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Risk of sump plugging. Experimental program on chemical effects

J.M. Mattéi, Y. Armand, I. Vicena, B. Soltesz, J. Batalik, V. Gubco, M. Liska, A. Klementova,
EUROSAFE 2005, Brussels, November 7-8, 2005,
Rapport DSR 89

Summary

The assessment of the operational characteristics of the filtration function used during the recirculation phase of safety injection system (SI) and containment spray system (SS), in the event of a primary system break in the containment, has been performed by the "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN) for the French pressurized reactors (58 reactors). Those one have been designed according with the RG 1.82 (rev. 1). The IRSN has focused in particular on the CPY series, 900 MWe 3 loops pressurized water reactors (28 reactors).
A general overview of the literature has been conducted between October 1999 and November 2000, which resulting in defining an approach methodology and in writing technical specifications for an experimental program of studies on the sump plugging risk. Those studies have been carried out for the different sizes of primary breaks: large, intermediate and small LOCA. For each size of break, located on the welding between SG and hot leg, the different characteristics of generated debris, flows of SI and SS systems have been defined. Methodology of event trees has been used to define the characteristics of the debris and their arrival point at the bottom of the containment using the results of the performed tests.
To estimate the risk involved in each case, the following points were studied:
_ An inventory of debris generated. This consists of the following types:
_ Glass wool thermal insulation fibers (spherical model: L = 12.D),
_ Paints, particulates and dusts present in the containment,
_ Vertical transfer of debris,
_ Structural modification of debris in the containment,
_ Horizontal transfer of debris at the bottom of the containment,
_ Filtration efficiency and modification of sump hydraulics / air and debris ingestion,
_ Operation of equipment with mud and air.
The subjects giving rise to important questions have been collected and a corresponding full-scale experimental program was chosen in order to answer to questions raised from a preliminary study. The following points were currently under experimental investigation:
_ IVANA loop (VUEZ / SLOVAKIA): grinding of fibrous debris on the grating system (mechanical action of falling water),
_ VITRA loop (EREC / RUSSIA): horizontal transfer speed of debris,
_ MANON loop (VUEZ / SLOVAKIA): pressure drop and air and debris ingestion at the sump filters,
_ ELISA loop (VUEZ / SLOVAKIA): chemical action of water, effect of temperature, establishment of different correlations.
Consequently, important phases related to the sump plugging issue have been developed for the last years. Nevertheless, questions are remaining opened in particular the LOCA induced long-term debris, as a combined action of the temperature and the chemistry of the water, in correlation with the nature of the water. Those are so-called chemical effects issue.
During last decades, the chemical durability of glass was and is frequently discussed in connection with the vitrification process of high level nuclear waste material. In minds of people from Nuclear Power Plant (NPP) field as well as for those from the glass science and technology, this is the most important connection between the glass chemical durability, and glass itself, on one side and the nuclear energy on the other side.
A great deal of the theory of the process of glass corrosion by liquid media was born within this connection – see e.g. the numerous publications in Nuclear Waste Management Journal.

Nowadays, a new phenomenon was pointed out in a new manner to this old theme, namely the problem of the sump screens clogging during the Loss of Coolant Accident (LOCA) in Pressurized Water Reactor (PWR) based NPP. In this case, a huge amount of debris coming from the glass fiber insulation is generated by the jet effect of the water vapor. Together with other kinds of debris (concrete, dust, paints, metal chips) the glass fibers are trapped on the containment sump screens. The crude hydrodynamic conditions of the solution falling down from the spraying system cause the mechanical disintegration of the fibers.

This phenomenon was thoroughly studied in IRSN with respect to the possible clogging of the containment sump screens. But moreover, the alkaline (pH=9.3 at 60°C-25°C) sodium borate solution of the spray system causes a significant chemical corrosion-dissolution of the glass accompanied with a back-precipitation of the dissolved glassy material, that can be in some cases accompanied by the generation of a thermodynamically unstable intermediates in gelatinous (i.e. colloidal) forms. The precipitated matter (and the gelatinous forms) is trapped in the thin layer of the filtration bed adhering to the screens and causes the screen clogging.

Three years ago (i.e. in the year 2003), this glass fiber dissolution phenomenon was identified and partially quantified simultaneously in USA (University of New Mexico & Los Alamos National Laboratories) and in the Vitrum Laugaricio Glass Centre (VILA) in Trenčín (Slovakia).

The results obtained led to the identification of serious risks for the safety of NPP in a specific LOCA situation, specially in case of small diameter fibrous glassy material, where all the glass volume can be chemically changed (corroded) in relatively short time.

On the other hand, the models can be applied for the initial stages of the glass dissolution and open new research fields for the glass industry.
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