20
4.1
External Dose
The aerial survey (Tipton & Meibaum, 1981) provided DOE with
important information on exposure to fallout in the Northern Mars
Islands. As the survey proceeded south and east from Bikini Atoll],
seat of the Bravo shot, the external exposure rate fell { Table 4.0
the
#1).
It was calculated for 1 meter above ground level.
At Rongelap Atoll
(Figure 4.1 #1), the islands fell into fou
exposure groups (microreoentgens per hour) from north to south: Naen,
Yugui, Lomuilal (28-43 pR/h), Eniaetok, Kabelle, Gogan (10-27 uR/hp;
Busch, Borukka, Gabelle, Tufa (5-9 pR/h); Rongelap and Arbar (4.1-#.5
pR/h).
The external dose (whole-body), was calculated from exposure
by ny
assuming 1 roentgen = 0.7 rem (Kerr, 1980). For Rongelap Island
the
annual dose was .028 rem, well below the EPA guide of .170 rem/year;
other major islands were also below the guide (Table 4.1 #1).
8
There is also a shallow dose to be considered, that due to beta rays
which travel for short distances into those parts of the body that] are
near or in close contact with the soil and that are unshielded.
eir
contribution is considered to be negligible (Note 9).
These estimated external gamma-ray dose rates are maximal ones.
Indoors the rate is reduced by about 50%.
Likewise, the rate is
reduced
by about 50% in the immediate vicinity of houses owing to the cor
gravel that is spread around them (Shingleton et al, 1987 and RobiBon et
al, 1982b).
Other annual contributions to external dosage which are not ifhcluded
come from cosmic radiation (.028 rem) and medical exposure.
In summary, the contribution of fallout to the total externa
radiation dose at Rongelap Island in 1978 was approximately .028
rpm per
year uncorrected for the shielding within or around buildings, whifh
would decrease it by 25% or more.
The 30-year whole-body dose woufd be
-590 rem allowing for spontaneous decay, but not shielding.
Environmental decay such as leaching of radionuclides from the soil
reduce this estimate still more, but was not allowed for.
would