The Accelerator Coupled System Dynamics

In order to mitigate unprotected transients in Accelerator Driven Systems (ADS), 'highly coupled' ADS, also named Accelerator Coupled Systems (ACS), have been recently proposed at the French Commissariat à l'Energie Atomique (CEA) [1-3]. In ACS, the coupling between the neutron kinetics and the external source is enhanced by feeding the driving accelerator with a fraction of the power produced in the system core. A similarity between the dynamics of these highly coupled subcritical and the dynamics of ideal critical systems has been also indicated by the ACS proponents. However, up to now, ACS features have been investigated in absence of self regulating mechanisms and by considering the spallation neutrons as a 'supplementary' group of delayed neutrons. In the present paper, firstly a fully analytic solution of ACS kinetics in the absence of thermal feedback phenomena is presented and discussed. The solution of the kinetic equations is obtained analytically resorting to a procedure similar to the one used to solve the neutron slowing-down equations in the presence of elastic scattering leading to the classic Paczek's function. Oscillations can come into play for suitable values of the coupling coefficient. Afterwards, ACS dynamics in the presence of thermal feedback phenomena are investigated by using the Tieste-Minosse code elaborated at ENEA Casaccia. Transients induced in ACS by reactivity insertions and by an unprotected loss of the primary coolant flow (ULOF) are considered. Results indicate that particularly rapid temperature transients are not always avoided in ACS. On the contrary, a long-term similarity between the dynamics of an ACS and those of an exotic critical system is verified.