This paper presents thermal hydraulic modelling and simulation in the absorber tube of a Linear Fresnel Reflector (LFR) solar thermal system is carried out. The system is modelled using two-phase flow simulation software, RELAP5/MOD3.4. Although, RELAP5 is very commonly used in nuclear engineering design and simulation, it can be used for the simulation of solar thermal systems. Unlike other thermal systems like refrigerators and nuclear industries, there is significant heat loss from the surface of the absorber tube in a solar thermal system, which varies significantly with the temperature of the absorber tube wall. The recently developed temperature dependent heat loss has been incorporated in RELAP5 for variable net heat flux studies. The implementation of the temperature dependent heat losses has been verified by comparing the results obtained from RELAP with those obtained by Homogeneous Equilibrium Model. Parametric studies are carried out using verified RELAP model for different values of heat flux, mass flux, inlet subcooling and inlet pressure. The developed model can be considered as an effective tool for better and effective absorber LFR tube design under designed conditions. (C) 2015 Elsevier Ltd. All rights reserved.