Patent GB-1476937-A (GB)
Title: Method and apparatus for controlling anuclear reactor
IPC: G21C7/36
Filed: 1974-05-17
Abstract: 1476937 Controlling nuclear reactors COMBUSTION ENGIne 17 May 1974 [22 May 1973] 22155/74 Heading G6C Violation of a design limit in a nuclear power generating system is prevented by calculating the value of a function of several system parameters and the projected value of the function due to projected changes in the parameters. The function used is the "Departure from Nucleate Boiling Ratio" (DNBR) which is a function of several system parameters including coolant pressure, coolant mass flow rate, reactor power &c. For safe operation the value of DNBR should not fall below a fixed value, say 1.3. The calculations may be performed by hard wired analogue devices or special purpose digital computers. The arrangement uses four independent channels, only one of which is described below, and majority logic. A core protection calculator calculates the value of DNBR from samples of signals indicative of system parameters, the samples being taken every two seconds. The value of each of the parameter signals is also monitored continuously, or at least at a rate substantially higher than the sampling above, and is compared with the most recent sampled value. Any change in each parameter is multiplied by a factor to give a conservative estimate of the resulting change in DNBR and all the resulting DNBR changes are summed and used to update the most recent value of DNBR from the sampled signals. The rate of change with time of the parameter, to changes in which the value of DNBR is most responsive, is also derived, and multiplied by the partial derivative of DNBR with respect to the parameter and by a period of time, chosen to allow predicted limit violations to be detected and a reactor shut down to be initiated without a limit violation occurring, to give a projected value of DNBR. The core protection calculator thus provides a high speed calculation of DNBR and projected DNBR which is highly responsive to changes in system parameters but of limited accuracy. A further, slower, but more accurate calculator is used to ensure that the system is operated with a sufficient safety margin in one of three ways. In the first a value of DNBR incorporating a margin, say 1.8, is chosen and from this value and the real value of all but one of the parameters, a value of the remaining parameter, e.g. reactor power, is calculated and displayed to an operator who ensures that the actual value of the parameter remains below the displayed value. Alternatively the reactor power calculated as above, the maximum power rating of the reactor, and a value of power calculated from a different expression are fed to a device which selects the lowest. Actual power is compared with the selected value and the reactor automatically shut down if necessary. In the second the reactor power is calculated as above but using the limit value of DNBR (i.e. without a margin) and the resulting value is modified to give the margin. In the third the value of one of the parameters is modified as a function of the condition of the system to give the margin and used to calculate a value for reactor power. In general the reactor system is operated in a steady state in accordance with the result of the more accurate calculation above with the core protection calculator being used to shut down the reactor in the event of an accident, e.g. loss of power to the coolant pumps. The shut down action of the core protection calculator may be de-sensitized to avoid unnecessary shut downs by using the accurate value for DNBR (including the margin) and updating this value in accordance with the monitored parameter changes (as above). The higher of the two updated DNBR values is then selected and is added to the projected change in DNBR to produce a second projected DNBR. The lower of the two values is then selected and, if it is equal to or below the limit value, is used to shut down the reactor. A second form of the core proection calculator is described briefly and receives as inputs the system parameters one of which is modified in accordance with its rate of change to provide a projected value of DNBR. The updating is then performed on the projected value. Specifications number 1,476,938 and 1,476,939 are referred to.