SuperCritical-Water Reactor NPP Concept: No-Reheat Cycle Option

Research activities are currently conducted worldwide to develop Generation IV nuclear reactor concepts with the objective of improving thermal efficiency and increasing economic competitiveness of Generation IV Nuclear Power Plants (NPPs) compared to modern thermal power plants. The Super-Critical Water-cooled Reactor (SCWR) concept is one of the six Generation IV options chosen for further investigation and development in several countries including Canada and Russia. Water-cooled reactors operating at subcritical pressures (10 – 16 MPa) have provided a significant amount of electricity production for the past 50 years. However, the thermal efficiency of the current NPPs is not very high (30–35%). As such, more competitive designs, with higher thermal efficiencies, which will be close to that of modern thermal power plants (45 – 50%), need to be developed and implemented. Previous studies have shown that direct cycles, with no-reheat and single-reheat configurations are the best choice for the SCWR concept. However, the single-reheat cycle requires a nuclear steam-reheat, thus increasing the complexity of the reactor core design. Although preliminary results show that the thermal efficiency of the no-reheat cycle is approximately 2% lower than that of the single-reheat cycle, the less complex core configuration may prove to be a major factor when selecting the most suitable design. This paper investigates the main parameters and performance in terms of thermal efficiency of a SCW NPP based on a no-reheat, direct cycle with heat regeneration. When compared to the single-reheat cycle, the no-reheat configuration has a more simplified design: the Intermediate-Pressure (IP) turbine section is eliminated and the exhaust from the High-Pressure (HP) turbine is directly routed to the inlet of the Low-Pressure (LP) turbines. The cycle also consists of a condenser, nine feedwater heaters, a topping de-superheater, associated pumps, and the nuclear source of energy, i.e., the SCWR. In general, the major technical challenge associated with a SC no-reheat turbine is the high moisture content in the LP turbine exhaust. A thermal-performance simulation reveals that the steam quality at the exhaust from the LP turbine is approximately 81%. However, the moisture can be reduced by implementation of contoured channels in the inner casing for draining water and moisture removal stages. The overall thermal efficiency of the cycle was determined to be about 50% (assumptions are made to account for turbine and pump efficiency losses). Furthermore, important safety parameters such as bulk-fluid temperature, sheath temperature and fuel-centerline temperature are calculated for a non-uniform cosine Axial Heat Flux Profile (AHFP) along a generic fuel channel of the no-reheat SCWR concept.