GB1172110A - Fluid-Cooled Nuclear Reactors - Google Patents

Abstract

1,172,110. Nuclear power plant. NUCLEAR DESIGN & CONSTRUCTION (1966) Ltd. 15 Aug., 1968 [30 Nov., 19671, No. 54469/67. Heading G6C. In a method of operating a nuclear power plant, changes in power output are initiated as required and the control parameters of the reactor coolant circuit automatically varied in dependence upon the changes in power output in such a manner that at a selected position within the reactor core the dimensional changes, due to the variation in the control parameters, of the fuel element canning are matched with the dimensional changes, due to thermal cycling produced by the changes in power output, of the fuel within the canning. The power plant comprises an advanced gas-cooled reactor 1, a boiler 2, a main turbo-alternator 3 having a high-pressure cylinder and a low-pressure cylinder and a condenser 4 associated with the latter, a feed pump 6 and a gas circulator 9. The fuel elements (not shown) are in the form of UO 2 pellets housed in stainless steel cans. The steam generated in the boiler 2 passes to the high-pressure cylinder of the main turboalternator 3 via a throttle valve 13 which under normal operating conditions is controlled by motors 15, 16 to maintain the boiler steam pressure at a preset value. Under certain emergency conditions the valve 13 is also controlled by a governor 14. The steam flow passes through the high -pressure cylinder of the main turbo-alternator 3 and is then returned to the boiler 2 for reheating. It is fed to the lowpressure cylinder and thereafter condensed in condenser 4 from which it returns via deaerator 5, feed pump 6 and regulator 8 to the boiler 2. The feed pump 6 is driven by a turbine 7 which utilizes steam bled from the highpressure cylinder of the main turbo-alternator 3 via a throttle valve 17. The position of the valve 17 is adjusted by a motor 20 to give the feed flow demanded by the operator. To give automatic load following, a control facility is provided to allow the feed flow demanded by the operator to be automatically varied by a signal proportional to the difference between the actual grid frequency and the nominal grid frequency. This control frequency comprises a sensing device 21 which feeds a signal proportional to the grid frequency to a comparator 22 which produces a signal proportional but opposite in sign to the grid frequency error This signal is fed to a device 23, as is also a manual signal representing the normal feed flow demand for a required power output from the plant, the combined signal (i.e. the total feed flow demand signal) produced by the device 23 being fed to a comparator 25 which compares it with one proportional to the actual feed flow obtained from a sensing device 26. The resultant error signal is fed to a shaping network 27 which controls the motor 20 and thereby the throttle valve 17 so that the turbine 7 and thereby the feed pump 6 are caused to change speed until the actual feed flow equals the total demanded feed flow. The change in reactor inlet gas temperature is detected by a sensing device 28, the signal from which is compared with one proportional to the total reactor inlet gas temperature demand from a device 30 by a comparator 29, the error signal from which is fed to a shaping network 31 which controls a motor 10 by means of which inlet guide vanes (not shown) are adjusted and thereby the gas flow through the circulator 9. The gas flow is changed until the reactor inlet gas temperature attains its demanded value. The resultant change in the outlet gas temperature is detected by a sensing device 34 and by means of devices 35, 36, 12 equivalent to devices 29, 31, 10, respectively, a control rod 11 is positioned so that the outlet gas temperature attains its demanded value. As a result of these operations, the reactor power is changed in sympathy with variations in grid frequency and automatic load following is thereby obtained. The variations in reactor gas temperature needed to alleviate the effect of reactor power changes on the fuel elements are obtained automatically as follows: the total inlet gas temperature demand signal is produced by the device 30 from the addition of a manual signal 32 proportional to the operator's demand temperature and a signal from a device 33 which is a function of the total feed flow demand. An input signal to device 33 proportional to the total feed flow demand is obtained from the device 23. In this way the total demanded inlet gas temperature can be varied with power in any predetermined manner by the appropriate choice of the relationship between the total feed flow demand signal and the output signal, this relationship being built into the device 33. Thus, as the reactor power automatically varies due to the variation of the total feed flow demand initiated by the comparator 22 in response to grid frequency variations, the reactor gas inlet temperature also automatically varies in a preselected manner. In a similar manner the reactor outlet gas temperature is also automatically varied as required by the action of devices 37, 38 and a manual signal 39, which are equivalent in operation to devices 30, 33 and manual signal 32, respectively. Variations of the inlet and outlet temperatures cannot be chosen which give complete matching of the thermal expansions for all the fuel elements in the core. Because the extent of fatigue damage due to thermal cycling tends to increase as canning temperatures increase and as the highest canning temperatures occur approximately two thirds of the way up a channel from the inlet end, it is preferable to choose the variations in coolant gas temperatures to give complete matching of thermal strains in the fuel elements situated at this point in the various channels of the core.