Abstract
One of the transients that have received considerable attention in the safety evaluation of RBMK reactors is the partial break of a group distribution header (GDH). The coolant flow rate blockage in one GDH might lead to excessive heat-up of the pressure tubes and can result in multiple fuel channels (FC) ruptures. In this work, the GDH flow blockage transient has been studied considering the Smolensk-3 RBMK NPP (nuclear power plant). In the RBMK, each GDH distributes coolant to 40–43 FC. To investigate the behavior of each FC belonging to the damaged GDH and to have a more realistic trend, one (affected) GDH has been schematized with its forty-two FC, one by one. The calculations were performed using the 0-D NK (neutron kinetic) model of the RELAP5-3.3 stand-alone code. The results show that, during the event, the mass flow rate is disturbed differently according to the power distribution established for each FC in the schematization. The start time of the oscillations in mass flow rate depends strongly on the attributed power to each FC. It was also observed that, during the event, the fuel channels at higher thermal power values tend to undergo first cladding rupture leaving the reactor to scram and safeguarding all the other FCs connected to the affected GDH.
Highlights
The Reactor Bolshoy Moshchnosty Kanalny (RBMK) is a graphite-moderated boiling water reactor
This choice permits the investigation of the behavior of each fuel channels (FC) connected to the affected group distribution header (GDH) and having a more realistic trend
The present paper presents results of a hypothetical GDH flow rate blockage event in the Smolensk-3 RBMK
Summary
The Reactor Bolshoy Moshchnosty Kanalny (RBMK) is a graphite-moderated boiling water reactor. The coolant flow rate blocking in one GDH might lead to excessive heat-up of the pressure tubes and can result in multiple fuel channels (FCs) ruptures. The pressure tube (PT) and the fuel cladding temperatures reach the acceptance criterion (at nominal pressure, i.e., 7 MPa) few seconds after flow stagnation The overpassing of this threshold should cause the PT rupture, pressure increase into the reactor cavity and scram generation signal. The temperature failure criterion in the form of the tube rupture temperature in dependence on the channel pressure has been established for pressure tubes of RBMKs on the basis of experimental data This dependence varies with the increase in the heating rate. Each header distributes coolant to 40–43 bottom water pipes These pipes are provided with isolation and control valves between the GDH outlet and the fuel channel inlet. Valves are installed upstream and downstream MCP to isolate the pumps and to allow natural circulation in the loop constituted by FC-outlet, SD, SH, PH, GDH, and FC inlet
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