ML20033F854

Variability of Natural Background Radiation

ML20033F854
Person / Time
Issue date:	09/30/1988
From:	Mark C Advisory Committee on Reactor Safeguards
To:
Shared Package
ML20033D930	List: ML20033D929 ML20033D959 ML20033D961 ML20033D972 ML20033D999 ML20033F826 ML20033F827 ML20033F830 ML20033F831 ML20033F832 ML20033F833 ML20033F837 ML20033F843 ML20033F850 ML20033F852 ML20033F853 ML20033F854 ML20033F856 ML20033F857 ML20033F858 ML20033F859 ML20033F861 ML20033F863 ML20033F875 ML20033F882 ML20033F926 ML20034C493
References
NUDOCS 9004030260
Download: ML20033F854 (32)

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.y TABLE OF CONTENTS l l..

PAGE u

VARIABILITY.0F NATURAL BACKGROUNO RADIATION.....,,........................

1 l

y l,

1.

NATURAL BACKGROUND...................................................

1

'I 1..

Cosmic' Rays'...........

......................................... 1-3J p

it' External Terrc5triat............................................ 6.

l

'111 Internal..................,.....................................

6-11 11; CELEBRATE-D HOT SP0TS.................................................

11-14 1

I l l. SOME LOCA L SU RV EY S................................................... 15 l

'i

'EML 0ata.........,.......................,......................

15-16 1

L ii. Hat $urvey -or Nuclear Pwe r Pl ant Sites,....................... ' 16-18 l

iii The'Los. Alamos Survey..............

................,........... 18-19 in Washington 0C......,...............

19-20 v-Variability.....................................................

20-21 IV. DBSERVATIONS AND' COMMENTS...........

4 21...............................

l

.V.

CONCLUSION...........................................................

25-26 l

Note Concerning Radiation Units.....................

................ 26-27 Bibliographical Note..................................................

27-29.

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VARIABILITY OF NATURAL BACKGROUND RADI ATION The average annual exposure of persons in the United States to radiation _ from i

natural background sources is often said to be "about 100 millirem" whole-body

-dose equivalent.

Though it is usually pointed out that actual' exposures differ from one region'_of the country to another, and that the 100 mrem value is an estimate of a population-weighted average, many references include little to

.indicato the extent of the variations actually encountered.

As a result, there is some room for the impression that this nominal 100 mrem is a sort of natural

' constant -- much like that of normal body temperature (37'C) -- and that any appreciable departure above this norm is associated with seriously undesirable i

consequences.

In the present discussion it is intended, first, to des: ribe the l

generally familiar range of natural background (particularly as= experiencea in B,'

the U.S.), and than to bring to attention some of the more fine grained aspects

-of its variability. These naturally-occurring variations warrant consideration L

in assessing the significance of incremental perturbations of the radiation D

levels to which people may be exposed.

I.

Natural Background j,

This consists of three major components:

(i) Cosmic Rays, (ii) External L

Terrestrial, anci (iii) Internal. These are described separately.

L (i) Cosmic Rays In the lower atmorohere (altitudes less than a few km) the radiation frcm this source is mostly provided by muons and high energy (very penetrating)

. electrons-There ere other particles in the flux, including neutrons.

The number of neutrons (at low altitudes) is small compared to the number of muons and electrons, but because of their large quality factor (Q) or relative biological effectiveness (RBE), which - 'at least in' UNSCEAR-1982 -- has been taken to be 10 for neutrons as compared with unity for-muons or: electrons, the-neutrons contribute appreciably (about 10%) to the dose equivalent in tissue,

.even at sea level. This contribution increases with altitude 'and at 3 km (9,650 f t)- the neutron component contributes about 25% of the f.otal biological dose. - (More recently, the NCRP has decided that the value of Q for neutrons might lie between 5.and 20.

The total level of the cosmic radiation (in rems) may,:then, finally be rated somewhat differently than in some of the values used below.*)

i At high altituds (altitudes greater than about 10 km, which are accessible only to.higb flying aircraft or space vehicles) there is a strong dependence of the cosmic ray flux (or dose) on the geomagnetic latitude -- the flux being many times larger at the magnetic pole than at the equator.

However, on the inhabit 6d portions of the earth's surface (altitudes less than ~5 km) the variation with geomagnetic latitude is much smaller; and for the continental U.S. (essentially all lying between 40 and 60* N geomagnetic latitude) the variation with latitude is only a percent or so. This will be ignored in the sequel.

• Set oncendtid Note Concerning Radiation Units, (p. 26).

1 E

m

1 g

At any particular location on the surf ace of the continental U.S., the cosmic radiation may be considered as uniform in time.

Though there are temporal variations associated with the 11 year sun-spot cycle, with solar flares, and i

with changet in atmospheric pressure and 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' considered, i

The significant variation in cosmic ray exposure is the variation with altitude. This results from the difference in thickness of the atmospheric blanket. On this account the tissue dose equivalent from cosmic rays at il altitudes of 1s 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, respectively.

The average. cosmic i

ray dose rate out-of-doors at sea level is 29 mrem /yr.

Since people spend a considerable fraction of their time indoors, and since structures provide at j

least some shielding, it has been estimated that for the U.S. the average f

exposure received by the pupulation is about 10% smaller than the exposure j

out-of-doors. The average exposure rate at sea level has thus been taken to be

,j 26 mrem /yr. Taking into account the distribution in altitude of the U.S.

j population,.the average dose equivalent rate from cosmic rays has been estimated to be 28 mrem /yr.

This is the number included in the assessment that J

the average annual exposure in the U.S. is about 100 mrem /yr.

l More than 80% of the U.S. population lives at altitudes less than 0.3 km

(~1,000 ft), and for these the cosmic ray dose rate is within a mrem /yr, or so, j

of the countrywide average. About 10 million live at altitudes 3 1 km, where the cosmic ray dose rate (out-of-doors) exceeds 40 mrem /yr.

More than five million live at altitudes > 1.3 km Jor whom the cosmic ray dose rate exceed, 45 mrem /yr. Cities included in this group are:

Salt Lake City, Albuquerque, Reno, Colorado Springs, and Denver.

(For Denver, altitude 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 12 km, for whom the out-of-doors cosmic ray dose exceeds 60 mrem /yr. There are many small settlements'in the Rockies (e.g., Silverton, Colorado, 2.8 km) at altitudes of about 3'km.

In particular, for Leadville, Colorado (altitude 3.1 km) and nearby Climax (altitude 3.4 km), in or near which a total of about 10,000 persons reside, the cosmic ray dose rate would 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.

These are at lower geomagnetic t

latitudes.than apply in the U.S.

As a rough allowance, in designating cosmic J-

. ray dose rater. for these cities, the doses from the detailed dose-altitude t

curve drawn for the U.S. have been reduced by the same fraction as the sea-level l,

doses for the relevant geomagnetic latitude.

The particular dose-altitude curve j

used is that presented in NCRP-45 (1975). These high-altitude cities include:

Johannesburg, alt. 1.8 km, population ~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; and Quito, alt. 2.85 km, t

population ~.75 million, dose rate ~100 mrem /yr.

There is also La Paz and the Altiplano region of Bolivia.

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'In the Altiplano the altitude ranges from 3.5 to 4 km, and about 75% of

- Bolivia's total population of 6 million live in this region. In addition to La-Paz at 3.6 km, population (La Paz Department -- that is, the city, plus the surrounding administrative area) 1.9 million, there fs the city of Oruro at 3.7 km, Lake Titicaca and its surrounding settlements at 3.8 km, and the city -

of_ Potosi at 3.9 km, population (Potosi Department) ~0.8 million.

Thus, in the A'itiplano region there are 4 million, or so, people for whom the' cosmic ray dose-rate is in the range 150 to 200 mrem /yr.

(ii) External Terrestrial At any location on the earth's surface persons are exposed to some flux of radiation (mostly. photons) from the decay of radioactive elements contained in the soil and rocks.' The main primordial sources are X-40, Th-232, and U-238;-

though, in tne case of Th and V, the major part of the radiation encountered is provided by the radioactive daughters in their decay chains. The radiation flux at any location will vary depending on whether the soil is. wet or dry, covered-with snow or not, subjected to changing barometric pressure, and 50 forth; but these fluctuations will average out over the year. The significant variation is that applying from place to place due to differences in the local abundance of the primordial elements.

Most of the radiation to which people are exposed is transmitted directly into tne air from the near-surface rocks and soil as they reside in place.

Almost all the radiation reaching the atmosphere originates in the topmost 25 or 30 centimeters of the soll.

On a mass basis the elements potassium, thorium, and uranium in the materials of the earth's crust are, respectively, something like two percent, and 12 and

~

4 parts per million. The number of atoms per gram of potassium-(atomic mass t

l -

~40) is six times larger than D t of thorium or uranium (atomic mass ~240).

l The isotopic abundance of K.4r. (t% only radioactive isotope of potassium) is

-1.2 x 10-4 The atomic ratios :; F 40, Th-232,- and U-238 in the earth's crust are, consequently, about as 4:3:1. With half-lives of 1.26 x 109, 1.4 x'1010, and 4.5 x 108 years, tne number of. disintegrations per unit time of K-40, L

Th-232, and U-238 are about in the ratio of 15:1:1.

In ninety percent of the disi_ntegrations of K-40 a s particle (maximum energy ~1.3 MeV) is emitted, and-almost all of these are absorbed -in the soil close to the source.

However, in the remaining 10% a y-ray (energy 1,46 MeV) is emitted, and some of these.will

= penetrate to the atmosphere from the above it can be seen that in material having the average composition of the earth's crust there are about 1.5 y-ray emitting disintegrations of K-40 per disintegration of Th-232 or U-238 --

which are essentially equal. Th-232 and U-238 are the parent nuclei of-decay series with ten or a dozen daughters having relatively short half-lives.

Assuming a state of radioactive eouilibrium (which doesn't always apply) each of the daughters in the series will disintegrate at the same rate as the parent nucleus. These series disintegrations release about 40 or 50 MeV of energy, but all but about 2 MeV of this energy is carried by a and S particles and deposited in the immediate vicinity of the source.

Aboct 30% of the energy carried by y rays is in low energy quanta (less than 1 MeV) which are strongly attenuated in the soil.

In the thorium series there is a 2.6 MeV y-ray emitted about 36% of the time, but in the uranium series there are no y-rays with such a high energy. Thus, thorium contributes more than uranium to the tarrestrial background radiation.

The average concentrations of these elements in near-surface soil is somewhat lower than in the earth's crust; but in UNSCEAR-77 it is estimated that the world average radiation level at one meter above the u

3 1

c

.a surf ace is about 40 mrad /yr:

15 from potassium,15 from thorium, and 10 from uranium.

As already suggested the actual background radiation rate.from one location to another may vary considerably from this average depending on the composition of the soil or rocks nearby.

On the basis of extensive surveys the U.S. has been divided into three distinguishable regions with respect to terrestrial radiation backgrounds.

These are:

(1) The Atlantic and Gulf Coastal Plains Area -- a coastal belt of from'one to a few hundred miles in width extending south and west from'Long Island to Texas, including between 15 and 20% of the U.S. population, and within which the terrestrial radiation is said to provide an absorbed dose rate in outdoor air of between 15 and 35 mrad /yr, with a population-weighted average i

f taken to be 23 mrad /yr; and (ii) Middle America, or The Noncoastal Plain Area,

,L the region extending north and west from the Coastal Plains Area to the Pacific coast (except for a relatively small. island around Denver and the Colorado

e Plateau).

In this region, which includes about 80% of the U.S. population, the natural terrestrial background exposure rates range from 35 to 75 mrad, with the average taken to be 46 mrad /yr; and (iii), the Denver, Colorado Area, 1

including some part of the East Front of the Rockies and the Cobrado Plateau, al in which the terrestrial exposure ranges f rom 75 to 140, and for which the d

average is taken to be 90 mrad /yr.

4 4

Much of the support for this regional breakdown is provided by the ARMS survey.

l. /

ARMS refers to the Aerial Radiological Measurements Surveys of the l

W radioactivity in the vicinity of government-sponsored nuclear facilities,

[

conducted for the AEC between 1958 and 1963. Areas about 100 miles on a side around each of 25 locations were surveyed on a one-mile grid to map the i

terrestrial radiation background. About 30% of the population of the U.S. was aj comprised within these areas.

Ib A' range of radiation rates was observed in each area.

For some of the lj locations,- half or more of the area was noted as having rates more than

!15 mrad /yr from the mean for the area.

For each area, the mean rate was taken l'

to be applicable to the population of that area. For those portions of the L

-country not covered by ARMS, the regional average exposures noted above were used to determine a population weighted average of ~40 mrad /yr for the outdoor j

absorbed dose rate in air for the U.S.

[

This terrestrial radiation is mainly composed of p rays with an energy of one

f to'two MeV.

This radiation is attenuated by the materials in structures, and,

-since people spend more than two-thirds of their time indoors, and even though there may be some external dose from the building materials themselves, a fac-te t of,0.8 bas been applied to the outdoor dose in estimating the. actual aver-

. age exposure people receive.

In addition, because of the shielding provided to

.the vital organs (gonads, bone marrow, etc.) by the outer tissues of the body,-

1 a further factor of 0.8.has been used in converting the terre'strial dose in air

~1 to the equivalent biological whole-body dose rate. With these factors,-the population-weighted countrywide average dose equivalent from terrestrial l

radiation to persons in the U.S. has been taken to be 26 mrem /yr. This is the number used in the assessment that the background radiation dose in the U.S. is

~100 mrem /yr.

4 Surveys of background terrestrial radiation levels have also been made in other countries.

Because of differences in instrumentation and procedures, not all 3

4 l

.of. these' survey resulta are directly comparable, and not all have been carried through to the point of developing a population-weighted average. From having smaller ~ areas the surveys of some of the countries'are geographically more complete than present U.S. surveys; and, in addition, at least some have been conducted more systematically.

Notwithstanding these differences, some of, the values-listed in UNSCEAR-82 showing the results of the surveys of about fif teen countries are indicated below.

The values quoted are for absorbed dose in out-door air in mrad /yr, which may be compared with the U.S. average of 40, already noted.

The lowest average values (32-33) are for Canada, Denmark, Poland; the highest (70-80) for. France, Romania, Switzerland, East Germany (GDR).

In most cases ranges'are given.

The highest of the high range values are: Norway, 950; Italy, 435; West Germany (FRG), 315; France, 250; GOR, 235. For bottom of

-the range values, several were less than 10, incluoing: Japan, Italy, FRG, France, Austria.

Not to be cheated out of having something special about it, the bottom of the range for Ireland is listed as Zero -- which could, of course, actually apply to a peat bog.

In a few cases, population weighted indoor to outdoor ratios are 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 Canada are not from UNSCEAR, but from the report of an extensive Canadian survey completed in 1984.) At least on the basis of the data shown in UNSCEAk 1982, the U.S. value for indoor-outdoor ratio would s

appear to be one of the least well supported, being based on results 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).

Indeed, the value for this factor for the U.S. may well deserve further consideration.

(In its forthcoming report, NCRP proposes to change this' factor from 0.8 to 1.0.)

From~this weiter of data, along with data concerning the worldwide distribution of the primordial elements, UNSCEAR-82 concluded that, for external terrestrial L

background,-a reasonable value for the global average of the absorbed dose rate in outdoor-air would be~ about 44 mrad /yr, and that a value of 1.2 would be e suitable global average for the indoor outdoor ratio.

The total environmental e-posure to external radiation consists of the sum of the cosmic ray and the terrestrial components.

For the Continental U.S., as already indicated, the population-weighted average of this sum'is 28 + 26 = 54 mrem /yr.

In a survey conducted in 1971 by the Lawrence Livermore Laboratory at 107 weather stations throughout the U.S. (but not including any locations at altitudes higher than that of Flagstaf f, Arizona ~7000 f t), the range in this quantity was from a low of about 35 mrem /yr to a high of about 150 mrem /yr.

The low values applied in-southern Florida, where the cosmic component was small (sea-level, less than 40' N. geomagnetic latitude), and the terrestrial component was also very low. The high values applied at Colorado Springs, Colorado (alt. ~6150 f t) which has fairly high components, both cosmic and

. terrestrial; and Bishop, C&lifornia (alt. ~4150 ft) with a moderate cosmic component, but very high terrestrial.

Flagstaff, Arizona, with the highest cosmic component of the locations included in this survey, had a rather low terrestrial component, and a total exposure to external radiation of only about 90 mrem /yr.

In Hawaii (near sea level, and only 20' N geomagnetic latitude)

-the cosmic component was smaller than in Florida and the terrestrial components were also very low; so that external radiation provided about 30 mrem /yr for the locations monitored.

In the reports examined, no measurements were given 5

a of the terrestrial component of external radiation for the high-lying settlements in Colorado (>7000 f t altitude).

There is, however, a general tendency for the external terrestrial radiation at such locations to be high --

~

in part, no doubt, because of the presence of rock near the surface, or of the i

exposure of bare. rock.

It therefore seems likely that among these settlements, 4

which already have a cosmic ray exposure in excess of 100 mrem /yr, there will

[

be some for which the total environmental exposure is >200 mrem /yr.

.L i

-(iii) Internal

-The exposures from internal sources of radiation may conveniently be considered 1

in three classes:

(a) that.from normal constituents of the body (principally.

i potassium); (b) that from radionuclides lodged in the body (uranium, etc.); and-(c) exposures from inhaled radionuclides (radon and its daughters).

s

- I' '

(a) The concentrations of the normal constituents of the body (such as H, C, or K) are maintained at fairly constant levels by the be4 's state of physio-i logical equilibrium. They are consequently largely inoependent of such factors 1-as diet or geographical location.

In the absence of temporary man-made pertur-1 bations -- such as tritium releases, nuclear explosions, and so forth -- the j

isotopic composition of such elements in the body will be the same as that in L

the biosphere.

l~

Cosmic rays provide a steady source of a large variety of radionuclides --

mostly produced at high altitudes. -These mix with the lower atmosphere and other components of the biosphere and the deep ocean-reservoir, and have established and maintained for a very long time a stable concentration in the various parts of the environment.

The concentration of any particular L

cosmogenic radionuclide in any particular component of the environment depends strongly on the half-life of the nuclide (along with other factors, such as solubility).

I-In the biosphere (the lower atmosphere, surface waters, plant life, etc.), the four most abundant cosmogenic radionuclides are C-14, Na-22, Be-7, and H-3.

Except for Be, these' elements are essential constituents of the body. The total internal dose delivered by these radionuclides is about 1 mrem /yr, and is j.

H-3, 0.001 mrem /yr..(Though not a body constituent, Be-7 may be ingested or-almost s'l provided by C-14; being, in particular:

C-14, ~1; Na-22, ~0.02; and inhaled, and is estimated to provide an internal dose of about 0.008 mrem /yr.)

The total dose from all other cosmogenic nuclides is thought to be less t M

.001 mrem /yr.

Potassium is an essential constituent of the body, with an abundance of about 2 grams per kilogram of total body weight.

Strictly speaking, the 2 gm level applies only to young males (age ~20) i falls essentially.1,inearly with time over the next 60 years to about 1.6 gir.

'a females, after age 20, the j-potassium concentration at all ages is N y about 75 to 80% of that.in males --

in part, possibly, because of the difference in proportion of adipose tissue in 1

which the potassium concentration. is relatively low (only about 0.5 gm/kg).

There is an' appreciable variation in potassium concentration from one organ of the body to another (~4 gm/kg in red marrow, 2 in testes, 0.5 in bone) and a corresponding variation in doses to the different organs. However, for an

'j assumed average concentration of 2 gm/kg body weight, the whole-body dose j

equivalent has been estimated to be ~18 mrem /yr.

i.

6 d

r; c

f o

y F

Essentially all the S-particles from the decay.of. K-40 will be aosorbed in the.

L' body;.but more than_ half of the t's will escape.

Because 4f this, each person carries a small-' radiation field'around with him.

This, no doubt, is the basis-for the jocular comment that there is some hazard (from radiation) in sharing a double bed. The hazard, of course, is not very great, being on the order of-only a-tenth of a mrem /yr in bed.

However, since a nearby body would screen about 100 of the solid angle from the normal external terrestrial radiation of

~30 mrem /yr, it might better be'said that sharing a double bed has a favorable etfect.

~

After K-40 the most prominent nonseries primordial radionuclide is Rb-87.

This U

nuclide emits only S particles (maximum energy 0.27'HeV) so it is significant

'(if at all) on.ly as a source of internal dose.

Considering the factors of ele-mental' abundance in the earths crust, isotopic fractions, nalf-lives,'and energy per.. disintegration, the dose from Rb-87 would be about fifteen times smaller than that from K provided the concentration in.the body relative to that in the earth's crust should be the same.

From measurements of rubidium in the body it has been concluded (UNSCEAR-77) that the dose rate from Pb-87 is about 0.4 mrem /yr.

This is about forty times smaller than that from K-40, in addition to K-40 and Rb-87, there are about twenty other nonseries primordial radionuclides in the material of the earth's crust. Considering their element:31 abundances, isotopic frections, etc., their rates of energy release per gram of

-terrestrial material range from a few percer.t down to many orders of magnitude smaller than that from Rb-87.

The contribution of these to internal dose may-consequently be ignored.

p In summa.7, the dose rate from radioactive constituents of the body-(K-40, C-14, H-3, etc.), is from 18 to 20 mrem /yr.

Finally', in this discussion of natural backgrounds, it'is not intended to l

discuss the ef fects of the testing of nuclear explosives et ept as these may have affected items in the natural background. -The immediate effect of nuciear testing (from the mid-1950s to'1963) was to release in the atmraphere-'large quantities of' radioactive fission fragments (such as Sr-90 and Ct-137)' which were not;otherwise present in the environment.

These will decay Dr have L

decayed) to. inconspicuous levels providing the present ban on testieg in the l

atmosphere continues. As to tne isotopes already considered in conne: tion with 1

natural background, the effects were as follows:

for Rb-87 *- even though this o

is a direct fission product -- the amount added =s much less than one percent of.the natural abundance of this nucleus in the upper mill 1mster of the earth's crust. Eight of the other primordial radioisotopes are also direct fission r

fraaments.

For these, also, the contribution from testing was a very small fraction of the abundance of these isotopes in the topmost layer of the earth's-j.

crust.

K-40 is not a fission product, so there was no effect on that.

C-14 is not a fission fragment, but it is formed by the capture of neutrons in-the nitrogen of the otmosphere. H-3 (tritium) is also not a fission fragment, but.is a_ residue of.the burning of thermonuclear fuel. The inventory of C-14 in.the biosphere was approximately doubled as a consequence of weapons' testing.

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' 7

e

=

3 gb,;

f 1 half life about 5,700 years) because of the process of equilibration with-the

~

_ contents of the deep ocean reservoir.

This~ process is believed-to proceed with a mean life 'of about 7 years, or so.

The present (1986) level of C-14 in the biosphere is about 20% larger than the " natural" level of C-14.

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 nuclear explosions conducted in the atmosphere prior to 1963. With a half-life of 12.3 years, the amount of injected tritium will by now have been reduced by a factor like 5; o

'but, it still completely masks the effect of " natural" tritium, and will continue to dominate for the next hundred years or so.

Even at that, of course, it is a rather small term in the total exposure to natural radiation.

-(b) Apart from the radioactivity associated with essential constituents of the body, there is some internal dose resulting from the ingestion of " foreign" radionuclides in the environment. The amount of these is not homeostatically i

controlled, but depends on their concentration in materials (air, water, and food) taken into the body.

The items of particular concern here are the parental ~ thorium and uranium and some of their daughters, such as radium.

Their gaseous daughter, radon, will be discussed separately later.

Though the amount of these elements taken up in the body was once, no doubt, rather directly,related to the concentration of these elements in the local environment,'that.is no longer so much the case.

It is still true that some of the underground water in Iowa and Illinois, as well as at other locations in the couatry, has an unusually high radium content; but an increasing fraction L

of such water is now treated before it reaches a consumer.

More significantly, l

with the greatly increased use of canned and packaged foods (which may be 1:

processed anywhen in the country) and the countrywide distribution 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 ingestion of.the series radienuclides it seems appropriate to use the average values estimated l

for the U.S.

Quite apart. f rom the (relatively) straightforward matter of assessing the average uptake of uranium and thorium (and daughters), the matter l'

of correlating this with a whole-body equivalent dose requires composing a number of-radically. dif ferent ef fects: the ingested radionuclides spend some time in the stomach, some time in the bloodstream, and some end up deposited in the gonads and on the bone surfaces. The amount of thorium ingested is probably about the same as that of uranium;_but the retention of thorium in the body is very much smaller.

As a consequence, most (80 or 90%) of the internal dose from the secies radionuclides is-provided by uranium and its daughters.

In the following discussion the estimates compiled in the 1975 report, NCRP-45, L

wi'll be presented; but at the end of this section on internal expbsure some l'

comparison will be made between these estimates and the newer (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 would not greatly affect 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 from uranium.

Thus, with the exception of the dose resulting from inhaled radon (and daughters), the dose equivalent rate i

from internal sources is about 26 mrem /yr - ~20 from K-40, and 7, or so, from 8

l

L uranium, etc.

This is the number assumed in the assessment that the average

' dose to persons in the U.S. is about 100 mrem /yr.

(c) The main additional source of internal radiation is that resulting from the inhalation of radon and its short-lived daughters.

Radon appears at nearly the same rate in both the uranium and thorium decay series, and is the only gaseous element in these series.

In the uranium series, the isotope Rn-222 is an alpha-emitter with a half-life'of 3.8 days. This allows time for an appreciable fraction of the radon formed near the surface to migrate into the atmosphere and to be carried about by the wind.

In contrast, the isotope Rn-220, which appears in the thorium series, has a half-life of only 55 sec, so

,?

that it does not succeed in migrating from the soll to an extent which warrants consideration in :omparison with the 3.8-day Rn-222.

Radon is an inert onatomic gas - one of the " noble" gases, which engage in l

few, if any,-chem, cal reactions. Once released to the atmosphere these atoms i

move freely about and the products of their decay appear as single atoms and attach themselves either to some molecule in the air or to an aerosol particle and thus remain suspended in the air for a considerable time.

Radon decays by a-emission; and if this occurs while the radon atom is still suspended in air

~there is no direct effec' on human exposure. The immediate daughters of Rn-222 (Po-218, Pb-214, Bi-214, Po-214) have short half-lives (f rom 0.16 msec, to 27 min) and their decays are also likely to occur while the atoms are still sus-pended in the air. The first and last of these decays are by a-emission; so 1

that, again, there will be no direct ef fects on exposure to humans -- unless, of course, the original radon atom, or one of these daughters hcd been taken l

into the body by inhalation and the energy of the subsequent decay were i

L deposited there. However, the second and third daughters are S-emitters, and truir disintegrations are accompanied by a large fraction of the gamma-ray

.j energy appearing in the uranium decay series. Thus, even if these' l

disintegrations occur while the daughters are still suspended in the air they i

i vould provide some external exposure to humans -- though not a very important component from radon concentrations normally encountered in outdoor air. The (temporary) end product of this group of decays is the (relatively) long-lived Pb-210 (21 years).

This undergoes two S-decays followed by the emission of an o particle, which terminates the uranium series in the stable isotope Pb-206.

l l

There are essentially no gammas associated with the decay of Pb-210; so this isotope contributes only to internal exposure. That could result either from the inhalation of air in 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 deposited.

The former is by far the more important route for exposure to radiation from Pb-210.

~

Very little radon is emanated from the surface of the ocean, and on this account the concentration in coastal air is low and variable -- depending on whether the air is moving from inland or from the sea.

In the continental air c

mass, the level of radioactivity is about 150 pCi (pico-curies: 1012 Ci) per

~

cubic meter. A large fraction (>2/3) 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 surfaces of the pulmonary. tract -- and particularly to

' the wails of the hair-like passages in the segmental bronchioles.

The dose rate b the tissues of the lung from this cause has (in ERP-45) been estimated

as being about 90 mrem /yr, and to the bronchial epithelium about 450 mrem /yr.

9 e

g, 1

Using the weighting f actor recommended by the ICRP (whcle-body dose equivalent

^

at 0.12 times the dose to the lung tissue), the whole-cody equivalent dose from exposure of the lung tissue would be about 11 mrem /yr.

If one applies the ICRP-recommended weignting f actor of 0.08 to the dose to the bronchial epithe-lium, this would add an additional 36 mrem /yr to the whole-body dose equivalent. Adding to the 80 mrem /yr already identified (28 cosmic, 26 external and 26 internal), we have an average natural background exposure for persons in the U.5. of rather more than 100 mrem /yr, without taking into account the proposed revision of the indoor / outdoor factor from 0.8 to 1.0, which would raise the external component from 26 to 32 mrem /yr.

Up-to this point the exposure to inhaled radionuclides (radon,'etc.), has been

~

~

described only in terms of persons breathing outdoor air..In fact, of course,

. people' spend a major fraction of their timc indoors, and t M radon levels in dwellings may be quite dif ferent (usually higher) than the redon levels out-of-doors.

Radon seeps into dwellings from the soil in which the basement

.is embedded, from the materials of construction -- such as cinder blocks --

and, because the rate of exchange of air in dwellings is intentionaily much smaller than the rate of exchange of air outdoors (in houses weatherproofed for energy conservation, a great deal smaller), the radon concentration in inde."

air may run much higher than in the ambient air outside.

The effects of t.Y s have not been considered here as part of the " natural background," since they are, in fact, technologically enhanced and could (in principle, at least) be controlled. They do, nevertheless, provide an additional source of radiation c

to which the population is exposed.

Some (quite partial) surveys have been conducted.

These ao not yet begin to be adequate to establish an average level for indoor radon exposure for the U.S.

From the surveys which have been made examples have been found in which the indoor radon levels were ten, or more, times larger than the continLntal outdoor average.

Such a level would imply an equivalent whole-body dose 'trger than the average already identified by a i

hundred - or even more -- mrem /yr.

As stated earlier, the components of the dose equivalent rates from natural background radiation as given above are derived from the data provided in NCRP-45.

In its 1982 report the bNSCEAR directed much more attention to radon than it had in previous reports; saying, in particular: " Inhalation is now recognized to be the most important pathway," -- and "on average about one-half the effective dose equivalent from natural sources of radiation is now calculated to be due to the presence of radon in the air inside buildings."

In the January 1987 draft of a %

ing NRCP report, the dose equivalent values for cosmic radiation, terrev 21 gamma radiation, and the internal dose frnm cosmogenic radionuclides and K-4) are changed very little. But there are marked changes in the components when e the exposure is provided primarily by a-radiation:

the uranium contribution to internal radiation, and, most particularly, the dose attributed to inhaled radon. These changes were in part occasioned by the increase from Q=10 to Q=20 for a-radiation; but they were also af fected by new data showing higher concentrations of Pb and Po-210 in bone, by higher estimates for the tissue dose from radon decaying in the body, and particularly by including some allowance for the higher level of radon indoors as compared to outdoors.

More specifically, the contribution of uranium to the internal exposure is now being rated as about 10 to 15 mrem /yr whole-body cMse equivalent (rather than the value of about 7 noted above); and the dose rate proposed for the bronchial epithelium is 2,450 mrem /yr (rather 10

.j

P

~ than the'450 suggested in NCRP-45).

Applying the weighting factor of 0.08 to tFe dose to the bronchial epithelium would add about 200 mrem /yr to the whole-body dose equivalent.

In summary, the draft version of the forthcoming report provides Pi estimate of the total average annual exposure to a member of the population of the U.S. from sou ces of natural background radiation of 300 mrem.

.It should be noted that there is some continuing controversy about the proper weighting factor for the whole-body equivalent of dose to the bronchial epi-thelium.

In addition, as already mentioned, the data to establish a country-wide c.erage of' indoor radon concentration is still far from complete --

=though additional surveys on this point are in progress. For both of these feasons the estimate of the contribution from the bronchial epithelium to the total _ dose must be regarded as still in question,

^

II' Celebrated Hot Scots There are-locations in whien the natural background of terrestrial radiation is much higher than those so far referred to.

A particularly notable one is the Kerala Coast.

(The state of Kerala is on the west coast of India near the sou-thern tip.) In a narrow strip, extending 100 miles, or so, along the beach, numerous patches of monazite sand are exposed.

(The mineral monazite consists of highly insoluble phosphates.of cerium and other rare carth elements in var-lous proportionsr, usually accompanied by some thorium and, on occasion, small amounts of uranium, and their daughts s.')

The most 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 -- the highest known in the world. Sout 70,000 persons live in this section.

There is, of course, considerable varia-tion in the external terrestrial exposure received by the people residing in this region (some of the dwellings -- which are mostly made of coconut straw and wood -- being located directly on patches of monazite, and some not; some residents being employed outside the high background area, while others spend nst of their time near home). However, on the basis of radiometric surveys, th< average exposure to terrestrial radiation for the 70,000 persons in the region ^ has been estimated to be about 380 mrem /yr.

For about 17,000 persons

the exposure has been estimated to exceed 500 mrem /yr.

It exceeded 1,000 mrem /

yr for more-than 4,000 persons; and it exceeded 2,000 mrem /yr for about 500.

People have been living in this part of India for hundreds of years.

It is very densely populated, and it would seem unlikely that there has been any large influx of people from outside for a.long time.

In all probability most of the present residents have generations of ancestors who also lived in this region.

Some preliminary epidemiological studies base been made, and more are planned.

'Still - at least as reported up through about 1980 -- no statistically signi--

'ficant evidence has been found of effects resulting from the unusually high background radiation to which the population of the Kerala Coast has been exposed.

Impressive deposits of monazite sands also occur on some of the beaches of Brazil, about 200 miles northeast of Rio de Janeiro.

In particular, in the town of Guarapari -- which has a resident population of 12,000 persons, and a summer tourist population of 30 to 40 thousand -- it has been estimated that the average annual exposure rate to external terrestrial radiation in the town is_aoout 550 mrem /yr.

Along the beach of this health resort there are patches 11

y of " 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.

There is a small agricultural area in China about 100 miles southwest of Canton in which an appreciable concentration'of monazite has been deposited by alluvial action. About 80,000 Dersons reside in the high-radiation area, and over 90%

of these have had'six or.more generations of forebears who lived in the same area. - There are similar long-established villages at distances of only 10, or i

so, km where the concentrations of U and Th in tu soil are from 3 to 10 times-smal.ler; and these have provided a control group. TMugh the external terres-trial radiation level is four times greater in the high-radiation area than in the control area the total whole-bocfy exposure (including cosmic and internal I

components) is only 2.4 times greater, being about 230 and 95 mrem /yr, respec-tively. Extensive medical surveys have been made of the two population groups to obtalet data concerning such f actors as morbidity and mortality rates from malignancies, spontaneous abortion rates, and the incidence of herecitary and congenital diseases.

In addition, more than 20,000 individuals from eacn group were examined to check for. dif ferences in chromosomal at'errations, leukemia, and measures of growth and development.

In a number of instances the results for the'two groups were essentially identical, and in.no case was a statistically significant difference observed. Although no appreciable effect was found the Chinese Radiation Research Group which conducted the studies concluded that the size of the pop'ulation group was too small to show minor increments of detrimental effects at such low doses.

In addition to the monazite beaches there is a region in Brazil with very high

' terrestrial background radiation in a distinctly dif ferent geological setting.

This is a volcanic area about 200 miles west (inland) from Rio and extending i

north.from the city of Pocos de Caldas to Arax& where there are intrusions

~

containing minerals having close to two percent thorium oxide and over one percent uranium oxide.

Radiation levels up to twice those noted in the streets of Guarapari have been measured near Arax&, and on a small uninhabited hill --

the Morro do Ferro -- near Pocos de Caldas absorbed dose rates in air up-to 24 rads /yr have been reported. No large population groups appear to be exposed j

continually to the very high radiation background in this region.

In France locations providing absorbed dose rates in air of'about 1.75 rads /yr l

are not uncommon, and the discovery of a quite small area providing a rate of L

over 80 rad /yr has been reported. There are 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 Rome, many people spend appreciable time.there, where it is reported that the paving stones provide up to something like 400 mrem /

yr.

The Fichtelgebirge is a granitic mountain near the northeast border of l

Bavaria, There are several towns or villages on the slopes of thia mountain.

On the streets of these villages the terrestrial y-ray exposdre ranges up to more than 500 mrem /yr -- the highest known in the FRG.

In Grand Central Station in New York City -- which was built with granite from l{

the Mil 1%one Quarry in Connecticut -- there are locations where the external terrestrial dose rate is about 525 mrem /yr.

Stone from the same source was i

l used in constructing the foundation for the Statue of Liber s in New York harbor, l

and this also provides a high radiation exposure.

(Wnile it m operating --

from about 1740 to 1960 -- the Millstone Quarry was a favored source of building 1

material since it was immediately adjacent to the shore, and reck could be 1

12 x

k L

g,

4

. 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 (absorced dose rates in air up to 150 mrad /yr, or so) can also be found in other granitic regions of New England, and, indeed, wherever else similar T

~ rock may be found at the surface.

A different' setting'for high terrestrial L

background rad bcion is presented by the phosphate deposits in Florida.

From tMs appreciat'y uraniferous material terrestrial background radiation levels 8

of absorbed dose rates in air up to 150 mrad /yr have been observed.. Deposits of phosphate rock occur throughout the world.

Among the major phosphate-proc'ucing areas the deposits in South Carolina, Wyoming, and some of-those in Br62il have higher concentrations of uranium than those in Florida, while a number of othe,rs are comparable to the ones in Florida.

The remaining type of situation resulting in unusually Mqh exposures. to natural background radiation (excluding the circumstances affecting underground miners) l 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 sandstone or frutured granitic rock may accumulate concentrations of radium very much higher than the concentration in the material through which=the water l

i has been flowing. At-locations where such water may emerge to the surface one has the makings of a " radium 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 radioactivity were ever considered.

Of interest here is the fact that not only do some of the " waters" carry a level of radioactivity which would now be regarded as distinctly unhealthy, but the J

radon decay product of the radium in the water is released to the atmosphere L

and provides an unusually high level af exposure to the population in the neigh-I borhood.

There are reports concerning a few notable radioactive hot springs.

Foi eample, the springs at Tuwa, a village in India about 200 miles north of Bombay, have a high concentration of Ra-226.

In the air close to the main spring at Tuwa, the y-ray dose (from the short-lived radon daughters) has been reported to be about 10, or more, rad /yr. At a distance of about a dozen kilometers (and several villages) dowr. wind, this exposure rate falls to ~750 mrad /yr.

Similarly, in the city of-Ramsar, a resort on the Caspian coast of Iran, population >10,000, there e

is an area-of a few square kilometers around the radium-bearing springs (which emerge in downtown Ramsar) within which levels of absorbed dose in air have been l

measured ranging from 1.75 to over 40 rads /yr.

The springs at Badgastein, Austria (about 50 miles south of Salzburg) have received the most extensive and detailed studies of radioactivity, 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 accounts have been written (including one by Paracelsus, printed in 1562) describing the therapeutic ef fects of the baths at Badgastein.

Badgastein gained in popularity, so that by 1940, 30,000 visitors were reported, and by 1970, about a million baths per year were administered.

By this time, also, about 300 hotels were said to be operat ko % the region to accommodate i

13 e

e y' j:

l visitors, and the permanent population of Badgastein and environs was about 12,000.

In 1904 the presence at Badgastein of " emanation" (as radon was then known) was established by P. Curie and colleagues.

Subsequent studies-have determined that, although the amounts of U, Th, and Ra in the spring water are not excep-tionally high, the Rn-222 content is outstanding.

For most of the visitors, or spa patients taking only a few treatments, the dose received is low (from a few,

.q to a few tens', of mrem).

For patients taking a "whole. cure"'(a dozen 2-hour 1

sessions in the " thermal gallery" in which the Rn-222 concentration is 3,000 pCi/1),

the dose to the lung tissue is about 900 mrem - and several times more to the bronchioles. By inhalation _of Rn-222 the 5 or 6,000 permanent in:.abitants of Badgastein prope - where the springs are located - - receive from 0.7 to 1.5 rems /yr (in lung tissue).

The bath attendants, other personnel connected with the treatment facilities, and, particularly, the doctors attending patients in the " thermal gallery" (a group of only a few hundred persons) receive from 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 for ilpha particles.) Surveys have been made to compare the ger.eral health of residents of Badgastein with that of groups living in similar circumstances -- but not having any enhanced radiation exposure.

These resulted in the conclusion that the longevity of the Badgastein residents was not less, and the incidence of cancer was not greater, than that for the other population groups. As of 1972 studies to identify possible radiation-1 induced anomalies 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 l

i, of cell damage.

For doses larger than somewhere between 0.3 and 1.0 rem /yr, there was an increasing incidence of (for example) broken chromosomes.

Presumably, L

such studies at Badgastein have by now been extended.

l There are many other well-known hot springs, or mineral springs, which have not been discussed at recent symposia on high natural environmental radiation.

l This could be because they have been studied, and found not to have.radiolog-ical features of interest; or because specific studies have tiot yet been made.

1 Among these are the springs at Bath, in southwest England -- a spe well-known and used since Roman days.

At about the same time as Curie made his findings-at Badgastein, J.J. Thomson (who discovered the electron in 1897) reported the existence of copious amounts-of " emanation" 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 York -- though it has been pointed out 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 several hundred times the radium content recommended (since 1962) by the NCRP as " maximum permissible," have been making regular use of this more rrdioactive water for perieds up to 50 or 60 years without any apparent deleterious effects.

Reports concerning the radioactive p.roperties (if any) of the springs in Vichy, France (famous since Roman times) or at Hot Springs, Arkansas, or Warm Springs,

^

Georgia, and many other locations could also be interesting.

14

p; e

IIll.

Some Local Surveys Partial' results from four sets of observations of environmental radiation are cescribed.

The measurements reported are the sum of contributions from terres-trial gammas and cosmic raciation to the exposure in air -- mostly in outdoor air.

The cata considered were drawn from:

(i) reports of EML -- the Environ-mental Measurements Laboratory of the U.S. DOE; (ii) NUREG-0837, the quarterly reports of the NRC TLD Direct Radiation Monitoring Network; (iii) the annual ~

reports of the-Los Alamos Environmental Surveillance Group; and (iv) readings taken in the course of a mini survey made by the author in downtown Washington, DC, in the early summer of 1986.

(i) EML Data Over many years members of the staff of the EML (initially the AEC's Health and Safety Laboratory --'HASL) have studied a very wide range of aspects of environ-mental radiation. Here, only three parti.A ar projects are referred to.

The first of these is a program initiated in Lne fall of 1971 to monitor continuously the-exposure level in outdoor air.

Thermoluminescent dosimeters (TLD's) were set up near four residential locations in the suburbs of New York City, and were mom tored on a monthly basis. The sites were (roughly) in directions west, north, and east, and at distances between about 15 and 30 miles, from Central Park.

These locations are all close to sea level (cosmic radiation exposure about 29 mrad /yr in outdoor air) and in the Coastal Plain region (average exposure to terrestrial radiation previously said to be 23 mrad /yr). On this basis the exposure at these locations would be about 52 mrad /yr.

The 10 year average exposures measured ranged from 53 to 60 mrad /yr -- in acceptable conformance with the nominal regional value.

The annual averages at a given site were observed to fall in the range (maximum-minimum / minimum) of only about 10%, but the measurements for a given month showed differences of as much as 40% from one year to another at a given site.

Such differences were attributed mainly to differences in the annual snow cover and rainfall.

One consequence of such variability is that it may be difficult to obtain a precise measurement,of the size of some increment in exposure' level (such as might result from reactor operation or other non-natural source of radioactiv-ity) -- at least.on the basis of.TLD readings, and particularly if the incre-ment is small compareo to the background.

The TLD registers the sum of the incremental and background exposures integrated over some period of time. To assess the increment it is necessary to subtract the background contribution from the total reading.

Since the background may vary, and cannot be read separately,'the background centribution will have to be assumed on some basis, i.

l' ano this may leue rooth for considerable uncertointy in the actual size of the increment. This could, of course, be greatly improved by the use-of more elaborate detectors, such as a spectrometer which could identify source iso-topes; but such equipment is not attractive for use in field monitoring.

The second EML project to be mentioned here is their continuous monitoring over several years of the natural radiation exposure rates at Shoreham, NY, and the EML station at Chester, NJ.

The Shoreham site is on the North Shore of Long Island, at sea level, and in the Coastal Plain region for terrestrial radiation.

Chester, NJ.is a little more than 90 miles west of Shoreham, at an altitude of l

t-15 m

I i

about 750 f t, and near the eastern f ringe of. the Middle America terrestrial region.

The annual average exposure rates measured by EML were 59 mrad /yr at

'~

Shoreham and about 109 mrad /yr at Chester.

The cosmic ray components will have been 29 and about 31 mrad /yr, respectively, so that the terrestrial components were about 30 and 78 mrad /yr. The terrestrial level at Shoreham is well within the range (15 to 35) previously ascribed to the Coastal Plain region, but the level at Chester is just above the range (35-to 75) ascribed to the Middle j

America region. The-50 mrad /yr dif ference in exposure rates is smaller than many of the variations identified earlier, but it is of interest to find that

'it applied betweer. locations which would not normally be thought of as widely

.i separated nor in different geographical provinces of the country. Actually the 1

50 mrad transition is much sharper than indicated by the Chester-Shoreham comparison since two of the residential sites discussed above, for which the average annual ' exposures measured by EMt were within one mrad of that reported for Shoreham, are less than 25 miles east of Chester.

3.

?

The third EML pr.oject considered is their sponsorship of a series of-Inter-national Intercomparisons of Environmental-Dosimeters.

Eight such exercises were held between 1974 and 1986 with participants from 130, or so, laboratories t-

'n from over 30 countries.

The TLD exposure readings were compared with each other and with control readings on continuously monitoring high pressure ionization

]

chambers. Many factors contributed to differences in the results obtained in the intercomparison.

These included effects from differences in packaging --

where both wall thickness and ambient temperature of the luminescent element i'

affected the readings; differences in calibration methods; in spectral response

-- as for example between terrestrial gammas and comic radiation; problems with signal loss, or " fading," for some phosphor types, and a few others.

The conclusion from the intercomparison series was that mer 85% of the partic-ipants obteined results within !30% of the delivered 9 po W es and that about half the results were within 110%.

Some of tM tast exptures included in this observation were at higher levels than typical environmental levels, and in general the percentage spreads in the readings are somewhat larger at lower exposure levels. This is partly because the corrections which must be applied for exposure during transportation and storage of the TLD's constitute a larger f;

fraction of the total.

For this reason, also, TLD readings of background over short periods -- much less than a month, say -- will not be very accurate.

It follows that exposures reported from different countries or different laboratories may not be fully comparaule. However, it may be expected that surveys made by a single organization using standard procedures and equipment will provide fairly good data on the differences in exposure levels from place to place or from time to time.

(ii) NRC Survey of Nuclear Power Plant Sites Since August 1979 (a few months af ter the accident at TMI-2); the NRC has main-tained a network of TLD's around every licensed nuclear power plant site in the country, both those under construction and those in operation. In each case about 40 detectors are emplaced in a reasonably unif m azimuthal distribution at various distances from the plant -- nominally,16 within 2 miles of the plant, but outside the plant boundary, 16 between 2 and 5 miles, and 8 between 5 and 20 miles from the plant.

The detectors are collected every three months and 16 y

7 a

l-

l n.-

replaced with fresh onts, and ttJ readings from the exposed detectors are reported in the quarterly series NUREG-0837.

The cosmic component is uniform over the extent of the array at any particular site, so that any variation in a single array will be entirely due to differ-

- ences in the terrestrial background - unless ef fluents from an operating plant should lead to higher readings on detectors close to the plant in the downwind L :}

direction.

However, there is little evidence of a general pattern of this sort in the data collected, and for plants still under construction there is no such h

considtration, zonal average is for one of the two outer zones _ rather than for At two of the sites detectors unusually close to the plant also had the zone.

highest readings in the array.

in the following, as has the data from a fThe data from these stations has been ignored readings which were obviously erroneous - ew other stations which provided i:

such as indicating levels smaller than that of the cosmic component alone, or levels which, for one particular quarter, were much higher than that for any other station in the array while, L

for other quarters, the level at the same station was not outstanding, in NUREG-0837.the exposure rates for the absorbed dose in outdoor air are listed l

in terms of mR/ quarter; but these are converted below to mrad /yr.

In the 4th quarter of 1993 arrays were operated at 69 sites, but from trouble in collecting ti]e data needed to normalize tt.a detector readings at 12 of these sites corrected data are available for only 57 sites.

The avera rate for 'the 57 sites during this quarter was about 66 mrad /yr -ge exposurein reasonable i

i agreement with the 70 mead /yr (30 cosmic plus 40 terrestrial) previously identi-fied as the country-wide population weighted average value for the exposure rate in outdoor air.

(Fort St. Vrain) to a low of 42 mrad /yr (Catawba).The site average rates ran reading for a single station was 135 mrad /yr at a location 13 mi from Fort St.The l

Vrain.

Of particular interest in the present discussion is the range of readings among L

the various detector stations within the limited extent of a single array.

L In the 4th quarter of 1933 the average over the 57 sites of the difference between the highest and lowest exposure rates recorded at each site was 34 mrad /yr.

i:

Amongst the sites this difference ranged from a low value of 16 mrad /yr to a high of 59-mrad /yr. This maximum spread was between two stations in the Surry array where one station, 3.7 mi from the plant, recorded a rate of 39 mrad /yr,

[

.while the otrer, 11 mi from the plant and 13 mi from the first, recorded a rate l

of 98 mrad /yr Of course, the rates recorded at theca stations -- as for almost all stations - change from one quarter to the next as do the differences between them; but during the whole of 1983 the difference in exposure at these two sta-tions was 42 mrad.

l This same difference in exposure for the year also occurred between two stations, only 1.5 mi apart, in the array at North Anna.

While the maximum spread within an array was fcund at Surry in the 4th quarter of 1983, for the other quarters of the year (first through third) the maximum spreads were as follows:

McGuire (56), Surry (57),-and North Anna (54).

Along with these maximum spreads, in the NUREG-0837 data for 1983 differences in exposure i

rates of more than 40 mrad /yr between stations in a single array were recorded i

at more than two dozen sites.

In half of these instances the stations involved h

were less than 10 mi apart.

Except for the Far South East (as in Florida, for example, where the terrestrial background on undisturbed land is generally ton small to allow room for variations as large as 40 mrad /yr) these arrays had an 1

17

T

\\

'l 8

essentially country wide distribution:

from the Pacific coast, through the mid-continental region, to the Atlantic.

Finally, considering that in the NUREG-0837 survey there were nominally 8 sta-c tions between the 5 and 20 mile circles around the plant, on the average each station' reported on an area of about 145 square miles. There is no reason to suppose that the extremes in the naturally-occurring exposure rates within the arrays would necessarily be picked up in this survey.

(iii) The Los Alamos Survey s

For many years the Environmental Surveillance Group of the Los Alamos Laboratory has monitored a large number of locations in the technical areas of the Labora-1; tory, and also in the surrounding neighborhood, for the presence of a long list of possible radioactive and chemical contaminants in the air, soil, and water.

As a part of this operation they have maintained an array of TLDs to monitor the cosmic ray and terrestrial radiation background. A number of these TLD sta-g tions are outside the perimeter of the technical area at locations where normal Laboratory operations would not affect the readings of the dosimeters.

Seven of these outside stations are deployed in the townsite; and these are all in generally similar (mesa-top) terrain, and are all at an altitude close to 7250 f t (2,200 m). They are all located within an area somewhat less than 7 square miles, 7

i, and the extreme distance between any two of these stations is only 3.5 miles,

]~

These seven sta'tlons thus constitute a rather compact array.

The measurements reported are believed to be within 4 ercent of actual levels, n-The TL0s register the sum of the absorbed dose in outdoor air from the cosmic and terrestrial backgrounds -- with the exception of the cosmic ray neutrons, to which the particular detectors used are not sensitive. To obtain the tatal 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 of NCRP-45 at 2,200 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, c

Again from NCRP-45, the average exposure rate to cosmic radiation (excluding neutrons) at 2,200 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 a

116 mrad /yr. The average exposure from terrestrial radiation is, then, j.

56 mrad /yr.

I Over any particular time period the cosmic background will, of course, be uni-form across this compact array, though over a six year period the level will 1

change somewhat as a consequence of the 11 year solar activity cycle. At the

-[

geomagnetic latitude of the Continental U.S., this variation has a maximum j

amplitude of less than-10% of the mean level.

Changes in the array average such as that between 1980 and 1981 (from 123 to 100 mrad /yr.),- or that between 1982 and 1983 (f rom 109 to 131 mrad /yr.) will have resulted from changes in the terrestrial background. Presumably such shifts are to be accounted for by p

differences in precipitation, snow cover, and so forth -- and, indeed, there was 30% more precipitation in 1982 than in 1983:

21.7" vs. 16.7."

However, the size of the changes from 1982 to 1983 was by no means the same at each station, ranging from +11 mrad /yr. to +35 mrad /yr.

Another curious example of a station-to-station variation occurred between 1984 and 1985.

The array average exposure j_

was 116 mrad /yr for each of these years; but, while the exposure at one station i.

dropped from 135 to 120, that at another, only 1.2 miles away, increased from i

115 to 1366 mrad /yr.

18 l-

p h

- - = - - - - -

E s

The spread 'between the highest _ and lowest readings in 1980 was only 25 mrad /yr; i

'but for each of the other annual periods this spread ranged between 30 and 40 mrad /yr - even' within the very limited extent of this array.

During the six annual intervals considerea, the lowest exposure was recorded at one or the 1

other of two stations, while three different stations were involved in proyiding the highest reading of the year.

These examples, culled from the results of the Los Alamos survey, point up'the j

-fact that there is much more variability in the natural background radiation --

_(

both over time, and in space ~ than is brought to mind by references to countrywide,

{

or even regional, averages.

(iv) Washington, DC Being at sea' level, Washington nas a cosmic ray dose rate (including neutrons) close to 30 mrem /yr.

Since the neutron component in this cosmic ray flux is quite small the difference between rads and rems is also small, and-may be j

ignored. Washington'is in.the Coastal Plains Region for which the outdoor l

exposure rate to terrestrial radiation is said to be between 15 and 35 mrad /yr.

The natural background dose rate in Washington should, then, be between 45 and 65 mrad /yr.

Still, some avestion on this point _is suggested by Alvin Weinberg's-neasurement in May 1979 of a dose rate of 250 mrem /yr during a hearing in the Dirksen Senate 0,ffice Building, j

j Having this in mind, a hand portable radiation rate-meter was taken on several l

}

short excursions during May, June, and July of 1986.

The resulting observa-tions cannot be considered to constitute a survey, since they were made in the.

course of visits to a somewhat random selection of targets.

The rapid time-response of the rate meter made it attractive to take many'of the readings en passant, so the precision of the readings sas not impressive -- something like

!1~pR/hr.

Still, the measurements were probably sufficiently accurate to per-mitLthe grouping into the rather broad exposure ranges indicated below.

The rate-meter was calibrated in pR/hr; but that has been converted to mrad /yr using-

)

1 pR/hr = 8.76 mR/yr = 7.6 mrad /yr.

The following is a summary of the results of this mini-survey. The numbers given refer to exposure rates in ambient air in mrad /yr.:

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 La 18th'and G; the 5tn floor of the Hart Senate Office Building; at street

- i level inside the new Presidential Plaza at 19th & I.

Rates close to 75 i;

were found aloag First Street, SE. ; on the steps and among the columns in L

f ront of the Supreme Court; the northwest doorway of the Russell Senate L

Office Building; the interior of the Lincoln Memorial; and the street in front of-1717 H Street, as well as in the lobby and the large conference room on the 10th floor.

o 75-90. Examples were.found along a number of streets (18th Street, I Street, Pennsylvania, and 20th); the lobby of the Lombardy Hotel; the i

19 l-p u

i i

lobby of the National Science' Foundation Btilding; and both on the street level and the lower level of the Farragut West-Metro station.

+

90-115.

Rates in this range were found on upper floors of both the Dirksen and the Russell Senate Office Buildings; 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 concrete structural columns; outside the base of the Washington Monu-ment; the-lobby of the Hay Adams; lobby of the New Executive Orrice Build-ing; upper floors of the Lombarcy Hotel; the men's rooms and corridors on-the 10th and lith floors of 1717 H Street (about 15 higher when passing concrete 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.

115-150.

Inside the Washington Monument at ground level; beside the Reflecting Pool; in Lafayette Square (about 30 higher than on the other side of Pennsylvania Avenue); the street in front of the New Executive Office Building; on some sections of sidewalk such as that paved with bricks on Madison Place, and the section paved with ornamental stone slabs at 17th and H--e both being about 30 higher than nearby sections with concrete walks.

+

150-200.

In this range were the entryway at the southeast corner of the Presidential Plaza; the porte-cochere on the east side of-the Capitol; the I

walk by the Viet Nam Memorial; and the steps from the Reflecting Pool up to the Lincoln Memorial.

>200.

On crossing Madison Place from the east side of Lafayette Square (rate ~150) one can go through the porch of the Law Courts Building (rate

~265) into a delightful patio (rate ~240) and on into the lobby (rate ~120).

On starting up the steps to the Library of Congress from First Street, SE.

-(rate ~75) one comes to the first landing (rate ~150), then the second

)

landing (rate ~225), and then the doorway (rate ~380) and on into the lobby (rate ~115).

On approaching the north entrance of the Old Exacutive Office l:

Building one leaves the sidewalk on Pennsylvania Avenue (rate ~115), goes through a gateway in the fence (rate ~165), crosses a flagstone paved patio i

(rate ~190):and up to the top of the steps (rate ~400) and into the lobby i;

(rate ~135).

Apart from these observations there is Weinberg's Senate Hearing. room (rate ~250).

1 (v) Variability o

Differences in natural background exposure rates of 50 to more than 100 mrad /yr have already been identified in earlier sections of this discussion.

Such,'for example,' as that in the cosmic radiation background between locations at sea-level and locations at an altitude of 3 or 4 km., and as,.also, that in the l'

terrestrial ~ background between the Coastal Plain and the Denver, Colorado Regions. Reference to these instances suggests bread, sweeping changes as-between some location and another location a continent or part of a continent away.

20

4 6

However, the variations noted in the local surveys just described make it clear thet the broad contours of the radiat{on intensity surf ace are overlaid by an irregular, fine-textured network of variations of-appreciable size.

It is not necessary for an individual to travel from the East Coast to Denver in order to encounter large changes in his rate of exposure to background radiation. Con-siderable variations will be experienced 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 the NRC survey), by individuals residing in one house or in another house a few blocks away (as from the Los Alamos survey),

or by individuals crossing from one side of the street to the other (as in Washington).

Of course, the " countrywide, population-wairhted, average annual exposure" is a perfectly well-defined concept which 4 en ful for some purposis, even if there should-not be an individual anywhere who actually receives jus' that exposure for one year, let alone two years running.

IV. Observations and Comments We know that extreme exposures to radiation can be fatal, and we know a fair amount about the levels which produce lethal ef fects in a short time. We even know that there is some risk that an exposure about twenty. times smaller than one resulting in a prompt fatality -- a whole-body exposure, that is, of some-thing like 20 or 30 rem of low-LET radiation delivered in a short time -- may, with a rather poorly known probability, initiate processes which result in fatal-ity years later.- However, there is a gap of about two orders of magnitude between the dose. levels for which observational data are available and the levels pro-vided by natural sources. As stated in UNSCEAR-77, "It must be emphasized, however, that such estimates" (referring to their estimate of ~10" f atal malig-nancies/ person rad) "are derived predominantly from rates observed following absorbed doses of over 100 rads," and "In particular, at low doses in the region of those received annually from natural sources, no direct information is avail-able as to the level of induction of malignancies that might apply."

The human species. has, of course, been receiving this natural background radia-tion, including variations of the sort already described, through the whole pcH od of evolutionary time.

Over that period it has evolved from primitive life forms, through the earliest hominids, and on to modern man.

In the course

- of this it will have experienced a large number of mutations, of which some

' fraction will have been induced by natural background radiation.

It seems to be generally felt that the outcome of this process has been favorable.

In its development the species has accommodated to the factors found in its er,/ironment; and for many of these factors there is a range of exposure levels

- which arrame to-D:3 optimi for the well-being of the organism. ' Frequently this range is in the neighborhood of the levels usually encountered.

Exposures (or supplies) at levels within this range may be either neutral, or beneficial, or essential to the organism's well-being; whereas great deficiencies may be detri-mental or fatal, as may great excesses. Such is the case, for example, for the physical factors of heat, light, sound, and moihture.

It is also the case for many chemical substances such as Vitamin A, and even materials containing arse-nic and selenium. These, and many other substances, are essential in trace amounts but are deleterious or lethal at even moderate doses.

21 0

N e

An agent having beneficial effects at low levels which would not be indicated by interpolation from its known deleterious effects at high levels is sometimes-referred to as "hormetic." It is not known whether low-LET radiation is hormatic for the human organism, but it would not be greatly surprising if such should be the case.

A large number of examples of radiation hormesis have been observed in~ a wide range of plant and animal species -- at least as gauged by such factors as growth rate, fertility, and longevity.

Included in these observations is a series in which the rate of proliferation of a colony of bacteria increased as -

the radiation-level was raised to fourteen times the natural background,- but decreased both as the radiation level was raised still further, and as it was reduced by a f actor of six below natural background by 10 cm of lead shielding.

Suggestive as such observations may be, they are, of course, by no means conclu-sive as to an hormetic effect of radiation on the specific and complex system constituting human tissue.

y In contrast with this there is no doubt that a single quantum can damage a cell or induce a mutation.

A quite enormous number of experiments have been conducted on cell colonies and various types of animals showing deleterious effects of exposures of ~10 rads, or more.

Such observations are also suggestive; but, as pointed out in UNSCEAR 77, they do not yet provide any direct information concerning the incidence of carcinogenesis in man resulting from exposures in the general range of natural background levels.

There are many gautionary statements by many authorities calling attention to-the lack of actual knowledge on this last point.

Those which appear in BEIR III include the following:

"The Committee does not know whether dose rates of gamma or X-rays of about 100 mrad /yr are detrimental to man."

"The quantitative estimation of the carcinogenic risk of low-dose, low-LET radiation is subject to nunierous uncertainties. The greatest of these concerns the shape of the dose-response curve."

"For the most part, the available human data fail to suggest any specific dose-response model "

The collective influence of the uncertainties which apply "is such as to deny great credibility to any estimates that can now be made for low-dose, low-LET radiation."

For its illustrative computations of the lifetime risk from whole-body exposure the Committee chose the situations of a single exposure to 10 raos, and a continuous lifetime exposure, to-1 rad /yr, and then said:

"Below these doses, the uncertainties of ~ extrapolation of risk were believed by some members of the Committee to be too greet to justify calculation."

In the face of these and other similar warnings that there is no factual basis for any particular estimate of the risk which might apply as a result of an

' exposure in the range of one to a few hundred mrad, precise values are routinely asserted for such quantities by regulators (and others) on the basis of the 22 Y

e no-threshold linear hypothesis.

equa ll;,

Of course, different precise values are employed is open to some choice. firmly asserted by differatnt estimators since the slo For example, the average risk of inducing a fatal malignancy was taken as being "in the region of 10"*/ rad" in UNSCEAR-77; but the coefficient multiplying 10-4 per man-rem has t,een Jariously taken to be:in the estimate of the number of fatalities 1985), and 3.75 (40 CFR 193, EPA, 1987).1980, and 10 CFR 20, N Though these assorted values display the lack of any absolute technical basis for the assumptions, to a very large extent the no-threshold linear hypothesis has been accorded the status of an axiom.

from this hypothesis.

Two important corollaries follow proportional to the dose (that is, the increment in exposure over natura of the incremental exposure or the level of the background. ground p (The medical com-(NCRP,1987) to add 53 mrem,yr to the average exposure of the

ponent, U. S. ).

The second corollary is that the collective consequences of an incremental exposure of a population are directly proportional to the integral of the incremental exposures. incremental number of man-rem delivered, independently of th For hypothetical incidents, at least, the incremental meteorological factors, individual behavior, and so forth. total of man-r Not everyone subscribes to the linear hypothesis.

There are those who hold that at low exposures the dose-response curve is concave downwards and lies above a straight line from the origin so that the effects at low doses will be larger - possibly much larger -- than indicated by the linear hypothesis.

There are also those -- including the majority of the BEIR III Committee - who consider it probable that the true response curve is concave upwards, lies below a straight line through the origin, and that the effects at low doses will be less - possibly considerably less -- than indicated by the linear hypcthesis.

And then, as mentioned above, there are those who hold that radiation may be hormetic.

constitute any risk at all.For those, exposures in some range of low doses would not nece of each of these dose-response models; but, as already noted, the BEIR II mittee concluded "the available human data f ail to suggest any specific dose-response model."

Whether the fashionable linear hypothesis represents any biological reality or not, it does have two features in its favor.

are made) is that it is wonderfully economical of regulatory thought.One (for which fe (that most commonly urged in its defense) is that it is said to be " prudent "

The other This neo-technical term - like its sister regulatory term " conservative" -- is frequently invoked to provide an unasshilable license to make mistakes However, other considerations must also be taken into accountas th

, as long estimate on one side of an equation is unlikely to be of assistance in striking

, and a " prudent" a (truly) is, reflecting the exercise of good judgment. prudent balance between conflicting c

, that 23

i

~p

, +.

j W

h The ALARA principle may be taken as an example.

Here the intention is to reduce risk, but the quantity which can be directly affected by actions taken is potential radiation exposure.

It becomes progressively harder (and more expen-sive) to reduce the potential exposure the lower the level at the start.

The resources devoted to the exercise of ALARA are themselves of interest to the public; but the ALARA principle is open-ended.. To avoid the indefinite and ulti-mately pointless iteration of cost-benefit analyses along with ef forts to devise means of reducing potential exposures to ever lower levels, there is an obvious need for some sort of floor for the further imposition of ALARA. Of course, along with ALARA and the linear hypothesis we do have the $1,000 per man-rem convention; but this merely intercalibrates the scales for risk estimates and costs, albeit in a somewhat arbitrary fashion.

It enables cost-benefit analysis.

and may serve to support a decision that some particular mechanical measure estimated to reduce exposures by such and such an amount is not " worthwhile."

But it goes no way towards saying when further study may be laid aside. Neither the linear hypothesis -- and particularly not the presumed prudtace of the risk estimates derived from it - nor the $1,000 per man-rem convention provide any logical assistance in specifying a reasonable floor for ALARA.

Similar conditions apply to attempts to establish a de minimis dose. While this and a floor for ALARA have much in common they are by no means identical.

The one refers to a level at which further efforts at reduction would not be mandatory.

The other refers to a dose level at which the consequences (if any) would be deemed trifling and would not warrant consideration - either by regu-lators, individuals, or society.

It has been urged that the establishment of a de minimis level would serve a number of useful purposes, such as providing a cut of f for regulatory ef forts (presumably including estimation of collective doses), providing limits for control programs, and, conceivably, assisting in

-developing a better public understanding of the significance of radiation exposure.

Any level selected will have to meet a number of conditions, among them that it can be adequately measured, but also that it be capable of gaining public acceptance.

The desirability of an official de minimis dose has been discussed for many p

years among many groups.

Most frequently such discussions have started (and often ended) with attempts to decide on an " acceptable risk." Levels for the l

risk of premature death in the range of 10-s to 10~5/yr have been mentioned in this connection and, with the help of the linear hyp3 thesis, a corresponding l

range of dose levels:

froin less than one to over a hundred mrem /yr.

It is, of course, far from clear what an " acceptable" risk level may be, or even if there is one. Much has been made of the fact that even in so-called " safe" industrial settings the (occupational) risk may run as high as 10"/yr; and, since this appears to be acceptable, presumably any risk appreciably smaller -- such as 10'8, say -- ought to be acceptable too? However, this may not cover the situa-tion.

For one thing, the risks in familiar settings have not been so flamboyantly identified, debated, and belabored as they have for radiation, and may to a large extent be accepteo unknowingly.

For another, there is nothing to say that risks similar to or even smaller than those applying to more familiar activities would be deemed acceptable for radiation - partly in view of its being pictured as more mysterious, but at least partly because the official assumptions have tended to endorse an unlimited and unreasoning fear.

Such psychological factors could well interpose great difficulties for any risk-based 24

y b

w,o

~

spproach to settling on a reasonable prescription for a de minimis dose.

r There is also the more basic ~ point tnat to proceed from a pre assigned risk to an associated dose in a logical way one really needs a fairly well-founded dose-response correlation; but, as BEIR-111 has tirelessly told us, we do not have one.

V.

Conclusion

. Confronted with a zone of ignorance a few decades wide we have adopted the simplest possible hypothesis for use as a convenient bridge.

A priori it woula seem rather unlikely that a linear dose-response function would actually provice a very' good description of such a complex biological relationship as that between carcinogenesis and radiation exposure, Perhaps the best that can be said of it is that there is a majority opinion that it provides a " prudent" description.

This cannot be regarded as an intellectually satisfactory oasis for important decisions, particularly when there are some quite relevant facts which are known with certainty.

Amongst these is the wide variation in the exposures people receive naturally.

As a sufficient gesture towards " prudence" one can leave aside the most extreme situations -- such.as the Andes, the Kerala Coast, or.even the hearing rooms in the Dirksen Senate Office Building -- and still find that millions of people dwell in low-LET radiation fields with levels from 50 to more than 100 mrem /yr larger than the averages usually assigned to the natural background of low-LET radiation.

large contingents of the species are detrimental in any way.There is n Such variations are similar in nature to the other inhomogeneities which mark the planet we inhabit:

differences in weather and climate, sunlight, altitude, water, ice, and so forth.

and in which we will continue to live.They are features of the environment in which we For the particular factor of exposure to radiation the natural background, and its. variations, provide the most certain guide and basis we have (and, quite possibly, the most certain guide we ever may have) for consideration of such matters as' appropriate levels for a floor to ALARA and for a de minimis dose.

(These, of course, need not be the same.) 'Though the guidance which diight be drawn from observation of the broad spectrum of natural variations would have a real basis, that would not point clearly at any precise value that should be used for the purposes we are considering.

It is not proposed to try to specify such a value here.

However, from the evidence which has been presented it would appear that any attempt to argue for a smaller level than 50 or 100 mrem /yr would have to construct its support on the mystical basis of the linear hypothesis.

L The main intent of this discussion is to urge that we base our actions and l

decisions to the extent possible on things known. Quantitative statements concerning risk do not fall in that category.

Particularly offensive in this respect are the statements frequently emitted by regulators to the effect that

.so and so many " excess fatalities" will result from such and such an incre-mental exposure, or be saved by this or that proposed new procedure.

Such statements are not only without foundation -- derived, as they are, by the application of some simple scale factor of unknown validity -- but they also tend to give the unwarranted and revolting impression that fatalities are the coin of the realm in nuclear affairs.

It is really of great importance that l-comments concerning risks, fatalities, and so forth should be as factual as l

possible.

They should leave no room for doubt as to what the real situation l

is, namely:

that neither we, nor anyone else, knows the precise relationship 25 l

n t

.3 +

between dose and response at low doses; but that, at least within the range of

'l y~~

exposures discussed above, there is as yet no evidence of any detrimental effects on man.

NOTE CONCERNING RADIATION UNITS 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") of 100 ergs /gm. To provide a correlation between the radiation flux and the dose, it is necessary to specify the material considered. Thus, one speaks of "an absorbed dose in air" of so many rads.

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 some-what more energy per gram in tissue than in air.

This difference, however, is rather small (only A) and is usually ignored; so that, to a reasonable approxi-mation, a radiation exposure providing an absorbed dose in air of one rad would be said to provide one rad of absorbed dose in tissue.

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 the body equivalent to those from one rad of X-rays within a given energy range. One rad delivered.by a particles is more damaging then one rad from X-rays, evet though the amount of ionization per gram produced by the two would be the same.

In the case of the a particle most of its energy is deposited in a very short distance at the end of its path, resulting in t. very high level of ionization within a quite small volume. On this account an a-particle is said tu have a high Linear Energy Transfer (LET), and the radiation it provides is referred to as high-LET radiation.

By contrast, an X-ray deposits its energy.more uniformly along the length of its (longer) path, and X-rays (as well as y-rays and 6 particles) provide what is called low-LET radiation.

To take account of the differences in biological effects, a factor -- variously referred to as the Relative Biological Effectiveness (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 in rems. By definition, Q=1 for X-rays; and is also taken to be unity for y-rays and S particles.

However, for a particles, it is now officially agreed to take Q=20.

A value of Q between 5 and 20 is currently assigned to neutrons.

One further convention is necessary, a particles as well as s-particles, deposit their energy in a quite thin layer of tissue immediately adjacent to their source.

Thus, they do not provide a "whole-body" dose, but only a dose to the organ (ur small portion thereof) in which the source of such radio-L activity 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 dose onto a common scale at a level judged to represent.an equivalent overall effect. The ICRP l

has devoted extensive efforts to developing a system of " weighting factors" whereby the dose to any particular region of the body can be converted to an appropriate value of whole-body dose equivalent (0.E.).

This is called the i

" effective" D.E.

Thus, for example, the effects of a dose of x rem to the lung tissue is taken to be adequately represented by 0.12 x rem of whole-body D.E.

Obviously, the sum of all the weighting factors for the different organs, or l

26 1

[.

p p

..1%

p u

regions of. the body judged to be significant, must equal unity - so that a dose of x rem to each significant center will, when added up, equate to a whole-body 0. E. of xrem.

j As an additional comment, it may be noted that the classical unit for radiation older and even recent reports. exposure -- the Roentgen (R) -- is no longer in use, th of ionization produced; in particular:The Roentgen was defined in terms of the amount one esu of charge in one cubic centimeter of air at standard temperature and pressure. This is equivalent to an absorbed dose in air of 87. ergs /gm (0.87. rad), or of 93 ergs /gm in tissue.

At least in discussions of natural background 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 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.

Finally, just as one could (if one chose) calibrate velocity in terms of furlongs per fortnight, there is the unit of the Working Level (WL) to calibrate a-activity in air, and the Working level Month (WLM) for the integrated exposure to such radioactivity.

The WL is defined as a concentration of short-lived radon daughters which' would result in the release of 1.3 x 105 Hev of a energy per liter of air.

The populatioh of the radon daughters Po-218, Pb-214,81-214, and Po-214 in radioactive equilibrium with 100pCi/1 of Rn-222 would release 1.3 x 106 Mev of a-energy per liter, and would thus provide one WL.

The WLM is defined as the exposure to one 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, departures f rom 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 decay products are attached (or not attached) to the dust particles.within the air, as well as the physiclogical distribution and retention of. the 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 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.

BIBLIOGRAPHICAL NOTE 1.

General references.

The following are noted in the taxt:

NCRP-45, " Natural Background Radiation in the United States."

o National Council on Radiation Protection and Measurements.

Report No. 45, Washington D.C. (1975).

o UNSCEAR.

United Nations Scientific Committee on Effects of Atomic Radiation.

Report on " Sources and Effects of Ionizing Radiation." New York: United Nations. (1972,1977,1982).

27

.,;c y O

)

-.o BEIR'. National Research Council', Committee on the Biological Effects of Ionizing Radiations.

"The Effects on Populations of Exposure to Low Levels of lonizing Radiation." 1972 (BEIR-I), 1980 (BEIR-III),

Washington D.C. = National Academy Press-.

i

'o ICRP-26.

Recommendations of the International Commission os Radiological-Protection.

Publication No. 26. Pergamon. (1977).

Though not noted in the text, an excellent discussion of this whole field o

?

is presented'in:,

Eisenbud, M. " Environmental Radioactivity." 2nd Ed., 1973; 3rd Ed., 1987 Academic Press.

2.

Particular Topics. Details may be found in the following.

P For Section 1:

o Beck, H.L., de Planque, G.

"The Radiation Field in Air Due to Distributed Gamma-Ray Sources in.the Ground." USAEC Report HASL-195 (1968).

o Oakley, D. T.

" Natural Radiation Exposure in the United States." USEPA report ORP/SID 72-1 (1972).

o Grasty, R. L., et al.

" Natural Background Radiation in Canada." Bulletin 360, Geological Survey of Canada (1984).

Report of the Minister of the Interior of the Federal Republic of Germany o

presenting the results of a survey of the background radiation (both outdoors and indoors) in the FRG. (1977).

[" Die Strahlenexposition von Aussen in der Bundesrepublik Deutschland Durch Naturliche Radioaktive Stoffe im Freien und in Wohnungen."]

For Section II:

o Papers in: the Proceedings of the First (and.Second and Third)

International Symposium on the Natural Radiation Environment. Three symposia were held in Houston, Tx.; in Apr. 1963, Aug. 1972, and Apr.

1978. The Proceedings were published under the titles:

"The Natural Radiation Environment," J.A.S. Adams and W.M. Lowder (editors), Univ. Chicago Press (1964).

"The Natural Radiation Environment - II," J.A.S. Adams, W.M. Lowder, and T.F. Gesell (editors), USERDA CONF - 720805 (1972).

"The Natural Radiation Environment - III," T.F. Gesell a'nd W.M. Lowder (editors) US DOE CONF - 780422 (1978).

o Papers in: Natural Radiation Environment, Proc. Second Special Symposium on Natural Radiation Environments, Bombay, Jan. 1981, K.G. Vohra, U.C.

Mishra, K.C. Pillai and S. Sadasivan (editors). John Wiley & Sons.

(1982),

o Muller, R, " Natural Radiation Background vs. Radiation from Nuclear Power Plants." Journal of Environmental Sciences, July / August 1972.

28

9 for Section III:

1 Gulbin, J. and de Planque, G., " Ten Years of Residential TLD Monitoring."

o Rad. Prot. Dosimetry, 6, 299-303. (1984).

yde Planque, G., and Gesell, T,F., " Environmental Measurements with o

Thermoluminescence Dosemeters -- Trends and Issues." Rad. Prot. Dosimetry,

~17, 193-200. (1986).

High Background Radiation Research Group.

" Health Survey in High o-Background Radiation Areas in China." Science, 209, 877-880, 1980, NUREG-0837. NRC TLD Direct Radiation Monitoring Network.

(Quarterly o

Reports) Vol. 2, 1982; Vol. 3, 1983.

Annual Reports of the Environmental Surveillance Group, Los Alamos o

National Laboratory. (1980-1985).

For Section IV:

Special Issue on Radiation Hormesis. Health Physics, 52,5. (1987).

4 o

Adams, J.A.S., in Natural Radiation Environment, Special Second Symposium.

o (1981).

Schiager, K.J. et al. Special Report "De Minimis Environmental Levels:

o Concepts and Consequences." Health Physics, 50, No. 5, 569-579,1986.

r e

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t

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[*%

UNITED STATES U,f

[e NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON NUCLEAR WASTE '

.g WASHINGTON D.C,20885 -

e, 4.....

May 3, 1989 The Honorable Lando W. Zech, Jr.

i Chairman' U.S. Nuclear Regulatory Comission Wash.ington, D.C. 20555

Dear _ Chairman Zech:

j

SUBJECT:

PROPOSED COMMISSION POLICY ON EXEMPTIONS FROM REGULATORY CONTROL l

During its ninth meeting, April 26-28, 1989, the Advisory Comittee on Nuclear Waste (ACNW) met with members of the NRC staff to discuss the proposed Commission Policy on Exemptions from Regulatory Control.

We-also had the benefit of the document referenced. This matter was also a subject for discussion at several of-our previous meetings.

We most '

recently comented to you on this matter on December 30,-1988.

As a result 'of our' review, we believe the latest version of the proposed Pclicy Statement has successfully addressed a number of formerly unre-solved issues. Areas that still need to be strengthened and/or clari-fied are listed below:

L 1.

The Policy Statement should state unequivocally that practices (including sources and devices)- that are candidates for exemption should not, taking into consideration all such practices, result in mrem (about 0.1 mSv) greater than a small fraction [i.e., about an-annual dose rate. per year] of the long-term annual dose limit

[100 mrem (1 mSv) per year] for individual members of the public.

Although this could mean that the dose rate from individual sources might. approach 10 mrem (0.1 mSv) per year, suitable adjustments will need to be made where a given population group might be exposed to multiple sources.

2.

Another important consideration, particularly in terms of releases j

L of radioactive materials -into the environment which represent an.

irretrievable action, is the associated longer-term dose comitment l

l to'the affected population.

In essence, the proposed policy must I

take into consideration both the annual dose and the dose comit-ment.

continue to believe that the permissible annual collective dose i.

limit should be reduced as the allowable dose rate to members of i

M the public from individual practices increases.

We urge that this approach be made a part of the Policy Statement.

S/4...to EDO for Appropriate Action..Cpys to: RF.. 89-0436 l

,[

• 3-The Honorable Lando W Zech,'Jr. May 3, 1989 i

4 Although differences-in'.the dose rates to meshers of the public from natural background sources can be used to provide perspective, we believe that such differences should not be used as-a justi.

fication for setting dose rate limits'.for practices being con-sidered for exemption.

The Policy Statement should be modified to reflect this limitation.

Sincerely, k

Dade W. Moeller Chairman

Reference:

Memorandum dated April 13, 1989 from Bill M.-Morris, Office of Nuclear Regulatory Research (RES), for Raymond F. Fraley, ACRS, transmitting Preliminary RES Draft of Proposed Comission Policy or. Exemptions from Regulatory Control 4

l 1.

u l

.