The Small-Sample Calorimetric System (Fig. 1) is a portable instrument designed to measure the thermal power produced by radioactive decay of plutonium-containing fuels. The mass of plutonium in a sample is related to the measured power through the weighted average of the product of the isotopic decay energies and the decay constants of the plutonium isotopes present. The uranium content of the fuel will not affect the measurement, since the thermal power produced by the uranium nuclides is

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... gnificant when compared to plutonium. This is the result of the longer half-life of uranium relative to the Pu isotopes. The principal radiations from plutonium are alpha particles and low-energy photons. As a consequence of the low penetration power of this radiation, greater than 99.9% of the decay energy will be degraded to tharmal energy measurable by the small-sample calorimeter. (A more detailed discussion of the physical principles involved in calori- metry is included in A Portable Calorimeter System for Nondestructive Assay of Mixed-Oxide Fuels t ISPO-16.) The small-sample calorimeter is capable of measuring samples producing power up to 32 milliwatts at a rate of one sample every 20 minutes. This power corresponds to samples containing approximately 10 g of low burn-up plutonium oxide. The measurement precision for the baseline power [a(P 0 )/P 0 ] is better than 0.1%. Samples containing 1.5 g of plutonium (2.5 mW) have to be assayed within this precision. The small-sample calorimeter can assay unknowns having milligram amounts of plutonium; however, the measurement precision may deteriorate. The instrument is contained in two packages; a data-acquisition module consisting of a microprocessor with an 8K-byte nonvolatile memory, and a mea-u u 5J P. I QJ