Volume 76, Issue 1

In case of a LOCA within the containment the released water/steam jet can fragment insulation material from the leaking pipe and from pipes in the neighborhood. The released insulation material and other materials like e.g. dust and paint chips are washed into the sump. A part of this debris reaches the sump strainers after starting ECCS pumps. Some material will be retained by the strainers depending on the mesh size or hole diameter of the strainers. The residual material penetrates the strainers and reaches downstream components and especially fuel elements. Therefore it has to be demonstrated that fuel elements are sufficiently cooled after LOCAs with release of insulation material and other materials. Several events since 1992 have shown the possibility of strainer clogging. This article discusses generic aspects and some international approaches to sump clogging issue and related tasks. An overview of the status quo and comparison of different approaches and background is given.

The facility is designed to investigate processes following a loss of coolant accident (LOCA) in the region of the reactor sump as well as downstream the sump strainers. The following parameters have been considered: The debris transport and sedimentation behavior in the reactor sump region; pressure loss caused by debris agglomeration on the sump strainer; the influence of strainer geometry and size of the strainer openings on the pressure loss; back-flushing ability of sump strainers; pressure loss caused by debris bypassing the sump strainer (downstream effect); the influence of erosion and corrosion processes on the pressure loss behavior. The experimental studies led to the development of an accepted procedure to handle a LOCA for KWU PWR plants. For the EPR™ the efficiency of the debris retention concept in case of a LOCA was demonstrated.

The investigation of insulation debris transport, sedimentation, penetration into the reactor core and head loss build up becomes important to reactor safety research for PWR and BWR, when considering the long-term behaviour of emergency core cooling systems during loss of coolant accidents. Research projects are being performed in cooperation between the University of Applied Sciences Zittau/Görlitz and the Helmholtz-Zentrum Dresden-Rossendorf. The projects include experimental investigations of different processes and phenomena of insulation debris in coolant flow and the development of CFD models. Generic complex experiments serve for building up a data base for the validation of models for single effects and their coupling in CFD codes. This paper includes the description of the experimental facility for complex generic experiments (ZSW), an overview about experimental boundary conditions and results for upstream and down-stream phenomena as well as for the long-time behaviour due to corrosive processes.

The investigation of insulation debris generation, transport and sedimentation becomes important with regard to reactor safety research for PWR and BWR, when considering the long-term behaviour of emergency core cooling systems during all types of loss of coolant accidents. A joint research project on such questions is being performed in cooperation between the University of Applied Sciences Zittau/Görlitz (HSZG) and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The project deals with the experimental investigation of particle transport phenomena in coolant flow and the development of CFD models for its description (see [10–12]). While the experiments are performed at the University at Zittau/Görlitz, the theoretical modelling efforts are concentrated in Rossendorf. In the current paper, the basic concepts for CFD modelling are described and feasibility studies are presented. The model capabilities are demonstrated via complex flow situations, where a plunging jet agitates insulation debris.

During a postulated cold leg LOCA with hot leg ECC injection, a limited amount of small fractions of the insulation material after passing the sump strainers can enter the upper plenum and can accumulate at the fuel element spacer grids, preferably at the uppermost grid level. This effect might affect the ECC flow into the core and could result in degradation of core cooling. The CFD simulations show that after starting the sump mode, the ECC water injected through the hot legs flows down into the core at so-called “brake through channels” located at the outer core region where the downward leg of the convection role had established. The hotter, lighter coolant rises in the center of the core. As a consequence, the insulation material is preferably deposited at the uppermost spacer grids positioned in the break through zones. This means that at the beginning the fibers are not uniformly deposited over the core cross section.

Corrosion investigations were carried out using hot-dip galvanized steel samples in order to identify the corrosion mechanisms as well as the nature of corrosion products formed. Corrosion of grating treads may exacerbate ECCS strainer clogging problems during LOCA events in LWR. For the related experiments, tailored bench-scale facilities have been constructed. The results of batch experiments as well as of long-time corrosion investigations in the Korr VA test facility suggest that there is a multi-stage corrosion process. The first stage comprises the dissolution of the Zn layer in the coolant forming Zn 2+ ions, which can generate crystalline zinc borate in boric acid solutions, if the ion concentration exceeds the saturation concentration. The base material (steel) is cathodically protected as long as the zinc layer is present. During the second stage, the base material dissolves forming insoluble corrosion particles, which can subsequently lead to acceleration of the strainer clogging. The main influences on corrosion were identified as impact of the water onto the corroding surface, water chemistry and Zn surface/coolant volume ratio.

In 1992, strainers on the suction side of the ECCS pumps in Barsebäck NPP Unit 2 became partially and temporary clogged with mineral wool after steam jet induced releases of parts of the mineral wool insulation. Although Barsebäck NPP Unit 2 is a Boiling Water Reactor (BWR) this event induced large investigations to under-stand und maintain strainer clogging effects after a loss-of-coolant accident for all reactor types. Especially for the German Pressurized Water Reactors (PWR) a program was launched by the German Utilities together with the plant manufacturer. The final estimations of all theoretical and experimental results caused different steps of modifications of the insulation material, the strainer area and mesh sizes in most of the German PWR including the E.ON PWR. Moreover, interactions and procedures were carried out such as back flushing actions to remove the mineral wool from the strainers supported by an additional differential pressure measurement at the strainers. All measures were implemented to assure the performance of emergency core cooling during the containment sump recirculation mode and to improve the NPP safety.

The incident, which happened in the Swedish nuclear power plant Barsebäck 2 on 28 th of July, 1992 brought about a re-evaluation of the design requirements for the control of loss-of-coolant accidents (LOCA) involving the release of insulation material that were valid in Germany up to that time. The current paper provides an overview on the aspects that need to be taken into consideration for analyses as well as a description of the important boundary conditions. In giving details of the analyses, the actual state is also described. It is also illustrated that sufficient examinations and tests have been performed for the German PWR plants and corresponding measures have been taken providing a sufficient basis for a final positive evaluation of the issue “LOCA involving release of insulation material”.

The following contribution describes the main features of the demonstration of the control of loss of coolant accidents with release of insulation material under consideration of actions taken in German BWR. The general criteria for the assessment as well as comparable individual aspects are adopted from the RSK statement (374 th meeting) for PWR from July 2004. After the Barsebäck event in 1992 administrative regulations were implemented for securing the plant cleanliness as well as modifications to strainers of the ECCS and the containment (e.g. the introduction of perforated plates in the control rod chamber). This contribution illustrates that sufficient examinations and tests have been performed for German BWR for a final positive evaluation that the amount of fibers which enter the core remain below the amount at which core cooling is no longer guaranteed. Currently, for one BWR an additional optimation of the ECCS systems (here the introduction of a so-called “sandwich” wire cloths in the ECCS strainers) has currently been applied for. Through this the core entry of insulation material – even for fibers with a covering small lengths spectrum – can be further reduced and currently existing safety margins further increased.