Chernobyl Nuclear Accident Congressional Hearings Transcript

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In addition. It also appears that the RBMK reactors are
susceptible to reactivity accidents (a rapid increase in power
with loss of water) which could lead to rapid steam formation and
the overpressurizatlon of the system.
The 1 oss-of-cool ant accidents of concern for the MHTGR
involve the depressuri zat i on of the core, along with the loss of
all active means of circulating the coolant or of removing heat
from the core. Since in the HTGR the fuel particles are physically
and thermally coupled with the massive core and reflector
graphite, the fuel temperature increases •^ery slowly. Because of
the slow heatup of the reactor and its low power density, sufficient
heat can be removed from the core by passive radiation
and conduction to the surroundings that the modular HTGR core
temperatures never reach temperatures at which the ceramic cladding
will fail or release fission products within the system.
A large sustained air-graphite reaction is also precluded
since the modular HTGR is designed to be contained within
a below ground silo. This type of installation limits the
quantity of air that would be available to sustain an air- graphite
reaction, even should a double rupture of the vessel occur
and the graphite becomes exposed. Further, studies of the combustion
of HTGR fuels have shown that the fuel particles retain
their capability to retain fission products even if the graphite
and outer pyrocarbon layer has been burned away.
Chemical reactions which could lead to an explosion are
essentially precluded in the HTGR since the system remains blanketed
with the inert helium coolant and the core components do
not chemically react with each other. The introduction of water
into the system could lead to the formation of hydrogen and
carbon monoxide. The rate at which this reaction takes place is
insignificant below about 2000°F (well above the temperatures
normally present in the MHTGR core). Further, the reaction is
endothermic (absorbs heat) and is sel f-1 imi 1 1 nq .