Abstract: The relationship between the activity of ¹³⁴Cs and ¹³⁷Cs in the main loop and the activity of ¹³⁴Cs and ¹³⁷Cs in ruptured fuel was analyzed when the cladding of fuel elements of a ship reactor was broken．The relationship between the activity of ¹³⁴Cs and ¹³⁷Cs and the burn-up of a fuel was also analyzed． With this analysis, the calculation formula of ruptured fuel’s burn-up can be found and it provides the theoretical basis to locate the fuel elements with broken cladding further.