AN AERIAL RADIOLOGICAL SURVEY OF

EG&G Letter Report NRC-8103 April 1981

MICHIGAN CHEMICAL CORPORATION ST. LOUIS, MICHIGAN

DATE OF SURVEY: 19-25 MAY 1980

Kent Hilton Project Scientist

This work was performed by EG&G for the United States Nuclear Regulatory Commission through an EAO transfer of funds to Contract Number DE-AC08-76NV01183 with the United States Department of Energy.

The Aerial Measurements System, operated by EG&G for the United States Department of Energy, was used to conduct an aerial radiologic survey of a 75 km2 area over St. Louis, Michigan and a 10 km2 area on the west shore of Lake Hurons' Saginaw Bay directly west and north of the Bay City State Park, Michigan. The purpose of this survey was to determine typical background radiation levels within the local area and to locate any areas which might contain above-background radioactivity (within the detection sensitivities of the airborne system).

The survey employed an array of 20 sodium iodide gamma radiation detectors mounted externally on a Boeing BO-105 helicopter. Flight lines, spaced 76 m apart, were flown at an altitude of 46 m and an airspeed of approximately 30 m per second.

Michigan Chemical Corporation's St. Louis, Michigan facility had, until 1971, an NRC license for handling radioactive materials and was, for a period of time, engaged in processing certain rare earth elements. At the time of the survey the plant was closed for cleanup.

Some Michigan state agencies were concerned about 12 or 13 specific locations where material formerly used by the Michigan Chemical Corporation may have been deposited. The areas surveyed included all these locations. However, only the plant site itself showed a large departure from the radiation intensity of natural background emitters found in this area. The data over the plant site indicated an excess of thalium-208, a daughter in the thorium-232 chain .

All the data were processed for gross count (total radiation intensity) and for excess 2oBTI. This latter process counted the photons in the 2.61 MeV photopeak window of 20BTI which were in excess of those expected from the natural baqkground. The point-to-point natural background was defined such that a constant ratio of thalium-208 to potassium-40 was maintained equal to the most probable value of this ratio from the whole area.

Figures 1 and 2 present the exposure rate isoradiation contours superimposed on a USGS map of the St. Louis and Bay City areas. Letter labels were used to identify discrete count rate intervals. Terrestrial exposure rate values (in microroentgen per hour at the 1 meter level) were inferred from the gross count rate data using the approximate conversion factor of 1100 counts/second equals 1 /lR/h. An additional 4 /lR/h due to cosmic ray contributions were added to the terrestrial values to obtain the total exposure rate.

Terrestrial exposure rate values inferred from aerial data generally agree quite well with ground measurements for large uniform areas, or areas with small variations in radiation intensity typical of normal background. For local anomalies, the aerial data will indicate lower values than those actually measured on the ground. This is due to the large-area averaging property of the airborne system. The average background exposure rate for the area was 6-10 tJR/h, including the cosmic ray contribution.

Figures 3 and 4 present the locations where excess 20BTI was observed. Excess 20BTI generally indicates the presence of thorium ore, either a natural outcropping or accumulations of thorium-bearing minerals associated with certain milling or mineral extraction processes. Excess thalium-208 as defined in the algorithm can also mean simply that the amount of the natural potassium-40 isotope is relatively low at a particular location .

Figure 3 shows seven locations marked with arrows which contain excess 208TI, in addition to the main plant area in St. Louis.

Figure 4 shows two locations with excess 208TI in the Bay City area.

Only ground measurements made at these locations can determine whether the excess 208TI detected is due to variations in the natural background of the area or to ore residues transported to the site.

,--------,

'I,

, , , , , . /8

L. --~ I

• • I I I I

. ~

C k

I'

c ______ --___________ .. __ ~ _____ .. ___ --~----I I I -

OROSI COUNT 11OP\.ETH lin GAMMA

IXPOIURI RATI' LITTER AT 1 m LIVII. LABEL pRJh

A ~

B 6-8 I! C 6-10 II 0 10-13 I ~ E >13

* Averoged over detector·lloI"",,l-Ylew ., 48 m altitude and oxtrapolated to '''a 1 mat ... level.

tnctudes4pRJhco.mlcrayexpoour. 'Ite.

ri~

,... 1. EXPOSURE RA TE ISOPLETHS SUPfRIMPOSED ON A USGS lAAP OF THE ST. LOUIS, MICHIGAN AREA

GROSS COUNT ISOPL!Tl1 KEY

GAMMA EXPOSURE IIATE'

AT 1 m LEVEL

1~13

.. Averaged oyer dlJtKlor-hlld-or· View II 46 m altitude and a"trlpotllad to the' mlt.r level

Includes "J.lRIh cosmiC ray l.pOaure rate

Rgur.2. EXPOSURE RA TE ISOPLETHS SUPERIMPOSED ON AN AERIAL PHOTOGRAPH OF THE AREA NEAR BAY CITY STATE PARK. MICHIGAN

11

n T ..

• , o

\

----------------_.------------_ .. _-----------------I I I I I

i-·--··-·~~~-~----·~~··-··-··-~

..... So USGS MAP OF ST. LOUIS. MICHIGAN AREA SHOWING LOCA TlONS WHERE EXCESS -n WAS OBSERVED

Figure.. AN AERIAL PHOTOGRAPH OF THE AREA NEAR BAY CITY STA TE PAR . MICHIGAN SHOWING THE tOCA nONS WHERE EXCESS 20tTi WAS OBSERVED

DISTRIBUTION

NRC

B. H. Weiss (15)

DOE/OES

L. J . Deal (1)

DOE/NV

J . K. Magruder (1)

EG&G

z. G. Burson, LVAO (1) J . F. Doyle, LVAO (1) L. K. Hilton, LVAO (1) H. A. Lamonds, SBO (1) R. E. Lounsbury, WAMD (1) L. G. Sasso, LVAO (1) T. p. Stuart, LVAO (1) W. J. Tipton , LVAO (1)

LIBRARIES

AMO (8) Las Vegas .(1) Santa Barbara (1)