CFD studies of the insulation material transport inside the reactor pressure vessel under LOCA conditions


CFD studies of the insulation material transport inside the reactor pressure vessel under LOCA conditions

Höhne, T.; Grahn, A.; Kliem, S.

In 1992, strainers on the suction side of the ECCS pumps in Barsebäck NPP Unit 2 became partially clogged with mineral wool because after a safety valve opened the steam impinged on thermally-insulated equipment and released mineral wool. This event pointed out that strainer clogging is an issue in the course of a loss-of-coolant accident (LOCA). In the LOCA event inside the containment, energetic pressure waves and fluid jets would impinge upon components in the vicinity of the break and could release thermal insulation and dust. In addition, debris can be created by chemical reactions between reactive ECC solutions and the materials in the containment. These reactions may result in additional debris such as disbanded coatings and chemical precipitates being generated [1].Through transport mechanisms such as entrainment in steam/water discharge from the break and wash down due to condensate flow in the containment, a fraction of the generated debris and other material in the containment would be transported to the containment sump. Subsequently, if the ECCS is operating in the sump recirculation mode, the debris suspended in the containment sump would begin to accumulate on the sump strainers. A small part could penetrate trough the strainers and could be transported towards the core. The accumulation of this suspended debris on the sump strainers can create an almost uniform covering on the strainer, which would tend to increase the head loss across the strainer. On account of this phenomenon, modifications of the insulation material, the strainer size and the mesh size at the strainers were carried out in most of the German NPPs to assure the emergency core cooling during the sump recirculation mode. Moreover, differential pressure measurements and back flushing procedures were implemented to remove the mineral wool from the strainers, before the differential pressure exceeds a certain level. Nevertheless, it cannot be completely ruled out, that small fractions of the insulation material are transported into the RPV. During hot leg ECC injection, the fibres enter the upper plenum and can accumulate at the fuel element spacer grids, preferably at the uppermost grid level. This effect might cause a reduction of the ECC flow into the core and degradation of core cooling.
It was the aim of the numerical simulations to study where and how many mineral wool fibres are deposited at the upper spacer grid. The 3D, time dependent, multi-phase flow problem was modelled applying the CFD code ANSYS CFX. Best practice guidelines were used to specify the grid size, turbulence model and the multiphase model. The spacer grids were modelled as a strainer, which completely retains all the insulation material reaching the uppermost spacer level. In this model, which was validated against experiments at HTWS Zittau, the accumulation of the insulation material gives rise to the formation of a compressible fibrous cake, the permeability of which to the coolant flow is calculated in terms of the local amount of deposited material and the local value of the superficial liquid velocity. The CFD simulations have shown that after starting the sump mode, the ECC water injected through the hot legs flows down into the core at so-called “breakthrough channels” at the outer regions of the core, which were first recognized in UPTF experiments. In contradiction the hotter, lighter coolant rises in the centre of the core. As a consequence, the insulation material is preferably deposited at the uppermost spacer grids positioned in the breakthrough zones. This means that the fibres are not uniformly deposited over the core cross section.
Further investigations are necessary to determine the accumulation of insulation material for a longer period of time. Also steam production in the core or re-suspension of the insulation material during back flow should be considered.

Keywords: CFD; UPTF; ECC; RPV

  • Lecture (Conference)
    CFD4NRS-4, The Experimental Validation and Application of CFD and CMFD Codes in Nuclear Reactor Technology, 10.-12.09.2012, Daejeon, Korea
  • Contribution to proceedings
    CFD4NRS-4, The Experimental Validation and Application of CFD and CMFD Codes in Nuclear Reactor Technology, 10.-12.09.2012, Daejeon, Korea
    CD-ROM

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Publ.-Id: 16623