquake of 9magnitude shook the power plant and as well as the city of Tokyo and Honshu some of the systems had failed to operate and lead to damage after the earthquake comes in 46-49ft Tsunami that hit and flooded the power plant to knee length later on the entire power plant was flooded with sea water that caused a decreased amount of cooling water seawater was present all the generators that produced cooling water were shut down and were only restarted 2-3 days after the meltdown the 3 safety systems had failed to cooperate that caused the rods to over-heat and self-melt,soon the TEPCO concludes that another meltdown was possible for the reactionary in 38 hours after days passed the reactor was still flooded with seawater and shutdown with heated rods that have melted the workers have caught an over-limit dose or radiation 2000-6000mSv of 400 is more then enough to kill a person. and have identified an amount of radiation in air, water,soil and food as a result of the damage in 300km radius has an amount of radiation all people that lived close to reactor have been evacuated.