High temperature gas-cooled reactor has the advantages of high thermal efficiency (40%~4 1%), deep burnup (up to 20 MW/t uranium) and high conversion rate (0.7~0.8). Because helium has good chemical stability, good heat transfer performance and low induced radioactivity, it can safely take out the residual heat after shutdown and has good safety performance. 10 MW high temperature gas-cooled experimental reactor;
With the support of the National "863" Program, since the mid-1980s, China has carried out the research and development of 10MW high-temperature gas-cooled experimental reactor, which was built to be critical in February 2000 and connected to the grid at full load in 20031October. China has made breakthrough achievements in the research and development of high temperature gas-cooled reactor technology, and basically mastered the core technology and system design integration technology. This scientific and technological achievement has caused extensive influence at home and abroad, which has made China enter the international advanced ranks in high-temperature gas-cooled reactor technology. On June 5438+ 10, 2006, the State Council officially issued the Outline of the National Medium-and Long-Term Science and Technology Development Plan (2006-2020), and listed "Demonstration Project of Large Advanced Pressurized Water Reactor and High Temperature Gas Cooled Reactor Nuclear Power Plant" as a major national project.
The fourth generation advanced nuclear energy system
The Fourth Generation International Forum on Nuclear Energy Systems (GIF) recently officially released the 20 13 annual report. The annual report covers the research and development progress made by GIF member countries, as well as the progress reports of six systems, including ultra-high temperature gas-cooled reactor, sodium-cooled fast reactor, supercritical water-cooled reactor, gas-cooled fast reactor, lead-cooled fast reactor and molten salt reactor. The concept of "the fourth generation advanced nuclear energy system" has been put forward internationally. This kind of nuclear energy system has good inherent safety. Once an accident happens, it will not cause harm to the public. It can compete with other power generation modes economically and has the advantage of short construction period. High temperature gas-cooled reactor is expected to become one of the technologies of the fourth generation advanced nuclear energy system.
The research and development of high temperature gas-cooled reactor in China began in the mid-1970s, and the main research unit is Tsinghua University Nuclear Research Institute. High-temperature gas-cooled reactor is recognized by the international nuclear energy community as a reactor with good safety characteristics. After the Three Mile Island nuclear accident, the safety of nuclear reactors in the world has been improved, and the probability of core melting has been significantly improved. At present, the core melting probability of nuclear power plants in the world can reach the level of "satisfactory power plants" indicated by solid lines in Figure 2, and some nuclear power plants have reached the level of "excellent safety power plants". The design requirement of core melting probability of advanced light water reactor is 10-5/ reactor. It is put forward in the document "User Demand of Power Company" formulated by American Electric Power Research Institute (EPRI). Modular High Temperature Gas Cooled Reactor (MHTR) is an innovative reactor type, and its estimated core melting probability is lower than 10-7/ reactor. This is far below the core melting probability required for advanced light water reactors in 2000.
High temperature gas cooled reactor (HTGR) uses excellent coated granular fuel, which is the basis of its good safety. Uranium fuel is divided into many small fuel particles, each of which is covered with a layer of low-density thermal medium carbon, two layers of high-density thermal medium carbon and a layer of silicon carbide. The diameter of coated particles is less than 65438±0mm, and the coated particle fuel is uniformly dispersed in the matrix of graphite moderating material, and a spherical fuel element with a diameter of 6cm is made (see Figure 3). The cladding completely blocks the fission products produced in the cladding particles. The experiment shows that the fuel of cladding particles still maintains its integrity after being heated at the high temperature of 1600℃ for hundreds of hours, and the release rate of fission gas is still lower than 10-4. High temperature gas-cooled reactor has the following basic safety features:
1. 1 Inherent safety characteristics of reaction transient In the whole temperature range, the reaction temperature coefficient (the sum of fuel and moderator temperature coefficient) of high temperature gas-cooled reactor core is negative, and the fuel temperature coefficient with transient effect is also negative. Therefore, the reactor core can rely on its inherent reactivity feedback compensation ability to realize automatic shutdown under any positive reactivity introduction accident. The accidents of introducing positive reactivity into high temperature gas cooled reactor mainly include:
(1) joystick misoperation; (2) The tube of steam generator is broken, and water enters the core, which leads to the enhancement of moderating ability and the introduction of positive reaction accidents; (3) Positive reaction accidents caused by the overspeed rotation of the primary circuit fan and the drop of the average temperature at the hot end of the coolant.
The accident analysis results show that the increase of reactor power leads to the increase of fuel element temperature, while the negative reactivity temperature coefficient can quickly restrain the increase of power, and the maximum fuel temperature is far below the maximum fuel element temperature limit.
1.2 Thermal design of modular high-temperature gas-cooled reactor core with passive safety characteristics of waste heat load takes into account that the core cooling does not need a special waste heat cooling system under accident conditions, and the decay heat of the core can be transferred to the surface cooler outside the reactor pressure vessel through passive mechanisms such as heat conduction, convection and radiation, and then the core waste heat is discharged to the atmosphere (final heat sink) by the air cooler through natural circulation.
When the primary coolant pressure loss accident occurs, the residual heat of the core can no longer be discharged by the main heat transfer system, and the decay heat of the core can only be discharged by the passive residual heat discharge system mentioned above, which will inevitably raise the temperature of the fuel element in the core center area. In order to limit the maximum temperature of core fuel elements to the temperature limit of 1600℃, the core power density and core diameter of modular high temperature gas-cooled reactor will be limited.
The realization of passive unloading function of modular high temperature gas-cooled reactor residual heat basically eliminates the possibility of core melting accident and has passive safety characteristics.
1.3 multiple barriers to prevent radioactive release defense in depth and multiple barriers are the basic safety principles of all nuclear power plants. As the first barrier of modular high-temperature gas-cooled reactor, the maximum temperature of core fuel element is limited to 1600℃ under all operating and accident conditions. Below this temperature, the pyrolytic carbon layer and dense silicon carbide coating remain intact, which can make the gaseous and metal fission products almost completely remain in the coated fuel particles. Moreover, the fissile material is dispersed into many small fuel particles in large quantities, which form a barrier independently and have high reliability.
The pressure boundary of the primary circuit is the second barrier to prevent the leakage of radioactive materials. The pressure boundary of the primary circuit consists of the following pressure vessels: reactor pressure vessel, steam generator pressure vessel and hot gas conduit pressure vessel connecting the two pressure vessels. The possibility of penetration and rupture of these pressure vessels can be ruled out.
Because the fuel element temperature exceeds 1600℃ and a large number of fission products will not be released under any working conditions, and the radioactivity level of the primary coolant is very low under normal operating conditions, even if the primary coolant is completely released into the surrounding environment in the pressure loss accident, the impact on the surrounding environment is very small. Therefore, the design of modular high temperature gas-cooled reactor adopts the design concept of "containment" instead of containment. The "containment body" is different from the containment. The containment does not require air tightness and full pressure, nor does it need functions such as spray depressurization and combustible gas control, so the system is greatly simplified.
The containment function of high temperature gas-cooled reactor is realized by the main cabin with certain sealing performance. The leakage rate under the pressure difference of 10kPa is less than 10-2/ day. Under normal operating conditions, the negative pressure in the main cabin is maintained by the exhaust system to prevent radioactive substances in the main cabin from diffusing into the reactor building, and the exhausted air is filtered and discharged from the chimney; When there is a serious accident of coolant pressure loss and the pressure in the primary cabin exceeds 10kPa, the explosion-proof membrane of the emergency exhaust pipeline will automatically open, and radioactive materials will be directly discharged into the atmosphere from the chimney without filtration. Because the consequences of direct release of radioactivity are not serious, the pressure in the first loop cabin immediately drops to normal pressure after a short time, and the system filters and discharges again, which can prevent a large amount of radioactive fission materials from being directly released into the environment when an accident occurs and avoid the risk of a large amount of radioactive release. Modular pebble bed high temperature gas-cooled reactor adopts the characteristic of passive waste heat discharge, which greatly enhances the safety, but its single reactor power is greatly limited. Because the pebble bed high temperature gas-cooled reactor can provide high temperature helium at 950℃, making full use of its high temperature helium potential to obtain higher power generation is the main development direction to improve its economic competitiveness. The direct circulation mode of helium turbine is the main development direction of high-efficiency power generation in high-temperature gas-cooled reactor.
The high-temperature gas-cooled reactor nuclear power plant designed by South Africa's ESKOM Company adopts the direct circulation mode of helium turbine [1, 2], and the high-temperature helium coolant at the primary outlet directly drives the helium turbine to generate electricity, with the reactor pressure of 7MPa and the helium outlet temperature of 900℃. The high-temperature helium drives the high-pressure helium turbine first, then the coaxial compressor, then the low-pressure helium turbine, and finally the main helium turbine to output electricity. After the whole cycle, the pressure of helium will drop to 2.9MPa, and the temperature will drop to 57 1℃. In order to pressurize helium to the inlet pressure of the primary circuit of the reactor, it needs to be cooled to 27℃ by regenerator and preheater, then boosted to 7MPa by two-stage compressor, and then returned to the other side of the heater to be heated to 558℃ and then returned to the inlet of the reactor core. This process is shown in Figure 5. The power generation efficiency of this cycle mode can reach 47%.
The main advantages of the circulating system are: the system is simple, all power systems are integrated in three coaxial pressure vessels, and the cost is low; The possibility of water inflow accident in the core is avoided; High thermal cycle efficiency. The direct circulation mode of helium turbine is the development direction of high-efficiency power generation in high-temperature gas-cooled reactor. However, at present, there are many projects that need to be researched and developed for this technology, mainly including:
(1) Develop fuel elements with high quality and low release rate (ensure low radioactive level entering the turbine power generation system);
(2) The development of vertical helium turbine technology, including: magnetic bearing, start-stop bearing, treatment of contact metal surface in high temperature helium atmosphere;
③ Develop efficient (98%) plate-fin regenerator technology.
From the point of view of technical feasibility, there are currently two methods to replace helium thermal cycle:
3. 1 direct combined cycle mode
The circulation process is shown in Figure 6. High temperature helium at 900℃ and 6.9 MPa first drives a helium compressor turbine to drive the coaxial compressor, and then drives the main power generation helium turbine to output electricity. Helium gas at the outlet passes through a once-through steam generator to heat water at the other side to generate steam. The generated steam drives the turbine generator to output power. Helium is pressurized to 7.0MPa, 183℃ by the compressor after passing through the once-through steam generator, and returns to the core inlet. The combined cycle power generation efficiency of helium turbine and steam turbine in this system can reach 48%.
The main advantage of this circulating system is that it does not need to adopt an efficient regenerator, which avoids a technical problem. However, due to the use of helium? Combined steam cycle increases the investment cost of the system, so the possibility of water inflow accident in the core cannot be ruled out.
3.2 indirect combined cycle
The indirect combined cycle flow shown in Figure 7 is as follows: The high-temperature helium gas at 900℃ at the reactor outlet passes through the intermediate heat exchanger (nitrogen gas is heated at the secondary side), cooled to 300℃, and then returned to the reactor core inlet through the helium fan. The nitrogen on the secondary side is heated to 850℃ through the intermediate heat exchanger, thus realizing the combined cycle of gas turbine and steam turbine. The power generation efficiency of this cycle is 43.7%.
Because nitrogen is used as working medium, mature gas turbine technology can be adopted, which has good feasibility under the basic conditions of existing technology. However, the increase of investment cost cannot rule out the possibility of water inflow accident in the core.
From the comparison of the above cycle processes, it can be seen that helium thermal cycle can obtain higher power generation efficiency, and according to the development level of technology, an appropriate cycle process can be selected. The output power of modular high temperature gas-cooled reactor is limited due to passive waste heat emission, and the maximum thermal power can only reach 200 ~ 260 MW. Its output power can only reach 100MW, which is less than that of PWR nuclear power plant. However, the economic analysis results of the 100MW high-temperature gas-cooled reactor designed by South Africa's ESKOM Company show that its power generation cost is very competitive compared with the large-capacity PWR nuclear power plant, which can be comparable to the local cheap coal-fired power cost. The main factors are as follows:
① High power generation efficiency: its power generation efficiency is about 25% higher than that of PWR nuclear power plant.
② Short construction period: 100MW high-temperature gas-cooled reactor adopts modular construction mode, which can shorten the construction period to two years, reduce the interest during the construction period, and reduce the construction investment by about 20% compared with the construction period of 5-6 years for PWR nuclear power plant;
(3) The system is simple: the passive safety characteristics of HTGR make the system very simple, and there is no need to install emergency cooling system and containment in PWR nuclear power plant, which saves the construction investment.
④ High safety: It has inherent safety characteristics, and traditional risks such as core melting will not occur in the most serious accident.