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Editor's Note: This is part of the IEEE Spectrum special report: Fukushima and the Future of Nuclear Power.

This past April, when the Japanese government and Tokyo Electric Power Co. (TEPCO) jointly unveiled their plan to bring the damaged reactors of the Fukushima Dai-ichi nuclear power plant to a cold shutdown and gain control of the release of radioactive materials, they set a tentative completion date for mid-January 2012. And "tentative" had to be the operative word, for the obstacles TEPCO faced—and to some extent still does face—are challenging in the extreme. They include:

Fuel rod meltdowns in reactors 1, 2, and 3 due to loss of cooling systems following the 11 March earthquake and tsunami; Severe damage to the upper levels of reactor buildings 1, 3, and 4 and slight damage to building 2, stemming from hydrogen explosions; High levels of radiation and contaminated rubble, making working conditions hazardous and difficult; Thousands of metric tons of contaminated water accumulating on the site and leaking out of the reactors.

In the No. 1 reactor, the overheated fuel may have eroded the primary containment vessel’s thick concrete floor, and it may have gotten almost within a foot of a crucial steel barrier, the utility said the new simulation suggested. Beneath that steel layer is a concrete basement, which is the last barrier before the fuel would have begun to penetrate the earth. Some nuclear experts have warned that water from a makeshift cooling system now in place at the plant may not be able to properly cool any nuclear fuel that may have seeped into the concrete. The new simulation may call into question the efforts to cool and stabilize the reactor, but the Tokyo Electric Power Company, or Tepco, says it is not worried more than eight months after the accident.

The findings are the latest in a series of increasingly grave scenarios presented by Tepco about the state of the reactors. The company initially insisted that there was no breach at any of the three most-damaged reactors; it later said that there might have been a breach, but that most of the nuclear fuel had remained within the containment vessels. “This is still an overly optimistic simulation,” said Hiroaki Koide, an assistant professor of physics at the Kyoto University Research Reactor Institute, who has been a vocal critic of Tepco’s lack of disclosure of details of the disaster. Tepco would very much like to say that the outermost containment is not completely compromised and that the meltdown stopped before the outer steel barrier, he said, “but even by their own simulation, it’s very borderline.” “I have always argued that the containment is broken, and that there is the danger of a wider radiation leak,” Mr. Koide said. “In reality, it’s impossible to look inside the reactor, and most measurement instruments have been knocked out. So nobody really knows how bad it is.”

Still, a spokesman for Tepco, Junichi Matsumoto, said Wednesday that the nuclear fuel was no longer eating into the concrete, and that the new simulation would not affect efforts to bring the reactors to a stable state known as a “cold shutdown” by the end of the year.    “The containment vessel as a whole is being cooled, so there is no change to our outlook,” Mr. Matsumoto said at a news conference. 

Tokyo Electric Power Co. is considering changing the method of injecting water into the No. 3 reactor at its hobbled Fukushima No. 1 nuclear power plant as the current system isn't cutting it. The No. 3 reactor is consuming nearly three times the coolant water that the No. 1 and No. 2 reactors are taking to cool down their fuel rods, as a considerable amount is missing the target. TEPCO said that the pressure vessels in the No. 1 through No. 3 reactors, where fuel meltdowns have occurred, currently have temperatures at the bottom between about 90 and 120 degrees. In the meantime, the amount of water pumped in daily to maintain the temperatures at these levels is about 216 tons for the No. 3 reactor, as opposed to 84 tons for the No. 2 reactor, which is about the same size and contains roughly the same number of fuel rods, and 91 tons for the No. 1 reactor, which is smaller.

The question is, why is this discrepancy occurring? TEPCO said that in all three reactors, coolant water is being injected from outside the shroud, a major component covering the core. Analysis conducted so far has hinted at the possibility that, unlike in the No. 1 and No. 2 reactors, part of the melted fuel in the No. 3 reactor did not fall through to the bottom of the pressure vessel but has stayed on the grid-like core support plate. The current injection method cannot pump water into there, resulting in inefficient cooling and increasing the amount of radioactive water. The new water injection method under consideration is based on the use of an emergency cooling system called a "core spray." It can pour water down like a shower above the fuel rods, resulting in more efficient cooling and the use of less coolant water, TEPCO said. Much has been learned about the state of the cooling pipe systems since workers regained access to the reactor buildings. On Aug. 3, TEPCO conducted tests on the operability of valves along the piping.

We plan to make decisions in two or three weeks," a TEPCO official said.

Achieving a "cold shutdown" of a nuclear reactor is not difficult as long as the reactor is not broken. A cold shutdown is defined by experts as a situation in which nuclear reactors whose operations are suspended are being stably cooled down and the temperatures in them are kept below 100 degrees Celsius. However, it is no easy task to achieve a cold shutdown at the tsunami-hit Fukushima No. 1 Nuclear Power Plant where fuel has melted and holes have developed in damaged reactors.

Goshi Hosono, state minister for the prevention of nuclear accidents, told the International Atomic Energy Agency (IAEA) annual general meeting under way in Vienna that Tokyo will do its best to achieve a cold shutdown of the stricken reactors at the plant by the end of this year. His remark suggests that the government intends to bring forward its target of achieving a stable cool-down of the troubled reactors and of substantially reducing the amount of radioactive substances released from the plant by January 2012.

The temperature at the bottom of the No. 1 reactor's pressure vessel has been stabilized at less than 100 degrees Celsius, and that of the No. 3 reactor has recently been kept below that level. Hosono appears to have made the remark at the IAEA conference while keeping in mind these positive signs. It is a matter of course for the government to try its utmost to bring the crippled reactors under control as soon as possible, and it is important for it to show its determination to achieve this goal to the international community.

As a radiation metrology and nuclear safety expert at Kyoto University's Research Reactor Institute, Hiroaki Koide has been critical of how the government and Tokyo Electric Power Co. (TEPCO) have handled the nuclear disaster at the Fukushima No. 1 nuclear plant. Below, he shares what he thinks may happen in the coming weeks, months and years. The nuclear disaster is ongoing. Immediately after the crisis first began to unfold, I thought that we'd see a definitive outcome within a week. However, with radioactive materials yet to be contained, we've remained in the unsettling state of not knowing how things are going to turn out.

Without accurate information about what's happening inside the reactors, there's a need to consider various scenarios. At present, I believe that there is a possibility that massive amounts of radioactive materials will be released into the environment again. At the No. 1 reactor, there's a chance that melted fuel has burned through the bottom of the pressure vessel, the containment vessel and the floor of the reactor building, and has sunk into the ground. From there, radioactive materials may be seeping into the ocean and groundwater.

The use of water to cool down the reactors immediately after the crisis first began resulted in 110,000 cubic meters of radiation-tainted water. Some of that water is probably leaking through the cracks in the concrete reactor buildings produced by the March 11 quake. Contaminated water was found flowing through cracks near an intake canal, but I think that's just the tip of the iceberg. I believe that contaminated water is still leaking underground, where we can't see it. Because of this, I believe immediate action must be taken to build underground water barriers that would close off the nuclear power plant to the outside world and prevent radioactive materials from spreading. The important thing is to stop any further diffusion of radioactive materials.

From inspectors’ abandoning of the Fukushima Daiichi nuclear power plant as it succumbed to disaster to a delay in disclosing radiation leaks, Japan’s response to the nuclear accident caused by the March tsunami fell tragically short, a government-appointed investigative panel said on Monday.

Officials of Japan’s nuclear regulator present at the plant during the quake quickly left the site, and when ordered to return by the government, they proved of little help to workers racing to restore power and find water to cool temperatures at the plant, the report said.

The panel attacked the use of the term “soteigai,” or “unforeseen,” that plant and government officials used both to describe the unprecedented scale of the disaster and to explain why they were unable to stop it. Running a nuclear power plant inherently required officials to foresee the unforeseen, said the panel’s chairman, Yotaro Hatamura, a professor emeritus in engineering at the University of Tokyo. “There was a lot of talk of soteigai, but that only bred perceptions among the public that officials were shirking their responsibilities,” Mr. Hatamura said.

Fukushima has three nuclear reactors exposed and four fuel cores exposed," he said, "You probably have the equivalent of 20 nuclear reactor cores because of the fuel cores, and they are all in desperate need of being cooled, and there is no means to cool them effectively.

TEPCO has been spraying water on several of the reactors and fuel cores, but this has led to even greater problems, such as radiation being emitted into the air in steam and evaporated sea water - as well as generating hundreds of thousands of tons of highly radioactive sea water that has to be disposed of.

"They are pouring in water and the question is what are they going to do with the waste that comes out of that system, because it is going to contain plutonium and uranium. Where do you put the water?"

Workers at the Fukushima Daiichi nuclear energy facility achieved a major milestone this week as recirculating cooling was restored to the used fuel storage pools at the last of the four damaged reactors.

marking the first time since the March 11 earthquake and tsunami that the pools at all four reactors have used recirculating cooling rather than water injection

TEPCO plans to train approximately 4,000 workers in radiation safety by the end of the year.

The main trade group for the nuclear power industry, the Nuclear Energy Institute, spent $580,000 in the first quarter lobbying federal officials about financial support for new reactors, safety regulations and other issues, according to a disclosure report. That's 32 percent more than the $440,000 the trade group spent in the second quarter of last year and 6 percent more than the $545,000 it spent in the first quarter of 2011. The nuclear crisis in Japan last March brought about by the earthquake and tsunami led to calls for tighter safety regulations for nuclear plants in the United States.

NEI, based in Washington, lobbied the government on measures designed to ensure the nation's 104 commercial reactors can withstand natural disasters. It also lobbied on a measure that would require nuclear operators to transfer radioactive spent nuclear fuel from cooling pools inside or near reactor cores to dry casks further from the reactors. In the Japanese nuclear accident, crowded pools of spent nuclear fuel overheated when the nuclear station's cooling power was knocked out. NEI also lobbied the government over environmental regulations. Congress is considering measures that would delay new clean air and clean water rules and curb the Environmental Protection Agency's ability to issue rules by forcing the EPA to factor in the cost of their implementation in addition to medical and scientific evidence.

There also are several measures under consideration that would block the EPA from regulating greenhouse gases. Nuclear power generation produces no greenhouse gases and none of the airborne toxins such as mercury that EPA clean air rules target. But many nuclear plants use outdated cooling systems that consume enormous amounts of water. Replacing those cooling systems with newer systems that use less water is expensive. NEI also lobbied for funds for research and development for smaller, cheaper reactors and other nuclear technologies.

Though nuclear power produces electricity with little in the way of carbon dioxide emissions, it, like other energy sources, is not without its own set of waste products. And in the case of nuclear power, most of these wastes are radioactive.1 Some very low level nuclear wastes can be stored and then disposed of in landfill-type settings. Other nuclear waste must remain sequestered for a few hundred years in specially engineered subsurface facilities; this is the case with low level waste, which is composed of low concentrations of long-lived radionuclides and higher concentrations of short-lived ones. Intermediate and high-level waste both require disposal hundreds of meters under the Earth’s surface, where they must remain out of harm’s way for thousands to hundreds of thousands of years (IAEA, 2009). Intermediate level wastes are not heat-emitting, but contain high concentrations of long-lived radionuclides. High-level wastes, including spent nuclear fuel and wastes from the reprocessing of spent fuel, are both heat-emitting and highly radioactive.

When it comes to the severity of an accident at a nuclear facility, there may be little difference between those that occur at the front end of the nuclear power production and those at the back end: An accident involving spent nuclear fuel can pose a threat as disastrous as that posed by reactor core meltdowns. In particular, if spent fuel pools are damaged or are not actively cooled, a major crisis could be in sight, especially if the pools are packed with recently discharged spent fuel.

Elements of success

The operator of the Fukushima Daiichi nuclear power plant said Thursday that the amount of water injected into the crippled Nos. 1 to 3 reactors temporarily dropped below the level regarded as necessary to keep the fuel inside cool. Although the sharp drop in the water volume did not last long enough to push up the temperature of the reactor pressure vessels, Tokyo Electric Power Co. said it will investigate the cause of the incident because water injection is the “most important” system to ensure the stable condition of the reactors.

The impacts on the ocean of releases of radionuclides from the Fukushima Dai-ichi nuclear power plants remain unclear. However, information has been made public regarding the concentrations of radioactive isotopes of iodine and cesium in ocean water near the discharge point. These data allow us to draw some basic conclusions about the relative levels of radionuclides released which can be compared to prior ocean studies and be used to address dose consequences as discussed by Garnier-Laplace et al. in this journal.(1) The data show peak ocean discharges in early April, one month after the earthquake and a factor of 1000 decrease in the month following. Interestingly, the concentrations through the end of July remain higher than expected implying continued releases from the reactors or other contaminated sources, such as groundwater or coastal sediments. By July, levels of 137Cs are still more than 10 000 times higher than levels measured in 2010 in the coastal waters off Japan. Although some radionuclides are significantly elevated, dose calculations suggest minimal impact on marine biota or humans due to direct exposure in surrounding ocean waters, though considerations for biological uptake and consumption of seafood are discussed and further study is warranted.

there was no large explosive release of core reactor material, so most of the isotopes reported to have spread thus far via atmospheric fallout are primarily the radioactive gases plus fission products such as cesium, which are volatilized at the high temperatures in the reactor core, or during explosions and fires. However, some nonvolatile activation products and fuel rod materials may have been released when the corrosive brines and acidic waters used to cool the reactors interacted with the ruptured fuel rods, carrying radioactive materials into the ground and ocean. The full magnitude of the release has not been well documented, nor is there data on many of the possible isotopes released, but we do have significant information on the concentration of several isotopes of Cs and I in the ocean near the release point which have been publically available since shortly after the accident started.

We present a comparison of selected data made publicly available from a Japanese company and agencies and compare these to prior published radionuclide concentrations in the oceans. The primary sources included TEPCO (Tokyo Electric Power Company), which reported data in regular press releases(3) and are compiled here (Supporting Information Table S1). These TEPCO data were obtained by initially sampling 500 mL surface ocean water from shore and direct counting on high-purity germanium gamma detectors for 15 min at laboratories at the Fukushima Dai-ni NPPs. They reported initially results for 131I (t1/2 = 8.02 days), 134Cs (t1/2 = 2.065 years) and 137Cs (t1/2 = 30.07 years). Data from MEXT (Ministry of Education, Culture, Sports, Science and Technology—Japan) were also released on a public Web site(4) and are based on similar direct counting methods. In general MEXT data were obtained by sampling 2000 mL seawater and direct counting on high-purity germanium gamma detectors for 1 h in a 2 L Marinelli beaker at laboratories in the Japan Atomic Energy Agency. The detection limit of 137Cs measurements are about 20 000 Bq m–3 for TEPCO data and 10 000 Bq m–3 for MEXT data, respectively. These measurements were conducted based on a guideline described by MEXT.(5) Both sources are considered reliable given the common activity ratios and prior studies and expertise evident by several Japanese groups involved in making these measurements. The purpose of these early monitoring activities was out of concern for immediate health effects, and thus were often reported relative to statutory limits adopted by Japanese authorities, and thus not in concentration units (reported as scaling factors above “normal”). Here we convert values from both sources to radionuclide activity units common to prior ocean studies of fallout in the ocean (Bq m–3) for ease of comparison to previously published data.

Federal regulators have been working closely with the nuclear power industry to keep the nation's aging reactors operating within safety standards by repeatedly weakening standards or simply failing to enforce them, an investigation by The Associated Press has found.Officials at the U.S. Nuclear Regulatory Commission regularly have decided original regulations were too strict, arguing that safety margins could be eased without peril, according to records and interviews.The result? Rising fears that these accommodations are undermining safety -- and inching the reactors closer to an accident that could harm the public and jeopardize nuclear power's future.

Examples abound. When valves leaked, more leakage was allowed -- up to 20 times the original limit. When cracking caused radioactive leaks in steam generator tubing, an easier test was devised so plants could meet standards.Failed cables. Busted seals. Broken nozzles, clogged screens, cracked concrete, dented containers, corroded metals and rusty underground pipes and thousands of other problems linked to aging were uncovered in AP's yearlong investigation. And many of them could escalate dangers during an accident.

Despite the problems, not a single official body in government or industry has studied the overall frequency and potential impact on safety of such breakdowns in recent years, even as the NRC has extended dozens of reactor licenses.Industry and government officials defend their actions and insist no chances are being taken. But the AP investigation found that with billions of dollars and 19 percent of America's electricity supply at stake, a cozy relationship prevails between industry and the NRC.Records show a recurring pattern: Reactor parts or systems fall out of compliance. Studies are conducted by industry and government, and all agree existing standards are "unnecessarily conservative."

The Japanese government seems to be "instructing" TEPCO not to release certain information in English.TEPCO submitted the report to its regulatory agency Nuclear and Industrial Safety Agency (NISA) "on the measures to continue water injection into reactors of Units 1 to 3 at Fukushima Daiichi Nuclear Power Station" on August 3. It's in Japanese only, and it may or may not be translated into English.According to TEPCO:We have provided a Japanese press release version of the instruction document received from NISA. However, at this time we have reserved the right not to provide an English version due to potential misunderstandings that may arise from an inaccurate rendering of the original Japanese text. We may provide the English translation that NISA releases in our press releases. However, in principle we would advise you to visit the NISA website for timely and accurate information.(From TEPCO's English press release on August 3 explaining why they are releasing the information only in Japanese.)The 34-page Japanese report is here.

The report talks about the fuel inside the Reactor Pressure Vessels;It talks about the reactors as if they were sound;It states that zirconium will start to interact with water at a certain temperature (1,200 degrees Celsius).

Most likely, there is no fuel left inside the RPVs at Fukushima I Nuke Plant. Even if there is, it is not fuel any more but "corium" - fuel, control rods, instruments, whatever inside the RPV, melted together. TEPCO has already admitted that there are holes in the RPV, and holes in the Containment Vessels. There is no zirconium left because there is no cladding left.

On May 4th 2011 the Potrblog team described the Fukushima 3 explosion and spent fuel rod ejections as the result of spent fuel cooling water becoming supersaturated with hydrogen

"It is also likely that the water in the fuel cooling pond was supersaturated with hydrogen. When the explosion occurred, the hydrogen in solution in the cooling pond water would have frothed up, burned, and deflagrated, sending the fuel rods out of the top of the holding pond. Visualize a bottle of Coke dropping; the resultant the carbon dioxide coming out of solution and shooting out of the end; now instead of carbon dioxide picture it as burning hydrogen shooting out"

Empirical lab research by the University of Tokyo and the Japanese Atomic Energy Agency now confirms the generation of hydrogen in such fuel pools. They attribute the explosion in Fukushima 4 to hydrogen generated in fuel pools. The POTRBLOG team suspects that unlike Fukushima 3, the Fukushima 4 explosion did not manifest a large BLACK mushroom cloud because the Fukushima 4 explosion triggered before the fuel pools could become supersaturated with Hydrogen and Oxygen.This early triggering, as compared to Fukushima 3, would have limited damage to the roof line of the Fukushima 4 building. For POTRBLOG's full analysis on the Unit 3 explosion see:Was the Fukushima Daiichi #3 EXPLOSION a detonation or deflagration?

Since the end of June when the contaminated water treatment system started the operation, total 50,000 tonnes of groundwater have seeped into the reactor buildings and turbine buildings at Fukushima I Nuke Plant. Now, the total amount of contaminated water (highly contaminated water plus not-so-highly contaminated, treated water) at the plant has grown from 127,000 tonnes at the end of June to 175,000 tonnes as of October 18, according to Asahi Shinbun.Does TEPCO have any plan to stop the flow of groundwater into the reactor buildings and turbine buildings, which just adds to the amount of highly contaminated water to be treated and stored? TEPCO is fast running out of storage space, even with cutting down more trees to make room for the storage tanks.Other than spraying the low-contamination, treated water on the premise, the answer is no. No plan, as TEPCO is running out of money that it is willing to spend on Fukushima I Nuke Plant.From Asahi Shinbun (10/19/2011):

It has been discovered that the contaminated water has increased by 40% in 4 months inside the reactor buildings and turbine buildings at Fukushima I Nuclear Power Plant, with the inflow of ground water of about 50,000 tonnes. The flow still continues. TEPCO may run out of storage space for the treated, still-contaminated, water. There is also a possibility of the highly contaminated water overflowing from the buildings if a problem at the water treatment facility and a heavy rain coincide.

According to the calculation done by Asahi Shinbun based on the data published by TEPCO, about 450 tonnes of ground water per day have been flowing into the buildings of Reactors 1 through 4 since the end of June when the contaminated water treatment facility started the operation. It is considered that there are damages in the walls of the buildings.

The operator of Fukushima-1 nuclear power plant aims to restore the cooling systems of the reactors at the troubled facility within three months and achieve “cold shutdown” of the plant in six to nine months.

The water, which has been pumped into the reactors to cool them down, started leaking into the basements of the facility’s buildings through the cracks that appeared as a result of the earthquake. The level of water reached 85cm below ground level on Sunday in reactor 2 and threatens to overflow into the ocean. It happened despite the efforts to decant some of the water to a condenser tank at the premises of the reactor.

''We will do our utmost to curb the release of radioactive materials by achieving a stable cooling state at the reactors and spent fuel pools,'' said TEPCO’s Chairman Tsunehisa Katsumata at a news conference on Sunday, as cited by Kyodo news agency.