MIT NSE Nuclear Information Hub

On August 23, a magnitude 5.8 earthquake struck the Piedmont region of the U.S. East Coast near Mineral, Virginia. This was an intraplate earthquake – most earthquakes are interplate, meaning that they occur on fault lines that bound tectonic plates. Intraplate earthquakes tend to be much less frequent and much smaller in magnitude than interplate earthquakes. In fact, the most recent earthquake of larger magnitude to strike anywhere in the U.S. east of the Rockies occurred 114 years ago (one of equal magnitude occurred 67 years ago in upstate New York).

The North Anna Nuclear Generation Station, a nuclear plant located about 11 miles from the epicenter, automatically shut down both of its reactors as a safety precaution. Although off-site power was lost, four on-site diesel generators provided sufficient power to reactor safety systems. When one of these generators failed, a fifth backup generator was activated. Off-site power was restored later on the day of the earthquake, and the reactors will likely resume normal operation as soon as possible. No significant damage occurred, and no radioactive material was released.

In short, the nuclear plant and its safety systems functioned properly. Even this rare earthquake was within the design basis of all U.S. nuclear plants.

It is important to understand that Richter magnitude is only one of many factors that contribute to the seismic damage risk of a nuclear plant. Earthquakes of equal magnitude can cause vastly different ground shaking behavior, both in terms of ground shaking frequency and ground acceleration magnitude. Although Richter magnitude corresponds to the total energy released in an earthquake, that energy can be released and propagate in a variety of ways. For example, many people on the U.S. West Coast, who are much more familiar with earthquakes than their East Coast compatriots, were surprised that a mere 5.8 earthquake centered in Virginia could be felt in Massachusetts. Had a 5.8 earthquake occurred in San Diego, people in Los Angeles would probably never know it. Indeed, the older, more solid and connected geology of the Piedmont allows for seismic waves to propagate beautifully and freely. In contrast, California is a disjointed geologic hodgepodge that dissipates seismic energy. Seismologists usually quantify all of this by evaluating the probability that the ground at a certain geographic location will exceed a certain acceleration within a certain period of time. See the map below, which shows the ground acceleration value that has a 10% probability of being exceeded in a 50-year period. For a reference point, the acceleration of gravity is 9.8 m/s2.

GLOBAL SEISMIC HAZARD MAP

download pdf of map

Of course, plant design has a large effect on seismic risk. Nuclear plants, just like all important structures, are designed to withstand larger earthquakes (usually quantified by ground acceleration, not Richter magnitude) on the West Coast than on the East Coast. Nuclear engineers who specialize in probabilistic risk assessment (PRA) quantify “plant fragility” by evaluating the probability of plant damage as a function of ground acceleration. See the figure below for a simple flow chart of factors that contribute to the seismic risk (or damage probability) of a nuclear plant.

Factors that contribute to the seismic risk (or damage probability) of a nuclear plant

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Fukushima Lessons Learned – MIT NSP-025

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The International Atomic Energy Agency (IAEA) has published a preliminary
summary of their fact-finding mission to three nuclear power stations affected
by the earthquake and subsequent tsunami. The original document can be found
here.

Some of the key findings include:

• “Hydrogen risks should be subject to detailed evaluation and necessary mitigation systems provided.”

  This refers to how it is believed that hydrogen entered Unit 4, which has experienced spent fuel pool heating, but was on shutdown for maintenance at the time of the incident. It is now believed that ductwork shared between Units 3 & 4 provided a pathway for hydrogen generated by Unit 3 to enter Unit 4 and reach dangerous levels. This means that this possibility must be investigated in other plants that share these design aspects, and sytems to vent any buildup of hydrogen must be devised. The hydrogen buildup warrants a careful look at hydrogen venting capabilities for any plants that could suffer from the same design flaw.

• “The tsunami hazard for several sites was underestimated. … Defence in depth, physical separation, diversity and redundancy requirements should be applied for extreme external events, particularly those with common mode implications such as extreme floods.”

  Two terms in this point require some explanation. The first, “Defence in depth,” refers to having multiple, redundant, diverse and independent safety systems in place, especially in the case of a single incident that can affect many systems, known as a “common mode” incident. “Common mode” refers to thefact that one incident (such as the tsunami) can disable many safety systems at once. Nuclear power stations will have to be re-analyzed to ensure that, within reason, no single incident or chain of events can disable enough safety systems
  to cause a major malfunction.

• The IAEA mission urges the international nuclear community to take advantage of the unique opportunity created by the Fukushima accident to seek to learn and improve worldwide nuclear safety.

  The IAEA uses this opportunity to call for the world to learn from the Fukushima incident, in order to improve safety of all other nuclear plants. They see this as a learning opportunity, and there is indeed much information to be acquired by analyzing the situation as it develops.

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Fukushima Lessons Learned (MIT-NSP-025)

In the wake of the Fukushima crisis, there has been much discussion of how to site future nuclear plants in locations that are relatively less vulnerable to earthquakes. Although we offer no opinions or recommendations on this issue, we will provide the following map.

The green dots represent all commercial nuclear plants in the world that are currently operating, under construction, or officially on order. There are 222. The only plant omitted is Russia’s portable floating power station Akademik Lomonosov (due for deployment in Kamchatka), for which the siting issue is not particularly pertinent.

The red dots represent all earthquakes of magnitude at least 7.0 that occurred from 1973 through 2010. There were 520 such earthquakes. These data points were provided by USGS, which has collected standardized worldwide earthquake data since 1973.

As you can see, an overwhelming majority of the world’s nuclear plants are located quite far from regions in which large earthquakes typically occur. The main exception is eastern Asia and especially northern Japan.

In fact, the mean distance* from a nuclear plant to the nearest earthquake shown is 785 miles. The mean distance from a large set of random points on the (land only) surface of the earth to the nearest such earthquake is 741 miles. The median distance from a nuclear plant to the nearest such earthquake is 809 miles. The median distance from the same large set of random points to the nearest such earthquake is 682 miles.

*Here we used the “great circle” distance, which is the shortest distance between two points on the surface of a sphere.

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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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