Question: During which construction project did 250,000 workers reach their lifetime radiation limits?
About the article "Chernobyl disaster", section "Debris removal".

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In recommendations by the NAS in 1967, it was noted that radiation protection in human space flight is philosophically distinct from the protection practices of terrestrial workers because of the high-risk nature of space missions.
This report by the NAS from 1967 did not recommend "permissible doses" for space operations, noting the possibility that such limits may jeopardize the mission, but instead estimated what the likely effects would be for a given dose of radiation.
In 1970, the NAS Space Science Board recommended guidelines for career doses to be used by NASA for long-term mission design and human operations.
At that time, NASA employed only male astronauts and the typical age of astronauts was 30 to 40 years.
A "primary" reference risk" was proposed that was equal to the natural probability of cancer over a period of 20 years following the radiation exposure (using the period from 35 to 55 years of age); this was essentially a doubling dose.
The estimated doubling dose of 382 rem (3.82 Sv), which ignored dose-rate reduction factor, was rounded to 400 rem (4 Sv).
The NAS panel noted that its recommendations were not risk limits, but, rather, a reference risk, and that a higher risk could be considered for planetary missions or a lower level of risk for a possible space station mission.
Ancillary reference risks were described to consider monthly, annual, and career exposure patterns.
However, the NAS recommendations were implemented by NASA as dose limits that were used operationally for all missions until 1989.
At the time of the 1970 NAS report, the major risk from radiation was believed to be leukemia.
Since that time, the maturation of the data from the Japanese atomic-bomb survivors has led to estimates of higher levels of cancer risk for a given dose of radiation, including the observation that the risk of solid tumors following radiation exposure occurs with a higher probability than that of leukemias, although with a longer latency period before expression.
Figure 4-5 illustrates the changing estimates of cancer risks for an average adult worker since 1970.
Together with the maturation of the atomic-bomb data, reevaluation of the dosimetry of the atomic-bomb survivors, scientific assessments of the dose response models, and dose-rate dependencies have contributed to the large increase in risk estimate over this time period (1970 - 1997).
The possibility of future changes in risk estimates can, of course, not be safely predicted today; it is possible that such changes Normal 0 false false false EN-US X-NONE X-NONE could potentially impact NASA mission operations.
Thus, protection against uncertainties is an ancillary condition to the ALARA principle, suggesting that conservatism be exercised as workers approach dose limits.
By the early 1980s, several changes had occurred that led to the need for a new approach in defining dose limits for astronauts.
At that time, NASA requested that the NCRP reevaluate the dose limits that were to be used for LEO operations.
Considerations included the increases in estimates of radiation-induced cancer risks, the criteria for risk limits, and the role of the evolving makeup of the astronaut population from male test pilots to a larger, diverse population of astronauts (~100), including mission specialists, female astronauts, and career astronauts of higher ages who often participate in several missions.
In 1989, NCRP Report No.
98 recommended age- and gender-dependent career dose limits using a 3% increase in cancer mortality as a common risk limit.
The limiting level of 3% excess cancer fatality risk was based on several criteria, including a comparison to dose limits for ground radiation workers and to rates of occupational death in less-safe industries.
It was noted that astronauts face many other risks, and that an overly large radiation risk was not justified.
It also should be noted that the average years of life loss from radiation-induced cancer death, which is about 15 years for workers over age 40 years and 20 years for workers between age 20 and 40 years, is less than that of other occupational injuries.
A comparison of radiation-induced cancer deaths to cancer fatalities in the U.S. population is also complex because of the smaller years of life loss in the general population, where most cancer deaths occur above 70 years.
In the 1990s, the additional follow-up and evaluation of atomic-bomb survivor data led to further increases in the estimated cancer risk for a given dose of radiation.
Recommendations from the NCRP, while keeping in the basic philosophy of risk limitation that had been in the earlier report, advocate significantly lower limits than those that were recommended in 1989.
Table 4-9 provides examples of career radiation limits for a career duration of 10 years, with the doses assumed to be spread evenly over the career.
The values from the previous report are also listed for comparison.
Both of these reports specify that these limits do not apply to Exploration missions because of the large uncertainties in predicting the risks of late effects from heavy ions.
The NCRP Report No.
132 noted that the use of comparisons to fatalities in the less-safe industries that were advocated by the NCRP in 1989 were no longer viable because of the large improvements that had been made in ground-based occupational safety.
Table 4-10, which shows an update to such a comparison, demonstrates that, indeed, the decreased rate of fatalities in the so-called less safe industries (e.g., mining, agriculture) would suggest a limit below the 3% fatality level today as compared to that in 1989.
The most recent reviews of the acceptable levels of radiation risk for LEO, including that provided during a 1996 NCPR symposium and the recent NCRP Report No.
132 on the LEO dose limits, instead advocate that comparisons to career dose limits for ground-based workers be used.
It is also widely held that the social and scientific benefits of space flight continue to provide justification for the 3% risk level for astronauts who are participating in LEO missions.
In comparison to the limits that have been set by NASA, the U.S. nuclear industry uses age-specific limits that are gender-averaged, which is of sufficient accuracy for the low doses received by nuclear workers.
Here career limits are set at a total dose-equivalent that is equal to the individual's age × 0.01 Sv.
It is measured by the NCRP that ground workers who reach their dose limits would have a lifetime risk of about 3%, but note the difference in dose values corresponding to the limit is due to differences in how the radiation doses are accumulated over the worker's career.
The short term (30-day and 1-year) dose limits set by NASA are several times higher than those of terrestrial workers because they are intended to prevent acute risks, while the annual dose limits of 50 mSv (5 rem), which are followed by U.S. terrestrial radiation workers, control the accumulation of career doses.
The exposures that are received by radiation workers in reactors, hospitals, etc.
rarely approach dose limits with the average annual exposure of 1 to 2 mSv, which is a factor of 25 below the annual exposure limit and significantly less than the average dose for a 6-month ISS mission (100 mSv).
Similarly, transcontinental pilots, although they are not characterized as radiation workers in the U.S., receive an annual exposure of about 1 to 5 mSv, and enjoy long careers without approaching the exposure limits that are recommended for terrestrial workers in the U.S..
Under these conditions, ground-based radiation workers are estimated to be well below the career limits, even if a 95% CL is applied.
As space missions have been relatively short duration in the past, thereby requiring minimal mitigation, the impact of dose limits when space programs actually approach such boundaries, including the application of the ALARA principle, has been unexplored. 
Major news source reporting at least 2 TEPCO employees confirmed dead from "disaster conditions" following the earthquake.
"The two workers, aged 21 and 24, sustained multiple external injuries and were believed to have died from blood loss, TEPCO said.
Their bodies were decontaminated as radiation has been spewing from the plant for three weeks.
"A Japanese Research Company was assigned to find out the health effects and casualties caused by the disaster.
They found that some deaths were early, during evacuation processes, while other deaths gradually happened after the disaster.
The agency found out that the cause of these early deaths were due to the disruption of hospital operations, exacerbation of pre-existing health problems and the stress of dramatic changes in life.
It is stated that the vast majority of people who died during their evacuation were elderly.
45 patients were reported dead after the evacuation of a hospital in Futaba due to lack of food, water and medical care as evacuation was delayed by three days.
The Associated Press reported that fourteen senior citizens died after being moved from their hospital which was in the Fukushima plant evacuation zone.
On 14 April 2011, it was reported that the oldest resident of Iitate, a 102-year-old, committed suicide rather than to leave following the announcement of his village's evacuation.
In a nuclear accident situation it is essential for authorities to understand and communicate the direction that contamination is spreading and where it may be deposited on land.
Given this information, as well as basic knowledge of the risks of radiation, residents would not feel unnecessary anxiety.
The wind measurably increased the radiation levels up to 100 miles away from the disaster site.
Radioactive iodine, which can lead to increased risk of thyroid cancer if absorbed into the body, was released into the air along with other fission products.
To counteract the radioactive iodine the distribution of potassium iodide is used, as it prevents the absorption of the potentially dangerous radioisotopes of that element.
Since Chernobyl, distributing potassium iodide to children has been a standard response when risk of radiation release is high.
According to the Japanese Government, over 160,000 people in the general population were screened in March 2011 for radiation exposure and no case was found which affects health.
Thirty workers conducting operations at the plant had exposure levels greater than 100 mSv.
In April 2011, the United States Department of Energy published projections of the radiation risks over the next year for people living in the neighborhood of the plant.
Potential exposure could exceed 20 mSv/year (2 rems/year) in some areas up to 50 kilometers from the plant.
That is the level at which relocation would be considered in the USA, and it is a level that could cause roughly one extra cancer case in 500 young adults.
Natural radiation levels are higher in some part of the world than the projected level mentioned above, and about 4 people out of 10 can be expected to develop cancer without exposure to radiation.
Further, the radiation exposure resulting from the accident for most people living in Fukushima is so small compared to background radiation that it may be impossible to find statistically significant evidence of increases in cancer.
As of September 2011, six workers at the Fukushima Daiichi site have exceeded lifetime legal limits for radiation and more than 300 have received significant radiation doses.
Workers on-site now wear full-body radiation protection gear, including masks and helmets covering their entire heads, but it means they have another enemy: heat.
, 33 cases of heat stroke had been recorded.
In these harsh working conditions, two workers in their 60s have died from heart failure.
As of September 2012, there were no deaths or serious injuries due to direct radiation exposures.
Cancer deaths due to accumulated radiation exposures cannot be ruled out, and according to one expert, might be in the order of 100 cases.
A May 2012 United Nations committee report stated that none of the six Fukushima workers who had died since the tsunami had died from radiation exposure.
According to a 2012 Yomiuri Shimbun survey, 573 deaths have been certified as "disaster-related" by 13 municipalities affected by the Fukushima nuclear disaster.
These municipalities are in the no-entry, emergency evacuation preparation or expanded evacuation zones around the crippled Fukushima nuclear plant.
A disaster-related death certificate is issued when a death is not directly caused by a tragedy, but by "fatigue or the aggravation of a chronic disease due to the disaster".
According to a June 2012 Stanford University study by John Ten Hoeve and Mark Z. Jacobson, the radiation released could cause 130 deaths from cancer (the lower bound for the estimater being 15 and the upper bound 1100) and 180 cancer cases (the lower bound being 24 and the upper bound 1800), mostly in Japan.
Radiation exposure to workers at the plant was projected to result in 2 to 12 deaths.
The radiation released was an order of magnitude lower than that released from Chernobyl, and some 80% of the radioactivity from Fukushima was deposited over the Pacific Ocean; preventive actions taken by the Japanese government may have substantially reduced the health impact of the radiation release.
An additional approximately 600 deaths have been reported due to non-radiological causes such as mandatory evacuations.
Evacuation procedures after the accident may have potentially reduced deaths from radiation by 3 to 245 cases, the best estimate being 28; even the upper bound projection of the lives saved from the evacuation is lower than the number of deaths already caused by the evacuation itself.
These numbers are very low compared to the estimated 20,000 casualties caused by the tsunami itself, and it has been estimated that if Japan had never adopted nuclear power, accidents and pollution from coal or gas plants would have caused more lost years of life.
Finally, there has been a widely critiqued paper published by members of the controversial Radiation and Public Health Project which attempts to ascribe the natural annual cycle of rising and falling adult and infant mortality rates in the United States to Fukushima fallout, suggesting about 14,000 have died.
Those who have responded to this paper in the literature have however noted a number of errors, among them include, that this figure was based on an assumption of acute deaths from low radiation doses.
There is no known mechanism for this, and more notably, "the cities under study with the lowest radiation fallout have the highest increases of death rates in the 14 weeks following Fukushima, while the Californian cities that would have received larger doses saw a decrease in death rate growth" and concluded that "innumerable factors other than radiation" were likely responsible for the major part of the variation in US mortality around the time of the nuclear disaster.
The author of the initial paper which attempts to draw a link between infant mortality in the US and the Fukushima accident, Joseph Mangano and his colleague Ernest J. Sternglass, both of the Radiation and Public Health Project, were also active publishing work attempting to draw a causality between infant death rates in Pennsylvania due to the Three Mile Island accident(TMI-2) in 1979, but likewise, these earlier papers conclusions have failed to be corroborated by any other peer reviewed paper or follow up epidemiology study, with Sternglass's paper being widely critiqued.
In their final 1981 report, the Pennsylvania Department of Health, examining death rates within the 10-mile area around TMI for the 6 months after the accident, said that the TMI-2 accident did not cause local deaths of infants or fetuses. 
The worst of the radioactive debris was collected inside what was left of the reactor, much of it shoveled in by liquidators wearing heavy protective gear (dubbed "bio-robots" by the military); these workers could only spend a maximum of 40 seconds at a time working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris.
The reactor itself was covered with bags of sand, lead and boric acid dropped from helicopters: some 5,000 metric tons of material were dropped during the week that followed the accident.
At the time there was still fear that the reactor could re-enter a self-sustaining nuclear chain-reaction and explode again, and a new containment structure was planned to prevent rain entering and triggering such an explosion, and to prevent further release of radioactive material.
This was the largest civil engineering task in history, involving a quarter of a million construction workers who all reached their official lifetime limits of radiation.
By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents.
A unique "clean up" medal was given to the workers.
Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area.
During the construction of the sarcophagus, a scientific team re-entered the reactor as part of an investigation dubbed "Complex Expedition", to locate and contain nuclear fuel in a way that could not lead to another explosion.
These scientists manually collected cold fuel rods, but great heat was still emanating from the core.
Rates of radiation in different parts of the building were monitored by drilling holes into the reactor and inserting long metal detector tubes.
The scientists were exposed to high levels of radiation and radioactive dust.
After six months of investigation, in December 1986, they discovered with the help of a remote camera an intensely radioactive mass in the basement of Unit Four, more than two metres wide and weighing hundreds of tons, which they called "the elephant's foot" for its wrinkled appearance.
The mass was composed of sand, glass and a large amount of nuclear fuel that had escaped from the reactor.
The concrete beneath the reactor was steaming hot, and was breached by solidified lava and spectacular unknown crystalline forms termed chernobylite.
It was concluded that there was no further risk of explosion.
Adverse effects.
The radiation used in CT scans can damage body cells, including DNA molecules, which can lead to cancer.
A single CT scan will expose a patient to enough radiation to cause cancer, while children who received multiple CT scans were found to be three times more likely to develop leukemia and brain cancer.
The Institute of Medicine found that such imaging radiation, along with hormone therapy, were, until recent years, the "leading" causes of breast cancer."
The most common cancers caused by radiation exposure are thought to be lung cancer, breast cancer, thyroid cancer, stomach cancer and leukemia.
According to the National Council on Radiation Protection and Measurements, between the 1980s and 2006, the use of CT scans has increased sixfold (600%).
The radiation doses received from CT scans are 100 to 1,000 times higher than conventional X-rays.
A study by a New York hospital found that nearly a third of its patients who underwent multiple scans received the equivalent of 5,000 chest X-rays.
Some experts note that CT scans are known to be "overused," and "there is distressingly little evidence of better health outcomes associated with the current high rate of scans.
"Early estimates of harm from CT are partly based on similar radiation exposures experienced by those present during the atomic bomb explosions in Japan during the second world war and those of nuclear industry workers.
A more recent study by the National Cancer Institute in 2009, based on scans made in 2007, estimated that 29,000 excess cancer cases and 14,500 excess deaths would be caused over the lifetime of the patients.
Some experts project that in the future, between three and five percent of all cancers would result from medical imaging.
A person's age plays a significant role in the subsequent risk of cancer.
Estimated lifetime cancer mortality risks from an abdominal CT of a 1-year-old is 0.1% or 1:1000 scans.
The risk for someone who is 40 years old is half that of someone who is 20 years old with substantially less risk in the elderly.
The International Commission on Radiological Protection estimates that the risk to a fetus being exposed to 10 mGy (a unit of radiation exposure, see Gray (unit)) increases the rate of cancer before 20 years of age from 0.03% to 0.04% (for reference a CT pulmonary angiogram exposes a fetus to 4 mGy).
A 2012 review did not find an association between medical radiation and cancer risk in children noting however the existence of limitations in the evidences over which the review is based.CT scans can be performed with different settings for lower exposure in children with most manufacturers of CT scans as of 2007 having this function built in.
Furthermore, certain conditions can require children to be exposed to multiple CT scans.
Studies support informing parents of the risks of pediatric CT scanning. 
The exposure of a human body to ionizing radiation has the same harmful effects whether the source of the radiation is a medical X-ray machine, a nuclear power plant or radiation in space.
The degree of the harmful effect depends on the length of exposure and the energy density of the radiation.
The ever-present radiation belts extend down to the altitude of manned spacecraft such as the International Space Station (ISS) and the Space Shuttle but the amount of exposure is within the acceptable lifetime exposure limit under normal conditions.
During a major space weather event which includes a burst of solar energetic particles, the flux can increase by one to several orders of magnitude.
There are areas within ISS where the thickness of the spacecraft surface and equipment can provide extra shielding and may keep the total dose absorbed within lifetime safe limits.
For the Shuttle, such an event would have required an immediate termination of the mission.