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Qualification tests have been conducted of a measurement system for determining the pressure of certain fuel rods in the loss-of-fluid-test (LOFT) reactor. Because of physical size (0.35-in. OD by 5.5-in. length) and operational characteristics, an eddy current device was selected as the most promising measurement transducer for the application. The sensor must operate at pressure up to 17.2 MPa (2500 psig) and at temperatures up to 800°F. During the reactor transient caused by loss of coolant flow, sensor temperature and applied pressure will vary rapidly and significantly. Consequently, qualification tests included subjection of the sensor to rapid depressurization, temperature transients, and blowdowns in an autoclave, as well as to calibrations and various slow temperature cycles. Calibration data show the sensor responds linearly for applied pressure, but with both temperature-dependent sensitivity and zero pressure output signal. Temperature compensation of signal output values was achieved with use of a derived temperature-dependent algorithm to transform the dc output voltage to indicate pressure. Because of the very rapid (up to 100°F/s) temperature changes imposed, the sensor erroneously indicated pressure by a maximum of 18% of reading during some transient tests. In addition to the influence of magnitude and rate of temperature change on the observed pressure measurement error, a very significant factor pertains to the spatial distribution of the temperature field immediately adjacent to the sensor envelope. As proven during 700 hours of qualification tests, precision and stability of the sensor should be satisfactory for LOFT.

The design and operational characteristics of a microwave frequency instrument capable of measuring strain of internally pressurized nuclear fuel cladding are described. Intended for use during the characterization of mechanical properties of advanced cladding alloys, the instrument should yield continuous strain measurements for in-core tests. Tests in a laboratory oven at 1000°F indicate the method permits strain resolution of several microinches, and accurately measures inelastic strain. Strain measurement errors attributable to physical separation of the sensor and specimen are eliminated by design of the two elements as an integral unit.

Qualification tests conducted for a period of 700 hours of each of three displacement measuring (LVDT) sensors confirmed applicability of the design for use in Loss-of-Fluid-Test (LOFT) reactor. Operationally, the sensor satisfied all specified requirements for LOFT. Even for imposed temperature transients at rates up to 100°F/s, the indicated displacement remained within the allowed maximum error band of ±10% of reading. The 0.625-in OD by 5.5-in long sensor exhibited a linearly related signal output variation for displacement variations of up to 1-in range. Long term operation at temperatures of 100°F to 800°F caused no perceptible permanent change of operating characteristics. Furthermore, operation of the displacement sensor during severe and varied qualification testing justifies the following conclusions: * All qualification sensors yielded linear variations of signal output with displacement changes for a range of 1-in and exhibited a change of sensitivity of less than 5% over the temperature range of 100 to 800°F. * For imposed temperature transients, by use of either laboratory ovens or an autoclave, the maximum measurement error of ±2% of reading remained. less than the allowed value (±10% of reading) for inaccuracy of LOFT displacement sensors. * The sensor speed of response to rapid displacements of the core easily satisfied the specified 100 millisecond requirement for LOFT. * Vibrations expected to occur during a LOFT transient should incur only minimal spurious signal response, as vibration tests at reactor system frequencies caused no discernable signals superimposed on the quiescent value.

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 × 107R/hr) irradiation facility, and in high temperature (1000°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 × 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 1000°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.

Operating theory and characteristics given in this paper concern a dual property measurement sensor and associated instrumentation for use with liquid metal cooled, fast breeder nuclear reactors. The sensor may also be applied in other similar severe environments. Pressure variations are sensed by deflection of a thin diaphragm which forms one end wall of a resonant cylindrical microwave cavity. Temperature changes cause the cavity size, and thus its resonant frequency to vary. Both physical properties may be continuously monitored as each relates to a distinct mode of cavity resonance. Precise digital readout of the resonant frequencies indicated a sensor sensitivity to pressure of about 9 MHz/psi (for null pressure scheme of operation), and to temperature of 350 kHz/°F. As both the sensor and connecting waveguide are entirely of metal, direct immersion in a liquid metal coolant should be possible.

Gain conditions are obtained for a microwave parametric amplifier in which the resonant structure is filled with a nonlinear dielectric material. The gain equation is applied to the case of a toroidal resonator of strontium titanate dielectric. At a signal frequency of 10 Gc/sec, the gain threshold is at a pump power of 7.5 W. If efficient energy transfer is to be realized between pump and signal, resonant modes of the dielectric amplifier must be chosen appropriately. The necessary conditions on mode geometry have been illustrated.