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Mayak disaster had long lived isotopes, but overall it still less that Chernobyl in volume.
Mayak has more ecological problems though. In 50s they dumped less active waste straight into river, then understood that this isn't good at all and started to dump it into isolated lake. Then, at drought period, strong winds pulled radioactive dust from shores. Then they started to fill lake with concrete and ground, and finished only recently.
Small accidents also happened regularly. So, Mayak is something like small, but persistent Chernobyl.