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IMPACT ON PEOPLE AND THE ENVIRONMENT
Release

RELEASES FROM ACCIDENTS INVOLVING SOURCES

 

Up to this point we have only considered releases from facilities.

You may have noticed when you looked at the Manual that there were two ways of defining a Level 5 and Level 4 release. If not, look at pages 17 and 18 of the Manual.

We need to consider how to rate events involving the dispersion of material from a dangerous radioactive source, for example as a result of a fire.

Operators of radioactive sources use a concept of D value, which is very similar to radiological equivalence. The definitions for release are derived from this concept.

Major accident
Serious accident

There is no need to define releases from sources at Level 6 and 7
as it is physically impossible to obtain such a release.

The last paragraph of page 15 of the Manual briefly explains D values. The next slide summarizes the use of D values in the INES methodology.

Level 5
"An event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to a release to the atmosphere of the order of hundreds to thousands of terabecquerels of 131I."
or
"An event resulting in a dispersed release of activity from a radioactive source with an activity greater than 2 500 times the D2 value, for the isotopes released."

Level 4
"An event resulting in an environmental release corresponding to a quantity of radioactivity radiologically equivalent to a release to the atmosphere of the order of tens to hundreds of terabecquerels of 131I."
or
"An event resulting in a dispersed release of activity from a radioactive source with an activity greater than 250 times the D2 value, for the isotopes released".

The D values are a level of activity above which a source is considered to be ‘dangerous’ and has a significant potential to cause severe deterministic effects if not managed safely and securely. The D2 value is, "the activity of a radionuclide in a source that, if uncontrolled and dispersed, might result in an emergency that could reasonably be expected to cause severe deterministic health effects." Appendix III lists D2 values for a range of isotopes.