The analysis of Unit 2 yielded even more mysteries. During the actual accident, Unit 2 was initially a success story compared to its siblings. Even though it lost battery power, the reactor’s RCIC system operated for nearly three days. Under conventional modeling assumptions, it would have failed around an hour after the eight-hour batteries ran out. Therefore, the analysts had to force the RCIC system to keep operating under abnormal conditions in their models, even though they didn’t understand why. However, they also didn’t know how well it had worked. After the RCIC system failed, workers took several hours to start seawater injection; by then, core damage was well under way. As with Unit 3, there was so much uncertainty about the amount of water that reached the core that no simulation could predict with any degree of confidence how much fuel was damaged and whether it melted through the vessel.
Also unexplained was the fact that, although Unit 2’s RCIC system was transferring heat from the core to the torus, the measured pressure within the torus was not increasing as much as it should have been. In fact, some of the models had to assume there was a hole in the torus just to make sense of the data. This led to speculation that there had indeed been a hole in the torus from early on in the accident or that the torus room had flooded with water from the tsunami, which helped to cool it down.
If it was a hole, it would have had to have formed well before workers heard the mysterious noise at Unit 2 on the morning of March 15 that made them think an explosion had damaged the torus. That was the only way to explain the pressure data. In any event, TEPCO later concluded that the noise didn’t originate at Unit 2 at all, but was an echo of the explosion at Unit 4 that occurred at approximately the same time. The rapid pressure drop that instruments recorded in the Unit 2 torus was attributed by TEPCO to instrument failure. A remote inspection made months after the accident did not reveal any visible damage to the torus.
So Unit 2 was apparently the only unit operating at the time of the earthquake that did not experience a violent hydrogen explosion. One possible reason for this is in the “silver lining” category: the opening created in the Unit 2 reactor building by the Unit 1 explosion prevented the buildup of an explosive concentration of hydrogen gas.
As for the Unit 4 explosion, most evidence indicates that the culprit was not hydrogen released from damaged fuel in the spent fuel pool. In fact, the pool was never as seriously in distress as the NRC, TEPCO, and many people around the world feared.
Inspections of the spent fuel in the pool using remote cameras did not see the kind of damage that would have been apparent if any fuel assemblies had overheated and caught fire. And samples of water from the pool did not reflect the radioactivity concentrations that would accompany damaged fuel. TEPCO was so sure of this finding that it pursued another theory for the explosion: hydrogen had leaked into the Unit 4 building during the venting operations in Unit 3 next door.
Like Units 1 and 2, Units 3 and 4 shared a stack for gas exhaust, and some of the piping from the two units was interconnected. Although closed valves ordinarily would have kept one reactor’s exhaust from getting into the other, the valves were not operating properly as a result of the blackout. TEPCO found further evidence for this theory when it measured higher radiation levels on the downstream side of the gas filters at Unit 4 than on the upstream side, indicating that radioactive gas flowed from the outside of the building to the inside.
This didn’t mean, however, that the Unit 4 pool was never in danger. TEPCO believed that the pool had lost up to ten feet of water early in the accident as the result of sloshing or some other unknown mechanism, causing the water temperature to shoot up toward the boiling point. Fortuitously, there was a second large pool of water—the reactor well—sitting on top of the reactor vessel. The reactor well is connected to the spent fuel pool through a gate and is filled during refueling to keep fuel rods submerged at all times. When the water in the spent fuel pool dropped, water flowed from the well into the pool, buying time until an effective external water supply was established.
The Unit 2 spent fuel pool may not have been as lucky. It was not one of the pools that commanded much attention during the early days of the accident. Yet when sampling of the pool water was conducted in April 2011, higher than expected levels of radioactivity were detected. Even more startling was a relative absence of iodine-131 in the pool water compared to the amount of cesium-137. If the radioactivity had originated in the reactor cores, more iodine-131 would have been detected. This led officials in the White House Office of Science and Technology Policy to conclude that there was either mechanical or thermal damage to some of the spent fuel in the pool.
Given all the uncertainty, there is little wonder that analysts still do not know exactly how radioactivity was released into the environment, when it was released, and where it came from. There were multiple and sometimes overlapping periods of radioactive releases from the different units.
Most experts agree that large releases on March 14 and 15, coupled with precipitation, were ultimately responsible for the extensive area of contamination stretching northwest from the plant to Iitate village. TEPCO has argued that the venting operations did not contribute significantly to radiation releases and therefore that the reactor toruses were effective in scrubbing fission products from the steam that was vented. The company claims that Unit 2 was the source of the largest release, coming not from the torus through a hole that may or may not exist but from the drywell, which underwent a rapid drop in pressure during the day on March 15. In this view, even the reactor building explosions at Units 1 and 3, as dramatic as they appeared, did not contribute as much to off-site releases, mainly because the buildings could do little to contain radiation even when they were intact.
However, other analysts have looked at the same data and concluded that the venting did cause large releases and that scrubbing in the torus was not effective. This is a crucial technical issue for the U.S. debate over whether filters should be installed on the hardened vents at Mark I and Mark II BWRs. Until forensic investigations narrow down the various possibilities, though, all of these claims remain in the realm of speculation.
Ultimately, based on off-site measurements and meteorological data, it appears that Fukushima Daiichi Units 1 through 3, on average, released to the atmosphere less than 10 percent of the radioactive iodine and cesium that the three cores contained. That would be generally consistent with the results from computer modeling of station blackouts in studies like SOARCA. But there are so many unexplainable features of the accident right now that the similarity in results may be mere coincidence.
What is clear is that, in terms of the amount of radiation released, the Fukushima Daiichi accident was far from a worst-case event. This meant that the direst scenarios that the National Atmospheric Release Advisory Center (NARAC) estimated for Tokyo and parts of the United States, based on much higher radiation releases, never occurred. Fukushima will not challenge Chernobyl’s ranking as the world’s worst nuclear plant accident in terms of radioactive release, although it will remain classified a level 7 accident by the IAEA.
The difficulties analysts have explaining what happened in 2011 at Fukushima are only compounded when they use the same computer models to predict the future. In other words, when computer models cannot fully explain yesterday’s accident, they cannot accurately simulate tomorrow’s accident. Yet the nuclear establishment continues to place ever-greater reliance on these codes to develop safety strategies and cost-benefit analyses.
GLOSSARY
AC (alternating current) power: The most common form of electrical power, such as that provided to household appliances plugged into wall outlets. Operating nuclear power plants supply AC power to an electrical grid for use by residential and industrial customers.
Advisory Committee on Reactor Safeguards: An independent committee that advises the U.S. NRC. The committee is made up of experts in a variety of fields and is responsible for reviewing NRC safety studies, license applications, a
nd advising on proposed standards.
B.5.b: Designation for orders issued by the NRC after the September 2001 terrorist attacks, requiring additional safety equipment to be available as backup in the event of a fire or explosion caused by an airplane crashing into a nuclear facility. B.5.b is the title of a section within the NRC’s Order for Interim Safeguards and Security Compensatory Measures of February 2002.
beyond-design-basis accidents: Also called “severe” accidents. Accident sequences that are possible but that were not fully considered in the reactor design process because they were judged to be too unlikely (see design basis).
boiling water reactor (BWR): A nuclear power reactor design in which water flows upward through the core, where it is heated and allowed to boil in the reactor vessel. The resulting steam then drives turbines, which activate generators to produce AC power.
cold shutdown: A reactor coolant system at atmospheric pressure and at temperature below 200 degrees Fahrenheit following a reactor cooldown.
common-mode failure: Multiple failures of structures, systems, or components as a result of a single phenomenon.
containment building: A steel-reinforced concrete structure that encloses a nuclear reactor. It is intended to minimize the release of radiation in the event of a design-basis accident. In a boiling water reactor, the containment consists of two parts: the drywell and the wetwell.
control rod: A rod that is used to control power output by controlling the nuclear chain reaction rate inside a reactor. This rod contains material that easily absorbs neutrons and can suppress nuclear fission when inserted between the fuel assemblies. In an emergency, all control rods fully insert into the reactor core to stop the nuclear chain reaction.
core: See reactor core.
cost-benefit analysis: A systematic economic evaluation of the positive effects (benefits) and negative effects (non-benefits, including monetary costs) of undertaking an action.
criticality: The normal operating condition of a reactor where the nuclear chain reaction is sustained. A reactor achieves criticality (and is said to be critical) when each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions.
DC (direct current) power: The electrical current produced by batteries and from inverters that transform AC power to DC power.
decay heat: When an atomic nucleus fissions, two smaller nuclei are commonly formed. Many of these fission by-products are unstable and release radiation seeking to become stable. These radioactive emissions generate thermal energy called decay heat. Even after a reactor core is shut down and its nuclear chain reaction stopped, it continues to generate substantial amounts of decay heat that can cause fuel damage from overheating if not removed at a sufficiently high rate.
defense-in-depth: Multiple independent and redundant layers of defense to compensate for potential human and mechanical failures so that no single layer is exclusively relied upon. For example, the array of emergency pumps installed to cool the reactor core during an accident, the containment building that minimizes radiation escaping from a damaged reactor core, and the emergency plans that evacuate people in event of a radiation release are each defense-in-depth layers.
design basis: The range of conditions and events taken explicitly into account in the design of a facility, according to established criteria, so that the facility can withstand them without exceeding authorized limits by the planned operation of safety systems.
dose: A measure of the energy deposited by radiation in a target.
dosimeter: A small portable instrument used to measure and record the total accumulated dose of ionizing radiation.
dry cask storage: A passive means of storing reactor fuel that provides radiation shielding. After several years in a spent fuel pool, fuel can be transferred to a cask, typically made of steel, concrete and other materials. Once in the casks, the fuel is cooled by natural airflow.
drywell: In a boiling water reactor with a Mark I containment, the drywell houses the reactor vessel. It resembles an inverted incandescent lightbulb and is made of a steel shell surrounded by steel-reinforced concrete.
emergency core cooling systems (ECCS): Reactor system components (pumps, valves, heat exchangers, tanks, and piping) specifically designed to remove decay heat from the reactor core in the event of a failure that drains or leaks the normal cooling water.
emergency diesel generators: Equipment permanently installed at nuclear plant sites that burns diesel fuel oil to generate AC power to supply emergency core cooling systems and other emergency equipment when off-site power is unavailable.
emergency planning zone: An area of approximately ten-mile radius surrounding a nuclear power plant where the principal exposure sources would be whole body external exposure to gamma radiation from the plume and deposited material, and inhalation exposure from the passing radioactive plume.
external event: Potentially damaging events originating from outside a nuclear plant site or outside of safety-related buildings. Typical examples of external events for nuclear facilities include earthquakes, tornadoes, tsunamis, and aircraft crashes. The NRC also considers plant fires to be “external events” even if they originate within plant buildings.
feed and bleed: A process in which makeup water is added to a reactor vessel when the closed-loop cooling system is malfunctioning. The makeup water absorbs heat given off by the nuclear fuel and is allowed to boil, or “bleed,” away.
FLEX: Short for “diverse and flexible mitigation capability,” the program was devised by the U.S. nuclear industry in the months following the Fukushima Daiichi accident. It comprises a variety of portable equipment that can be rapidly installed in or deployed to a nuclear facility in the event of an accident or natural disaster to provide backup electrical and cooling systems.
fuel assembly: A set of fuel rods loaded into and subsequently removed from a reactor core as a single unit.
fuel pellet: In light-water reactors, a thimble-sized ceramic cylinder (approximately ⅜-inch in diameter and ⅝-inch in length), consisting of uranium (typically uranium oxide, UO2), which has been enriched to increase the concentration of uranium-235 (U-235). Reactor cores may contain up to 10 million pellets, stacked in the fuel rods and arranged in fuel assemblies.
fuel rod: A hollow tube more than twelve feet long made from a metal alloy containing zirconium that is filled with fuel pellets and included in a fuel assembly.
hardened vent: A metal pipe designed to withstand the higher pressure that may occur inside containment during an accident that can be used to discharge (vent) the containment atmosphere to an elevated point. The normal ventilation system for lower pressures uses sheet-metal ducts similar to those used in homes and offices.
heat sink (also ultimate heat sink): The nearby river, ocean, or lake used to cool the nuclear plant. During normal operation, one unit of electricity is generated and two units of waste heat are rejected to the heat sink for every three units of thermal power produced by the reactor core. During accidents after a successful scram, a smaller amount of water from the ultimate heat sink is required to cool the reactor core and emergency equipment.
high-pressure coolant injection (HPCI) system: Part of the emergency core cooling systems. HPCI is designed to inject substantial quantities of water into the reactor while it is at high pressure. Like the reactor core isolation cooling system (RCIC), it can run without AC power as long as DC power is available. HPCI uses steam produced by the reactor core’s decay heat to spin a turbine connected to a pump.
hypocenter: The point of the earth’s crust where a rupture initiates, creating an earthquake.
International Nuclear and Radiological Event Scale (INES): A scale developed by the International Atomic Energy Agency to communicate in a consistent way the safety significance of nuclear and radiological events. Events are classified on a scale with seven levels, ranging from level 1 (an “anomaly”) with little danger to the general population, to a level 7 (a “major accide
nt”) with a large release of radioactive materials and widespread health and environmental effects. The Chernobyl and Fukushima nuclear accidents were both level 7 accidents.
MELCOR: A computer code developed for the NRC by Sandia National Laboratories. It models the progression of severe accidents in boiling water and pressurized water reactors.
meltdown: Large-scale melting of nuclear fuel rods in the reactor core.
millirem: One thousandth of a rem (0.001 rem). A measure of radiation dose. (See rem.)
millisievert: One thousandth of a sievert (0.001 sievert). A measure of radiation dose (see sievert).
Nuclear Regulatory Commission, U.S. (NRC): An independent agency created by the Energy Reorganization Act of 1974, replacing part of the former Atomic Energy Commission (AEC). The NRC is made up of five commissioners appointed by the president, who serve staggered, five-year terms. One member serves as chairman and acts as principal executive officer. The commission formulates policies, develops regulations governing nuclear reactor and nuclear material safety, issues licenses and is responsible for overseeing reactor and nuclear material safety.
off-site power: Power supplied to a power station via transmission lines from the electrical power grid.
operating basis earthquake: The earthquake that, considering the regional and local geology and seismology, and specific characteristic of local subsurface material, could reasonably be expected to affect a nuclear plant during its operating life.
potassium iodide: A compound that provides protection to the thyroid from exposure to radioactive iodine-131, which is one of the fission products that can be released in a meltdown. Potassium iodide would not give any protection against any other radioactive isotope that may be released in a meltdown at a nuclear power plant.
primary containment: See containment building.
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