RRFM 2009 Transactions - European Nuclear Society

Q o (α) is ratio of the resonance integral (I) to the cross section at 2200 m/s corrected for
non-ideality of epithermal neutron flux (α)
Details of the conversion of Q o to Q o (α) can be found in De Corte and Simonits (2003).
ENAA procedure is employed to reduce the influence of interfering elements with high
thermal neutron capture cross section in the analysis of elements of interest with high
resonance integrals. The enhancement of radionuclide of interest is normally expressed as
the advantage factor (AF), which is calculated as the Cd-ratio [R CD (1)] of interfering
element divided by the Cd-ratio [R CD (2)] of element of interest.
RCD(1)
AF =
(2)
R (2)
CD
Results and Discussion
Two input decks were constructed for the current HEU core of NIRR-1 with and without
Cd-liner in one of the large outer irradiation channels. The k eff multiplication factors were
found to be 1.0042±0.0003 and 1.0047±0.0003 respectively. From these data, it can be
shown that the loss of core excess reactivity due to the presence of the Cd-liner in one of
the large outer irradiation channel is 0.5 mk. As such, the effect of installation of a Cdliner
in one of the large outer channel on reactor excess reactivity is negligible thus
preserving uniformity of the neutron flux in ten irradiation channels of commercial
MNSR facilities. Furthermore, data of neutron spectral distribution in 640 group energy
structure in the large outer irradiation channel with and without Cd-liner are displayed in
Fig. 2. As can be seen in the figure, the neutron spectral distribution in the channel with a
Cd-liner is without the thermal neutron energy component with a cut-off energy of
approximately 0.5 eV.
In order to further assess the suitability of NIRR-1 for the ENAA procedure, Cd-ratios
and the advantage factors were calculated for some elements of interest on the basis of
the spectrum parameters of an outer irradiation channel of NIRR-1 and the relevant
nuclear data of nuclides of interest taken from De Corte and Simonits, 2003. Results are
presented in Table 1 for I, Br, C and K with Na considered as the interfering element. The
advantage factors obtained were calculated to be greater than unity for the elements of
interest using the outer irradiation channel of NIRR-1 with the following neutron
spectrum parameters, f = 48 3±3.3 and α =-0.029±0.003 (Jonah et al. 2005). These values
are consistent with experimental data obtained by Stuart et al. 1981 using the
SLOWPOKE-2 reactor at Dalhousie University, Halifax, Canada. Since the advatnge
factor provides information on the degree to which an element of interest will be
enhanced, the provision of Cd-lined irradiation channel will lead to improved utilization
of commercial MNSR facilities.
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