Fukushima

Sharing a chapter of my book, Critical Thinking; What a Senior Reactor Operator Wants You to Know About Nuclear Power.

The Great Tōhoku Earthquake of 2011 was the fourth most powerful earthquake ever recorded, measuring a staggering magnitude 9.0. The quake moved the island of Honshu 8 feet to the east and caused 19,759 deaths, with over 2500 people still listed as missing.
What the earthquake did not do was cause the meltdowns at the Fukushima Daiichi nuclear power plant on Japan’s east coast. The site experienced shaking beyond the design of the units there, and they all automatically tripped as designed for large earthquakes. However, nuclear plants in Japan are anchored into solid rock and are designed with significant margin to actual limits during seismic events. There are no indications that any of the main safety functions of the plants were disabled by the earthquake itself.
The earthquake did disrupt offsite power to the station. As with all nuclear plants, the Fukushima plant had several emergency diesel generators designed to start on a loss of power from the offsite grid and power the station’s emergency buses. The emergency diesel generators worked as expected following the earthquake.
Indications for the reactors all appeared normal following the trips caused by the earthquakes. There was no sign that the earthquake had caused any kind of leak or break in the RCS. Had the event ended here, nothing bad would have happened, and no one outside of Japan would know about the nuclear power plant at Fukushima Daiichi.
The earthquake occurred along a subduction zone where the Pacific plate dives beneath northeastern Japan. As the plates suddenly broke free and snapped into their new locations, sections of the seafloor suddenly rose several meters. This sudden upward thrust shoved the water above it, sending tsunami waves radiating outwards.
The largest wave was approximately 130 feet high and hit Iwate Prefecture to the north of Fukushima. The first tsunami hit the plant approximately 40 minutes after the initial earthquake and was only 13 feet high, smaller than the design basis wave of 18.7 feet. The 19-foot breakwater easily handled the first wave.
The next wave was at least 25 feet high, but the exact height is unknown as the tide gauge fails at that level. The highest wave that hit the site was approximately 50 feet high based on water marks on the buildings at the site. Needless to say, the 19-foot seawall did little to protect the site against these waves.
The flooding from the tsunamis did what the earthquake could not. The intake structure where cooling water was taken from the Pacific and pumped into the station was damaged to the point of uselessness. The turbine and reactor buildings flooded. The emergency diesel generators tripped off, either due to being flooded themselves (the EDG rooms were in the turbine buildings and were built below grade) or from a loss of cooling. At this point, the Fukushima Daiichi power station was experiencing a station blackout. Two operators died in the flooding.
As discussed earlier, nuclear power plants require electricity to run the safety equipment needed to protect the core. This usually comes from the power grid (destroyed by the earthquake) or from the emergency diesel generators (destroyed by the tsunami). Without electrical power, the pumps that remove the decay heat do nothing, and the heat begins to build up.
A small portion of the electrical system survived on Unit 6 (there were 6 reactors at Fukushima). Using a recently installed air-cooled diesel and cross-ties between the units, operators in Units 5 and 6 were able to reestablish power and cooling. Units 5 and 6 were safely decommissioned in 2014.
Unit 1 lost core cooling almost immediately. Estimates put fuel uncovery at 3 hours after the earthquake. For nearly 15 hours following the tsunami, there was no injection flow into the Unit 1 reactor. Nearly all of the fuel melted and fell to the bottom of the reactor vessel. (This mixture of fuel and control rods and other reactor internals is referred to as corium.) Core cooling was eventually reestablished by using a fire truck to pump seawater through the reactor.
Unit 2 lost core cooling approximately 70 hours after the earthquake. The reactor core isolation cooling system (RCIC) had so far functioned remarkably well with no power available to it and no operator interaction. RCIC runs off a steam turbine, with the reactor itself providing the steam. (Fukushima was a boiling water reactor, so no steam generators. The reactor is the steam generator. I know it’s weird. We’ll talk later.) There are several theories as to why RCIC stopped working 3 days into the event, but the results were predictable. The core in Unit 2 had no cooling for approximately 7 hours, and most of the fuel melted and fell to the bottom of the vessel.
Unit 3 lost cooling 36 hours after the earthquake. High-pressure coolant injection (HPCI) tripped off, and operators were unable to restart it. Reactor pressure was too high for injection using fire trucks. It took operators 7 hours to open a valve to relieve the pressure and restore injection flow. Injection flow was again lost 3 hours later when they ran out of fresh water, and attempts to switch over to seawater were delayed due to an aftershock and evacuation order. Injection flow was restored an hour later; however, the water level in the reactor was well below the top of the active fuel, and core damage had already occurred. They would lose injection flow at least one more time. All fuel in Unit 3 melted and fell to the bottom of the vessel.
The conditions facing the operators at Fukushima were more extreme than at any other time than at Chornobyl. Radiation levels in the control rooms rose to as high as 1.2 rem/hr. To put that into perspective, any area with a dose rate that high in a power plant in the US is locked and requires special permission to access. Two control room operators received total doses of over 65 rem. I have received less than 1 rem in my entire career.
Conditions outside the control rooms were worse. Following hydrogen explosions from Units 1 and 3, radioactive debris was thrown around the area. Some of it had dose rates as high as 1000 rem/hr. During venting of Unit 3 containment, a dose rate of over 16000 rem/hr was recorded. A dose of 500 rem in a short period of time is generally considered fatal.
The operators showed significant ingenuity in their efforts to combat the event. With the loss of the onsite power systems, they lost most of their ability to see and control the power plant. To address this, they sent workers out to the parking lot to take batteries out of cars. By stringing together these batteries, they were able to restore some indications and operate some equipment, such as the relief valves that lowered reactor pressure to allow for seawater injection.
This all took place while the rest of Japan was dealing with one of the worst natural disasters in its history. Outside help was extremely limited and took forever to reach the site. Roads were destroyed, airports were shut down, and emergency services were overwhelmed. The disaster at the nuclear station was only part of a much larger ordeal.
The main cause of the Fukushima nuclear disaster was the design of the plants. The original estimate of the largest possible earthquake in the Japan trench system was a magnitude 8. This was never fully reassessed, despite multiple magnitude 9 earthquakes occurring in similar subduction zones. The design tsunami for the site of 3.1 meters was based on the maximum wave height observed following the 1960 earthquake in Chile. Due to the lack of data on earthquakes and tsunamis generated by the Japan Trench, these possibilities were not included in the site’s flooding design.
In the years leading up to the 2011 earthquake, the operator of Fukushima, TEPCO, had performed other analyses on possible tsunamis at the site. One analysis had determined that a 15-meter wave was possible. This analysis was under review at the time of the event, and no changes had been made to protect the site against larger waves other than raising the height of the motors of certain pumps. This change made no difference to the accident that occurred.
Had a new seawall been built to protect against a 15-meter wave, or critical equipment been better protected or raised to a higher elevation, the cores at Fukushima would have survived the fourth most powerful earthquake ever recorded with no damage.
In the US, the NRC response to Fukushima required every nuclear station to prepare to respond to beyond design basis events such as what happened at Fukushima. Plants have special FLEX equipment stored in hardened warehouses, kept ready in the event that something happens that can’t be handled with the installed emergency systems at the plant. This includes things like portable diesel generators designed to connect to key electrical buses and construction equipment to move debris.
The industry, through INPO, created two regional staging centers where more of this equipment is kept and has plans in place to deliver it to a stricken site should the need arise.
One surprising takeaway from Fukushima is that no one died from acute exposure to radiation. One man died several years later from lung cancer that the Japanese ministry of Health determined could be linked to his work at Fukushima and ordered his family be paid compensation. Although the Japanese government concluded the worker’s lung cancer was work-related for compensation purposes, establishing that an individual cancer was caused by radiation exposure is scientifically very difficult, particularly for solid cancers with many other potential causes.
Another surprise is how little radiation Fukushima released when compared to Chornobyl, despite three cores melting, whereas only a single core was damaged at Chornobyl. Estimates for radiation released by Chornobyl are around 5300 PBq (P is for peta, or 1 quadrillion, and Becquerels is a measure of radioactivity). Fukushima, by contrast, is estimated to have released approximately a tenth of that, at 520 PBq.
The reason for this difference is the presence of a containment building. Despite the damage that was sustained to the containment buildings at Fukushima, they still kept much of the radioactive material inside, as they are designed to do. Chornobyl was built inside a building more like a modern industrial building than a robust steel and concrete containment structure. When the reactor exploded, this building was destroyed, and the reactor core was exposed to the environment directly, something that never occurred at Fukushima. This is the main reason why so much more radiation was released to the environment in Ukraine and the surrounding areas.
Fukushima is proof that nature can always hit us harder than we can build. We’ll move on to Chornobyl and see what happens when human beings decide to do things they aren’t supposed to with an operating nuclear reactor.

If you like this chapter and are interested in the book, here is the link again:

Critical Thinking; What a Senior Reactor Operator Wants You to Know About Nuclear Power.