Probabilities of inherent shutdown of unprotected events in innovative liquid metal reactors Page: 3 of 13
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The uncertainty in predicting the effectiveness of inherent shutdown in
preventing severe core damage in innovative reactors designed for passive
shutdown results from three broad contributing areas of uncertainty: (1) the
inability to exactly predict the frequency of initiating events with potential
to challenge the safety systems and require inherent shutdown; (2) the
approximation of representing all such events by a selected set of "generic
scenarios"; and (3) the inability to exactly calculate the core response to
the selected generic scenarios.
This paper outlines the approach and methods used to address these
contributing areas for the loss of flow (LOF) and transient overpower (TOP)
scenarios in support of risk assessments for the innovative design projects
featuring metal fuel cores in pool configurations. In particular, this paper
focuses on uncertainties in core temperatures due to the inability to
precisely characterize the core response to LOF and TOP initiators. From
sensitivity studies and estimates of the underlying uncertainties, probability
distributions for safety margins can be propagated, and the risk can be
calculated. The report also discusses supporting work, cites preliminary
results, and identifies initiatives being taken to reduce the key
contributions to uncertainties in the effectiveness of inherent shutdown,
given that an unprotected scenario has occurred.
Uncertainties in the Frequency of Unprotected Events
The frequency of events requiring inherent shutdown is a function of
(1) the frequencies of those transients or other abnormal events (e.g.,
earthquake, fire) signalling control rod scram; and
(2) the concomitant probabilities of unsuccessfully terminating fission
power production by inserting control rods, or if applicable, by
activating alternate neutron poison features.
The data base for estimating frequencies of the challenging transients or
abnormal events includes published LWR transient data (e.g. ATWS (anticipated
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Mueller, C.J. & Wade, D.C. Probabilities of inherent shutdown of unprotected events in innovative liquid metal reactors, article, January 1, 1988; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1061430/m1/3/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.