Method for controlling a pressurized water nuclear reactor during stretchout

Abstract

A method for controlling a pressurized water nuclear reactor is provided, including core producing thermal power, sensors for acquiring the mean temperature of the primary coolant and for calculating the thermal power, actuators for controlling the axial distribution of power, the control method including: a first control phase for controlling the reactor during normal operation by controlling the mean temperature of the primary coolant so as to make it correspond to a reference temperature profile (P.sub.ref) dependent on the thermal power of the reactor; and a second control phase, referred to as stretchout, that occurs after normal operation of the reactor in order to control the reactor in stretchout by controlling the axial distribution of power, the mean temperature varying freely in a temperature range delimited by an upper limit and a lower limit.

Claims

1. A method for controlling a pressurized water nuclear reactor comprising: a core producing thermal power; control rod clusters and a boron injection system configured to control the mean temperature of the primary coolant from the core and an axial distribution of power; the control method comprising: a first control phase comprising a step of controlling the reactor during normal operation by moving the control rod clusters in the core so as to make the mean temperature of the primary coolant correspond to a reference temperature profile (Pref) dependent on the thermal power of the reactor; and a second control phase, referred to as a stretchout phase, that occurs after normal operation of the reactor, when the fuel of the core is used up such that a concentration of boron in the primary coolant is below 50 ppm when the thermal power of the nuclear reactor is 100% of nominal power, said second control phase comprising a step of controlling the reactor by movements of the control rod clusters, wherein the mean temperature of the primary coolant evolves freely in a temperature range having an upper limit and a lower limit, wherein the upper limit of the temperature range corresponds to the reference temperature profile (Pref) during normal operation of the reactor, and wherein the lower limit of the temperature range corresponds to the reference temperature profile (Pref) during normal operation of the reactor with a shift of Y C., Y being between 5 and 50, or the lower limit corresponds to a fixed temperature equal to a reference temperature at 100% of nominal power with a shift of Z C., Z being between 10 and 50.

2. The method for controlling a pressurized water nuclear reactor according to claim 1, wherein in the stretchout phase, the control of axial distribution is brought about by moving the control rod clusters situated above a mid-height of the core such that lower ends of the control rod clusters move between an upper part and the mid-height of the core.

3. The method for controlling a pressurized water nuclear reactor according to claim 1, wherein in stretchout phase the control of the axial distribution of power is automated.

4. The method for controlling a pressurized water nuclear reactor according to claim 1, wherein Y is between 5 and 30.

5. The method for controlling a pressurized water nuclear reactor according to claim 1, wherein Z is between 20 and 30.

6. The method for controlling a pressurized water nuclear reactor according to claim 1, wherein the second control phase further comprises: a first sub-phase during which the thermal power of the reactor is kept at 100% of nominal power by increasing an opening of turbine inlet valves; and a second sub-phase during which the turbine inlet valves are fully opened and the thermal power of the reactor decreases.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the invention will become clearer from the description that is given thereof below, for indicative purposes and in no way limiting, with reference to the figures.

(2) FIG. 1, described previously, illustrates the different temperature regulation programmes in the course of stretchout phase as well as the evolution of temperature as a function of power in the course of this phase during operation at maximum power according to the prior art.

(3) FIG. 2 illustrates an example of evolution of temperature as a function of power during a drop in load that occurs in the course of the stretchout phase according to the invention.

(4) FIG. 3 illustrates the evolution of temperature as a function of power during a drop in load that occurs in the course of the stretchout phase according to the prior art.

DETAILED DESCRIPTION

(5) As described previously in FIG. 1, when the reactor goes into stretchout after a cycle of normal operation, a first phase consists in conserving as much as possible the thermal power of the reactor at 100% of nominal power (PN). This first phase is illustrated in FIG. 1 between points A and B. The maintaining at 100% of nominal power is achieved by increasing the opening of the turbine inlet valves as the pressure of the secondary circuit (and the mean temperature) drops. The end of this first phase corresponds to reaching the full opening of the turbine inlet valves (point B).

(6) The turbine inlet valves being completely open, the power can no longer be maintained at 100% when the steam pressure drops. The maximum thermal power of the reactor thus drops as the mean temperature of the primary coolant drops and thus as the steam pressure drops. This phase is represented in FIG. 1 between points B and H. It occurs after the first phase described previously and corresponds to the evolution of the power of the reactor at full power limited by the steam pressure at the turbine. This limit is designated hereafter turbine limit and is represented by the dotted line referenced LT in FIG. 1.

(7) According to the prior art, in the course of this A-B then B-H phase described previously, the temperature programme is shifted by a programme P.sub.1 to P.sub.i+1 as the temperature drops when the temperature reaches the lower limit of the dead band of the temperature programme P.sub.i.

(8) According to the method of the invention, the evolution of the temperature of the reactor at maximum power is also limited by the steam pressure at the turbine. On the other hand, the temperature evolves freely and is no longer regulated according to a temperature regulation programme P.sub.i. Nevertheless, when maximum power is maintained in the course of stretchout, the evolution of temperature as a function of power remains identical to the evolution described previously according to the prior art.

(9) FIGS. 2 and 3 illustrate the gain provided in stretchout by the control method according to the invention, particularly during a drop in load.

(10) More particularly, FIG. 2 illustrates an example of evolution of temperature as a function of power according to the invention during a drop in load that occurs in the course of the stretchout phase described previously.

(11) As a comparison, FIG. 3 illustrates the evolution of temperature as a function of power during a drop in load that occurs in the course of the stretchout phase according to the control method of the prior art.

(12) With reference to FIG. 2, during the realisation of a drop in load, for example with a low stage at 80% of nominal power, from point C with the control method of the invention controlling exclusively the axial distribution of power by insertion of the control rod clusters, the mean temperature of the primary coolant is typically going to increase a little given the cumulative effects of the insertion of the rod clusters for controlling the axial distribution of power and the compensation of the other effects of reactivity by the mean temperature. This drop in load is rapid (several minutes) and is represented in FIG. 2 between points C and D. At the low stage of the drop in load (80% of nominal power), xenon growth is offset by the possible drop in the mean temperature without modification of the nominal power (point D to G). Thus, following this drop in load, the reactor can continue to operate at the low stage as long as the mean temperature has not reached the turbine limit LT (point G). Once the temperature reaches the turbine limit, dependent on the xenon concentration in the primary coolant, the mean temperature can drop (when the xenon concentration is high) consequently leading to a drop in the thermal power of the reactor (point G to H) or may also increase (when the xenon concentration is low) making it possible to achieve a rise in power to reach the maximum power possible for example by following the turbine limit LT (point G to H, or even C). Throughout the phase, the axial distribution of power is continued to be controlled by movements of the control rod clusters.

(13) Thus, following a drop in load, as described previously, the control method according to the invention enables the reactor to be made to operate longer at its maximum power.

(14) Moreover, the control method according to the invention also enables a rise in power following a drop in load, as represented by the dotted line between points E and F, as long as the turbine limit is not reached. In an identical manner, when the temperature reaches the turbine limit, the variation in temperature (increase or decrease) will depend on the xenon concentration at the moment the turbine limit (point F) is reached.

(15) It is also possible to define the upper and lower limits of evolution of the mean temperature of the primary coolant. The upper limit may for example be the reference temperature regulation profile as a function of the power of the reactor during normal operation P.sub.ref. The lower limit may for example be the temperature regulation profile as a function of the power of the reactor shifted by Y C., with Y comprised between 5 and 50, and preferentially between 5 and 30, and corresponding to the temperature profile P.sub.n at the end of the stretchout phase. As an example, the duration of the stretchout phase is generally 30 days and may last up to 60 days.

(16) In a variant, the lower limit, referenced T.sub.min in FIG. 2, corresponds to a fixed temperature equal to the reference temperature at 100% of nominal power with a shift of Z C., Z being comprised between 10 and 50, and preferentially between 20 and 30.

(17) The temperature range thereby limited by the upper and lower limits is practically contained in the range which has been the subject of a safety study in stretchout phase according to the method of the prior art.

(18) In situations where the mean temperature of the primary coolant reaches the upper limit or the lower limit of the temperature range in which the temperature can vary freely, it is possible to intervene by acting as a priority on the rod clusters if that is possible, then on the power and finally on the boron concentration if it is not possible to act on the power.

(19) It is possible to use as actuators for controlling the temperature of the primary coolant both the control rod clusters and the boron injection system. The same is true for the actuators for controlling the axial distribution of power.

(20) The sensors for acquiring the mean temperature of the primary coolant are for example sensors for measuring the temperature of the primary coolant situated in the hot branch and in the cold branch of the primary circuit (the mean temperature then being calculated by determining the mean of the hot branch and cold branch temperatures).

(21) The thermal power may for example be calculated using the difference in temperatures measured by the sensors situated in the hot branch and in the cold branch of the primary circuit.

(22) Thus, as an example, when the mean temperature reaches the upper limit of the temperature range, the first action consists in inserting the rod clusters if that is possible. Nevertheless, if the axial distribution of power is heading too much towards the bottom of the core (that is to say that there is a greater flux in the bottom of the core than in the top), an insertion of the control rod clusters will not be possible because that would even further unbalance the axial distribution of power, then it is possible to increase the power if it is not already at its maximum power and potentially extract at the same time the control rod clusters. If it is not possible to increase the power because it is already at its maximum limit, then a boration action may be carried out.

(23) If the temperature reaches the lower limit, for example from point H in FIG. 2, as a priority the control rod clusters are extracted if the rod clusters are not at the upper limit in the core and if the axial distribution of power is not heading too much towards the top of the core. If an extraction of the control rod clusters is not possible, then the power is reduced and the rod clusters are inserted if necessary.

(24) Using the principle of regulation according to the prior art, the regulation of the mean temperature of the reactor would have imposed regulating the drop in temperature following the drop in load following the reference temperature profile and thus to reduce the thermal power of the reactor. For comparison, the behaviour of the reactor according to the same conditions but controlled according to the principle of control of the prior art is represented in FIG. 3.

(25) When a drop in load is realised from point C identical to that described previously with reference to FIG. 2, the control method according to the prior art is going to impose regulating the temperature so that the mean temperature of the primary coolant corresponds to a temperature profile Pi. This drop in load is represented in FIG. 3 between points C and D. Following this drop in load, the drop in the mean temperature is going to continue to be regulated according to a same temperature profile (points D to E) implying a drop in power of the reactor taking account of the necessity of controlling the temperature without perturbing too much the axial distribution of power. This in practice can cause the operator to stop the reactor shortly after this drop in load.

(26) Furthermore, according to the prior art, it is not possible to operate at constant power up to the turbine limit following a drop in load.

(27) The control method in stretchout phase is applicable whatever the control mode of the reactor. Thus, if the reactor has several types of control rod clusters with different neutron absorptivities, the regulation of the axial distribution of power in stretchout phase is identical but with an additional freedom of adjustment which makes it possible to further optimise the control of the reactor during this stretchout phase.

(28) The control method according to the invention has been particularly described with a linear temperature regulation programme, nevertheless, the invention is also applicable whatever the profile of the temperature regulation programme.