Both the TMI and Chernobyl nuclear facilities, which experienced core meltdown, suffered hydrogen explosions.Let us compare their details with the explosions at Fukushima.

First, the time of explosion.There is a big difference.The explosion at Chernobyl occurred within several minutes of the nuclear accident.In contrast, the hydrogen explosion did not occur until around ten hours later at TMI, and one whole day later at Fukushima Unit 1.It took three days before Fukushima Unit 3 experienced the explosion.The time lag represents the difference of time that took these facilities to suffer core meltdown.

Zircaloy, used in fuel cladding, reacts with water at a high temperature to generate hydrogen and heat.When this reaction intensifies, the resulting heat melts down the reactor core.The rapidly increased amount of hydrogen produced in the reactor buildings that exploded was proof that the reactor core had melted down.

At TMI, the explosion occurred inside the containment vessel.The amount of hydrogen associated with the explosion indicated that 50-60% of fuel cladding had reacted, about the same as the analyzed ratio of reaction as that in Fukushima.The explosion at TMI did not have any external impact because of the large capacity of the containment vessel, a characteristic of PWR.

There is universal fear of radiation spreading in a hydrogen explosion.However, it was not until three days after the explosion that the state governor, forced by the voices of concerned residents, advised voluntary evacuation.By then, the explosion had long ended. So ironical.

In fact, plant operators heard the sound of an explosion but dismissed it as the sound of a ventilation damper slamming shut.This misunderstanding led to the evacuation of some 160,000 people.

The explosions at Chernobyl occurred at several locations on the site. This was because pipes ruptured at various places.The biggest explosion happened right atop the reactor.The upper shield measuring 17 meters in diameter and weighing 1,600 tons rotated in mid-air right above the reactor.

This exposed the top surface of the reactor to the atmosphere, igniting the graphite in the core.The fire sent radioactive materials high and into the jet stream, causing extensive contamination across the globe.The explosions at Chernobyl contributed significantly to the expansion of the nuclear disaster, which is a major difference from the TMI accident.It is difficult to determine where the Fukushima accident stands in this regard.

Because BWR has a small containment vessel, it is filled with nitrogen gas to avoid explosion.This is why the explosion at Fukushima occurred outside the containment vessels but within the reactor buildings where oxygen was present. Since the explosions happened outside the containment vessels, they had little impact on reactor facilities inside and the discharge of radiation.Yet, this conclusion only applies to Unit 1.

The explosion at Unit 1 happened when on-site workers were connecting a power vehicle to the small part of the power panel that survived the tsunami at Unit 2, and finishing up emergency work to arrange water injection into Units 2 and 3.The blast instantly damaged the power vehicle and hoses.If it had not been for the Unit 1 explosion, Unit 2 and Unit 3 could have avoided their core meltdown.The verdict on this matter should be left up to the Investigation Committee on the Accident.

The Unit 1 explosion blew off the blowout panels of the adjacent Unit 2, leaving holes on the reactor building’s exterior walls.Hydrogen at Unit 2 escaped into the outer atmosphere through these holes.This is why, despite core meltdown, Unit 2 escaped an explosion.In this sense, Unit 2 just barely averted the worst.

BWR has a small containment vessel.To overcome this weakness, it has a semi-sealed reactor building that acts as the 5th containment wall.Even though the reactor building was built very solidly, it turned out to be detrimental in this case.

The more solid the structure was, the more it made the explosions fierce.The video footage shows that the blast destroyed the building's beams and pillars at Unit 3.It was indeed a massive explosion.If the structure had at least some windows, the explosion could have been as minor as a few broken windowpanes.

The sturdier the structure was, the more the contained hydrogen raised the internal pressure, which caused a reverse flow of hydrogen into the adjacent unit to cause an explosion.The reverse flow occurred at the joint of exhaust ducting.The explosion at Unit 4, which was in shut down at the time, was by-blow from Unit 3's blast.

As we have examined, hydrogen explosions come in various forms.Be it a case of a by-blow or a lucky escape, the slightest twist of fate could lead to a fortunate or unfortunate result.It is just how life goes.

he best approach to preventing hydrogen explosion is to have large ventilation holes, which would also create an opening that allows outside-the-box approaches.