Study On Radiation Characteristic Above Reactor Cover Of Accelerator Driven Subcritical System

Accelerator Driven subcritical System (ADS), a combination of accelerator, subcritical reactor core and spallation neutron target, is considered as one of the future advanced nuclear systems by IAEA and has attracted many countries’ attention due to its high waste transmutation ability and inherent safety. Radiation shielding, which is one of important barriers to ensure nuclear safety, is a key study in ADS design. The coupling of accelerator, spallation target and sub-critical reactor makes the radiation outside the reactor of ADS, especially above the reactor cover, much more serious, due to particles leakage through beam duct and secondary radiation induced by beam losses. Besides, because of a great variety of particle type in ADS, the available methods and tools of shielding calculation are hardly applicable.With Super Monte Carlo Simulation Program for Nuclear and Radiation Process (SuperMC) and some other codes,, the dissertation will focus on the key problems study for radiation characteristic at upper section of accelerator driven subcritical system. The main research and innovation work of this dissertation includes:(1) Improvement of source term for ADS transport simulation:There are two methods applied for ADS radiation shielding calculation currently. Direct simulation method such as MCNPX simulation is very time-consuming and difficult to satisfy the efficiency requirement of actual engineering design. For two-step Monte Carlo simulation, there is a larger approximation for present the distribution of spallation neutrons. According to the cascade characteristic of interaction between high energy particle and target nuclides, the paper firstly proposed a method of identifying and filtering source neutrons based on "child-father" relationship of secondary particles produced in spallation process, eliminating the repeat neutrons and improving the accuracy of spallation neutrons source. The test results indicate that this method could improve efficifency and accuracy of whole transport simulation process.(2) Radiation shielding for upper section of ADS:Based on the reference model of a 10 MW LBE cooled reactor coupled with a 250 MeV/10 mA proton accelerator, the contribution on radiation field above the reactor cover from particles penetrating cover, beam duct leakage and secondary radiation induced by beam losses were analyzed in detail. It was found that secondary radiation induced by beam losses could not be neglected in the ADS shielding design. A preliminary shielding proposal for the containing compartment and beam duct of ADS was put forward and analyzed based on the radiation characteristic mentioned above.(3) For future possible demonstrated ADS driven by energetic charged particle, the impact of energetic charged incident particle on characteristics of radiation field above the reactor cover were investigated. Firstly, turnoff energy was determined by studying the variation of spallation neutrons yields per energy with the energy of incident particles. And then the assessment of impact of particles penetrating cover, beam duct leakage and secondary radiation induced by beam losses on radiation field above the reactor cover was conducted under the condition of given power and sub-criticality of the core. The results indicated that the neutrons flux and total dose above reactor cover slightly increased and the zone where the dose was domained by secondary particles induced by beam loss was extremely expanded.