ML20043C061
| ML20043C061 | |
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| Issue date: | 06/30/1987 |
| From: | Mark C Advisory Committee on Reactor Safeguards |
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| FRN-53FR49886, RULE-PR-CHP1 ACRS-GENERAL, NUDOCS 9006010259 | |
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Text
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' i}'...,y Copies of this report have been provided to the members of the ACRS but
-(u.
H the Committee.as a group. has not reviewed the document. The. technical./
i content of this draft report should be considered to have neither ACRS i
approval or disapproval.
l V
4 REPORT TO ACR$ MEMBERS i
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I' VARIABILITY OF NATURAL BACKGROUND RADIATION l
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I l-CARSON MARK, ACRS MEMBER 1
JUNE 1987 l
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VARIABILITY OF NATURAL BACKGROUND RADIATION 3-v.
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1 i
The average annual exposure of persons in the United States to radiation from natural background sources is often said to be "about 100 elllirem" 1
whole-body dose equivalent. Though it is usually pointsd out that actual i
exposures differ.from one regior, of the country to another, and that the 100 mrem value is an estimate of a population-weighted average, many references include little to indicate the extent of the variations actually As a result, there is some room for the impression that this encountered.
nominal 100 mrem is e sort of natural constant -- much like that of no L
i bodytemperature(37'C.)--andthatanyappreciabledepartureabovethis L
1 l'n the present norm'is associated with seriously undesirable consequences.
\\
.l discussion it is intended, first, to describe the generally familiar range l'
of netural background (particularly as experienced in the U.S.), and then n
l' to bring to attention some of the more fine-grained aspects of its vari-These may warrant consideration in assessing the significance of ability.
incremental perturbations of the radiation levels to which people may be j
i exposed.
e
,g, I
1.
Natural background consists of three major components:
- 1) Cosmic Rays, ii) External Terrestrial, and iii) Internal. These are described separately, 1)
Cosmic Rays i
i Intheloweratmosphere(altitudeslessthanafewkm.),theradiation from this source is mnstly provided by muons and high energy (very penetrating) electrons; There are components of other particles in the flux, including neutrons. Thoughthenumberofneutrons(atlow altitudes) is small compared to the number of muons and electrons, because of their large quality factor (Q) or relative biological t
effectiveness (RBE),which--atleastinrecentreports,suchas j
UNSCEAR-1982 -- has been taken to be 10 for neutrons as compared with unity for muons or electrons, the neutrons contribute appreciably This (about 10%) to the dose equivalent in tissue, even at sea level.
)
contributionincreaseswithaltitude,andat3km.(9850ft.)the neutron component contributes about 25% of the total biological dose, (Within the.past year -- and after the above was written -- the NCRP l
has decided that the value of Q for alpha particles ought to be taken as 20 -- rather than 10 as had been previously assumed -- and that, The total for neutrons, the value of Q might lie between 5 and 20.
level of the cosmic radiation (in rems) may, then, finally be rated L
somewhat differently than the values just stated, or than in some of the numbers used below.)
I l
(
i Athighaltitudes(altitudesgreaterthanabout10km.,whichare accessibleonlytohigh-flyingaircraftorspacevehicles),thereisa strongdependenceofthecosmicrayflux(ordose)onthegeomagnetic latitude -- the flux being many times larger at the magnetic pole than j
However, on the inhabited portions of the earth's a' the equator.
1 surface (altitudes less thana. $ km.), the variation with geomagnetic L
l latitude is much smaller; and, for the continental U.S. (essentially
.f alllyingbetween40'and60'Ngeomagneticlatitude),thevariation-This will be ignored in the f
with lat.itude is only a percent or so.
I' sequel.
l At any particular location on the surface of the continental U.S., the cosmic radiation mey be considered as unifom in time. Though there l
are temporal variations associated with the 11-year sun-spot cycle.-
with solar flares, and with changes in atmospheric pressure and L
temperature, these are either of limited extent (near the surface at U.S. latitudes) or are of short duration. They may consequently be incorporated in some average value, and will not be further i
considered.
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I The significant variation in cosmic ray exposure is the variation with.
This results from the difference in thickness of the altitude.
On this account the tissue dose equivalent from atmospheric blanket.
cosmic rays at altitudes of 1, 2, or 3 km. above sea level are larger than the exposure at sea level by factors of about 1.35, 2.2, and 4.0, The average cosmic ray dose rate out-of-doors at sea respectively.
1 Y
h
1
- k C '.,
,4, Since people spend a' considerable fraction of f
level is 29 mrem /yr.
their time indoors, and since structures provide at least some shield-ing, it has been estimated that for the U.S. the average exposure the exposure received by the population is about 10% smaller that The average exposure rate at sea level has thus been out-of-dnors.
taken to be 26 mrem /yr. Taking into account the distribution in altitude of the U.S. population, the average dose equivalent rate from.
This is the number cosmic rays has been estimated to be 28 mren/yr.
included in the assessment that the average annual exposure in the U.S. is about 100 mrem /yr.
More than 80% of the U.S. population lives at altitudes less than 0.3 km. (~ 1000 ft.), and for these the cosmic ray dose rate is within a About 10 million live at
-mrem /yr., or.so, of the countrywide average.
altitudes El km., and for these the cosmic ray dose rate L
Fivemillion(ormore)liveat l
-(out-ofdoors) exceeds 40 mrem /yr.
altitudes *1.3 km. for whom the cosmic ray dose rate exceeds 45 Salt Lake City Albu-j mrem /yr. Cities included in this group are:
(ForDenver, altitude querque, Peno, Colorado Springs, and Denver.
1.6 km., population 1.5 million, the cosmic ray dose rate is 50 mrem /yr.). More than 100,000 live in cities -- such as Durango,
~
Gallup, Flagstaff, and Santa Fe -- at altitudes E2 km., for whom the There are many small out-of-doors cosmic ray dose exceeds 50 mrem /yr.
settlements in the Rockies (e.g., Silverton, Colorado 2.8 km.) at P
In particular, for Leadville, Colorado i
altitudes of about 3 km.
i (altitude 3.1 km.) and Climax (alt. 3.4 km.-), in, or near which, a t
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i
. 4.
n total.of about 10,000 persons reside, the cosmic ray dose rate would J
be 120-150 mrem /yr.(out-of-doors).
In this same general connection, outside the U.S. there are a number of cities with large populations at quite high altitudes. Th6se are at lower geomagnetic latitudes than apply in the U.S.
As a' rough e
allowance, in designating cosmic ray dose rates for these cities, the doses from the detailed dose-altitude curve drawn for the U.S. have been reduced by the same fraction as the sea-level doses for the relevant geomagnetic latitude. The particular dose-altitude curve c
used is that presented in NCRP-45 (1975). These high altitude cities include: Johannesburg, alt.1.9 km., pop. ~ 2 million, dose rate ~60 mrem /yr.; Mexico City, alt. 2.5 km., population + 18 million, dose rate - 80 mrem /yr.; Bogota, alt. 2.6 km., population ~4 million, dose rate ~ 85 mrem /yr.; a% Quito, alt. 2.85 km., population ~.75 millior, dose rate ~100 mrem /yr. There is also La Paz and the L
Altiplano region of Bolivia.
In the Altiplano the altitude ranges from 3.5 to 4 km., and about 75% of Bolivia's total population of 6 1
million live in this region.
In addition to La Paz at 3.6 km., pop-l ulation (La Paz Department -- that is, the city, plus the surrounding administrativearea)1.9million,thereisthecityofOruroat3.7 km., Lake Titicaca and its surrounding settlements at 3.8 km., and the city of P t mi at 3.9 km., population (Potosi Department) + 0.8 r
million.
.us, in the Altiplano region.there are 4 million, or so, people for whom the cosmic ray dose rate is-in the range 150 to 200 mrem /yr.
..':e t,
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ii) External terrestrial i
At any location on the earth's surface persons are exposed to some fluxofradiation(mostlyphotens)fromthedecayofradioactive elements contained in the soil and rocks. The main primordial sources are K-40, Th-232, and U-238; though, in the case of Th and U, the major part of the radiation encountered is provided by the radioactive The radiation flux at any particular daughters in their decay chains.
incation will vary depending on whether the soil is wet or dry, covered with snow or not, subjected to changing barometric pmssure, and so forth; but these effects may be thought of as fluctuations which will average out over the year. The significant variation is that applying from place to place as a consequence of differences in Most of the radiation the local abundance of the primordial elements.
l considered is transmitted directly into the air from the near-surface Almost all the radiation reaching the scil as it resides in place.
atmosphere originates in the topmost 25 or 30 centimeters of the soil.
On a mass basis the elements potassium, thorium, and uranium in the materials of the earth's crust are, respectively, something like two The number of atoms per gram percent, and 12 and 4 parts per million.
of potassiun (atomic mass ~40) is six times larger than that of The isotopic abundance of thorium or uranium (atomic mass ~ 240).
K-40 (the only radioactive isotope of potassium) is 1.2 x 10' l
The atomic ratios of K-40 Th-232, and U-238 in the earth's crust are, u
n
..m-e
'.;o e
With half-lives of 1.26 x 10', 1.4 x i
consequently, about as 4:3:1.
10, and 4.5 x 10' years, the number of disintegrations per unit time 10 In of K-40. Th432, and U-238 are about in the ratio of 15:1:1.
ninety percent of the disintegrations of K-40 a [-particle (maximum energy v 1.3 MeV) is emitted, and almost all of these will be ab-However, in the remaining 10%
sorbed in the soil close to the source.
a[ ray (energy 1.46MeV)isemitted,andsomeofthesewillpene-From the abova it will be seen that in trate to the atmosphere.
material having the average composition of the earth's crust there are -
[ ray-emittingdisintegrationsofK-40perdisintegration about 1.5 Both Th-232 and of Th-232 or U-238 -- which are essentially equal.
l U-238 are the parent nucleus of a decay series with ten or a dozen Assuming a state of daughters having relatively short half-lives.
l radioactiveequilibrium(whichdoesn'talwaysapply)eachofthe daughters in the series will disintegrate at the same rate as the i
These series disintegrations release about 40 or 50 parent nucleus.
MeV of energy, but all but about 2 MeV of this energy is carried by hnd[particlesanddepositedinthe'immediatevicinityofthe About 30% of the energy carried by [ rays is in low energy source.
quanta (less than 1 MeV) which are strongly attenuated in the s In the thoriun series there is a 2.6 MeV [ ray emitted about 361 of the time, but in the uranium series there are no [ rays with su high energy and no [-rays with energy 72 MeV which appear in mor Thus, thorium contributes more I
than 51 of the series disintegrations.
than uranium to the terrestrial backgrour.o radiation in the atmosph 1
.~
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For above soil having the average composition of the earth's crust.
l such soil, the radiation level at one meter above the surface wo 20 from potassium, 20 from thorium, and 10 from about 50 mrad /yr.:
f As already suggested the actual background radiation rate l
uranium.*
from eno location to another may vary considerably from this average depending on the composition of the soil er rocks nearby.
1 On the basis of extensive surveys, the U.S. has been divided into three distinguishable regions with respect to terrestrial radiation be.ekgrounds. These are:
- 1) The Atlantic and Gulf Coastal Plains Area -- a coastal belt of from ont. to a few hundred m extending South and West from t.ong Island to Texas, including bj 15 and 20% of the U.S. population, and within which the terre radiation is said to provide an absorbed dose rate in outdoor between 15 and 35 rrad/yr., with a population-weighted averag to be 23 mrad /yr.; and ii) Middle America, or The Noncoastal Area, the region extending North and West from the Coastal l
d to the Pacific coast (with the exception of a relatively small is In this regien, which arour.d Denver and the Colorado Plateau).
includes about 80'a of the U.S. population, the natural terrest background exposure rates range from 35 to 75 mead, with the ld taken to be 46 mrad /yr.; and iii), the Denver, Colorado Area, i ing some part of the East Front of the Rockies and the Colo l
1
- See Appended Note Concerning Radiation Units, (p. 34)
"e r,-
. < ~.m.-
e
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c i
Plateau, in which the terrestrf n1 exposure ranges from 75 to 140. and i
for which the average is taken to be 90 mrad /yr.
Much of the support for this regional breakdown is provided by the ARMS refers to the Aerial Radiological Measurements ARMS survey.
f Surveys of the radioactivity in the vicinity of government-sponsored nuclear facilities, conducted for the AEC between 1958 and 1963.
2 Areasabout100milesonaside(es10,000mi)-aroundeachof25 locations were surveyed on a one-mile grid to map the terrestrial
+
radiation background. About 30% of the population of the U.S. was comprised within these areas.
In a few A range of radiation rates was observed in each area.
L instances the radiation rates were rather uniform -- to the extent thatnofractionoftheareawasnotedashavingaratemorethanf15 On the other hand, for some L
mrad /yr. from the mean rate for the area.
of the locations, half or more of the area was noted as having rates 4
For each area, more then 1;15 mrad /yr from the mean for the area.
the mean rate was taken to be applicable.to the population of that l'
area; and, ir, this way, the exposure rate to terrestrial background radiation was estimated.
For those pc-tions of the country not covered by. ARMS, the regional average exposures noted above were used l
to determine a population-weighted average of e* 40 mrad /yr. for the outdoor absorbed dose rate in air'for the U.S.
Thisterrestrialradiationismainlycomposedof)I$rayswithan n
i j
s This radiation is attenuated by the materi-energy of one to two MeV.
als in structures, and, since people spend more than two-thirds of J
their time indoors, and even though there may be some external dose J
from the building materials themselves, a factor of 0.8 has been l
applied to the outdoor dose in estimating the actual average exposure i
in addition, because of the shielding provided to the people receive, vitalorgans(gonads,bonemarrow,etc.)bytheoutertissuesofthe body, a further factor of 0.8 has been used in converting the terres-i trial dose in air to the equivalent biological whole body dose rate.
With these factors, the population-weighted countrywide average dose L
[
equivalent from terrestrial radiation to persont, in the U.S. has been U
This is the number used in the assessment taken to be 26 mrem /yr.
that the background radiation dose in the U.S. is sw 100 mrem /yr.
L Surveys of background terrestrial radiation levels have also been made
[
Because of differences in instrumentation and L
- in other countries.
procedures, not all of these surve/ results are directly comparable, and nrt all have been carried through to the point of developing a f
From having smaller areas the surveys of 1
population-weightcd average.
some of the countries are geographically more complete than present U.S. surveys; ard in addition, at least some have been ennducted more Notwithstanding these differences, some of the values systematically.
listed in UNSCEAR-1982 showing the results of the surveys of about I'
fifteen countries are indicated below. The values quoted are for c
L absorbed dose in outdoor air'in mrad /yr., which may be compared with The lowest average values the U.S. average of 40, already noted.
(32-33) are for Canada, Denmark, Poland; the highest (70-80) for
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a m-m
--m m
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France; Romania, Switzerland, East Gemany (GDR).
In sett cases l
ranges are given. The highest of the high range values are:
- Nomay, j
950; Italy, 435; West Gemany (FRG), 315; France, 250; GDR, 235. For bottom of the range values, several were less than 10, including:
Japan, Italy, FRG, Frence, Austria. Not to be cheated out of having something yecial about it, the bottom of the range for Ireland is j
listed as zero -- which could, of course, actually apply to a peat bog.
i In a few cases, population-weighted indoor to-outdoor ratios are l
listed. With the exception of the GDR which lists 0.8 (the same value assumed for the U.S.), these ratios are all larger than unity --
ranging from 1.65 for Austria to 1.08 for Canada.
(The values for Canade are not from UNSCEAR, but from the report of an extensive Canadian' survey completed in 1984.) At least on the basis of the data i
shown in UNSCEAR-1982, the U.S. value for indoor-outdoor ratio would appear to be one of the least well supported, being based on results a
L from only about 270 dwellings as compared with the Norway value of 1.12 (2000 dwellings), or the FRG value of 1.36(30,000 dwellings).
-i Indeed, the value for this factor for the U.S. may well deserve i
further consideration.
(In its forthcoming report, NCRP proposes to change this factor from 0.8 to 1.01) l From this welter of data, along with data concerning the worldwide r
distribution of the primordial elements, UNSCEAR concluded that, for external terrestr 41 background, a reasonable value for the global average of the absorbed dose rate in outdoor air would be about 44 I
- . ',, o
)
4 1
u b l average trad/yr., and that a value of 1.2 would be a suitable glo a 4
for the indoor-outdoor ratio.
The total environmental exposure to external radiation consists of the For the Conti-sum of the ecsmic ray and the terrestrial components.
nental U.S., as already indicated, the population-weighted average of In a survey conducted in 1971 by the this sum is 28+20=54 mrem /yr.
Lawrence livennore Laboratory at 107 weather. stations throughout th U.S. (but not including any locatiers at altitudes higher than that of Flagstaff, Arizona a 7000 ft.), the range in this quantity was from a The low low of about 35 mrem /yr to a high of about 165 mrem /yr.
-values applied in southern Florida, where the cosmic component was small (sea level, less than 40" N. geomagnetic latitude, and the The high terrestrit.1 component was also very low: ~ 30% of average).
values applied at Colorado Springs, Colorado (alt.~ 6150 ft.) which l-has fairly high components, both cosmic and terrestrial; and Bishop, I
California (alt.~ 4150 ft.) with a moderate cosmic crvaponent, but Flagstaff, Arizona, with the highest cosmic very high terrestrial.
- omp ment of the locatie..s included in this survey, had a rather terrestrial component, and a total exposure to external radiation o In Hawaii (near sea level, and only 20' N.
only about 105 mrem /yr.
geonagnetic latitude) the cosmic component was smaller th
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f and the terrestrial components were also very low; so that externa l :
radiation provided somewhat less than 30 mrem /yr for the location:
In the reports examined, no inessurements were given of the L
monitored.
terrestrial component of external radiation for the high-lying l
i
, 33,
I l
- c.
- i.
There is, however, a l
l settlements in Colorado (77000 f t. altitude).
I general tendency for the external terrestrial radiation at such locations to be high - is i rt, no doubt, because of the presence of It therefore rock near the surface, or of the exposure of bare rock.
f I
seems likely that among these settlements, which already have a cosmi I
ray exposure in excess of 100 mrem /yr., there will be some for which the total environmental exposure is F 200 mrom/yr.
Fint11y, in the survey veferred to (altitudes up only to ~ 7000 ft.),
f the cosmic component varied by a factor of cnly a little more than l-The exposure to total external radiation varied by a factor of a
(
two.
l The terrestrial component varied by a factor little more than four.
of more than five (excluding the extreme case of Key West vs. Colorado Springs, where the factor was ten).
In most cases, the terrestrial Consequently, the ennponent was larger than the cosM c component.
terrestrial component is dominant in detemining the place-to-place variations in the exposure to external environmental radiation.
iii) Internal The exposures from internal sources of radiation may conveniently be a) that from nomal constituents of the considered in three classes:
body (princip11y potassium); b) that from radionuclides lodged in th body (uranium, etc.); and c) exposures from inhaled radionuclides (radonandite. daughters).
I 1
i
.- I,
, 34,
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a) The total quantity, and also the concentrations in various organs of the body, of any of the normal body constituents (such as H. C, or r
K) are maintained at fairly constant levels by the body's state of i
physiological equilibrium. They are consequently largely independent f
of such factors as diet or geographical location.
In the absence of temporary man made perturbations -- such as tritive releases, nuclear-explosions, and so forth -- the isotopic composition of such elements i
in the body will be the same es that in the biosphere.
Cosmic rays provide a steady source of a large variety of These mix with radionuclides -- mostly produced at high altitudes.
the lower atmosphere and other components of the biosphere and the deep ocean reservoir, and have established and maintained for a very long time an equilibrium concentration in the various parts of the i
The concentration of any particular cosmogenic environment.
l radionuclide in any particular component of the environment depends L
l strongly on the half-life of the nuclice (along with other factors.
L suchassolubility). Some, with short half-lives, scarcely survive to' l
reach the deep ocean reservoir; some, with much shorter half-lives, r
scarcely penetrate the troposphere:.
t In the biosphere (the lower atmosphere, surface waters, plant life, etc.),thefourmostabundantcosmogenicradionuclidesareC-14, i
Na-22, Be-7, and H
- Except for Be, these are essential constituents The total internal dose delivered by these essential of the body.
constituents is about I mrem /yr., and is almost all provided by C-14; i
t
+
- . c.
15 -
l l
being, in particular: C-14, ~ 1; No-22, ~ 0.02; and H 3, 0.001 (Though not a body constituent, Be-7 may be ingested or mrom/yr.
.i inhaled, and is estimated to provide an internal dose of about 0.008 l
mrem /yr.) The total dose from all other cosmogenic nuclides is thought to be less than.001 mrom/yr.
Potassium is an essential constituent of the body, with an abundance usually taken to be about 2 grams per kilogram of total body weight.
Strictly speaking, the 2 gm. level applies only to young males (age a C) and fails essentially linearly with time over.the next 60 years In females, after age 20, the potassium concentra-to about 1,6 gm.
tit,n at r.11 ages is only about 75 to 601 of that in miles -- in part, posribly, because of the difference in proportion of adipose tissue in L
whichthepotessiumconcentrationisrelativelylow(onlyabout.5 i
gm/kg.). Thete is an appreciable variation in potassium concentra-j tion from one organ of the body to another (
- 4 gm /kg in red i
marrow, 2 in testes. 0.5 in bone) and a corresponding variation in
~However, for an assumed average doses to the different organs.
concentration of 2 gm./kg body weight, the whole-body dose equivalent p
htas been estimated to be ~ 18 mrem /yr.
Essentiallyallthe[-particles (witharangeintissueofonlisne or two m.) will be aosorbed in the body; but more than half of the
['swillestne. Because'of this, each person carries a small radiation fiald around with him. This, no doubt, is the basis for the l-jocular coment that there is some hazard (from radiation) in sharing t
, c e
J The hazard, of course, is not very great, being on the a Bouble bed.
order of only a tenth of a mrem /yr. in bed. However, since a nearby f
f body would screen about 10% of the solid angle from the normal exter--
nel terrestrial radiation of ~30 mrem /yr., it might better be said
)
that sharing a double bed has a favorable effect.
After K-40 the most prominent nonseries primordial radionuclide is Though the chemistry of rubidium is similar to that of rubidium-87.
potassium it is not known whether or not rubidium is an essential 10 constituent of the body. Rb-87 decays with a half-life of 4.8 x 10 yr., and emits only [-particles, with a maximum energy of 0.27 MeV.
Consequently, rubidium is significant (if at all) only as a source of Putting together the factors of atom ratio in the internal dose.
l earth's crust, energy ratio, and decay rate one would conclude that the dese rate from rubidium would be about 50 times smaller tha from K-40, provided the concentration of rubidium in the body relative to the concentration in the earth's crust were about the same i
From measurements of the rubidium concentratiun in the for potassium.
body, it has been concluded that the dese rate from Rb-87 is about one This-mrem /yr -- which is about 20 times smaller than that from K-40.
lends credence to ignoring the possible effects of'the remaining primordial radienuclides whose activity in the earth's crust is a hundred, or more, times smaller than that in rubidium.
In sumary, the dose rate from radioactive constituents of the body t
(K-40,C-14.H3,etc.)isfrom18to20 mrem /yr.
l 4
h y
f.
'd.
17 Finally, in this discussion of natural backgrounds, it is not intended l
to discuss the effects of the testing of nuclear explosives except as these may have affected items in the natural background. The most t
obvious and innediate effect of nuclear testing (from the mid-1950s to i
1963)wastointroducelargequantitiesof-radionucl.idesoccurringas fissionfragments(suchas$r-90andCs-137)whichwerenototherwise present in the environment. The effects of these will recede (or have f
receded)toinconspicuouslevelsprovidingthepresentbanontesting in the atmosphere continues. As to the isotopes already considered in for connection with natural background, the effects were as follows:
Rb-87 -- even though this is a direct fission product -- the amount added was much less than one percent of the natural abundance of this Potassium-40 is-nucleus in the upper millimeter of the earth's crust.
not a fission product, so there was no effect on that. The inventory of carbon-14 in the biosphere was almost doubled, so the previously ascribed one mrem /yr from this source could have been raised to something between 1.5 and 2 mrem /yr. This incremental effect will decrease much more rapidly than it would merely as a result of the radioactive decay of C-14 (half-life about 5700 years) because of the process of equilibration with the contents of the deep ocean reser-This process is believed to proceed eth a mean life of about 7 voir.
The present (1986) level of C-14 in the biosphere is years, or so.
probably something ~ 4e 20% larger than the ' natural" level of C-14.
i t
It has been estimated that the global inventory of tritium (H-3) was increased by a-factor of between several hundred and a thousand by the l
s
l
.a.
j With a nuclear explosions conducted in the atmosphere prior to 1963.
half-life of 12.3 years, the amount of injected tritium will by now l
have been reduced by a factor like 5; but, it still completely masks
'i the effect of " natural" tritium, and will continue to dominate for the Even at that, of course, it is a rather next hundred years nr so.
small tem in the total exposure to natural radiation.
t f
Apart from the radioactivity associated with essential constitu-b) ents of the body, there is some exposure to radiation -- some internal dose -- resulting from " foreign" radionuclides in the environment which may be ingested, imbibed, or inhaled and which may subsequent The amount of these is not homeostatically con-lodge in the body.
trolled, but will depend rather directly on their :encentration in The items of materials (air, water, and food) taken into the body.
Their particular concern here are the parental thorium and uranium.
I j.
gaseous daughter, radon, will be discussed separately later.
Though the amourt of these elements taken up in the body was once, no l
doubt, rather directly related to the concentration of these elements It is in the local environment, that is no longer 50 much the case.
still true that some of the underground water in Iowa and 1111reis, as well as at other locations in the country, has an unusually high l
l radium content; but an increasing fraction of such water is now More significantly, with the L
treated before it reaches a consumer.
greatly increased use of canned and packaged foods (which may b processed anywhere in the country) and the countrywide distribut Y
b
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. ig.
...(
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system for produce of all sorts, the U.S. food supply has become homogenized to a very large extent. Consequently, in discussing the uptake by ingestien of the series radionuclides it seems appropriate L
to use the average values estimated for the U.S.
Quite apart from the (relatively) straightforward matter of assessing theaverageuptakeofuraniumandthorium(anddaughters),thematter L
of correlating this with a whole-body equivalent dose requires compos-1 int, a wr.uer of radically different effects: the ingested radienuclides spend some time in the stomach, some time in the blood-P stream, and some end up deposited in the gonads and on the bone i
The amount of thorium ingested is probably about the same surfaces.
as that of uranium; but the retention of thorium in the body is very L
I:
Asaconsequence,most(80or90%)oftheinternaldose
[
much smaller.
from the series radionuclides is provided by uranium and its daughters.
In the following discussion, the estimates compiled in the 1975 report, NCRP-45, will be presented; but at the end of this section on i
L internal exposure some comparison will be made between these estimates andthenewer(1986-87) estimates being considered by.the NCRP. From NCRP-45, then, the ingestion of the primordial series radionuclides results in a whole-body equivalent dose rate of about 7 mrem /yr.
Uncertainties and differences which could readily affect this estimate
-- whether from differences in estimates of the uptake, or differences in assessing the doses to various organs -- would not greatly affect l
..[
7 b
the estimate of the total dose from internal sources since this is dominated by the dose from K-40, which is about twice as large as that Thus, with the exception of the dose resulting from from uranium.
inhaled radon (and daughters), the dose equivalent rate from internal sources is about 26 arem/vrm - ~20 from K-40, and 7 or so, from This is the number assumed in the assessment that the uranium, etc.
average dose to persons in the U.S. is about 100 mrom/yr.
.u, The main additionai source of internal radiation is that resulting c)
Radon from the inhalation of radon and its short-lived daughters.
appeers at nearly the same rate in both the uranium and thorium decay In the series, and is the only gaseous element in these series.
uranium series, the isotope Rn-222 is an alpha-emitter with a half-This allows time for an appreciable fraction of the life of 3.8 days.
radon fomed near the surface to migrate into the atmosphere and to be In contrast, the isotope Rn-220, which carried about by the wind.
appears in the thorium series, has a half-life of only 55 sec., so that it does not succeed in riigrating from the soil to an extent which warrants consideration in comparison with the 3.8-day Rn-222.
Radon is an inert monatomic gas -- one of the " noble" gases, which Once released to the engage in few, if any, chemical reactions.
atmosphere, these atoms move freeiy about and the products of their decay, though solids, appear as single atoms and attach themselves either to some molecule in the air or to an aerosol particle and thus' Radon decays by remain suspended in the air for a considerable time.
f I
(
i
.i.,..
' t-
- t 1
1
-emission; and if this occurs while the redon atom is still sus-The pended in air there is no direct effect on human exposure.
imediate daughters of Rn-222 (Po-218 Pb-214, Bi-214, Po-214) have
)
shorthalf-lives (from0.16 msec,to27 min.)andtheirdecaysare also likely to. occur while the atoms are still suspended in the air.
l Thefirstandlastofthesedecaysareby[-emission;sothat,again, 1
l there will be no direct effects on exposure to humans -- unless, of 1
]
course, the original radon atom, or one of these daughters, had been taken into the body by inhalation and the energy of the subsequent' However, the second and third daughters decay were deposited there.
are [-emitters, and their disintegrations are accompanied by a large l
^
fraction of the gama-ray energy appearing in the uranium decay Thus, even if these disintegrations occur while the daughters series.
are still suspended in the air they would provide some external i
exposure to humans -- though not a very important. component from radon t
The(temporary) concentrations normally encountered in outdoor air.
end. product of this group of decays is the (relatively) long-lived Pb-210(21 years). Thisundergoestwo[-decaysfollowedbythe I
emission of an hparticle, which terminates the uranium series in
[
There are essentially no games associated L
the stable isotope Pb-206.
with the decay of Pb-210; so this isotope contributes only to int rnal I
L That would result either from the inhalation of air in
)
exposure.
which Pb-210 were still present after the decay of Rn-222, with some fraction of the Pb-210 being lodged in the body, or from the ingestion
[
of plant-life growing on soil in which the Pb-210 had been depos'./ J.
1 1
~ - -
e 1
?@,
j
~
- The former is by far the more important route for exposure to
)
radiation from Pb 210.
1 i
I Very little radon is emanated from the surface of the ocean, and on this account the concentration in coastal air is low and variable --
i depending on whether the air is moving from inland or from the sea.
i In the continental air mass, the level of radioactivity is about 150
.]
pCi(pico-curies: 10-12 Ci) per cubic meter. Alargefraction(y2/3).
of the radon inhaled is exhaled before it decays, but the solid radon daughters (the 21-year Pb-210 and the 140-day Po-210) attach to the i
surfaces of the pulmonary tract -- and particularly to the walls of the hair-like passages in the segmental bronchioles. The dose rate to thetissuesofthelungfromthiscausehas(inNCRP-45)beenestimat-ed as being about 90 mrem /yr., and to the bronchial epithelium about 450 mrem /yr. Using the weighting factor recomended by the ICRP.
(whole-body dose equivalent at 0.12 times the dose to the lung tis-sue),thewhole-bodyequivalentdosefromexposureofthelungtissue would be about 11 mrem /yr.
If one applies the ICRP-recomended weighting factor of 0.08 to the dose to the bronchial epithelium, this would add an additional 36 mrem /yr, to the whole-body dose equivalent.
Adding to the 80 mrom/yr. already identified (28 cosmic, 26 external
-- about to be revised, presumably, to ~32, and 26 internal), we have an average natural background exposure for persons in the U.S. of r?ther more than 100 mrem /yr.
Up to this point, the exposure to inhaled radionuclides (radon, etc.)
In has been described only in terms of persons breathing outdoor air.
t
- ts.
S s.
j I
fa*ct, of course, people spend a major fraction of their time indoors, and the radon levels in dwellings may be quite different (usually higher) than the redon levels out-of-doors.
Redon seeps into dwell-1 ings from the soil in which the basement is embedded, from the materi-i als of construction -- such as cinder blocks -- and, because the rate l
of exchange of air in dwellings is intentionally much smaller than the rate of exchange of air outdoors (in houses weatherproofed for energy conservation,agreatdealsmaller),theradonconcentrationinindoor air may run much higher than in the ambient air outside. The effects of this have not been considered here as part of the
- natural back-ground," since they are, in fact, technologically enhanced and could (inprinciple,atleast)becontrolled. They do, nevertheless, previde an additional source of radiation to which the population is exposed.
Some (quite partial) surveys have been conducted. These do y
not yet begin to be ad&quate to establish an average level for indoor i
rodon exposure fer the U.S.
From the surveys which have been made, examples have been found in which the indoor radon levels were ten, or l
L more, times larger than those applying to the continental outdoor j
average. Such a level would imply an equivalent whole-body dose larger than the average already identified by a hundred -- or even more -- mrem /yr.
As stated earlier, the components of the dose equivalent rates from i
natural background radiation as given above are derived from the data provided in NCRP-45.
In its 1982 report the UNSCEAR directed much more attention to radon than it had in previous reports; saying, in
4..
,o -
particular: " Inhalation-is now recognized to be the most important pathway." -- and "on average about one-half the effective dose equiva-1ent from natural sources of radiation is now calculated to be due to the presence of radon in the air inside buildings."
i 1
In the January 1987 draft of a forthcoming NRCP report, the dose equivalent values for cosmic radiation, terrestrial ganna radiation, and the internal dose from cosmogenic radionuclides and K-40 are I
changed very little. But there are marked changes in the components where the exposure is provided primarily by [ radiation: the uranium contribution to internal radiation, and, most particularly, the dose attributed to_ inhaled radon. These changes were in part occasioned by i
the increase from Q=10 to Q=20 for fradiation; but they were also affected by new data showing higher concentrations of Pb and Po-210 in L
bone, by higher estimates for +.he tissue dose from radon decaying in l
the body, and particularly by including some allowance for the higher level of radon indoers as compared to outdoors.
More specifically, the contribution of uranium to the internal exposure is now being rated es about 10 to 15 mrem /yr. whole-body dose equivalent (rather L
than the value of about 7 noted above); and the dose rate proposed l
for the bronchial epithelium is 2450 mrem /yr. (rather than the 450 I
suggestedinNCRP-45). Applying the weighting factor of 0.08 to the dose to the bronchial epithelium would add about 200 mrem /yr. to L
the whole-body dose equivalent.
In sumary, the draft version of the e
,---em e-----
.-y
+ -,
v-
3 fofthcoming report provides an estimate of the total average annual exposure to a member of the population of the U.S. from sources of natural background radiation of 300 arem.
It should ba noted that there is some continuing controversy about the proper weighting factor for the whole-body equivalent of dose to the bronchial epithelium.
In addition, as already mentioned. the data to establish a countrywide average of indoor radon concentration is still i
far from complete -- though additional surveys on this point are in
?
For both of these reasons the estimate of the contribution L
progress.
from the bronchial epithelium to the total dose must be regarded as I'
still in question. Nevertheless, assuming that we already have in i
hand a real value for the number of health effects per man-rem, and r
thatthenewscaleforcorrelatingre.4and.[..radiationiscorrect, the number of health effects attributable to natural background must be increased considerably.
1
[
l i
l' I. Celebrated Hot Spots i
There are locations in which the natural background of terrestrial radiation is much higher than those so far referred to. A particular-j
.ly notable one is the Kerala Coast.
(The state of Kerala is on the west coast of India near the southern tip.)
In.a narrow strip, extending 100 miles, or so along the beach, numerous patches of f
i G
l
.::y e +
1 monazite sand are exposed.
(The mineral monarite consists of highly ~
3 insoluble phosphates of cerium and other rare earth elements in.
various proportions, usual'y accompanied by some thorium'and, on occasion, small amounts of uranium, and.their daughters.) The most i
concentrated deposits are found in a 30-mile'section of the strip; and there the monazite contains from 8 to 10.5 percent thorium by weight l
-- the highest known in the world. About 70,000 persons live in this i
section.- There is, of course, considerable variation in the external f
i terrestrial exposure received by the people residing 'in this region L
(some of the dwellings -- which are mostly made of coconct straw and F
woou -- being located directly on patches of rione.zite, and some not; i
some residents being employed outside the high background area, while.
]
othersspendmostoftheirtimenearhome). However, on the basis of' o
g radiometric surveys, the average exposure to terrestrial radiation for r
5 the 70,000 persons in the region has been estimated to-be about 380 L
For about 17,000 persons the exposure lhas been estimated to 4
mrem /yr.
- e exceed 500 mrem /yr. It exceeded 1000. mrem /yr. for more than 4000 persons; and it exceeded 2000. mrem /yr. for about 500, People have
.been living in this part of India for hundreds of years.
It is very.
3j densely populated, and living conditions are not particularly attrac-tive.
It would seem unlikely that there has been any large influx of
- 7 fi' people from outside for a'long time.
In all probability most of the presert residents have generations of ancestors who also lived in this t'
region. Some. preliminary epidemiological studies have been made, and L
more are planned. Still -- at least as reported up throu5h about 1980
,7
-- no statistically significant evidence has been found of effects K
5
< /.1 f
resulting from the unusually high background radiation to which the population of the Kerala Coast has been exposed.
V Impressive deposits of monazite sands also occur on some of the In beaches of Brarii, about 200 miles northeast of Rio de Janeiro.
particular, in the town of Guarapari -- which has a resident popula-1 tion of 12,000 persons, and a summer tourist population of 30 to 40 gp thousand -- it has been estimated that the average annual exposure rate to external terrestrial radiation in the town is about 550
{
Along the beach of this health resort there are patches of
)
mrem /yr.
" black sand" (particularly favored by the tourists) on which the radiation levels are from five to ten times higher than in the streets of the town.
w In Brazil, also, there is a region with very high terrestrial back-
{l grouno radiation in a geological setting distinctly different from the Q
This is a volcanic area about 200 miles west monazite beaches.
f (inland) from Rio and extending north from the city of Pocos de Caldas 5
In this region there are intrusions of minerals containing
_2 s
to Araxa.
close to two percent thorium oxide and over one percent uranium oxide.
N Radiation levels up to twice those noted in the streets of Guarapari 1
have been measured near Araxa, and on a small uninhabited hill -- the
{
MorrejoFerro--nearPogosdeCaldasabscrbeddoseratesinairup to 24 rads /yr. have been reported. No large population groups appear to be exposed continually to the very high radiation background in
+his region.
E p
- I l
g.ygg,
jgg c.
eL V.,./
In* France, locations providing absorbed dose _ rates in air of about.
1.75 reds /yr. are'not-uncommon, and:the discovery of a quite small-There are
- area providing a. rate of,over 80 rad /yr, has.been reported.
.also locations in Paris where one may receive a biological dose of up to 350' mrem /yr. Though no one actually lives in St. Peter's Square in v
Rome, many people spend appreciable time there, where it is reported The that the paving stones provide up to something like 400 mrem /yr.
. +
Fichtelgebirge is a granitic mountain near the northeast-border.of Bavaria. There are several towns or villages on the slopes of this On the streets of these villages the. terrestrial [ ray mountain.
exposure ranges from 200 to more than'500 mrem /yr. -- the highest in the FRG.
In Grand Central Station in New York City -- which was built with' granite from the Millstone-Quarry in Connecticut _-- there are loca-
.. ~.. -. _ _ _ _
tiens where the external terrestrial dose rate is about 525-mrem /yr.
Stone from the same source was used in constructing the foundation for, the Statue of Liberty in New York harbor, and this also provides a (Whileitwasoperating--fromabout1740 high radiation exposure.
to 1960 -- the Hillstone Quarry was'a favored source of building material since it was immediately adjacent to the shore, and rock could be transported readily to locations on the East Coast. The radiation exposure of persons working in this quarry must have been quite-high.) High radiation levels (absorbed dose rates in air up to 150 mrad /yr., or so) can also be found in other granitic regions of New England.
=
m
~?t9 ~'
~
l h
,~
v A different. setting for high terrestrial background radiation is.
j 1
~
presented by the phorphate deposits in Florida.
From this' appreciably uraniferous material terrestrial background radiation levels of-x absorbed dose rates in air up to 150 mrad /yr. have been observed.
(These instances of elevated terrestrial radiation levels in the U.S.
6 c
evidently constitute exceptions to-ttie ' levels assigned in the broad-brush partitioning of the country into three regions for terres-m g
trial radiation estimates -- and even provide exceptions to the ranges j
usually quoted for these regions. However, with the grand objective-of developing a countrywide average the data have (necessarily) been subjected to such a severe process of weighting, smoothing, ar.
everaging that relatively local' anomalies are largely suppressed.)
There are many anomalous situations in other parts of-the world, beyond those mentioned above, where the particular composition of the
~
rocks and soil results in unusually high terrestrial radiation levels.
fio doubt there are many more which have not yet been identified --
including inhabited as well as uninhabited locations.
l The remaining type of situation resulting in unusually high exposures to natural background radiation (excluding the circumstances affecting_
underground miners) has to do with water.
In'the ionization states most usually occurring in natural settings, radium is much more soluble and mobile than either uranium or thorium. On this account-water -- and particularly warm water -- flowing through beds of i
sandstone or fractured granitic rock may accumulate concentrations of i
2
__._-.A_____-_---_-_.-___.,----_______._------______--
,w
O'1g = '. k
= i 30. -
e radium very much higher than the concentration in the material' through -
l which the water has been flowing. At locations where such water say.
emerge to the-surface one-has the makings of a " hot spring,"'a." radium.
1 spring,"-or -- where the neighboring population is sufficient to support it -- a " spa."
Locally notable " hot. springs" occur in all parts of the world. Many of these became famous as " health resorts" long before the existence of radium was known, and before measurements of levels of radioactivi-ty were ever considered. Of interest here is the fact that not only-t do some of the " waters" carry a level of radioactivity which would now I-"
be regarded as distinctly unhealthy, but the radon decay product of L
3 the radium in the water is released to the atmosphere and provides an p
unusually high level of exposure to the population in the-p E
neighborhoed.
There are reports concerning a few notable radioactive hot springs.
[
For example,' the springs at Tuwa, a village.in India'about 200 miles north of-Bombay, have a high ~oncentration of Ra-226.
In the air' i
c i
close to the main spring at Tuwa, the /I~raydose(fromtheshort-L lived radon daughters) has been reported to be about 10, or more, At'a distance of about a dozen kilometers (and several rad./yr.
villages) downwind, this exposure rate falls'to a-750 nrad/yr.
I Similarly, in the city of Ramsar, a rescrt on the Caspian coast of Iran, population 7 10,000, there is an area of a few square kilometers around the radium-bearing springs (which emerge in downtown Ramsar) 1:
~
4
~~
.J 31'.i
~
^
~ ' ~ ~ ~ ' ~
~~
+
u f ',, g /,,,.;
wifhin which levels of absorbed dose in air have been measured ranging from 1.75 to over 40 rads /yr.
1 t
.l l
ThespringsatBadgastein, Austria (about50'milessouthofSalzburg) i ii
?
.have received the most extensive and detailed studies of rad oact v -
ty, both as to:the " waters," and as to the surrounding neighborhood.
This famous spa' has been known as a " watering place" for'more than six hundred years. Already in the 18th century several thousand persons travelled.there each year for treatment. Over the centuries many l
l-accounts have been written (including one by Paracelsus, prir.ted-in i
1562)-describing +he therapeutic effects of the baths at Badgestein.
~
Badgastein. gained in popularity, so that by 1940 30,000 visitors were reported, and, by 1970, about a million baths per year were admints-tered. By this time,- also, about 300 hotels were. said to be operating in the region to accomodate visitors, and the pemanent population of Badgastein and environs was about 12,'000, i
I i
in 1904 the presence at Badgastein'of " emanation" (as radon was then b
known) was established by P. Curie and colleagues.
Subsequent studies have detemined that, although the amounts of U, Th, and Ra in the-spring water are-not exceptionally high, the Rn-222 content is out-i standing..For most of the visitors, or spa patients taking only a few-treatments, the dose received is low (from a few, to a few tens, of mrem).
For patients taking a "whole cure" (a dozen 2-hour sessions in u
the "themel gallery" in which the Rn-222 concentration is 3000 I>
pCi/1), the dose to the luny tissue b s V vem -- and several
- === w
+
'f k,- l
- 32.-
. w
' times more to the-bronchioles. By inhalation of Rn-222 the 5 or 6000-pemanent inhabitants of Badgastein proper -- where the springs are located--receive'from0.7.to1.5 rems /yr.(inlungtissue). The bath attendants, other personnel connected with the treatment facili-ties, and, particularly, the doctors attending patients in the "ther-malgallery"(agroupofonly.afewhundredpersons)receivefrom-about two, up to several tens, of rem /yr. (to-lung' tissue) -- or did receive such exposure until about 1970 when some corrective measures are said to have been placed in effect.
(The dose levels reported in-this, and previous paragraphs, are all in the "old scale" using Q=10 foralphaparticles.) Surveys have been made to compare the general health of residents of Badcastein with that of groups living in similar circumstances -- but not having any enhanced radiation expo-These resulted in the conclusion that the longevity of the sure.
Badgastein residents was not less, and the incidence of cancer was not greater, than that for the other population groups. As of 1972, some_
cytogenetic studies (to identify possible radiation-nduced1momalies in cells) had led to the tentative conclusion that at dose levels up to somewhere between 0.3 and 1.0 rem /yr. there was no clear evidence of cell damage.
For doses larger than somewhere between 0.3 and 1.0 rem /yr.,therewasanincreasingincidenceof(forexample) broken chromosomes.
Presumably, such studies at.Badgastein have by now been extended.
1 There are many other well-known (or not so well-known) hot springs, or mineral sprinos, which have not been discussed at recent symposia on v
I
' - 33 4
. high natural environmental radiation. This could be because they have been studied, and found r.ot to have radiological' features of interest; i
or-because specific studies have.not yet been made.
?
Among these are the springs at Bath, in southwest _ England -- a spa well-known and used since Roman days. At about the same time as Curie m
made-his findings at Badgastein, J. J. Thomson (he who discovered the electron in 1897) reported the existence of copious amounts of_"emana-
- tion at Bath, and suggested that the salubrious properties of the waters there might be due to their radioactivity. With respect to the waters at Saratoga Springs, New Ynrk'-- though it has been pointed out i
i that the waters bottled and distributed from there come from a spring having low to moderate-radioactive content -- some of the long-time residents, preferring the water from a different spring having seviral
.hundred times the radium content reconnended (since -1962) by the-NCRP as " maximum pennissible," have been making regular use of this more c
radioactive water for periods up to 50 or 60 years without any appar-ent &leterious effects. Reports concerning the radioactive proper-ties (if any) of the_ springs in Vichy, France (famous since Roman times) or at Hot Springs, Arkansas, or Wann Springs, Georgia, and many other locations could also be interesting.
III. Fine-grained Variations i
Three sets of data are considered; i) some of the measurements listed i l
3 p
._. cy l
in'the-quarterly reports of the WRC TLD Direct Radiation Monitoring-Network; ii)'some of the measurements appearing in the annual reports 1
oftheLosAlamosEnvironmentalSurveillanceGroup;and111)some results from a mini-survey made by the author in downtown Washington, D..C., during the early summer of 1986.
r i)
NRC Survey of Nuclear Power Plant Sites Since August 1979 (a few months after the accident at TMI-2), the NRC-t has maintained a network of radiation detectors around every licensed nuclear power plant site in the country, both those under construction-and those in opetation, in each case some forty or so detectors are emplaced at various distances (from a few tenths of a niile to 10 cr 20 g
miles) fron the plant, and in a reasonably uniform azimuthal distribu-The detectors are collected every three months and replaced tion.
with new ones, and the readings from the exposed detectors are report-L ed in the quarterly series NUREG-0837.
Such detectors record the sum of the background exposure from cosmic i-The L
rays and terrestrial radiation at the location of the detector.
cosmic component is uniform over the extent of the array at any particular site. Any variation in a single array will, consequently, be entirely due to differences in the terrestrial background -- with the possible exception that effluents from an operating plant might lebd to higher readings on detectors close to the plant in the i
.-a r
9
--M 35 --
- ,7
- p downwind direction;_but there is no strong evidence of a general' pattsrn of this sort-in the data collected.
For sites still under construction there is no such considtration; and there are a fair -
number of such plants in the NUREG-0837 data.
Indeed, for at,out half
- the sites the average of the readings of the detectors in the upwind direction are as.arge, or larger, than the average far the detectors within two miles of the plant.- So far as the radial zones are con-
- cerned (which are: within 2 miles of the plant, but outside the site boundary; 2 to 5 miles; and greater than 5 miles) there are twice as many instances in which the highest average is for one of the two-outer zones, rather then for the innertnost-zone.
In the 4th quarter of 1983 arrays were operatei :.t 69 sites. Because of trouble-in collecting the deta needed to nomalite the readings of the detectors at 12 of thes,e sites, corrected data are available for enly 57 sites.
In NUREG-0837 the exposure rates for the absorbed dose in outdoor air are listed in tems of mR/ quarter; but these are converted below to mrad /yr. The accuracy of the readings is believed ~
to have a standard deviation of no more than four to five percent.
The everall average ambient exposure' rates for the 57 sites was about 66 mrad /yr. This is in reasonable agreement wit:) the 70 mrad /yr. (40 from terrestrial, and 30 from cosmic radiation without reduction for.
structural shielding) previously identified as the countrywide population-weighted average value of this quantity.
For individual sites the average exposure rates ranged from a high of 108 mrad /yr.
(for Fort St. Vrain) to a low of 42 mrad /yr. (Catawba).
1
._E___.2______.._m_____._____2____m_______m__.____m__
'h 4
=. 36 -
h-y i
.The point of_ particular. interest here is the range of readings et the,
?
various detector locations within the limited area of a single array.
7 At'two of the sites' detectors unusually close to the plant also had-
)
g the highest readings in the array. Since these could have been affected by effluents from the plant the readings from these stations have been ignored.
In addition, there are a few instances in which
,1 the exposure rate listed for a single station appears to be anoma-lously high -- higher, that is, by some tens of mrad /yr. than that for i
any other station in the array for one particular time period, though-not for others. These readings also have been set aside. With these p
emendations to the NUREG-0837 data for the 4th quarter of 1983, there-L i
was no site for which the difference between the highest and lowest The
-exposure rates recorded in the array was smaller than 16 mrad /yr.
average of this difference for the 57 sites was 34 mrad /yr. 'The highest value was 59 mrad /yr. -- which occurred between two stations-in the Surry array where one station (3.7 miles from the plant)-
recorded on expcsure rate of 40 mrad /yr. while another (11 miles from 4
the plant) showed 99 mradlyr.
The exposure rates at most of the stations in an array change from one quarter to the next.
For example, starting with the 4th quarter,of 1982 (IV-82) and continuing through the four quarters of 1983 (to IV-83) the exposure rates at the station in the Surry array, which-usually -- though not always--- provided the highest reading were (in mrad /yr.): 82, 79, 95, 65, 99. At another high-reading station in the Surry arra,r, about 6 miles from the first, these readings were:
(l
. _ ~
<t u.
37 j s, - ;,,,
n, l
-79,_80, 89, 65, 89. The highest-reading station at Watts Bar i
(non-operating)showedasimilarpattern: 84, 77, 86, 70, 91; as did E
the highest-reading station in the array at Fort St. Vrain:
134, 142..
130', 112, 136. As a somewhat curious fact, an increase in the rates at the high-reading stations by 20 mrad /yr., or inore, between-111-83 and IV-83 was reported for sites as variously located as Diablo Canyon, Palo-Verde, Clinton, Limerick, Pilgrim, and others, in addi--
+
tion to those mentioned above.
e In view of the se/donal shifts just noted, it may be expected that the 3
j array showing the largest spread in exposure rates would not always be l
u found at the same site.
ForIV-83this.wasatSurry(59 mrad /yr.);
but for the periods'from IV-82 tc 111-83 these were, respectively,-
found in the. arrays at Summer, McGuire, Surry, and North-Anna. These naximum' spreads were all in the range of 55 to 63 mrad /yr.
L 1
Variations in natural background exposure rates of 60 mrad /yr. have J
- i already been identified: as, for example, that in the cosmic radia-tion background between locations at sea level and locations at an altitude of about 3 km. (9800 ft ); and-as, also, in connection with the background exoosure to terrestrial radiation in difierent regions of the country, for which 60 mrad /yr. is just the difference between the lower range of exposure proposed for the Coastal Plains Region (15' mrad /yr.)andthelowerrangeproposedfortheDenver,ColoradoRegion (75 mrad /yr.). However, the 60 mrad /yr, difference siscussed here 1
h I
~"
,,. ; j'g-(which is entirely due to differences in the terrestfial background) is that bp. ween locations no more than 10 or 20 miles apart.
In connection with the ARMS survey (where the unit-areas considered were about 8 times larger than the areas of the site arrays in the NRC survey) variations larger than ! 15 arem/yr. from the mean have already been referred-to.
In the NUREG-0837 data, also, during the period from IV-02 to IV-83 variations in exposure rates f 40 mrad /yr.
within the confines of a single array were recorded for one or more quarters at 23 sites in addition to.the four mentioned above.
If one excludes Florida-(where the highest readings listed -- about 50 wet,iyr. -- are themselves so low as not to allow room for in-site variatiers as large as-40 mrad /yr.) these 23 additional sites had an essentially countrywide distribution: from the Pacific Coast, through the mid-continental region, and on to New England.
Finally, with about 40 stations in an area with a radius of 20 miles there would.on the average, be one station per 30 square miles.
'Actually the distribution is not uniform, there being, nominally, 16 detectors within a two-mile radius,16 between-2 and 5 miles, and 8 beyond 5 miles; so that the average spacing is much wider in the outer There is no reason to scppose that ne extremes in the natural-zone.
ly occurring exposure rates within the 20-mile region around the sites would tiecessarily be picked up in the NUREG-0837 survey.
i f
- H.
. 39..
-e
/- * ?
l t
o e
ji) :The Los Alamos Survey i
L For many years the Environmental Surveillance Group of the'Los Alamos p
Laboratory has monitored a large number of locations in the technical
- i areas of the Laboratory, and also in the surrounding neighborhood, for the presuce of a leng list of possible radioactive and chemical contoninants in the. air, soil, and water. As a part of this operation I -;
they hav'e maintained an array of thermoluminescent dosimeters-(TLDs)-
A to monitor the cosmic ray and terrestrial radiation background.
number of these TLD stations are outside the perimeter of the techni-cal area at locations where nomal Laboratory operations would not -
L affect the readings of.the dosimeters.
Seven of these outside sta-g tiens are deployed in the townsite; and these are all in generally similar. (mesa-top) terrain, and are all at an altitude close to 7250 ft.,(2200 m.).
They are all located within an area somewhat lest than y
7 square miles, and the extreme distance between any two of these u
b o!
stations is only 3.5 miles. These seven statiens thus constitute a The measuraments reported are believed to'be rather compact array.
within 4 percent of actual levels.
l The TLDs register the sum of the absorbed dose in outdoor dir (with no allowance for shielding) from the cosmic and terrestria' backgrounds.
with the exception c,f the cosmic ray neutrons, to which the partic-ular cietectors used are not sensitive. To obtain the total background exposure it is necessary to add 11 mrem /yr. to the TLD readings.to allow for the neutron component (as taken from the dose-altitude' curve o
1
-l E
w-t
- :.1
-- g-of NCRP 45 at 2200 m.) The. total exposures for the calendar year recorded by the TLDs at each station are listed in the annual reports of the-Surveillance Group. The exposures are entered in terms of mrem /yr.; but these will be tha'same as arad/yr...since the types of radiation seen by the TLD's all have a quality factor of unity. Again from NCP.P-45, the average exposure rate to cosmic radiation (excluding neutrons) at 2200 m. altitude is 60 mrad /yr. The average of the TLD readings for all seven stations over the six-year period from 1980 through 1985 is 116 mrad /yr. The average exposure from terrestrial i
radiation is, then, 56 mrad /yr.
-Over any perticular time period the cosmic background will, of course, be~ uniform across this compact array; but the level could change f
somewhat over a six-year period. However, the main variation in l
cosmic ray intensity accompanies the 11-year solar activity cycle ano,.
at the geomagnetic latitude.of the Continental U.S., this variation Such a 3
has a maximum amplitude of less than 10% of the mean level.
change in the array average as that between 1980 and 1981 (from 123 to l
100 mrad /yr.),orthatbetween1982and1983-(from109to131 mrad /yr.) will have resulted from changes in the ter:estrial back-Presumably such shifts are to be accounted for by differences G
ground.
in precipitation, snow cover, and so forth -- and, indeed, there was i
30% more precipitation in 1982 than 1983,'21.7" vs. 16.7" -- however, the size of the changes e s by no means the same at each station, A
Y i
ranging from +11 mrad /yr. to +35 mrad /yr.
It must be realized, of course, that quite local effects, such as.the existence of an l
+
4 l
n-J,
) f')
ikj
[ d{
4
~
.y as
. 41 -
l 71
? b' inversion layer -- and particularly in an area as'meteorologica11y 4
complicated as the mesas at Los Alamos - can have quite significant (and unpredictabir.: reflections'on the exposures applying at one
. location or enother..Another interesting example of a station-to-station variation occurred between 1984 and 1985. The j
array average exposure was 116 mrad /yr. for each of these years; but, _
s while the exposure at one station dropped from 135 to 120, that at another, only 1.2 miles away, increased from 115 to 136 mrad /yr. The j
fir-station had the highest reading in the array in_1984, but fell to a Ird place in 1985 whereas the second station advanced from fourth place in 1984 to first place in 1985.
The spread between the highest and lowest readings in 1980 was only 25 mrad /yr.: but for each of the other annual periods this spread ranged between.30 and 40 mrad /yr. -- even within the very limited extent of i
this array. During.the six annual-intervals considered, the lowest 7
exposure was recorded at one or the other of two stations, while three different stations were involved in providing the-highest reading of L
L
=the year.
These examples,-culled from the results of the Los Alamos survey, point up the fact that there is much more variability in the r.atural E
l background radiation -- both over time, and in space -- than is brought to mind by references to countryride, or even regional, averages.
-r<
I
..- : =
~.
J-7, ;,.
Patenthetically, in NUREG-0837-a plan is referred to for running in additional TLDs in the event of some off-nomal incident. The obser-vations just noted, along with those reported in connection with the HRC survey, raise some question as to just what such additional TLDs-J (on previously unmonitored sites) will be able to establish. Some-
! thing, possibly, in the event of changes in the ten, or few tens,'of mrad /yr. range; but rather little for smaller changes.
(
c.
iii) Washington,D.C.
l Being at sea-level, Washington has a cosmic ray dot.e rate (including neutrons) close to 30 mrem /yr. Since the neutron component in this l-cosmic ray flux is quite small the difference between rads and rems is l'
also.small, end may be ignored. Washington is in the Coastal Plains
. Region for which the outdoor exposure rate to terrestrial radiation is said to be between 15 and 35 mrad /yr. The natural background dose l
l rate in Washington should, then, be between 45 and 65 mrad /yr. Still, some question on this point is suggested by Alvin Weinberg's measure-
>i ment in May 1979 of a dose rate of 250 mrem /yr. during a hearing in L.;:
the Dirksen Senate Office Building.
k p
1; Having this in mind, a hand-portable radiation rate-meter was taken on several short excursions during May, June, and July of 1986, mostly in the near neighborhood of 1717 H Street. The resulting observations cannot be considered to constitute a survey, since they were made in The the course of visits to a somewhat random selection of targets.
t f
- 43 %
e; rapid time-response of the rate-meter made it attractive to take many-of the. readings en passant, so the precision of the readings was not impressive -- something like tif R/hr. Still, the' measurements were probably sufficiently accurate to permit the grouping. into the rather broad exposure ranges indicated below. The rate-meter was ealibrated in/<R/hr;butthathasbeenconvertedtoerad/yr.using1pR/hr.=-
8.76 mR/yr. = 7.6 mrad /yr.
The The following is a summary of the results of this mini-survey.
nur:bers given refer to exposure rates in ambient air (whether outside orinside)in-mrad /yr.:
V 60-75. The lowest rate observed was about 60. This was found in a variety of locations: the doorway of the older World Bank Building at 18th and G; the 5th floor of the Hart Senate Office Building; on-the ground floor inside the new Presidential Plaza at 19th & I, both in the southwest corner and at the foot of the elevator bank near the Rates close to 75 were found along First Street.
southeast corner.
S.E.; on the steps and among the columns in front of the. Supreme Court; in the northwest doorway of the Russell-Senate Office Building;.
in the interior of the Lincoln Memorial; and cn the street in front of 1717 H Street, as well as'in the lobby and in the large conference room on the 10th floor.
75-90. Examples were found along a number of st.eets (18th Street, I Street, Pennsylvania, and 20th); in the lobby of the Lombardy Hotel;
4
.,e
. 44..
' 6 3
the. lobby of the National Science Foundation Buildihg; both on the-street level and the lower level of the Farragut West Metro station; and in the Comissioners' meeting room,11th floor,1717 H Street.
^
90-115. Rates in this range were found on the 4th floor of the Dirksen Senate Office Building and the 3rd floor of the Russell Senate Office Building; on the street level.of the new World Bank Building at 18th and Pennsylvania (then under construction) except that the rate of"about 90 increased to about 115 on walking past the structural columns; outside the base of the Washington Monument; the lobby of the Hay Adams; lobby of the New Executive Office Building; the 5th and'8th floors of the Lombardy Hotel; the men's rooms and corridors on the 10th and lith floors of 1717 H Street (about 15 higher than the meeting rooms when passing structural columns); the roadway of East Capitol near the foot of the steps to the Capitol; the sidewalk along Pennsylvania Avenue near the White House fence.
Inside the Washington Monument at ground level; beside 115-150.
the Reflecting Pool; in Lafayette Square (about 30 higher than on.the othra side of Pennsylvania Avenue); on the street in front of the New Executive Office Building; on some sections of sidewalk such as that paved with oricks on Madison Place, and the new section paved with ornamental stone slabs at 17th and H -- both being about 30 higher than nearby sections with concrete walks.
%,p
{
l
- 150-200. In this range were the entryway at the southeast corner i
of the' Presidential Plarat the porte cochere on the east side
- f the 1
Capitol; the walk by the Viet Nam Memorial; and the steps from the Reflecting Pool up to the Lincoln Memorial.
J
- 7 200.. On' crossing Madison Place from the east side of Lafayette Square (rate M 50) one can go through the porch of the Law Courts Building (rate ~265) into a delightful patio (rate 4 240) and on into the lobby (rate *120). On starting up the steps to the Library of Congress from First Street, S.E. (rate +75) one comes to the first ~
1anding (rate *150), then the second landing (rateM25), and then the doorway (rate *380)andonintothelobby(ratev115). On approaching-the north entrance of the Old Executive Office Building one leaves the sidevolk on Pennsylvania Avenue (rateM15), goes through a gateway in the fence -(rate *165), crosses a flagstone-paved patio (ratead90) and up to the top of the steps (ratea400) and through the doorway into the lobby (rate *135). Apart from these observations there is Weinberg's Senate Hearing room (rate -250).
In reciting these scattered observations it is not intended to charac-terize Washington, D.C. so much as to illustrate the fact that it is not necessary for an-individual to travel from the East Coast to Denver in order to encounter wide variations in his rate of exposure Considerable variations will be experienced to background radiation.
by a stationary individual in many locations, by individuals traveling a-few miles to the store in many parts of the country (as evidenced by l
46 -
'L,
- (
i ei the NRC survey), by individuals residing in one house'or in another 1-house a few blocks away'(as from the Los Alamos survey), or by indi-vidualscrossingfromonesideofthestreettotheother(asin Washington). Of course, the " countrywide, population-weighted, average annual exposure" is a perfectly well-defined concept which is useful for some purposes, even if there should not be an individual
>s anywhere who actually receives just that exposure for one year, let
- (
alone two years running.
IV. Observations and Comments t
j We know that extree exposures to radiation can be fatal, and we know i
a fair amount about.the levels which produce lethal effects in a short time. We even know that there is some risk that an-exposure about
-l r
twenty times smaller than one resulting in a prompt fatality -- 6 whole-body exposure, that is, of something like 20 or 30 re-of
(
low-LET radiation delivered in a short time -- may, with a rather poorly known probability, initiate processes which result in fatality However, there is a gap of at least two orders of years later.
magnitude between the dose levcis for which we have any such knowle As stated in UNSCEAR-77 a
and the levels.provided by natural sources.
"Itmustbeemphasized,however,thatsuchestimates"(referringto their estimate of ~ 10-4 fatal malignancies / person-rad) "are derived.
predominantly from rates observed following absorbed doses of over rads," and "In particular, at low doses in the region of those re-ceived annually from natural sources, no direct information is
)
- x L..'
f,
' L+.
-i 47 % -
availab1'e as to the_ level of induction of malignancies that might apply.' On this same general point the BEIR-III Consnittee states that "The Comittee does not know whether dose rates of gansna or X-rays of about 100 mrad /yr, are detrimental to man." These cautionary state-y; ments confirin the existence of a shore to our knowledge, and a sea of ignorance beyond.
However, the Committees' statements do not call very specific atten-tion to the fact that the human, species has been steeped.in something like today's natural background through the whole period of 'evolu-tionary time.
In fact, for a large part of that period the flux could well have been higher than that estimated for.today since, until quite recently, our ancestors had little benefit from structural shielding.
If it be argued that a cave might constitute a quite impressive 1 structure, it should be recognized that in some cries, at least, the terrestrial component of the-background would be markedly higher than we are used to.
Indeed, there are some who hold that the surprisingly rapid transition from the earliest hominids to modern man in less than 10 generations could not have occurred without some extra biological 6
assistance -- as, possibly, from radiation. Of course, for those who know that man was created in the image of God just 5,991 -- or, perhaps, 5,748 -- years ago, evolutionary considerations are irrele-vant,'there being no need for them. And in modern times, as evidenced earlier in these comments, there have been considerable population groups exposed to-several times the generally assumed background level of 100 mrad /yr. with no identified deleterious effects.
1
48 -
it:is implicit in the statement of the BEIR-III v
-Whatever may apply, Comittee that radiation at about the natural background level may be neutral to. man's well-being, that there might be some non-detrimental, or-compensating, or conceivably some favorable aspects of such radia-:
At least the BEIR Whether this is true-or not, who is to say?
tion.
Committee 1 caves the question open.
regulatory To return to more factual matters, one thing is certain:
They must fill them in with something bodies cannot abide a vacuum.
To ensure that the need for action not.be held up
-- evidence, or no.
by a mere lack of actual knowledge, our response has been to adopt, and apply, a enuple of theorems:
That absolutely all exposure to radiation is detrimental -- which I.
has the corollary that all exposures should be reduced to a level =
as low as reasonably achievable (the Al. ARA principle); and That the risk of a detrimental health effect is directly propor-II.
tional to the dose at all levels -- which has the corollary that the collective number of health effects will be proportional to the total of person-rem occasioned by some particular action or event.
No doubt there is considerable virtue in Corollary I -- at least at some levels -- though no levels are specified in the statement of the.
principle; and there is the evident difficulty in applying the i
5
+
i=
49.-
- y ]*,,;
jq principle that it is so much easier to-detemine 'what may be achiev-able than to agree on what may be reasonable.
Whether Corollary II has any quantitative relevance to reality or not, i
it does have the seductive attraction of being arithemotically simple It is convenient for cost-benefit arithmetic; and is used to apply.
by both regulators and intervenors to assert.that so.and so many delayed fatalities will result (or be saved) by coing (or not doing).
-whatever may be under_ discussion. And, from the point of view of government officials, it also has the. inestimable advantage that it can be held to be " conservative" (a catchword.for overstating the negativeaspects). The conservative nature of that opinion is not quite universally accepted; but it does have the support of a large majority of the BEIR Comittee.
However, apart from the question of their actual relation to reality, the application of these theorems and their corollaries is quite expensive to society. A particular example may be found in the criteria laid on by the Environmental Protection Agency (EPA) for the Since a perfomance of a repository for high-level radioactive waste.
demonstration of compliance with criteria based on a limitation of-exposures to individuals would be either extremely difficult or impossible, the Agency has adopted criteria based on limitations of
~
Assuming.
the amount of radioactivity released from the repository.
such radicactivity to be distributed.in the biosphere, they (the EPA By Staff) have estimated the number of person-rem which might result.
i
+
- "9
e i
< -.,., - ~
- 50.-
i c
+
the second theorem, this is converted to a number of fatalities (at-the rate of 60 fatalities per year for a one mrom/yr increase in-s exposure,ofthe.U.S. population). In~ its most comprehensible fom the '
Agency's criteria come out as not more than 1,000 premature deaths in~
5 the first 10,000. years after the disposal of the waste from 10 tons-of heavy metal -- that is, an average of no more than 0.1 deaths per L
year.
An additional" application of the ALARA principle is not specifically i
required; but it has already been implicitly applied -- as evidenced by their remark that repositories with this property appear to be achievable, and their further remark that "if mined geologic reposito-ries were not capable of providing such good protection the Agency might have chosen considerably different standards." The Agency also pointed out that in their consideration of several model repositories the projected population risk was at least a factor of ten smaller than that required by its criteria, Public coments were invited before the Agency's criteria were adopted as a rule. Some, as could be anticipated, were to the effect that the proposed standards were insufferably lax; while others -- including those from the Agency's Scientific Advisory Board (SAB) -- pointed out that the proposed standards were totally inconsistent with the risks society accepted from other activities, and that there would be great I
difficulties in demonstrating compliance with the proposed containment 1
I l'
1 i-
- 51 ' -
4=
4,, g )".
.j 1c I j!
requirements. ' The SAB recomended in'particular that the " level of i
protection" be relaxed.by at least a factor of. ten, v
t L
Naturally the' Agency declined to follow any such suggestion. After i
all, a government agency is under a compulsion to establish a "conser-1 vative" position when fatalities are in question -- even if the so-called " fatalities" are a figment of its own imagination, or, as in y;
this case, arise merely from a-literal, application of the second'
-theorem.
Whether, as the SAB claimed, the criteria adopted are far too strin-From
~
gent may be judged in the light of the following observation.
the earlier discussion of natural background, uranium is responsible E
for a total of about 30 mrem /yr.: about ten each from external terres-trial, internal exposure, and inhalation of outdoor continental air.
u
- With EPA's correlation factor, 30 mrem /yr, would occasion about 2000 premature fatalities per year in the U.S. population. Almost all of this is provided by uranium residing at depths of about 30 cm., or less, below the surface of the soil -- since the-gamma rays and radon must escape to the atmosphere.
1 Assuming a-concentration of 4 ppm, the amount of uranium in the I
topmost 30 cm. of soil in the U.S. is about 3 x 10 tons -- though the t
surficial concentration of uranium in U.S. soil is probably apprecia-
' bly smaller than 4 ppm. By EPA's method of estimation there would be 5
6, or more, fatalities per year from 10 tons of uranium in U.S. soil.
~~ ~
~~
~
J+
L"-
- +
c I
This-amount of uranium, after irradiation in a nuclear reactor and disposal in a repository, is to-be given a level of protection by which the number of fatalities is kept below 0.1 per year -- or, more realistically, below 0.01 per year. Neither the factor of 60 -- nor
.the more: probable factor of 600 -- would seem necessary for protecting
.j health and safety, even if the fatalities hypothecated were known to i
be real.
Finally, does this " level of protection," as derived from our theo-j rems, cost anything extra? The EPA claims that such protection may be-available "without significant effects on the cost of disposing of these wastes." This may be true for the cost of the actual construc-tion of a repository; but it is not at all true for the far from inconsiderable effort needed to characterize and qualify a site to meet the criteria -- the focus of all the effort undertaken to dete, i
as well as for sore years to come -- or to monitor a' site after disposal to " detect deviations from expected performance."
l.
[
The effects of the application of our theorems show up persistently;in a great variety of other situations:
in the application of.the Safety Goal Policy, in backfit decisions, in the Severe Accident Policy, in L
It doesn't matter whether money Emergency Planning, and many more.
and effort expended on conjectural risks is provided by Government (and collected in taxes) or by the: utilities (and collected from consumers)orbyintervenors(and.collectedincontributions), itis And all this as a conse.quence of the unbridled imposed on society.
i,.w; W. *
- 53~-
.l 33
,y application of our theorems -- which may_in fact be amongst the great n
hallucinations of the twentieth century.
i t
Clearly this whole matter cries out for new thought. We badly need a floor for ALARA. We badly need an officially-promulgated g minimis j
The theorems we have been using are just that:_ theorems; and' se.
we'are using them far outside the range in which there is any evidence that they apply. And yet, by their very form, they are incapable of providing any assistance in meeting needs of the sort mentioned.
Were we able to bring ourselves to be reasonable in this connection we would decide somewhat as follows: We do not know whether or not We radiation is harmful at the levels provided by natural background.
do know that, no matter what they may do, persons are exposed to-variations of 40 or 50, or more, mrem /yr. in the rate at which they g 1, 4
We therefore conclude that doses below 50 mrem /yr. _
receive radiation.
(or some such level
- along with a consonant dose level for. single posure) are below regulatory concern; are not to be taken into l-I (regulatory) account in any way -- either in the application of ALARA, or in'the integration of presumed effects of some presumed event; and are, in fact, g minimis.
Judged in the light of the circumstances described earlier in this I
L report such a conclusion would be consistent with any and all known Proposals of this general kind have often been made before facts.
[
f (cf., for example, Adler, H.I. and Weinberg, A.M., Health Physics 34, 1978; et al.). But perhaps the most cogent consideration in support 1
Q X_,
.p.
',i. *, A.
- .n
.,r ofthis'viewistobefoundinanoldsaw(slightlymodifiedforthe occasion) "What'you don't know won't hurt you -- unless you start believing that you do."
NOTE CONCERNING RADIATION UNITS i
The units used throughout this discussion are the " rad" and the ' rem."
The rad is the unit for. energy deposited by ionizing radiation of any type in any material. One rad refers to an exposure resulting in an absorbed energy (or." dose")of100 ergs /gm. To provide a correlation between the radiation flux and the dose, it is necessary to specify the material Thus, one speaks of "an absorbed dose in air" of so many rads.
considered.
Since there are more electrons per gram in biological tissue than in air (resulting from the larger proportion of hydrogen in tissue) a given flux of radiation will deposit somewhat more energy per gram in tissue than in L
. This difference, however, is rather small (only 7%) and is usually air.
p
_ ignored; so that, to a reasonable approximation, a radiation exposure l
providing an absorbed dose in air of one rad would be said to provide one c
l rad of absorbed dose in tissue.
1 The rem is the unit used to calibrate biological effects in human tissue.
One rem is the dose from any radiation that produces biological effects in l
the body equivalent to those from one rad of X-rays within a given energy L
L,*, '
+
4 l
. Different types of radiation cause different effects in biologica range.
For example, one rad delivered by [ particles is more damaging tissue.-
than one rad from X-rays, even though the amount of ionfration per gram associated with the two would be the same.
(Thedifferenceresultsfrom the difference in the Linear Energy Transfer-(LET), that.is, the distribu-tion of the ionization along the path of the ionizing ray, or particle.
This is reasonably unifom in the case of low-LET radiation, such as the
-X-ray, but heavily concentrated near the end of the path in'the case of the
,(particle.) To take account of the differences in-biological effects, a factor -- variously referred to as the Relative Biological Effectiveness l
(RBE) or Quality Factor -- and, in current writing, usually designated Q --
is introduced, by which the dose in rads is multiplied to obtain the dose That is, the dose equivalent (D.E.) in rems equals Q times the in rems.
dose in rads, where the value of Q depends on the type of radiation. By definition, Q=1 for X-rays; and is also taken to be unity for [ rays and
[-particles. However,for[ particles,itisnowofficiallyagreedto It will scarcely concern us here, but a value of Q between 5 1
take Q=20.
and 20 is currently assign u to neutrons.
L Onefurtherconventionis.necessary.[-particles,aswellas[-particles, deposit their energy in a quite thin layer of tissue imediately ad,iacent Thus, they do not provide a "whole-body" dose, but only a to their source.
dose to the organ (or small portion thereof) in which the source of such radioactivity may be located.
In order,to assess the relative biological hazards of the effects of radiation delivered by various means to various parts of the body, one needs some way of translating any particular organ
4 i.....
~ tg M t
,;e dose onto a comon scale at a level judged to represent an equivalent 4
The ICRP has devoted extensive efforts to developing a overall effect.
L
[
system of " weighting factors" whereby the dose to any particular region of This
.the body can be converted to an appropriate value of whole-body D.E.
1 s
[
is called the " effective" D.E.
Thus, for example, the effects of a dose of -
4 yrem to the lung tissue is taken to be adequately represented by 0.12 4
x rem of whole-body D.E.
Obviously, the sum of all the weighting factors
[
for the different organs, or regions of the body judged to b'e significant.
must equal unity -- so that,a dnse of xtem to each significant center will, when added up, equate to a whole-body D.E. of xrem.
l P
As an additional coment, it may be noted that the classical unit for i
l radiation exposure -- the Roentgen (R) -- is no longer in use, though it The Roentgen was defined in appears in mary older and even recent reports.
one esu of terms of the amount of ionization produced; ir, particuler:
charge in one cubic centimeter of air at standard temperature and pressure.
Thisisequivalenttoanabsorbeddose.inairof87 ergs /gm.(0.87 rad),or At least in discussions of natural background of 93 ergs /gm. in tissue.
radiation (where air and tissue are the media of interest) the rough approximation 1 R = 1 rad is frequently used. Today, there is the newer SI unit for energy deposited -- the Gray (Gy). One Gray is the exposure resulting in the deposition of one joule / kilogram (rather than 100 l[
ergs /gm.) so that 1 Gy = 100 rads.
Similarly, the SI unit for dose equiva-lent in biological tissue -- the Sievert (SV) -- is such that 1 Sv = Q Gy =
100 rems.
l
[ [ [,,,.
57 --
ej c['
v Finally, just as one could (if one chose) calibrate. velocity in tems of i
furlongsperfortnight,thereisthe(unspeakable)unitoftheWorking
. Level (WL) for the exposure to hactivity in air, and the Working Level Month (WLM) for the integrated exposure to such radioactivity'. The WL is defined as a concentration of radon and short-lived decay products which would result in the release of 1.3.x 10 MeV of kenergy per liter of air.
5 A concentration of 100 pCi/1 of Rn-222 in equilibrium with its daughters Po-218,'Pb-214, Bi-214, and Po-214 would provide one WL. The WLM is defined.as the exposure to cne WL 'for 170 hours0.00197 days <br />0.0472 hours <br />2.810847e-4 weeks <br />6.4685e-5 months <br />. As with any attempt to correlate the concentration of airborne radioactivity with the dose delivered to any particular organ (such as the lung) resulting from inhalation, the steps are more than a'little' complicated;: requiring, as they do, either knowledge or assumptions concerning breathing rate, departuret from radioactive equilibrium (which essentially always apply except in situations where the air is quite stagnant), the particle sizes of the aerosols involved, and the extent to which the individual radon p
l
?
decay products are attached (or not attached) to the dust particles within l
the air, as well as the physiological distribution and retention of the-L materials inhaled. On the basis of averaging assumptions on each of these points it has been taken that one WLM corresponds to a dose of about 12-14 i
I ll rem to the segmental bronchioles. With the ICRP weighting factor of 0.08,
'one WLM corresponds to a whole-body dose equivalent of about I rem.
l-
.