NUCLEAR HEATING REACTOR AND ITS APPLICATION

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1 NUCLEAR HEATING REACTOR AND ITS APPLICATION Zhang Yajun* and Zheng Wenxiang INET, Tsinghua University, Beijing China Abstract The development of nuclear heating reactor (NHR) has been carried out by the Institute of Nuclear Energy Technology (INET), Tsinghua University since 1980 s. A number of advanced passive safety features are adopted in the design of the NHR. Emergence core cooling system and any off-site emergency actions such as sheltering, evacuation, relocation and decontamination are not necessary for the NHR. NHR has widely potential applications in the fields of district heating, seawater desalination and industrial processing. The retrospection of R&D on the NHR, the main technical and safety features, the economic competitiveness and prospect will be discussed in this paper. Retrospection of R&D on the NHR in China The R&D on the key technology for the nuclear heating reactor (NHR), such as nature circulation, integrated design, passive residual heat removal and hydraulic control rod driving system, were carried out in the early eighties by the Institute of Nuclear Energy Technology (INET) of Tsinghua University. A 5MW experimental NHR (NHR-5) was begun to construction on March The initial fuel loading was achieved on Oct. 9, The NHR-5 went to the first criticality on Nov. 3, The rated power was reached on Dec. 16, Since then, the reactor had been successfully operated for district heating until the main objectives were actualized. In the meantime, a number of experiments have been carried out on the NHR-5 to investigate its safety features and the multiple functions for the NHR, such as electricity generation in co-generation mode as well as air conditioning and seawater desalination using nuclear heat. Chinese energy authorities have been impressed with the excellent performance shown by the NHR-5. A number of cities and utilities are very interested in introducing the NHR into their local energy system. Therefore, a commercial sized NHR with an output of 200MW th (NHR-200) has been developed by INET. The first NHR-200 demonstration plant will be built in Northeast China. The construction permit has been issued by Chinese National Nuclear Safety Administration (NNSA). Main Technical and Safety Features of NHR-200 Both NHR-5 and NHR-200 are vessel type light water reactor with fully power 48

2 natural circulation capacity. The main design parameters of NHR-5 and NHR-200 are briefly summarized in Table 1. For the NHR, it should be located in the vicinity of the user due to the consideration of heat transportation and economy. The distance between the reactor and the populous area is not a protecting factor any longer. Therefore, no off-site emergency action, such as sheltering, evacuation, relocation and decontamination, is the one of the fundamental safety objectives in case of all credible accidents. In other words, the radioactive release from the NHR-200 in any case has to be reduced to such low levels that off-site emergency action is not necessary. To meet the requirements of the fundamental safety objectives, a series of technical measures are adopted in the design of the NHR-200. The main technical and safety features can be briefly summarized as follows. Table 1 Main Design Parameters of the Nuclear Heating Reactor NHR-5 NHR-200 Thermal power MW Primary system pressure MPa Core inlet/outlet temperature C 146/ /210 Ave. linear heat rate kw/m Volumetric power density kw/l Number of fuel assemblies Number of control rods Active core height m Active core diameter m Inventory of UO 2 t Enrichment of initial core % 3 1.8/2.4/3.0 Refueling enrichment % 3 3 Intermediate circuit temperature C 102/142 95/145 Intermediate circuit pressure MPa Heating grid temperature C 90/60 130/80 Integrated Design One of the fundamental design criteria for the NHR-200 is that the core should be always covered by coolant. Therefore, the integrated design concept is adopted. Both reactor system and the primary circuit, including primary heat exchanges, are arranged in the reactor pressure vessel (RPV). The RPV is designed to meet leak-beforebreak requirement. All of the penetrations are on the upper part of the RPV and the largest diameter of the penetrations of coolant pressure boundary is 50mm. Therefore, large LOCA can be excluded, small LOCA will not make the core uncovered and safety injection system is not necessary. Full Power Natural Circulation Cooling The coolant circulates due to the density difference between hot and cold regions 49

3 inside the RPV at all power levels so that the primary circulating pumps can be eliminated and the higher system reliability can be ensured. Dual Vessel Structure A guard vessel encloses the primary system. The guard vessel will ensure the flooding of the reactor core without any emergency cooling actions in case of a very unlikely failure of the RPV. It means that a steel containment fits tightly around the RPV and two isolation valves, which are driven by different driving-means, are located inside and outside of the guard vessel respectively so that the core will be covered under any postulated coolant leakage within it. The results of LOCA analysis for the NHR-200 are listed in Table 2. Low Temperature, Low Pressure and Low Power Density The reactor is designed with the features of low temperature, low pressure and low power density. In addition, there is no boric acid in the coolant during normal operation. Gadolinium oxide as a burnable poison is used to control the reactivity along with the B 4 C control rods. A boric acid injection system as a secondary shutdown system will be operated when the event of anticipated transient without scram (ATWS) occurs. Hydraulic Driving Mechanism of the Control Rods The system intakes coolant from RPV. The coolant from the RPV will be pressured by circulation pump and filtered by filter located at the outlet of the pump. Then, it will be distributed by the control units and drive the step cylinder, which connects to control rod. Therefore, the control rod will be driven by hydraulic according to the requirement of operation. This design meets the requirement of fail-safe principle i.e. based on the natural law, gravity, control rods will drop into the core automatically in case of lose of power supply, depressurization of RPV, pipe break and pump shut down events. This design simplifies the reactor structure and eliminates the accident of rapid rod ejection. Table 2 Results of LOCA Analysis for NHR-200 Accident circumstances Events * Ultimate Amount of Amount of water pressure in RPV water lost remained above the core MPa t t ~ *Event 1: φ50 pipe break inside the guard vessel. Event 2: φ50 pipe break outside the guard vessel followed by failure of isolation. Event 3: Small crack (~1cm 2 ) at the bottom of the RPV. Event 4: Safety valve stuck open. Event 5: ATWS initiated by loss of offsite power and safety valve stuck open. 50

4 Storage of Spent Fuel in the RPV About two cores of spent fuel assemblies are stored on the racks around the active core to provide more than 15 years of interim storage for spent fuel. This design can simplify the refueling device and bring some benefit in nuclear design. No Emergency Cooling System Because of the safety features and technical characteristics of NHR, the possibility of large LOCA caused by the breaking of primary pipe can be excluded and the core will be covered by coolant under any postulated accident conditions. Therefore, emergency core cooling system is not needed in the NHR-200. Natural Circulation for Residual Heat Removal The residual heat removal system, that is the most important safety system in NHR-200, is designed as two independent trains. The design capacity for each train is not less than 1.5% of rated power. Each train is composed by a triplex loop, which are the primary loop, intermediate loop and air-cooler. Non-active residual heat removal is achieved by natural circulation completely. Self-Pressurized Performance The primary pressure, composed by the saturate steam pressure corresponding the core outlet temperature and a certain inventory of nitrogen, can be stably maintained at the designed level. Therefore, the pressurizer with its complicated control system adopted in PWR can be eliminated. Preventing Heat User from Rad-Contamination The multi-barriers, including fuel cladding, primary pressure boundary, guard vessel and a secondary confinement or containment, compose the multi-defenses against the release of radioactive material. In addition, the nuclear heat supply system is composed by a triple loop, i.e. the primary circuit in RPV, an intermediate circuit and the heating grid or other heat user circuit. The operating pressure in the intermediate circuit is higher than that in RPV, so that the contamination of radioactivity can be prevented from and the safety of the heat user can be ensured. Economic Competitiveness of NHR-200 Using NHR-200 as a heat source has a better economic competitiveness relative to conventional heat sources. The economic competitiveness of NHR-200 represents mainly on the following fields: Simplified System As mentioned above, the integrated design and fully power natural circulation cooling are adopted in the design of NHR-200. Therefore, there is no primary circulating pumps, primary pipeline, pressurizer, steam generator and boron condensation and dilution system. The system configuration of NHR-200 is much simpler than that of 51

5 large power plant. Higher Safety Standard One of the fundamental safety objectives of NHR-200 is no off-site emergency action such as sheltering, evacuation, relocation and decontamination in case of all credible accidents. The PSA results show that the CDF of NHR-200 is lower than The achievement of the fundamental safety objective is due to the inherent and passive safety features of NHR-200. Higher Availability The higher plant reliability will be ensured based on the simplified system design adopted in NHR-200. Therefore, the higher availability can be achieved. For example, NHR-5 had achieved a high availability of 99% (factual operation days divided by planned operation days) during the first three-year operation for space heating. Less Rad Waste Production The designed rad waste production is very few relative to large nuclear power plant. For example, the rad waste produced in NHR-5 during the first three-year operation could be disregarded, the amount of liquid waste was 8.5m 3 with β radioactive level of 14Bq/l and the radioactive level of gaseous influent at the outlet of the stack was as low as the background. Lower Professional Collective Dose The design objective of the professional collective dose is lower than the limit set in the relative safety codes, regulations and guides. The average professional collective dose was 5.67mSv-man per year. The environmental surveillance proved that there was no influence on the environment in around region by the operation of NHR-5. Shorter Construction Time The shorter construction time for the NHR-200 will be achieved due to its simplified design. A 36 months construction time should be achieved according to the design. To analyze the economy of NHR-200 further more, the economic competitiveness using a NHR-200 as a heat source to couple with a heating grid was compared with the oil-fired heating plant in the Chinese economic condition. The result shows that the heating cost of NHR-200 will be lower than that of oil-fired heating plant at the condition of average interest rate of 7.65% and that the heating cost will be decrease with 5% if the interest rate drops per 1%. The Cost comparison between NHR-200 and oil-fired heating plant is listed in Table 3. For the another important application, the main parameters of NHR-200 perfectly match the requirements on the heat source for seawater desalination processes. Two kinds of interface design between NHR-200 and a multi-effect distillation (MED) plant are conducted, one is single water production and another is water electricity cogeneration. For single water production, the low-pressure steam generated in the inter- 52

6 mediate circuit steam generator will be directly introduced to the MED plant. A GOR of is designed and the daily fresh water production will be 165,052m 3. For water electricity co-generation, the steam will first be used for electricity generation. And then, the steam extracted from the last stage of the turbine with lower temperature and pressure will go to the MED process. The maximum fresh water output will be 132,065 m 3 /d with a GOR of and the generated electric power will be 13.45MW. The levelized water price may reach to 0.93 $/m 3 for single water production and $/m 3 for co-generation in case of the 5% interest rate. Table 3 Cost Comparison between NHR-200 and Oil-Fired Boiler Plant NHR-200 Oil-fired boiler plant Oil RMB /tonne / ,280 price US$/Barrel Fuel cost, RMB /GJ Production cost, RMB /GJ Total cost, RMB /GJ : The oil price means CIF (Cost, insurance and freight) The following factors have the more influence on the water price: output of water, investment and service life of water plant, investment and service life of the reactor. Further analysis indicates that the most sensitive factor is the capacity factor of the water plant and the reactor. The result of sensitivity analysis on the water price indicates that the water price will be dropped in case of the low interest rate and less investment of NHR desalination plant as well as high availability of the plant. Several special advantages would be achieved using a NHR for seawater desalination. Firstly, the scale of the NHR-200 desalination plant is more suitable for the demand of potable water about a hundred thousands m 3 /d. Secondly, due to the inherent and passive safety features of the NHR itself, the NHR desalination plant can be constructed near large cities and industrial consumers. It would lead to a decrease in the cost of the water pipe network significantly. At last, under the suitable condition, several NHR-200s could be combined to supply heat and electricity to a large-scale seawater desalination plant for cities and industrial districts with large fresh water requirements. The combined NHR-200s desalination plant can not only ensure the continuity of the water production but also improve the economy by sharing of common facilities and service systems including infrastructure, maintenance facilities, reduction of staff and so on. Prospect Coal is the most important primary energy in China. The coal annual consumption in China is about 30% of the total coal consumption in the world The proportion of the coal annual consumption relative to the amount of primary energy is about 75%. Because of the distribution, exploiting process, and quality of coal, the Chinese energy industry is facing a lot of serious problems. 53

7 Except the problems on production and transportation, the coal exploitation results ground surface caving in, a mass of gangue, and a large quantity of wastewater. Moreover, unrestricted use of coal has resulted in serious impact on environment. CO 2 is the main greenhouse gas. It is estimated that CO 2 produced by coal burning is 85% of the total released into atmosphere in China. SO 2 is the secondary byproduct by coal burning. In China, total amount of tones of SO 2, in which about 80~90% was produced by coal, was released in It is the primary reason of acid rain. The third main source of environment pollution produced by coal is soot. The quantity of coalsoot released that is more than 70% of the total countrywide is the main reason of the floating pollution at atmosphere. In addition, the residue after coal burning, of which more than tones will be let off countrywide, is also to pollute environment. Even for radioactivity, the level produced by coal burning is higher than that by nuclear heating. Table 4 Environment Impact by Different Heating Plants, ton/year Fossil-fired boiler NHR-200 Oil Coal Gas Thermal power 200MW 200MW 200MW 200MW Carbon dioxide 200, , ,600 0 Sulfur oxide 1,800 6, Heavy metal Nitrogen oxide 619 1, Radioactivity(mSv/person year) Soot , Residue , High level rad waste ~1 Transportation for fuel 100, , m 3 /year ~1 Therefore, nuclear heating is helpful to reduce the release of Greenhouse gas and to protect environment because no baleful gases, such as CO 2, SO 2 etc., will be released during the process. The level of radioactivity that is about one thirtieth of the coal-fired boilers in the same scale is lower than the value envisioned by people. It can be resulted from Table 4 that environment will benefit greatly from using nuclear fuel to replace coal as heating source. With the same thermal power, an NHR-200 can reduce the environment pollution by decreasing the release of tones of CO 2, tones of SO 2, tones of nitrogen oxide, tones of soot and tones of residue. From the point of view of environmental protection, it can be considered that nuclear energy is a very clean energy resource. Consequently, the NHR-200 can be considered as the new generation nuclear reactor with a number of advanced and inherent safety features. The R&D on NHR-200 exploit the new field for the application of the nuclear energy. As a safe, clean and economic energy resource, NHR-200 could be used in certain areas and helpful to improve energy structure and environmental condition in China. 54