A Level 7 Incident

After the explosion involving reactor facility 4, the decision was made to evacuate all “non-essential” personnel and 650 workers left the site, leaving just 70 to continue the stabilization work.

In addition to the three reactors, there was also concern that the spent fuel pools might be losing water thus allowing the fuel to overheat.  Additional fire apparatus and helicopters were used, without much effect, in an attempt to keep the pools full.  Eventually, concrete pumping trucks were flown in and used to maintain safe water levels though spent fuel was never in serious danger.

Despite the valiant efforts of plant workers,  the fuel in all three operating reactors was uncovered for significant amounts of time and melted as a result.  In some cases fuel rods were exposed early in the incident and eventually this occurred at all three reactors. The extremely high temperatures associated with the uncovered fuel contributed to the liberation of hydrogen gas which resulted in three separate explosions.  These explosions resulted in much of the area being highly contaminated from debris strewn around the site.

Over 100 plant worker received doses in excess of 10 rem and two received doses over 65 rem.  Capability to monitor plant or personnel radiation levels was lost early in the incident as the fixed system was either swept away in the tsunami or rendered inoperable because of the lack of power.  Contamination was widespread, including at the Emergency Response Center.  (Radiation levels were so high there that the windows had to be covered with lead.)

It is interesting to note that the only method to be successfully employed to inject water into the three failed reactors was using fire apparatus though this was ultimately ineffective and was hampered by pressure differentials and the inability to obtain a constant water supply.  In addition, radiation levels were high enough that apparatus was left to run unattended and at one point ran out of fuel.

The Fukushima disaster is run-through with irony.  The original bluff on which the plant is located was 35 meters high but was reduced to 10 meters in order to position the plant on bedrock to better withstand earthquakes and to reduce the cost of pumping seawater for cooling, thus making it vulnerable to a quake-induced tsunami.  A plant that produced massive amounts of electricity was undone when it lost that very power.  Key back-up systems were so intricately entwined with the systems that were designed to replace that the loss of one meant the loss of the other.

In the most highly regulated industry in the world, rules and procedures were necessarily discarded as humans fought to overcome design limitations and the effects of nature and physics.

Nature won, again.

Sources:

Institute of Nuclear Power Operations

New York Times

Wikipedia

INES

Explosions

As March 12^th 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 13^th, 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 15^th, 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

Flying Blind

The incoming tsunamis, much larger than planned for, had destroyed back-up electrical generation capability when the diesel generators were swamped and electrical switch gear wetted.  Electricity was vital in order to monitor the conditions within the reactor vessels and to take actions to prevent a catastrophic event.

Plant personnel were now faced with an extremely complex set of variables related to the requirement to control the heat associated with the nuclear fuel.  Water was used to control the temperature but the loss of electricity meant that the ability to replenish or circulate that water was severely hampered.  Monitoring the level of the water to ensure that fuel remained covered was difficult and spotty.  The heated water would eventually vaporize into steam creating escalating pressures in the vessels which could lead to an uncontrolled release of radioactive gases to the reactor containment structure.  Since the reactor vessel and containment structures had pressure limitations, failure to control rising pressures caused by escalating temperatures could also cause a catastrophic event.

Complete boiling off or loss of all water surrounding the fuel rods would result in their degradation, eventual melting and the potential release of very high amounts of radiation if the containment systems failed.

A potential also existed for the steam being created by high temperatures to react with zirconium in the vessel material to create large amounts of hydrogen, an extremely explosive gas. Such an explosion would also result in the release of radioactive gases and the potential dissemination of highly radioactive debris across a wide area.

These scenarios were true for each of the three operating reactors and there were concerns, as well, about the integrity of the stored spent fuel cooling systems.

Plant battery systems also failed because of flooding and grounding.  Critical temperatures and pressures were spiking.  Complicating an already complex scenario was the fact that the plant was effectively isolated and a much larger area was attempting to deal with its own overlapping emergencies.  Outside help, if it arrived at all, would be slow in coming.

Workers moved into an improvisation mindset as they attempted to control the reactors through “unorthodox” methods.  They were forced to rely on “creativity” and work experience as they derived solutions that were “unique” and completely outside of design guidance. They swept the administration building for design documents, plans and drawings and created a work center to brainstorm methods for injecting vital cooling water into the reactors. At one point they were reduced to scavenging auto and truck batteries and cables and bringing them to the control rooms in order to supply enough DC power to monitor reactor water levels.

They looked to fire control systems (stationary pumps and fire apparatus) as a means to inject water into the vessels to control temperature.  Neither scenario proved easy.  Of the three fire engines on site, one was damaged by the tsunami; another was blocked by an oil tank and a de-energized security gate, leaving just one for the initial attempt.

The tsunamis had strewn the site with debris, blown off manhole covers and generally created an extremely hazardous working area.  Building interiors were pitch black, often flooded and there was effectively no monitoring of radiation available.

Controlling reactor temperature and pressure would mean finding reliable water sources and successfully locating and manipulating a complicated series of valves using whatever was at hand. Water would have to be introduced and pressure controlled and released.  Crews would be learning as they went as they determined that key valves were either air or electrically activated and that they all must be operated properly in order to achieve the desired result.

Tomorrow:  Part Four, Explosions

Fukushima Nuclear Power Plant

The plant is located on the northeast coast of Japan about 120 miles from Tokyo.  It consists of six (6) GE-designed Boiling Water Reactors.  The steam energy from the boiling water is used to drive turbines which create electricity.  The steams cools and condenses back into water for re-use.  The 860-acre site is some 30 feet above sea level on a bluff overlooking the sea.

Construction planning for the site and reactors included consideration for protection against both earthquakes and tsunamis.  The tsunami from the 1960 Chilean earthquake that measured 10.2 feet when it came ashore nearby was used as a reference guide.

Breakwaters had been installed in the harbor to mitigate tsunami effects. (Many of the purpose built breakwaters were ineffective in the Tohoku quake and have even been implicated in worsening flooding in adjoining areas by re-directing the waves.) In 2002 the inundation protection level was increased to 18.7 feet when some back-up equipment was placed on higher ground. 

In addition, the plant was equipped with Emergency Diesel Generators (EDGs) in order to operate cooling systems for the reactors if AC power was lost. Control rooms for the reactors were created in paired groupings of 1 and 2, 3 and 4, 5 and 6.  They shared some common facilities and travel between the paired control rooms was easy.

The site also included storage for spent nuclear fuel utilizing dry cask storage and a separate area where fuel was stored in water-filled pools.

The severe shaking associated with the quake damaged the electrical infrastructure on-site and caused the collapse of electrical transmission towers.  It also exceeded the built-in vibration parameters and resulted in the three on-line reactors going into emergency mode with control rods fully inserted and effectively shutting down.  The diesel generators activated in order to allow for cooling to continue and to ensure that the fuel rods remained covered with adequate water. Any anomalies noticed in the reactor shutdowns were monitored and solved.  The heat produced in this mode was less than 10% of normal reactor operation but still enough to be of significant concern.

The quake struck at 1446 Japan time and the first of the tsunamis came ashore at 1527. These tsunamis, one over 45 feet high and well above any plant protection features, rolled over the harbor breakwaters and in and through the plant, inundating reactor buildings and flooding basements and other key areas.

The surging water knocked out all of the diesel generators except one, immersed electrical switch gear and panels causing shorting and grounding, and critically disabled the control rooms for the three operating reactors.

The ability to control the reactors began to degrade rapidly as AC and DC power faded and the control rooms, nerve centers for protecting the reactor cores, went completely dark.

Tomorrow:  Part Three, Flying Blind

Today begins a five-part series on the events which unfolded at the Fukushima Daiichi Nuclear Power Plant as workers tried to simultaneously stabilize three out-of-control reactors after the deadly earthquake.  Fukushima will surely stand as one of the most complex emergency events ever as technicians improvised solutions in an attempt to counteract the run-away physics created by a total loss of power within the facility.  Without electrical power they lost all systems used to monitor and control the temperature and pressure of the reactors, containment vessels and fuel.

This past March 11^th at just before 3PM, the largest earthquake ever to strike Japan occurred 43 miles east of Tohoku in the ocean where the Pacific plate is sliding under the Japanese Island of Honshu.  The Tohoku mega-quake is the fourth or fifth largest quake on record at 9.0 and shifted the earth on its axis between 4 to 10 inches.  The stupendous amount of energy released in the earthquake which lasted over three minutes is estimated to be 600,000,000 times that of the atomic bomb detonated at Hiroshima.

At least 15,833 persons are confirmed dead and thousands more remain missing.  500,000 buildings were damaged or destroyed and significant parts of the infrastructure, including Sendai Airport were severely damaged.

The devastating quake and the chaos that it caused was in many cases dwarfed by the extraordinary damage coming from a series of seven tsunamis which struck in some places 30 to 45 minutes later.  These tsunamis ranged from 10 to 128 feet in height, inundating huge areas of land and literally wiping out entire towns and villages in their wake.  Satellite photos taken before and after the waves show the immense carnage.

In the hours after the quake and the tsunamis, the country was reeling as strong aftershocks continued to occur.  With utter devastation in evidence it could be said that the worst was over, but in one place it was just about to begin.

Tomorrow:  Part Two, Fukushima Nuclear Power Plant