Keyword:TOLERANCE

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TOLERANCE itol atol {1.0E+04} rtol {2.0E-03}


This keyword is used to set absolute (atol) and relative (rtol) tolerances that are passed to the LSODES solver to control the convergence of the solution. If itol = 0 the tolerances are applied to the main inventory calculation and if itol = 1 they are applied to pathways calculations. The keyword may be used twice to adjust both pairs of tolerances. If the keyword is used twice or more for a given itol, then the last values specified will be used.

The atol parameter is significant in relaxing the accuracy requirement on the results for the minor constituents of an inventory and to avoid excessive demands on the solver. If accurate results are required for minor constituents of the inventory, indicated by the setting of a small mind parameter, then atol should be reduced as well.

It is an important part of any numerical study to establish that the significant radiological output quantities are converged. Repeating calculations with itol=0 and smaller atol and rtol, and comparing results must be part of any serious investigation. One indication that radiological predictions should be treated with caution until convergence studies have been performed is if the ? flag is set in the inventory table output for nuclides that appear in the dominant nuclide table.

Another indication of poor convergence is that is the total inventory in a pathway calculation adds up to more than 100%. If this occurs, then warning messages will be issued in the runlog file. The pathways calculations use the LSODE solver on the subset of nuclides appearing in a pathway, and are likely to give better predictions for minor constituents than the main solver where convergence of main constituents dominate, so first try reducing the tolerances for itol=0, and then for itol=1 until convergence is achieved.

Example usage:

< -- Control phase -- >
...
FISPACT
* Title of the simulation
< -- Initial phase -- > 
...
TOLERANCE 0 5.0E3 2.0E-6
TOLERANCE 1 1.0E2 2.0E-3
< -- Inventory phase -- > 
...

In this case the absolute tolerance is reduced by a factor of two and the relative tolerance is reduced by 1000 compared with the default values for the main inventory calculation and the relative tolerance is reduced by a factor of 50 in the pathways analysis.