In accelerator driven sub-critical system (ADS), the high-energy proton beam produced by accelerator is used to strike the target nucleus, and generate spallation neutrons as external neutrons to drive and maintain its operation. The power level and the safety of ADS are susceptible to the instability of proton beam, such as beam overpower (BOP) which is treated as the typical transient accident for ADS system. In ADS core, the sudden increases of power and temperature will be caused by BOP accident, which may exceed the safety limits of materials, threatening ADS safety.

This study aims to investigate the transient safety characteristics of the eXperimental accelerator driven system (XADS) under BOP accident.

The multi-physics coupling code MPC-LBE, in which the fuel pin heat conduction (HC) model and the point reactor kinetics (PK) model were coupled with self-developed computational fluid dynamics (CFD) code, was employed to simulate BOP accident of XADS. Firstly, the MPC-LBE simulation model of XADS was constructed and then the steady state condition was established. On this basis, the double and triple BOP accident cases were simulated, and the safety boundary of BOP accident conditions was also evaluated.

For the simulation results, in the double and triple BOP cases, the reactor powers increase to only 1.88 and 2.7 times of the original ones, respectively. The maximum temperature of the cladding is about 843 K in the triple BOP case, exceeding its safety limit.

Conclusions can be drawn that the negative temperature feedback effect plays an important role in protecting the reactor from power sharp rise, and the double BOP case can be treated as the BOP safety boundary of BOP accident in XADS.