ISSN Online: 2688-7231
ISBN Print: 978-1-56700-497-7 (Flash Drive)
ISBN Online: 978-1-56700-496-0
Proceedings of the 25th National and 3rd International ISHMT-ASTFE Heat and Mass Transfer Conference (IHMTC-2019)
Thermal Hydraulic Evaluation of Anticipated Transients Without Scram (ATWS) Scenario for VVER-1000
Anticipated Transients Without Scram (ATWS) is considered as Design Extension Condition that could happen if complete failure of SCRAM, i.e. the inability to automatically drop control rods into the core on reactor trip signal, takes place during transients. Kudankulam Nuclear Power Plant (KKNPP) has two operating VVER-1000 Pressurized Water Reactors. The mitigation of ATWS events takes place by passive and active safety systems designed for KKNPP. An ATWS signal could be generated by an observed failure to reduce neutron flux less than set value within a short time after any reactor trip (scram) signal. Boration of the primary circuit by safety systems after ATWS signal, bring the reactor to a stable condition with nearly zero fission power. Transient analysis with such safety systems are of paramount importance since the rate of boron mixing (both actively and passively) in the core is important with respect to reactor safety. Transient analysis of ATWS events has been analyzed using Thermal Hydraulic computer code RELAP-5/MOD 3.2 for KKNPP. RELAP5/MOD3.2 uses a one-dimensional, two fluid, non-equilibrium, six equation hydrodynamic model with a simplified capability to treat multi-dimensional flows. To analyze system thermal-hydraulic behavior under ATWS scenarios a detailed nodalisation was developed for KKNPP. ATWS mitigation systems were also modelled and the rate of boron addition by these systems in the core during such events was verified. Reactivity feedbacks due to fuel temperature, coolant temperature and boron concentration change in the core are simulated using control parameters coupled with neutronic point kinetics model. Initiating transient of Turbine trip along with ATWS have been analyzed to establish the effectiveness of boron injection safety systems. This paper contains the modelling and verification aspect of the required systems and evaluation of the thermal hydraulic parameters such as boron concentration, core reactivity, maximum fuel & clad temperature, primary & secondary pressures, departure from nucleate boiling ratio(DNBR). These parameters are checked against the applicable acceptance criterions.