Brief account of the effect of overcooling accidents on the integrity of PWR pressure vessels Page: 4 of 20
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MECHANISMS AND ACCIDENTS RESULTING IN
There are two basic mechanisms for creating thermal shock to the PWR
pressure vessel.^ One is the injection of cold fluid, and the other is de-
pressurization of the primary or secondary system which reduces the satura-
tion temperature. Cold water can be injected into the primary circuit by
means of the emergency core cooling systems and into the secondary circuit
by means of the feed-water systems, while depressurization can be the
result of stuck-open valves, excessive power demands and pipeline failures.
In either of these cases cool water eventually enters the vessel through a
main coolant line, as indicated in Fig. 1 and passes down through the down-
comer region, coming in contact with the inner surface of the vessel wall
on the way to cooling the core.
Large decreases in pressure do not necessarily accompany a reduction
in coolant temperature, and for those cases where they would, the primary
system could be repressurized with the emergency core cooling system.
Thus, it is possible for thermal-shock effects and high pressure to coexist.
Four classes of postulated PWR accidents that can result in pressurized-
thermal-shock situations are the large-break loss-of-coolant accident
(LBLOCA), the small-break loss of coolant accident (SBLOCA), a main steam-
line break (MSLB) and a runaway feed-water transient (RFT).^ The 1978
Three Mile Island accident is an example of an SBLOCA (stuck-open primary-
system relief valve), and the 1978 Rancho Seco accident is an example of
an RFT. In the case of an LBLOCA the primary system pressure would drop
very quickly to nearly one atmosphere and would remain there; thus, vessel
integrity would be retained to the extent of being able to maintain coolant
in the vessel.
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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. (digital.library.unt.edu/ark:/67531/metadc1074282/m1/4/: accessed September 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.