After rupture and meltdown of the Number 4 Reactor at Chernobyl, radiation level within the building were estimated to be 5.6 roentgens per second (R/s), or 20,000 R/hr. In modern SI units, this is equivalent to 200 Gray per hour (Gy/hr). The unprotected workers would have received the threshold dose for gastrointestinal radiation sickness of 3 Gy in one minute, and the LD50/30 acute lethal dose of 5 Gy in less than two minutes. (The LD50/30 is the lethal dose for 50% of the exposed population in 30 days). Dosimeters capable of reading such high levels were destroyed or unavailable. Conventional biological dosimeters such as "Geiger counters" "peg out" at >1 mR/s, the detector "locks" and ceases to report discrete decay events ("clicks"). As a result, actual radiation levels inside the reactor building were under-reported by a factor of > 5,000 x.
The low dosimeter
readings led reactor crew chief Aleksandr Akimov to
assume that the reactor was intact. Akimov stayed with his
crew in the reactor building until morning, sending members of
his crew to try to pump water into the reactor. None of them
wore any protective gear. Akimov and 27 others died from
radiation exposure within three weeks.
In the immediate area in
what is now an Exclusion Zone, approximately 6,000 excess
cases (above background) of juvenile thyroid cancer, including
15 fatalities, are attributed to radio-iodine 131I , one of the major products of
a reactor accident*. As many as 100 excess cancer
deaths in the immediate area are attributed to fallout of
neutron-activated reactor materials. Chernobyl remains the
only industrial nuclear accident in which people died directly
from acute radiation sickness.
*Prophylactic treatment
for exposure to radio-iodine consists of mega-doses of "cold"
iodine 127I in tablet or liquid form. This
saturates the thyroid, so that it cannot absorb additional
radio-iodine. The biological half-life on "cold"
iodine in the human body is about 80 days, versus the physical
half-life of 8 days for radio-iodine 131I.
This means that of the course of 80 days, the radio-iodine
from a reactor accident decays to 2-10
= 1/1024 of the original amount, while only 1/2 of the
cold iodine is excreted. One of many lessons from Chernobyl is
that the Soviet Union delayed announcing the accident to its
civil population, such that no prophylactic steps were taken.
Elsewhere in western Europe, early detection of the disaster
triggered pre-planned distribution of cold iodine within
hours.