These two incidents rank as the two most serious disasters in the age of nuclear power. They both involved a meltdown of the core of the reactors, though to differing degrees, and the resultant release of radiation made large amounts of territory uninhabitable in both Japan and the present day Ukraine. In both cases, those who responded to the disasters took extraordinary risks, often with fatal results, in order to limit the scope of the disaster. Both the reactor at Chernobyl and the one in Fukushima were old, practically on the verge of being deactivated when the disasters occurred.
There were some key differences. Fukushima was the result of a tsunami, and while the reactor was poorly located right along a tsunami and earthquake-prone coastline, maintenance and operation of the facility was within acceptable international standards. They just didn't expect the disaster, and had no backup system for it or protection against it.
The Chernobyl meltdown, on the other hand, happened as a result of faulty engineering and human error. Gauges measuring water flow failed to reveal a dangerously exposed and overheated core, until it was too late to prevent disaster.
In both cases, the governments of the USSR and Japan withheld information from their populations and the world until late in the disaster.