DEVELOPMENT OF STEAM INJECTOR FEEDWATER HEATER SYSTEM

Transcription

1 7th International Conference on Nuclear Engineering Tokyo, Japan, April 19-23, 1999 ICONE-7371 DEVELOPMENT OF STEAM INJECTOR FEEDWATER HEATER SYSTEM Shuichi Ohmori* and Michitugu Mori, Tokyo Electric Power Company, 4-1, Egasaki-cho, Tsurumi-ku, Yokohama , Japan Phone: , Fax: Tadashi Narabayashi, Mikihide Nakamaru, Yutaka Asanuma, and Makoto Yasuoka Toshiba Corp. 8, Shinsugita-cho, Isogo-ku, Yokohama , Japan Phone: , Fax: In order to realize the simplified BOP system of BWR plant, the authors developed the basic designs of feedwater system simplified by application of Steam Injector. Basic specification of simplified steam injector feedwater heater system (SI-FWH) was developed based on system designs and thermal efficiency evaluation. Test facility was constructed based on system specification selected in accordance with analysis results of plant thermal efficiency. The steam pressures used in the selected best system for low pressure feed water heaters are 0.05MPa, 0.10MPa, 0.21MPa and 0.40MPa are almost the same pressures as the current ABWR's. Improved steam injectors for feedwater heater system were studied by thermal hydraulic analyses and the test model attained the 1 st and 2 nd stage specifications. The 3 rd and 4 th stage injectors succeeded in raising the water temperature to 136 C. The parallel multistage steam injectors rationalize the feedwater system by eliminating the twelve neckheaters, and prevent the chrome ion, which is dissolved from the stainless steel heater tube, introducing into the RPV. Keywords BWR, Steam injector, Feedwater heater system, Plant thermal efficiency, Multistage steam injectors 1 INTRODUCTION Development of technologies for simplifying the components of nuclear power plants is needed to improve the reliability of these plants and reduce their cost, as stated Wilkins (1990). The steam injector (SI) is a passive jet pump that has no movable part and drives feedwater by supersonic jet. Fig.1 shows the working mechanism of the steam injector, which attains higher discharge pressure than the supply steam pressure. As Narabayashi et al., (1991,1994 and 1997) clarified, when normal-temperature water is injected from the water jet nozzle at the axial center and steam is supplied to the annular steam nozzle composed of the outside of the water jet nozzle and the mixing nozzle inlet, the steam becomes a supersonic flow in the mixing nozzle and accelerates the water jet, producing a high-speed water flow at the throat. In the after-throat process, steam condensation is completed and the flow changes into a single-phase water flow which is decelerated by a diffuser, with its pressure rising to a high 1 Copyright 1999 by JSME

2 level in accordance with Bernoulli's principle. In addition to its function as a pump, the SI works as a heat exchanger through direct contact between steam and water. This provides the SI with capability to serve also as a direct-contact feedwater heater that heats up feedwater by using extracted steam from the turbine. As it is a compact equipment, the SI is expected to bring about great simplification and materials-saving effects, while its simple structure ensures high reliability of its operation, thereby greatly contributing to the simplification of the power plant. With this in view, we developed a simplified feedwater system using steam injectors and proved its viability by a small-scale model test in a bid to significantly rationalize the balance of plant (BOP) for the turbine feedwater and condensation system. Steam Mixing Nozzle Water Water Jet Nozzle Throat Diffuser Fig.1 Principle of the steam injector 2 CURRENT STATE AND PROBLEMS OF FEEDWATER AND TURBINE Fig. 2 shows the configuration of the feedwater and condensation system for the advanced boiling water reactor (ABWR) at the present stage. The feedwater temperature and pressure of the system are approximately 42 C(deg.C) and 0.3 MPa at the condensate demineralizer outlet, 49 C and 2.8 MPa at the high-pressure condensate pump outlet, 139 C and 2.2 MPa at the low-pressure feedwater heater (LPHTR) outlet, 156 C and 8.7 MPa at the feedwater pump outlet, and 216 C and 7.5 MPa at the high-pressure feedwater heater (HPHTR) outlet. The pressure of extracted steam for feedwater heating varies as 0.05MPa, ABWR RPV 7646t/h 7624t/h 2.4MPa 415t/h 216'C 186'C 1.3MPaTD-RFP 19MW 566t/h 156'C 139'C HP Turbine 0.4MPa 209t/h 0.21MPa 198t/h 117'C MSH 0.1MPa 216t/h 97'C LP Turbine Cond. 0.05MPa 313t/h 75'C RFPT 1356MW LPCP 3MW GEN. AESJ CF/CD HPCP 5.7MW 8.7MPa 2.2MPa HP HTR RIP 10units 13MW <_f 2f" HPDT MD-RFP HPDP 4.8MW LP HTR A,B,C 3X4=12units LPDP 0.9MW 49'C LPDT 2.8MPa 42'C 0.29MPa Fig.2 Feedwater heater system of ABWR 2

3 0.1 MPa, 0.21 MPa, 0.4 MPa, 1.3 MPa and 2.4 MPa in increasing order from the lowpressure side, while the flow rate of feedwater is around 4,500 t/h at the low-pressure condensate pump outlet and about 7,600 t/h at the reactor inlet. Issues to hopefully improved for the existing system could be summarized as follows: The existing feedwater system has 12 LPHTRs, about 2 meters in diameter and some 14 meters in length, with four of them installed in each of three systems as neckheaters at the top of steam condensers (See Fig. 3). This LPHTR arrangement has reduced the required installation area but increased the height of the condensers and the turbine building. Each LPHTR is provided with a pullout space (an empty space) for equipment repairs, including the replacement of heat exchanger tubes, and this involves inefficiency in plant layout. The feedwater heaters involve the elution of a chromium ion from the heat exchanger tubing, which causes the adhesion of cladding dust on equipment in the reactor. Besides, the feedwater heaters pose technical problems in maintenance and inspection, including the need for large-scale replacement of system components resulting from the degradation of the heat exchanger tubing. Fig. 3 Low pressure heaters (LPHTR) of ABWR 3 STUDY ON SIMPLIFIED FEEDWATER SYSTEM A study was conducted on the design of the simplified feedwater system using steam injectors, followed by an analysis of plant thermal efficiency. Then an attempt was made to select fundamental system specifications based on promising system configurations. 3.1 Study on Configurations of Simplified Feedwater System and Analysis of Plant Thermal Efficiency The study found that the highest thermal efficiency and the least decline in electric output could be achieved by replacing all LPHTRs with four-stage steam injectors having steam extraction pressure equal to that for the existing ABWR system. The simplified feedwater system is equipped with four-stage steam injectors which are supplied steam from each turbine steam extraction points of 0.05 MPa, 0.1 MPa, 0.21 MPa and 0.4 MPa, and heat up condensate of 42 C to 65 C at the first stage, 90 C at the second stage, 115 C at the third stage, and 139 C at the fourth stage (See Fig. 4). These feedwater temperature increases 3

4 were attained within the each available range of a feedwater temperature set, confirmed with the test results. In this instance, the electric output of the plant declined to 1,351 MWe, down about 5 MWe from that of the existing ABWR. If the SI outlet temperature is raised to 75 C at the first stage and 97 C at the second stage, however, the plant electric output could be increased to a level equal to or larger than that of the existing ABWR which stands at 1,356 MWe now. /2C /2C /2C /2C ` ` $WHHGT 6CPM /2C,GV&# /2C ` /2C /2C `` ` ` ` /WNVKUVCIG 5VGCO +PLGEVQTU `.2&6 Fig. 4 Comparison between LPHTR and SI-FWH 3.2 Examination of Materials- Saving Effects Based on the scale-model tests, an attempt was made to determine approximate dimensions of a full-scale steam injector for the simplified feedwater system and to study the layout of the actual turbine building. The required amount of materials for the feedwater 6$ JGKIJV O TGFWEG O.2*64 5+(9* $WHHGT 6CPM (a) Current ABWR (b)si-fwh plant Fig.5 Layout comparison showing height reduction of turbine building 4

5 system, including a buffer tank for transitional measures described later in more detail, could be cut back to approximately one third by replacing all the 12 LPHTRs (each measuring about 2 meters in diameter and 14 meters in length) with six four-stage steam injectors (including jet centrifugal deaerators) that are approximately 0.6 meters in diameter and 20 meters in length. Use of the proposed steam injector will also reduce the height of the turbine building by some 3.5 meters because it eliminates the need for neck-heaters now installed on the top of the steam condensers (See Fig. 5). The steam injector, which has no heat exchanger tubes like those of the conventional feedwater heater, helps significantly reduce the sources of Cr ion-a kind of cladding dust deposited on reactor equipment. Another advantage of the proposed steam injector is in the use of easily replaceable internal nozzles which eliminates the need for large-scale replacement of system components, such as feedwater heaters. 4 SCALE MODEL TESTS 4.1 Single-Stage Operation Test An operation test was conducted with a reduced-scale model prepared for each stage of the proposed four-stage steam injector for the simplified feedwater system. Fig. 6 shows an external view of the testing facility used for the experiment. The test models were prepared on a scale of approximately 1 to 9.2. Fig. 7 summarizes the results of the test which was conducted in accordance with the policy that the steam injector be operated simply as a compact direct-contact heat exchanger, without seeking for an excess pumping head to the SI. ÝUVnÞPF OQFGN ßTF OQFGN àvj OQFGN Fig. 6 Scale model test fasility The first stage test for the proposed SI proved its capability to work with an ultra-lowpressure steam of 0.03 MPa, an inoperative range for conventional steam injectors. Under this condition, according to the test findings, the new steam injector could be operated in parallel with an actual plant when its load reached 60% or more of the rated capacity. The steam injector also proved capable of heating feedwater from 42 C to 65 C at a rated steam pressure of 0.05 MPa. The second stage test found that the steam injector could satisfy the specified thermohydraulic requirements by its capability to work with 0.1 MPa steam at a feedwater temperature of 65 C, an inoperative range for conventional steam injectors. Examination of the third stage SI found that when steam pressure is lower than feedwater 5

6 pressure, a steam injector with a water jet nozzle at the center is more advantageous than other versions, based on the test data proving capable of working at a steam pressure of 0.23 MPa and a feedwater pressure of 0.39 MPa. Probably the proposed injector could attain a discharge temperature of 115 C with an expanded steam passage. The fourth stage test was conducted with a center-steam-nozzle version of the injector that holds the steam jet at the center, because this stage TF VJ has a high steam pressure of PF +PNGV 9CVGT 0.4 MPa. The findings UV 1WVNGV 9CVGT indicate that the steam injector could heat feedwater from 109 C to 136 C with 0.47 MPa steam. 9CVGT 6GORGTCVWTGÔ`Õ 5VGCO 2TGUUWTG/2C Fig. 7 Test results of single stage SI 4.2 Parallel Operation Test A test was conducted on parallel operation of steam injectors that is required for their application to actual plants. In this test, we examined whether the steam injectors, when in parallel connection, could operate stably and whether these injectors were subject to mutual interference, especially the impact of a trip at one of them on others. As shown in Fig. 8, parallel steam injectors proved capable of performing stable parallel operation in a wide range of steam 5+# 5+$ pressure. Even if one of the steam injectors tripped due to a drop in the flow rate of steam supply, according to the test findings, they could restart and resume parallel operation. 6KOG UGE Fig. 8 Test results of parallel SI &KUEJCTIG RTGUUWTG MIEO 4.3 Deaeration Test The simplified feedwater system with steam injectors is a direct-contact feedwater heater that heats up feedwater using steam extracted from the turbine. Accordingly the system involves the mixing of non-condensable gas produced in the reactor core with low-dissolvedoxygen feedwater deaerated in the main steam condenser, and consequently it increases the concentration of dissolved oxygen in the feedwater. To solve this problem, a deaeration test 6

7 was conducted on the system with the aim of deaerating non-condensable gas from feedwater. The upper limit of dissolved oxygen in feedwater is 500 ppb, according to studies by the Electric Power Research Institute and some other organizations, but the maximum allowable limit of dissolved oxygen concentration is set at 200 ppb for ABWR plants from the viewpoint of protecting the initial nuclear fuel. Taking into account likely changes in concentration, steps are taken to hold down dissolved oxygen below 100 ppb. When the concentration of dissolved oxygen falls below 20 ppb, meanwhile, carbon steel loses its oxidized coating which, under this condition, is eluted into a ferrous ion, resulting in the thinning of structural steel members. It is desirable, therefore, that the concentration of dissolved oxygen in feedwater be held in a range from 30 ppb to 100 ppb. Large deaeration tanks are often installed at general thermal power plants but, in the research project discussed herein, we studied a jet centrifugal deaeration process that does not require any large deaeration tank. As shown in Fig. 9, a test was conducted on a jet centrifugal deaerator that was designed to expand the surface area of vapor-liquid boundary by accelerating a water jet into drops with a jet nozzle and facilitate the deaeration of the fluid by the agitating effects of a relative velocity difference between vapor and drops. The test findings indicate that the proposed deaeration technique is capable of reducing the concentration of dissolved oxygen from 2,300 ppb to 29 ppb. $WHHGT 6CPM UV PF 5+U 5VGCO 8GPV 4GEKTEWNCVGF 5VGCO %GPVTKHWICN 'NDQY TFVJ 5+U &1,GV 0Q\\NG &# UVGCO &1 Fig. 9 Principle of jet centrifugal deaerator 5 STUDY ON SYSTEM APPLICABILITY TO ACTUAL PLANTS 5.1 Examination of Operating Modes (during Plant Startup) Through the scale-model tests described in the preceding section, the proposed steam injector was proved to be capable of working with ultra-low-pressure steam of 0.03 MPa, i.e., 60% of the rated steam pressure of 0.05 MPa for the first stage. This means that an operating mode, in which the steam injectors are actuated at 60% of the rated load can be used for the feedwater system. We think that plant output can be stepped up rather smoothly by an operating mode that calls for raising the thermal output by high-pressure feedwater heaters during each startup of the plant, with all stages of the steam injectors bypassed en masse, and then actuating the injectors when the plant output reached around 60% of the rated capacity. 7

8 5.2 Examination of Transients If steam extraction from the turbine is suddenly lost due to a turbine trip or some other event, a feedwater system using steam injectors is no longer expected to maintain a water supply and consequently the event will result in the loss of feedwater. To prevent such trouble, the feedwater system should be provided with an integrated bypass line for all stages of the steam injectors so that SI operation can be quickly switched to bypass system operation in the event of the loss of steam extraction from the turbine. Taking into account the time required for switching to the bypass line, a buffer tank should also be installed on the downstream side of the steam injectors to maintain a sufficient flow rate of feedwater for a certain period of time after a turbine trip. 6 CONCLUSIONS AND FUTURE TASKS The following are conclusions drawn from the study on the simplified feedwater system using multistage steam injectors: An analysis of plant thermal efficiency and a series of scale-model tests found it feasible to dispense with all the 12 LPHTRs used for the conventional feedwater system, while maintaining thermal efficiency equal to that of existing ABWR plants. Replacement of feedwater heaters by steam injectors has the following merits: i. Improvement of the system reliability through the elimination of LPHTRs, resulting in the prevention of long plant outages ii. iii. Simplification of the system, leading to a cutback in initial plant cost Reduction of the required workforce for plant maintenance and inspection and the improvement of plant maintainability and inspectability, leading to the curtailment of the maintenance cost Future Tasks Further research efforts will have to be exerted to improve the thermohydraulic performance at each stage of the steam injectors, e.g., raising the operating water temperature and discharge pressure and expanding the available range of heating. Four-stage scale model tests and single-stage full-scale tests will have to be conducted as a step toward the development of a four-stage full-scale steam injector. REFERENCES Wilkins, D. R., Status of Advanced Boiling reactors, Proc. of the 7th ANS Pacific Basin Nucl., Conf. (San Diego, 1990). Narabayashi, T., et al., Proc. of Int. Conf. on Nuclear Engineering, ICONE-1, a-4, (1991). Narabayashi, T., et al., Proc. Int. Conf. on New Trends in Nucl. Systems Thermohydraulics Vol. 1, (Pisa, 1994). Narabayashi, T., et al., Proc. of Int. Conf. on Nuclear Engineering, ICONE-3, , (Kyoto, 1994). Narabayashi, T., Mizumachi, W., and Mori, M., Nuclear Engineering and Design, Vol. 175, , (1997). 8