Source terms for severe accidents

Iodine, moreover, was assumed to be mainly in elemental form (I2, 91 per cent), for five per cent in particulate form (particles or aerosols) and for four per cent in the form of organic iodide (methyl iodide and similar compounds). 

It is surprising to consider that these simple rules have dominated a large part of the nuclear safety 

The engineered safety features, for example, have been optimized for the removal of elemental and organic iodine, while the closure time of the isolation valves has been established on the basis of the immediate release from the core. The Technical Information Document 14844 (TID) releases, as they were then named, have been used for the verification of the resistance to radiations of equipment inside the containment, as well as for the evaluation of control room habitability after an accident and for the design of liquid and gas sampling systems. 

After publication of the Rasmussen report (1975) and the TMI accident, the validity of the old and glorious TID was questioned, much research on the subject was carried out and, in 1992, after years of debate in all the scientific and regulatory centres all over the world, a NRC report was published (USNRC, 1992a) containing a new proposal of source term , which should replace the HD. The new proposed releases for a PWR are shown in Table 5-3, expressed as a fraction of core inventory. The releases for a BWR are slightly different. 

The new proposal derives from the consideration of the sequences studied in USNRC (1990) and USNRC (1992b), and intends to represent an average of meaningful cases. The releases due to interaction of the molten core with concrete (late out of vessel releases) are those deriving from the assumption of an absence of water above the molten layer. 

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