Nuclear units and applications

Transcription

1 Nuclear units and applications Activity The rate of nuclear disintegrations is known as the activity. Activity is the total number of disintegrations in a sample. It is measured using the becquerel (Bq), which is one disintegration per second. A gram of radium, for example, produces about 3.7 x disintegrations per second, or 3.7 x Bq of activity. An older unit of activity that is still sometimes used is the curie (Ci), which represents the activity of one gram of radium. Table E shows the recommendations of the European Union for maximum exposure of citizens to radioactivity. Table E shows the details of the specific activity (activity per kilogram) in various foods. TABLE E Selected Reference Action Levels of Radioactivity Human ingestion of 14 C from food Natural radioactivity ( 40 K) in fish Human ingestion of 40 K from food Natural radioactivity ( 40 K) in one cubic meter of sea water Radioactivity of potatoes Natural radioactivity in whiskey Natural radioactivity in milk Cesium in milk (level proposed by ecologists) Cesium in milk (proposal from European Parliament) Cesium in milk (proposal by World Health Organization) Cesium in milk (proposal by Euratom experts) Radioactivity of 131 Cs which corresponds to the tolerated annual dose limit of 5 millisieverts 100 Bq per day 100 Bq per kg 100 Bq per day 12,000 Bq Bq per kg 50 Bq per liter 80 Bq per liter 1 Bq per liter 100 Bq per liter 1,800 Bq per liter 4,000 Bq per liter 400,000 Bq Source: European Economic Community, 1987

2 Energy, Ch. 20, extension 2 Nuclear units and applications 2 TABLE E Uranium and Thorium Series Elements in Food (mbq/kg) Element Food Source 238 U 230 Th 226 Ra 210 Pb 210 Po 232 Th 228 Ra 228 Th 235 U Milk Products Meat Products Grain Products Leafy Vegetables Root Vegetables, Fruits Fish Products Drinking water Source: UNSCEAR 2000, Ref. 8, Annex B, Table 15 The activity of a sample depends on the time, because it depends on the number of nuclei N that can decay, and that number changes with time as N(t) = N 0 e -t/τ, where τ is the mean life of the nucleus. This may also be expressed as N(t) = N 0 e -λt, where λ is the decay rate of the nucleus. (This simply means that τ = 1/λ.) The longer τ, the fewer decays experienced by a group of nuclei in the amount of time t. If the time is long, then the nuclei must not be decaying very fast, that is, the decay rate λ is small. Conversely, if λ is large, then the nuclei decay rapidly and the mean life τ is short. In short, the activity is given by A(t) = A 0 e -λt. The connection between the activity and the number of particles is just A(t) = λn(t),

3 Energy, Ch. 20, extension 2 Nuclear units and applications 3 or, the activity is the decay rate times the number of nuclei that can decay that are present. To illustrate, suppose that the activity of 100,000 nuclei is 4000 Bq, so that there are 4000 decays every second. The decay rate must be 4000 Bq = 0.04 per second The mean life for this hypothetical nucleus is therefore τ = 1/λ = 1/(0.04 per second) = 25 s. If we know the mean life of a free neutron is minutes, we can say that the decay rate is λ = 1/τ = 1/(886.7 s) = /s = x 10-3 /s. If we had a collection of 100,000 free neutrons at time t, we would have an activity of A(t) = ( x 10-3 /s)(100,000) = Bq. Fig. E The decay of nuclei shows that the number of original nuclei decreases with time. It must be emphasized that we cannot follow any individual nucleus and predict whether it will decay. Decay description is based on statistics of large numbers. We can describe the behavior of large numbers of nuclei statistically. The decays are random, so small

4 Energy, Ch. 20, extension 2 Nuclear units and applications 4 numbers of nuclei will decay in unpredictable ways, but the behavior of large numbers of nuclei are well-described by the parameters λ (or τ) and N described above. Figure E reproduces the decay curve we saw in Fig that tells us the number of decaying nuclei. In Fig. E20.2.2, we see that the number of daughter nuclei (progeny)increases with time. Fig. E The decay of nuclei shows that the number of daughter nuclei increases with time. Note that this picture is the upside down version of Fig. E The sum of the two is a constant. The sum of the decaying nuclei and the decay products (daughter nuclei) is the total number we originally had in the collection. Exposure Activity by itself is not enough to allow determination of the health effects of radiation, because just knowing something decays does not tell us what effect it will have. Clearly, if a lot of energy is deposited into some matter by a particle such as a fission decay product that it is passing through, it will do more damage to the matter than one that

5 Energy, Ch. 20, extension 2 Nuclear units and applications 5 deposits little energy. The gray (Gy), which is a measure of absorbed energy, is the unit of exposure (officially, absorbed dose ) from gamma radiation losing 1 J/kg of material (such as tissue). The röntgen or roentgen (symbol R), named for the discoverer of x rays, measures the ionization of gamma radiation: this is a unit of exposure from x or radiation producing mc of charge in 1 kg of air (originally, producing one esu unit of charge per cubic centimeter in dry air at 0 C and sea-level atmospheric pressure), corresponding to an energy loss of J/kg = 87.7 mj/kg of air. This unit is being used less and less, since it may be replaced by the gray. The rad (0.010 Gy), which represents deposition from exposure to 1 röntgen in soft body tissue, was once in common use, but it too is becoming less and less prevalent in the literature. (9) Another (minor) problem with the rad is that it may be confused with the symbol for the radian, the unit of angle. Since the gray is 100 rads, it is easy to convert rads to grays. Dose The unit of dose (officially, dose equivalent ) is the sievert (Sv), which is equivalent in biological effect of 1 gray of gamma rays. The more common unit in use at present in the United States is the rem (röntgen equivalent, man), sieverts; but it is slowly being superseded by the sievert. Often official American figures are given in millirem; 1 mrem is 10 µsv. The amount of energy deposited per unit length, known as linear energy transfer (LET), is a measure of the ionizing ability of radiation. This means high-let, or charged particle, radiation is more damaging than low-let radiation. (9,10) LET is high for alpha particles, lower for beta particles, and lowest for gamma rays. This affects the dose and

6 Energy, Ch. 20, extension 2 Nuclear units and applications 6 must be included. So if you were to read that the average dose to Americans is 360 mrem/yr, you could find that the equivalent is 3600 µsv/yr = 3.6 msv/yr. The excess cancers attributable to this natural background is R excess = 8.5 x 10-2 cancer deaths/person-sievert. (11) A person-sievert is a collective measure of dose; it could be a one sievert dose to one person, a 0.1 sievert dose distributed among 10 persons, a 0.01 sievert dose distributed among 100 persons, a microsievert dose distributed among 1,000,000 people, and so on. The effect of small doses is assumed to be additive. According to UNSCEAR, the lifetime dose risk of solid cancers from 1 Sv of low LET radiation is for males and for females. The lifetime risk of leukemia from 1 Sv of low LET radiation is (8) The world average dose from natural causes is 2.4 msv, (8) compared to 3.6 msv for Americans. For comparison to these backgrounds, someone living in Denver, Colorado would experience a dose rate of 1.2 µsv/h, or 10.5 msv/yr. Americans have a higher than world average dose because many of us live at high altitudes. Worldwide, most people live near sea level. A person undergoing a full set of dental x rays would receive an additional dose between 100 and 390 µsv per set. (12) People living in houses made with stone, concrete, or masonry, receive an additional annual dose is 70 to 250 µsv (according to UNSCEAR, (8) in a Finnish wooden house there is an activity density of 70 Bq/m 2 /h, while in a masonry house, it is 90 Bq/m 2 /h). The Environment, Nuclear Safety, and Civil Protection division of the European Commission, in Radiological Protection Principles Concerning the Natural Radioactivity of Building Materials (1999), estimates that an apartment-dweller in a concrete structure receives an excess annual dose of about 0.25 msv, and recommends that a level of 200 Bq/m 3 of radon not be exceeded in any construction project. The

7 Energy, Ch. 20, extension 2 Nuclear units and applications 7 Environmental Protection Agency recommends a dose rate from enclosed air of not to exceed 0.58 µsv/h, or 0.02 WL (see below); this would mean a dose of 5 msv/yr if someone lived every minute in the room. An airline passenger flying at 8 km over the U.S. gets a dose at a rate of about 5 to 8 µsv/h; (8) at 12 km altitude, perhaps as high as 14 µsv/h. (4,11,13,14) A European flight might involve a dose rate of 30 to 45 µsv/h. (8z) Many people get an additional dose due to their jobs. Airline crew members and frequent flyers receive doses of around 5 to 6 msv/yr, (13) and airline crews have been designated as radiation workers by the European Union. (4) A person working in a nuclear power plant would probably receive a dose of 3 msv/yr (the Nuclear Regulatory Commission s limit is 50 msv per year for occupational exposures). (13) However, people working in an aboveground building would receive a dose of 4.8 msv/yr due to the radon entrapped (see Extension 20.7, Radon). See the Occupational exposures section below. Below, we discuss the uranium sequence. It and two other sequences provide radioactive progeny in the environment. The effect of these three sequences for humans is presented in Table E

8 Energy, Ch. 20, extension 2 Nuclear units and applications 8 TABLE E Effect of the Uranium, Thorium, and Actinium Series Elements Food Activity Inhalation Ingestion Element Rate (Bq/yr) Dose (µsv/yr) Dose (µsv/yr) 238 U U Th Ra Pb Po U Th Ra Th Source: UNSCEAR 2000, Ref. 8, Annex B, Tables 16, 17, 18 As we have mentioned, the linear energy transfer (LET) is a measure of the energy deposited per length traveled. For example, a from 60 Co, with an average energy of 1.25 MeV, sets showers of electrons moving through tissue; the electrons have an LET of 250 ev/µm. A 2-MeV α produces about 250 kev/µm. The α particle is much more disruptive than the electrons because it can ionize so much more easily. Biologically speaking, it causes more damage. So it must be included in dealing with dose, which is the biological effect of exposure. Table E shows the value of the quality factor for various radiations. To interpret the table: For electrons and gamma or x rays, 1 sievert results from exposure to ~ 1 gray. For alpha particles at fission energies, 1 sievert results from exposure to ~ 1/20 gray. For

9 Energy, Ch. 20, extension 2 Nuclear units and applications 9 typical protons and neutrons, 1 sievert results from exposure to ~ 1/10 gray. (The same quality factor ratios prevail between the rem and the rad.) TABLE E Linear Energy Transfer Quality Factors (Effectiveness) Radiation factor Electrons (beta particles) 1 X rays, rays 1 Neutrons 5-20 α particles, fission fragments 20 Source: International Commission on Radiological Protection, 1990 Recommendations of the International Commission on Radiological Protection ICRP Publication 60 (Oxford, England: Pergamon Press, 1991). Working level The total alpha energy emissions are given in terms of working levels (WL); an exposure to 1.3 x 10 5 MeV per liter of air (or about 3.7 disintegrations per liter of air, or about 3700 disintegrations per cubic meter) is called a WL. (15,16) One working level at equilibrium concentrations is about 3700 Bq/m 3. Typical background levels are 5 x 10-3 WL, and the EPA recommends a maximum of 0.02 WL in a home (about 75 Bq/m 3, far lower than the European Commission recommendation discussed above). Table E shows how the risks to health of exposure to alpha daughters in air depend on the total number of disintegrations. (The working level month is one working level for one month. It is a dose equivalent to about 4.9 millisieverts.)

10 Energy, Ch. 20, extension 2 Nuclear units and applications 10 TABLE E Lifetime Risk of Lung Cancer Mortality According to Various Studies Study Cancer Deaths Per Million Person WL Months BEIR IV 350 BEIR III 730 UNSCEAR NCRP 130 This definition of working level is a result of studies done on uranium miners described in the Biological Effects of Ionizing Radiation (BEIR) reports. (17-19) It is the cumulative exposure to radon daughters is important in determining what dose of radiation the miners had received. A unit had to be developed that took into account the progeny that gave rise to a certain amount of deposited energy of α particles from the shortest-lived of the progeny. The Navajo nation s reservation is in an area with rocky outcrops that contain thorium and uranium. The maximum exposure of the Navajo was 0.04 WL, much lower but still twice as high as the maximum recommended. (20) The affected states will have to deal with this problem. (21) The DARI In 2002, Charpak and Garwin published their book (originally published in French) Megawatts and Megatons: A Turning Point in the Nuclear Age? (22) In this book and an article, (23) they made the case for a new unit of dose to be called the DARI (acronym for the French Dose Annuelle due aux Radiations Internes). They say that information provided to the public about radiation dose from industry is inadequate to an intuitive and

11 Energy, Ch. 20, extension 2 Nuclear units and applications 11 correct understanding of relative risk in part because radiation exposure is expressed in units that non-specialists find difficult to comprehend. (23) They define a unit that would provide such a comfortable measure for people. They point out that there is an inescapable dose coming from human bones and tissue from internal radioactive nuclei (about 90% is potassium-40, most of the remainder is carbon-14, and there are smaller contributions from other elements). The DARI would be the dose from internal irradiation by these radioactive materials, which they calculate to be 0.17 msv. They round this to 0.2 msv as the DARI. The ICRP implies that exposure to one DARI carries a probability of incurring lethal cancer of 7 x (23) If a lethal cancer shortens life by 16 years on average, Charpak and Garwin calculate that a DARI dose would shorten a life by about one hour. (23) Experts seem to agree that this is not helpful for the scientific literature, while it may be very helpful in allowing the public to perceive the extent of the danger from artificial and natural sources. The annual dose from the environment is around 5 to 10 DARI, and the dose from a chest x ray is 5 DARI, a CAT scan delivers a dose of 40 DARI, and 25,000 DARI is lethal. (23) The dose allowed nuclear workers (500 DARI), has the same effect that these workers would get from smoking one-half a pack of cigarettes per day. (23) Charpak and Garwin were concerned about how the media distorted and the public responded to the risk from accidental releases of radiation. While not minimizing the risks of radiation, they wish to have perspective in comparison to voluntary risks such as smoking and involuntary risks such as from industry and automotive emissions. Clearly, having the DARI as a benchmark would allow a person to judge a risk of 0.01 DARI, 1 DARI, 10 DARI, and 1000 DARI in perspective. According to their calculations, a French citizen s life is shortened by about 6 minutes per year because of the 80% French

12 Energy, Ch. 20, extension 2 Nuclear units and applications 12 dependence on nuclear energy, minuscule in comparison to the loss from supplying this energy from coal-fired plants. (See also Extension 20.12, Comparing nuclear and fossilfuel energy risks.) More on using half-life and mean life Here we give several examples of the use of mean life and half-life, and the connection to activity. In the first example, what might happen if radioactive phosphorus were incorporated in body tissue because of ingestion of phosphorus in food (very little naturally-ingested phosphorus is radioactive because of the exceedingly short mean life). In the second example, we examine radiocarbon dating. Consider what we can learn from knowing about how radioactive decay proceeds. Suppose a source contains two phosphorus radionuclides, 32 P (τ = 14.3 d) and 33 P (τ = 25.3 d). We can find the present ratio of the activities of the two isotopes only if we know how many nuclei of each type there are, or if we know their ratio. Call the decay rate of phosphorus-32 λ 32 and that of phosphorus-33 λ 33. Then the ratio of activities is λ 32 N 32 /λ 33 N 33. Suppose 15% of the decays are found to come from 33 P. We can then say that 0.15 = λ 33 N 33 /(λ 32 N 32 + λ 33 N 33 ), which means that 0.15(λ 32 N 32 + λ 33 N 33 ) = λ 32 N 33, or 0.15 λ 32 N 32 = 0.85 λ 33 N 33, or λ 32 N 32 = 5.67 λ 33 N 33.

13 Energy, Ch. 20, extension 2 Nuclear units and applications 13 Since τ = 1/λ, we can say that N 32 = 5.67 λ 33 λ 32 N 33 = 5.67 τ 32 τ 33 N 33 = d 25.3 d N 33 = 3.20 N 33. There must be 3.2 phosphorus-32 nuclei for every phosphorus-33 in the sample. As time goes on, though, the ratio will change, and more activity will come from phosphorus-33. This occurs because N(t) = N 0 e -λt for each of these nuclei. Since the mean life is smaller for phosphorus-32, it will decay away faster. The initial dose to a person would first come mostly (~75%) from the phosphorus-32; later it would come more and more from phosphorus-33. To know the actual activity, we would have to know the number of nuclei of either sort. Suppose the amount of phosphorus-32 and phosphorus-33 totals just one-billionth of a gram (1 ng). We may find the number of atoms by realizing that the atomic mass of the phosphorus is u and u; call it on average 32.0 u and 33.0 u. Then the average atomic mass of the atoms present will be average atomic mass = [3.2(32 u) + 33 u]/4 = u. Dividing 1 ng by u = (1.67 x kg) = 5.65 x kg, we find the number of phosphorus atoms in 1 µg to be 1 ng N = N 32 + N 33 = 5.65 x kg = kg 5.65 x kg = 1.77 x There are therefore (at this time) 4.2 x atoms of phosphorus-33 and 1.35 x atoms of phosphorus-32. The respective activities are 1.35 x 1013 A 32 = = 1.09 x d 7 Bq and A 33 = 4.2 x d = 1.92 x 10 6 Bq. The total activity is A 32 + A 33 = 1.29 x 10 7 Bq. The normal activity of a person due to natural radioactivity in the person s body is around 3000 Bq.

14 Energy, Ch. 20, extension 2 Nuclear units and applications 14 Therefore, ingestion of just 1 ng of this phosphorus increases a person s activity by a factor of 4283! This shows that ingestion of even a picogram (10-12 g) of this radioactive phosphorus would have an effect on a person s radioactivity. It would increase the activity by a factor of over 4. To hold the dose to less than a 10% additional effect, the amount ingested would have to have an activity of only 300 Bq, and one could ingest a mere 2.3 x kg only a few hundred million atoms of this mix of phosphorus! There are many isotopes used in determining geological and archaeological dates. Uranium and argon dating is used for old rocks. In archaeology, which focuses on the near past of human history, the preeminent method of dating materials is radiocarbon dating. Cosmic rays create some carbon-14 all the time from atmospheric nitrogen. This isotope, 14 6C, is known as radiocarbon. At the present time, about 1.3 in every carbon atoms in the atmosphere is carbon-14. Eventually the carbon-14 beta decays and becomes normal nitrogen again. The half-life of radioactive carbon is 5,730 years. Living things take in and give out carbon while they live, in equilibrium with the atmosphere. When the living thing dies, the radiocarbon content is frozen, and slowly decays. From the amount of 14 6C in a material, its approximate age can be found. Suppose we have a wood sample of mass 2.00 kg. What is the activity of new cut wood? What is the activity of 5,700 year old wood (that is, after one half-life)? What is the activity after two half-lives? Wood, which is made of cellulose, has the chemical formula C 6 H 11 O 5. Therefore, using the atomic masses from the periodic table of elements (Fig. 7.3), wood is about (6)( ) (6)( ) + (11)( ) + (5)( ) = , or 44% carbon. Of that sample, 0.88 kg is carbon. In that carbon, there will be

15 Energy, Ch. 20, extension 2 Nuclear units and applications 15 (0.88 kg)(1.3 x ) = 1.14 x kg of carbon-14. The mass of a carbon-14 atom is u. Therefore, the number of carbon-14 atoms is 1.14 x kg N 14 = ( )(1.67 x kg) = 4.90 x We can now find the activity, since we know the half-life, 5,730 years. To do this, we need the connection between half-life and mean life. We know N(t) = N 0 e -t/τ = N 0 ( 1 2 )t/half-life. This means that the logarithm of the second and third terms are equal and so -t/τ = (t/half-life) ln 2, or τ = half-life/ln 2 = half-life/ We want the activity, so we want to know the decay rate, which is λ. This is simply λ = 0.693/half-life = 0.693/5730 yr. Therefore, the activity of the carbon-14 is A 14 = (0.693/5730 yr) x (4.90 x ) = (0.693)(4.90 x 1014 ) 1.81 x s = 1878 Bq. This is not a large activity, but should easily be detected. After one half-life, the activity will be 939 Bq, after two half-lives (about 11,400 yr, near the start of recorded history) 469 Bq. Secular equilibrium Consider the decay of a long-lived (large τ) nucleus such as uranium-238, which has a mean life of 6.45 billion years. Its α-decay daughter is Th, which also happens to be radioactive. Thorium-234 s daughter is also radioactive, and so on until the series ends with the stable nucleus Pb. Consider a specific volume of rock: The activity of the uranium-238 is given by the decay rate times the number of uranium-238 nuclei in that

16 Energy, Ch. 20, extension 2 Nuclear units and applications 16 volume. Assuming that that is a large number, there will accumulate a large number of thorium-234 nuclei. But the thorium-234 decays as well, and has an activity. After a long time, the number of thorium-234 nuclei will be just about constant, because the number of nuclei decaying will be balanced by new nuclei decay products of uranium-238. The number stays the same because nuclei are being supplied at the same rate they are decaying. Therefore, the activity of the thorium will be equal to that of the uranium. The number of thorium s daughters, protactinium-234 will grow until there is a dynamic balance between new protactinium-234 nuclei from the beta decay of thorium-234 and the loss due to the beta decay of protactinium-234 into uranium-234. In other words, there is a dynamic equilibrium that has the activity of each element in the decay chain the same as all the other ones (this is only a very slight exaggeration) A 1 = λ 1 N 1 = A 2 = λ 2 N 2 = A 3 =... This balance is known as secular equilibrium. We expect to find this in stable rock formations, for example. Knowledge that secular equilibrium holds allows measurement of total activity by measurement of any one component of the decay chain. Of particular interest is radium-226, one of the members of the uranium-238 decay chain. It has a very short mean life compared to all the other nuclei in the chain, about 2,300 years. This means that only small amounts accumulate in secular equilibration. Mass for mass, though, it has the greatest activity of all the elements in the chain. Radium-226 s daughter, radon-222, is the only gas in the chain, which is discussed further in Extension 20.7, Radon. While there are other radon isotopes, only radon-222 has any health consequences.

17 Energy, Ch. 20, extension 2 Nuclear units and applications 17 Occupational exposures There are many workers exposed to radiation because of their jobs. The Nuclear Regulatory Commission keeps records of occupational exposure (Fig. E20.2.3). In 1999, 129,951 individuals were monitored, of whom about half had measurable doses. The NRC finds that businesses with multiple locations consistently have individuals receiving dose in the higher dose ranges and routinely have 20% to 30% of the collective dose delivered to individuals above 2 rem, or 20 msv. (24) In 1999, the average measurable dose to workers in nuclear-related industries was 2.5 msv. (24) This is about 12.5 DARI (see above), only a bit above the average annual worldwide dose of 10 DARI. However, averages can hide details, and some workers received a dose far in excess of this. Figures E through E show details of dose received by workers in various types of industry. Radiographers (Fig. E20.2.3) use radioactive materials (usually emitters) in camera-like devices to do nondestructive testing. Some such work is done in the field (for example, on an oil rig), which limits the availability of shielding. Radioactive sources for radiographic work and other purposes must be manufactured, and these workers are profiled in Fig. E Workers in low-level waste disposal may be exposed at a waste disposal site; their doses are shown in Fig. E At nuclear plants and some government facilities, workers deal with spent fuel. Their doses are shown in Fig. E Finally, some workers manufacture fuel rods for nuclear utility plants; their doses are shown in Fig. E Note the fluctuations from year to year, which can be quite large (especially for workers who work with dry storage casks).

18 Energy, Ch. 20, extension 2 Nuclear units and applications 18 Fig. E Occupational exposure to workers and number of radiography workers exposed. (U.S. Nuclear Regulatory Commission, Ref. 24, Fig. 3.1) Fig. E Occupational exposure to workers and number of radioactivity manufacturing and distribution workers exposed. (U.S. Nuclear Regulatory Commission, Ref. 24, Fig. 3.4)

19 Energy, Ch. 20, extension 2 Nuclear units and applications 19 Fig. E Occupational exposure to workers and number of low-level waste disposal facility workers exposed. The NRC stopped collecting these data after (U.S. Nuclear Regulatory Commission, Ref. 24, Fig. 3.7) Fig. E Occupational exposure to workers and number of spent fuel storage workers exposed. (U.S. Nuclear Regulatory Commission, Ref. 24, Fig. 3.9)

20 Energy, Ch. 20, extension 2 Nuclear units and applications 20 Fig. E Occupational exposure to workers and number of fuel cycle license holder workers exposed. (U.S. Nuclear Regulatory Commission, Ref. 24, Fig. 3.11) Some workers had radiological intakes that required monitoring and reporting of internal dose. Table E shows the distribution of internal dose from 1994 to The term CEDE refers to a committed effective dose equivalent, NRC-ese for dose delivered internally. TABLE E Internal Dose (CEDE) Distribution, Year Number of Individuals with CEDE in the Ranges (rem) Meas Total with Meas. CEDE Collective CEDE (person-rem) Average Meas. CEDE (rem) , ,338 1, , ,959 1, , , , , , , , , Source: Ref. 24, Table 3.10

21 Energy, Ch. 20, extension 2 Nuclear units and applications 21 A measurable CEDE is any reported value that is greater than zero. Most of the internal doses are received by individuals who work at fuel plants manufacturing nuclear fuel. One person at a Westinghouse Electric Company fuel fabrication facility had a dose in 1999 of rem (26.93 msv) from uranium-234, uranium-235, and uranium-238. The highest total dose was rem ( msv = Sv), and was delivered to this same individual at Westinghouse. Clearly, workers are exposed to more radiation than the general population. In 1999, five people came to within 5% of the total dose limit, in addition to the one individual who exceeded the limit. Only a few exposures were greater than half the NRC limit. (24) One good trend visible in these data is that the total collective internal dose is decreasing over the years studied.