Evaluation of the Fast-Response, Small-Sample Calorimeter

This paper describes an evaluation of the measurement reliability of a portable, fast-response, small-sample calorimeter. The main thrust of the study was to determine the measurement confidence to be expected over the long-term, since only short-term tests, under laboratory conditions, had previously been made of the instrument. An initial series of measurements was made of a small group of well-characterized plutonium samples varying in size from approximately 0.1 to 10.0 g and covering the capacity of this small-sample calorimeter. The major effort consisted of a study conducted over a period of 16 months during which 15 different 3-g size plutonium samples were periodically received and measured calorimetrically. These results were compared with careful chemical and mass spectrometric analyses made during the same period. A third part of the evaluation consisted of a study to examine the reliability of the calorimeter when operated with the new and more sophisticated controller called the PCdriven DAS (i.e., Personal Computer Driven Data Acquisition System). The controller provides improved handling of the measurement sequence and has the capacity to mathematically predict the thermal equilibrium endpoint of a measurement. Endpoint prediction, if accurate, can considerably reduce the measurement time. So, to test the accuracy, a 3-g sample was repetitively measured over a period of seven weeks to compare predicted endpoint values to final endpoint values. Conclusions reached from the three parts of the study indicate the following: 1) Samples ranging in size from 0.1 to 10 g of plutonium are measurable, but the best accuracy and precision is obtained with the 3- to 10-g size. 2) Over the long term, 3-g-size samples can be measured at a precision of± 0.5%. 3) Endpoint prediction by the PC-driven DAS yields reliable values and allows a measurement to be completed in 45 minutes.