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Loss of residual heat removal has occurred in several PWRs during mid-loop operation after plant shutdown, and is now recognized as an accident situation whose relevant physical phenomena require improved understanding. Several cases have been selected for investigation in the frame of the BETHSY program, with the main purpose of providing an experimental basis for the assessment of safety codes which have been so far especially involved with accident transients initiated during full-power operation. The major results from four experiments are presented in this paper, which addresses two main kinds of physical problem: mechanisms for, and rate of, primary mass inventory reduction in the case of manways opened at various locations in the primary coolant system; cooling of the plant when the primary is only half-open through vent paths and one steam generator is available to remove decay heat in reflux condenser mode with the presence of non-condensable gas.

The condensation process in steam generator (SG) U-tubes with the presence of non-condensible gas (nitrogen) has been investigated on the Bethsy integral test facility. Bethsy test 7.2c, dealing with the reflux condenser mode, consists of a succession of steady states where the effects of the amount of nitrogen, core power and secondary pressure on the primary-to-secondary heat transfer have been analysed. Examination of the non-condensible gas distribution in the SG U-tubes leads to the identification of an active zone where condensation takes place and a passive zone with a high concentration of nitrogen where no heat transfer occurs. The experimental data serve as a basis for safety code assessment. The first verification of these situations has been performed with the Cathare code, and results are presented and discussed in the following.

For several years an extensive programme of separate effect tests related to PWR safety has been conducted in France and in particular at the Nuclear Center of Grenoble. Recently the BETHSY integral test facility - three identical loops, full height and pressure - has been constructed with the main objectives of contributing to the verification of the CATHARE calculation code of accidents and to the validation of the physical bases of Emergency Operating Procedures. So far several tests have been (November 1988) carried out, among which natural circulation under various conditions (single phase, two-phase, symmetric or asymmetric conditions, different core power, variable steam generator liquid level), 2 inches cold leg break, steam bubble formation and collapse in the upper head. Summarized results of some of these tests are introduced and compared to a first CATHARE calculation as far as the 2″ cold leg break is concerned.

In a PWR core the power is not identical from an assembly to another and this may result in 2D phenomena which have to be studied in order to know whether it is necessary to take them into account in safety codes or not and, if yes, to define the simplest models. suitable for that. An experimental programme has been conducted during which numerous aspects were investigated. Boil up and reflood tests (constant and oscillating flow rate) have been performed which show the effect of various parameters and somewhat clarify the problem of fall back and cross flows as well as their consequences on quench front propagation and heat transfer in the dry region.