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Much recent news and many questions directed at the blog have centered around the detection of plutonium in soil and water surrounding the Fukushima reactors. This post will outline our current knowledge of the situation, as well as potential impacts on the environment and on human health.

Measurements to date

On March 21 and 22, five soil samples around the site indicated the presence of isotopes of plutonium. Two of these samples contained the isotope Pu-238. Typically, a large ratio of Pu-238 to the other isotopes indicates that the material has been produced in a reactor. This is because the production of Pu-238 requires one of these processes to take place:

• Successive neutron captures in U-235 produce U-237. U-237 decays to Np-237, with a half-life of 6.75 days. Np-238 captures another neutron and decays to Pu-238.
• A fast moving neutron causes a Pu-239 nucleus to eject an additional neutron.

This second reaction can take place during the detonation of a nuclear weapon, but is rare. The first is next to impossible in a nuclear detonation, as the weapon blows itself apart before the necessary series of captures and decays can occur. This is why the detection of Pu-238 at two sites indicates that material at those sites came from a reactor. There is not sufficient information at this time to determine whether the material originated from the MOX-fueled unit 3, or from one of the other cores.

Because Pu-238 was not detected at the other three sites, it is thought that the plutonium at those sites is a remnant of past nuclear weapons tests. For reference, the natural rate of plutonium decay in Fukushima City is 0.61 Bq/kg, or 0.61 decays per second in each kilogram of soil. The quantities being measured on the reactor site are at roughly double this same level, according to TEPCO.

Transport Pathways

As stated, the pathway which was taken by the plutonium to the soil of the reactor site is not clear at the present. However, it is generally transported via one of two pathways:

• Adhesion to particulate matter, like smoke.
• Solution or suspension in water.

Whichever route led to the disposition of plutonium on the reactor site, it would be difficult for such plutonium to be transported over great distances. Its high mass means that it is not easily aerosolized, even by fire. Mention has been made of the fact that plutonium will, under the right conditions, burn. However, this burning occurs during plutonium metal’s conversion to plutonium oxide. As the plutonium within each reactor is already in an oxide form, it has no such tendency to burn. Finally, plutonium is not very water-soluble. Under optimal conditions, the solubility of plutonium metal in water is around 55 microgram/L. The solubility of plutonium oxide is even lower.

Could plutonium be transported away from the reactor site, under the current conditions? Potentially, yes. However, it would likely be in minute quantities that have no impact on human health.

Impact of Plutonium on Human Health

As a radiation hazard, plutonium is a danger when ingested or inhaled. This is because it’s an alpha emitter. Alpha particles, while they can be stopped by the skin or a sheet of paper, can severely injure very delicate structures of the body, such as the alveoli in the lungs, or the lining of the gastrointestinal tract. Plutonium is a bone-seeker, but is not efficiently absorbed by the body because of its low solubility in the body’s fluids. The vast majority of ingested plutonium (greater than 99%) is excreted within a week of ingestion. Between 5% and 60% (estimates by different agencies vary) of inhaled plutonium stays within the body, with the rest being exhaled immediately.

In addition, plutonium is chemically toxic like other heavy metals. A number of estimates have been circulated regarding how much plutonium is fatal to humans, many of which have no evidence to support them. Experiments using lab rats have indicated that 50% of those rats die within a month after injection of 700-1000 micrograms of plutonium per kilogram of body weight. This would translate to 47.7-68.2 mg of plutonium injected into a 150-pound person. Since the efficiency of plutonium uptake by inhalation or ingestion is low, the dose needed to actually cause illness or death would be correspondingly higher.

It’s unknown whether the results of experiments on rats translate directly to human exposures. No human has ever died from acute uptake of plutonium. Our information on the health effects of plutonium on humans is derived from the case studies of plutonium workers, who sustain very low doses over a period of decades; a series of studies on chronically ill patients; and the histories of atomic bomb survivors, whose doses are confounded by exposures to a whole host of other radioactive isotopes.

Sources: Journal of Radiological Protection, Los Alamos National Laboratory, TEPCO,  American Chemical Society.

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It seems that there is a lot of confusion as to what isotopes, radioisotopes, nuclides, and radionuclides are.  First, we have to go back to chemistry class and remember the periodic table of elements, which lists all of the chemical elements in an organized fashion.

The periodic table reports each element with its average properties.  Each chemical element on the periodic table has a distinct number of protons.  The reason we say “average properties” here is because each element has a number of different isotopes.  The word “isotope” indicates an equal number of protons, hence the prefix “iso” and the letter “p” in the name (note that isotones represent nuclides with the same number of neutrons).  For example, hydrogen (1 proton) consists of 3 natural isotopes: hydrogen (0 neutrons), deuterium (1 neutron), and tritium (2 neutrons).  The same is true of uranium, where U-235 is an isotope that can undergo fission.  The number 235 represents the sum of neutrons and protons that make up the nucleus of the uranium atom (92 protons and 143 neutrons).  The term “nuclide” is just a general name for any isotope of a chemical element.

The prefix “radio” in front of “isotope” and “nuclide” refers to radioactivity.  This indicates the spontaneous transformation (decay) of unstable nuclides to more stable ones.  In order to accomplish this, nuclides may emit a spectrum of particles including alpha particles, beta particles (electrons or positrons), neutrons, gamma rays (photons), or x-rays.  In order to characterize the probability of a nuclide decaying, each radionuclide has a half-life. The half-life of a radionuclide is the expected time it takes for one half of the amount of one isotope to decay into another isotope.  In terms of radiation safety, it is desirable for unstable nuclides to eventually decay to stable nuclides.  The amount of radionuclide present, when there is no source producing it, undergoes an exponential rate of decay.

Activity is another term that is used when talking about radioisotopes.  Activity, measured in the unit of Bequerel (Bq), is the number of decays occurring per unit time. It is not necessarily equal to the rate at which particles are emitted. For example, cobalt-60 emits both beta and gamma radiation each time it decays.  The activity of an isotope also follows a similar exponential trend as shown above.  It is also often expressed in units of Curie (Ci), where 1 Ci = 3.7 x 1010 Bq.

There is also a big difference between nuclear reactions and chemical reactions.  Nuclear reactions are quite different for different isotopes of the same element, while chemical reactions are quite similar for different isotopes of the same element.  All isotopes of the same element (I-127, I-131, and I-135 are all isotopes of iodine) have similar chemical interactions, but they could result in different health effects due to different levels of radioactivity.  This is because chemical reactions involve changing electron configurations in the atom.  Since all isotopes of a given chemical element have the same electron configuration, they will have similar chemical reactions.  A good example is the use of iodine tablets.  Different isotopes of iodine will have similar chemical interactions in the body.  Therefore, if the body is already saturated with non-radioactive iodine, it is already full and radioactive iodine has a lower chance of being absorbed.  For nuclear reactions, each isotope of an element will have different nuclear reaction characteristics.  For example, slow neutrons have a much higher chance of causing fission in U-235 than in U-238.

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On prediction of “worst case” scenarios

The blog has received a great number of questions surrounding worst case scenarios. This is not surprising given that such scenarios, with varying degrees of scientific merit, have been advanced in the media. The intent of this blog is to educate, using our best available information, and so we intend to refrain from making predictions of our own. We do, however, want to review some of the terminology used in these predictions, and describe the methods used by government agencies and scientific organizations to determine what actions must be taken to inform the public.

Meltdown

The term meltdown describes melting of the zirconium alloy cladding, and uranium oxide (or mixed oxide, in the case of Unit 3) fuel pellets. These two structures are the first two barriers to fission release, since radioactive fission products normally exist as either solids within the fuel pellet, gases within pores in the fuel pellet, or gases that escape the pellet but remain in the cladding. When a reactor is shut down, these fission products continue to decay, generating heat. This amount of heat is produced at first at 7% of its initial rate, and then decreases as the isotopes responsible for generating it decay away. If this decay heat is not removed by cooling water, the fuel and cladding increase in temperature.

At temperatures above 1200 C, the corrosion reaction which is constantly ongoing in the zirconium cladding accelerates dramatically. The reaction’s products include zirconium oxide, hydrogen (for more on this hydrogen, see our post “Explanation of Hydrogen Explosion at Units 1 and 3), and heat. This heat continues to both fuel the corrosion reaction, and to prevent the fuel rods from being cooled.  Because of the self-catalyzing nature of this reaction, safety systems are usually actuated in such a fashion as to provide a large margin of safety to the clad reaching 1200 C.

If multiple failures prevent these actions from being taken, as was the case at Three Mile Island, the fuel rods heat up until the uranium oxide reaches its melting point, 2400-2860 C (this figure depends on the makeup and operating history of the fuel). At this point, the fuel rods begin to slump within their assemblies. When the fuel becomes sufficiently liquid, slumping turns to oozing, and the “corium” (a mixture of molten cladding, fuel, and structural steel) begins a migration to the bottom of the reactor vessel. If at any point the hot fuel or cladding is exposed to cooling water, it may solidify and fracture, falling to the bottom of the reactor vessel.

A similar sequence of events takes place if cooling to spent fuel pools is not maintained, but at a reduced rate of progression.

Breakthrough: Operating Experience and Experiment

With the fuel at or above temperatures of 2400 C, there exists the possibility that the fuel could cause damage to the reactor vessel. The melting point of the steel making up the vessel is in the neighborhood of 1500 C. In addition, the vessel in question may have been weakened by its exposure to seawater. The sodium chloride within seawater accelerates the corrosion of steels, but usually on the order of weeks or months, not days. Nevertheless, some uncertainty as to the condition of the vessel does exist.

In the event that molten corium does, as has been the case in some experiments, penetrate the lower head of the reactor vessel, it will drop onto the concrete basemat of the containment and spread out as far as possible.  The interaction of corium with concrete is known to produce a buildup of non-condensable gases within the containment, a process called molten-core concrete interaction (MCCI).

In the wake of the Three Mile Island accident, a number of agencies undertook programs to determine experimentally how corium would behave when placed into contact with a concrete reactor pad. These experiments have been used to measure concrete ablation, and also the rate of generation of non-condensable gases. Over the past twenty years, these studies have focused on quenching of the corium with water.

The experiments are performed by producing a melt of un-irradiated uranium dioxide (extremely low levels of alpha radioactivity, easily avoided by the experimenters), zirconium alloy, and structural steel, in the proportions that would be present in a reactor core. This melt is sent through a nozzle used to simulate a pressure vessel lower head breach, and dropped onto concrete. Measurements are taken during the hours-long experiment using thermocouples and camera equipment, and the solidified material is examined after completion.

The experiments have shown that without water quenching, corium under conditions similar to those present at Fukushima Dai-ichi will ablate the meters-thick concrete pad at a rate of just millimeters per minute. Gases would build up within the containment at a rate which would require filtered ventilation of the containment in order to prevent rupture.

If, however, water is supplied to quench the corium as it spreads onto the reactor floor, the ablation occurs at 5-7% of the pre-quench rate, and production of gases is suppressed. The rate of ablation continues to undergo fits and starts, as the corium forms a solid crust, and then this crust is broken and re-formed.

Again, this summary is intended to explain the different pathways which molten fuel could potentially take. We do not aim to predict what’s going on in each of the reactors and spent fuel pools in question.

Analysis: How it’s done, what it means

The experiments described previously are used to validate, or confirm the results of, calculations which predict what will happen to a reactor or spent fuel pool’s fuel if it should melt down. These calculations are then used to provide the source term for an advection calculation, which predicts doses at sites removed from the plant as a function of time.

These calculations involve complex interactions between a number of different factors, such as

• The method of release: Explosive, or a slow, steady stream? Carried away by air currents, smoke, or water? How high off the ground?
• Weather patterns, both local to the site and further away
• Physical geography, both local to the site and further away

Like the methods used to model the disposition of molten fuel, these methods are validated against the best available data, which include both real-life experience like post-Chernobyl data, and the results of small-scale experiments.

The calculated doses are used by the agencies which calculate them, national and local governments to make decisions about when to evacuate or apply “take-cover” orders to people at different distances removed from the situation. Again, we recommend that our readers close to the facility heed the instructions issued by their governments.

A note about predictions of future radiation doses: in recent days a map has circulated the internet, purporting to predict high doses to the Western U.S.  This map bears the seal of the Australian Radiation Service, which did not produce it. The map has been refuted by the U.S. NRC, and experts state that it more closely resembles predictions for doses after deployment of a nuclear weapon than those for a situation such as that unfolding at present.

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The high levels of radiation braved by workers at the scene in Fukushima Daiichi appear to have reduced after the expansion of the workforce and announcements of infrastructure improvements to come.

In recent days emergency managers were faced with an extremely complicated task to prioritise jobs across all four struggling reactor units in the main part of the site, while a skeleton operating crew maintained the status of units 5 and 6 about two hundred metres away.

There have been about 50 staff engaged in pumping seawater into the reactor cores and primary containment vessels of units 1, 2 and 3. From time to time these need to vent steam, which causes radiation to rise across the site and required the workers to move to a safer location.

Another 130 were also on site, according to reports, including soldiers from the Japan Self Defence Force.

Normally nuclear workers are allowed to receive a dose of 20 millisieverts per year, although in practice they often receive very much less. If that limit is exceeded in any year, the worker cannot undertake nuclear duties for the remainder.

A helicopter shot of Fukushima Daiichi 3 earlier today

In emergency circumstances safety regulators allow workers to receive up to 100 millisieverts with the same conditions applying, that they must leave the site should that limit be reached. The 100 millisievert level is roughly the point at which health effects from radiation become more likely. Below this it is statistically difficult to connect radiation dose to cancer rates, but above this the relationship starts to become apparent.

Under a special allowance from the Nuclear and Industrial Safety Agency, workers at Fukushima were permitted doses of up to 250 millisieverts. Managers must be careful to make the best use of those experienced workers with the most detailed knowledge and experience of the plant.

The small workforce battled to spray water into the damaged buildings of units 3 and 4, working when and where they could to avoid exceeding those radiation dose limits. World Nuclear News understands that the army fire engine was able to “deliver 30 tonnes of water” towards or into unit 3’s fuel pond, but this is not confirmed, nor is the expected drop in radiation levels expected to accompany it. However, Tokyo Electric Power Company has been able to significantly expand the workforce and a range of other activities are now taking place.

External power, diesels coming

The Ministry of Economy Trade and Industry said at 8.38pm that a cable was being laid to bring external power from transmission lines owned by Tohoku Electric Power Company. This was to be connected when radiation levels had died down after a planned venting operation at unit 2.

In addition, one of the emergency diesel units can now be operated and will be used to supply unit 5 and 6 alternately to inject water to their used fuel pools. Later, the power will be used to top up water in the reactor vessels.

Casualties among power plant workers

• Two Tepco employees have minor injuries.
• Two contractors were injured when the quake struck and were taken to hospital, one suffering two broken legs.
• A Tepco worker was taken to hospital after collapsing and experiencing chest pains.
• A subcontract worker at an “important earthquake-proof building” was found unconscious and was taken to hospital.
• Two Tepco workers felt ill whilst working in the control rooms of Fukushima Daiichi units 1 and 2 and were taken to the medical centre at Fukushima Daini.
• Four workers were injured in the hydrogen explosion at Fukushima Daiichi 1. They were all taken to hospital.
• Eleven workers (four Tepco workers, three subcontract workers and four members of Self Defence Force) were hurt following a similar explosion at Fukushima Daiichi 3. They were transferred to the Fukushima Daini plant. One of the Tepco employees, complaining of pain in his side, was later transferred to hospital.
• The whereabouts of two Tepco workers, who had been in the turbine building of Fukushima Daiichi unit 4, is unknown.
• Only one casualty has been reported at the Fukushima Daini plant. A worker in the crane operating console of the exhaust stack was seriously injured when the earthquake struck. He subsequently died.

Contamination cases

• One Tepco worker working within the reactor building of Fukushima Daiichi unit 3 during “vent work” was taken to hospital after receiving radiation exposure exceeding 100 mSv, a level deemed acceptable in emergency situations by some national nuclear safety regulators.
• Nine Tepco employees and eight subcontractors suffered facial exposure to low levels of radiation. They did not require hospital treatment.
• Two policemen were decontaminated after beng exposed to radiation.
• An unspecified number of firemen who were exposed to radiation are under investigation.

http://www.world-nuclear-news.org/RS_Progress_by_on-site_workers_1703111.html

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Mixed Oxide (MOX) Fuel

Uranium occurs naturally on earth. We mine it, refine it, and use it as fuel in nuclear reactors. All other elements that can sustain the nuclear fission chain reaction, including plutonium, do not occur naturally on earth*. The only way to obtain plutonium is to produce it from other elements through neutron radiation. Neutrons in a nuclear reactor produce power through the fission of uranium, but they also convert some of the uranium into plutonium. Thus, all uranium-fueled reactors contain at least some plutonium.

After spent fuel is removed from a reactor, the Japanese nuclear industry sometimes reprocesses it by separating the plutonium and uranium from the fission products, which are (useless) light elements that arise when uranium splits in the fission reaction. The separated plutonium and uranium are often mixed to make new “fresh” fuel that has a higher relative plutonium concentration than the original spent fuel. This is called mixed oxide (MOX) fuel, as the uranium and plutonium are ceramics in oxide chemical form (UO2 and PuO2). Uranium-fueled reactors also typically use the oxide form of uranium (UO2). It is crucial to understand that uranium oxide fuel contains plutonium just as MOX fuel does. The only difference is the relative concentrations of the two elements. MOX fuel is one way in which the nuclear industry can “recycle” nuclear fuel in order to use the earth’s uranium resources more efficiently and responsibly.

 

*As a side note, the element thorium occurs naturally on earth and can be used to produce uranium, although it cannot sustain the fission chain reaction itself.

 

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What is radiation? Where does it come from and what is it used for?

Radiation is energy that propagates through matter or space. Radiation energy can be electromagnetic or particulate. Radiation is usually classified into non-ionizing (visible light, TV, radio wave) and ionizing radiation. Ionizing radiation has the ability to knock electrons off of atoms, changing its chemical properties. This process is referred to ionization (hence the name, ionizing radiation). Ionizing radiation is the main concern for health effects since it can change chemicals’ properties in the human body.

Radiation comes from many sources including cosmic rays from the universe, the earth, as well as man-made sources such as those from nuclear fuel and medical procedures. Radiation has been used in many industries including diagnostic imaging, cancer treatment (such as radiation therapy), nuclear reactors with neutron fission, radioactive dating of objects (carbon dating), as well as material analysis.

Ionizing radiation and its effects on the human body

There are four main types of ionizing radiation: electrons (also known as beta), photons (mostly gamma ray and X-ray), charged particles (alpha) and neutrons. In a nuclear reactor, the radiation is formed due to the decay of radioactive isotopes, which are produced as part of nuclear reactions inside the reactor.

Each ionizing radiation type interacts with the body differently but the end results are similar. When radiation enters a body, it can deposit enough energy that can directly damage DNA or cause many ionizations of atoms in tissues that would eventually cause damage to critical chemical bonds in the body. The mechanisms of how radiation damages tissues and the degree of damage of each type of radiation are different. However, the amount of radiation needed to cause permanent damage to the tissue depends on the total dose to the body, the type of radiation, and the amount of time it takes to get that amount of radiation (dose rate). Also, depending on the total dose and/or dose rate, the effect can be acute (happen right away such as radiation burns, sickness, nausea) or delayed (long-term, such as cancer ).

What are the health effects of various doses/dose rates?

Radiation dose is measured in Rad or Gy (1Gy = 100 Rad). However, the most often reported two units that have been mentioned in the media are Sievert (Sv) and Rem (1 Sv = 100 Rem). These are defined as dose equivalent, which accounts for the different effects each type of radiation have on the body. The Sievert and Rem are units used by regulatory authorities to control radiation release and exposure. The table below lists the different amount of radiation you can get from your normal activities.

Source
of RadiationDose in millirem (mrem) Dose in milliSv (mSv)
Background
(average in U.S.)~360 (1 yr)
~3.6 (1 yr)
Chest
X-ray~8 (per X-ray)~0.08 (per X-ray)CT
scan of abdomen~800 (per CT) ~8 (per CT) A
cross country flight in the U.S.2-5
0.02 – 0.05
Regulatory
limit for radiation workers5000 (1 yr) 50 (1 yr)

note: 1 Rem = 1000 millirem; 1Sv = 1000 millisievert; 1 millisievert = 1000 microsievert

It is important to note that the health effects of radiation exposure vary for different doses.  It is important to note dose is different from dose rate. Dose refers to the total amount of exposure, while dose rate refers to the exposure per unit of time (typically per hour). The dose numbers provided in the following discussion are not exact numbers, but instead general averages. An acute dose (received in a few days) above 250-400 Rem (2.5 – 4.0 Sv) is considered to be lethal for at least half of the population exposed. Not much is known about doses between 50 Rem and 250 Rem (500 mSv and 2500 mSv), but the exposed person will experience acute radiation sickness. The symptoms of such exposure can include nausea, vomiting, diarrhea, burns, and hair loss, but may or may not lead to near term death. Below this level, no acute symptoms have been observed. For radiation exposure of less than 50 Rem there is the potential for delayed effects such as non-specific life shortening, genetic effects, fetal effects, and cancer, but little is known about the long term consequences of exposures in this range. For doses less than 25 Rem there are not enough data to determine if such an exposure can cause any long-term effects on human health at all.

Lethal radiation dose compared to dose from normal activities.  Again, these numbers reflect cumulative dose, not dose rates.  To determine cumulative dose, multiply the dose rate by the time exposed:

Cumulative Dose = Dose Rate x Time Exposed

Radiation released from reactors at Fukushima and what it means

The radioactive fission products from the affected reactors include noble gases (xenon and krypton), volatile radioactive isotopes (iodine-131 and cesium-137) and non-volatile fission products. As mentioned before, these radioactive products release radiation as they decay. Therefore, over exposure and/or contact with them is dangerous. The noble gases are usually not of a big concern since they are inert, and tend to impose very small doses. Non-volatile fission products usually stay within the fuels so that is not much of a concern to the general public either. The fission products of most concern are the volatile ones such as I-131 and Cs-137 since they can be dispersed in air and get carried far away by wind from the affected reactors.

Iodine-131 is a radioactive isotope that releases beta particles (electrons). Concentration of iodine-131 in the thyroid has been shown to cause thyroid cancer. Therefore, it is a big concern if too much iodine-131 gets out of the reactor and falls to the ground away from the affected reactors. This can contaminate food, water, and animal products such as milk. The Japanese government has distributed iodine pills to people in the affected area. These iodine pills contain stable iodine-127, which does not cause cancer. When people take these iodine pills their bodies absorb the stable iodine to a level that prevents or limits the absorption of I-131, which helps to prevent the risk of thyroid cancer. Another fact about radioactive iodine-131 is that its half-life (the time it takes for half of it to decay to another nuclear isotope) is only about 8 days. This means that after about three months, almost all of the radioactive iodine-131 would have decayed away.

Cs-137, also emits a beta particle as it decays. Exposure to Cs-137 can also increase the risk of getting cancer but that again depends on the dose and the dose rate. However, Cs-137 causes a much longer term contamination problem because its half-life is about 30 years. Depending on the amount of Cs-137 that is released, and the regulations for acceptable elevated background radiation levels, the area contaminated with Cs-137 may not be inhabitable for a long time.

How to minimize radiation exposure

The rules of thumb for minimizing your exposure are to use time, distance, and shielding to your advantage. Shorten the time of your exposure to radiation, stay as far away from the radioactive source as reasonably possible (radiation goes down quickly as a function of distance, ~1/r2), and provide more shielding between you and the source. This is one of the reasons the people very close to the reactors were required to evacuate very early on after the earthquake. Also, the government recommended people between 20 and 30 km to stay indoors (because their houses provide extra shielding from some of the radiation – beta, alpha), and minimize their time outdoors to limit their exposure.

We strongly urge that our readers in the region follow the instructions of their local governments regarding if, when, and how to take cover or evacuate.

Radiation dose rate history at the Fukushima Daiichi site perimeter

The figure below was taken from the NY Times on 3/16/11:

http://www.nytimes.com/interactive/2011/03/16/world/asia/20110316-japan-quake-radiation.html?ref=asia

Note that dose comparisons are shown to provide perspective on how much dose people receive over a year, or during a one time exposure like a CT scan.

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Units 1 and 2: TEPCO has released estimates of the levels of core damage at these two reactors: 70% damage at Unit 1 and 33% at Unit 2. They have also stated that Unit 1 is being adequately cooled.

Outlook: It is difficult to make conjectures at this point about the final disposition of the damaged fuel without further information. However, during our only operating experience with a partially melted and subsequently cooled core, Three Mile Island, the fuel mass was fully contained by the reactor vessel, resulting in minimal radiation release to the public. A decision is currently being made on how to best supply cooling water to Unit 2.

Unit 3: At 8:34 AM JST, white smoke was seen billowing from the roof of Unit 3. The source of this smoke was not investigated because workers were evacuated due to radiation levels. These levels had been fluctuating during the early morning hours before rising to 300-400 millisievert/hr around the time that the smoke appeared. It was unclear at the time whether these rising levels were a result of some new event at Unit 3, or were lingering as a result of Unit 2’s recent troubles.

Outlook: In order to provide some perspective on worker doses to this point, radiation sickness sets in at roughly 1000 millisieverts. A future post will deal further with the health effects of various amounts of radiation. Response to the smoke seen at Unit 3 appears to be in an information gathering phase at this point. Chief Cabinet Secretary Yukio Edano speculated that the smoke from Unit 3 might be the result of a similar wetwell explosion to that at Unit 2, but there is not enough information currently available to support or refute that statement.

Units 4-6: Flames at Unit 4 were reported to be the result of a pump fire, which caused a small explosion that damaged the roof of Unit 4 (See TEPCO’s press release on the most recent fire at http://www.tepco.co.jp/en/press/corp-com/release/11031606-e.html) . Efforts at Units 4-6 are focused on supplying cooling water to the spent fuel storage pools. Temperatures in these pools began to rise in the days after the quake. At the time of the quake, only Unit 4’s core had been fully offloaded to the spent fuel pool for maintenance; roughly 1/3 of the cores of Units 5 and 6 had been offloaded. This explains in part why the temperature in Unit 4’s pool has risen faster than at the other reactors: it has a higher inventory, both in fuel volume and in heat load.

Outlook: The fuel within these pools needs to remain covered with cooling water in order to prevent the low levels of decay heat present from causing it to melt, and also in order to provide shielding. Boiling of the water results in reduction of the water level in the pools, so if/when the pools get hot enough for boiling to begin, water needs to be added to replace what boils off. The staff of Unit 4 plan to begin pumping water to the spent fuel pool from ground level as soon as radiation levels from Unit 3 are low enough for them to return. This pumping operation should be relatively easier than injection of cooling water into the reactor vessels at Units 1-3 because the pools are at atmospheric pressure.

Sources: TEPCO, World Nuclear News

UPDATE (11:48 AM EST): A report by the Federation of Electric Power Companies of Japan indicates that radiation levels as a result of the Unit 4 fire were higher than those reported previously. Radiation levels early this morning at the outside of Unit 3 measured at 400 millisieverts/hr. At the present time however, radiation levels at the boundary of the facility are 1530 microsieverts/hour. We will continue to update as further reliable information is available.

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Explanation of Nuclear Reactor Decay Heat

Nuclear reactors produce electricity in a similar way to conventional coal plants in that they heat steam to drive a turbine that spins an electric generator.  However, they differ on how that heat is produced.   Coal plants burn coal to heat a boiler that produces the steam while nuclear reactors use nuclear fission to create the heat.  The Fukushima reactors are boiling water reactors (BWRs) that produce the steam directly in the reactor core, which then drives the turbines.

The heat in an operating reactor is produced mainly by the fission of fissile isotopes such as uranium-235 and plutonium-239.  When a neutron causes one of these isotopes to split, a large amount of energy is released, which is then deposited in the fuel, cladding, coolant, and structures.  On average, approximately 80% of the energy released in a fission reaction is imparted to the two or more fission products and these deposit their energy in the fuel since they have a very short range.   The rest of the energy is released in the form of neutrons, and other forms of radiation.

When there is a SCRAM, where all the control rods are inserted and the reactor is shutdown, the fission reactions essentially stop and the power drops drastically to about 7% of full power in 1 second.  The power does not drop to zero because of the radioactive isotopes that remain from the prior fissioning of the fuel.  These radioactive isotopes, also called fission products, continue to produce various types of radiation as they decay, such as gamma rays, beta particles, and alpha particles.  The decay radiation then deposits most of its energy in the fuel, and this is what is referred to as decay heat.   As these radioactive isotopes continue to decay, more and more of them reach a stable state and stop emitting radiation, and thus no longer contribute to the decay heat.

The decay heat must be removed at the same rate it is produced or the reactor core will begin to heat up.  The removal of this heat is the function of the various reactor core cooling systems that provide water flow through the reactor core and then reject the heat elsewhere.  However, at the Fukushima site the integrity of these systems were compromised by the large tsunami that resulted from the earthquake, and made it difficult for the operators to keep up with removing the decay heat.

The amount of the decay heat expected at various times after shutdown is well known.  Below is a figure and a table that show an estimate of the decay heat of Fukushima Units 1-3 in MW as time has progressed since the earthquake.  This data is not produced from measured data on the actual reactors at Fukushima, but from using a simplified model that estimates the behavior relatively well.  The data is likely to be somewhat conservative, so a more accurate model (i.e. 2005 ANS Standard or ORIGEN simulation) would correctly estimate lower powers.

Approximate reactor decay heat vs. time.  The curves begin after the SCRAM of the reactors (complete and rapid control rod insertion) that occurred immediately after the earthquake.

Tabulation of approximate decay heat for the Fukushima reactors from 1 second after the scram caused by the earthquake until 1 year after the event.

Date/Time (Fukushima Time)Fukushima Daiichi-1 Decay Heat (MW)Fukushima Daiichi-2 & 3 Decay Heat (MW)Percent of Full Reactor Power3/11/11 2:46 PM89.8153.16.45%3/11/11 2:47 PM42.572.53.05%3/11/11 2:48 PM34.759.12.49%3/11/11 2:50 PM29.249.82.10%3/11/11 3:00 PM21.937.31.57%3/11/11 3:30 PM16.928.81.21%3/11/11 8:00 PM10.718.20.77%3/12/11 8:00 AM8.013.60.57%3/12/11 8:00 PM7.011.80.50%3/13/116.711.50.48%3/14/115.89.90.42%3/16/114.98.30.35%3/20/114.06.90.29%4/1/113.05.20.22%7/1/111.62.70.11%10/1/111.22.00.08%3/11/120.81.40.06%

Fukushima unit 1 has an electrical rating of 460 MWe and units 2 and 3 have an electrical rating of 784 MWe.  However, due to various thermodynamic and practical constraints, the efficiency of the plants is only about 33%.  Therefore, they have thermal ratings (MWth) about 3 times that of the electrical ratings and this thermal energy is the energy that must be removed, and is what is shown in the figure and table above.  The decay heat drops off very slowly after about 1 day where the decay power is already below 1% of the operating power of the reactor.  After a year the decay power is about 0.06% of the operating power of the reactor.

If the decay heat is not removed then the reactor fuel begins to heat up and undesirable consequences begin as the temperature rises such as rapid oxidation of the zircaloy cladding (~1200C), melting of the cladding (~1850C), and then the fuel (~2400-2860C).

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Spent nuclear fuel pools

Spent nuclear fuel (SNF) refers to fuel after it has fueled a reactor.  This fuel looks like new fuel in the sense that it is made of solid pellets contained in fuel rods.  The only difference is that SNF contains fission products and actinides, such as plutonium, which are radioactive, meaning it needs to be shielded.   Just as with the fuel rods in a shutdown reactor, the SNF produces decay heat because most of the decay radioactivity from the fission products and actinides is deposited in the fuel and converted into thermal energy (aka heat).   As a result, the SNF also needs to be cooled, but at a much lower level than fuel in a recently (<12 hours) shutdown reactor as it produces only a fraction of the heat.   In summary, the SNF is stored for a certain time to: 1) allow the fuel to cool as its decay heat decreases; and 2) shield the emitted radiation.

To accomplish these goals, SNF is stored in water pools and large casks that use air to cool the fuel rods.  The pools are often located near the reactor (in the upper floors of the containment structure for a BWR Mark-1 containment).  These pools are very large, often 40 feet deep (or larger depending on the design).  The pools are made of thick concrete, lined with stainless steel.   SNF assemblies are placed in racks at the bottom of these pools, so almost 30 feet of water covers the top of the SNF assemblies.  The assemblies are often separated by plates containing boron which ensure a neutron chain reaction cannot start.  The likelihood of such an event is further reduced because the useful uranium in the fuel has been depleted when it was in the reactor, so it is no longer capable of sustaining a chain reaction.  The water in the pool is sufficient to cool the SNF, and the heat is rejected through a heat exchanger in the pool so the pool should stay at fairly constant average temperature.  The water depth also ensures the radiation emitted from the SNF is shielded to a level where people can safely work around the pools.

Under normal operating circumstances, spent fuel can be stored in the pools indefinitely. An active cooling system is in place to remove the residual decay heat and the water also provides effective radiation shielding. The amount of fuel that can be stored into the pool can vary according to the capacity of the pool itself, but most spent fuel pools are design to be able to store many reactor cores at once.

During the refueling operation the reactor is shut down, all the areas between the reactor and the spent fuel are flooded with water (to provide radiation shielding) and fuel elements are moved one by one from the reactor to the spent fuel pool where they are re-racked. Refueling can occur every 12-18 months and during a single refueling shut down, up to one third of the fuel elements of the core are replaced. All the operations are conducted remotely under water through cranes and special equipment to avoid radiation exposure to the workers.

The spent fuel is usually stored in the spent fuel pool for a number of years, depending on the spent fuel capacity and on regulations, and after that period they are usually dry stored in concrete casks located on the site outside the reactor buildings.

If there is a leak in the pool or the heat exchanger fails, the pool temperature will increase.  If this happens for long enough, the water may start to boil.  If the boiling persists, the water level in the pool may fall below the top of the SNF, exposing the rods.  This can be a problem as the air is not capable of removing enough heat from the SNF so the rods will begin to heat up.  If the rods get hot enough, the zirconium-based cladding will oxidize with the steam and air, releasing hydrogen which can then ignite.  These events would likely cause the clad to fail, releasing radioactive fission products like iodine, cesium, and strontium.  It is important to note that each of these occurrences (cooling system failure, pool water boiling, fuel rod overheating in air, zirconium oxidation reaction) would each have to last sufficiently long in order to cause an accident, making the total likelihood of a serious situation very low.

The most significant danger if such an event were to occur is that there is no robust containment structure (like the one housing the reactor,) surrounding the SNF pool.  While SNF pools themselves are very robust structures, the roof above each pool is not as strong and may have been damaged, meaning the surface of the pool may be open to the environment.  As long as the water covers the fuel, this does not pose a direct threat to the environment, however it does allow for a possible dispersion of these fission products if a fire were to occur.  But if the water level stays above the fuel, the threat of a large dispersion event is low.

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Explosion at Unit 2

It was reported earlier today that the explosion at Unit 2 of the Fukushima Daiichi plant damaged the suppression chamber.  As discussed in the previous post, the suppression chamber/torus (i.e. donut shape vessel containing water) is used to depressurize the reactor.   The suppression pool is designed to condense the hot steam from the reactor, but can only do so as long as sufficient cold water remains in it.  It should also be noted that the suppression pool is part of the primary containment.

Hydrogen gas from the cladding oxidation with steam collected in the suppression pool and ignited.  This scenario differs from those of units 1 and 3 where the explosion occurred outside the primary containment in the upper part of the reactor building.  The reasons why the steam/gas mixture was not released to the reactor building are still not clear.  This breach of primary containment is certainly more serious than the situation in units 1 and 3.  Seawater is still being pumped in the containment and the reactor vessel.  At this time radioactive releases from unit 2 have been similar to the ones seen from units 1 and 3.

Fire at Unit 4 spent fuel pool

Recent reports by TEPCO indicate that an oil leak in a cooling water pump was the cause for the fire that burned for approximately 2 hours on Tuesday.  On Wednesday morning (local time), another fire broke out, but it is reported the fire is not at the spent fuel pool.  The cause is still unknown.

Reactor spent fuel pools

Spent fuel pools are used to cool down used nuclear fuel after it is removed from the reactor.  The used nuclear fuel still contains residual heat from the radioactive decay of the fission product and must be stored in a cooled pool of water until intermediate or ultimate disposal.  If insufficient cooling is provided to the pools, the water boils potentially exposing the spent fuel.  As the temperature increases, the cladding would oxidize with the steam releasing hydrogen which can then ignite.  This would also create fuel failures, releasing radioactive gases such as iodine, cesium and strontium.

It should be noted that unit 4 was under a 105-day long outage and that the fuel in the reactor had been moved to the spent fuel pool.  Reports throughout the day indicated that the temperature of the spent fuel pool was increasing.

Current reports also indicate that the temperatures in the spent fuel pools of units 5 and 6 are also increasing.

http://nei.cachefly.net/newsandevents/information-on-the-japanese-earthquake-and-reactors-in-that-region/

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