Safety inherent negative reactivity control

The ultimate mechanism to ensure public protection from the consequences of postulated ATWS are the inherent negative reactivity feedback when the reactor system temperature increases, and the heat removal function of PSDRS. The analyses of the selected ATWS are conducted to assure the effectiveness of inherent safety features in the KALIMER design. The events considered are Unprotected control rod withdrawal (UTOP), Unprotected loss of heat sink (ULOHS), Unprotected loss of primary flow (ULOF), and combinations of those events. 

The elimination of soluble boron control together with the adopted parameters of the fuel lattice provide negative reactivity coefficients on the fuel and coolant temperature negative steam and integral power coefficients of reactivity in the entire range of operating parameters, which altogether secures inherent safety features of the reactor core. These inherent safety features ensure power self-control in a steady state reactor operation, power rise self-limitation under positive reactivity insertions, self-control of the reactor power and primary coolant pressure and temperature self-limitation in transients, as well as the limitation of the heat-up rate in reactivity-initiated accidents. 

Inherent safety and ease of control. Any liquid fuel which expands on heating gives an immediate negative temperature coefficient of reactivity. This effect is not delayed by any heat-transfer process. The rate of expansion is limited only by the speed of sound (shockwave) in the liquid. This instantaneous effect tends to make the reactor self-regulating. Adjustment of fuel concentration can be used as an operating control. 

Several inherent and passive safety features are incorporated in compact high temperature reactor. Due to negative temperature coefficient of reactivity, the power of the reactor comes down without necessitating any external control in case of increase in core temperature. The reactor also adopts passive systems like removal of core heat by natural circulation of liquid metal coolant in the main heat transport circuit, passive regulation and shut down systems. The reactor is also able to remove heat passively by way of conduction in the reactor block and by radiation and natural convection from the outer surface of the reactor during loss of heat sink. The paper deals with the details of passive systems incorporated in the AHWR and CHTR and the analysis performed for these systems. 

All reactivity coefficients for U core have negative values, which means this core design holds inherent safety characteristics. In particular, the sodium void effect also shows a negative value, which is a different tendency in a typical SFR because of plutonium-free core. For the diversity of a shutdown system, two types of control assemblies are arranged to secure sufficient shutdown margin. 

The KALIMER design highly emphasizes inherent safety, which maintains the core power reactivity coefficient negative during all modes of plant status and under accidental conditions as well. The reactivity feedback mechanisms consist of Doppler, thermal expansion of the fuel and coolant, thermal bowing of the core, thermal expansion of the core structure and core support structure, and thermal expansion of the control rod driveline. These effects result from either the physics laws, or both the physics laws and core design. 

---