Physics conditions of passive autonomous frequency control operation in conventional large-size PWRs

Abstract

The non-base load operation of nuclear power plants is expected to be unavoidable in future due to the increasing shares of the intermittent renewables. In this paper, the physics conditions required for a passively autonomous frequency control operation (PAFO) in a 3400 MWt conventional pressurized water reactor (PWR) design is investigated. The PAFO scheme allows a PWR to passively achieve the requested power maneuverability for primary and secondary frequency regulations on the electrical grid. This study is carried out for a conventional coolant system with a typical critical boron concentration, which yields a less negative coolant temperature coefficient (CTC) than that in a soluble-boron-free (SBF) core. The PAFO scheme is expected to be more challenging in the conventional PWRs than in an SBF small modular reactor, particularly for the secondary frequency regulation. This is mainly due to the weaker coolant reactivity feedback and the higher fuel temperature, which yields a stronger Doppler reactivity feedback during the passive power transients. Numerical simulations of PAFO are performed, for typical power ranges and power ramping rates required for frequency regulations, using a lumped PWR model that is solved by an in-house Fortran-95 computer code. In conventional PWRs, the progressive dilution of the soluble-boron throughout the cycle and the fuel depletion vary the reactivity coefficients. Therefore, we perform sensitivity analyses on CTC, FTC, and Xe worth to assess the feasibility of PAFO throughout the fuel cycle. In addition, sensitivity analysis on the fuel temperature is done to study its role in altering the reactor performance during PAFO.