RCS Depressurization and Loss of Flow No Trip 3D Neutronic Transient Analyses

Abstract

To be able to correctly analyze an RCS Depressurization or Loss of Flow accident with no reactor trip, the Westinghouse 3D Transient Neutronics tool RAVE™ was used. Plants which utilize all-digital Instrumentation and Control (I&C) systems may be required to consider a complete failure of the digital protection system. A backup analog system is provided in the event of a failure of the digital system; however the backup analog systems do not carry all safety trip signals. Therefore, it is postulated that the plant might experience an accident event in which no immediate trip signal is generated and protection would rely on operator action. To allow adequate time for the operator to recognize the malfunction and take action to manually trip the plant, it is assumed that operator action will not occur for ten minutes from the initiation of the transient. These kinds of transients may be analyzed with point kinetic transient codes; however, due to the lack of 3D neutronic feedback with a core height dependent moderator temperature coefficient and its effect on the axial and radial power distributions, it may not be possible to show that Departure from Nucleate Boiling (DNB) does not occur. Using 3D Neutronic transient analyses methods (RAVE™), it was shown that neither of these transients (RCS Depressurization or Loss of Flow) would generate DNB violations, even though the point kinetics version is showing the opposite.