The nominal source term concept can help the assessor to interpret the data from the installed instruments provided the potential errors caused by isotopic mix are borne in mind. The response of the installed instrumentation to the nominal source term should be calculated, based upon an initial estimate of a four hour linear release. As time progresses it should be possible to obtain a better estimate of the likely release duration and hence a better estimate of the source term.
A number of the radioisotopes which are likely to be released in a major accident have relatively short half-lives (see Table 3.1). As such the time between reactor shutdown and the release will affect the isotopic composition of the release. When calculating the expected response to a given source term the effect of delay before release due to, perhaps, hold up in the containment, should be taken into account.
For
any installed dose rate monitor, a plot should be made of the dose rate against time so that a time profile of the release as measured by that monitor will be obtained.
Third Stage
The first and second stages of the assessment were based upon information from the reactor operators, installed equipment and prejudgements about isotopic mix, etc. In the third stage, monitoring teams can be sent out to obtain the required plume measurements. The first priority should be collect one or more air samples using activated charcoal, or silver zeolite, cartridges together with gamma dose rate measurements from positions close to the assessed plume centreline. The samples should be assessed for gross beta activity and analysed by gamma spectrometry. These early samples together with their associated gamma dose rate measurements will allow the following to be assessed:
TABLE 7.1
SUGGESTED
NOMINAL SOURCE TERM
Radionuclide
Krypton-85 Krypton-85m Krypton-87 Krypton-88
Activity (becquerels)
1.3 X 1014
5.6 X 1015
1.0 X 1016
1.4 X 1016
Strontium-89
3.4 X 1012
Strontium-90
1.8 X 1011
Zirconium-95
1.2 X 1012
Ruthenium-103
5.3 X 1012
Ruthenium-106
1.3 X 1012
(i)
isotopic mix of the volatiles;
Tellurium-132
9.7 X 1013
(ii)
cloud concentration of the volatiles at the time of sampling;
Iodine-131
6.8 X 1013
Iodine-132
9.9 X 1013
(iii)
the ratio of cloud concentration of the volatiles to gamma does rate at the sampling point;
lodine-133
1.4 X 1014
Iodine-135
1.3 X 1014
(iv)
(v)
the potential dose to a member of the public at the sampling point assuming the release con- tinues for four hours, (unless other information is known about the release duration);
identification of the areas affected.
Xenon-133
4. I X 1016
Xenon-135
1.0 X 1016
Caesium-134
3.9 X 1012
Caesium-137
2.3 X 1012
45
Barium-140
Lantharium-140
Cerium-144
Neptunium-239
This is a very large source term and could only arise
following a beyond design basis accident, typically a core melt followed by basemat failure.
6.1 X 1012
1.3 X 1012
7.2 X 1011
I.5 X 1013