Power Raise Through Improved Reactor Inlet Header Temperature Measurement at Bruce A Nuclear Generating Station

Abstract

For optimal performance, CANDU reactors are operated within a small reactor inlet header (RIH) temperature range during normal operation. The upper limit on reactor inlet header temperature is one of several parameters which affects critical heat flux in the reactor channel, and hence the integrity of the fuel. This limit, including an allowance for instrument accuracy, is a nuclear safety requirement which cannot be exceeded. As the boilers and feedwater preheaters have aged they have become less efficient at removing heat. The boiler primary side divider plate may also be allowing some inlet-to-outlet bypass flow on the primary heat transport (020) side. Over time this has caused the boiler outlet temperature and the reactor inlet temperature to slowly rise. At present the Bruce A units are de-rated to compensate for the increased RIH temperature. A more accurate temperature measurement would reduce the uncertainty allowance in the safety analysis and allow the station to raise the RIH temperature to a value closer to the safety limit. Hence, it could increase the plant power output and, therefore the plant's revenue. Based on the present operating state of the Bruce A units, increasing the RIH temperature by 1 degrees C would increase the Boiler Pressure 83 kPa, which could increase the output by 15 MW per unit. Currently, the nominal RIH temperature is 250 degrees C, and includes an inaccuracy of as much as +/- 3 degrees C. This inaccuracy is made up of process flow stratification, instrument tolerances and drift, and environmental effects. A detailed analysis of the measurement loop errors was performed and it was demonstrated that by using commercially available digital technology, the measurement loop error can be reduced to +/- 0.72 degrees C. This could result in an improvement of 2.28 degrees C or 34 MW per unit. The use of calibrated high quality dual element RTD's (Resistance Temperature Detectors) installed in the existing spare thermowells provides redundancy at low cost. Using different transmitter manufacturers on opposite legs of the RTD temperature measurement loop provides diversity in measurement and permits the use of commercially available 'smart' digital transmitters at no extra significant cost or safety impact.