Abstract: According to the collected lifetime data, the reliability assessment of the sodium pump mechanical seals for Russian fast reactor was completed by Weibull distribution. And then the reliability information of the sodium pump mechanical seals was obtained. The reliability assessment based on the collected lifetime data of domestic mechanical seals was completed by optimal confidence limit method, whose assessment results were compared with those of Russian sodium pump mechanical seals. The calculation results show that the reliabilities for Russian sodium pump mechanical seals and domestic mechanical seals are no less than 0.947 3 and 0.874 4, respectively, with 0.8 one-sided confidence limits of 25 000 operation hours. The results of the reliability assessment show that the reliability of the sodium pump mechanical seals for Russian pump is better than the mechanical seals’ on domestic pump, but the gap between the two is not significant.