Fukushima: Lessons Learned (Part Four)

Explosions

As March 12th dawned the reactors continued out of control with the plant largely reliant on the built-in design safeguards to cope with surging pressures and temperatures.  There was a vital need for electricity so that operating parameters could be observed and maintained, if possible.  In addition, aligning valving for water injection or controlled release would require electricity or compressed air in many cases. Portable generators had been located and were arriving at the plant by barely passable roads as they were too heavy for helicopter transfer.

The first generator to arrive was positioned near Reactor 2.  A lengthy and nearly 1-ton electrical cable system was being hand positioned by 40 workers when a violent explosion occurred.  The cause was probably a build-up of hydrogen gas and the blast injured five workers and wrecked the cable and generator and other equipment being used in the cooling effort.  The area was strewn with highly radioactive debris.  Everyone was evacuated to the Emergency Response Center to re-group and for accountability.

Twenty-four hours into the incident all three active reactors were unstabilized and the effort to control them had suffered a serious setback.

On March 12 the decision was made to vent one of the reactors to the atmosphere as a way to create conditions favorable for fire apparatus to attempt to inject cooling water into the reactors in order to keep the fuel covered.  The high pressures in the containment system were above the fire pump discharge pressure capability making water injection impossible.  System pressures would have to be lowered in order to facilitate cooling. (At one point pressures were over twice the design limit of the container.) Venting was accomplished by an arduous and complicated process of aligning valves, some manually. It was done after close-by civilian evacuation and shelter-in-place farther from the plant was completed.

As previously mentioned, the quake and the tsunami had essentially destroyed the facility radiation monitoring capability.  In fact, personnel were forced to share dosimetry and to try to track exposure accordingly.  After the explosion and venting it became clear that parts of the facility were highly contaminated.

When teams were sent out to attempt to align valve systems for venting or cooling their work times were limited, in some cases to just 17 minutes, to keep exposure to acceptable emergency levels.  Volunteers were employed for the most hazardous work.  Several workers had already received over 10 rems and the radiation levels in some areas were as high as 16,000 mrems per hour.  Personal protective equipment (PPE) included the use of firefighter turnout gear and self-contained breathing apparatus.

Workers slept on the floor and rations were limited to biscuits and soup.  Control room workers were forced to wear PPE in the control room environment and some received significant exposures as the incident escalated.  (Some of the most severe exposures, up to 67.8 rem, were to control room workers and over 100 workers received exposures in excess of 10 rem.)

At 11AM on March 13th, about 63 hours into the incident, a second explosion occurred, again, from hydrogen being liberated, this time in Reactor 3.  The explosion damaged the fire apparatus being used in the operation to control containment pressure in Reactor 2.  It also spread more contaminated debris creating an environment where it was now unsafe to staff the remaining fire apparatus being used for coolant operations.  The engines were left to run unattended. In addition, there was a stop in operational activity as workers were again assembled in the Emergency Response Center for accountability.

Shortly after midnight on March 15th, there was yet another explosion in the Reactor 4 building.  Reactor 4 was not operational at the time of the event and it is thought that the explosion, probably hydrogen induced, was caused by a backflow of gas from shared piping coming from Reactor 3 when valves failed in the open position after the loss of electrical power.

Tomorrow:  Conclusion

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