The first experiment series included five Small Break LOCA tests with break in one hot leg of PACTEL. Three break sizes (2, 4 and 6 mm or 0.5, 2 and 4.4%, respectively) were used. Three tests included secondary system depressurization as an accident management measure. The operators of the loop also depressurized the secondary system by opening a steam generator relief valve. The core power in the tests was 80 kW corresponding to 1.8% of the scaled thermal power of the reference reactor. In all tests, only PSIS provided ECC water to the core. The initial temperature of the water in the CMT was 40°C. The primary pressure used in the tests was lower than the nominal operation pressure of PACTEL. Maximum operation pressure of the passive accumulators determined the upper limit of the experiment pressure (3.8 MPa).

Munther [4] and Munther et al. [5, 6, 7] have published the results of these experiments. The main results of the first test series can be summarised as the following:

• three flow modes of CMT operations were present in the experiments: recirculation mode with water circulating through the PSIS lines, oscillating phase with two-phase flow in to the tank and injection phase with flow of steam in to the CMT.

• the ECC water injection stopped totally several times during injection, due to rapid vapour condensation in the CMT.

• flow reversed in the broken cold leg leading to flow of cold water from the downcomer towards the break

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Although some problems with condensation in the CMT occurred no core heat-up was detected. It should be mentioned here that the CMT used in the test was very large, about twice as large as the scaled volume of four accumulators of the reference plant. On the other hand, the volume of the CMT is about the same as the volume of the rest of the loop. All the tests in this first series were terminated before the CMT was totally empty.