BS IEC 60737:2010 pdf – Nuclear power plants – Instrumentation important tosafety – Temperature sensors(in-core and primary coolant circuit)-Characteristics and test methods.
temperature measuring device based on the use of two conductors of different metals welded together at their two ends to form two junctions
NOTE On. junction 1.1 the temperature Sit, of int.r.sL the other at a stabi, cold t.inp.ratur.. Tb. Bqnal of a Inermocouple arises from me Seebeck erlect Which generates a voltage that varies wlh the temDeiature biween lb. junctions
the rmowell
protective jacket for RTDs, thermocouples, and other temperature sensors, The thermowell is also used to facilitate replacement of the temperature sensor
IIEC 62385, 3.19j
4 General considerations
4.1 R.qulr.m.nts for measurements
Temperature is a fundamental parameter related to the nuclear process in a reattv,. It Ldib t measured with specific sensors to perform the following main safety functions;
— to monitor the temperature of the cooling system and to follow the operating conditions with regard to the design parameters:
— to measure the thermal power of the reactor when the temperature measurement Is combined with the coolant flow rate measurement;
— to monitor the temperature of the fuel elements in order to avoid a boiling incident or melting of the fuel element itself.
Temperature measurements are required from the fuel, moderator, coolant or structural members supporting the core. They are used for control purposes, for the protection system. for shut-down and accident monitoring or for the provision of more general information about the reactor or its components.
In a power reactor with a core which has large physical dimensions, it may be important to monitor not only mean temperatures but also spatial temperature distributions. Measurements at particular positions may be used for the control of specific parts of the reactor core to ensure adequate safety margins for protection system parameters or to provide for optimum fuel utilization.
Some in-core measurements may also be necessary for reasons such as protecting the fuel from damage caused by local disturbances in coolant flow or by transients in local power density. In most cases, temperature sensors are used to measure temperature directly, but applications do arise in which information is derived from fluctuations in temperature. An example of the latter is the derivation of coolant flow by correlation of the fluctuations obtained from a spaced pair of sensors,
The measurement of in-core temperature for water reactors is important for reactor efficiency and fuel burn up, and may be achieved through probes inserted into specific channels of the reactor, or by permanently installed detectors. The measurements of these sensors are normally taken at routine intervals, followed by calculations to assess the conditions at each monitored fuel channel.
In all these applications, the environment is demanding and the performance of the temperature measurement system is either important to safety or to operation.
The temperature signals may be measured In a continuous or discontinuous manner depending on the application. This will not usually affect the design of the In-core installation.
The coefficient B is relatively small (about 6 x i01 C—2) so that the resistance varies almost linearly with temperature, and It is taken to be linear for the rest of this standard.
RTDs can be difficult to measure because they have a relatively low resistance that changes only slightly with temperature (less than 0,4 UPC). To measure these small changes in resistance accurately, special connection configurations should be used that minimize errors from lead wire resistance.
Typically, an RTD can be used with three different wiring configurations: 2. 3 or 4 wires. The wiring configuration has a direct impact on accuracy. The 4 wires configuration offers the best accuracy.
The sensitive element of an RTD Is a metallic wire or a metallic coating on an insulating material. Due to the principle of measurement, the sensitive element shall be protected in a sheath filled with a mineral insulating material.
Because an RTD is a passive resistive device, a current has to be passed through the sensitive element to produce a measurable voltage. This current causes the RTD to internally heat, which appears as an error. Self heating is typically specified as the amount of power that will raise the RTD temperature by 1 C, usually expressed In mWlC.
The self heating can be minimized by using the smallest possible excitation current. The amount of self heating also depends on the medium in which the RID is immersed.
RTDs are constructed in a number of forms and offer greater stability, accuracy and repeatability in some cases than thermocouples.