‘Vent’ refers to an opening that allows ventilation.It is a technical term and has not made it into the ‘Kojien’ Japanese dictionary as it is not a term commonly used in Japan.The term attracted attention most recently from the nuclear accident, when the failure to control the vent for the containment vessel led to hydrogen explosions.

Breakdown in the cooling function, as in the case of this accident, sends the temperature and pressure in a reactor soaring due to the decay heat at the core, even if the reactor itself manages to shut down.Then, safety valves open to release steam into the containment vessel to prevent the excessive buildup of pressure in the reactor.

The purpose of a containment vessel is to keep in radiation at the time of an accident.It keeps the released steam inside, causing the pressure of the containment vessel buildup.

In an accident of a postulated level, the steam condenses into water due to natural cooling and the effect of cooling equipment, keeping the pressure inside the containment vessel at a moderate level.

However, in the last accident, the station blackout lasted for as long as ten days, surpassing the 8 hours supposed for the design.With cooling systems that operated without power going out of operation and natural cooling reaching its limit, the pressure inside the containment vessel reached around 8atm, making it difficult for the containment vessel to bear the stress.Weak sections could crack or tear to trigger uncontrolled discharge of radiation to the environment.This must not happen.It is a far better option to open the vent and systematically depressurize the containment vessel, with careful monitoring of wind directions.This is the purpose of the emergency vent operation.

Even in emergency, releasing radiation to the outside environment is a task that no one in the nuclear industry is willing to carry out.This was the reason behind procrastination shown by reactor personnel and the government alike.

At Unit 1, by the time the vent valves were opened, it was just one hour before the explosion.Data suggested a rapid depressurization of the containment vessel, causing many to believe that the operation was successful.However, it was the case of too little too late.By then, hydrogen had already leaked and filled the reactor building.It was because the start of operation was delayed and the operation itself took time.

At Unit 2, even though the valves were opened, the rupture disk inserted into the pipe failed to break, hampering vent.It was perhaps due to this fault that, in the early morning of March 15, the containment vessel became damaged, triggering the release of a large amount of radioactive materials into the environment.

At Unit 3, the vent operation was carried out several times on March 13 onwards to successfully depressurize the containment vessel.However, the success, achieved despite lack of sufficient equipment, was maintained only until the supply of DC current ran dry, which shut down the cooling system and initiated core melting.This led to rapid generation of hydrogen, which leaked from the containment vessel to cause an explosion.

Unfortunately, the vent operation, which plant personnel put substantial efforts into during the accident, did not turn out to be effective.It failed to cool the reactor cores or prevent hydrogen explosions.

The transcripts of video conferencing that took place at the time of the accident were made public the other day.They indicated that the power station’s site superintendent was using safety relief valves and vents to depressurize the reactors and injecting water with firefighting pumps.It was, indeed, a major gamble, not mentioned in the plant’s manual.

If that was the aim, why didn’t plant personnel carry out the vent operation without hesitation?Regardless of complaint from the Chair of the Nuclear Safety Commission or visit by the Prime Minister, there was no room for hesitation on the battlefield.

Only God knows what the outcome would have been.Earlier vent and release of hydrogen from the containment vessel could have averted the explosion at Unit 1.That would have changed the subsequent course of events at Unit 2 and Unit 3.We could not regret enough about the initial procrastination.

Despite the explosions, the containment vessels at Unit 1 and Unit 3 mostly maintained their air-tightness, with radiation released via the vents.The vent system channeled gas through an around 3-meter deep pool of water at the bottom of the containment vessel, thereby reducing the amount of released radioactive materials in a wash down effect.However, highly radioactive materials from Unit 2 escaped directly from the damaged parts of the containment vessel, resulting in high radioactive concentration due to no wash down effect.

What is the difference in the amount of released radioactive materials?When the containment vessel at Unit 2 became damaged on March 15, the contamination dose along the perimeters of the power station jumped from several μSv per hour to several hundred μSv per hour.This fact alone indicates that the vent system, passed through water, reduces radioactive concentration by one-hundredths.Instead, the power station inserted a rupture disk.They cannot escape serious blame.

Incidentally, now the amount of cesium released is just 10 million Bq per hour, or one 100-millionth of the worst figure recorded (on March 15 last year).