Patent Number: 039473198
Section: summary

The invention relates to a nuclear reactor plant intended for supplying heat to a steam generator, which plant comprises at least two circuits for conveying this heat, these circuits being hydraulically separated but thermally coupled to each other by means of heat exchangers, specifically one water-steam-circuit equipped with a steam generator, a feed water pump which is included in a feed water conduit with a feed controlling valve as well as with a steam turbine, and one reactor-circuit equipped with a primary circulation pump and charged with a non-water, heat-transferring medium, in such a way that the heat exchangers in the operative state exhibit a relatively small pressure drop. The control systems hitherto known for such a reactor system have been marked by a high degree of complexity. Applicant has arrived at the insight that it is also possible to attain very good results with a control system of very simple design. According to the invention, this aim is reached by dimensioning the feed water conduit with the feed water control valve contained therein in such a way that the pressure drop along this feed water conduit at full load is greater than 10 bars. In proportion as the pump characteristic is steeper, this pressure drop can become somewhat smaller without any objection. If a pump is selected having a steep curve indicating the correlation between lift and output, this will furthermore provide the advantage of such a pump being lower in price. In a given design, use may be made, for example, of a feed pump marked by an almost linear correlation between lift and output. This curve may be actually curved or may be a virtually straight line which, at a constant pump speed, indicates the correlation between lift (=pump pressure) and output, and is in this connection called steeper in proportion as, with a given drop in output, the lift of the pump rises. This curve is in the following sometimes designated as "pump characteristic." Since it is sometimes not a simple matter to realize a fairly appreciable pressure drop in a control valve, use may be made, if required, of a hydraulic turbine. In this liquid turbine, at least part of the pressure drop can be realized which is necessary for the stability of the system. An additional advantage of this is that energy is recovered in the liquid turbine. The liquid turbine, which often needs to comprise only one step, can advantageously be accommodated in the housing of the feed water pump, the blade wheel of this turbine being fastened on the rotor of the pump. A bypass valve, arranged in a bypass of the turbine, in such case regulates the amount of feed water flowing through the turbine. In many cases, the aforementioned hydraulically separated but thermally coupled circuits are separated from each other not only by heat exchangers but also by an additional heat-transferring circuit. This measure will be taken specifically in the case of a nuclear reactor plant provided with a sodium-cooled reactor. In such a case, the precaution is taken of providing an additional heat-transferring circuit for hydraulically separating the primary circuit and the steam-water-circuit; said additional or so-called secondary circuit being equipped with a secondary circulating pump and containing a non-water heat-transferring medium. The three hydraulically separated circuits then are thermally coupled by means of an intermediate heat exchanger for conveying heat from the primary or reactor-circuit to the secondary circuit and the steam generator. For a correct understanding of the following, it is pointed out that a sodium-cooled reactor, on account of the secondary cooling circuits with prolonged dead times (time lags) for heat transportation, gives rise to processes which are difficult to control. In this respect, it has been considered that the use of a steam-water-separator in the sodium-heated steam generator presents advantages with respect to the control of temperature in the steam generator. A consequence of using such a water-steam-separator, however, is that this steam generator gives rise to the formation of a positive feedback between live steam pressure and feed water flow. An important element of the present invention is the insight that this undesired feedback can be simply eliminated by taking the required measures for the pressure drop through the feed water conduit and the control valve, to have a value of at least 10 bars. The aforementioned steam separator is necessary, because the evaporator of the steam generator produces somewhat wet steam. The excess water is separated from the mixture in the steam-water-separator, so as to return it to the feed water preheaters. It has been found in practice that several factors are decisive for the stability of the control system of the steam generator. This steam generator system, consisting of a feed water pump, a control valve, an evaporator, a steam-water-separator, a superheater and a turbine-inlet-valve exhibits -- without the control circuits -- a negative feedback effect on account of the thermal behaviour thereof. In certain circumstances, however, the same system exhibits a positive feedback effect on account of the hydraulic behaviour. The presence of a negative thermal feedback (back coupling) can be observed by an increase in the flow of feed water through the evaporator. Since, in this case, more water must be heated to boiling temperature, less steam will be produced, resulting in a drop of the flow of steam through the superheater. Since the conditions of steam admission to the superheater through the steam water separator must be kept constant, the power transmitted from the sodium to the steam is proportional to the quantity of steam flowing per unit of time through the superheater. A reduced steam production gives a higher sodium outlet temperature at the superheater, causing more power to become available for producing steam in the evaporator. As a result, the entire system will rapidly find its new state of equilibrium. The fact, however, that a positive hydraulic feedback can also arise can be understood by realizing that a change occurs in the position of the turbine inlet valve. An increase in the valve passage of the turbine causes an increase in the amount of steam flowing through the steam conduit, as well as a pressure drop in this steam conduit, in the superheater and in the steam-water-separator. Now the mass flow through the evaporator is directly proportional to the square root of the pressure difference that prevails between the steam-water-separator and the feed water pump. This pressure difference increases as a result of the decrease of pressure in the steam-water-separator. The eventual increase of the feed water flow causes a decrease in the steam flow through the superheater, which in turn causes a decrease of the live steam pressure. In this manner, variations can occur in the steam pressure without leading to a new stable state. It is a fortunate circumstance that the increase in the amount of feed water likewise produces a decrease of the feed water pump pressure as a result of the pressure output characteristic of this pump. If this decrease is of the same order of magnitude as the decrease in pressure of the steam-water-separator, a new, stable adjustment can indeed be reached in operation. The output characteristic of the great majority of feed water pumps exhibits a slight inclination at low outputs. Accordingly, the opposing effect is too small for our purpose. Measures must therefore be taken for the increase in feed water flow, resulting from the pressure drop in the steam-water-separator, to be reduced at low loads. This can be done in a simple manner by introducing an extra pressure drop or resistance in the feed water conduit. According to calculations, a pressure drop of 20 bars at a load of 30 percent is sufficient for ensuring a stable behaviour of the steam generator. This pressure drop should be about 10 bars at full load for attaining the same stabilizing effect. According to the invention, the control system is furthermore so designed that a control impulse coming from the measured amount of water separated in the external water separator gives an impulse to the feed water control valve. A steam generator system equipped with such a control circuit exhibits the following behaviour: A drop of the live steam pressure owing for instance to a greater turbine steam flow, causes an increase in the feed water flow. This causes an increase in the draining of condensate collected in the steam-water-separator. The control circuit over this condensate drainage will slightly close the feed water valve, resulting in a decrease of the feed water flow as well as in a decrease of the steam condensate drainage and an increase in pressure, because of an increase both in the flow of steam to the superheater and the supply of heat. This pressure increase brings about a further decrease of the feed water flow, and thus a still further decrease in the drainage from the water separator. This decrease will again induce the control circuit to open the feed water valve still further, until a correct and stable state has fairly rapidly been established. Now if the coupling factor between the live steam pressure changes and the change in the drainage of condensate from the steam water separator is too high, this control circuit can become unstable. This can be remedied by reducing the coupling factor by increasing the pressure drop in the feed water conduit. It has already been explained in the above that other considerations have also led to the finding that a pressure drop of 20 bars will ensure at all loads a stabilizing dynamic behaviour of this control circuit. According to a preferred embodiment of the invention, the set-point controls of the regulators for the tertiary and for the primary circuit are disconnected, whereby the process has become well regulable. This is preferably accomplished by taking such measures that a control impulse coming from the measured steam pressure, or the measured amount of steam per unit of time, or a combination of these measured values, corrects the mass flow of the secondary circuit, by influencing the speed of the secondary circulating pump. This measure can be effectively supplemented by causing a control impulse from the mass flow measured in the secondary circuit to correct the mass flow of the reactor circuit by influencing the speed of the primary circulating pump. Finally, it has been found effective to cause a control impulse from the mass flow measured in the secondary circuit to adjust the set point of the reactor temperature regulator. With the use of the control method described, according to which the reactor outlet temperature changes as a function of load, the temperature of the live steam does not have to be separately regulated. Calculations have shown that this temperature during very fast load changes, such as, for example, 10 percent of load in 5 seconds, changes by only about 6.degree.C, during a very short time, approximately 30 seconds. The control system according to the invention can be load-following as well as load-forcing. According to the latter method, the secondary sodium pump is controlled with a desired power signal, and the live steam pressure is constantly controlled with the turbine valve, so-called prepressure (initial pressure) control. With the aid of the following figures, two embodiments of the invention will be explained in further detail.