Month: April 2011

News Updates, 4/15/11

An earthquake (magnitude 5.4) was reported at Hamadori, Fukushima Prefecture on 4/13. No unusual events were reported at any of the nearby nuclear power stations as a result of this.

Injection of fresh water into Units 1-3 continues, and their temperatures continue to drop steadily, while remaining above the level considered to be “cold shutdown”.

The removal of contaminated water from Unit 2 to its condenser for storage was initiated April 12, then stopped April 13, in order to check for leaks. We don’t have word on whether the process has restarted since then or whether any leaks were identified.

After spraying fresh water onto the spent fuel pools of Units 3 and 4, TEPCO collected samples of the water from Unit 4. That water showed much higher than normal quantities of radioactive iodine and cesium isotopes. No other radioisotopes were reported as being observed.

Workers are attempting to minimize discharge of water to the open sea by using steel plates to close off the facility’s seawater intake.

Dose rates to the public at the 30 km from the site continue to vary between 0.1 microsievert/hr and 26 microsievert per hour (in one isolated region, identified by MEXT reading location 32). No further updates on seawater measurements or food products are available since our last post.

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Today the Japanese Nuclear and Industrial Safety Agency revised its INES rating of the Fukushima Daiichi event. The previous assessment treated the events at each of the ailing reactors as separate: the core damage to units 1-3 resulted in an assignment of a 5 (accident with wider consequences) for each reactor; the problems at unit 4’s spent fuel pool were assigned a 3 (serious incident). NISA is now treating the situation as a single event, assigned a rating of 7 (major accident). This rating is still being assessed as information about the disposition of radioactive materials originating at the reactor site comes in.

Because the rating is now the same as that assigned to the Chernobyl accident, the blog has received a number of questions about how the events at Fukushima differ from it. We present a sequence of events at Chernobyl, along with links to some denser technical matter for interested readers, and an IAEA report on the human costs of the disaster. For comparison, it’s been estimated that the radiation released by the Fukushima reactors is 1/10th that released to the environment at Chernobyl.

Chernobyl

On April 26th 1986, the most serious nuclear accident in history took place at Unit 4 of the Chernobyl power plant located 130 km north of Kiev, Ukraine.  The site had four RBMK-1000 reactors.  These reactors are graphite moderated boiling water reactors and did not have a containment structure. Reactor containment is the large and thick concrete and metal structure surrounding the nuclear reactor. Its purpose is to protect the reactor from external damage, and to contain radioactivity in case of a significant reactor failure. By regulation, all western BWR and PWR reactors have to have a containment.  Additionally, the RBMK design also had a very large and positive coolant void reactivity coefficient, meaning that as the coolant (i.e. water) temperature increases, the reactor power increases. This positive coefficient is not present in BWRs or PWRs.

A brief summary of the events is presented here, a detailed description can be found at http://www.world-nuclear.org/info/chernobyl/inf07.html.    The following document (http://www.iaea.org/Publications/Booklets/Chernobyl/chernobyl.pdf) also includes information about the health and environmental effects of Chernobyl accident.

On April 25th, unit 4 was to shut down for routine maintenance, prior to which a safety test was to be performed.  The test was to evaluate how long the turbine would spin and supply power to the main water pumps in the event of a loss of electrical power.  This test had been tried in the past and new adjustments were made to allow the pumps to be powered longer.

After a few delays due to demand for electricity from the grid, the test was performed by an inexperienced crew.  The test was to be performed at ~30% of full power.  A series of operator actions, including the disabling of automatic shutdown mechanisms, positioned the reactor in a very unstable condition, in which the reactor was at very low power. As operators withdrew control rods in an attempt to increase the power to the level necessary for the test, the reactor heated up. The reactor’s positive void reactivity coefficient resulted in a rapid increase in power.  Control rods were inserted in order to staunch this increase in power. The unusual design of these control rods, which had graphite “followers” (recall that graphite is a moderator) worsened the situation by increasing power at an even more rapid rate. The result was a power excursion of between 100 and 500 times full power as the rods were inserted into the reactor.

This large power surge caused the fuel to disintegrate.   As the fragmented fuel interacted with the steam/water mixture, a steam explosion occurred. This blew off the reactor’s massive vessel top (1000 tons) which penetrated the reactor building concrete, and dispersed burning graphite and fuel.   This initial explosion and the subsequent fire sent a plume of radioactive gas and particulates into the environment. Further explosions were caused by production of hydrogen in clad/steam chemical reaction.

The radiological consequences of the Chernobyl incident were severe. The radioactive plume that emanated from the reactor contained not only volatile radioactive nuclides (such as Iodine-131, Cesium-137) which have been observed around Fukushima, but also many non-volatile ones, which were in the disintegrated fuel pieces. This plume got carried far away by wind and deposited radioactive particulates over many places in the northern hemisphere. 31 of the plant operators and firefighters got lethal radiation doses. The risk of cancer to surviving staff members and to residents of the 30 km evacuation zone is predicted to have approximately doubled as a result of exposure. An important thing to note about the Chernobyl accident is that the evacuation was not started until a nuclear reactor in Sweden (1000 km away) detected elevated radiation levels.

About 97% of the radioactive nuclides found in spent or partially spent fuel remain inside the fuel rods, as long as they do not melt. Of those, a fraction are noble gases (such as Xe-135), and many are solid materials. When the fuel melts, the noble gases escape the fuel and leak to the environment; however, due to being noble gases they do not react chemically, and disperse in the atmosphere. Iodine-131 (deposits in thyroid), Cesium-137 (30-year half-life) and Strontium-90 (replaces calcium in bones) are the three most significant non-gaseous fission products. Due to the explosion of the reactor vessel in the Chernobyl accident, these products were released as well, thus significantly contributing to the dose to the public.

Chernobyl unit 4 is now enclosed in a large concrete shelter which was erected quickly (by October 1986) to allow continuing operation of the other reactors at the plant.  The last reactor, unit 3, was shut down in 2000.  A New Safe Confinement structure is due to be completed in 2014. It is being built adjacent to the facility and then will be moved into place on rails.

 

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News Brief, 4/11/11

Aftershocks
The region surrounding the troubled nuclear power station has suffered two aftershocks in recent days. The first, a magnitude 7.1 on the Richter scale, took place on 4/7/11. This aftershock injured thousands, and killed four, but no issues in maintaining supply of cooling water were experienced at Fukushima Dai-ichi.

The second aftershock, rated a 6.6 on the Richter scale, occurred at 08:16 UTC this morning. As a result of the aftershock, workers at Fukushima Dai-ichi were temporarily evacuated to the facility’s earthquake shelter. The event caused loss of offsite power to the water injection pumps for Units 1, 2 and 3. Power to these pumps was restored approximately 50 minutes later. No change in radiation readings was observed as a result of either aftershock.

Ongoing Mitigation Efforts
Management of contaminated water is still ongoing. Water from turbine buildings and trenches is being pumped into the condensers of the units, into the plant’s wastewater treatment facility, and into temporary storage tanks being brought in for this purpose. In order to make way for this highly contaminated water to be stored, low-level radioactive water was discharged from the wastewater treatment facility and from the “sub-drain” pits of units 5 and 6. This operation was completed on April 10.

Nitrogen is being pumped into the containment vessel of Unit 1. This is being done in order to displace oxygen, which could allow ignition of any hydrogen that has built up or builds up in the future. We have no information on the current level of concern  about hydrogen buildup.

In order to prevent the spread of radioactive materials which have been deposited on the ground at the reactor site, workers have sprayed an anti-scattering agent. This agent prevents the material from being swept up from the ground as dust, and carried away from the site.

Radiation Readings
Monitoring is ongoing by a variety of organizations: TEPCO, IAEA, and MEXT, among many others. Radiation levels in most prefectures continue to hover at or slightly above background. However, radiation dose rates in a few isolated areas are higher. Yukio Edano named these locations: Katsuo, Kawamata, Namie, Iitate, and Minami Soma, most of which are inside the zone already ordered to evacuate or take shelter. The dose rates reported by the IAEA in these areas have a maximum of 1.6 microsieverts per hour. For comparison, the average background rate is 0.05-0.1 microsieverts per hour.

Monitoring of food and water from the region is ongoing. In a few prefectures, iodine and cesium isotopes have been detected in quantities below regulatory limits. An advisory against giving water to infants is in place in just one village in Fukushima prefecture. Food samples have for the most part shown either no detectable contamination, or levels below regulatory limits. Of 157 samples of foods, one sample of seafood (sand lance) and three samples of shiitake mushrooms, all originating from Fukushima prefecture, exceeded regulatory limits on I-131.

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Safety Limits: What are they? How are they determined?

Much of the discussion concerning radiation levels and radioactive material releases has been presented in the context of safety limits set by a regulator. Examples of such limits include the I-131 limit for drinking water (210 Bq/L) or an annual occupational radiation dose limit (0.05 Sv). What is often left out of these discussions is how these limits were determined and what exceeding a limit implies. This post is intended to provide a general description of the implications of safety limits.

What is a Safety Limit and how are Safety Limits determined?

Safety limits are designed to protect the public from a potential harm and are often set well below the point of potential danger to prevent that point of danger from being accidentally reached. Safety Limits are determined in two steps. First, by identifying the amount of exposure to any given agent, above which causes a health effect to be observed. This amount is determined for the most vulnerable members of the population, and considers the effects of both short and long-term exposure. That resulting number is then divided by a safety factor to ensure that the public is never exposed to dangerous levels. The reason for the safety factor is so the regulator will have time to fix the problem before the levels reach a point that can cause harm to the public, if for whatever reason, the safety limit is exceeded. The more uncertain the dividing line between safety and harm is, the larger the safety factor used to protect the public.

Key Principles of Radiation Protection at Low Radiation Exposure

The probabilistic nature of low-dose radiation health effects makes it impossible to derive a clear distinction between ‘safe’ and ‘dangerous’ level of radiation. This also creates difficulties in explaining the control of radiation risks. The major policy implication is that some finite risk, however small, must be assumed and a level of protection established based on what is deemed acceptable. This leads to a system of protection based on three key principles recognized by the International Commission of Radiation Protection (ICRP) and endorsed by the US National Council on Radiation Protection and Measurement (NCRP) and all other national agencies:

 

–          Principle of Justification, based on the analysis of benefit versus risk of exposure;

–          Principle of Optimization of Exposure, based on the ALARA (As Low As Reasonably Achievable) principle;

–          Principle of limitation of exposure to any person;

 

The ICRP, in its latest Recommendations on Radiological Protection, stated that for radiation doses below around 100 mSv in a year, the increase in the incidence of stochastic effects is assumed to occur with a small probability and in proportion to the increase in radiation dose over the background dose. The use of this so-called linear-non-threshold (LNT) model is considered by the ICRP and by NCRP the best practical approach to managing risk from radiation exposure and commensurate with precautionary principle, being a prudent basis for radiological protection at low doses and low dose rates. However, uncertainties on the over-conservatism on this judgment are recognized by the ICRP and the NCRP, which have stated the need for further evaluation based on new research results.

Despite the fact that the actual onset of latent cancer and other long term effects in relationship to radioactivity exposure is unknown, we do know that those effects are not statistically significant at very low doses. In simpler terms, the number of cancers caused by exposure to low doses of radiation is so small that we can’t sort it out from the noise – the natural rate of cancer incidence.

In 1980, the US National Council on Radiation Protection and Measurement (NCRP) published a report examining and quantifying the dose rate effect.  In examining all laboratory data regarding tumor induction published at that time, they found that lowering the dose rate from acute (eg 180 mSv/hr) to about 4.8 mSv/hr reduced the rate of tumor generation by an average factor of 4. They called this the ‘dose rate effectiveness factor’, DREF.  When the irradiations were much longer term irradiations, comprising “a significant or sizeable fraction of the life span” an even larger reduction in effect was observed, an average of a factor of 10; this was called the ‘protraction factor’ (PF). With few exceptions, the dose rates used in all of the laboratory studies cited in NCRP 64 used ‘low dose rates’ at least a factor of 4000 times higher than normal background dose rates. It is the results of these experiments and others like them, plus corresponding safety factors, which are used to establish regulatory limits on dose and dose rate to the general public.

However, what is of interest today in Japan are dose-rates more like 10, 30, or 100 times background.  What about these dose rates?  The problem noted by the NCRP was that deleterious effects of these very low dose rates could not be observed. In fact, low doses and low dose rates led to increased longevity rather than the decreased lifespan seen at higher doses and dose rates.  In addressing the apparent life lengthening at low dose rates, the NCRP interpreted this effect as reflecting “a favorable response to low grade injury leading to some degree of systemic stimulation.”  They go on to state that “…there appears to be little doubt that mean life span in some animal populations exposed to low level radiation throughout their lifetimes is longer than that of the un-irradiated control population.” In the future, the accurate examination of residents of high background radiation areas around the world might generate the needed information on this phenomenon, which is termed “radiation hormesis”. Based on the presently available data, residents of high background radiation areas (sizeable population is exposed up to 20 mSv per year from natural background) do not appear to suffer adverse effects from these doses.

Areas characterized with background radiation significantly higher than average can be found in Iran, Brazil, India, Australia and China. In the U.S., the population of Denver receives more than 10 mSv per year from natural background.

 

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News Brief, 4/6/11

TEPCO has reported that as of 5:38 AM JST, the leakage of water from Unit 2’s supply cable pit has stopped. Before and after photos supplied by the IAEA are shown below:

After it was found that the leak had stopped, TEPCO continued to reinforce the crack by applying additional sealant. They are now considering the injection of additional “liquid glass” coagulant as an added measure of safety.

Release of low-level radioactivity water from the facility’s water treatment facility to the ocean is underway. This measure is intended to prevent a potentially much more serious release of contamination to the ocean by allowing the more radioactive water currently flooding the reactor buildings to be stored. The IAEA states that this operation is likely to last no more than five days.

Monitoring of radiation levels in air and water surrounding the plant is ongoing.  Levels of radioactive iodine and cesium levels at the site continue to show an overall downward trend: http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110406e8.pdf

Radiation levels in seawater immediately adjacent to the facility, near where the leak from Unit 2 occurred, have shown an increase in recent days: http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110406e4.pdf. However, these levels are expected to decrease now that the leak has ceased. Officials in Japan are monitoring the levels of contamination to fish in the region, and at the time being, no vessels (including fishing vessels) are permitted within 30 km of the nuclear power station. The U.S. FDA has stated that it will carefully check all fish imported from Japan to ensure that it meets regulatory limits.

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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.

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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.

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