Page 133 - Radiochemistry and nuclear chemistry

Radionuclides in Nature 119

FIG. 5.8. The distribution of the airborne radioactivity from Chernobyl.

is 3 - 24 months depending on latitude (greatest at the equator). The average surface
deposition is 2500 Bq/m 2 at 40 - 50~ decreasing towards north an south (< 800 at >
70~ and at < 20~ The global integrated deposition density for 241pu is ,-440 Bq/m 2
and the air concentration is -- 0.8 mBq/m 3.
Because the atmospheric tests stol~ ~ in 1990, the fission products have decayed for >
10 y, leaving almost only 90 Sr, 1~ I~^
Cs and Pu "active'. Comparing with global natural
radioactivity the contributions of these isotopes to our environment is negligible today,
except for T and 14C.

5.10.2. Nuclear power plant accidents

In 1957 a fire developed in one of the gas cooled graphite moderated rectors at Windscale,
UK. The main radionuclides released were (in TBq) 131I 700, 137Cs 20, 89Sr 3 and 9~
0.3. The largest air concentration close to the plant was 20 kBq/m 3. The Cs activity
deposited was _< 4 kBq/m 2. The ground activity is now back to normal.
In 1979 a partial core melt-down occurred in a reactor at Three Mile Island power station
(TMI), Harrisburg, Pennsylvania. Although the fission products released from the core
were almost completely contained within the building, the Xe and Kr and some iodine (--, 1
TBq 1311) were released. No deposition occurred on ground outside the building.