CANDU Combined Cycles Featuring Gas-Turbine Engines

Abstract

In the present study, a power-plant analysis is conducted to evaluate the thermodynamic merit of various CANDU combined cycles in which continuously operating gas-turbine engines are employed as a source of class IV power restoration. It is proposed to utilize gas turbines in future CANDU power plants, for sites (such as Indonesia) where natural gas or other combustible fuels are abundant. The primary objective is to eliminate the standby diesel-generators (which serve as a backup supply of class III power) since they are nonproductive and expensive. In the proposed concept, the gas turbines would: (1) normally operate on a continuous basis and (2) serve as a reliable backup supply of class IV power (the Gentilly-2 nuclear power plant uses standby gas turbines for this purpose). The backup class IV power enables the plant to operate in poison-prevent mode until normal class IV power is restored. This feature is particularly beneficial to countries with relatively small and less stable grids. Thermodynamically, the advantage of the proposed concept is twofold. Firstly, the operation of the gas- turbine engines would directly increase the net (electrical) power output and the overall thermal efficiency of a CANDU power plant. Secondly, the hot exhaust gases from the gas turbines could be employed to heat water in the CANDU Balance Of Plant (BOP) and therefore improve the thermodynamic performance of the BOP. This may be accomplished via several different combined-cycle configurations, with no impact on the current CANDU Nuclear Steam Supply System (NSSS) full-power operating conditions when each gas turbine is at maximum power. For instance, the hot exhaust gases may be employed for feedwater preheating and steam reheating and/or superheating; heat exchange could be accomplished in a heat recovery steam generator. as in conventional gas-turbine combined- cycle plants. The commercially available GateCycle power plant analysis program was applied to conduct a thermodynamic evaluation of various CANDU gas-turbine combined cycles. For the evaluation, a minimal number and size of gas-turbine engines were considered, specifically, 4x50 MWe (based on CANDU 6). With this set of gas turbines, it is calculated that a relatively high level of reliability of class IV power restoration can be attained. The results from the GateCycle analysis indicate that certain CANDU combined cycles can generate over 940 MWe (net) with an overall thermal efficiency of up to 37% (which is about 4 percentage points higher than that of the current CANDU 6). Hence, the proposed concept may significantly enhance the competitiveness of future CANDU plants. This is especially important in light of: (a) advancements in combined-cycle technology and (b) recent studies on the thermal coupling of gas turbines with future light water reactors.