Abstract
Provision of passive means to reactor core decay heat removal enhances the nuclear power plant (NPP) safety and availability. In the earlier Indian pressurised heavy water reactors (IPHWRs), like the 220 MWe and the 540 MWe, crash cooldown from the steam generators (SGs) is resorted to mitigate consequences of station blackout (SBO). In the 700 MWe PHWR currently being designed an additional passive decay heat removal (PDHR) system is also incorporated to condense the steam generated in the boilers during a SBO. The sustainability of natural circulation in the various heat transport systems (i.e., primary heat transport (PHT), SGs, and PDHRs) under station blackout depends on the corresponding system's coolant inventories and the coolant circuit configurations (i.e., parallel paths and interconnections). On the primary side, the interconnection between the two primary loops plays an important role to sustain the natural circulation heat removal. On the secondary side, the steam lines interconnections and the initial inventory in the SGs prior to cooldown, that is, hooking up of the PDHRs are very important. This paper attempts to open up discussions on the concept and the core issues associated with passive systems which can provide continued heat sink during such accident scenarios. The discussions would include the criteria for design, and performance of such concepts already implemented and proposes schemes to be implemented in the proposed 700 MWe IPHWR. The designer feedbacks generated, and critical examination of performance analysis results for the added passive system to the existing generation II & III reactors will help ascertaining that these safety systems/inventories in fact perform in sustaining decay heat removal and augmenting safety.
Highlights
INTRODUCTIONIn the 700 MWe PRESSURISED HEAVY WATER REACTOR (PHWR), the primary coolant heavy water under pressure removes (with partial boiling at channel exit) the fission heat generated in the reactor core and transfers it to the secondary coolant (light water) in the steam generators (SGs)
TO 700 MWe PRESSURISED HEAVY WATER REACTOR (PHWR)In the 700 MWe PHWR, the primary coolant heavy water under pressure removes the fission heat generated in the reactor core and transfers it to the secondary coolant in the steam generators (SGs)
station blackout (SBO) scenario includes the loss of all the operating pumps, that is, 4 primary circulating pumps (PCPs), primary pressurising pumps (PPPs), and all the boiler feed pumps (BFPs)
Summary
In the 700 MWe PHWR, the primary coolant heavy water under pressure removes (with partial boiling at channel exit) the fission heat generated in the reactor core and transfers it to the secondary coolant (light water) in the steam generators (SGs). As the primary coolant flows over the fuel bundles placed inside the channels, it picks up the fission heat in four passes through the reactor core. The recirculation flow from the steam drum, after mixing with the feed water, flows down the annulus (downcomer) It rises up through the main boiling zone, as it picks up the heat. In the 700 MWe PHWR, for controlling the PHT system pressure, a pressuriser (surge tank) is provided along with the Feed/Bleed system for maintaining the coolant inventory. At 100% full power steady state, the reactor core inlet temperature is 266 C, the core outlet temperature at the ROH is 310 C, and the reactor core exit quality is around 3% only
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