Patent Number: 046559922
Section: summary

BACKGROUND OF THE INVENTION This invention relates to ultrasonic temperature measurement. The invention has arisen in consideration of the severe problems that arise in measuring temperature of coolant flowing in a fast-fission nuclear reactor cooled by liquid sodium. Monitoring of coolant temperature is not only vital for normal control purposes but it is also vital for safety purposes as temperature trends and transients can foreshadow the onset of incidents like blockage of coolant flow which might cause solid nuclear fuel to melt if corrective or preventative action is not taken. Various solutions to these problems have been considered such as reliance on flowmeter or thermocouple readings or by the acoustic detection of coolant boiling. However, with these known solutions uncertainties may arise especially where the normal flow is low, or there is cross-flow in which a normal flow could mask an abnormal flow. These uncertainties are increased when, as is often the case, the flow or temperature measuring device cannot be located precisely at the most sensitive localities (which are usually the discharge points of flow from defined channels into bulk zones) because of access or obstruction problems. Whilst, current practice does not itself involve any hazards, as the acceptable margin of safety can be suitably achieved, improved standard is continuously being sought. The present invention provides such an improved standard by the use of ultrasonics which may either replace or work in harness with known systems. The use of ultrasonic techniques to measure temperature is well known--see, for example, British Patent Specifications Nos. 2114299, 2002118, 1300159, 1202182, 1178529, 1178385 and 1035763. Patent No. 1300159, for instance, is specifically concerned with a device for ultrasonic measurement of the temperature of liquid metal coolant within a nuclear reactor. Such a device suffers from the drawback that it is invasive in the sense that the hardware is physically located at the position at which the temperature measurement is to be made and therefore interferes with the flow conditions prevailing at that position. FEATURES AND ASPECTS OF THE INVENTION According to the present invention there is provided a method of measuring temperature within a body of fluid in which ultrasound is transmitted through the fluid and the time taken for the ultrasound to traverse a known distance is translated into a corresponding temperature value, said method being characterised in that, to measure temperature at a selected zone or zones within said body of fluid, which zone or zones are demarcated by elements located at a known separation distance or distances within a containment structure for said body of fluid; (a) ultrasound is launched into the fluid medium at a location which is physically discontiguous with said zone(s) whereby the ultrasound propogates towards said zone(s) and undergoes reflection by said elements; and (b) the resulting ultrasound echoes are identified as being derived from a particular zone or zone(s) and the elapsed time therebetween is translated into a temperature value. The invention takes as its starting point the known fact that the sonic velocity is sodium (or other liquid for that matter) is a function of its temperature. The invention uses ultrasonic beams from an interrogating ultrasonic transducer or transducers which can be sighted on said elements (for example the opposite sides of a channel containing nuclear fuel swept by sodium coolant or reflectors specifically in the channel). By measurement of the time difference between echoes received back from the elements demarcating each zone, together with a knowledge of the distance apart of the elements, sonic velocity can be calculated and hence the means temperature between those points determined. It is also possible, because of the short time required to make a measurement, to monitor temporal fluctuations ("temperature noise" as it is sometimes called) which is a sensitive method of detecting local overheating. The beams from the interrogating transducers may be sighted to be penetrating or to be at a glancing incidence, the latter being very advantageous for measuring temperatures at an outlet from a channel. Where a number of zones lie on a common line which can be sighted along by a single interrogating transducer beam, it is possible for a number of temperature measurements to be taken and presented or recorded simultaneously. Interrogating transducers may be individual to each zone, or they may be arranged, e.g. on a sweep arm, so as to scan a multiplicity of zones.