In technological processes for solidification of high-level wastes it is necessary to establish remote control of the apparatus and mechanisms, to monitor the technological parameters, and to monitor the quality of both the intermediate product and the final parts. In many cases, the properties of the intermediate products can be judged from the parameters of the technological process being monitored, while tests of the final parts which must be put into long-term storage or buried in geological formations, require an independent technical solution. Nondestructive methods of monitoring, for example, the acoustic-emission method, based on the detection of signals emitted from solids to which a force is applied [1], are being increasingly used. In standard methods of ultrasonic flaw detection the amplitude of the acoustic signal with the crack length increasing by 0.01-0.1 mm is hundreds of times greater than the change in amplitude of an ultrasonic signal reflected by the same growing crack. The maximum sensitivity achieved under laboratory conditions in investigations of the growth of a fatigue crack reached 0.025 ~m/cycle [1]. It should also be noted that ceramic piezoelectric transducers, employed for recording acoustic signals, also have high radiation and thermal resistance [2]. The laboratory apparatus for recording the acoustic signals consisted of separate units and devices, connected with one another according to the scheme shown in Fig. 1. The heating element (not shown in the figure) consisted of a vertical resistance furnace, which made it possible to perform experiments with the temperature of the heating zone reaching 1050~ Depending on the experimental conditions, we installed in the heating zone either a melter-crucible with a die at the bottom to obtain glass granules or the top (hot) part of a ceramic sound duct with a platform on which the experimental ceramic sample was placed. The initial product for preparing the glass granules consisted of presynthesized (prefound) glasses with different composition. The ceramic samples consisted of cylindrical tablets, which were pressed out of ground clay and mixed with additives simulating real high-level wastes. Phosphate and borosilicate glasses with different founding temperature were used in the experiments with glass granules. The finished glass samples were placed in a crucible, heated up to a definite temperature, and granulated by the method of free drop efflux. The temperature in the drop-formation zone was measured. The fused-glass drops formed either fell on a receiving area of the sound duct, where they were formed into granules and cooled to a certain temperature, or they were directed by a mechanical system into a collector container. After cooling, the granules were held for an additional time on the receiving area and then were removed. This process was repeated many times. During the experiment the spectra of the amplitudes of the envelopes of the acoustic signals were recorded continuously. Time was measured from the signal recorded at the moment a glass drop fell on the receiving area of the sound duct. Figure 2 displays typical spectra of granules which had defects in the form of cracks of different size. The defects vary over a wide range: from microcracks, visible only under high magnification, to chips and large cracks, observed by the unaided eye. For example, granules whose spectra are presented in Fig. 2a consisted of sectioned spheres, whose bases and volumes were penetrated by cracks of length up to several millimeters and chips. Figure 3 displays the spectra of glass granules obtained by cooling of phosphate glass drops on a heated area of a sound duct. The granule whose acoustic emission spectrum is displayed in Fig. 3a contained microcracks on its base and several microcracks in the volume; the spectrum of the amplitudes, which is shown in Fig. 3b, corresponds to a visually "ideal" granule. All granules of the same composition and the same size, whose acoustic emission spectra correspond to the range between the positions a and b in Fig. 3, had the same (within the limits of accuracy of the measurements) mechanical strength under a static load. The objective of the experiments with ceramic samples was to determine from the acoustic signals the possibility of observing defects, formed during the heat-treatment process. The samples were placed on the working area of the ceramic sound duct, which was placed in the heating zone of the resistance furnace, and heated or cooled with different
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Mar 1, 1995
John N Koblanski 
Open Access