DYN3D/ATHLET calculations of a boron dilution transient during natural circulation conditions


DYN3D/ATHLET calculations of a boron dilution transient during natural circulation conditions

Kliem, S.; Rohde, U.

Slugs of under-borated, or even boron-free, coolant can be formed in the primary circuit during different accident scenarios, e. g. due to a malfunction of the chemical and volume control system, or due to an SBLOCA with partial failure of the safety injection system. An unintentional, or even unavoidable, decrease of boron concentration in the reactor core leads to a reactivity increase and may result in a power excursion, a so-called boron dilution transient. Whether re-criticality of the scrammed reactor occurs, and how high the power peak is, depends among other factors, in a sensitive manner, on the mixing of the too weakly borated water with the ambient coolant in the downcomer and the lower plenum on its way to the core. For an adequate simulation of such boron dilution transients, 3D neutron kinetic core models coupled with advanced thermal hydraulic system codes are necessary. Such analyses should be completed by a simulation of the above mentioned coolant mixing processes inside the reactor pressure vessel.

An earlier developed fast running semi-analytical coolant mixing model has been recently extended to natural circulation conditions. A parameter study, incorporating stationary and transient core calculations, was carried out for a hypothetical boron dilution event in a pressurized water reactor during the operation of the residual heat removal system, using the coupled 3D neutron kinetics/thermal hydraulics code system DYN3D/ATHLET in combination with this coolant mixing model. According to the boundary conditions, this analysis is carried out at atmospheric pressure conditions and a coolant temperature of 25 °C. By varying the initial slug volume, it was found in stationary calculations that, for the given core configuration, slugs of less than 6 m³ do not lead to re-criticality. Transient core calculations from 8 to the maximum initial slug volume of 15 m3 show a significant reactivity insertion and over-criticality. The resulting power excursion is mitigated and stopped by Doppler and moderator density feedback. The power increase in all calculations is accompanied by significant coolant boiling in single channels. It should be pointed out, that the thermal hydraulics simulations at low pressure are quite challenging because of the high density difference between water and steam and therefore, high sensitivity of void fraction versus coolant enthalpy. Although in all calculations local coolant boiling occurs over several tenths of seconds, the integrity of the fuel is always ensured according to the calculation results.

Keywords: boron dilution; natural circulation; neutron kinetics/thermal hydraulics coupling; coolant boiling

  • Contribution to proceedings
    NURETH-12 - International Topical Meeting on Nuclear Reactor Thermal Hydraulics, 30.09.-04.10.2007, Pittsburgh, United States
    Proceedings of the 12th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, CDROM, paper 053, 0894480588
  • Lecture (Conference)
    NURETH-12 - International Topical Meeting on Nuclear Reactor Thermal Hydraulics, 30.09.-04.10.2007, Pittsburgh, United States

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Publ.-Id: 9594