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 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

Posted in Fukushima

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 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.

Yield Fission Product Half-life
6.8% cesium-133/134* 2 years
6.3% iodine-135 / xenon-135 7 hours
6.3% zirconium-93 1.5 million years
6.1% cesium-137 30 years
6.1% molybdenum-99 / technetium-99** 200,000 years
5.8% strontium-90 30 years
2.8% iodine-131 8 days
2.3% promethium-147 3 years
1.1% samarium-149 not radioactive
0.7% iodine-129 15 million years
0.4% samarium-151 90 years
0.4% ruthenium-106 1 year
0.3% krypton-85 11 years
0.2% palladium-107 7 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.

Posted in Basics

Status Update – 3/20/11 at 3:30 pm EDT

Stabilisation at Fukushima Daiichi

20 March 2011
FIRST PUBLISHED 2.05pm GMT
UPDATE 1: 2.56pm GMT Units 5 and 6 in cold shutdown, tsunami heights
UPDATE 2: 3.30pm GMT IAEA graph of fuel pond temperatures

Workers on site have succeeded in increasing the stability of the Fukushima Daiichi reactor units with units 5 and 6 now in cold shutdown. Pressure built up within unit 3 but a more significant venting does not seem necessary now.

External power has now been connected to unit 5 and 6, allowing them to use their residual heat removal systems and transfer heat to the sea. This has been used to cool the fuel ponds and bring the units to cold shutdown status, meaning that water in the reactor system is at less than 100ºC.

An extended operation to refill the fuel pond took place at unit 3, with the Hyper Rescue crew spraying for over 13 hours. Radiation levels 500 metres north of the reactor showed a decrease from 3.44 millisieverts per hour to 2.75 millisieverts per hour, indicating a measure of success in refilling the pond. A similar operation is planned for later today at unit 4 and the surface temperatures of the buildings appear to be below 100ºC.

At units 1 and 2, external power has been restored. Tokyo Electric Power Company (Tepco) said it would restore functions in the central control room shared by the units so that accurate readings could again be taken from the reactor system. Next, workers will check the condition of the water supply systems to the reactor and the used fuel pond. With luck these will be able to go back into operation as they had been immediately after the earthquake on 11 March.

External power for units 3 and 4 should be in place ‘in a few days time’, said Tepco.

Tsunami data

The Fukushima power plants were required by regulators to withstand a certain height of tsunami. At the Daiichi plant the design basis was 5.7 metres and at Daini this was 5.2 metres.

Tepco has now released tentative assessments of the scale of the tsunami putting it at over 10 metres at Daiichi and over 12 metres at Dainii.

The plant sites were inundated, causing the loss of residual heat removal systems at both sites as well as emergency diesel generators at Daiichi.

During the day, the company had noted a pressure increase within unit 3, warning that venting may be required. Preparations were underway to open the relief vale on the torus suppression chamber, or if that had proven full of water, another valve on the reactor containment. These operations had been expected to release a more significant amount of radioactivity than earlier venting, but Tepco has since said the higher pressure is stable and it does not need to vent.

Radiation monitors in the area near the plant have shown gradually decreasing readings since spikes on 15 March following the loud noises and apparent rupture of the torus of unit 2. While levels were above natural background radiation they have not posed a threat to public health. Most people within 20 kilometres were evacuated before that time and people in a further ten-kilometre radius has been warned to stay indoors. Evacuation centres have been supplied with potassium iodide tablets that would protect people’s thyroid glands in the case of a serious release of radionuclides from the reactor cores.

Despite contradictory comments by the US Nuclear Regulatory Commission to US politicians and media, most observers in nuclear industry and regulation consider the measures taken by Japanese authorities to be prudent.

Researched and written
by World Nuclear News

http://www.world-nuclear-news.org/RS_Stabilisation_at_Fukushima_Daiichi_2003111.html

Posted in Fukushima

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 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/

Posted in Fukushima

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 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.

Posted in Basics

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, 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.

Posted in Radiation Protection

News update, 3/18

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: http://www.mext.go.jp/component/a_menu/other/detail/__icsFiles/afieldfile/2011/03/18/1303727_1716.pdf.

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.

Posted in Uncategorized