PSI - Issue 60

2

Rahul Mittal et al. / Procedia Structural Integrity 60 (2024) 604–613 Rahul Mittal/ Structural Integrity Procedia 00 (2019) 000 – 000

605

1. Introduction

The Primary Heat Transport (PHT) System transfers the heat generated in the core during normal operation to the steam generators, which produce saturated steam from secondary feed water for driving the turbine-generator. Function of main PHT System is to extract the heat produced in the nuclear reactor by circulating pressurized heavy water through the reactor coolant channels. During normal operation, the heat is used to generate steam for power production. The PHT system of 700 MWe Indian PHWR is divided in two independent and identical closed loops, each comprising of two SGs, two Primary Coolant Pump (PCP), two Reactor Outlet Headers (ROH) and two Reactor Inlet Headers (RIH) along with feeder pipes connecting coolant channel assemblies to the reactor headers. The heat transport medium (primary coolant) is pressurized heavy water and is circulated through and around the fuel bundles in the coolant channel assemblies, by the primary coolant pumps via reactor headers, feeder pipes and associated piping. Steam Generators transfer heat from primary system to secondary system. Two SGs are provided in each of primary loop. The Steam Generators of 700MWe PHWRs are vertical mushroom type design without integral pre-heater. They are recirculation type heat exchangers, having inverted U tubes for primary flow. Simplified flow diagram of PHWR is shown in Figure-1. Essential components of SG: Primary head, tube-sheet, cylindrical shell housing the tube bundle, extended steam drum which accommodates the steam separators and dryers. The primary head is sub-divided into two chambers viz., primary inlet chamber and primary outlet chamber using a divider plate. Man-holes are provided in primary head for in-service inspection of tubes and maintenance work plugging of tubes, if required. Four hand hole nozzles are provided just little above the tube sheet in the secondary cylindrical shell for visual inspection of the tube bundle, tube sheet and for tube sheet lancing. The internals necessary for steam drying can be inspected through a manhole in the upper steam plenum. Installation and replacement of steam separators or vanes of steam drier is through the manhole, if required. The U-tubes of tube bundles are rolled into primary side of cladded low alloy steel tube sheet and then seal welded. The secondary side of the steam generator consists of a shell, Feed Water Nozzle (FWN), Steam Outlet Nozzle (SON), moisture separator, drier, tube bundle and shroud. Egg-crate grid supports have been provided on the tubes. Refer Figure-2 for schematic diagram of Steam generator showing all the internals. Functioning of SG: The shell/secondary side is designed for a natural recirculation. Heavy water/primary fluid enters the steam generator at the inlet nozzles and passes through the tube bundle in the hot leg and exits through primary outlet at cold leg. The feed water at 180 o C (at Full power) enters the steam generator through two feed water nozzles provided diametrically opposite in the steam drum at higher elevation above the tube sheet. It flows down through an annulus just inside the shell (down comer), where it mixes with water coming from the separators. The water enters the heating surface (tube bundle) at the bottom, and it rises through the steam generator. The steam-water mixture enters the steam water separators — where steam is passed through to the driers and then to the steam nozzle; the water is recirculated, through the down comer, to the bottom of the heating surface. Steam Generator is a Safety Class-1 component and designed & fabricated as per the requirements of ASME Section III, Subsection-NB. The principal material of construction for secondary side pressure boundary is of low alloy carbon steel. Design parameters of 700MWe PHWR stream generators are given in Table-1.

Table 1. Design Parameters of 700MWe PHWR Steam Design Parameters No. of Steam Generators per unit

Four

Type

Vertical U-tube with integral steam drum (Mushroom-shaped)

Pressure Boundary Components Material

Low alloy Carbon Steel

Tube Material

Incoloy-800

Design Pressure / Temperature Tube Side Shell Side

126 kg/cm 2 / 326°C 55 kg/cm 2 / 270°C