RELAP5 Analyses of ROSA/LSTF Experiments on AM Measures during PWR Vessel Bottom Small-Break LOCAs with Gas Inflow

Abstract

RELAP5 code posttest analyses were performed on ROSA/LSTF experiments that simulated PWR 0.2% vessel bottom small-break loss-of-coolant accidents with different accident management (AM) measures under assumptions of noncondensable gas inflow and total failure of high-pressure injection system. Depressurization of and auxiliary feedwater (AFW) injection into the secondary-side of both steam generators (SGs) as the AM measures were taken 10 min after a safety injection signal. The primary depressurization rate of 55 K/h caused rather slow primary depressurization being obstructed by the gas accumulation in the SG U-tubes after the completion of accumulator coolant injection. Core temperature excursion thus took place by core boil-off before the actuation of low-pressure injection (LPI) system. The fast primary depressurization by fully opening the relief valves in both SGs with continuous AFW injection led to long-term core cooling by the LPI actuation even under the gas accumulation in the SG U-tubes. The code indicated remaining problems in the predictions of break flow rate during two-phase flow discharge period and primary pressure after the gas inflow. Influences of the primary depressurization rate with continuous AFW injection onto the long-term core cooling were clarified by the sensitivity analyses.

Performance Analysis of AP1000 Passive Systems during Direct Vessel Injection (DVI) Line Break

“…Figure 12 shows the development of mixture level due to the loss of coolant inventory in the core region that, along the transient time, is still above the core height; thus, no core uncovery occurs. The core is cooled by the two-phase mixture [23] as proven by the cladding temperature reduction along the time. That result also conforms with the NOTRUMP calculation.…”

ROSA/LSTF experiment on a PWR station blackout transient with accident management measures and RELAP5 analyses

“…Set point pressures for opening and closure of the PZR SV are 17.26 and 17.06 MPa, respectively, and those of the SG SVs are 8.68 and 7.69 MPa referring to the corresponding values in the reference PWR. Initial SG secondary-side collapsed liquid level was set to 10.3 m that corresponds to the medium tube height, similar to the authors' experiments on the PWR small-break loss-of-coolant accidents (Takeda, et al, 2013;Takeda, 2014).…”

ROSA/LSTF Tests and Posttest Analyses by RELAP5 Code for Accident Management Measures during PWR Station Blackout Transient with Loss of Primary Coolant and Gas Inflow

“…This prevents noncondensable gas (nitrogen gas) used for pressurization of ACC tanks from entering the primary system. If the isolation of ACC system is failed after the coolant injection initiation, ingress of nitrogen gas to the primary system takes place following the completion of ACC coolant injection, suggesting that degradation in the condensation heat transfer in the SG U-tubes should affect the core cooling [13,14]. The nitrogen gas enters cold legs through ECCS nozzles first and then migrates to hot legs through the vessel downcomer and hot leg leak simulating lines that connect the downcomer to the hot legs, as shown in Figure 1.…”