Anatomy of Japan’s Nuclear Crisis

The massive 9.0 magnitude earthquake that hit the northeast coast of Japan March 11, 2011 and the awesome tsunami it generated created a nuclear crisis for Japan and nuclear power worldwide. The release of dangerous radioactivity¹ from damaged reactors at Fukushima Dai-ichi (Fukushima-I) nuclear power plant is of particular concern. In the following I try to explain what may have happened to and in the reactors, and examine some potential outcomes.

Figure 1. Fukushima Dai-ichi plant, pre-earthquake. Tokyo Electric Power Co.

 

Fukushima-I, one of the world’s largest nuclear power plants (installed capacity 4,696 megawatts), consists of 6 reactors (Fig. 1) located close to the ocean for convenient access to cooling water. In Fig. 1 the reactor buildings are box-like structures, No. 4 is the closest building in the photograph with Nos. 5-6 in order at the far end. Tokyo Electric Power Company.

As our previous blog explained, the northeastern coast of Japan is a subduction zone, where the Pacific plate is being shoved under the North American plate, occasionally giving rise to major earthquakes.² The earthquake epicenter was about 109 miles off-shore, northeast of Fukushima.³ It caused many minutes of severe earth-shaking⁴ in a coastal belt that extended southward to Tokyo. In the first week alone, the area also experienced three aftershocks of magnitude 7 or greater, and 49 of magnitude 6 or greater, adding to the potential for additional shaking and tsunami damage.⁵

The earthquake automatically shut down three operating Fukushima-I plant reactors (Nos. 1-3) by inserting control rods to stop the nuclear fission chain reaction (http://www.answers.com/topic/chain-reaction). The remaining 3 reactors (Nos. 4-6) had been shut down before the earthquake for maintenance. Whether shut down or not, continuing to cool the reactors is essential because radioactive materials in the fuel rods, produced prior to shut down, will continue to decay and produce heat. The temperature of the fuel will continue to rise unless cooled.⁶ Waste spent-fuel⁷ rods stored in on-site pools also require constant cooling.

But the shock of the earthquake disconnected the plant from the regional electrical grid, preventing it from receiving AC power. Damage from the huge tsunami following the initial shock further disrupted connections to the national power grid, causing “a station blackout”—loss of external power to the entire plant. The tsunami also flooded the plant’s backup power generators, compromising its cooling systems. Backup battery power lasted only about 8 hours, inadequate for a weeks-long loss of principal power sources. When cooling water stopped flowing, fuel rods in the shut-down reactors and in the spent fuel ponds began overheating.

Figure 2. Cut-away view of Fukushima-I reactor type. Blue color represents cooling water. Wikimedia Commons

A diagram of the Fukushima-I type of reactor and its housing in Fig. 2 shows the various components involved in this disaster: the reactor vessel contains the active fuel rods and nuclear reactions; primary containment is a free-standing steel container drywell, orange line; secondary containment is a concrete shield wall surrounding the reactor vessel; and the spent fuel pool holds used-fuel rods. The wetwell or torus, is probably steel-walled and may be part of the primary containment, and the reactor building is what you see from outside. All these components provide the heat that generates electricity in turbines, housed nearby in turbine halls.

The fuel rods consist of ceramic pellets of materials made of easily-split (fissionable) atoms. Fukushima-I reactors 1, 2, and 4-6 fuel rods contained uranium, but rods in reactor 3 contained mixed-oxide (MOX) of uranium plus plutonium. The rods were surrounded by a sheath (or cladding) of a stable zirconium (metal) alloy. So long as cooling water flowed through the reactor, the sheath did not overheat and so contained the fuels and fission products.

When the Fukushima-I reactors lost power, cooling water stopped flowing, so water in the reactor vessels heated to boiling and turned to steam. The water levels fell, exposing at least the upper parts of hot fuel rods. The zirconium alloy sheath then chemically interacted with steam to form zirconium oxide and hydrogen, causing the sheath to break down. This chemical interaction is highly exothermic, releasing large amounts of heat and raising the fuel temperature even more rapidly. In a positive feedback loop, greater heating speeds the deterioration of zirconium alloy sheaths,⁸ releasing hydrogen and radioactive fission products.

Hydrogen and steam were building up in the reactor vessels, threatening to reach pressures that could rupture them, so the plant operators chose to vent some of these gases to the atmosphere. Hydrogen is highly explosive, so the plant was designed to carry hydrogen and steam through pipes and vent some distance from the plant. Instead hydrogen accumulated within the reactor buildings and eventually exploded. Hydrogen explosions caused severe damage to the unit 1 reactor building on March 12, to unit 3 on March 14, and to units 2 and 4 on March 15.⁹

There are two potential sources for the hydrogen:  fuel rods in the reactor vessels and spent fuel rods in cooling pools. Hydrogen coming from reactor vessels would mean either that a breach had opened up in the primary containment releasing hydrogen, steam, and radioactive fission products into the reactor buildings, or disruption to the buildings’ internal venting systems by earthquake shaking. In contrast the spent fuel pools, also dependent on the failed cooling systems, are outside the primary containment (Fig. 2). If their cooling water levels dropped sufficiently to expose the rods, they could emit hydrogen, steam, and radioactive fission products directly into the reactor buildings. A buildup of hydrogen in the buildings needed only a spark to detonate the flammable gas mixture.

Various experts have suggested ways that primary containment might have breached, including potential weaknesses in piping, decay of organic seals, and disturbance of the steel containment vessel’s seal (Fig. 2).¹⁰ Other suggestions include the possibility of fractures in and melting through containment structures. The processes of sheath deterioration, hydrogen production, and releases of radioactive fission products from fuel rods in the reactor vessels can eventually let fuel pellets spill into the bottom of the reactor vessel without melting.¹¹ These sheath reactions are exothermic and may produce sufficient heat to melt the sheath. Whether melted or disaggregated, the radioactive fuel that gets into the bottoms of the reactor vessels is no longer affected by the fission-limiting effects of the control rods and is in danger of restarting heat-producing fission reactions.

Locating spent fuel pools inside the reactor building is a particular weakness of the Fukushima-I reactor design. The internal pools are vulnerable to earthquake disruption, with the potential for spilling cooling water and exposing the stored used-fuel rods. Once exposed, the used fuel rods would proceed to generate steam which, if hot enough will interact with the zirconium sheath, releasing hydrogen. This process might have contributed all or some of the hydrogen that exploded in the reactor buildings. Even if the pools were not principal contributors, the explosions so disrupted the reactor buildings that radioactive fission products from the spent fuel pools are now being emitted directly into the atmosphere (Figs. 3a and 3b).¹²

Figure 3a. Satellite photo of reactor buildings 1-4. Green circles identify steam plumes from reactor buildings. DigitalGlobe

Use of mixed uranium/plutonium (MOX) fuel in Reactor 3 poses a special problem with its spent fuel storage because of the high toxicity of plutonium. Small amounts of plutonium have apparently been released and contaminate nearby soil.¹³

Discovery of high levels of radioactivity in areas as far as 36 miles from the plant site has caused officials to enlarge evacuation zones, increasing public concern.¹⁴ The reactors remain unstable and cooling systems are unrestored. The initial desperate efforts to cool the reactors and spent fuel pools by dumping of seawater on the plants in air drops and from fire engine pumps has been replaced by somewhat improved cooling with imported fresh water. The large amounts of salt deposited as the seawater turned to steam remains a serious problem of clogging the cooling systems of some reactors.

This poorly controlled flooding of the reactors has resulted in radioactive waters running off to the ocean, with daily reports of increasing contamination of seawater, and seepage into groundwater.¹⁵

Figure 3b. Oblique aerial view of reactor buildings 1-4, right to left. Steam plumes are from buildings 2 and 3, as in Fig. 3a. The severe damages to reactor buildings 3 and 4 allow direct release of radioactivity from spent fuel ponds to the atmosphere

These unresolved problems and the continuing releases of radioactivity to the environment led officials on April 12 to boost the crisis rating from 5 to 7, the highest rating on an international scale of nuclear accidents.

To stabilize the reactor sites now will require removal (and disposal) of huge amounts of contaminated water. Previous disposal plans never included the construction of decontamination plants and storage facilities, so current highly contaminated waters are being dumped in the ocean to make room for even more highly contaminated waters in the limited storage now available.¹⁶

In addition, the continued reports of short-lived iodine-131 contamination in and beyond Fukushima-I carry worrisome implications. The spent fuel in pools at the time of the earthquake would not contain much iodine-131 (half-life 8 days), so its appearance suggests it is coming from either a breached primary containment or fission reactions somewhere else in the plants, which are generating iodine-131 (and other radionuclides). If derived from a breached containment, the amounts of iodine-131 should be small, and should decline significantly from the date of reactor shut down.

Another possibility is that fission reactions are taking place in accumulations of fuel pellets that fell to the floors of reactor vessels or of spent fuel pools due to catastrophic deterioration of fuel rod sheaths, or in pools of melted fuel rods.¹⁷ The worst-case results of those potential events vary from melts burning through reactor vessels and into a water-filled torus (see Fig. 2) and either being cooled by the water (good) or causing violent steam explosions (bad).

If melts from fuel rods were to enter a dry torus, they could melt through the bases of the reactor buildings and migrate to groundwater beneath the reactors. This scenario could result in cooling of the melts by groundwater or violent steam explosions, resembling the phreatic volcanic eruptions that occur when hot rising magma (natural rock melts) encounters groundwater. The violent explosion scenarios would worsen the Fukushima disaster by orders of magnitude.¹⁸

We are all hoping for a less extreme outcome: successful stabilization of the reactors, that re-establishes controlled cooling to prevent overheating and further fuel deterioration in the reactors and spent fuel pools. Using seawater for emergency cooling of four of the six reactors has rendered them useless, so “stabilization” is merely a disaster-prevention step. It cannot be viewed as a solution to the eventual requirement to disassemble the damaged plants and their contaminated environs, and dispose of them in a manner that protects the environment and future populations for at least several hundred years. The suggested time frame allows for the decay of cesium-137 and strontium-90 to a minimal hazard level, and assumes no significant amount of plutonium-239 in the mix.

The eventual cleanup also will include removal and safe disposal of a very large amount of soil and rock in the unsaturated zone above the water table, which likely is heavily contaminated. Estimates of 30 years and $12 billion cost to scrap the damaged plants, based on very limited experience in Japan, are likely conservative.¹⁹

Acknowledgements

I have profited greatly from technical advice from Vernon Brechin, nuclear watchdog par excellence, Jane Nielson, geologist, and Ernest Goitein, former nuclear engineer.

Endnotes

¹Limited information from new reports mostly identify iodine-131 (half-life 8 days) and cesium-137 (half-life about 30 years). Many other short-lived radioisotopes likely are being produced but decay quickly. The short-lived isotopes remain problems only so long as they are being actively released. Strontium-90 (half-life about 29 years) and very small quantities (so far) of Plutonium-239 (half-life 24,100 years) also are reported. Even though Strontium-90 and Cesium-137 together dominate the reactors’ long-lived fission products, Strontium-90 is generally not being reported from Fukushima-I. One rule of thumb estimates that longer-lived radioisotopes remain hazardous for 5 times the half-lives, others use a factor of 10 to estimate the hazard lives. Thus, cesium-137 and strontium-90 are the most abundant longer-lived isotopes being released and will remain serious problems for 150 to 300 years. Plutonium-239 and its decay products will be with us beyond the foreseeable future (H.G. Wilshire, J.E. Nielson, and R.W. Hazlett, The American West at Risk: Science, Myths, and Politics of Land Abuse and Recovery (New York, Oxford University Press, 2008), Chapters 7, 10)

²Jane Nielson, Nature Bats Last, blog http://theamericanwestatrisk.wordpress.com/

³Details about location of the Sendai earthquake epicenter: U.S. Geological Survey, Magnitude 9.0 – Near The East Coast Of Honshu, Japan, 2011 March 11 05:46:23 UTC. http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/usc0001xgp.php)

⁴Unconfirmed reports give as much as 5 minutes of severe earth shaking (David Biello, Anatomy of a Nuclear Crisis, Yale Environment 360, 21 March 2011), but a filmed record of liquifaction during the Sendai earthquake in a landfill-based Tokyo park began after liquifaction started and lasted for 3 minutes, 8 seconds. Time of activity elapsed before and after film was made is not known

⁵262 aftershocks of magnitude 5 or greater occurred within the first week, 49 of magnitude 6 or greater, and 3 of magnitude 7 or greater (National Aeronautics and Space Administration, Earth Observatory), http://www.nasa.gov/topics/earth/features/japanquake/quake-intensity.html. A 7.1 magnitude aftershock that cut power to northern Japan occurred 07 April 2011 as a reminder that the story as yet has no end.

⁶Euan Mearns, Fukushima Dai-ichi Status and Slow Burning Issues, The Oil Drum, 25 March 2011. http://www.theoildrum.com/node/7706#more; Biello, Anatomy of a Nuclear Crisis

⁷Unfortunately, the industry term ‘spent fuel’ is misleading. The fuel starts out having quite low levels of radioactivity. The longer the fuel spends in an operating reactor the more highly radioactive fission products build up in it. Fuel that has been in the reactor for two years may be twice as radioactive as fuel that’s only been in the reactor for one year. So as the fuel becomes increasingly more spent it’s radioactivity increases. Once removed from the reactor the radioactivity decreases rapidly in an exponential curve so that its level of radioactivity may be considerably down a year later when the next load of partly irradiated fuel is removed from the reactor. A month after the removal of the recent fuel load, its radioactivity may still be ten times greater than the fuel that’s been aging in the spent fuel pool for a full year (Vernon Brechin, written communication, April 2011)

⁸Arjun Makihajani, Post-Tsunami Situation at the Fukushima Daiichi Nuclear Power Plant in Japan: Facts, Analysis, and Some Potential Outcomes, Institute for Energy and Environmental Research, 14 March 2011; Euan Mearns, Fukushima Dai-ichi Status and Potential Outcomes, The Oil Drum, 17 March 2011. http://www.theoildrum.com/node/7675#more

⁹Jenna Fisher, The Christian Science Monitor, TEPCO To Decommission Fukushima Reactors: Japan Nuclear Timeline, 30 March 2011. http://www.csmonitor.com/World/Asia-Pacific/2011/0315/TEPCO-to-decommission-Fukushima-reactors-Japan-nuclear-timeline; Fukushima 1 Nuclear Accidents, Wikipedia. http://en.wikipedia.org/wiki/Fukushima_I_nuclear_accidents

¹⁰Dave Lochbaum, Possible Cause of Reactor Building Explosions, Union of Concerned Scientists, 18 March 2011; Euan Mearns, Fukushima Dai-ichi Status and Pronosis, The Oil Drum, 31 March 2011, http://www.theoildrum.com/node/7722#more

¹¹A description of the fuel rods disintegration in the reactor core as “catastrophic disintegration of the cladding structural integrity” rather than melting is given in Euan Mearns, Fukushima Dai-ichi Status and Potential Outcomes

¹²In testimony before Congress, Gregory B. Jaczko, Chairman, U.S. Nuclear Regulatory Commission, stated the Commission’s belief that there had been a hydrogen explosion in Reactor 4 [March 15] due to uncovering of fuel rods in the spent fuel pool. This destroyed the secondary containment. In the Commission’s opinion, the spent fuel pool was dry (http://www.nrc.gov/about-nrc/organization/commission/comm-gregory-jaczko/0317nrc-transcript-jaczko.pdf). Subsequently, TEPCO claimed that the spent fuel pools have been filled with water, but high radiation levels prevent access for verification. The vulnerability of spent fuel storage facilities is well known: Makhijani, Post-Tsunami Situation at the Fukushima Daiichi Nuclear Power Plant; National Research Council, Safety and Security of Commercial Spent Fuel Storage: Public Report, National Academies Press, 2006; Keith Bradsher and Hiroko Tabuchi, Danger of Spent Fuel Outweighs Reactor Threat, New York Times, 17 March 2011; Robert Alvarez, Safeguarding Spent Fuel Pools in the United States, Institute for Policy Studies, 21 March 2011

¹³Justin McCurry and Suzanne Goldenberg, Fukushima Soil Contains Plutonium Traces, According to Japanese Officials, The Guardian, 29 March 2011. The reported distinction between plutonium from atmospheric weapons testing and that originating in the MOX fuels of the power plant is apparently based on ratios of Pu-238/Pu-239, low in bomb tests, higher in MOX fuels (Vernon Brechin, written communication, 30 March 2011)

¹⁴Aerial surveys of radiation around Fukushima-I revealed hot spots as far as 36 miles from the plant with radiation levels that exceed international standards for immediate evacuation (Jim Smith, A Long Shadow Over Fukushima, Nature, 472 (7), 5 April 2011). Very high levels of cesium-137 will require evacuation for a very long period of time.

¹⁵Contamination of groundwater about 50 feet below the Fukushima-I reactors was reported on March 31, giving values for iodine-131 of 10,000 times the legal limit. TEPCO, the facility owner, reports on contaminant concentrations have been severely criticized; the company claims the iodine-131 value was checked and is correct, but is not sure of values for other contaminants. Since the location of the plant is so close to the ocean it is probable that groundwater in the surficial unconfined aquifer flows directly to the ocean. It is also likely that contaminants in the unsaturated zone in soils above the water table will migrate to the water table and then to the ocean over time. The rates of migration are not known (see Wilshire, H.G., Nielson, J.E., and Hazlett, R.W.,The American West at Risk: Science, Myths, and Politics of Land Abuse (New York, Oxford University Press, 2008),, Chapters 7, 13).

¹⁶World Nuclear News, Tepco’s Plans for Water Issues, 01 April 2011; Mari Yamaguchi and Yuri Kageyama, Search for Radiation Leak Turns Desperate in Japan, Associated Press, 04 April 2011

¹⁷Accidental restarting of nuclear fission chain reactions long after reactor shutdown may be causing formation of very short-lived chlorine-38 (half-life 37 minutes) by neutron absorption of stable chlorine-37 in seawater pumped into the reactors (F. Dalnoki-Veress, with an introduction by Arjun Makhijani, What Caused the High Chlorine-38 Radioactivity in the Fukushima Daiichi Reactor #1?, Asia-Pacific Journal , 30 March 2011, http://www.ieer.org/)

¹⁸Justin Elliott, Japan’s Nuclear Danger Explained, Salon, 18 March 2011

¹⁹Shigeru Sato, Yuji Okada and Tsuyoshi Inajima, Tepco’s Damaged Reactors May Take 30 Years, $12 Billion to Scrap, Financial News, 29 March 2011, http://hotstocksforyou.com/2011/03/tepcos-damaged-reactors-may-take-30-years-12-billion-to-scrap/; see also, Yamaguchi and Kageyama, Search for Radiation Leak, 04 April 2011