Inherent to all source term predictions are large uncertainties, that amount to at least one, often several orders of magnitude. This holds true for source terms computed by modern mechanistic models and for thumb rule predictions, in particular. The above thumb rules are chosen to represent the upper (95th-percentile) bound of the uncertainty interval. Real releases can be expected to turn out lower than what is predicted by the rule of thumb.
The above discussion applies to the atmospheric source term, which means release to the atmosphere. The thumb rule formalism applies to near-term releases of gasborne radioactivity in the first hours up to first day of an accidental event, until monitor data become available as discussed above.
Spills to the ground of radioactive liquids, the aquatic source term, is not further treated here. Consequences of liquid releases are in the short term local.
Environmental Consequences
The source term provides input data for dose calculations by hand or by use of computer models like e.g. LENA. The results inform on doses incurred or avoided in specific areas by application of emergency protective measures like iodine tablets, sheltering, evacuation.
Consideration in the case of Ignalina NPP on the applicability and effectiveness of various counter-measures and estimations of final consequences are reflected in the following Chapters 5 and 6 of this Handbook.
The thumb rules can as illustrated help answer the questions:
Will there be a release ?
How large can it be ?
When can it occur ?
The question finally:
How long will the release last ?
will in practice be answered by the operators actions to cool the core and its debris by water filling and spraying inside the accident confinement system and by closing valves in the leak paths.