Patent Number: 054208971
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fast reactor, and more particularly, to a fast reactor having a reflector control system for controlling a reactivity of a core by utilizing a neutron reflector. One example of conventional fast breeder reactor is shown in FIG. 47. Referring to FIG. 47, a fast breeder reactor 10 is provided with a columnar core 11 which is supported by a core barrel 12 disposed outside the core 11 and a reactor vessel 13 is disposed further outside the core barrel 12. A guard vessel 14 for protecting the reactor vessel 13 is disposed outside the reactor vessel 13 and a reflector 15 is disposed further outside the guard vessel 14. A coolant passage 16 through which a primary coolant flows downward is formed between the core barrel 12 and the reactor vessel 13. An electromagnetic pump 17 is disposed perpendicularly above the core, and an intermediate heat exchanger 18 and a decay heat removal coil 19 are disposed further above the electromagnetic pump 17. In the actual operation of the fast breeder reactor 10 of FIG. 47, the primary coolant such as liquid sodium fills the reactor pressure vessel 13 and plutonium in the core is then fissioned. This core 11 contains plutonium and depleted uranium, and heat is generated in accordance with the fission of the plutonium, thereby emitting neutrons. The emitted neutrons are reflected by the reflector disposed so as to surround the outer periphery of the guard vessel 14 and are then absorbed by the depleted uranium to thereby produce plutonium. The thus produced plutonium is again fissioned and the heat is generated. In accordance with burn-up of the core, the reflector 15 is relatively vertically moved while maintaining a critical state of the core 11, whereby the burn-up gradually progresses and generates the heat for a long time. The primary coolant moves upward in the reactor vessel 13 as shown by solid arrow in FIG. 47 by the actuation of the electromagnetic pump 17, decends in the coolant passage 16 through the intermediate heat exchanger 18 and then again flows in the electromagentic pump 17 through the core 11. The primary coolant passes the core 11 while absorbing the heat generated in the core 11 and the heat is transferred to the intermediate heat exchanger 18. A secondary coolant flows into the intermediate heat exchanger 18 through an inlet tube 20 as shown by broken arrow in FIG. 47 and, in the intermediate heat exchanger 18, the heat exchanging operation is carried out between the primary coolant and the secondary coolant. The heat from the core 11 is taken outside the reactor vessel 13 through an outlet tube 21, which is then utilized as a power source. However, in the conventional fast breeder reactor 10 of the structure shown in FIG. 47, since there is not provided a neutron shield in the reactor vessel and the reflector is disposed outside the reactor vessel, the reactor vessel and the reflector diffuse a large amount of heat inside a shielding structure accommodating the fast breeder reactor. In order to remove this heat, the shielding structure of the conventional fast breeder reactor must be provided with a cooling equipment having large capacity, thus providing a significant problem. Furthermore, since the conventional fast breeder reactor radiates a large amount of neutrons outside the reactor vessel and gas such as argon and nitrogen in an atmosphere in the shielding structure is activated, it is necessary to provide the activated gas containment vessel for preventing the gas from discharging externally in an environment under severe management, resulting in further enlargement of an entire reactor arrangement, thus also providing a problem. Still furthermore, in the conventional fast breeder reactor, since a neutron irradiation amount to the reactor vessel during a life time of the reactor exceeds 10.sup.23 nvt (E&gt;0, 1 MeV), stainless steel is not used and expensive crominium steel is to be used, thus also providing an economical problem. Still furthermore, in the conventional fast breeder reactor, since the electromagnetic pump is disposed directly above the core, a large thermal strain is caused to the electromagnetic pump by the heat of the liquid sodium highly heated by the core and the life time for maintaining required reliability is then shortened, and accordingly, in the conventional fast breeder reactor, the shortening of the life time of the electromagnetic pump adversely affects the life time of a small sized fast breeder reactor itself. Still furthermore, in the conventional fast breeder reactor, since the intermediate heat exchanger as well as the electromagnetic pump is disposed directly above the core, it is necessary to disassemble and remove the electromagnetic pump and the intermediate heat exchanger at a fuel exchanging time, resulting in a complicated and troublesome disassembling and removing working and a possibility of giving accidental damage to these elements is also increased. Still furthermore, in the conventional fast reactor having a reflector moving structure, in order to enhance a controlling ability of the neutron reflector, it is obliged to elongate the length of the neutron reflector itself. However, the elongation of the neutron reflector increases its weight, and moreover, affects the core structure itself, and accordingly, it is not desired to elongate the length of the neutron reflector in various view points. Particularly, in so-called a incore reflector type fast reactor in which the neutron reflector is arranged in the reactor vessel, it is difficult to use an elongated neutron reflector from the view point of the incore structure, thus remarkably providing the above problem. FIG. 48 is an illustration showing a structure of a conventional nuclear power plant 30 including a control system therefor. Referring to FIG. 48, a core 32 is accommodated in a reactor 31 and the core 32 generates heat through a fission chain reaction and heats a primary coolant passing the core. The heated primary coolant is fed into an intermediate heat exchanger 34 through a primary coolant high temperature side line 33 and, in the intermediate heat exchanger 34, heat exchanging operation is performed between the primary coolant and a secondary coolant to transfer the heat to the secondary coolant. After the heat exchanging operation, the primary coolant having the lowered temperature is again circulated into the reactor 31 through a primary coolant low temperature side line 35. Such circulation of the primary coolant is carried out by means of a coolant circulation pump 36. The secondary coolant having a raised temperature through the heat exchanging operation is transferred to a steam generator 38 as a load heat exchanger through a secondary coolant high temperature side line 37 and heats a water in the steam generator 38. The secondary coolant having temperature lowered in the steam generator 38 is circulated into the intermediate heat exchanger 34 through a secondary coolant low temperature side line 39. Such circulation of the secondary coolant is performed by means of a secondary coolant circulation pump 40. The water heated through the heat exchanging operation in the steam generator 38 changes to a steam, which is fed to a turbine 42 and drives the same to thereby generate power. The water is fed to the steam generator 38 by means of a water feed pump 43 through a water feed line 57 and feed water flow rate Gw is regulated by a feed water flow rate regulating valve 44. Power control in the conventional nuclear power plant 30 is performed in the following manner. The control system of the nuclear power plant 30 comprises a power setter 45 for setting a power, a reactor power control unit 47 for controlling a control rod 46, a primary coolant flow rate regulator 48 for regulating the flow rate of the primary coolant, a secondary coolant flow rate ragulator 49 for regulating the flow rate of the secondary coolant, and a feed water flow rate regulator 50 for regulating the feed water flow rate Gw to the steam generator 38. The reactor power control unit 47 operates and processes a driving speed of the control rod in response to a power setting signal from the power setter 45, with a reactor outlet temperature detected by a temperature detector 51 being as a feddback signal and a neutron flux level detected by the neutron detector 51 being an auxiliary signal, and then controls the vertical movement of insertion or withdrawal of the control rod 46 in accordance with the operated and processed result. The power of the reactor 31 is regulated by vertically moving the control rod 46. The primary coolant flow rate regulator 48 controls the revolution number of the primary coolant circulation pump 36 in response to the power setting signal form the power setter 45 with the flow rate of the primary coolant detected by the primary coolant flow rate detector 53 being a feedback signal. The flow rate of the primary coolant is regulated by changing the revolution number of the primary coolant circulation pump 36. The secondary coolant flow rate regulator 49 controls the revolution number of the secondary coolant circulation pump 40 in response to the power setting signal from the power setter 45 with the flow rate of the secondary coolant detected by the secondary coolant flow rate detector 54 being as a feedback signal. The flow rate of the primary coolant is regulated by changing the revolution number of the primary coolant circulation pump 40. The feed water flow rate regulator 50 controls an opening degree of the feed water regulating valve 44 in response to the power setting signal from the power setter 45 with the feed water flow rate detected by the feed water flow rate detector 55 being as a feedback signal and a steam temperature detected by the steam temperature detector 56 being as an auxiliary signal. The feed water flow rate to the steam generator 38 is regulated by changing the opening degree of the feed water flow rate regulating valve. As described above, in the conventional nuclear power plant 30, the inserting, i.e. charging, amount or degree of the control rod 46, the flow rates of the primary and secondary coolants and the feed water flow rate to the steam generator 38 are set by the power setter 45, and in order to maintain the set values regarding these factors, the power setter 45, the reactor power control unit 47, the primary coolant flow rate regulator 48, the secondary coolant flow rate regulator 49 and the feed water flow rate regulator 50 are operated, thereby maintaining the value of the aimed power. However, the control system of the conventional nuclear power plant is composed of the power setter, the reactor power control unit, the primary coolant flow rate regulator, the secondary coolant flow rate regulator and the feed water flow rate regulator, thus being complicated in its structure. Furthermore, since the reactor power control unit directly operates the control rod, there is a fear of erroneously withdrawing the control rod due to a failure of the reactor power control unit itself. This problem has been commonly considered to the case of a fast breeder reactor in which the power is roughly adjusted by driving the reflector and a fear resides in an erroneous operation of the reflector. SUMMARY OF THE INVENTION A primary object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art and to provide a fast reactor of small size capable of less diffusing heat and neutrons externally of a reactor, which are absorbed by a shielding structure having a simple construction and a cooling equipment, and capable of effectively utilizing the heat. Another object of the present invention is to provide a neutron driving structure capable of enhancing a reactivity controlling ability of a neutron reflector without elongating the reflector itself. A further object of the present invention is to provide a nuclear power plant having a compact structure capable of eliminating the problems encountered in the prior art as described above and capable of finely adjusting the power of the power plant by roughly adjusting the power by driving the reflector with a constant speed and regulating the feed water flow rate to the steam generator. These and other objects can be achieved according to the present invention by providing, in one aspect, a fast reactor characterized by comprising a core composed of nuclear fuel, a core barrel surrounding an outer periphery of the core, an annular reflector surrounding an outer periphery of the core barrel, a partition wall surrounding an outer periphery of the annular reflector and supporting the core barrel by a supporting structure arranged radially of the fast reactor, the partition wall constituting an inner wall of a coolant passage for a primary coolant, a neutron shield surrounding an outer periphery of the partition wall and disposed in the coolant passage, a reactor vessel surrounding an outer periphery of the neutron shield and having an inner wall constituting an outer wall of the coolant passage, and a guard vessel surrounding an outer periphery of the reactor vessel. Further, for achieving the above objects, the reactor of the present invention of the reflector control type, in which the reactivity of the core is controlled by adjusting leakage of neutrons from the core by vertically moving the neutron reflector arranged outside the core of the reactor immersed in the coolant, is characterized in that the periphery of the core positioned above the neutron reflector is surrounded by a substance having a neutron reflecting ability lower than that of the coolant. Furthermore, for achieving the above objects, the nuclear plant of the present invention includes a neutron reflector disposed in the fast reactor and driven with a constant speed to maintain a burn-up in the core by changing a burn-up range of the core for roughly adjusting a thermal power of the fast reactor, and a plant control unit for changing a temperature of the primary coolant at an inlet of the fast reactor by adjusting a feed water flow rate of the steam generator and finely adjusting the thermal power of the fast reactor in accordance with a temperature feedback effect. The plant control unit comprises: a thermal power calculation section for calculating a thermal power of the steam generator in response to inputted steam temperature at an outlet portion of the steam generator, steam pressure at the outlet portion thereof and steam flow rate; a thermal power control section for comparing the thermal power calculated by the thermal power calculation section with a set value of the thermal power of the steam generator and setting a feed water flow rate signal; and a flow rate control section detecting a feed water flow rate, comparing the detected feed water flow rate with the feed water flow rate signal set by the thermal power control section and setting a signal relating an opening degree of a feed water flow rate regulating valve to thereby control the feed water flow rate. According to the fast reactor of the present invention, since the reflector is disposed closely to the outer periphery of the core, the neutrons are effectively reflected and the burn-up and the breeding of the nuclear fuel can be hence effectively performed. Further, since the reflector is itself immersed in the primary coolant, the heat generated by the reflector is utilized as a power of the fast reactor, thus improving the running efficiency of the reactor. Next, since the neutron shield of the fast reactor of the present invention is disposed inside the reactor vessel and in the coolant passage, the heat generated by the neutron shield can be utilized as a power of the reactor and less amount of the neutrons is irradiated in and out of the reactor vessel. Accordingly, the irradiation of the neutrons to the reactor vessel can be reduced, whereby the reactor vessel can be formed of a stainless steel being a cheap material, thus achieving an economical advantage. Moreover, sealing requirement for the shield structure containing the fast reactor and the heating of the cooling equipment associated with the shield structure and a radiated air in the shield structure can be alleviated, thus making compact the shield structure and the cooling equipment. Still furthermore, according to the present invention, since the core disposed above the neutron reflector is surrounded by a substance having a neutron reflecting ability lower than that of the coolant, at the beginning of life (BOL) at which the neutron reflector is positioned below the reflector, the periphery of the core is covered by that substance to suppress, to a lower value, the reactivity in comparison with a conventional structure in which the entire surface of the core is covered by the coolant, thus enhancing the enrichment of the fuel. Further, in the case where the neutron reflector is moved upward, the reactivity is increased by the change of the relative positions of the neutron reflector and the core and the range surrounded by the coolant is gradually widened while reducing the range surrounded by that substance, whereby the reactivity due to the difference of the neutron reflecting abilities of both portions displaced between that substance and the coolant. Still furthermore, according to the nuclear plant according to the present invention, since the reflector of the fast reactor is driven at a predetermined speed, the control unit for the control rod or the reflector, which is required for the conventional structure, can be eliminated, and moreover, the power of the fast reactor can be roughly controlled by the reflector, thus preventing the reflector from erroneously operating on a failure of the reflector control device. Still furthermore, according to the nuclear plant of the present invention, the actual power of the steam generator is calculated by the plant control unit in accordance with the steam temperature, the steam pressure and the steam flow rate and the difference between the set power value of the power plant and the feed water flow rate to the steam generator is also calculated thereby, thus controlling the feed water flow rate to the steam generator. According to the control of the feed water flow rate to the steam generator, the power of the fast reactor can be controlled by the temperature feedback effect. Namely, in a case where an actual power of the power plant is larger than the set value, the feed water flow rate to the steam generator is reduced, and accordingly, the temperature of the primary coolant at the inlet port of the reactor is increased through the secondary coolant, the intermediate heat exchanger and the primary coolant, resulting in the lowering of the fission chain reaction in the core and hence decreasing the power of the reactor. On the contrary, in a case where the actural power is smaller than the set value, the feed water flow rate is increased, and accordingly, the temperature of the primary coolant at the inlet port of the reactor is decreased through the secondary coolant, the intermediate heat exchanger and the primary coolant, resulting in the increasing of the activation of the fission chain reaction in the core and hence increasing the power of the reactor. According to this temperature feedback effect, the plant control unit can finely adjust the power of the fast reactor.