The QUENCH program, performed at Forschungszentrum Karlsruhe, Germany, is dedicated to out-of-pile studies of the initiation and progression of damage during core reflood of a degraded commercial nuclear reactor. Main work in this program is spent on the investigation of the material behavior of the solid structures. However, for the deeper understanding of the integral tests, especially of the quench phase, as well as for computational support of the tests and for the validation of severe accident codes, a sufficient knowledge of thermal-hydraulics in the bundle during the quench phase is also mandatory. Though much instrumentation is available in the test section, information to interpret thermal-hydraulics is scare due to principal and technical reasons. The main objective of the present paper is to get a better idea of the reflood process, based on all available experimental data. For this purpose, the test QUENCH-06 is used because of the amount of available qualified experimental data and because of its special importance for code validation, this test being selected as OECD International Standard Problem (ISP) no. 45. At reflood initiation of QUENCH-06, some irregularity of water injection occurred due to the malfunction of a check valve. A thorough inspection and comparison of experimental data is presented in this paper to clarify details of the start of the quench phase. It is complemented by still more detailed computations with the in-house version of SCDAP/RELAP5 mod 3.2 than at the time of ISP-45. Apart from its relevance for this special test and for ISP-45, this work sheds light on the consistency of the involved experimental data. Besides to this investigation, the transition from two- to single-phase flow is examined in more detail than before, giving indications for the axial extension of the two-phase flow region with large droplets or a sensible fluid fraction and for the duration of two-phase flow near saturation temperature. Again, the consistency of data of various instrumentations is assessed. Despite of this success, a better instrumentation for thermal-hydraulics, mainly of void sensors in the lower part of the bundle, is desirable to facilitate interpretation of thermal-hydraulic aspects of the tests.

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