Assessment of Natural Circulation Interruption during Asymmetric Cooldown Transients

This paper presents assessment results for the Natural Circulation Interruption (NCI) phenomenon during the cooldown phase in a nuclear Pressurized Water Reactor (PWR). This phenomenon could take place due to several circumstances such as an asymmetric cooldown after the loss of the forced primary flow. Under NCI conditions, the connection of the Reactor Coolant System (RCS) to the Residual Heat Removal System (RHRS) and the homogeneous boration of the RCS could be hindered. Moreover, at very low or no primary flow rates and an operating Safety Injection (SI) system, a Pressurized Thermal Shock (PTS) could occur in the reactor vessel due to cold fluid stratification in the loops. It is therefore important to understand the cause of loop flow stagnation and what could be the appropriate operator actions in order to avoid such phenomenon. The main goal of the current study is to assess the effect of a cooldown strategy upon the single-phase NCI occurrence. For this purpose, two scenarios with asymmetric cooling between the reactor cooling loops were investigated: the first one concerns a Feed Water Line Break (FWLB) combined with a Loss Of Offsite Power (LOOP), while the second one is limited to the LOOP (or any other transient leading to the loss of the forced primary flow). The analyses were carried out using the CATHARE 2/V2.5_1mod8.1 thermal-hydraulic system code. The calculation results emphasize, mainly, the effect of the cooldown rate and the opening strategy of the main steam Atmospheric Discharge Valve (ADV) upon the occurrence of the NCI phenomenon.

VLASSENBROECK Jacques;  BOUSBIA SALAH Anis;  BUCALOSSI Andrea; 

2010-11-26

AMER NUCLEAR SOC

JRC56999

0029-5450,   

https://publications.jrc.ec.europa.eu/repository/handle/JRC56999,