News brief, April 4

News Updates, 4/4/11 TEPCO has identified a potential pathway by which water from Unit 2 may have been leaking into the Pacific Ocean. This pathway consists of a 20 cm crack in the concrete wall of a pit which holds electrical cables for the seawater intake pumps. Two efforts have been made to plug this…

News Updates, 4/4/11

TEPCO has identified a potential pathway by which water from Unit 2 may have been leaking into the Pacific Ocean. This pathway consists of a 20 cm crack in the concrete wall of a pit which holds electrical cables for the seawater intake pumps. Two efforts have been made to plug this crack, with limited success. The first was an attempt to pour fresh concrete over the breach, and the second made use of a polymer sealant. Crews are making use of tracer dyes in order to both track the flow of water out of the pit, and determine whether the repair is successful.

Efforts are additionally still underway to remove and store water from the basements of the reactor turbine buildings. In unit 2, storage space for this water is running low. As a result, the Japanese government has authorized the discharge of some 10,000 tons of low-level radioactivity water from its wastewater treatment facility, in order to make way for storage of the more highly radioactive water in unit 2’s turbine building basement. TEPCO also plans to release some 1,500 tons of similarly low-level radioactive water from Units 5 and 6, so that it does not damage vital safety equipment. The discharge was scheduled to begin at 7 PM JST.

While these numbers seem alarming, TEPCO has predicted that a person eating seaweed and fish from the waters immediately outside the plant every day for a year would receive a dose of just 0.6 mSv. The IAEA has asked for additional information on this planned discharge so that it may itself predict the impact on the environment.

The IAEA has reported the most recent results of tests on vegetables grown in Fukushima and neighboring prefectures. In 133 of 134 samples tested, radiation was either not detected or was detected at levels lower than regulatory limits. A single sample of mushrooms from Fukushima province exceeded regulatory limits on radioactive iodine and cesium. The IAEA is continuing its own program of air and water monitoring, in addition to that already being performed by TEPCO and MEXT.

Like this:

Like Loading…

News updates, April 1

News Brief, 4/1/11 Status of Reactors Efforts are still underway to pump water from the turbine buildings of the reactors. Water in the turbine buildings is being pumped directly into the reactors’ condensers. The IAEA reported yesterday that the condenser of Unit 1 has been completely filled and that the pumping of water from Unit…

News Brief, 4/1/11

Status of Reactors

Efforts are still underway to pump water from the turbine buildings of the reactors. Water in the turbine buildings is being pumped directly into the reactors’ condensers. The IAEA reported yesterday that the condenser of Unit 1 has been completely filled and that the pumping of water from Unit 1’s turbine building has ceased.

All units are currently being cooled by injection of fresh water, using temporary pumps, with backup power supplies in place in case of further electrical power issues. TEPCO reports that water temperatures in the units are below 100 C in the pressure suppression chambers, and that no reactor coolant is being leaked to containment.

Radiation Measurements

Air monitoring data for the region can be found at http://www.mext.go.jp/english/radioactivity_level/detail/1303986.htm .

An isolated location outside the boundary of the evacuation zone displayed high levels of deposition of Iodine-131. The IAEA has announced that these levels, found in the village of Iitate, exceed their guidelines for evacuation. Local officials are assessing the situation.

Testing of various food products from the prefectures surrounding the damaged reactors show that all of the food product samples from outside Fukushima prefecture display either no radioactivity or levels below regulatory limits. However, 25 samples from the Fukushima prefecture did exceed Japanese regulatory limits.

Radiation levels in seawater immediately adjacent to the plant’s discharge canal rose yesterday (http://www.tepco.co.jp/en/press/corp-com/release/11033109-e.html) . The cause of this increase is still not known.

The EPA has continued to monitor the potential pathways for radiation exposure of the U.S. population. To date, radiation detected in milk is on the order of picocuries (10-12 Curie) per liter. This is 5,000 times lower than the FDA’s Derived Intervention Level. A Derived Intervention Level is the point at which the FDA would act to take the food in question out of our food supply. The level considers what fraction of the diet is made up by that food (very conservative numbers are used), how often it is drunk, and for how long people are exposed.

Similarly, the trace levels of radioactivity being detected in rainwater in the U.S. have been deemed far too low to be of consequence to human health. Levels in the air on the western coast of the U.S. continue to fluctuate about the natural level of background radiation, and are of no concern.

Like this:

Like Loading…

Measurements to date

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.

News Updates, March 30

News Brief, 3/30/11 Plutonium Found in Soil Soil samples collected from five locations around the Fukushima Daiichi site were found to contain trace amounts of plutonium. These trace amounts are in roughly the same quantities as the amounts left behind by nuclear weapons tests conducted prior to 1980, and are not considered a threat to…

News Brief, 3/30/11

Plutonium Found in Soil

Soil samples collected from five locations around the Fukushima Daiichi site were found to contain trace amounts of plutonium. These trace amounts are in roughly the same quantities as the amounts left behind by nuclear weapons tests conducted prior to 1980, and are not considered a threat to human health, according to JAIF. Only two of the sites are believed to contain plutonium originating in the troubled reactors, with the rest being the result of the nuclear weapons tests. It is not known from which reactor the observed plutonium might have come, or how it was deposited in the soil. A companion post will discuss this, and the health effects of plutonium.

Water Accumulations

Contaminated water has accumulated in the basements and turbine rooms of units 1-3, and the basement of unit 4. Efforts are underway to clean up and store this water, preventing it from entering the environment, and allowing crews to continue servicing the electrical connections in the basement of each reactor.

Each reactor building additionally has a trench outside it which is concrete-encased, and holds cables and piping for its associated reactor. The trenches outside units 1-4 have flooded with contaminated water. The trenches do not flow to the ocean, and are currently being sandbagged so that they do not overflow and carry radionuclides elsewhere.  TEPCO has released a nuclide analysis of trench 1 (http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110330e2.pdf) which shows that the trench contains low levels of fission products, and no uranium or plutonium.  Dose rates at the surface of this trench are around 0.4 milliSievert per hour. Dose rates at Unit 2’s trench are high, at 1000 milliSieverts per hour. This high dose rate indicates that the water has been in contact with molten fuel for some time. The pathway through which this water made it to the trench is not known at this time.

Measurements have been taken of seawater 30 km from the facility, and have indicated that only fission products, in small quantities, have made their way to the sea. These quantities, in amounts shown here (http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110330e7.pdf) are far too low to impact human health. Fish from the region have been tested, and a have shown levels of Cs-137 at or just above the level of detection. These levels are below those of concern for fish consumption. Experts from the National Research Insitute’s Fisheries Research group say that it’s too early to draw conclusions, as the situation may change rapidly, but that the situation should improve as the radionuclides decay and dilute in seawater.

Like this:

Like Loading…

News Updates, March 26

Plant Status The IAEA has shared that as of 05:15 UTC, Japanese authorities reported the following about the conditions of the six reactors: Unit 1: Workers have restored lighting in the control room, and recovered the ability to use some instrumentation. As of March 25, fresh water is being pumped into the pressure vessel instead…

Plant Status

The IAEA has shared that as of 05:15 UTC, Japanese authorities reported the following about the conditions of the six reactors:

Unit 1: Workers have restored lighting in the control room, and recovered the ability to use some instrumentation. As of March 25, fresh water is being pumped into the pressure vessel instead of seawater, in an effort to minimize corrosion.

Unit 2: Seawater injection continues and pressure in the reactor vessel is stable.

Unit 3: Workers are pumping fresh water into the reactor vessel, and seawater into the spent fuel pool. Fire fighters sprayed water into the building from outside yesterday.

Unit 4: Workers used a concrete truck to pour water into the spent fuel pool, while simultaneously pumping seawater through the spent fuel pool’s own coolant system.

Units 5 and 6: Both reactors are in cold shutdown, with fuel pool temperatures stabilized at acceptable levels.

Effects on Health and Safety

A TEPCO press release (http://www.tepco.co.jp/en/press/corp-com/release/11032503-e.html) dated March 25 estimated that three workers, who were laying electrical cable, received doses of around 170 milliSievert to the legs. Doses of these levels, when caused by beta radiation, often cause burns to the skin. The workers were transferred to the hospital, and decontaminated. TEPCO maintains that the workers did not heed the alarms of their radiation dosimeters, believing radiation levels to be low in the area. It has been speculated that rising radiation levels in water surrounding Unit 3 is the result of a leak; updates about this leak will be made as information becomes available. Much has been made of this leak in the media, as Reactor 3 is fueled by a mixture of uranium and plutonium. However, measurements taken of the water in the plant (the water to which workers were exposed) did not detect the presence of either uranium or plutonium—just fission products.

On March 25, Japanese authorities reported to the IAEA that they had recorded the radiation doses to the thyroids of 66 children living just outside the perimeter of the evacuation zone. These measurements are important because the thyroid tends to accumulate iodine, and radioactive isotopes of iodine make up much of the radiation field being measured far from the reactor site. In addition, children are especially sensitive. The measurements showed no significant deviations from background radiation levels in these children, 14 of whom were infants.

Seawater 30 km offshore from the facility has been tested for the presence of radioactive species. Measurements revealed the level of iodine-131 to be at their legal limits, and cesium-137 to be well below their legal limits. Because these levels dilute with increased distance, it would take months or years for cesium-137 to be detected on other Pacific shores, predicts the IAEA’s Marine Environmental Laboratory. The tiny quantities of radionuclides already being measured on foreign shores are as a result of atmospheric transport, not dispersion in seawater.

Like this:

Like Loading…

Status Update as of 3/22/2011

Heightened discharges as pump truck arrives 22 March 2011 Researched and written by World Nuclear News ORIGINALLY PUBLISHED: 1.08pm GMT UPDATE 1: 4.01pm GMT, Connection of power to unit 3 Discharges to sea from Fukushima Daiichi have shown increased levels of radioactivity, Tepco has reported, as it brings in a concrete pumping truck to secure water…

Heightened discharges as pump truck arrives
22 March 2011
Researched and written by World Nuclear News
ORIGINALLY PUBLISHED: 1.08pm GMT
UPDATE 1: 4.01pm GMT, Connection of power to unit 3

Discharges to sea from Fukushima Daiichi have shown increased levels of radioactivity, Tepco has reported, as it brings in a concrete pumping truck to secure water levels in used fuel ponds.

Tokyo Electric Power Company released the results of a half-litre sample of water taken 100 metres south of the discharge channel from damaged units 1 to 4.

Testing for a range of radionuclides showed amounts below regulatory limits for cobalt-58, iodine-132 and cesium-136. Detections were far above limits, however, for cesium-137, cesium-134 and iodine-131.

Local people began evacuation more than ten days ago and this is complete to a 20 kilometre radius. People in a further ten-kilometre zone have been warned to stay indoors. Pills to block the potential negative health effects of iodine-131 have been distributed to evacuation centers.

Iodine-131 has a half-life of eight days, so its potential danger reduces relatively quickly. Caesium-137 has a half life of 30 years, whereas the other isotope, caesium-134, has a half-life of two years. Additional monitoring at eight locations is to be carried out by the Japan Agency for Marine-Earth Science and Technology in conjunction with the Japan Atomic Energy Agency. Results from this are expected on 24 March.

Major pump begins work

At 5.17pm today efforts to refill fuel ponds at units 3 and 4 were upgraded significantly by the arrival of a concrete pumping truck of the kind usually used in construction. It will supply water at up to 160 tonnes per hour through a 58 metre flexible boom via remote control.

It is hoped that this extra reach, capacity and flexibility will enable Tepco to reduce issue of fuel pond levels and cooling until such time that normal systems can be brought back into operation.

External power connections have been made to units 1 and 2 and checks are underway to verify that systems will function when power is restored. A connection has been made for units 3 and 4 and cabling to the main power control centre has been completed.

Smoke

White smoke was seen rising from the broken reactor buildings of units 2 and 3 yesterday, causing a short-term evacuation of workers from around unit 3 to a safer area of the site.

Smoke at unit 2 has now reduced virtually to the point of invisibility, while at unit 3 it changed from grey to white and is dissipating. Tepco said that neither its monitoring of unit 3’s reactor system or radiation in the area showed nothing unusual.

http://www.world-nuclear-news.org/RS_Heightened_discharges_to_sea_as_new_pump_truck_arrives_2203111.html

Like this:

Like Loading…

Fission Products and Radiation

When a heavy atom (such as uranium or plutonium) undergoes fission, it splits into two lighter fission products. This splitting process also yields two or three neutrons, which can cause other heavy atoms to fission, as well as a huge amount of energy, which nuclear engineers convert into electric power. The two fission product atoms…

When a heavy atom (such as uranium or plutonium) undergoes fission, it splits into two lighter fission products. This splitting process also yields two or three neutrons, which can cause other heavy atoms to fission, as well as a huge amount of energy, which nuclear engineers convert into electric power.

The two fission product atoms are not the same two atoms every time. Nuclear scientists can predict the distribution of fission products through physical models, but generally this is measured experimentally to ensure accuracy.

When fission products are first produced, they are highly unstable and rapidly decay  (usually β- decay) multiple times until they become relatively stable nuclides with long half-lives. All this decaying generates quite a bit of energy, which we call decay heat. So even after the fission reaction completely stops (which it did immediately following the earthquake), fission products continue to produce energy for a long period of time. This energy is large enough to melt the fuel if the fuel is not cooled, and cooling the fuel has been what the reactor operators at Fukushima have been struggling with for several days.  The fission reaction was never out of control – only the decay heat cooling systems were out of control.

Although most fission products are considered waste, some are very important to the operation of a nuclear reactor and have specific uses. Two of the fission products, xenon-135 and samarium-149, are prolific neutron absorbers (called “neutron poisons”) and can substantially affect control of the fission reaction during normal operation. Others, especially molybdenum-99 which eventually decays to technetium-99m, are used to produce “medical isotopes” that are essential for diagnostic testing for numerous life-threatening illnesses. Each year, 40 million people worldwide undergo necessary testing with technetium-99m. If you’ve ever had a nuclear medicine procedure, the chances are high that what they put into your body came straight out of a nuclear reactor – and if they hadn’t put it into your body, it would have been considered nuclear waste!

Fission products remain inside the fuel under normal circumstances.  When fuel resides in the core, it contains an amount of fission products proportional to the total energy it generated.  When the fuel is depleted, it is moved to spent fuel pools and ultimately to dry cask storage, long-term repositories, or reprocessing facilities. At the Fukushima nuclear power plants, fuel inside the core (and possibly the spent fuel pools) is suspected to have likely been damaged.  Because of this, some fission products, especially the gaseous products, have likely been released. We do not currently have enough information to know exactly which (or in what amount) fission products have been released.

Not all fission products are harmful. Although a few are gaseous, which enables them to travel long distances through the atmosphere, most are not highly mobile and will thus remain localized near the reactor site. Although nearly all fission products emit radiation, only some are potentially harmful to humans.

The chart below lists various important fission products along with their yields – the frequency at which they are produced from fission. For example, 6.3% of fission events (on average) will produce xenon-135 (after the highly unstable fission products rapidly decay). The half-life is a general time scale for how long the listed radioactive fission product will exist before decaying to a more stable fission product. Note that cesium and iodine, which were detected near the Fukushima site, are by far the most frequently occurring radioactive fission product elements.

YieldFission ProductHalf-life6.8%cesium-133/134*2 years6.3%iodine-135 / xenon-1357 hours6.3%zirconium-931.5 million years6.1%cesium-13730 years6.1%molybdenum-99 / technetium-99**200,000 years5.8%strontium-9030 years2.8%iodine-1318 days2.3%promethium-1473 years1.1%samarium-149not radioactive0.7%iodine-12915 million years0.4%samarium-15190 years0.4%ruthenium-1061 year0.3%krypton-8511 years0.2%palladium-1077 million years

*Cs-133 is stable but has a high fission yield, but it will then produce Cs-134 from absorbing neutrons in the reactor and Cs-134 is radioactive with a ~2 year half-life.

**Half-life reported in the table is for Tc-99.  Mo-99 has a half-life of ~66 hours, which then decays to Tc-99m (metastable form of Tc-99) with a half-life of  ~6 hours.  The Tc-99m then decays to the Tc-99 with the 200,000 year half-life reported in the table.

Note that longer half-lives do not necessarily mean more danger. Some fission products have extremely long half-lives but emit very little radiation at any given time, so they are not dangerous. Other fission products emit huge amounts of radiation but exist for such a short period of time that they are not dangerous. How harmful a given fission product is to humans is a complicated function of half-life, radiation intensity, and various human biology factors.

Like this:

Like Loading…

Status Update – 3/19/2011 at 10:00 am EDT

UPDATE AS OF 10 A.M. EDT, SATURDAY, MARCH 19 (per NEI): At a March 19 news conference, Japan’s Chief Cabinet Secretary Yukio Edano said that sea water injection is continuing at reactors 1, 2 and 3 at the Fukushima Daiichi nuclear power plant. Preparations were being made to spray water into the used fuel pool…

UPDATE AS OF 10 A.M. EDT, SATURDAY, MARCH 19 (per NEI):

At a March 19 news conference, Japan’s Chief Cabinet Secretary Yukio Edano said that sea water injection is continuing at reactors 1, 2 and 3 at the Fukushima Daiichi nuclear power plant.

Preparations were being made to spray water into the used fuel pool at reactor 4, and an unmanned vehicle sprayed more than 1,500 gallons of water over seven hours into the used fuel pool at reactor 3, Edano said. He also said he believed that the situation at the reactor 3 fuel pool is stabilizing.

Some reactor cooling capacity has been restored at reactors 5 and 6 after the installation of generators at those reactors, Edano added.

Edano said that progress had been made on “a fundamental solution” to restore power at the Fukushima Daiichi nuclear power plant, with electricity expected to be restored at reactors 1 and 2 today and reactor 3 as early as Sunday.

Edano said that additional equipment was being transported to the site and that other means of providing cooling water to the pool is be examined.

Radiation dose at the west gate of the Fukushima Daiichi was 83 millirem per hour on March 18 at 7:10 p.m. EDT and dropped to 36 millirem per hour by 8 p.m. EDT, Edano said. Radiation levels have decreased since March 16. Although they are higher than normal, radiation levels near the reactors are within the range that allows workers to continue on-site recovery measures, the International Atomic Energy Agency said.

According to the IAEA, radiation dose rates in Tokyo and other areas outside the 30-kilometer zone remain far below levels which would require any protective action by the public.

All reactors at the Fukushima Daini nuclear power plant are in cold shutdown (See the Japan Atomic Industrial Forum website).

Radiation levels have increased above the federal government’s level in some food products from the Fukushima Prefecture and nearby areas. These levels were detected in samples of milk in Fukushima Prefecture and six samples of spinach in neighboring Ibaraki Prefecture, according to the Japan Atomic Industrial Forum. Edano said that if these products are consumed for a year, the total radiation dose would be equivalent to one CT scan.

Additional monitoring of food products is continuing in those regions.

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

Like this:

Like Loading…

What is criticality?

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…

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.

Like this:

Like Loading…

Plutonium in the Environment

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,…

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.

Like this:

Like Loading…