|Type||H - High priority request|
|Quantity||SIG - Cross section|
|Incident energy||Thermal - 100 eV|
|Status||Pending new evaluation or validation|
|Latest review date||28-Apr-2022|
Mr Cristian MASSIMI at UBOLOGNA, ITY
In nuclear industry gadolinium is used as neutron absorber having the highest thermal absorption cross section among all stable elements thanks to Gd-155 and especially Gd-157 isotopes. It is commonly used, in PWR or BWR, as burnable absorber in fresh fuel to compensate an excess of reactivity or as emergency shutdown poison in CANDU reactors. RPI measurements of the Gd-157 thermal capture cross-section  differ by 10% from the ENDF/B-VI.8 value and, thus, in ENDF/B-VII.1  the low energy cross section uncertainty was increased by up to a factor 2, i.e. from about 2-4% to 4-6%. There is no significant change in the latest ENDF/B-VIII.0 evaluation of these two isotopes. A sensitivity and uncertainty analysis was performed by ENEA to quantify the maximum impact of the uncertainty of the gadolinium isotopes cross sections on the criticality of a LWR system [3-5]. It showed that, in systems with a high number of gadolinium fuel pins, neutron capture of odd gadolinium isotopes contribution to keff uncertainty ranks high, just after the major U-235 and U-238 contributions.
Sensitivity and uncertainty analyses show that keeping the cross section uncertainty below 4% mitigates the impact on keff uncertainty at high-burnup, which is important for a good estimation of the residual reactivity penalty of a fuel assembly at the end of life.
Despite their importance, the capture cross sections of the odd Gd isotopes have not been extensively studied and are not known with the accuracy required by present-day nuclear industry. In the thermal energy range these cross sections contribute more than 99% to the total cross section. Complementary accurate transmission measurements are also required to constrain the capture cross section. Table 1 in Ref.  illustrates current discrepancies for 157Gd capture cross-section at thermal energy. Such contradictions should be clarified thanks to new measurements and evaluation.
Comment from requester
Beyond nuclear industry, nuclear astrophysics can benefit from the accurate knowledge of 155- and 157-Gd(n,g) cross section up to a few keV, as the even-Gd isotopes play an important role in the s-process nucleosynthesis. Therefore a consistent analysis of all Gd-nat isotopes from thermal up to a few keV is of high interest for both astrophysics and nuclear data evaluation purpose.
Pending new evaluation or validation (as of SG-C review of May 2018)
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