An indirect experimental technique for determining the expansion coefficient was developed to provide uncertainty estimates for expansion coefficient calculations. This technique uses an R, Z reactivity worth map synthesized from small-sample reactivity traverse measurements for major materials over the reactor core and blanket regions. The experimentally based expansion coefficients, representing the reactivity change due to uniform axial and radial expansion, are deduced by appropriately integrating measured worth profiles. This technique was evaluated in Phase A of the Zero Power Plutonium Reactor Assembly 5. Direct calculations of the expansion coefficients were performed, and results were compared with the experimentally determined values. The validity of the technique used to derive expansion coefficients from worth measurements was evaluated. It is concluded that the total expansion coefficients are reasonably well calculated; however, the calculated radial expansion coefficient was overestimated. Sources of possible systematic errors in the experimentally based values were studied. Based on the present experiment, an uncertainty of ±20% (90% level of confidence) on expansion calculations using ENDF/B-III data is estimated for a clean core configuration.