We treated the highly gamma- and neutron-irradiated nuclear accident victim by the criticality fission reaction. Conventional dosimetry methods posed us the risk of a treatment delay, and later established organ-oriented dose estimation METREPOL criteria is a helpful tool for rapid triage and stratification of the acute radiation syndrome (ARS) patients. Graft-versus-host disease (GVHD) is the most predominant life-threatening factor for ARS. Umbilical cord blood (UCB) provides us the rapid availability during the golden time for hematopoietic stem cell transplantation (HSCT) with lower risk of GVHD. Current advances of supportive care provide us the safer HSCT, so UCB overweigh the risk of engraftment failure and delayed hematopoietic recovery compared to the matched unrelated donor or related one haploidentical donor. UCB is an ideal stem cell source for HSCT of ARS, if the patients lack the suitable donor candidate.