Patent Number: 047284854
Section: summary

FIELD OF THE INVENTION The present invention relates to a method for regulating the pressure of the primary circuit during the shut-down phases of a pressurized water nuclear reactor, and more particularly for regulation during the so-called "collapse" phase of the pressurizer. DESCRIPTION OF THE PRIOR ART The primary cooling circuit of a pressurized water nuclear reactor includes one or several loops wherein water circulates under the action of a primary pump between the core of the reactor where it absorbs the heat produced by the nuclear reaction, and a generator where it gives up this heat to the secondary fluid which is vaporized therein to supply turboelectric machines. When the reactor operates at full power, the primary water is at a temperature of the order of 320.degree. C. in the core, and to keep it in the liquid state the pressure in the primary circuit is maintained at about 155 bars. This pressure is regulated by the action of a pressurizer which operates by heating or spraying cold water. If, for example, the pressure tends to diminish, a portion of the water of the pressurizer is vaporized by a heating rod system so as to increase the volume and the pressure of the steam cushion which occupies the upper dome of the pressurizer, which pressure is communicated to the whole of the primary circuit through an expansion pipe opening into the bottom end of the pressurizer. On the contrary, a reduction in the primary pressure is obtained by sprinkling cold water on the vapor phase at the upper part of the pressurizer, which has the effect of condensing a portion of this vapor. FIG. 1 shows, in extremely simplified diagrammatic manner, a prior art primary circuit denoted symbolically as a whole by 1 surrounded by a heavy mixed dash-dot line, which groups the core 2, a primary pump 3 and a steam generator 4. The expansion pipe of the pressurizer 5 is tapped on the hot water piping, between the core of the reactor and the steam generator. Quite usually, a so-called "reactor chemical and volume control" system (RCV system) is arranged in parallel with the primary circuit. The RCV system includes a discharge line 8 with two successive expansion stages. The first "high pressure" stage is constituted by a system of expansion orifices 9; the regulating valve 10 constitutes the "low pressure" stage. Each expansion stage is normally preceded by a cooling stage (not shown in the diagram). The discharge flow rate in the line 8 is a function of the number of orifices 9 put into service. The pressure in the discharge line is controlled by the valve 10, slaved to a pressure signal taken at 11 immediately upstream of the valve, and acting on the regulator 12. The RCV circuit then includes a volume control tank 13, normally provided with a level regulator (not shown in the diagram). The RCV circuit is closed by a charging line 14 comprising a pump 15 which reinjects into the primary circuit, through a charging regulating valve 17, the fluid discharged or the added supplementary water introduced at 16. The charging flow rate is servo-coupled through a regulator 18 to the water level measured at 19 in the pressurizer 5 and to the flow rate signal measured at 20 downstream of the valve 17. The circuit 22 for sprinkling cold water on to the vapor phase of the pressurizer is taken as a branch line from the charging line 14 and ends at the sprinkling control valve 23. When it is desired to shut down the reactor, after, of course, stopping the nuclear reactions by lowering the rods, it is necessary to bring the primary circuit back gradually to a lower temperature and pressure. In the primary phase, which enables it to be brought to about 30 bars and 180.degree., steam is discharged to the principal condenser or to the atmosphere, keeping at least one primary pump in operation for uniform cooling of all the loops. At the same time, a sprinkling follows in the pressurizer to reduce the pressure, while maintaining a minimum value necessary to permit the operation of the pumps. During this primary phase the primary water contained in the pressurizer is in a bi-phase liquid-vapor state at saturation equilibrium. In a subsequent second phase, and while keeping a primary pump in service, the cooling circuit of the reactor on shut-down is brought into play (RRA circuit) constituted by a closed circuit with tapping and delivery into the primary loops, a part of this delivery being effected at 25 (FIG. 1) in the RCV circuit. During this second phase however, wherein it is desired to reach a temperature of 70.degree., a pressure of at least 23 bars which is the minimum operating pressure of the primary pumps must be preserved. These conditions can only be reached in a mono-phase medium if it is desired to avoid thermal stresses which are too high on the expansion piping of the pressurizer. For this second phase it is therefore necessary to reabsorb the steam "bubble" located at the top of the pressurizer. Although in the initial bi-phase condition it is the pressurizer which regulates the primary pressure, in a subsequent mono-phase condition the primary pressure will be regulated by the expansion valve 10 of the discharge line, the charging valve 17 then having a constant flow rate setting since the liquid level at the pressurizer is constant and maximum. The flow rate from the discharge line 8 is no longer provided through the high pressure expansion orifices 9 which are provided for a large pressure drop, but rather by the delivery at 25 from the RRA circuit. However, the reabsorption of the steam bubble, between the bi-phase state and the mono-phase state, passes through an intermediate phase, and the bubble only occupies the very top part of the pressurizer, and during this phase the evolution of the liquid level cannot be followed since it is difficult to place measuring members in the upper part of the dome of the pressurizer close to the sprinkling ramps. In this intermediate phase the normal regulation by the low pressure discharge valve 10 presents the drawback of not taking the pressure from the primary directly as reference pressure, but of taking this from a circuit which is not representative of this primary pressure, either statically or dynamically. Hence it is not sufficient to control the pressure of the primary circuit reliably during the final reabsorption phase of the vapor bubble at the top point of the pressurizer. For rapid pressure variations the bubble constitutes an adiabatic system which does not absorb heat. Hence it can only be reabsorbed with difficulty under the action of pressure, since this reabsorption must pass through a reequilibration of the temperature, which is a long and poorly defined action. Moreover, the indirect observation of the evolution of the bubble by examination of the charging and discharging flow rates is falsified by variations in volume of the primary circuit under the effect of temperature variations. This imprecision on the evolution of the bubble can lead to unforeseen excess pressure in the primary fluid capable, for example, of resulting in the opening of cooling circuit valves on shut-down. OBJECT OF THE INVENTION It is an object of the invention to provide a method for regulating the pressure in the primary circuit during the phases of shut-down of a pressurized water nuclear reactor, which overcome the aforesaid drawbacks. It is another object of the invention to provide a method which enables effective and positive reabsorption of the latter portion of the vapor bubble by working at substantially constant pressure, and by monitoring the evolution of the bubble. Other objects and features of the invention will be apparent from the description which follows. SUMMARY OF THE INVENTION According to the invention, there is provided a method for the regulation of the pressure of the primary circuit during the shut-down phases of a pressurized water nuclear reactor, wherein, when the level of the liquid reaches the upper portion of the pressurizer where the detection of the level becomes inoperative, a constant level is maintained at the charging pipe, the discharging flow rate is adjusted by rapid acting regulation from a direct measurement of the pressure of the primary circuit, successive sprinklings at the pressurizer then follow and the reduction in the discharging flow rate which is caused immediately by the reduction in pressure due to the condensation in the pressurizer is detected, and this until a further sprinkling no longer has an effect on the discharge flow rate, the cooling then being able to continue with pressure regulation of the primary through the discharge valve. In a preferred embodiment of the invention, the method is rendered automatic by servo-coupling the flow rate of the sprinkling control valve from measurement of the discharge flow rate, by using a regulation with a slower reaction speed than that of the regulation of the discharge valve from the primary pressure.