Patent Number: 052710440
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a boiling water nuclear reactor, and particularly relates to a start-up process of a natural circulation nuclear reactor in which circulation flow rate is secured by the hydrostatic head difference between the outside and inside of a reactor core (hereinafter also simply referred to as "core"). In a current water nuclear reactor, cooling water is circulated into a core by recirculation pumps at the start-up time after ordinary shut-down of the nuclear reactor (hereinafter also simply referred to as "reactor"), and the cooling water is heated by nuclear reaction by withdrawing control rods to thereby make the cooling water high in temperature as well as pressure. At this time, since the core is being cooled by forced circulation, the cooling water is in a state of single-phase flow. Being heated, the cooling water is made to transit monotonously from the state of single-phase flow into a state of two-phase flow to make it possible to perform stable starting-up of the reactor. On the other hand, there is a boiling water reactor of the type in which at the start-up time, at least in a period from a non-critical state to a state in which an isolation valve is opened so that steam is discharged from the reactor, a core is cooled by natural circulation. For example, in a natural circulation reactor provided with no recirculation pumps, a hydrostatic head difference between the outside and inside of a shroud enclosing a core is used as driving force for the natural circulation of the cooling water in the core. Accordingly, if the cooling water is heated in the core by nuclear reaction at the start-up time after ordinary shut-down of the reactor, the cooling water outside and inside the shroud is circulated at a low flow rate by driving force generated by a density difference due to a temperature difference. When the water temperature becomes high and the subcool temperature at an inlet of the core becomes lower than the maximum subcool temperature to start boiling which is determined by physical properties and a circulation flow velocity, boiling is generated in the core. At this time, the hydrostatic head difference between the outside and inside of the shroud increases because of generation of steam bubbles to thereby increase the circulation flow velocity. By this, the quantity of cooling of the core increases so that the cooling water in the core comes back into the state of single-phase flow. This operation is repeated so that the state of single-phase flow and the state of two-phase flow are alternated to thereby generate flow fluctuations. This unstable phenomenon becomes remarkable under low temperature where the vapor-liquid density ratio is large and continues until the subcool temperature at the core inlet becomes lower than the minimum subcool temperature to cause unstable phenomena. In such an unstable phenomenon at low temperature two-phase flow, the degree of void reaction of nuclear fuel fluctuates because of occurrence of flow fluctuations so that there arises a problem that the stability of the core can not be improved. Further, in order to avoid such an unstable phenomenon at low temperature two-phase flow by making the temperature of the cooling water rise up in the state of single-phase flow to a temperature as high as possible while delaying the start of boiling, it is necessary to heat the cooling water with an extremely low quantity of heating by nuclear reaction for a long time. In this process, however, the circulation velocity in the core is so low that a phenomenon of thermal stratification occurs in the cooling water in a lower plenum in a pressure vessel and low temperature water stays in the lower plenum. Accordingly, when most of the cooling water becomes high temperature to start boiling, the low temperature water in the lower plenum flows into the core because of increase of the core circulation velocity to thereby generate a similar unstable phenomenon. Further, since the cooling water is heated with an extremely low quantity of heating by nuclear reaction, it takes a very long time for the start-up of the reactor to thereby extremely lower the economy in connection with the running of the reactor. In a conventional system for preventing such an unstable phenomenon at low temperature two-phase flow, as disclosed in JP, A, 59-143997 and JP A 59-217188, at the start-up time of a natural circulation reactor, heat is supplied from a house boiler used in service inspection to cooling water in a pressure vessel of the reactor to raise the temperature of the cooling water and thereafter heating by nuclear reaction is started to thereby prevent lowering of core stability due to flow instability in the low temperature two-phase flow. In an alternative conventional process, as disclosed in JP, A, 60-69598, the temperature of a coolant inside a pressure vessel is raised through a heat exchanger so that the subcool temperature at a core inlet is set within a range smaller than the minimum sub-cool temperature to cause unstable phenomena, and thereafter output increase is started to thereby secure the stability of the core at the start-up time of the reactor. In each of the above conventional techniques, equipment of heat is supplied by equipment inside/outside a housing and no improvement is made on the equipment of the reactor primary cooling system, the start-up process and the start-up characteristics. Further, heat of nuclear reaction is not used to raise the temperature of the cooling water, so that not only heat loss is generated for heat generation by a boiler and transportation of the heat but also in order to obtain the same quantity of heat as that of nuclear reaction, it is necessary to provide a large-scale boiler or it takes a long time for the start-up of the reactor, resulting in reduction in economy. Further, since a heat exchanger and a heat supply system are provided outside/inside a housing or inside a pressure vessel to increase the temperature of the cooling water, pipings and a control system are required to make the structure of the reactor complicated and there arises a problem that the economy and reliability can not be improved. Further, even in the case of employing a process in which cooling water is heated with an extremely low quantity of heat of nuclear reaction for a very long time to thereby avoid the unstable phenomena in low temperature two-phase flow, there arises a problem that it takes a very long time for the start-up of the reactor and the economy in connection with the start-up of the reactor can not be improved. SUMMARY OF THE INVENTION A main object of the present invention is therefore to provide a boiling water reactor and a start-up process of the reactor, in which the flow fluctuation and the reduction in core stability due to occurrence of unstable phenomena in low temperature two-phase flow at the start-up time of the reactor can be prevented so that it is made possible to perform stable start-up of the reactor. Another object of the present invention is to provide a boiling water reactor and a start-up process of the reactor, in which the start-up time is shortened and which is superior in economy and reliability. In order to attain the above objects, according to an aspect of the present invention, provided is a start-up process of a boiling water reactor having a pressure vessel in which a core loaded with nuclear fuel is incorporated, cooling water is retained and steam is generated, wherein the process comprises: (a) a first step of pressurizing the inside of the pressure vessel from the outside of the pressure vessel and heating the cooling water while keeping the cooling water in a single-phase flow state, at the time of starting-up of the reactor; (b) a second step of making the cooling water in the pressure vessel transit from the single-phase flow state into a two-phase flow state after the first step; and (c) a third step of heating the cooling water in the two-phase flow state. In the first step, the inside of the pressure vessel is pressurized and the heating is carried out, so that the subcool temperature of the cooling water in the pressure vessel becomes high to thereby prevent boiling of the cooling water and the cooling water is heated to a high temperature in the state of single-phase flow left as it is. As a result, the cooling water is in a condition where generation of unstable phenomena is suppressed when the cooling water transits into a state of two-phase flow. That is, by the rising of the cooling water temperature, the vapor-liquid density ratio becomes small and the density difference between the density in the state of single-phase flow and the density in the state of two-phase flow becomes small, so that the flow fluctuation becomes small. Further, since the temperature of the cooling water is high, the transit from the state of single-phase flow into the state of two-phase flow becomes easy, and the flow fluctuation becomes small. In the second step, therefore, the cooling water transits into the state of two-phase flow through the state in which generation of unstable phenomena is suppressed. Thereafter, the cooling water is heated in the state of two-phase flow so that the subcool temperature of the cooling water decreases to generate boiling to make it possible to obtain the rated reactor running temperature and pressure easily. According to the above start-up process, the flow fluctuation and the reduction in core stability due to occurrence of unstable phenomena in low temperature two-phase flow at the start-up time of the boiling water reactor can be prevented to thereby make it possible to perform stable and highly reliable start-up of the reactor. Further, it is possible to shorten the start-up time of the reactor to thereby improve the economy. Specifically, in the above first step, the cooling water may be kept in the single-phase flow state by controlling the pressure in the pressure vessel so as to make the pressure in the pressure vessel higher than the saturation pressure of the cooling water corresponding to the temperature of the cooling water in the pressure vessel. Thus, the cooling water can be surely kept in the single-phase flow state. The above first step may be a step in which the inside of the pressure vessel is pressurized solely first and thereafter the cooling water is heated while controlling the pressure in the pressure vessel. Alternatively, the first step may be a step in which the pressurization in the inside of the pressure vessel is started simultaneously with start of heating the cooling water so that the heating of the cooling water and the pressurization in the inside of the pressure vessel are carried out simultaneously and parallelly with each other. Specifically, in the above second step, the cooling water may be made to transit from the single-phase flow state into the two-phase flow state at least by controlling the pressure in the pressure vessel. More specifically, the second step may be a step in which the cooling water is made to transit from the single-phase flow state into the two-phase flow state by controlling the pressure in the pressure vessel so as to make the pressure in the pressure vessel gradually approximate to the saturation pressure of the cooling water corresponding to the temperature of the cooling water in the pressure vessel until predetermined pressure not higher than the rated running pressure of the reactor is reached. Thus, the cooling water can be made surely to transit into the two-phase flow state. In this case, preferably, the control on the pressure in the pressure vessel is performed so that the pressure in the pressure vessel is kept to be substantially constant or reduced to thereby make the pressure in the pressure vessel gradually approximate to the saturation pressure. Thus, it is possible to make the cooling water be in a state of saturation surely before the pressure in the pressure vessel reaches the rated running pressure of the reactor, so that it is possible to make the cooling water in the pressure vessel transit early into the state of two-phase flow to thereby shorten the time taken for transit to the state of two-phase flow and shorten the time taken for raising temperature by boiling in the saturation state thereafter. Thus, the start-up time can be further shortened. Further, it is preferable that when the control on the pressure in the pressure vessel is performed, the quantity of heat for heating the cooling water is reduced or heating the cooling water is once stopped. Thus, the heating in the core is suppressed to present boiling so that it is possible to make the cooling water transit into the state of two-phase flow in the condition that unstable phenomena are prevented substantially completely from occurring to thereby make it possible to perform stable start-up of the reactor. Further, the above second and third steps may be performed in a single step of continuously controlling the pressure in the pressure vessel so as to make the pressure in the pressure vessel reach the saturation pressure of the cooling water corresponding to the temperature of the cooling water in the pressure vessel at the rated running pressure of the reactor. Alternatively, the above second step may be a step of controlling the pressure in the pressure vessel so as to make the pressure in the pressure vessel reach the saturation pressure of the cooling water corresponding to the temperature of the cooling water in the pressure vessel at predetermined pressure not higher than the rated running pressure of the reactor, and the above third step may be a step of increasing the pressure in the pressure vessel by heating the cooling water. Further, preferably, the above first step may include a step of calculating a first critical thermal power in the single-phase flow on the basis of respective measured values of the temperature of the cooling water, the pressure in the pressure vessel, and the flow rate of the core to thereby set amounts of withdrawal of control rods for controlling a power of the core so that the thermal power of the core becomes not larger than the first critical thermal power, and the above third step may include a step of calculating a second critical thermal power in the two-phase flow on the basis of respective measured values of the temperature of the cooling water, the pressure in the pressure vessel, and the flow rate of the core to thereby set the amounts of withdrawal of the control rods so that the thermal power of the core becomes not larger than the second critical thermal power. Thus, it is possible to perform stable start-up of the rector and to further shorten the start-up time of the reactor. Preferably, the above second and third steps may include: a step of controlling related valves so as to keep the water level in the pressure vessel at a proper value on the basis of respective measured values of the pressure and water-temperature in the pressure vessel, the water temperature at an inlet of the core, the water level in the pressure vessel, the power of the core and the amounts of insertion of control rods; and a step of controlling respective openings of related values to make a flow rate of feed water proper on the basis of respective measured values of the power of the core and the subcool temperature at the inlet of the core. Further, preferably, the reactor related to the start-up process according to the present invention may comprise a main steam line for feeding steam generated in the core to a turbine, a feed water line for feeding condensate water condensed in a condenser after driving of the turbine into the pressure vessel as cooling water, a main steam isolation valve, a turbine steam stop valve for stopping a steam flow into the turbine, and a control valve for controlling a steam flow rate into the turbine which are arranged in the main steam line, a feed water pump and a feed water stop valve which are arranged in the feed water line, a turbine bypass line for connecting the main steam line to an inlet of the condenser at a portion of the main steam line between the main steam isolation valve and the turbine steam stop valve, a turbine bypass stop valve arranged in the turbine bypass line, control rods for controlling the power of the core, and a pressure regulator provided in at least one of the pressure vessel, the main steam line and the feed water line; and in the start-up process according to the present invention, the above first step may include a step of isolating the pressure vessel by closing the main steam isolation valve and the feed water stop valve, a step of pressurizing the inside of the pressure vessel by the pressure regulator, and a step of heating the cooling water in the single-phase flow state by withdrawing the control rods, and the above second and third steps may include a step in which the pressurization by the pressure regulator is released and the main steam isolation valve and the turbine bypass stop valve are opened so that the pressure in the pressure vessel is reduced and a water level is formed in the pressure vessel. Further, preferably, the reactor related to the start-up process according to the present invention may further comprise a feed water bypass line for connecting the feed water line, at a outlet side of the feed water pump, to the inlet of the condenser, and a feed water bypass stop valve arranged in the feed water bypass line; and in the start-up process according to the present invention, the above first step may further include a step of closing the turbine steam stop valve and the turbine bypass stop valve, opening the feed water bypass stop valve and operating the feed water pump to thereby circulate feed water to the condenser, and thereafter pressurizing the inside of the pressure vessel. Preferably, the above second step may includes a step of inserting the control rods to reduce the power of the core after increase of the cooling water temperature, and the above third step may include a step of withdrawing the control rods again to heat the cooling water in the two-phase flow state. Preferably, in the above first step, the cooling water is heated by nuclear reaction while keeping the cooling water in the single-phase flow state by making the pressure P1 in the pressure vessel satisfy the condition P1&gt;P2 while keeping the condition T2&lt;T1-Tb, where T1 represents the saturation temperature of the cooling water at the pressure P1, T2 represents the temperature of the cooling water, P2 represents the saturation pressure corresponding to the temperature T2, and Tb represents the maximum value of core inlet subcool temperature to start boiling. Alternatively, in the above second step, the cooling water may be made to transit from the single-phase flow state into the two-phase flow state by making the pressure P1 in the pressure vessel satisfy the condition P1&gt;P2 while keeping the condition T1&lt;T2+Ts, where Ts represents the maximum value of core inlet subcool temperature in a region in which stable boiling occurs, or may be made to transit from the single-phase flow state into the two-phase flow state by making the pressure P1 in the pressure vessel satisfy the condition P1=P2 while keeping the condition T1&lt;T2+Ts. Preferably, the reactor related to the start-up process according to the present invention may comprise an electric heater provided in at least one of the pressure vessel, the main steam line and the feed water line; and in the start-up process according to the present invention, in at least one of the above first, second and third steps, the cooling water may be heated by nuclear reaction and at the same time heated by the electric heater. Alternatively, in at least one of the above first, second and third steps, the cooling water may be heated by nuclear reaction and at the same time heated by heat due to rotation of a feed water pump being operated. Thus, by use of a step of heating by nuclear reaction and heating by any other method, the start-up time of the reactor can be further shortened. Further, the reactor related to the start-up process according to the present invention may comprise a start-up feed water line formed by bypassing a feed water line and having a start-up feed water stop valve, a cooling water outlet of the start-up feed water line being connected to a portion below the core in the pressure vessel; and in the start-up process according to the present invention, in at least one of the above first, second and third steps, the cooling water is heated by nuclear reaction and at the same time the cooling water is forcedly circulated to the core by a feed water pump through the start-up feed water line to thereby increase the core flow rate. Thus, by use of a step of heating by nuclear reaction and heating by any other method, the start-up time of the reactor can be further shortened. Further, in order to attain the above objects, according to another aspect of the present invention, provided is a boiling water reactor which comprises: a pressure vessel in which a core loaded with nuclear fuel is incorporated, cooling water is retained and steam is generated; pressure regulator means disposed outside the pressure vessel and made communicate with the pressure vessel for pressurizing the inside of the pressure vessel at the time of starting-up of the reactor; and control means for actuating the pressure regulator means to operate at the time of starting-up of the reactor. Preferably, the above pressure regulator means may include a pressurized tank connected to a feed water line, a high pressure gas tank connected to the pressurized tank, means provided between the pressurized tank and the gas tank for supplying a gas from the gas tank to the pressurized tank, and means for controlling discharge of the gas out of the pressurized tank. The above pressure regulator means may include a leakage test system connected to a feed water line for testing leakage of the pressure vessel and a reactor primary cooling water line. The above pressure regulator means may include a pressurized tank connected to a feed water line and provided therein with an electric heater. The above pressure regulator means may include a high pressure gas tank connected to one of the pressure vessel and a main steam line.