By Michał Wierzchowski, and Robert M. Field, KEPCO International Nuclear Graduate School
Nuclear Power Plants (NPPs) have been studied as a source of both electricity and heat for an array of cogeneration (cogen) configurations. These have included heat service for: (i) industrial processes (e.g., pulp and paper), (ii) food processing (e.g., corn milling and downstream operations), (iii) hydrogen production, (iv) heavy water production, (v) district heating, (vi) oil sands recovery, (vii) oil field production enhancement, and (viii) sea water desalination.
To date however, nuclear cogeneration applications have been limited, primarily to district heating in the former Soviet Union and Eastern Europe, and heavy water production in Canada. Based on the current global price for oil and energy, this technology is not economically viable for most countries.
With oil and fossil fuel prices known to be highly volatile, and the Paris Agreement calling for a reduction in fossil fuel use, the economics of heat supplied by nuclear power may return to favor. As a minimum, cogen design may improve NPP economics by increasing operational flexibility and diversifying the revenue stream. To prepare for such a scenario, this study investigates design considerations for a prototypical modern Pressurized Water Reactor (PWR) plant, the Korean designed and built Advanced Power Reactor (APR1400) (e.g., Shin Kori Units 3, 4; Shin Hanul Units 1, 2; Barakah Units 1, 2, 3, 4).
Nuclear cogeneration can impact balance of plant system and component design for the main turbine, and condensate, feedwater, extraction steam, and heater drain systems. The APR1400 turbine cycle is reviewed for a parametric range of pressures and flow rates of the steam exported for cogeneration to identify changes to the established design and to determine operational constraints.
ADVANCED POWER REACTOR 1400 Overview
The APR1400 is an evolutionary Pressurized Water Reactor (PWR) developed in the Republic of Korea2. The design is based on the previous Korean reactor technology, the Optimum Power Reactor (OPR1000) which was successfully deployed in South Korea. The APR1400 is licensed for 3981 MWt (core). The first APR1400 unit, ShinKori Unit 3 is currently undergoing startup testing.
Secondary System Description
The steam cycle of APR1400 is comprised of the main Turbine-Generator (T/G) and associated support systems, and the Main Steam (MS), Extraction Steam (ES), Condensate (CD), Feedwater (FW), and Heater Drain (HD) Systems. The turbine-generator system consists of one (1) High Pressure Turbine (HPT) and three (3) LPTs coupled with the half-speed, 4-pole main generator. The cross-around steam passes through moisture separator reheaters with two stages of reheat.
The APR1400 CD and FW systems consist of seven (7) points of heating. Extraction lines transport heating steam from the LPTs to the LP FWHs and the deaerator, and from the cross around piping and HPT to the HP FWHs. The arrangement of the steam cycle is illustrated in Figure 1.
APR1400 Coupled with a Desalination Plant
In 2009, the Emirates Nuclear Energy Corporation (ENEC) awarded a contract for four (4) APR1400 units to the Korea Electric Power Corporation (KEPCO). Construction on all four units is proceeding. Due to climatic conditions in the UAE, water shortages across the Middle East, and a rapidly growing population, the potential for seawater desalination coupled with the APR1400 plant has been previously investigated.
This analysis presents a number of economic and technical advantages of combined power and water generation using APR1400 technology. However, the study did not address design considerations related to the APR1400 steam cycle. Those issues are addressed here.
The study examines the possibility of coupling the current APR1400 design with a cogeneration application by exporting steam from the turbine cycle. Two locations for steam export were selected: (i) in the main steam line upstream of the Turbine Stope Valve (TSV) (High Pressure Export, or ‘HPE’), and (ii) from the cross around steam between the 1st and 2nd stage reheater bundle (Low Pressure Export, or ‘LPE’) [Figure 1]. The steam export interface was based on thermodynamic parameters in the cycle such as pressure and temperature. It was also assumed that there should be as little design modification as possible to simplify the licensing process and minimize the impact on operations.
Based on the pre-study, the decision was made that economic justification for the cogeneration projects requires a large and significant quantity of export steam. Two cases, one with ~500 and on with ~1,000 MWt of export steam heat were considered. The analysis for these values was performed using Microsoft Excel by respecting mass and energy balance in the cycle.
The base steam cycle is for the APR1400 design for the 60 Hz market as described in the Design Control Document4. However, for large quantities of LPE steam, the cross-around pressure is significantly depressed, and as described below, this results in a large power increase for the HPT and a severe challenge for the MFWP turbines. Therefore, a second case, the ‘modified’ T/G is also examined. For this case, the cross-around pressure for the VWO condition is raised ~15%. A higher cross-around pressure results in an overall production penalty (~7 MWe) but ameliorates several operational challenges for systems and components for both HPE and LPE steam conditions.
Design conditions were examined for the following systems: (i) MS System, (ii) Cross-Around Steam System, (iii) ES System, (iv) CD System, (v) FW System, and (vi) HD System. In addition, components subject to design challenges were reviewed, including: (i) Turbine-Generator, (ii) Feedwater Heaters, (iii) Moisture Separator Reheaters, (iv) Main Condenser, and (v) Main Feedwater Pump (MFWP) Turbines.
Steam cycle heat balances were computed for the configuration per Figure. 1 for both the baseline T/G and for the modified T/G with an increased cross-around pressure. Twelve cases were considered in all as follows:
- B – APR1400 Baseline T/G
- B-VWO – Valves wide open case
- B-100 – 100% reactor power case
- B-H1.6 -1,600,000 lbm/hr HPE steam baseline case
- B-H3.2 – 3,200,000 lbm/hr HPE steam baseline case
- B-L1.6 – 1,600,000 lbm/hr LPE steam baseline case
- B-L3.2 – 3,200,000 lbm/hr LPE steam baseline case
- M – APR1400 Modified T/G
- M-VWO – Valves wide open case
- M-100 – 100% reactor power
- M-H1.6 – 1,600,000 lbm/hr HPE steam modified case
- M-H3.2 – 3,200,000 lbm/hr HPE steam modified case
- M-L1.6 – 1,600,000 lbm/hr LPE modified case
- M-L3.2 – 3,200,000 lbm/hr LPE steam modified case
The thermodynamic parameters were calculated for each case and then used in further analysis of key steam cycle design conditions. As indicated below, large quantities of export steam present challenges for the design of systems and components. In particular, the following parameters experience significant adverse changes which may require re-design: (i) HPT shaft power, (ii) steam velocities in ES lines, and (iii) HD drain control valve capacity.
Table 1 presents the shaft power and electricity generation with and without cogeneration. Generally, LPE operations lead to considerably increased high pressure turbine shaft power and this is applicable to both baseline and modified turbine cases.
Due to changes in volumetric steam flow while operating with steam export, some of the steam lines may not be suitable for cogeneration operations. It was assumed that the current APR1400 design can accommodate a 5% increase in flow velocity within existing design margins. Table 2 presents results for steam line sizing analysis. If the calculated line velocity exceeds 105% of the baseline case, the line velocity is indicated as a percentage of the baseline velocity. For the case of MS and 2nd stage reheating steam (Rht. 2), there is no adverse impact to design associated with cogen operations. However, a significant velocity
Analysis was also performed for HD System and MSR drain control valve sizing. Again, a calculated value of less than 105% of the base case requirement was considered to be within normal design margins, and would not require a change to hardware. The analysis indicates adequate margin across all valve positions in the current design for the following cases: (i) B-H1.6, (ii) M100, (iii) M-H1.6, and (iv) M-L1.6. For other cases and valve positions, the increase in required valve capacity is from 8 to 34%.
Beyond the most significant impacts of cogen operations identified in Tables 2 and 3, a review of system and component design considerations is provided below. In general, design for cogen operations would start with design requirements for steam export and condensate return quantities and thermodynamic parameters and the interface locations with the APR1400 steam cycle. While the analysis supporting this paper did not identify significant design changes beyond those identified above, the review should encompass the scope described below.
MS System – As described previously, MS piping is not adversely impacted by cogen operations. However, additional analyses should be conducted for the safety and non-safety portions of the MS system. Due to bounding thermal, weight, seismic, and fluid transient loading conditions, no changes to the piping, piping supports, or components of the MS system is expected. A design review for cogen should include confirmation of:
- design pressure and temperature ratings,
- throttle margin revision,
- pipe stress and support load analysis,
- steam hammer loading, and
- containment isolation capability.
Cross-around Steam System – The T/G supplier is responsible for this piping as part of the scope of supply for the T/G. Impacts due to changes to steam pressure and mass flow rate can be significant, particularly for the LPE cases. A few design challenges were identified for this system including:
- Cold Reheat, and
- Velocity analysis for Low Export Pressure cases
- 1st Stage reheating steam velocity for all design conditions.
- Cross-Around Relief Valve (CARV) sizing and pressure setpoint.
ES System – Velocities increase in ES lines for every case with steam export. Velocity in these lines can impact Flow Accelerated corrosion (FAC) rates and vibration induced fatigue failures in expansion bellows. Any changes to ES line sizing will also impact sizing of inline valves. Therefore, the review of the ES System for cogen operations should include a review of:
- ES nozzles and line sizing,
- ES expansion bellows (e.g., design to EJMA criteria),
- Limiting steam velocity revision
- Stress loading
- ES Non-Return Valves (NRVs), and
- ES Block Valves
CD and FW Systems – It was determined that there is no adverse impact on the CD and FW systems. Flow rates remain with the existing design basis. CD pumps will see a slight decrease in flow. Analysis of the FWBPs and MFWPs indicates no material changes to the following parameters: (i) pump flow, (ii) NPSH ratio, (iii) pump speed, and (iv) flow vs. Best Efficiency Point (BEP) flow.
HD System – HD System drain control valves, both normal and emergency, are expected to see a significant increase in required capacity for steam export operations. This is associated with an increase in drain flows and a decrease in extraction pressures. For the HD System, the detailed design review should address:
- normal drain control valve sizing,
- emergency drain control valve sizing,
- line sizing, and
- pipe stress and support design.
T/G – The T/G vendor should be required to design for the full range of expected operations. A series of bounding heat balances should be prepared and provided to the vendor. The scope of review is the responsibility of the vendor. However, as a minimum the vendor would be expected to consider the following for impact:
- T/G shaftline analysis,
- rotor dynamic analysis
- HPT throttle margin,
- overspeed protection analysis,
- Turbine Water Induction (TWI) analysis,
- steam flow path design considerations,
- moisture management (including changes to crossing the Wilson line)
- blade design optimization
- Last Stage Blade (LSB) design and optimal exhaust area determination,
- low load performance analysis
- low load stress analysis
- self-excitation analysis (stall and unstalled flutter considerations).
FWHs – The FWH vendor should be directed to consider in the design the full range of conditions represented on design basis heat balance diagrams for limiting operations with and without export steam. Design considerations for the tube side include:
- tube side design pressure and temperature,
- tube side nozzle velocity limits,
- tube velocity limits, and
- pass partition plate differential pressure.
- Design considerations for the shell side include:
- shell side design pressure and temperature,
- shell side shell to bundle clearance and steam velocity,
- steam inlet and drain outlet nozzle velocity,
- drain inlet nozzle flux parameter,
- drain cooler inlet window velocity,
- operating vent capacity,
- condensing zone tube support plate spacing,
- drain cooler zone baffle plate spacing,
- impingement plate sizing,
- thermal centerline and nozzle locations,
- condensing zone tube vibration analysis for all design basis conditions, and
- drain cooler zone tube vibration analysis for all design basis conditions
MSR – In the design considered here, the MSR is configured for LPE steam extraction between the 1st and 2nd stage. For this and other impacts (e.g., high steam flows are reduced cross-around pressures), the MSR is one of the most impacted components under cogen operations. Design consideration of MSR should include:
- nozzle locations for the LPE case,
- steam shell side velocity,
- steam tube side velocity,
- 1st and 2nd reheat stages tube vibration analysis for all design basis conditions.
Main Condenser – The main condenser has design margin to accommodate all cases considered here. However, there is the opportunity to improve the efficiency of the steam cycle once the external cogen cycle is determined. In particular, the temperature of the returned condensate will determine the injection point and additional design features which may be warranted. If condensate is returned at a temperature lower than the saturation temperature in the condenser shell, it should be distributed in a tray system or sprayed into the condenser to preheat the returned condensate and to assist in reducing backpressure. If condensate is returned to the system at a temperature higher than the saturation temperature in the condenser, the return system should be provided with a high head pump in order to inject either downstream of FWH No. 1 or FWH No. 2. In either case, the additional components can serve to improve the cycle efficiency.
MFWP Turbine – The component most impacted by cogen operations is the MFWP turbine. Under either HPE or LPE operations, the cross-around pressure will experience a significant reduction of the ability of the turbine to operate with only low pressure steam admission. The design of this component should be studied separately.
As found in the standard industry design, the MFWP turbines can be supplied with either high pressure or cross-around steam, or a combination of the two. Under HPE or LPE operations, and without modification, the MFWP turbines would be expected to meet design duty, but would use a significant quantity of high pressure steam. This has two adverse impacts. The first is relatively minor, but is associated with reduced efficiency of the component. The second is significant and is associated with the flow of wet steam through the steam flow path. This can have very adverse consequences related to FAC and it is strongly advised that such operations should be avoided.
To address adequate capacity with reduced cross-around pressure (for either the baseline cases or the modified T/G cases) it is expected that the MFWP turbine would require a significant re-design to increase the low pressure inlet bowl coefficient. This will reduce efficiency when operating without steam export. As an alternative, a change in the drive system for the MFWP to a variable speed electric motor could be considered.
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