Thesis Open Access

Analytical and experimental study of chugging boiling instability: the CHUG project

Mambelli, Simone

Thesis supervisor(s)

Mikityuk, Konstantin; Prasser, Horst-Michael

One of the main aims of ESFR-SMART European project is the safety evaluation of a low-void Sodium Fast Reactor (SFR) core design, in particular the analysis of an unprotected loss of flow (ULOF) accident. Recent studies on the low-void SFR core show the occurrence of a stabilized chugging sodium boiling regime, that can be classified as a new safety measure acting as a level of defence preventing the severe accidents. In order to better understand and simulate the chugging boiling regime condition and to gather new experimental data, the ESFR-SMART project envisaged the construction of a new simple facility called CHUG to be planned and designed using water as sodium simulant. The main purpose of this Master's Thesis has been the design and realisation of the experimental water rig facility "CHUG" for the study of the phenomenology of chugging boiling. This included also the conduction of the first tests and the comparison between the experimental data and the results from simulations performed with thermal-hydraulics code TRACE and PSI's in-house CFD code (PSI-BOIL). The test section consists of a vertical pipe of few-centimetre diameter filled with light water at ambient temperature and atmospheric pressure, where high-pressure steam is injected upwards from the lower part. Pressure at the bottom of the section and the axial stratification of the water temperature are tracked through the use of appropriate sensors. The interaction between the injected steam and cold water are analysed to study the conditions of the chugging boiling regime occurrence and condensation-induced pressure waves. In parallel, analytical simulations of the experiment are conducted by the use of the thermal-hydraulics code TRACE to assess the validity of the code for the simulation of chugging boiling. Moreover, the behaviour of high-pressure steam bubbles in cold water will be investigated through the CFD code PSI-BOIL, highlighting the presence of inertia-driven bubble collapse and the partial suitability of water as sodium simulant. The experimental results highlight that the chugging phenomenon is well reproduced by CHUG experimental facility. The variation of the level of injection allows the reproduction of several regimes of pressure oscillations, which are in accordance with previous studies on Direct Contact Condensation. The characteristic design of CHUG facility allows the reproduction of different oscillation patterns along the same test. In particular, the strongest pressure spikes (up to 5 bar) occur towards the end of each test, when the enhanced axial thermal stratification induces the formation of vapour slugs rising through the test section and collapsing in the upper low-temperature area. This may represent a way to mock up the oscillating behaviour of sodium boiling in low-void core concepts. By comparing the experimental results with TRACE pre-calculations, it is proven that TRACE is able to reproduce the magnitude of the pressure peaks characteristic of chugging behaviour, but it is not capable of simulating the direct contact condensation phenomena occurring in the test section. In particular, TRACE predicts complete mixing of the water pool and no thermal stratification. This sets a difficult challenge for the simulation of thermally-stratified pools and the mock-up of real experimental conditions.

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