When it is used in the initial conditions section of the input file, this keyword causes the initial inventory to be printed to the output file. In the inventory phase, this keyword starts the solution of the inventory equations over the time interval specified and causes the results (isotopic, elemental, spectral) to be output. After the ZERO keyword, it also causes pathways and uncertainty results to be output. It is the standard method of producing output; other options are SPECTRUM (solves the inventory equations but does not output inventory, just the gamma spectrum) and STEP (solves the inventory equations but does not output anything). The time step is set to zero after the completion of the output, and so must be reset in subsequent steps using the TIME keyword.
Note that without using ATOMS, SPECTRUM, or STEP after each time interval, the rate equations will not be solved. Omission of these keywords will not result in an error and calculations will run, but the results will not be as expected.
< -- 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 ...