ID 114
Type H - High priority request
Target 83-Bi-209
Reaction (n,g)Bi-210g,m
Quantity BR - Branching ratio for isomeric states of residuals
Incident energy 500 eV - 300 keV
Accuracy 10 %
Field(s) ADS, Fission
Accepted date 09-Nov-2018
Status Work in progress
Latest review date 28-Apr-2022


Project (context)

MYRRHA and Gen-IV reactors with lead-bismuth coolants


Polonium-210, a rather short-lived alpha emitter (half-life 138.4 days), determines the radiological burden associated with the use of lead-bismuth heavy metal coolants of fast reactors, MYRRHA accelerator-driven system currently designed at SCK•CEN, Belgium is one of the examples. It is produced via neutron capture on 209Bi forming the ground state of 210Bi (half-live 5 days) which rapidly decays into 210Po. 210Bi has also long-lived (3 My) metastable state that can also be produced via neutron capture and that decays to 206Tl. Thus the knowledge of branching ratio between these two neutron capture channels is important to accurately predict the inventory of Polonium-210.


10% on the capture cross sections both to ground and metastable states of 210Bi.

The analysis performed in [Fiorito2018] reveals high sensitivity of 210Po production to the neutron capture cross section at first (801.6 eV) and neighbouring resonances up to 10 keV, as well as to the data around 100-300 keV where the neutron spectrum is peaked. The spread in prediction of 210Po concentration between nuclear data libraries reaches 40% while the propagation of existing covariance data for the capture cross section results in uncertainties between 5 and 20%. Furthermore, the covariance data on the branching ratio are not provided in the evaluated data files although they are likely to contribute significantly to the final 210Po concentration uncertainty.

Justification document

Significant differences between evaluations were observed in the neutron capture cross section and branching ratio, which were proved to pose a severe constraint for the correct 210Po content prediction in MYRRHA and other lead-bismuth cooled reactor concepts. The covariance matrices found in the 209Bi evaluations result in different uncertainty profiles and energy correlations. The 210Po concentration uncertainty (intended as one standard deviation) after one irradiation cycle of MYRRHA is ranging between 5% and 20%. This level of uncertainty does not cover the deviation between 210Po concentration values assessed with the evaluated files coming from different libraries, even when three standard deviations are considered. Furthermore, branching ratio covariances were not propagated since they are not provided in the evaluated files, although they are likely to add a significant uncertainty contribution to the polonium content.

Significant efforts are necessary to reduce the discrepancies between evaluated data. In particular, accurate measurements should be carried out to obtain the reliable branching ratio not only at thermal energies and for the first resonance at about 800 eV, but also in the energy ranges of interest for lead-bismuth-cooled reactors such as MYRRHA, as made clear by the sensitivity plots in [Fiorito2018].

Because of the disagreement between libraries, a consistent evaluation for the branching ratios should be elaborated that also includes the energy-dependent behaviour in the resonance region and above. Although the BROND-3.1 [Blokhin2016] evaluation seems providing most accurate branching ratio among evaluated libraries, it also relies only on few experimental energy points [Saito2003, Saito2004, Borella2008, Borella2011] in the range 800 eV – 10 keV. The future evaluation thus should be supported by additional experimental measurements.


  • [Saito2003] K. Saito, M. Igashira, T. Ohsaki, T. Obara, H. Sekimoto, Measurement of cross sections of the 210-Po production reaction by keV-neutron capture of 209-Bi, in JAERI Conf. Proc., Vol. 6, p. 133, 2003
  • [Saito2004] K. Saito, M. Igashira, J. Kawakami, T. Ohsaki, T. Obara, H. Sekimoto, Measurement of keV-Neutron Capture Cross Sections and Capture Gamma-Ray Spectra of 209Bi, J. Nucl. Sci. Technol. 41, 406 (2004)
  • [Borella2008] A. Borella, T. Belgya, E. Berthoumieux, N. Colonna, C. Domingo-Pardo, J.C. Drohe, F. Gunsing, S. Marrone, T. Martinez, C. Massimi, P. Mastinu, P.M. Milazzo, P. Schillebeeckx, G. Tagliente, J. Tain, R. Terlizzi, R. Wynants, Measurements of the branching ratio of the 209Bi (n,g) 210gBi/210mBi reactions at GELINA, Proc. Int. Conf. on Nuclear Data for Science and Technology ND-2007, Nice, France, EDP Sciences (2008), DOI: 10.1051/ndata:07431
  • [Borella2011] A. Borella, T. Belgya, S. Kopecky, F. Gunsing, M. Moxon, M. Rejmund, P. Schillebeeckx, L. Szentmiklosi, Determination of the 209Bi (n,g) 210Bi and 209Bi (n,g) 210m,g Bi reaction cross sections in a cold neutron beam, Nucl. Phys. A 850, 1 (2011)
  • [Blokhin2016] A.I. Blokhin, E.V. Gai, A.V. Ignatyuk, I.I. Koba, V.N. Manokhin, V.G. Pronyaev, New version of neutron evaluated data library BROND-3.1, Technical Report Yad. Reak. Konst. No.2, p.62, A.I. Leypunsky Institute for Physics and Power Engineering, Obninsk, Russia, 2016
  • [Fiorito2018] L. Fiorito, A. Stankovskiy, A. Hernandez-Solis, G. van den Eynde and G. Zerovnik, Nuclear data uncertainty analysis for the Po-210 production in MYRRHA, EPJ Nuclear Sci. Technol. 4, 48 (2018)

Review comment

In addition to 209Bi(n,g) branching ratio or activation measurements, complementary transmission measurements may be needed to fix the resonance parameters in the re-evaluation of the 209Bi capture cross section. Moreover, integral benchmarks dedicated to 209Bi activation/cooling in representative spectra are required to better assess the performance of evaluated data.

Entry status

Work in progress (as of SG-C review of June 2019)

Main recent references

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