Filtered Containment Venting System

Abstract

In the hypothetical event of a core melt accident a buildup of pressure will occur inside the containment.

In order to prevent loss of containment integrity as a result of overpressurization, nuclear power plants in the Federal Republic of Germany as well as in the most other European countries have been or will be backfitted with a system for filtered containment venting.

The Siemens venting system basically consists of a combined venturi scrubber with integrated downstream metal-fiber filter which is connected to the containment atmosphere via a pipe equipped with isolation valves (Figure 1). The venting system discharge pipe downstream of the filter is provided with a check valve and a throttling orifice. In the event of an accident requiring containment venting, the system can thus be operated in the sliding pressure mode at optimum flow velocities in the venturi scrubber and the metal-fiber filter. A second combined venturi scrubber version is designed to operate close to the atmospheric pressure venting containments with lower design pressures e.g. 2 bar absolute. These system designs result in high removal efficiencies and especially the sliding pressure version in an extremely compact scrubber unit.

In the event of an accident inside the containment requiring activation of the venting system, the isolation valves are set to open at a pressure approximately equivalent to the containment design pressure. In this way the pressure inside the containment is reduced and at the same time the radioactive containment atmosphere is filtered in the venturi scrubber prior to discharge to the environs via the plant stack.

As soon as the pressure inside the containment has dropped sufficiently, venting is terminated by closing the containment isolation valves. In the course of process qualification, hydraulic functional tests and removal efficiency tests for aerosols and iodine were performed on the scrubber unit at pressures between 1 and 10 bar, at temperatures between 50°C and 190°C and using air, steam and gas mixtures. The aerosol spectrum relevant for process design and testing varies depending on aerosol concentrations and the time at which venting is commenced. In Germany, the Reactor Safety Commission had specified that Sn0₂ with a mass mean diameter of approximately 0.5 μm should be used as an enveloping test aerosol.

To perform these tests, the JAVA scrubber test facility was erected enabling a complete section of the system to be tested on a scale of 1:1. The removal efficiencies of the entire combined scrubber unit were found to be greater than 99.95% for all the aerosols employed in the tests (BaSO₄, uranine and SnO₂ aerosols.

The iodine retention capability was demonstrated using iodine with a radioactive tracer. The removal efficiencies established for iodine were > 99% over the entire operating pressure and temperature range and at various venturi flow velocities.

These high removal efficiencies as well as the high aerosol loading capacity have been confirmed by the venting filter comparative tests performed as part of the international ACE program conducted by Battelle Northwest using other aerosolsunder atmospheric pressure conditions.

As the Siemens system uses a wet scrubber, it can also meet requirements for high aerosol loading capacity ( > 100 kg), removal of a large amount of decay heat from the scrubber unit (e.g. 480 kW) and high mass flows (e.g. 14 kg/s). The Siemens venting system is of passive design and over a prolonged period does not require any control actions apart from initiating opening of the isolation valves when containment venting is required.

Its compact configuration (see Fig. 2) makes it particularly suitable for backfitting into existing plants. In all of its venting system backfitting projects to date, Siemens has been able to install this venting system within the time scheduled for the annual refueling outage, without requiring any extension of the shutdown period.

The Siemens venting system has so far been installed in 14 PWR and BWR plants in the Federal Republic of Germany and Finland.