System and safety studies of accelerator driven transmutation ... - SKB

fundamental decay rate, determined by the α-value obtained for EC3, is displayed
together with the fitted function in Fig. 36. The results from the analyses of the experimental
data, including the obtained values of keff, Λ, and βeff in the different experimental
channels, are presented in Paper V.
1.E+00
Counts/pulse
10 0
1.E-01
10 -1
10 -2
1.E-02
10 -3
1.E-03
10 -4
1.E-04
1.E-05
10 -5
Experimental data
Fit of exponentials
Fundamental decay mode
0 5 10 15 20
Time [ms]
Fig. 36. Fit of exponentials to the experimental data in the experimental
channels in the
core (EC3). The straight line represents the fundamental
decay mode.
The Sjöstrand method
The Sjöstrand method, also called the area method, is based on the decomposition of
the neutron flux transient into two components; the prompt part and the delayed part,
which is illustrated in Fig. 37. The delayed neutrons are represented by the “delayed
area”, Ad, and the prompt neutrons by the “prompt area”, Ap. Starting with the point
kinetics equations for the prompt neutrons, the delayed neutrons and the delayed
neutron precursors, the reactivity
of the sub-critical system can after some algebraic
manipulation be expressed as
ρ A
= −
β A
eff
p
d
. (61)
This method has the advantage that the involved parameters are integrals, which
reduces the statistical errors. The approximation that the prompt and the delayed
neutrons are emitted with the same energy spectrum was made in the derivation of
Eq. (61), which may affect the accuracy
of the results somewhat. The results of the
analyses are presented in Paper V.
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