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An instrument for measuring radial strain of nuclear fuel cladding has been developed for use during in-core characterization tests of candidate materials. Employing a microwave sensor of coaxial geometry, a strain as small as several microinches can be detected. At the opposite extreme, the large dynamic range of the instrument permits measuring strain equal to ten percent of the original specimen diameter. In all configurations tested, the sensor comprises a hollow metal cylinder enclosing a test specimen positioned coaxially within the cylinder. Microwave energy absorbed by the sensor/specimen unit at particular frequencies relates to diameter of the specimen. Consequently, determination of specimen size results from measurement of the resonant absorption frequencies of the sensor. Tests to determine operational characteristics of the sensor and associated instrumentation have been conducted in a gamma (2 Ã 107R/hr) irradiation facility, and in high temperature (1000Â°F) laboratory ovens. Most recently concluded tests concerned evaluation of a microprocessor-controlled instrument design. As indicated by the instrumentation, elastic strain of a pressurizable specimen of 0.23-in. OD fuel cladding occurred linearly with stress at a modulus of about 30 Ã 106 psi. The gamma radiation appeared not to cause any short or long term effects upon sensor performance. During several tests of the sensor/specimen at 1000Â°F, indicated elastic and inelastic strain of 316 SS test specimens were monitored. Completely satisfactory data resulted even though a light oxidation of the sensor occurred during the initial high temperature test.