Assessment of RELAP5 MOD3.3 in predicting the flashing behavior of adiabatic steam-water flows

Several new reactor designs plan to utilize natural circulation flows in order to eliminate the use of pumps, allowing for continuous cooling without any external power. However, the lower operating pressures of natural circulation type reactors make them prone to flashing induced instabilities which significantly affect system operation. Therefore, having a system-analysis tool which can accurately model flashing phenomena is vital for safety analysis and licensing. One of the most common tools used by the United States Nuclear Regulatory Commission is RELAP5 MOD3.3. RELAP5 has been used habitually for system-level thermal-hydraulics modeling of traditional high-pressure nuclear reactor systems like the Pressurized Water Reactor and Boiling Water Reactor. However, several studies have shown that RELAP5 struggles in predicting two-phase parameters, such as void fraction, for low pressure regimes below 1 MPa. Moreover, few studies exist which have investigated the prediction capabilities of RELAP5 in modeling flashing flows. The prediction capabilities of RELAP5 MOD3.3 are investigated using a previous experimental dataset. The facility consists of an annulus having an inner diameter of 19.05 mm, and outer diameter of 38.10 mm. The dataset concerns 9 cases within a pressure range between 215.1 - 522.0 kPa. Measured parameters such as pressure, liquid temperature, void fraction, and gas velocity are compared to the results from a RELAP5 simulation. The RELAP5 simulation is modeled based on the dimensions of the unheated section of the annulus where only flashing occurs. Different types of boundary conditions are also investigated for properly matching the superheated conditions and void fraction at the inlet. Modifications to the Taylor bubble heat transfer coefficient are also implemented in the RELAP5 source code using a previously derived model. This model accounts for the effects of bubble geometry, and utilizes an updated liquid film thickness, and drift flux model for the prediction of Taylor bubble velocity. The static interfacial area correlation for Taylor bubbles in RELAP5 is also modified to better match the measured interfacial area for each case. The effects of implementing the modified Taylor bubble heat transfer coefficient in RELAP5 are compared with the experimental data with an emphasis placed on the void fraction prediction.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Nuclear, Plasma, Radiolgc Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2023

Author dc:creator

• Smith, Erik Richard

Contributors dc:contributor

• Brooks, Caleb
• Kozlowski, Tomasz

• Flashing
• Relap5

Statement dc:rights

• Copyright 2023 Erik Smith

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/120116

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F120116