Nuclear Engineering and Design 237 (2007) 1522–1525 A method to eliminate the effect of radiation on thermocouple performance Fawaz Ali, Lixuan Lu ∗ School of Energy Systems and Nuclear Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada Received 20 September 2006; received in revised form 22 January 2007; accepted 28 January 2007 Abstract In-core temperature measurements are pivotal in maintaining nuclear reactors in a safe state of operation. Thermocouples serve as the liaison in ensuring this safe state. The realization of the thermocouple’s full potential is hindered by the fact that thermocouples cannot be situated in areas with high radiation fields. Radiation has the potential of generating voltages in the thermocouple wires, hence producing an error in the temperature transmitter output. In this paper, a mathematical model is developed to quantify the effect that radiation from the Canada Deuterium Uranium (CANDU) Nuclear Power Plants (NPPs) has on the thermocouple temperature reading. Subsequently, a method to offset the effect of radiation on the thermocouple is proposed. Simulation is performed to verify the effectiveness of the proposed system. © 2007 Elsevier B.V. All rights reserved. 1. Introduction Temperature is one of the most fundamental process param- eters in typical industrial control environments. This holds true for Nuclear Power Plants (NPPs) as well, whereby the monitor- ing and control of temperature is pertinent to the safe operation of the plant. The two primary methods by which the thermal power is measured in Canada Deuterium Uranium (CANDU) NPPs are (1) measuring the heat output from the reactor and (2) measuring the heat input to the steam generators. With respect to the first method, it is imperative to understand that the useful heat output from a CANDU reactor is situated in the fuel chan- nels. Thus, to measure the thermal power, the heat transferred to the fuel coolant is obtained by determining the flow rate and the temperature difference from the fuel channel’s inlet and outlet. The second method computes the heat input to the steam gener- ators by measuring the steam flow and the feedwater flow and their respective temperatures (Bereznai, 2005). As can be seen, temperature measurements are of paramount importance when computing the thermal power output of the reactor. A simpli- fied thermocouple temperature transmission circuit in NPPs is shown in Fig. 1. ∗ Corresponding author. Tel.: +1 905 721 3111x3456; fax: +1 905 721 3046. E-mail address: Lixuan.Lu@uoit.ca (L. Lu). The fundamental principle of operation of thermocouples is known as the “Seebeck effect”, which means when two dissimi- lar metals are joined together, an electromotive force (EMF) will be developed between the endpoints A and B, as shown in Fig. 2. From this EMF, the temperature at the junction can be deter- mined. One fundamental problem associated with the use of ther- mocouples in the reactor core and in the heat transport system is the effect of radiation on the voltage reading (and subsequently, the temperature reading) from the thermocouple circuit. In order to overcome this shortcoming, the only documented method that has been used is “radiation shielding”. However, no technical specifications have been found based on shield- ing. To gain a better understanding of the effect of radiation on thermocouple performance, a model which takes radiation into consideration is developed in this paper. Following this, an innovative system that eliminates the effect of radiation on the thermocouple temperature output is proposed. Simulation is per- formed where radiation is considered to be the daughter nuclei and/or nucleon of fission products. The type J thermocouple is used to illustrate the functionality of the system. The results show that there is a significant improvement on the temperature measurement with the proposed system. 2. Modeling the effect of radiation on thermocouples To gain a better understanding of how radiation can affect the EMF of a thermocouple circuit, one must understand the 0029-5493/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2007.01.015