Vessel & Internals Flashcards
Explain the purpose of the Reactor Vessel and Internals
The reactor vessel is part of the PCS boundary and is a Fission Product Boundary.
Reactor Vessel Internals - Provide support for and orient the reactor core fuel bundles and control rods, absorb the control rod dynamic loads and transmit these and other loads to the reactor vessel flange, provide a passageway for the reactor coolant and support incore instrumentation.
Given a sketch of the Reactor Vessel and Internals, trace the normal primary coolant flow path through the reactor vessel, without error.
Draw:
Normal Flow - Coolant enters through four (4) inlet nozzles (cold legs) and is directed downward in the annulus region between the Core Support Barrel (CSB) and the Reactor Vessel. At the bottom of the CSB coolant is directed toward the center of the reactor vessel by the flow skirt. The coolant then travels upward through the core and into the Upper Guide Structure (UGS). The coolant then exits the reactor vessel via two (2) outlet nozzles (hot legs). A significant amount of coolant flows upward through the Upper Guide Structure (UGS) and into the upper head region prior to exiting via the hot leg nozzles.
Given a sketch of the Reactor Vessel and Internals, trace the bypass primary coolant flow path through the reactor vessel, without error.
Draw: Bypass Flow - Most is from coolant flowing upward through the region between the CSB shell and the core shroud. This flow minimizes thermal gradients, provides some head cooling and minimizes unacceptable chemistry conditions that might result from stagnant water.
Describe the operational design of the Reactor Vessel Flange Inner and Outer Seals
- Function to seal the vessel head interface.
- Two silver-plated, self-energizing, inconel O-rings. A self-energizing O-ring is one that is hollow with flow passages that allow the pressurized fluid inside the O-ring to assist the sealing force.
- Inner seal pressurized. The outer seal would be pressurized only if there were a failure of the inner seal.
What alarms are associated with the Reactor Vessel Flange Inner Seal?
EK-0767, “REACTOR VESSEL FLANGE INNER SEAL LEAKAGE”
- Setpoint: 1500 psig on PS-0101 . Can be verified by indication PI-0101
- Sensing line penetrates the reactor flange and terminates in the space between the two O-ring seals.
- Drain path is to the Primary Drain Tank that is normally isolated by CV-0101 being closed.
What alarms are associated with the Reactor Vessel Flange Outer Seal?
EK-0768, “REACTOR VESSEL FLANGE OUTER SEAL LEAKAGE”
- Sensing line is located outside the outer O-ring groove and does not constitute part of the pressure boundary. Setpoint: 120” water column on LS-0160.
- Drain path is to the containment floor drain. Normally isolated by SV-0160 which opens on LS-0160 actuation.
Describe the operational design of the Core Support Barrel
- Cylindrical with an inside diameter of 150 inches and a wall thickness of 1 inch
- Suspended by 4-inch thick flange from the reactor vessel core support ledge
- Carries the entire weight of the core and other internals
- A 1.5‑inch thick core support plate containing flow distribution holes and fuel assembly alignment pinholes is supported by a ledge inside the core support barrel and by 52 core support columns. The core shroud is attached to the core support plate and limits core bypass flow.
- Within the core support barrel are axial shroud plates and former plates that are attached to the core support barrel wall and the core support plate and form the enclosure periphery of the assembled core.
Describe the operational design of the Core Support Assembly
- It is the major support member of the reactor internals.
- Consists of the core support barrel, the core support plate and support columns, the core shrouds, the core support barrel to pressure vessel snubbers and the core support barrel to upper guide structure guide pins.
- Supported at its upper flange from a machined ledge in the reactor vessel flange. The lower end is restrained in its lateral movement by six core support barrel to pressure vessel snubbers.
Describe the operational design of the Core Shroud
- Follows the perimeter of the core and limits the amounts of coolant bypass flow.
- Consists of rectangular plates 5/8 inch thick, 145 inches long and of varying widths.
- The critical gap between the outside of the peripheral fuel bundles and the shroud plates is maintained by seven tiers of centering plates attached to the shroud plates.
- Holes in core support plate allow some bypass flow upward between core shroud and CSB - minimize thermal stress, eliminate stagnant pockets of coolant.
Describe the operational design of the Upper Guide Structure
- Consists of a flanged grid structure, 45 control rod shrouds, a fuel bundle alignment plate and a ring shim.
- Aligns and supports the upper end of the fuel bundles, maintains the control rod channel spacing, prevents fuel bundles from being lifted out of position during a severe accident condition and protects the control rods from the effect of coolant cross flow in the upper plenum. It also supports the incore instrumentation guide tubing.
- The periphery of the flange contains four alignment keyways, equally spaced at 90-degree intervals, which engage the core barrel alignment keys. The reactor vessel closure head flange is slotted to engage the upper ends of the alignment keys in the core barrel.
- The fuel bundle alignment plate is designed to align the upper ends of the fuel bundles and to support and align the lower ends of the control rod shrouds.
- Since the weight of a fuel bundle under all normal operating conditions is greater than the flow lifting force, it is not necessary for the upper guide structure assembly to hold down the core. However, the assembly does capture the core and would limit upward movement in the event of an accident condition.
Describe the operational design of the Core Stops
- Nine core support lugs (stops) are attached to the vessel lower wall.
- Designed to catch the core barrel in the event that the upper core supports failed. The Core Support Lugs contain a small yield pad that is intended to deform and absorb the energy of the falling core.
Describe the operational design of the Core Support Barrel Snubbers
- Sometimes referenced as CSB Stabilizing Lugs.
- Prevent flow-induced vibration and seismically-caused movement of the CSB.
- Six core support barrel snubbers provide a close fit between the core support barrel and the lower vessel wall.
Describe the operational design of the Flow Skirt
- Used to reduce inequalities in core inlet flow distributions and to prevent formation of large vortices in the lower plenum.
- It is a cylinder broken into 900 2.5-inch holes that break the large annular wall jet into approximately 900 2.5-inch diameter jets.
- Provides a nearly equalized pressure distribution across the bottom of the core support barrel.
- It is hung by welded attachments from the core stop lugs near the bottom of the pressure vessel and is not attached to the core support barrel.
Describe the operational design of the Fuel Assembly
Typical reload fuel bundle that consists of a square (15 by 15) array of 225 positions: 216 fuel rods, 8 Zircaloy-4 guide bars, and 1 Zircaloy-4 instrument tube
Describe the operational design of the Surveillance Capsules.
They measure the long term effects of temperature and radiation on reactor vessel materials
