Multidisciplinary design approach and safety analysis of ADSR cooled by buoyancy driven flows

Transmutation is useful to reduce the storing time and the amount of nuclear waste to be stored in the geological repository. Transmutation can be achieved in all types of reactors: thermal systems, fast systems, critical and subcritical systems. Fast spectrum systems have significant advantages because they offer higher transmutation efficiency in comparison to thermal systems. However, the addition of actinides to the fuel has adverse effects on safety parameters. For this reason, a subcritical reactor driven by an external neutron source has been proposed by CERN. The system should be able to operate with a steady power level without a self-sustained chain reaction and the shutdown of the external neutron source would offer rapid control against any power excursion in the core. This novel reactor concept has been introduced as the Accelerator Driven Subcritical Reactor (ADSR). The ADSR operating principle consists of feeding a subcritical core with an external neutron source. The neutron source is obtained from powerful collisions of a high-energy charged particles with a high Z target. The high-energy particles are protons produced by a particle accelerator, the protons travel through a beam tube and collide with a target. The spallated neutrons will start a chain reaction in the nuclear core and the rate will be dependent on the proton flux from he accelerator. The spallation target would be located in the middle of the reactor core. In this way, effective control can be executed through the accelerator's proton current. The use of ADSR in the fuel cycle offers interesting degrees of freedom in the core design, which can maximize the effectiveness of transmutation. Another advantage is that almost any fuel composition including those with low safety characteristics can be tolerated. ADSR is a promising technology that may benefit the society from different viewpoints. However, there are many technical issues to be solved before an ADSR system is realized. The development of the ADSR depends on the successful integration of different systems, for which a safety strategy has to be adapted. This dissertation contributes to the development of the ADSR technology by studying two important safety aspects: the time-dependent response during transient conditions and its influence over the structural integrity. For this purpose, thermalhydraulics, neutronics and structural models of the reactor have been integrated. The results have shown that the inherent safety attached to the subcriticality in an ADSR presents large benefits to avoid criticality accidents. Nevertheless, the development challenges for this technology remain in the core power control and the accelerator reliability. Regarding natural circulation cooling and flow enhancement with gas lift pump, it was found that the maximum core power is limited and the transient behavior does not satisfy the safety requirements for all scenarios