The passive residual heat removal (PRHR) system is an important innovative design of the advanced pressurized water reactor technology. Under accident conditions, PRHR system can transport decay heat in the form of natural circulation to ensure core cooling. However, the heat exchange function of PRHR system will be lost when the PRHR pipeline breaks. With development of the accident process, the coupling effect between different safety equipments of the passive core cooling system (PXS) will be affected. Besides, thermal hydraulic state of the reactor coolant system (RCS) will also be affected via complex interaction mechanism. As a result, the new thermal hydraulic phenomena occur, and thus ultimately affecting the accident mitigation capacity of the PXS.

This study aims to confirm the safety characteristics of PXS and identify the new thermal hydraulic phenomena of advanced passive nuclear power plant during accident with multiple failures.

A series of integral effect tests of loss of coolant accident (LOCA) were conducted on the advanced core-cooling mechanism experiment (ACME) facility. The influence of failure of PRHR HX flow and heat exchanging function on LOCA accident process were investigated on the basis of the test cases including PRHR pipeline break and cold leg (CL) break. The unique thermal hydraulic phenomena occurred during PRHR LOCA were explored, and their influence laws on the coupling effect among PXS safety equipments, and the influence laws on thermal hydraulic state of RCS were obtained.

The results show that there is a momentary reverse flow and heat transfer process in PRHR HX at the beginning of PRHR LOCA compared with typical CL LOCA. Besides, the natural circulation process between the core and steam generators (SGs) plays a critical role in cooling and depressurization of RCS, and its corresponding time-averaged heat transfer power is increased by about 30%. Besides, the asymmetric arrangement of PXS leads to a significant difference of transient thermal hydraulic state between the RCS branches, namely the PRHR cools the coolant via one RCS loop while two core makeup tanks (CMTs) inject the cold water to the core via the other RCS loop, and the pipeline resistance distribution shows a significant impact on the injection performance of safety equipment with low driven head such as CMTs.

The unique and important thermal hydraulic phenomena in the early stage of the accident, namely reverse flow and heat transfer process in PRHR HX and natural circulation process between core and SGs, are identified. The asymmetric arrangement effect will be more noticeable when the break occurs in PRHR pipeline.