WP 1: Temperature measurements and temperature sensors developments for the fission environment
Background
Within the next decade, a new generation of nuclear power plants will progressively replace those currently in use. To improve the efficiency of these new power plants they are required to operate at higher temperatures than the current generation (i.e. above 1000 °C compared with 500 °C - 800 °C, depending on the type of reactor). The current temperature sensing technology, largely based around mineral-insulated metal-sheathed type K thermocouples, is inadequate for the new challenges posed by the higher temperature environments. To maintain the safety and to ensure the long-term reliability of such plants it is therefore essential that new temperature measurement sensors and methods for in-situ measurement are investigated and developed.
State of the art
Temperature measurements in the nuclear industry are usually performed in challenging conditions due to high temperatures and ionising radiation. The temperature sensors are therefore prone to drift, transmutation or even breakage.
The best capabilities of measurements in this field in the harsh environment of a nuclear plant are at the level of several tens of Kelvin for temperatures around 1200 °C - 1500 °C. Various instruments were developed 30 to 40 years ago for use under severe conditions (neutron irradiation) but the stability of the instruments and accuracy of the measurements are still insufficient.
The development of specific contact sensors based on materials having very low neutron absorption cross section (molybdenum and niobium), or contact sensors based on a different measurement technique (acoustic thermometer) are among the possible solutions. These sensors are unaffected by transmutations or more adapted to harsh environments and should, in principle, exhibit lower drifts. These specific sensors aim to address the needs for improved measurements on-site in terms of accuracy and reliability.
Knowledge of the performance of these specific temperature sensors needs the support of National Metrology Institutes for the studying their typical metrological characteristics and developing new methods of self-validation and drift control.
Major facilities/equipment
NPL has extensive experience and in some areas world-leading capability in the following technical areas: The construction, calibration and assessment of thermocouples at high temperatures, the construction of high temperature fixed points above that of the copper fixed point (1084 °C) and primary acoustic thermometry. It has an extensive range of high temperature furnaces that can operate from 1000 °C -
3000 °C, high temperature ingot casting capability and access to and experience in all background technology required for practical acoustic thermometry.
CEM has facilities to calibrate thermocouples at fixed points (Sn, Zn, Al, Ag, Cu and Co-C) and/or by comparison to a radiation thermometer up to 1600 °C. Different types of furnaces (heat pipes and 3 zones) with good thermal profiles are available to perform the work together with set-up to assemble thermocouples and to perform their necessary electrical and heat treatments.
LNE is among the world leaders in the construction of high temperature fixed point cells above the silver point (962 °C) for contact and non-contact thermometry, and in the construction and metrological study of new types of thermocouples. LNE’s facilities in this field include thermocouples and fixed points construction equipment, several high temperature furnaces that cover the temperature range 960 °C -
3000 °C. LNE is also equipped with facilities dedicated to the calibration of thermocouples up to the copper (1084 °C) and gold (1064 °C) point using fixed point cells and up to the palladium point (1554 °C) using the wire-bridge method.
CNAM is involved in contact and non-contact high-temperature thermometry and has participated in determining the reference functions for noble metal thermocouples in the past. One of the facilities available at CNAM that will be of importance to the JRP is the thermoelectric measurement apparatus, which can help assessing the efficiency of the annealing applied to high-temperature thermocouples.
Aims of the work package
WP1 aims to develop several innovative methods to improve temperature measurement in the challenging environment posed by fission reactors. In brief, four main areas will be investigated; the traceability and assessment capability for new thermocouple types, new thermocouple types, including with self-validating capability. In addition a demonstrator of a highly innovative permanently drift-less thermodynamic method of temperature sensing will be developed.
Scientific tasks
Task 1.1: Development of reference fixed-point(s) above the copper point to determine thermocouple characteristics (CNAM, LNE, NPL)
Task 1.2: Mo/Nb type and/or other thermocouple studies (CEM, LNE, CNAM)
Task 1.3: Investigation of self-validation methodology for thermocouples (LNE, NPL, CNAM)
Task 1.4: Primary acoustic thermometry for in-pile validation studies (NPL)
Selected references
[1]. J. L. REMPE, S. C. WILKINS, "High Temperature Thermocouples for In-Pile Applications", Paper 143, The 11th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH-11), Popes' Palace Conference Center, Avignon, France, October 2-6, 2005.
[2]. J.-F. Villard, S. Fourrez, D. Fourmentel and A. Legrand "Improving High-Temperature Measurements in Nuclear Reactors with Mo/Nb Thermocouples" Int J Thermophys (2008) 29:1848–1857
[3]. Machin, G., "Realising the benefits in improvements in high temperature measurement", Acta Metrologica, Sinica, 29, p. 10-17, 2008, Invited keynote lecture at Tempbeijing 08
[4]. DePodesta, M., Sutton, G., Underwood, R., "Practical acoustic thermometry with acoustic waveguides", presented at Tempmeko & ISHM 2010, submitted to the International Journal of Thermophysics.