Mindaugas Valincius

Integrated assessment of failure probability of the district heating network

Abstract The aim of the research presented in this paper is the assessment of failure probability of the district heating network piping. The applied methodology for assessment of failure probability of the piping network energy systems includes three types of analyses: probabilistic mathematical, deterministic thermal-hydraulic and integrated deterministic–probabilistic structural integrity analyses. The analysis of Kaunas (Lithuania) district heating (DH) network was performed. First of all, the statistical analysis was performed and the piping with the highest failure rate was determined. The thermal-hydraulic analysis was performed and loads for deterministic–probabilistic structural analysis were calculated for the selected part of DH network. The integrated deterministic–probabilistic structural integrity analysis was performed in two steps—general structural integrity evaluation and probabilistic analysis of chosen piping part. Finally, the probabilistic mathematical method was applied for the integrated assessment of failure probability of the DH network piping. This method takes into consideration statistical information about Kaunas DH piping failure data, system structure, and pipe failure probability received by integrated deterministic–probabilistic structural integrity analysis.

Study and demonstration of pressure wave-based leak detection in a district heating network

Abstract A district heating network (DHN) is one of the most important infrastructures in cities and towns of countries with colder weather. Citizens generally use DHN services for hot water supply all year round and also for space heating during winter. It is important that in case of an accident this service would be restored as soon as possible, causing minimal damage or inconveniences for the customers. In addition, it is important to minimise losses of the DHN operator. This study demonstrated a possibility of leak location in DHN using the data from the pressure sensors in the network, employing the negative pressure wave (NPW) method. The mass balance in the DHN is measured at the heating source, which acts as a confirmation of a leak in a closed system. The experiments in the real DHN were performed and the data was used to trace the leak location. Numerical modelling tools were used to model pressure transients during pipe break accident under various conditions in order to predict the effectiveness and limitations of the leak detection system.

The Concept and RELAP5 Model of Thermal-Hydraulic System, Employing a Rapid Condensation for Coolant Circulation

The rapid condensation event is mostly considered a dangerous and undesirable side effect in thermal-hydraulic systems. This work demonstrates a different viewpoint, where condensation implosion is employed to perform mechanical work. Previous experimental study of the condensation implosion event, briefly presented in this article, showed that condensation implosion can be induced intentionally. These results were used as the basis for further investigations. In this work, a concept of the thermal-hydraulic system has been developed, where condensation-implosions-generated pressure difference could be used as a driving force. Numerical study has been performed to investigate the operation of the developed conceptual thermal-hydraulic system. A thermal-hydraulic computer code RELAP5 was selected for modeling the system operation. The RELAP5 code was found not able to predict the condensation implosion; therefore, a modified heat transfer model was implemented into the code. This modification allowed simulating the condensation implosion artificially in the thermal-hydraulic system and modeling the system response to the event. Final results show that a proposed circulation principle is possible and such a thermal-hydraulic system can operate.

Modelling of Severe Accident and In-Vessel Melt Retention Possibilities in BWR Type Reactor

One of the severe accident management strategies for nuclear reactors is the melted corium retention inside the reactor pressure vessel. The work presented in this article investigates the application of in-vessel retention (IVR) severe accident management strategy in a BWR reactor. The investigations were performed assuming a scenario with the large break LOCA without injection of cooling water. A computer code RELAP/SCDAPSIM MOD 3.4 was used for the numerical simulation of the accident. Using a model of the entire reactor, a full accident sequence from the large break to core uncover and heat-up as well as corium relocation to the lower head is presented. The ex-vessel cooling was modelled in order to evaluate the applicability of RELAP/SCDAPSIM code for predicting the heat fluxes and reactor pressure vessel wall temperatures. The results of different ex-vessel heat transfer modes were compared and it was concluded that the implemented heat transfer correlations of COUPLE module in RELAP/SCDAPSIM should be applied for IVR analysis. To investigate the influence of debris separation into oxidic and metallic layers in the molten pool on the heat transfer through the wall of the lower head the analytical study was conducted. The results of this study showed that the focusing effect is significant and under some extreme conditions local heat flux from reactor vessel could exceed the critical heat flux. It was recommended that the existing RELAP/SCDAPSIM models of the processes in the debris should be updated in order to consider more complex phenomena and at least oxide and metal phase separation, allowing evaluating local distribution of the heat fluxes.

Experimental Investigation and RELAP5 Modeling of Two-Phase Flow in Horizontal Rectangular Channel

The investigation of steam, water, and air flow characteristics in horizontal channel is a part of major investigations program at the Lithuanian Energy Institute. The objective of this program is to identify condensation effects on two-phase flow stability and to predict conditions when rapid condensation could be induced in two-phase condensable flow. This article presents investigation of steam–water and air multiphase flow in nearly horizontal rectangular channel. The experimental data for pressure drop and interfacial and wall shear stresses in the channel with uniform distribution of void fraction are presented in this paper. Overall channel dimensions are length = 1.2 m, width = 0.02 m, height = 0.1 m; however, the test section was about 0.84 m in length. Three different flow types were analyzed at atmospheric pressure: (1) single-phase air flow (height of the channel was reduced to 0.075 m); (2) non-condensable air–water two-phase flow at void fraction of 0.75; (3) two-phase steam–water flow at almost saturation conditions, and void fraction of 0.75. RELAP5 Mod3.3 code was selected to model test cases. Modeling results and experimental measurements show good agreement with each other. The developed model will be used for calculating different cases of the process.