## 1 Introduction  
Nuclear energy plays a key role in the development of clean energy and reduction of carbon emissions all over the world. The accelerator driven subcritical system (ADS) is a new nuclear energy system which could not only produce clean energy but also incinerate actinide nuclides and long-lived radioactive fission products [1]. The ADS device consists of a subcritical core, a high-energy proton accelerator and a neutron spallation target, in which the fission process is sustained by a spallation neutron source. Compared with critical nuclear energy systems (CNESs), ADSs have a better performance concerning nuclear waste transmutation due to a harder neutron spectrum. In recent years, ADS has been attracting more and more attention because of its superior neutronics and safety characteristics [2]. Conceptual designs of three types of experimental accelerator driven systems (XADSs) have been studied by the European Atomic Energy Community within its fifth framework program [3], which includes a zero-power subcritical facility YALINA, an 80 MW lead bismuth eutectic (LBE) cooled XADS, and a 50 MW multi-purpose hybrid research reactor MYRRHA for high-tech applications. A roadmap for developing Accelerator Transmutation of Waste (ATW) technology was presented by the United States Department of Energy (DOE) [4]; several researchers studied the physics design for using sodium or LBE as coolant of the ATW systems [5, 6]. The Japan Atomic Energy Agency (JAEA) hasinvestigated an 800 MW LBE-cooled subcritical reactor with a 1.5 GeV proton accelerator to transmute minor actinides [7]. Research on a hybrid power extraction reactor (HYPER) was carried out by the Korea Atomic Energy Research Institute (KAERI) to produce energy and transmute nuclear waste [8]. The China lead-based reactor (CLEAR) and the Chinese initiative accelerator driven subcritical system (CiADS) have been proposed by the Chinese Academy of Sciences for the transmutation of nuclear waste and sustainability of nuclear energy development [9; 10]. The conceptual design of a 10 MW LBE-cooled CLEAR has been completed, and a proton accelerator with 650 MHz multicell superconducting radio frequency (SRF) in the CiADS device and an accelerator-driven advanced nuclear energy system (ADANES) have been proposed [11]. The component of the ADS system and its operation principle are significantly different from those of the CNES system. To ensure safe operation and optimize reactor design of the ADS system, it is necessary to investigate its transient characteristics under accident conditions and analyze its advantages and disadvantages compared to the CNES system. Several codes have been developed for safety analysis of ADS by using a neutronics model coupled with thermal and hydraulic feedback effects [12]. Reference [PERSON] et al. [13] analyzed some safety characteristics for a lead-bismuth eutectic (LBE) cooled ADS using the extended SIMMER-III code. In addition, [PERSON] et al. [14] investigated the unprecedented blockage in a single fuel assembly and severe core-melt accidents with the updated SIMMER-III. The neutronics and thermal-hydraulics coupled simulation program was developed by the FDS team for the design and research of lead or LBE cooled ADS reactors [15], and three typical transient accidents were simulated with NTC code, such as beam trip and transient overpower condition [16]. Reference [PERSON] et al. [17] used the RELAP5 program for a safety analysis on loss of flow accidents and external source transients of an 800 MW ADS with the code modifications of the point-kinetics model and the thermal property package. These studies are mainly focused on the transient characteristics of ADS itself, but did not analyze the differences of inherent safety features between ADS and CNES. Reference [PERSON] [18] has built a lumped parameter model of the neutronics and thermal-hydraulics to demostrate the differences of dynamic behavior between ADS and critical fast reactor.