THERMODYNAMIC CONSIDERATIONS FOR SCW NPP CYCLES
Pioro, I., Naidin, M. and Zirn*, U.
Faculty of Energy Systems and Nuclear Science,
University of Ontario Institute of Technology
2000 Simcoe Str. North, Oshawa, Ontario, L1H 7K4 Canada
E-mail: and
*Hitachi Power Systems America, Ltd., 645 Martinsville Road, Basking Ridge, NJ 07920 USA
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Abstract
Currently, there are a number of Generation IV SuperCritical Water-cooled nuclear Reactor (SCWR) concepts under development worldwide. The main objectives for developing and utilizing SCWRs are: 1) Increase gross thermal efficiency of current Nuclear Power Plants (NPPs) from 30 – 35% to approximately 45 – 50%, and 2) Decrease capital and operational costs and, in doing so, decrease electrical-energy costs.
SCW NPPs will have much higher operating parameters compared to current NPPs (i.e., steam pressures of about 25 MPa and steam outlet temperatures up to 625°C). Additionally, SCWRs will have a simplified flow circuit in which steam generators, steam dryers, steam separators, etc. will be eliminated. Furthermore, SCWRs operating at higher temperatures can facilitate an economical co-generation of hydrogen through thermo-chemical cycles (particularly, the copper-chlorine cycle) or direct high-temperature electrolysis.
To decrease significantly the development costs of a SCW NPP, to increase its reliability, and to achieve similar high thermal efficiencies as the advanced fossil steam cycles, it should be determined whether SCW NPPs can be designed with a steam-cycle arrangement that closely matches that of mature SuperCritical (SC) fossil-fired thermal power plants (including their SC-turbine technology). The state-of-the-art SC-steam cycles at fossil-fired power plants are designed with a single-steam reheat and regenerative feedwater heating. Due to that, they reach thermal steam-cycle efficiencies up to 54% (i.e., net plant efficiencies of up to 43% on a Higher Heating Value (HHV) Basis).
This paper consists of three parts. The first part analyzes main parameters and performance in terms of thermal efficiency of a SCW NPP concept based on a direct single-reheat regenerative cycle. The cycle is comprised of: an SCWR, a SC turbine (consisting of one High-Pressure (HP) cylinder, one Intermediate-Pressure (IP) cylinder and two Low-Pressure (LP) cylinders), one deaerator, ten feedwater heaters, and pumps. Since this option includes a “nuclear” steam-reheat stage, the SCWR is based on a pressure-tube design. A thermal-performance simulation reveals that the overall thermal efficiency is approximately 50%. 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.
The second part analyzes 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: theIntermediate-Pressure (IP)turbinesectioniseliminated 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, the third part presents important safety parameters such as bulk-fluid temperature, sheath temperature and fuel-centerline temperature profiles along the heated bundle-string length, which were calculated for a non-uniform cosine Axial Heat Flux Profile (AHFP) along a generic fuel channel of the no-reheat SCWR concept.
1. INTRODUCTION
Currently, there are a number of Generation IV SCWR concepts under development worldwide (Pioro and Duffey, 2007). The main objectives for developing and utilizing SCWRs are: 1) Increase the thermal efficiency of current NPPs from 30 – 35% to approximately 45 – 50%, and 2) Decrease capital and operational costs and, in doing so, decrease electrical-energy costs.
SCW NPPs will have much higher operating parameters compared to current NPPs (i.e., pressures of about 25 MPa and outlet temperatures up to 625°C) (see Figure 1). Additionally, SCWRs will have a simplified flow circuit in which steam generators, steam dryers, steam separators, etc., will be eliminated. Furthermore, SCWRs operating at higher temperatures can facilitate an economical production of hydrogen through thermo-chemical cycles or direct high-temperature electrolysis (Naidin et al., 2008; Naterer et al., 2009).
Fig. 1: Pressure-Temperature Diagram of Water for Typical Operating Conditions of SCWRs, PWRs, CANDU-6 Reactors and BWRs.
Table 1. Major parameters of SCW CANDU® and KP-SKD nuclear reactors concepts (Pioro and Duffey, 2007).
Parameters / SCW CANDU / KP-SKDReactor type / PT
Reactor spectrum / Thermal
Coolant / Light water
Moderator / Heavy water
Electric power, MW / 1220 / 850
Pressure, MPa / 25 / 25
Inlet temperature, °C / 350 / 270
Outlet temperature, °C / 625 / 545
No. of fuel elements in bundle / 43 / 18
Length of bundle string, m / 6 / –
Max. cladding temperature, °C / 850 / 700
The SCWR concepts (Duffey et al., 2008a; Pioro and Duffey, 2007) follow two main types: (a) A large reactor pressure vessel (PV) with a wall thickness of about 0.5m to contain the reactor core (fuelled) heat source, analogous to conventional Light Water Reactors (LWRs); or (b) Distributed pressure tubes (PTs) analogous to conventional Heavy Water Reactors (HWRs).
Within those two main classes, PT reactors are more flexible to flow, flux and density changes than PV reactors. This makes it possible to use the experimentally confirmed, better solutions developed for these reactors. The main ones are fuel re-loading and channel-specific flow-rate adjustments or regulations. A design whose basic element is a channel or tube, which carries a high pressure, has an inherent advantage of greater safety than large vessel structures at supercritical pressures.
AECL (Atomic Energy of Canada Limited) and RDIPE (Research and Development Institute of Power Engineering, Moscow, Russia) are currently developing concepts of PT SCWRs (see Table 1).
To decrease significantly the development costs of a SCW NPP and to increase its reliability, it should be determined whether SCW NPPs can be designed with a steam-cycle arrangement that closely matches that of SC fossil-fired power plants (including their SC-turbine technology) that have been used extensively at existing thermal power plants for the last 50 years.
Previous publications have been mainly devoted to a general development of SCWR concepts (Pioro and Duffey, 2007). However, very few publications were dedicated to development of a general steam-cycle arrangement of a SCW NPP (Duffey et al., 2008b; Naidin et al., 2008; Duffey and Pioro, 2006).
2. General Considerations regarding SCW Npp Cycle
2.1. Review of SC Turbines
SC-“steam” turbines of medium and large capacities (450 – 1200 MWe) (Duffey et al., 2008b; Naidin et al., 2008; Pioro and Duffey, 2007) have been used very successfully at many fossil power plants worldwide for more than fifty years. Their steam-cycle thermal efficiencies have reached nearly 54%, which is equivalent to a net-plant efficiency of approximately 40 – 43% on a Higher-Heating Value (HHV) basis.
Table 2 lists selected current and upcoming SC turbines manufactured by Hitachi for reference purposes.
Table 2: Major Parameters of Selected Current and Upcoming Hitachi SC Plants.
First Year of Operation / Power Rating (MWe) / P (MPa) / Tmain / Treheat (°C) /2011 / 495 / 24.6 / 566/566
2010 / 677 / 25.5 / 566/566
809 / 25.4 / 579/579
790 / 26.4 / 600/620
2009 / 677 / 25.5 / 566/566
600 / 25.5 / 600/620
2008 / 1000 / 24.9 / 600/600
870 / 24.7 / 566/593
870 / 24.7 / 566/593
2007 / 1000 / 24.9 / 600/600
870 / 25.3 / 566/593
It should be noted that the absolute leaders among large-scale power plants in terms of thermal efficiencies are combined-cycle (i.e., tandem arrangement of gas turbine and subcritical-pressure steam turbine) gas-fired power plants with about 60% net plant efficiency on a Lower-Heating Value (LHV) basis or net-plant efficiencies of up to 54% on a HHV basis.
An analysis of SC-turbine data (Duffey et al., 2008b; Naidin et al., 2008; Pioro and Duffey, 2007) showed that:
· The vast majority of the modern and upcoming SC turbines are single-reheat-cycle turbines;
· Major “steam” inlet parameters of these turbines are: main or primary SC “steam” – P = 24 – 25 MPa and T=540–600°C; and the reheat or secondary subcritical-pressure steam – P = 3 – 5 MPa and T = 540 – 620°C.
· Usually, the main “steam” and reheat-steam temperatures are the same or very close (for example, 566/566°C; 579/579; 600/600°C; 566/593; 600/620°C).
· Only very few double-reheat-cycle turbines were manufactured. The market demand for double-reheat turbines disappeared due to economic reasons after the first few units were built.
2.2 Direct, Indirect and Dual Cycle Options
Since the steam parameters of a SCW NPP are much higher than those of current NPPs, several conceptual designs have been investigated to determine the optimum configuration. As such, direct, indirect and dual cycles have been considered.
In a direct cycle, SC “steam” from the nuclear reactor is fed directly to a SC turbine, as illustrated in a previously proposed cycle shown in Figure 2 (Duffey et al., 2008b). This concept eliminates the need for complex and expensive equipment such as steam generators. From a thermodynamic perspective, this allows for high steam pressures and temperatures, and results in the highest cycle efficiency for the given parameters. Current Boiling Water Reactor (BWR) NPPs are based on this concept.
Figure 2: Schematic of Direct Steam Cycle with Moisture Separator Reheater (MSR)
in SCWR Plant (Duffey et al., 2008b).
Figure 3: Schematic of Dual-Cycle with Reheat in SCWR Plant (Duffey et al., 2008b).
The indirect and dual cycles utilize heat exchangers (steam generators) to transfer heat from the reactor coolant to the turbine. They are currently used in Pressurized Water Reactors (PWRs) and CANDU power plants. The indirect cycle has the safety benefit of containing the potential radioactive particles inside the primary coolant. However, the heat-transfer process through heat exchangers reduces the maximum temperature of the secondary-loop coolant, thus lowering the efficiency of the cycle. A schematic of a previously proposed dual cycle is shown in Figure 3 (Duffey et al., 2008b).
Since increasing the thermal efficiency is one of the main objectives in the development of SCW NPPs, the direct cycle is further investigated in this paper.
2.3 Reheating Options for SCW NPP
A preliminary investigation of SCW NPP reheat options (Naidin et al., 2008) revealed the following:
· The no-reheat cycle offers a simplified SCW NPP layout, contributing to lower capital costs. However, the efficiency of this cycle was the lowest of all the considered configurations.
· The single-reheat cycle has the advantage of high thermal efficiency (compared to that of the no-reheat cycle) and reduced development costs due to a wide variety of single-reheat SC turbines manufactured by companies worldwide. The major disadvantage was the increased design complexity associated with the introduction of steam-reheat channels to the reactor core.
· While the double-reheat cycle had the highest thermal efficiency, it was deemed that the complicated nuclear-steam reheat configuration would significantly increase the design and construction costs of such a facility.
In conclusion, the double-reheat configuration is no longer considered of interest, while the most viable options are the no-reheat and single-reheat SCW NPP.
2.4 Regenerative Cycle
Another way of increasing the average temperature during heat addition is to increase the temperature of feedwater entering the SCWR. Since the reactor inlet temperature is approximately 350°C, it is obvious that a regenerative cycle needs to be implemented to increase the feedwater temperature from the condenser outlet (about 40ºC) to the reactor inlet conditions (350ºC).
In practice, regeneration is accomplished through feedwater heaters. Steam extracted from the turbine at various points is used to heat the feedwater to the desired temperature. The regeneration process does not only improve the cycle efficiency, but also improves the quality of the feedwater system by removing air and other non-condensable gases.
2.5 Turbine Options
Since the no-reheat and single-reheat cycles were deemed to be viable, a suitable turbine arrangement must be chosen as well. In a single-reheat configuration, the supercritical “steam” coming from the reactor flows to the HP turbine, where it expands and is exhausted back to the subcritical-pressure steam-reheat channels. Here, the steam temperature is increased to superheated conditions and the steam is allowed to expand through an Intermediate-Pressure (IP) turbine. Furthermore, the steam is carried through a cross-over pipe to the Low-Pressure (LP) turbine and exhausted to a condenser.
However, for a no-reheat cycle, the IP turbine is eliminated and the steam is transferred directly from the HP Turbine to the LP Turbines.
The LP turbines have large exhaust areas, because the steam is expanded to very low pressures for the purpose of extracting as much of the useful energy as reasonably possible. Due to the large volume of steam, the LP turbines have a double-flow configuration. The single-reheat cycle IP turbine is also a double-flow since the expected flow rate of steam is quite high.
From a shaft orientation perspective, the turbine-generator module can be classified as a tandem compound or cross compound. Generally, the cross-compound configuration consists of the HP and IP turbines located on the same shaft and driving one generator, while the LP turbines are on a different shaft, driving a separate generator. The speed of the HP and IP turbine shaft is generally 3600 rpm, while that of the LP turbine shaft is 1800 rpm (in a 60 Hz electrical grid). The slower speed of the LP turbine allows the implementation of longer last-turbine blades with expansion to higher moisture percentages and less exhaust losses, thus increasing the overall cycle efficiency (Black and Veatch, 1995). Due to this, the proposed turbine arrangement for the SCW NPP single-reheat cycle is the cross-compound option. However, it is to be noted that there is a higher cost associated with cross-compound turbine arrangements.