Japan Nuclear Reactor Explosion - pass the salt please?

[Sasquatchvnv]

Sounds like you pretty much know what you're talking about. In any case - this is a horrible mess with long lasting effects for at least the local people, and maybe the rest of us as well.

 

[The B]

Crazy as it sounds - a google search indicates that these reactors have the containment pools ABOVE the reactor... (multiple sources) But conflicting reports as to whether they have been blown.

they're NOT above the reactor- they're to the side, and guarded by steel and concrete framing. the actual tops of the buildings- the "attics," essentially, are the secondary containment "vessels" for each reactor. they're designed to take flash steam, hydrogen in the event of an emergency, and blow up if it comes down to it. while the hydrogen explosions have been gnarly and damaging, the area where the pools are kept are well below the blast zone.

i'll see if i can dig up a diagram in a bit.

 

[MA Duce]

Sounds like you pretty much know what you're talking about. In any case - this is a horrible mess with long lasting effects for at least the local people, and maybe the rest of us as well.

Yes..and the sad thing is we will deal with the aftermath by the renewed activity by the anti-nukes. OPEC must be just loving this.

 

[trainsktg]

i'll see if i can dig up a diagram in a bit.

Here's a diagram from the Nuclear Energy Institute.

<broken link removed>

Personally, I don't see how they can claim that a BWR is safer than a PWR, but a bunch of them are built this way.

Keith

 

[Sasquatchvnv]

Commonseletion requests/File:BWR Mark I Containment, cutaway.jpg - Wikimedia Commons

Here is the diagram that was posted on Wikipedia. Been deleted now...

 

[Blitzkrieg]

they're NOT above the reactor- they're to the side, and guarded by steel and concrete framing. the actual tops of the buildings- the "attics," essentially, are the secondary containment "vessels" for each reactor. they're designed to take flash steam, hydrogen in the event of an emergency, and blow up if it comes down to it. while the hydrogen explosions have been gnarly and damaging, the area where the pools are kept are well below the blast zone.

i'll see if i can dig up a diagram in a bit.

With all due respect and the possibility that you may be correct, you may also be totally wrong. We are not there (thankfully) and we really have no clue what is actually happening. Caution is the reaction of sane folks at this point: calm, reasoned caution and preparation for the worst

As of tonite I only have ambient radiation indicated on my dosimeter and my Geiger counter, (Lacey, WA area) and it looks at this point that the radioactive weather will go South of here, but I am watching carefully, regardless

 

[The B]

there's not going to BE any radioactive weather. it'd be the first time in history fallout of any harm- short term or long term- made it farther than 1,000 miles, and this is, by far, the NOT worst nuclear disaster earthlings have endured. japan is four thousand six hundred miles away, and across a very wet ocean, approaching a wet coast in the midst of spring moisture. i commend you for being prepared, but your prophecy of doom is ridiculous.

as to the fuel rods- they already admitted that unit 4's pool burned. that's a pretty honest admission, and is a worse scenario than having spent fuel rods ejected a few hundred feet in a hydrogen explosion. but neither will effect us.

 

[Colt Carbine]

Misconceptions and ignorance seem to be fueling a lot of hysteria and fear, all over the place. Yes, I was ignorant of all information posted below myself until a few days ago. Here is some stuff I found educating myself on nuclear energy. I hope some of these postings can help others understand.

No, this does not mean I know what is going on with these reactors. However, a basic understanding of how they function and what can actually happen if they did meltdown goes along ways.

I'm sure those who have been freaking out will continue to do so regardless of what information and facts are put before them to read.

Why A Nuclear Reactor Will Never Become A Bomb




By Alasdair Wilkins on March 17, 2011 at 10:14 AM

As Japan's Fukushima power plant continues to struggle with massive equipment failure and radiation release that could well reach Chernobyl levels, we can take some small comfort in the knowledge that a full-on nuclear explosion is completely impossible. Here's why.

Chain Reactions

Both nuclear reactors and nuclear weapons depend upon chain reactions. Such reactions require the presence of fissile materials, which are any atomic isotopes which can, when they undergo a particular nuclear reaction, create the raw materials necessary for the same reaction to repeat itself. There's only one naturally occurring fissile isotope, and that's uranium-235 &#8211; all other fissile isotopes, such as various plutonium isotopes, have to be artificially "bred" from natural isotopes.

So how does a chain reaction work? Let's consider the one involving uranium-235, which is the chain reaction used in nuclear reactors and many nuclear weapons. A free neutron hits a slow-moving uranium-235 isotope and is absorbed into it. Here one of two things can happen: the uranium will fission into two lighter, faster-moving isotopes, typically krypton-92 and barium-141, as well as some gamma radiation. The nuclear reactor is then able to absorb this energy, which is about three million times the energy the same amount of coal can produce in conventional burning.

Crucially, this reaction also creates additional free neutrons, which can then be absorbed into other uranium-235 isotopes and start the whole process over again. This is why, of the naturally occurring uranium isotopes, only uranium-235 is fissile &#8211; when uranium-238 undergoes such reactions, it can't release neutrons with the energy to start up a chain reaction.

As long as the reactions create an average of one or more neutrons, the chain reaction can go on indefinitely. Frequently, these reactions create more than one free neutron, which can cause the amount of energy being produced to increase over subsequent generations.

Safety Measures

To prevent a potentially dangerous buildup of energy, nuclear reactors build in huge numbers of fail-safes and redundancies. One of the better-known methods is the use of control rods, which are made from materials such as boron that absorb neutrons but cannot undergo nuclear reactions. In the event of a runaway energy buildup, these rods are often rigged to fall right into the heart of the reactor to absorb all the free neutrons and shut down the chain reaction. Mismanagement of these rods was one of many factors behind the Chernobyl disaster.

And yes, if all the fail-safes and redundancies do somehow fail to stop the heat buildup &#8211; as they did in Chernobyl, as they partially did at Three Mile Island, and as they might do in the current situation in Japan &#8211; there can be some pretty nasty effects. The most infamous threat is that of a nuclear meltdown, which is when the heat buildup causes the entire core to melt, damaging the protective structures to the extent that intensely radioactive materials can be released into the environment.

A meltdown obviously can have horrific short-term and long-term environmental effects, but what about an actual explosion? Could a nuclear reactor explode with the sort of force unleashed in the bombings of Hiroshima and Nagasaki? After all, Chernobyl exploded, didn't it? Thankfully, the answer to all this is no, a nuclear explosion is impossible, and the destructive blast at Chernobyl was actually just a steam explosion &#8211; and a good thing too, because a nuclear blast of the same magnitude could have turned Chernobyl from a horrific disaster to a full-on cataclysm. But again, such an explosion is totally impossible, and to understand why we have to look at the difference between nuclear reactors and nuclear weapons.

A Matter of Quality

Although we think of uranium as the most common fuel for nuclear reactions, that isn't strictly true. Natural uranium is pretty much completely useless for nuclear reactors, let alone nuclear weapons. This is because natural uranium is about 99.3 per cent composed of the isotope uranium-238, while just .7 per cent uranium-235, and only the latter is capable of sustaining nuclear chain reactions.

To make uranium usable for chain reactions, it needs to be enriched. This involves carefully separating out the uranium-235 from the uranium-238. The two have practically the same mass, particularly because uranium-235 is typically found in a compound state with fluorine, which nudges its mass to pretty much that of its big brother.

Nuclear reactors need low-enriched uranium, which is defined as anything with less than a 20 per cent concentration of uranium-235. Typically, nuclear power plants only need a 3-4 per cent concentration to have reactor grade uranium. Nuclear weapons, on the other hand, require highly enriched uranium for the sort of runaway chain reaction that can create a nuclear explosion. The cutoff for high enrichment is just 20 per cent, but the vast majority of nuclear weapons use uranium with a concentration of anywhere from 80 to 95 per cent. The bomb dropped on Hiroshima, for instance, used 80 per cent enriched uranium.

Critical Mass

So what's the real difference between low- and high-enriched uranium? Why couldn't low-enriched uranium create an explosion that's just not quite as severe as its high-enriched equivalent? For that, we must turn to another term that is frequently mentioned but infrequently understood, and that's critical mass. The term simply means that there's enough fissile material present to sustain a chain reaction, and a supercritical mass is where enough material is present for the fission rate to increase.

Although mass is obviously an important factor here &#8211; hence the name &#8211; it's possible to alter the point of criticality by varying other attributes of the material, including shape and density. A nuclear weapon is designed to release all its energy in one incredibly destructive blast, which means the material wants to be as densely packed with fissile material as possible, and the material should be packed into as homogeneous a sphere as possible.

That's absolutely nothing like the design of reactor cores, which is meant to produce a steady, controlled release of energy, and even the sort of energy buildup needed to produce a meltdown can't ever attain the speed and intensity needed for an explosive nuclear energy release. The geometric arrangement of uranium-235 in a nuclear reactor is just fundamentally not conducive to the spherical arrangement needed for an explosive chain reaction, and the amount of non-fissile uranium-238 in reactor-grade uranium also stops any runaway reactions dead in their tracks.

Why this matters

None of this is intended to minimise the very real dangers of nuclear reactor accidents. As seen in Chernobyl, meltdowns can have absolutely devastating environmental effects, and the nearby town of Pripyat remains uninhabitable twenty-five years after the accident. We don't yet know whether the current situation in Japan will reach Chernobyl levels &#8211; experts have at least seriously considered the possibility, but we still don't have a clear handle on the full extent of the danger.

Still, even in the midst of disasters of unimaginable proportions, it's crucial to try to maintain some nuance regarding the line between real fears and hysteria, and in the case of nuclear safety the best way to do that is to understand a little of the science behind the technology. The threat of a nuclear meltdown is worrisome enough without having to invoke the specter of the mushroom cloud.

 

[Colt Carbine]

BWR Mark I Containment sketch

1 Core with fuel rods
2 Concrete shield plug
3 Equipment pool
4 Drywell head
5 Fuel storage pool; spent fuel area
6 Refuelling cavity
7 Drywell flange
8 Reactor pressure vessel
9 Biological shield
10 Secondary concrete shield wall
11 Free standing steel drywwell
12 Radial beam
13 Concrete embedment
14 Jet deflector
15 Expansion bellows
16 Vent header
17 Downcomer pipe
18 Water (wetwell)
19 Embedded shell region
20 Basement
21 Reactor building
22 Refuelling platform
23 Refueling Bulkhead
24 Pressure supression chamber (runs in a torus around the reactor)
25 Vent (81 inch diameter)
26 Crane
27 Used Fuel
28 Coolant pipe
29 Cold water pipe (from generator)
30 Steam pipe (to generator)
31 Control rod drives
39 Control rods
40 Steam separators (water normally goes to this level)
41 Steam dryer
42 Vent and head spray

 

[Colt Carbine]

What is criticality?

Posted on March 18, 2011 9:52 pm UTC by mitnse

The words "criticality" and "re-criticality" have been used extensively in the media coverage. Criticality is a nuclear term that refers to the balance of neutrons in the system. "Subcritical" refers to a system where the loss rate of neutrons is greater than the production rate of neutrons and therefore the neutron population (or number of neutrons) decreases as time goes on. "Supercritical" refers to a system where the production rate of neutrons is greater than the loss rate of neutrons and therefore the neutron population increases. When the neutron population remains constant, this means there is a perfect balance between production rate and loss rate, and the nuclear system is said to be "critical." The criticality of a system can be calculated by comparing the rate at which neutrons are produced, from fission and other sources, to the rate at which they are lost through absorption and leakage out of the reactor core. A nuclear reactor is a system that controls this criticality or balance of neutrons.

The power of a reactor is directly proportional to the neutron population . If there are more neutrons in the system, more fission will take place producing more energy. When a reactor is starting up, the neutron population is increased slowly in a controlled manner, so that more neutrons are produced than are lost, and the nuclear reactor becomes supercritical. This allows the neutron population to increase and more power to be produced. When the desired power level is achieved, the nuclear reactor is placed into a critical configuration to keep the neutron population and power constant. Finally, during shutdown, the reactor is placed in a subcritical configuration so that the neutron population and power decreases. Therefore, when a reactor is said to have "gone critical," it actually means it is in a stable configuration producing a constant power.

A reactor is maintained critical during normal power operations. In other systems, such as a spent fuel pool, mechanisms are in place to prevent criticality. If such a system still achieves criticality, it is called "re-criticality". Boron and other materials, which absorb neutrons, are in place to make sure that this re-criticality does not occur. The added neutron absorbers substantially increase the rate of loss of neutrons, to ensure a subcritical system.

Most types of light water reactors (like the BWRs in Japan) use water to not only cool the reactor, but to also slow down neutrons. In these systems, slower neutrons cause the majority of fission reactions. Therefore, if the water boils off, neutrons will not slow down as much and the probability of fission reactions and power decreases, thus putting the nuclear system in a subcritical state.

If water heats up and vaporizes in a BWR reactor or spent fuel pool without cooling, the temperature increase of the water and eventual vaporization of water will tend to place the system in a subcritical condition. There are also large amounts of boron in these systems such as the control rods of the reactor, and various kinds of boron in the spent fuel pools. Additionally, steel structures supporting the spent fuel in the pool are sometimes made out of borated steel, which also contains large amount of boron. Even if the fuel does melt, the new geometric configuration will likely not be favorable for slowing down neutrons, so re-criticality is unlikely, even if water should be reintroduced to the system.

 

[Colt Carbine]

On prediction of "worst case" scenarios

Posted on March 17, 2011 3:55 pm UTC by mitnse

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.

Thankfully, operating experience with melted fuel speaks favorably. At Three Mile Island, approximately 50% of the core's nuclear fuel melted, and just 5/8 inch (out of 9 inches) of the reactor pressure vessel's internal surface was ablated. During the corium's contact with the bottom of the vessel, the vessel glowed red-hot for about an hour. The heat to which the vessel was exposed induced metallurgical changes in the steel, rendering it more brittle. Instrumentation penetrations in the lower vessel head also suffered damage. Nevertheless, the molten core was contained by the vessel.

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.

 

[Colt Carbine]

What is an isotope?

Posted on March 17, 2011 10:41 pm UTC by mitnse

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.

 

[Colt Carbine]

News update, 3/18

Posted on March 18, 2011 10:19 am UTC by mitnse

News Brief, 3/18/11, 10 AM EDT

Spraying of spent fuel pools at Units 3 and 4 is still underway. Visual inspection of Unit 4’s pool showed water in the pool, and so efforts have been temporarily focused upon Unit 3. While efforts at using helicopters to dump water onto the pools had been largely unsuccessful , army firetrucks used in putting out aircraft fires have been employed with some success. The elite Tokyo Hyper Rescue component of the Tokyo fire department has arrived on scene and is conducting missions of roughly two hours in length, during which they spray the pools for 7-8 minutes, wait for steam to dissipate, and spray again.

A cable has been laid from a TEPCO power line 1.5 km from the facility, which will be used to supply power to emergency cooling systems of the reactors at Units 1 and 2.

Backup diesel generators have been connected to cool the spent fuel pools at Units 5 and 6. As of 4 PM JST, temperatures in those pools have reached 65.5 and and 62 degrees Celsius.

Visual inspections have been conducted of both the central spent fuel pool, which contains 60% of the facility’s fuel, and the dry cask storage area. Water levels at the central pool have been described as “secured”, and the dry casks show “no signs of an abnormal situation”. More detailed checks of these areas are planned for the future.

A Japanese government agency has released the results of radiation measurements at dozens of monitoring posts. See the data here: <broken link removed> .

These measurements give doses in excess of background radiation, which is why some may appear low. High measurements at reading point 32 are thought to be the result of a controlled containment venting and a simultaneous fire which carried radioactive particles inland. Over the course of the incident, the general trend has been for weather patterns to sweep radioactive particles out to sea.

As a result of these radiation measurements and the ongoing work, the Japanese Nuclear and Industrial Safety Agency upgraded the event to a 5 on the INES scale. This is the same level as the Three Mile Island accident, and two steps below Chernobyl.

Resources: ANS Nuclear Café’; World Nuclear News,; IAEA; Ministry of Education, Culture, Sports, Science and Technology (MEXT).

Note: We earlier reported the temperature of spent fuel pool 6 as 84 degrees C. This was a typographical error. We apologize for the mistake.

 

[Colt Carbine]

Introduction to Radiation Health Effects and Radiation Status at Fukushima

Posted on March 16, 2011 11:39 pm UTC by mitnse

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 Radiation Dose 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 workers 5000 (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:

Radiation at Fukushima Daiichi - Graphic - NYTimes.com

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.

 

[trainsktg]

Orygun,

Thanks for taking the time to provide this information. Although basic in content, it is nonetheless more than adequate to allow interested laymen the ability to sufficiently educate themselves about the operation of a modern nuclear reactor and therefor better understand the situation in Japan. I hope folks here take the time to read it.

On a personal note, although I have an excellent job here in the States, I will be applying to nearly every company that has a chance of being mobilized in the next few months to assist in the decommissioning of these six reactors. The Japanese people are in a dire situation because of the effects of the overall disaster, and I do not think that they alone can supply the vast number qualified tradespeople, technicians, and engineers that will be necessary to quickly rebuild their country. I'd like to help with that if I am able .

Keith

 

[MA Duce]

Nice job Orygun. Although I'm sure some will not pass up any chance to continue to make a hysterical crises out of nothing regardless of the huge amount of information out there telling us the truth. Some will always choose to ignore logic and science if it gets in the way of their love of drama and devoted pursuit of ignorance.

 

[MA Duce]

Looks like the Japanese have got a permanent power link to two of the six reactors at Fukishima, and the coolant systems are operating. But the most heavily damaged one are still at risk....small victory, but victory nonetheless.

 

[The B]

and lo and behold.... no nuclear holocaust. whatever will i put my energy into now, now that the nuclear zombie uprising will not be happening?

guess i'll go get the .50 off the roof, pack up the claymores, and sweep up all those caltrops...

 

[MA Duce]

and lo and behold.... no nuclear holocaust. whatever will i put my energy into now, now that the nuclear zombie uprising will not be happening?

guess i'll go get the .50 off the roof, pack up the claymores, and sweep up all those caltrops...

Yeah......I just wish I hadn't downed those 500 PI tablets......don't ya just hate it when reality bites a big hole in your paranoia???

 

[The B]

Yeah......I just wish I hadn't downed those 500 PI tablets......don't ya just hate it when reality bites a big hole in your paranoia???

speaking of paranoia, this is your 666th post...