ABSTRACT
At the radiochemical Plant B, where irradiated naturally enriched uranium was processed3, the final products (free of uranium and fission products) had a plutonium concentration high enough to start spontaneous nuclear reaction. The technology used at Plant V possessed an intrinsic hazard of a spontaneous reaction at practically all processing stages. Therefore, the nuclear safety problem for the plant was closely connected, first of all, with plutonium concentration monitoring in every piece of processing equipment, vessels or canisters, pipelines or a smelter. Also, it had to be taken into account that process vessels might contain plutonium precipitates, solutions, etc. Critical masses of fission materials vary at different processing stages. Because of the low critical masses, the smelting process at Plant V was organized using smallsize equipment and small amounts of plutonium. Similar problems were encountered at Plant V while handling solutions and precipitates that accumulated in the processing equipment and also radioactive wastes: at that time no experimental data on a minimum critical mass of plutonium were available (only calculated data were at hand). It was only in 1951 that they determined that under certain conditions the critical mass of plutonium in 20-40 g/1 solutions was slightly more than 500 grams. Inaccurate laboratory analyses, instrumental errors, or just carelessness of personnel were the factors that might result in exceeding the safety exposure limit of 100-150 g set at that time. Therefore, the total concentration of plutonium in each process vessel and pipeline had to be monitored, taking into account the plutonium precipitation. At that time it was an enormously difficult task: safety levels were often exceeded, especially when handling plutonium solutions, and, therefore, spontaneous chain reactions occurred rather often.
