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.

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: 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: 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.

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.

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 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 (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.

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 (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 (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.

Many thanks to everyone who has been visiting our blog and sending us questions and comments since we started up a little over two weeks ago. Now that information on the situation at the Fukushima Daiichi power plant is becoming more widely available, we’re reducing the level of activity on our blog somewhat. We’ll continue to monitor your questions and comments and add to the content – posts, responses, and FAQs – but with less frequency than we did initially.

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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 (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.