The technology of removing cesium radionuclides from sodium coolant at the BN-350 fast reactor was realized in the form of two different types of cesium traps: stationary devices connected to the circuit and in-core devices installed into the core of reactor when it was not under operation. Carbon-graphite materials were used as sorbents in these traps to collect and concentrate radioactive cesium, accumulated in the BN-350 reactor circuits over the decades of their operation. The relatively small volume traps provided effective radiation-safe conditions for personnel working close to the primary circuit coolant and equipment during BN-350 decommissioning. These spent cesium traps represent solid radioactive wastes that must be treated before long term storage. The presence of chemically active sodium, potassium and cesium in the traps results in series of problems related to their long term storage and disposal in the Republic of Kazakhstan. As a consequence, the technology of filling spent cesium traps with lead/lead-bismuth alloy was evaluated. A set of experiments was implemented aimed at verification of calculations performed in substantiation of the proposed technology: filling a full scale cesium trap mock-up with sodium followed by its draining to determine the optimal regimes of draining; filling bench scale cesium trap mock-ups with sodium and cesium followed by sodium draining and filling with lead or lead-bismuth alloy at different temperatures and filling rates to chose the optimal regimes for filling spent cesium traps; implementation of leachability tests to determine the rate of cesium release from the filling materials into water. This paper provides a description of the experimental program carried out and the main results obtained.