Importance of coolant mixing for accident analyses using coupled codes using the coupled code DYN3D/ATHLET


Importance of coolant mixing for accident analyses using coupled codes using the coupled code DYN3D/ATHLET

Kliem, S.; Grundmann, U.; Höhne, T.; Rohde, U.

The 3D neutron kinetics core model DYN3D has been coupled to the thermohydraulics system code ATHLET. Different main steam line break analyses (MSLB) have been performed with this coupled code for a NPP with VVER-440 reactor and for the TMI-1 NPP. It is shown, that in both cases the mixing of the coolant from different loops strongly influences the reactor behaviour.

For the VVER-440, the MSLB analysis shows, that the assumption of no coolant mixing in the downcomer and the lower plenum leads to conservative results, while assuming homogeneous mixing, the consequences of the transient can be underestimated qualitatively. More realistic mixing models have to be used to obtain a best-estimate analysis of MSLB scenarios. Thus, a model describing the coolant mixing in VVER-440 type reactors under nominal flow conditions was applied in the analysis for this reactor type.

To check the applicability of this mixing model under the natural circulation conditions occuring in the considered MSLB scenario, the stationary three-dimensional flow distribution in the downcomer and the lower plenum of the VVER-440/230 reactor was calculated by help of the CFD-code CFX-4. Results of the MSLB analysis performed by DYN3D-ATHLET were used as boundary conditions for the CFX-calculation. To estimate the influence of the temperature distribution obtained by CFX, steady state calculations were carried out with the help of DYN3D, the keff - values were compared. It was found, that using the CFD results, the reactor remains sub-critical.

A full power MSLB analysis based on real plant design and operational data for TMI-1 NPP shows a return-to-power after the scram. The exclusion of the coolant mixing inside the reactor pressure vessel enhances this secondary power peak by about two times in comparison to the first calculation, where the coolant mixing is treated by means of an experimentally determined mixing ratio. The described variation of mixing applied in point kinetics calculations with compatible feedback coefficients shows the opposite effect on the obtained core power.

  • Lecture (Conference)
    Tagungsband der Fachtagung der KTG: "Wechselwirkungen Neutronenphysik und Thermofluiddynamik", pp. 513-525, Forschungszentrum Rossendorf, Januar 2000
  • Contribution to proceedings
    Tagungsband der Fachtagung der KTG: "Wechselwirkungen Neutronenphysik und Thermofluiddynamik", pp. 513-525, Forschungszentrum Rossendorf, Januar 2000

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Publ.-Id: 3075