One important safety aspect of any reactor is the ability to shutdown the reactor. A shutdown in an ADS can be done by stopping the accelerator or by lowering the multiplication factor of the reactor and thus by inserting negative reactivity. In current designs of liquid-metal-cooled GEN IV and ADS reactors reactivity insertion is based on absorber rods. Although these rod-based systems are duplicated to provide redundancy, they all have a common failure mode as a consequence of their identical operating mechanism, possible causes being a largely deformed core or blockage of the rod guidance channel. In this paper an overview of existing solutions for a complementary shut down system is given and a new concept is proposed. A tube is divided into two sections by means of aluminum seal. In the upper region, above the active core, spherical neutron-absorbing boron carbide particles are placed. In case of overpower and loss of coolant transients, the seal will melt. The absorber balls are then no longer supported and fall down into the active core region inserting a large negative reactivity. This system, which is not rod based, is under investigation, and its feasibility is verified both by experiments and simulations. 1. Introduction One of the most important safety features of all reactor types is the ability of shutting down under all circumstances. This is in particular true for GEN IV reactors because they are designed to be safer than currently existing reactors thus also the ability to shut down the chain reaction must also be more reliable. In current PWR reactors shutting down the reactor can be accomplished in two completely independent and diverse methods. The first method is the insertion of safety and control bars by means of gravitation or other passive methods. The second method is the dissolution of the neutron absorbing boric acid into the primary water. For the liquid-metal- and gas-cooled GEN IV reactors (LFR, SFR, and GFR), the second method cannot be used because there are no liquid absorbers that can be dissolved in sufficient quantity in the liquid metal or gas, and even if such an absorbent would exist, cleaning the liquid metal after a SCRAM would be very expensive. In a liquid-metal-cooled ADS, there are in principle two different ways to shut down the reactor. First the accelerator can be turned off, leading to a safe shutdown of the subcritical core. Second the reactor power can be lowered by decreasing the multiplication factor of the core by inserting negative reactivity. This is the same problem as inserting reactivity in a
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