4.3.1 Rule of thumb
Early in an emergency there is a need for a quick rough release predictions in support of planning of the emergency response. The rule of thumb provides a tool to assess releases to the atmosphere.
The rules of thumb focus on three major items:
The form and contents of the thumb rules and their basis are discussed below.
4.3.2 The Formalism s = e×L
The formalism is intended to provide the emergency staff with a tool for answering early in an accident the pertinent questions:
Will there be a release ?
How large can it be?
When can it occur?
How long can it last ?
"“Will there be a release ?”'' The fuel and containment are recognized as the main barriers that prevent release of radioactivity to the atmosphere. As long as the fuel is intact and the containment is leak tight, no significant release will occur.
"How large can the release be ?'' The rule of thumb answer is: “S/I = s = e×L”, where “S” is the release to the atmosphere expressed as a fraction of inventory “I”. The rule of thumb enables the emergency staff to early focus their attention on the three main factors that constitute the accident's release potential, that is: I, e and L.
S and I are quantities of radioactivity in Becquerel. They are here written in bold type. s, e and L are dimensionless fractions of inventory I ( Figure 4.2).
Fig. 4.2 Release prediction
The fuel core holds volatile noble gases, iodine and cesium. This is the inventory “I”. The Data Handbook provides plant specific amounts in Becquerel. In the course of the accident, the fuel can become heated and emit a fraction “e” from the fuel of its contents of noble gases, iodine, cesium in gasborne form to the containment. The emission “e” of gasborne noble gases, iodine, and cesium will increase with fuel temperature. The fuel temperature and extent of fuel damage will increase with the uncovery time, that is the time interval during which the fuel by accident stays in lack of water for cooling. Given a leak path, a fraction “L” of the emitted gasbome amount of iodine and cesium will leak from the containment to the atmosphere. For noble gases “L = l” as they sooner or later will escape. For iodine and cesium L will be smaller, less than 1.
Also for iodine and cesium there is a general tendency borne out by source term studies. Figure 4.2, that: