In the case of generators of steam from pressurized-water nuclear reactors the regulation of this water level is rendered difficult by the size of the flows or rates of recirculation necessary.
In the case of generators of steam from pressurized-water reactors the generator consists of an enclosure of large dimensions inside which are mounted tubes fixed into a tube plate and conveying the primary fluid which is water under pressure. The enclosure likewise receives feedwater which comes to fill the generator up to a certain level and circulates in contact with the tubes conveying the primary fluid during its time spent in the steam generator. This contact with the primary-fluid tubes enables vaporization of the feedwater in the upper portion of the steam generator, this steam being sent to the turbine and feedwater coming to replace in the steam generator the water which has been evaporated.
During the course of this dynamic process automatic regulation of the water level in the steam generator is necessary and safety devices are provided for the case where the water level deviates from the chosen reference level.
During the operation of the nuclear reactor disturbing elements intervene to produce more or less large variations in the water level. These disturbing elements are, for example, variations in the flow of steam as a function of the power required of the turbine, the flow and temperature of the feedwater and the temperature of the primary circuit, which likewise depend amongst other things upon the power level demanded with respect to the nominal power. Other elements may likewise intervene casually at the time of abrupt variations in load or of faulty operation of the reactor.
Hence the duty of the device for automatic regulation of level is to maintain this water level in the steam generator as constant as possible in spite of these disturbing elements during the operation of the steam generator. The level regulation device includes in general a unit enabling the measurement of the real instantaneous level in the steam generator, the comparison of this level with a reference level, the working out of a deviation signal proportional to the difference .xi. between the measured water level and the reference level and the introduction of this signal into a regulator which enables the inlet flow of feedwater into the steam generator to be modified by way of valves. In general two valves are employed of which one is employed for flows higher than 15 or 20% of the nominal flow of water and the other for flows lying between 0 and 15 or 20% of the nominal flow of feedwater.
The feedwater is itself put back into circulation by a circuit which collects the water recovered at the outlet of the condenser of the turbine and includes a set of reheaters which recover the residual heat in the steam before draining to the condenser.
Thus the temperature of the feedwater is a function of the power level demanded at the turbine.
The proportionality factor or gain by which the signal is multiplied, which represents the deviation in level for working out the signal introduced into the regulator which enables control of the valves is a linear function of the power level with respect to the nominal power, that is to say, of the ratio of the real power to the nominal power. In reality as there exists an unequivocal relationship between the temperature of the feedwater and the power level, under normal conditions of operation the parameter which is taken into account for the determination of the gain is this temperature of the feedwater.
In the regulation chains at present employed the linear variation of the loop gain is a function of the main parameter, that is to say, of the temperature of the feedwater which directly influences the dynamics of the process and is representative of its load level. In general a very low regulation loop gain is imposed at low load so as to ensure good damping.
In practice, in the devices considered above for regulation of the steam generators of pressurized-water reactors the gain varies between 1 and 8 when the power passes from the value 0 to the nominal value.
The result is that at low load and at low temperature of the feedwater the gain is a minimum, which considerably reduces the performance of the regulation device. This minimum gain does not enable the transitory disturbances to which the installation may be subjected, to be effectively compensated, with as a consequence poorly controlled evolutions of the water level which may have the effect of letting the process develop towards dangerous zones of operation which impose the entering into action of the safety systems of the installation.
These phenomena are particularly sensitive and troublesome in the case of the starting-up of the installation when the power and the feedwater temperature are low with the result that the low static gain does not enable the production of signals which are sufficient to subdue large disturbances during the increase in power. In reality the difficulties connected with the appearance of casual disturbances during the starting-up period lead operators to avoid the use of the automatic regulation device and to carry out starting-up manually and make necessary a considerable mobilization of operators.
Hence it is seen that the reduction of the gain in the regulation loop under the conditions when instabilities appear, introduces a risk of bringing about variations in level beyond safety limits and the setting into operation of the safety devices upon the secondary portion or upon the nuclear portion of the reactor through inadequacy of the actions upon the feedwater flow caused by signals of low amplitude.
In particular, if the rise in power demands a very rapid increase in the flow of steam the supply of water by the low-flow valve employed at low load may be insufficient and the steam demand may cause abrupt lowering of the water in the steam generator which may bring about emergency stopping of the reactor.