Formation of secondary minerals and Cs mobility in Hanford sediments were investigated under conditions similar to the Hanford tank leak in a dynamic flow system at 50A degrees C. The objectives were to (1) examine the nature and locations of secondary mineral phases precipitated in the sediments and (2) quantify the amount of Cs retained by the sediment matrix at 50A degrees C. To this end, Hanford sediments were packed into 10-cm long columns and leached with simulated tank waste consisting of 1.4 M NaOH, 0.125 M NaAlO2, 3.7 M NaNO3, and 1.3 x 10 (-aEuro parts per thousand 4) M Cs at 50A degrees C. Compositions of outflow solution were monitored with time for up to 25 days, and the columns were then segmented into four 2.5-cm long layers. The colloidal fraction in these segments was characterized in terms of mineralogy, particle morphology, Cs content, and short-range Al and Si structure. It was observed that cancrinite and sodalite precipitated at 50A degrees C. Approximately 53% Cs was retained in the column treated by the simulated tank waste at this temperature. Cesium retention in the column was lowered in the high ionic strength solution due to competition from Na for the exchange sites. This can be explained by alteration of distribution and number of sorption sites which reduces the selectivity of Cs for Na, and through the formation of cancrinite and sodalite. The formation of hydroxide complexes in highly alkaline solutions could also contribute to relatively poor retention of Cs by hindering ion exchange mechanism.