From FISPACT-II Wiki
This keyword allows the input of the irradiation or cooling time interval t (in seconds by default). The value of the time may be followed by one of the following keywords SECS, MINS, HOURS, DAYS or YEARS so that time units other than seconds may be used. Note that it is important when inputting times that it is the interval time, not the total elapsed time that is specified. Thus for cooling steps the time printed on the inventory is the sum of all the previous cooling time intervals after the keyword ZERO.
< -- Control phase -- > ... FISPACT * Title of the simulation < -- Initial phase -- > ... ... FLUX 1.0E15 ATOMS < -- Inventory phase -- > TIME 1.0 HOURS ATOMS TIME 1.0 YEARS ATOMS ... << cooling periods >> ZERO FLUX 0.0 ATOMS TIME 100.0 YEARS ATOMS ...