New generation elastic neutron absorber systems for new and existing design nuclear reactors

The proposed advanced Reactivity Control Method (RCM) is designated for control of the power field of nuclear reactor during its normal operation, start-up and normal and emergency shutdown by the uniform change of distribution of solid neutron-absorbing material concentration in the reactor core using the new spiral Elastic Reactivity Control Device (ERCD). The ERCD is the elastic absorber element implemented in form of cylindrical spiral with the variable, naturally closed, coil gap. Increase or reduction of coil gap is made by pulling or releasing the elastic spiral with the actuator drive. At each position of ERCD element the elastic properties of spiral ensure the uniform distribution of absorber material in the reactor core. Once the reactor emergency protection is triggered, the ERCD is 'inserted' in the reactor core from any of its positions by the accumulated power of elastic deformation of spiral. In this case ERCD coil gap L in the reactor core rapidly decreases, which gives fast and uniform increase of solid neutron-absorbing material concentration in the reactor core. The transparency of ERCD for thermal neutrons changes from 'gray' to 'black'. The ERCD emergency insertion time is mainly defined by the speed of elastic deformation spread along the spiral body. Generally, it is the combination of the gravity force and elastic deformation force, that ensures the 'insertion' of ERCD (decrease of coil gap) in the reactor core in case of other equipment failure (e.g., loss of power supply of actuator drive). Benefits of ERCD implementation are as follows: Flattening of power field gives higher uniformity of power distribution in the reactor core, which can be achieved even in the very beginning of the reactor campaign; Better utilization of fuel provided by higher degree of burnout; High performance emergency shutdown - fast introduction of negative reactivity into the reactor core without the axial distortion of power field by rapid and uniform change of absorber concentration in the reactor core; High efficiency of reactivity control is ensured by ERCD in the whole range of variation of absorber coil gap (concentration), compared to the low efficiency of the conventional control rod in the positions near to the edge of reactor core with low neutron flux; High reliability of ERCD and operational availability due to the inherent ability of spiral absorber movement in damaged/sagged CPS tubular guides or channels; Dynamic properties of ERCD are substantially better than of conventional control rods. The areas of application of advanced Reactor Control Method and ERCD are as follows: Nuclear reactors of new design; Modernization of CPS at the existing nuclear reactors. The implementation of ERCD in the existing PWR designs is one of the possible solutions that bring the proven technology and existing reactor core design to the next level of economic efficiency and increased passive safety. Replacement of conventional control rods by ERCD solutions gives more precise reactivity control solution than rods. The axial uniformity of absorber ensured by each ERCD in each moment of time provides the ideal conditions for power field control in the whole reactor core. The required number of ERCD to replace conventional control rods is determined specifically for each design. Key points of ERCD implementation in the Existing PWR Designs are: Basic design of FA remains without changes. Control Rods Drive Mechanism is the same and used for the ERCD placed in the existing control rod guides; ERCD groups are used for power control and compensation of reactivity effects; Power control - is implemented by ERCD groups, ensuring high grade uniformity of power distribution during the reactor start-up and the whole campaign. All transients are performed without distortion of power field; Fuel burnup - is compensated by increase of coil gap in ERCD groups. All other Control Rod groups (if remain in the specific design) are used for compensation of excess reactivity and used only in 2 positions: (1) fully withdrawn or (2) fully inserted (replace the boron control used for this purpose in the original design). During the campaign the CR groups are being withdrawn in stages, as necessary, to keep the ERCD reactivity controls in the required range. All ERCD and Control Rod groups are used for implementation of Emergency Shutdown. Liquid Poison (Boron) system is no longer required for reactivity control (can be left for safety reasons as a diverse shutdown system). Results of ERCD implementation: Improved uniformity of power distribution in the reactor core provides conditions for better fuel utilization; New safety margins are achievable due to the absence of power field distortion during transients; Xenon oscillations are avoided by the method of absorber concentration control; Boron-free control concept improves plant economy; Minimum scope of modifications to the in existing PWR design. In channel type reactors the reactivity control is performed by Control Rods operating in separate CPS channels that penetrate through the reactor core. Such channels can be cooled by CPS coolant flow. If the Liquid Poison is not used during the normal reactor operation, all reactivity control demands are fulfilled by Control Rods. They are operated individually to reach the necessary precision in reactivity control and power distribution in the reactor core. With the use of ERCD and conventional control rod located in the same CPS channel, the dual-purpose combined function channel can be implemented. In the dual-purpose channel, various combinations of ERCDs and control rods can be implemented, e.g., outer ERCD with inner control rod. The CPS modernization with full or partial replacement of existing control rods to the ERCD will yield in two independent (diverse) reactor shutdown systems