Brief account of the effect of overcooling accidents on the integrity of PWR pressure vessels Page: 3 of 20
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PWR accidents of particular concern are those that allow cool water
to come in contact with the inner surface of the pressure vessel at a
time when the primary-system pressure is substantial. The rapid cooling
of the inner surface results in high thermal stresses and a reduction in
the fracture toughness near the inner surface. This introduces the pos-
sibility of propagation of preexistent inner-surface flaws (sharp, crack-
like defects), and this possibility increases with reactor operating time
because of an additional reduction in fracture toughness that is the
result of neutron exposure. Because of this dependence on neutron expo-
sure, the portion of the vessel directly across from the reactor core,
where the neutron flux in the vessel wall is a maximum, is of greatest
Thermal shock by itself presumably cannot drive a flaw all the way
through the wall; however, if the primary-system pressure were substan-
tial, a potential for vessel failure could exist; that is, a preexistent
flaw, under proper circumstances, could penetrate the vessel wall and
provide a large enough opening to prevent flooding of the reactor core.
A complete evaluation of the OCA problem in terms of its threat to
pressure vessel integrity requires consideration of a number of factors,
including postulated accident initiating events, reactor system and
operator response to these events, specific design features of the
reactor vessel and core that affect fluence-rate and coolant-temperature
distributions adjacent to the inner surface of the vessel wall, sensi-
tivity of the vessel material to radiation damage, size and orientation
of preexistent flaws, and remedial measures. This paper examines pri-
marily the fracture-mechanics-related conditions that could lead to a
potential for vessel failure.
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Cheverton, R.D. Brief account of the effect of overcooling accidents on the integrity of PWR pressure vessels, article, January 1, 1982; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1074282/m1/3/: accessed March 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.