Izvestiya vuzov. Yadernaya Energetika

The peer-reviewed scientific and technology journal. ISSN: 0204-3327

Assuring reactor’s subcriticality following beyond design accident by simultaneous cool down and depressurization

3/25/2019 2019 - #01 Global safety, reliability and diagnostics of nuclear power installations

Sviridenko I.I.

DOI: https://doi.org/10.26583/npe.2019.1.08

UDC: 621.039.587

The article presents the results of analysis of emergency transient, when reactor is maintained in subcritical conditions by autonomous passive safety systems, providing for residual heat removal along with primary depressurisation. The method presents analytical modelling of beyond design accident with complete long-term blackout caused by loss of all in-house safety power sources of a NPP. The research has been performed on a model of WWER-1000/V-320 using RELAP5 Сode. Specific feature of the model under study is availability of two passive safety systems: Passive Residual Heat Removal System from primary circuit (PRHRS-R) and Passive Cooldown System of Pressurizer (РСS-PRZ). Heat exchangers based on evaporative condensing devices of closed type – two-phase thermosyphons are used as the heat exchange equipment that conducts heat transfer from the primary circuit to the intermediate circuit in these passive systems. Such heat exchange equipment provides high efficiency, reliability and safety of emergency heat sink. In a beyond design accident conditions the two systems provide for transition and maintaining the reactor in final safe conditions. The main objective of the research is to analyse the possibility of maintaining subcriticality during long term simultaneous operation of the above mentioned passive systems. The analytical modelling shows that parallel operation of the RHRS-R and РСS-PRZ provides for reliable and effective heat removal along with primary depressurisation down to 2 MPa. Despite that the residual pressure allows injection of only 50% of the design inventory of ECCS Hydro Accumulators (ECCS-HA), the boron concentration reaches the level when primary subcritiality along all stages of the accident is guaranteed. In addition, the operation of the above mentioned systems provides for safe cooldown rate, avoiding challenges to primary pressure boundary (Pressurised Thermal Shock). The result achieved takes into account release of positive reactivity due to Reactor de-poisoning. Highest level of reactivity reached –2.79 $.

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reactor safety beyond design accident subcriticality passive residual heat removal system