TITAN code development for application to a PWR steam line break accident : final report 1983-1984

Abstract

Modification of the TITAN computer code which enables it to be applied to a PWR steam line break accident has been accomplished. The code now has the capability of simulating an asymmetric inlet coolant temperature transient by employing different temperature transient forcing functions for different core inlet regions. Up to ten regions of the core can be considered and each region can have at most 50 channels. A total inlet coolant mass flow rate boundary condition option has been added to the code. Flow/coolant temperature transient and control rod transient can be simulated simultaneously by the code as necessary for a steam line break accident simulation. Also, the transient restart capability has been fixed which allows users to change core conditions during a transient calculation for various purposes. All these modifications have been tested by a ten-channel test calculation.Three steam line break accident simulations (YA-1, YA-2, and YA-3) with different pressure forcing functions have been performed. Each simulation included both closed and open-channel calculations. The steady-state results show that a 1-D thermalhydraulic analysis gives accurate results.Case YA-1 employed a pressure forcing function taken from a Yankee Atomic report. No boiling during the whole calculation was observed. Also, no significant difference between closed and open-channel calculations was found.Case YA-2 employed a reduced pressure forcing function with constant pressure after 45 seconds (because of the limitation of W-3 correlation data base). Boiling was observed around 42 seconds after the beginning of the transient. The MCHFR dropped to a value below 6 after boiling. The MCHFR went back to a high value ("30) at 50 seconds for the open-channel calculation while the MCHFR for the closed-channel case still remained below 6. The open-channel model provided a better condition of flow mixing among channels.Case YW-3 had the same pressure forcing function as that of case YA-2 except the pressure kept decreasing after 45 seconds. The MCHFR was about equal for open-and closed-channels. It is concluded that the closed-channel calculations may produce conservative core power values, but the effect on MCHFR is not always conservative