How Not to Destroy a Reactor Building
A lesson from Fukushima
– Part 4 of a 6-part series –
Part 3 of this series explored the vastly smaller land footprint required for nuclear fuel storage compared to the massive land and environmental impacts from alternative energy sources. I also discussed how properly managed nuclear fuel poses no harm to people or our environment – neither today nor hundreds of years from now. And a major point was made: nuclear fuel cannot explode. What, then, caused the explosions at the Fukushima Daiichi Nuclear Power Plant on March 12 and 15, 2011?
Hydrogen gas. It’s highly flammable and will readily explode when confined and concentrated. In that state, all it needs is air and a relatively low heat source to combust. A 2007 hydrogen explosion at an Ohio coal plant killed one and injured 10. An April 2020 hydrogen explosion at a gas production facility in North Carolina damaged 60 nearby homes. To get an up-close sense of hydrogen’s volatility, watch this September 2020 video of a street celebration in India.
To set the stage for hydrogen’s explosive role at Fukushima, let’s look at three confinement structures at the plant – all designed to hinder release of radioactive material.
To set the stage for hydrogen’s explosive role at Fukushima, let’s look at three containment structures at the plant – all designed to hinder release of radioactive material.
1. Reactor vessels (the brown cylinder at the center of the graphic).
At Fukushima, between 31 and 43 million fingertip-sized metal pellets (depending on the specific reactor) of low-enriched uranium fuel, and their assemblies, rested inside steel reactor pressure vessels. These vessels have six-inch thick walls, and are about 70 feet high and 17 feet in diameter. The stacked uranium pellets within them react with each other to release kinetic energy, or heat, which boils the surrounding water into high-pressure steam that turns turbines that, in turn, generate massive amounts of electricity.
2. Primary containment (the yellow bulb).
The hot reactor vessels are tucked inside three-foot-thick steel and reinforced leaded concrete primary containment structures that are approximately 60 feet in diameter (widest portion) and about 110-feet high. Of note, reactor pressure vessels and primary containments are both void of air, rendering hydrogen completely harmless.
3. Secondary containment (the green exterior).
Primary containments are surrounded by otherwise commonly constructed, square, eight-floor reactor buildings, within which employees accomplish the majority of their maintenance work.
As I stated in a 2013 op-ed on the safety and value of the Columbia nuclear plant in Washington, all six reactors at Fukushima survived the most powerful earthquake known to have ever hit Japan. The problems began when the nearly 50-foot tsunami knocked out emergency back-up power to three of the operating reactors. Without power, the ever-flowing water that completely covers and cools nuclear fuel to under 700°F (before providing high-pressure steam to turn the turbines) ceased to flow. Although some mitigation measures were taken, in the end the stagnant water remaining inside the vessels began to boil away, exposing fuel cores that heated up to more than 1,500°F.
The World Health Organization stated that the highest recorded dose rate outside the Fukushima facilities is unlikely to have an adverse impact on health.
The fuel and their supporting steel assemblies literally melted through the bottom of the reactor vessels – hence the term “melt down.” They dropped as enmeshed molten blobs of metal to the bottom of their respective primary containment structures – where they remain today.
In the process, the chemical reaction between the steam, melting zirconium rods (that hold the fuel pellets) and other metals generated hydrogen. The newly created hydrogen contributed to rising steam pressure within primary containments. To prevent containment damage, the steam was released – along with hydrogen and radioactive material – into the reactor buildings (secondary containment).
At Units 1 and 3, hydrogen and other gases were now in contact with air, yet trapped within their respective reactor buildings. (A dislodged exterior panel at Unit 2 allowed gases – and fission products – to escape that building.) Without electrical power to further scrub, filter and release these gases to outside atmosphere, they remained confined to the warm-air environment of secondary containment. All the while, hydrogen concentration increased. Although mobile generators eventually allowed employees to start filtering and releasing these trapped gases, it didn’t happen fast enough. Worse yet and unbeknownst to all, gases were accumulating in the relatively problem-free Unit 4 building, likely in part or whole from pipes interconnected with Unit 3.
Boom. Times three.
The NRC task force concluded that had Japan established similar post-9/11 measures, the Fukushima disaster would very likely have been avoided.
Hydrogen-air explosions blasted the roof of Unit 3 nearly 1,000 feet skyward, similarly devastated Units 1 and 4, and, in the process, spread radioactive particles throughout the countryside. Surprisingly, and according to the World Nuclear Association, the main source of radioactive release into the environment came from the building that didn’t explode – Unit 2 – from water leaking from primary containment. The primary containment structures in Units 1, 3 and 4 remain intact.
The World Health Organization stated that the highest recorded dose rate outside the Fukushima facilities is unlikely to have an adverse impact on health. Yet, as discussed in "The Godzilla Effect," the issue of radiation remains controversial with the general public, and many evacuees experience psychological stress over the unfounded fear of delayed or genetic radiation risks. As discussed in "The Myth of the Cows," Japanese authorities have therefore chosen the most conservative approach by restricting access to the area.
U.S. Industry Response
The U.S. Nuclear Regulatory Commission has since required, and operators have installed, passive (no power required) vent pipes on all U.S. boiling water reactors to ensure that hydrogen and other gases, in the event of an emergency, can safely pass from containment through radionuclide-scrubbing filters directly to the outside atmosphere. The NRC also inspected all U.S. nuclear plants soon after Fukushima to verify that resources and procedures established after the 9/11 terrorist attacks are properly maintained and exercised (they are). The NRC task force concluded that had Japan established similar post-9/11 measures, the Fukushima disaster would very likely have been avoided.
Nevertheless, the U.S. industry invested billions of dollars to establish two national response centers – one in Phoenix (which I’ve toured) and another in Memphis – to further ensure America never experiences a Fukushima-type disaster. The facilities are supported by dedicated ground and air transport capability to move equipment from either center to any nuclear plant in the country within 24 hours. The sizeable investment has had a negligible impact, however, on the affordable cost of nuclear energy due to the remarkably low price of relatively abundant nuclear fuel.
Pump, treat and release
Efforts to remove the fuel and debris from Fukushima’s primary containment structures were to begin this year. As with most everything, the COVID-19 pandemic altered those plans. Debris removal is now scheduled to begin next year.
Site remediation, however, requires working space currently occupied by vast fields of tanks holding more than 300 million gallons of treated water. Treating the water is not an option, but storing it is.
Fukushima’s reactor buildings and turbine building basements – all radioactive areas – are below sea level. Groundwater is seeping into them and, more worrisome and difficult, into underground wiring and piping areas that connect the buildings, becoming contaminated in the process.
Water must also be circulated through primary containments to continually cool the melted fuel debris. Workers pump the resulting contaminated water out of the ground and buildings at a rate of more than 45,000 gallons a day. The water passes through radionuclide-capturing filters before storage in aboveground tanks – some of which have leaked since day one. Fortunately, the principal radioactive element remaining in the otherwise filtered water is tritium, which is harmless in small quantities. Remaining radioactive elements are also too sparsely concentrated to harm people or marine life.
Japan’s latest solution is to pump it into the ocean (as other nuclear facilities normally do) over the next decade with no impact to the environment or the food chain.
Stay tuned as we notch down the turmoil to explore the reality of Three Mile Island in Part 5 of this series.