Patent Number: 050531883
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a reactor system according to an embodiment of the invention includes a reactor pressure vessel 20, a primary containment vessel 22 accommodating the reactor pressure vessel and disposed in a reactor building 21, a main turbine 24 disposed in a turbine building 23, a main steam piping 25, an inside main steam isolation valve 26 of a quick closure type, an outside main steam isolation valve 27 of a conventional type, a main steam stop valve 28, a main steam control valve 29 and a piping 31. The main steam piping 25 extends through the reactor building 21 and turbine building 24 to supply main steam generated in the reactor pressure vessel to the main turbine 24. The inside main steam isolation valve 26 can be closed within 0.1 second which corresponds to one thirtieth of a period of time required for the closure of a conventional inside main steam isolation valve, and is provided on the main steam piping 25 inside the primary containment vessel 22 to serve as a quick closure valve operable in case of a break accident of the main steam piping 25. The outside main steam isolation valve 27 is provided on the main steam piping 25 outside and near the primary containment vessel 22. The main steam stop valve 28 and the main steam control valve 29 are provided on the main steam piping 25 near the main turbine 24. The main steam stop valve 28 is a conventional quick closure valve, and the inside main steam isolation valve acts in a similar manner to the main steam stop valve 28. The piping 31 provided with a safety relief valve 30 is connected to the main steam piping 25 at a position between the reactor pressure vessel 20 and inside main steam isolation valve 26. As shown in FIG. 1, a portion of the main steam piping 25 extending from the outside main steam isolation valve 27 towards the reactor pressure vessel is designed in the aseismic highest class category I while the other portion of the main steam piping 25 extending from the outside main steam isolation valve 27 towards the main turbine, the main steam stop valve 28 and the turbine building 23 supporting them are designed in the seismic non-category I class. The reactor pressure vessel 20 in the embodiment of the invention is for a natural circulation reactor, and is of a simple construction to be dispensed with such piping 15 of reactor pressure vessel recirculation system and many steam separators 16 for forced circulation, unlike a reactor pressure vessel 14 of a prior forced circulation reactor shown in FIG. 3. Accordingly, the reactor pressure vessel 20 in the embodiment of the invention has in its upper construction only a solid portion one third of that in a prior forced circulation reactor. As shown in FIG. 2, the reactor pressure vessel 20 includes a reactor core 33, a cylindrical chimney 34 disposed above the reactor core and having a height of 9 meter, a dryer 35 disposed above the cylindrical chimney, an upper dryer tube 36 and a steam dome section 32. The cylindrical-shaped, upper dryer tube 36 is arranged in the steam dome section 32 above the reactor core 33, so that the steam dome section 32 has a larger volume than that in a prior boiling water type reactor which has the same level of output as that of the present reactor system. Therefore, an additional volume to be enlarged corresponding to a possible quick closure of the inside main steam isolation valve 26 caused in case of a break accident of the main steam piping can be made small, so that the transient phenomenon in the reactor pressure vessel 20 can be readily mitigated. In the embodiment of the invention, when breakage accidentally occurs on a portion of the main steam piping 25 on the side of the main steam turbine 24 from the outside main steam isolation valve 27, the inside main steam isolation valve 26 quickly closes to enable substantially reducing an amount of coolant flowing out of a broken portion of the main steam piping 25 as compared with that in a prior reactor system. In this case, how severe the transient pressure phenomenon in the reactor pressure vessel 20 upon the quick closure of the inside main steam isolation valve 26 is problematic. Since the volume of the steam volume section 32 in the reactor pressure vessel 20 and the volume between the inside main steam isolation valve 26 and reactor pressure vessel 20 are adequately ensured, however, it is possible to mitigate the transient pressure phenomenon in the reactor pressure vessel 20. An analysis will be given hereinbelow to a transient phenomenon in application of the present invention on a natural circulation reactor. Referring to FIG. 4, a calculation model of the reactor system employed for the analysis comprises the safety relief valve 30, main steam stop valve 28, inside main steam isolation valve 26 of a quick closure type, outside main steam isolation valve 27, steam dome section 32, feed water piping 40, downcomer 41, chimney 34, flow channel 43 of fuel assembly, fuel element 44 and lower plenum 45. The flow channel 43 of fuel assembly and fuel element 44 constitute the reactor core 33 as shown in FIG. 1. In FIG. 4, the same elements as shown in FIG. 1 are designated by the same reference numerals. A length of flow passage extending from the steam dome section 32 to the inside main steam isolation valve 26 is simulated be decreasing the volume of the flow passage, and quick closure of the inside main steam isolation valve 26 is simulated by rapidly decreasing an area of flow passage or the valve module to zero. Reactivities taken into consideration include void reactivity, doppler reactivity and reactivity of control rods. FIG. 5 shows a result of a transient analysis in the case of quick closure of the inside main steam isolation valve 26. Pressure in the reactor pressure vessel 20 is raised upon the quick closure of the inside main steam isolation valve 26, and then the safety relief valve 30 operates to suppress the highest pressure. The pressure rise in the reactor pressure vessel 20 is accompanied by decrease of voids in the reactor core and causes void reactivity increase, which results in an increase of neutron flux. However, the volume in the reactor pressure vessel for accommodating vapor is large to make a speed of pressure rise low and scram (insertion of control rods) corresponding to a signal of quick closure of the inside main steam isolation valve 26, so that increase in neutron flux is suppressed to the extent of 1.1 times as large as that during the normal operation of the reactor system. Heat flux of fuel assembly is also suppressed while synchronizing in a time lag with a change in neutron flux. On the other hand, the flow rate of circulating reactor coolant is once increased since pressure rise causes voids in the reactor core to decrease thus reducing the flow resistance of two phase flow (liquid phase and vapor phase). Thereafter, the voids are reduced due to reduction of heat flux of fuel assembly and the average density of reactor coolant in the reactor core is increased to reduce the flow rate of the coolant. As a result, the change of the minimum critical power ratio (.DELTA.MCPR) is suppressed to 0.05 since the flow rate of reactor coolant is large at the time of large power immediately after the transient phenomenon and the power is small at the time of small flow rate. The above value is adequately small in comparison with the thermal margin in the normal operation, and no boiling transition phenomena occurs even upon the quick closure of the inside main steam isolation valve. In this manner, it is confirmed that the provision of the inside main steam isolation valve 26 of a quick closure type in the interior of the primary containment vessel 22 offers no problem in terms of a transient characteristics to enable quickly closing the valve 26 upon a break accident of the main steam piping 25. Such quick closure of the inside main steam isolation valve 26 decreases an amount of reactor coolant flowing out. Thus even when the main steam piping 25 is accidentally broken at a position between the inside main steam isolation valve 26 and the main turbine 24 by an earthquake corresponding to the seismic class: Seismic Category I, the inside main steam isolation valve 26 of a quick closure type quickly closes to decrease an amount of reactor coolant flowing out of a broken portion of the main steam piping 25, thereby suppressing the exposure dose to a slight extent as compared with the prior art. Accordingly, a portion of the main steam piping 25 extending from the outside main steam isolation valve 27 towards the main turbine, the main steam stop valve 28 and the turbine building 23 supporting these members can be designed in the non-category I class to thereby facilitate reduction of an amount of structural materials for equipments and pipings and construction of the turbine building. The enlarged steam volume section of the reactor pressure vessel mitigates a transient phenomenon associated with pressure rise caused due to vapor generated in the reactor pressure vessel after a break accident of the main steam piping to ensure safety and stability of a reactor system. While the inside main steam isolation valve 26 is of a quick closure type in the above embodiment, the outside main steam isolation valve may be alternatively of a quick closure type, in which the length of a flow passage extending from the steam dome section to the outside main steam isolation valve is larger than in the above embodiment to further mitigate the transient phenomenon for an improved effect. Referring to FIG. 6, there is shown a reactor system of a conventional, typical boiling water type atomic power plant. Main steam generated in a reactor pressure vessel 1 flows from a reactor building 6 to a turbine building 7 through a main steam piping 2 disposed in a main steam tunnel 11, and is supplied via a main steam stop valve 8 and a main steam control valve 9 to a main turbine 10 for driving the same. The main steam piping 2 is provided with an inside main steam isolation valve 4 and an outside main steam isolation valve 5, both of which are disposed near a primary containment vessel 3. The main steam stop valve 8 acts to shut off a supply of main steam to the main turbine 10, and it takes 0.1 second to close the valve 8, so that it can close several tens of times as fast as the main steam isolation valves do. The inside and outside main steam isolation valves 4 and 5 are opened during a normal operation of the plant, and function to close in case of a break accident of the main steam piping 2 for the prevention of outflow of reactor coolant within a predetermined period of time. The inside and outside main steam isolation valves 4 and 5 are limited to 3 to 4.5 seconds in a period of time for closure. Therefore, some reactor coolant in the form of vapor will flow out of the reactor pressure vessel until the inside and outside main steam isolation valves have been fully closed. A portion of the main steam piping 2 on the side of the reactor pressure vessel 1 from the outside main steam isolation valve 5 is designed in the highest seismic class seismic category I class, and a portion of the main steam piping 2 on the side of the main turbine 10 from the outside main steam isolation valve 5 and the main steam stop valve 8 are designed in the seismic category I class on the basis of an evaluated exposure dose upon a break accident of the main steam piping, which takes into consideration a period of time (3 to 4.5 seconds) required for the closure of the main steam isolation valves. The turbine building which is required to have a shielding function is designed as a whole in the non category I class. Since the main steam piping 2 and the main steam stop valve 8 within the turbine building 7 are designed in the seismic category I class as described above, however, those elements which support these main steam piping and main steam stop valve are also designed corresponding to the seismic category I class to be of a firm construction. Referring to FIG. 7, there is shown a reactor system of a conventional underground type atomic power plant. Reactor building 3a and turbine building 7a are spaced away from each other due to the terrain, so that a main steam piping 2a disposed in a main steam tunnel 11a is large in length. The main steam piping 2a is provided with a quick closure valve 13a which is disposed between an outside main steam isolation valve 5a and a main steam stop valve 8a. In the drawing, the reference numeral 4a designates an inside main steam isolation valve, and 9a a main steam control valve. With this arrangement, steam and water flowing out upon a break accident of the main steam piping 2a are prevented from flowing into the reactor building 3a and the turbine building 7a. Construction downstream of the quick closure valve 13a is designed in the non category I class. While the invention has been described by way of an embodiment, it is to be understood that the invention is not limited to the embodiment but to the scope of the appended claims.