Heavy water moderator surrounding each fuel channel is one of the
important safety features in CANDU reactors since it provides an
in-situ passive heat sink for the fuel in situations where other
engineered means of heat removal from fuel channels have failed.
In a critical break LOCA scenario, fuel cooling becomes severely
degraded due to rapid flow reduction in the affected flow pass of
the heat transport system. This can result in pressure tubes
experiencing significant heat-up during early stages of the
accident when coolant pressure is still high, thereby causing
uniform thermal creep strain (ballooning) of the pressure tube
(PT) into contact with its calandria tube (CT). The contact of the
hot PT with the CT causes rapid redistribution of stored heat from
the PT to CT and a large heat flux spike from the CT to the
moderator fluid. For conditions where subcooling of the moderator
fluid is low, this heat flux spike can cause dryout of the CT.
This can detrimentally affect channel integrity if the CT
post-dryout temperature becomes sufficiently high to result in
continued thermal creep strain deformation of both the PT and the
CT.
A comprehensive mechanistic model is developed to predict the
critical heat flux (CHF) variations along the downward facing
outer surface of calandria tube. The model is based on the
hydrodynamic model of \cite{Cheung/Haddad1997} which considers a
liquid macrolayer beneath an elongated vapor slug on the heated
surface. Local dryout is postulated to occur whenever the fresh
liquid supply to the macrolayer is not sufficient to compensate
for the liquid depletion within the macrolayer due to boiling on
the heating surface. A boundary layer analysis is performed,
treating the two phase motion as an external buoyancy driven flow,
to determine the liquid supply rate and the local CHF. The model
takes into account different types of flow regime or slip ratio.
It is applicable for a calandria vessel as well, under a sever
accident condition where a thermal creep failure is postulated to
occur if sustained CHF is instigated in the surrounding shield
tank water. Model shows good agreement with the available
experimental CHF data. The model has been modified to take into
account the effect of subcooling and has been validated against
the empirical correction factors. / Dissertation / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/18045 |
| Date | 11 1900 |
| Creators | BEHDADI, AZIN |
| Contributors | LUXAT, JOHN C., Engineering Physics and Nuclear Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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