Chernobyl Nuclear Accident Congressional Hearings Transcript

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the fuel elements and light water coolant flowing past the
fuel elements. The pressure tube walls are about 0.16
inches thick and are located within 1/4 inch of the fuel
cladding; whereas the pressure vessel walls on U.S. LWRs are
about 6 1/2 to 8 1/2 inches thick, and are separated from
fuel and fuel cladding by about two feet of water and steel
thermal shielding. This means that during an accident,
overheated and damaged fuel elements on a Soviet RBMK reactor
have a much greater chance of penetrating this second
barrier of defense (the pressure tubes) than on a U.S. LWR
(a single large pressure vessel).
These first two Soviet differences, use of graphite as the
moderator and thin-walled pressure tubes, combine to form a
third distinction, the possibility of creating dangerously
hot-graphite/hot-steam reactions from a breach in the pressure
tube wall. This reaction releases carbon monoxide and
hydrogen gas, which can burn or explode in the presence of
oxygen.
As at TMI, when the core overheated, hydrogen was generated
from the chemical reaction between the hot zircalloy
cladding and the water or steam. However, two important
differences between U.S. and Soviet RBMK reactor designs
impacted the consequences of hydrogen production from zirconium-water
reactions. First, the Soviet design includes
two to seven times more zirconium inventory than U.S. LWRs,
and thus the potential for much more hydrogen production.
Second, the likelihood that oxygen can come in contact with
this chemically produced hydrogen in an accident is much
greater in the Soviet RBMK design. At the time of the TMI
accident, there was never any danger from explosion of the
hydrogen bubble in the pressure vessel. Oxygen gas was not
present in the reactor coolant system, so no explosion was
possible
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