Fast Reactors and Accelerator Driven Systems Knowledge Base

Conference Article: Investigations of neutronics of subcritical systems with the use of MCNP code

S. Taczanowski, M. Kopec

Abstract

As the tool particularly appropriate for investigations of neutronics of subcritical systems, the MCNP code (4b version) has been chosen in view of its great advantages of universality and wide spread. Two simplified models of the systems have been assumed in calculations - a full-scale spherical assembly and a smaller cylindrical experimental one. The neutron multiplication process has been examined in subcritical systems with various targets while following the generation dependent: neutron multiplication factor ki, number of neutrons and cumulative energy release. This has provided indications for right selection of the active cycles in the MCNP code for correct estimation of the keff. Then, the spatial distribution of fissioning (i.e., approximately that of heating) has been simulated. E.g., within a uniform system of keff amounting to 0.962, the span in specific fissioning has been found larger than in similar, exactly critical systems even by over 3 orders of magnitude, thus proving a drawback of the former. Finally, the time evolution of fissioning have been studied in detail - among others also the development of the neutron spectrum in the subcritical system after a 14 MeV neutron -shaped pulse. The particularities of neutron field trajectories according to the place of detector and time proved explainable. Instead, discrepancies between the provided neutron lifetimes by the MCNP in the KCODE and Fixed Source modes require further studies. In conclusion, we can state that the performed study has illustrated some properties of subcritical systems, contributed to their better understanding and of the used tool the MCNP code, thus allowing for better design and reliability of the respective validation experiments.

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key words: Fast Neutron Spectrum Systems, Nuclear Technology
Reference:
Proceedings of a Committee Meeting (TCM) on “Core Physics and Engineering Aspects of Emerging Nuclear Energy Systems for Energy Generation and Transmutation” held in Argonne, Illinois, U.S.A., 28 November - 1 December 2000
International Atomic Energy Agency, Vienna (Austria)
IAEA-TECDOC--1356, pp:228-239