Assessment of Natural Circulation Interruption during Asymmetric Cooldown Transients

• Published in: Nuclear technology Vol. 172; no. 2; pp. 179 - 188
• Main Authors: Vlassenbroeck, Jacques, Salah, Anis Bousbia, Bucalossi, Andrea
• Format: Journal Article
• Language: English
• Published: La Grange Park, IL Taylor & Francis 01.11.2010; American Nuclear Society
• Subjects: Applied sciences; Assessments; Asymmetry; Circulation; Cooling; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Fission nuclear power plants; Fuels; Installations for energy generation and conversion: thermal and electrical energy; Nuclear engineering; Nuclear fuels; Nuclear power generation; Nuclear reactor components; Nuclear reactors; France; Europe; natural circulation interruption; Coolant; Feed water; Unsteady state; Computation code; Reactor vessel; Injection; CATHARE 2; asymmetric cooldown; Natural circulation; Radioprotection; Nuclear reactor; Valve; Thermohydraulics; Stratification; Pressurized water reactor
• ISSN: 0029-5450; 1943-7471
• DOI: 10.13182/NT09-106

This paper presents assessment results for the natural circulation interruption (NCI) phenomenon during the cooldown phase in a nuclear pressurized water reactor. This phenomenon could take place because of several circumstances, such as an asymmetric cooldown after the loss of the forced primary flow. Under NCI conditions, the homogeneous boration of the reactor coolant system (RCS) and the connection of the RCS to the residual heat removal system could be hindered. Moreover, at very low or no primary flow rates and an operating safety injection system, a pressurized thermal shock 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 to derive accordingly the appropriate operator actions to avoid such a 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 feedwater line break 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 best-estimate thermal-hydraulic system code CATHARE 2/V2.5_1mod8.1, developed by Commissariat à l'Energie Atomique, Electricité de France, AREVA, and Institut de Radioprotection et de Sûreté Nucléaire. The calculation results mainly emphasize the effect of the cooldown rate and the opening strategy of the main steam atmospheric discharge valve upon the occurrence of the NCI phenomenon.