Document : n.a.Proprietary Class 3Page 1 The design of the PBMR Core Structures Mark Mitchell Pebble...

Document : n.a. Proprietary Class 3

The design of the PBMR Core Structures

Mark MitchellPebble Bed Modular Reactor (Pty) ltd.

Fifth International Nuclear Graphite Specialists Meeting

Plas Tan-Y-Bwlch, Maentwrog, Gwynedd, United Kingdom

12th – 15th September, 2004


Content

• Overview of the PBMR Plant design• Overview of the PBMR Reactor Unit• Description of the design of the PBMR Core

Structures– Core Barrel Assembly (CBA)

– Core Structures Ceramics (CSC)

• Current status of the project


PBMR Plant Design Overview

• The PBMR Main Power System (MPS), end 2003.

Reactor UnitHPT LPT

PTG

Recuperator

Pre-coolerInter-cooler

CBCS

CCS


Reactor

Core Barrel Conditioning

System

Maintenance Isolation/Shutdown Valve

Generator

Power Turbine

Recuperator

High Pressure Compressor

Low Pressure Compressor

Gearbox

Inter-CoolerCore

Conditioning System

Pre-Cooler

Main Power System - 2004


Brayton Cycle (PFD)


Main Power System T-S Diagram (HTR and VHTR)

Specific Entropy

Tem

per

atu

re

Additional energy gained

ROT

RIT

System Pressure Ratio = 3.2

ROT = 900°C cycleROT = 1200°C cycle


Main Power System in Building


RU Layout

Reactivity ControlSystem (RCS)

ReserveShutdown

System (RSS)

CoreUnloading

Device (CUD)

CoreStructures

(CS)

ReactorPressure Vessel

(RPV)

System Function

Core Structures (CS) To form and maintain the core geometry.

Reactor Pressure Vessel (RPV)

To contain the helium under pressure.

Reactivity Control System (RCS)

To control reactivity and shutdown the reactor

Reserve Shutdown System (RSS)

To shutdown the reactor

Core Unloading Device (CUD)

Remove the fuel elements from the core


RU Vertical Section

System Function

Fuel Line To feed fuel spheres to the core

Fuel Core The generate heat by nuclear fission

Bottom, Centre, Side & Top Reflector

To reflect neutrons back to the core

Control Rod (RCS) To control the reactivity

SAS Channel (RSS0 To shutdown the reactor

SAS Extraction Point To extract the SAS from the SAS Channel

Fuel Core

Control Rod

Fuel Line

SideReflector

CentreReflector

TopReflector

BottomReflector

SAS ExtractionPoint

SASChannel


RU Horizontal Section


Core Structures – Product Breakdown Structure• Core Structures (CS)

– Core Barrel Assembly (CBA)• Core Barrel• Top Plate• Upper and Lower Support Rings• Upper and Lower Lateral Guides

– Core Structures Ceramics (CSC)• Top Reflector• Side Reflector• Central Reflector• Bottom Reflector• Lateral Restraints


General arrangement of the CS.

• Identification of the major components of the CS.


Functions of Core Structures

• Provide and maintain core geometry• Provide and maintain flow path for fuel spheres• Provide and maintain openings for the Reactivity Control

and Shutdown Systems• Provide inlet and outlet flow paths for helium gas• Provide neutron reflection• Provide thermal neutron and gamma shielding• Provide and maintain specified heat transfer path


Core Barrel Assembly

• Core barrel assembly supports the CSC

• Provides thermal shielding to the RPV

• A Schematic representation of the CBS is provided in the sketch.

Core Barrel Side

Circumferential Restraints

Top Plate

Upper Support Ring

Core Barrel Lateral Guide

Vertical Support

Lower Support Ring

Lower Lateral Guide

Supported by RPV

Supported by RPV

Supported by RPV Metallic Portion of

the Defuel Chute

Vertical Support Bearing Vertical

Support Assembly

CBSS

CBSS Skirt


Core Barrel Support Philosophy

• Core barrel >22 m long and >18 m circumference

• Heats up to ~ 400°C and cools down during shutdown

• Uneven temperature distribution will cause barrel to bow

• Support design must allow for small amount of bowing

• Solution is the single vertical support system, coupled with lateral and seismic restraints.

Lateral Support Assembly

Core Barrel bowing allowed

Lateral Support Assembly

Single point Support at bottom


Layout of Core Barrel Assembly

CBSS

Circumferential Restraints

Later Upper Restraint Beam

Bearing Bracket

Core Barrel Side

Main coolant outlet interface

Main coolant inlet (X2) interfaces

CCS inlet

CCS outlet

Top Flange


Core Structures Ceramics (CSC)

• Top Reflector• Side Reflector• Central Reflector• Bottom Reflector• Lateral Restraints

• Materials selection and environmental conditions• Description of components


Fundamental Safety Functions - CSC

Safety Functions:• Control reactivity (Core geometry / Control and Shutdown

element access)• Remove core heat (Limiting case -> Passive heat removal

through core components)• Contain radioactive materials (No functions associated with

this.)


Design Lifetime Requirements

• Plant Design life: – 40 Calendar years

– 36 Full Power Years

• CSC Design life (Replaceable components)– Target lifetime - 24 Full Power Years

– Minimum lifetime – 18 Full Power Years.


Core Structures Materials

• Candidate materials:– Graphite

• NBG-10• NBG-12• NBG-32• NBG-18

– CMC’s (specifically C-C)

• SIGRABOND 1501 YR• SIGRABOND 2001 YR

– Insulation• Carbon• Fused quartz• Alumina


Graphite Materials Breakdown

• Grade NBG-10: Medium grain, isotropic pitch coke, extruded Locations used: - Contact areas between reflector & fuel sphere

- High flux regions, typically > 1012 n/cm2 EDNF - Long-life components

- Highly stressed & critical components

• Grade NBG-12: Medium grain, isotropic pitch coke, extruded(includes recycled graphite in mix)

Locations used: Mainly NBG-10 replacement(larger blocks possible)

• Grade NBG-32: Medium grain, isotropic pitch coke (coke differs from NBG-10/12), vibration mouldedLocations used: Base & top of centre reflector, larger

components possible


Material Exposure Conditions (HTR)

Component Material Temperature (ºC) Fluence (x1020

n/cm2 EDN)Normal Accident

Outer Central Reflector

NBG-10 500-1050 1600 280

Side Reflector NBG-12 500-900 1200 220

Tie Rods 1501YR 500 1000 <1020

Restraint Straps 2001YR 500 700 <1020

Bottom Insulation

Ceramic/ Carbon

550 550 <1020


Scope of Graphite Irradiation Tests

• Selected irradiation dose and temperature range cover property changes well in excess of expected limits for the extreme surfaces of the inner side and outer central reflector up to 24 FPY.

• This irradiation regime also covers the expected EOL property changes for the graphite materials.


BR (1)


BR (2)


BR (3)


Bottom Reflector Outlet Flow Design Sections

De-fuel cone area

Plenum area

Support block area

Spacers

Flow transition

Flow transition


Section Through Side Reflector

Outer side reflector sealingkeys

Inner reflector

Outer reflector

Inner reflector dowel

Riser channel

Inner reflector keys

Control rod channel

Control rod sleeve

Inner to outerreflector lock


Typical Inner Side Reflector Block

Keyway

Dowel

Control rodchannel

Sleeve locator

Inner face(to core)

Flow distubance groove

Control rod channel

Dowel

Keyway


Typical Outer Side Reflector Block

Dowels

Keyway

Seismic restraintlocation


Central Reflector

• The solid central reflector allows for insertion of shutdown elements• Decreases the amount of bypass flow• Outer diameter is 2 m

– Outer 400 mm thickness is affected by irradiation induced damage– Inner 1.2 m diameter is used as the primary load bearing structure

• The central reflector is supported on the core barrel bottom plate

• It is built up in two parts:

- Structural spine interlocking cross

- Vertically separated by spacer blocks to maintain single column principle

• Spine locked together by set of corner blocks• Corner blocks also hold final side blocks in place


Central Reflector Build Up (Cross Block)

NGB 12 Block

Centre coolingchannel

Dowel

Undercut

Keyway


Central Reflector Build Up (Cross and Spacer Blocks)

NGB 12 CrossBlock

Dowel

Undercut

Keyway

Middle supportblock

Middle supportblock



Cross BlockDowel

Undercutinterlocked

Keyway

Middle supportblock

Middle supportblock


Central Reflector Build Up(Cross Spacer and Corner Blocks)

Cross blocks

Dowel

Corner block

Corner block key andkeyway

Middle supportblock

Corner block

Side blockkeyway

Inspectionchannel

Surface coolantflow slot

RSS channel



Cross blocks

Dowel

Corner block

Corner block

Side block keykeyway

Inspectionchannel

Surface coolantflow slot

RSS channel

Side block

Side block


Principles behind the TR layout

• The principle layout of the Top reflector.

Core barrel topplate

Core barrelsides

Sidereflector

Topreflectorfixed tie

rods

RemoveableTop plate

plug

Ceramicrestraint

strap

Tie rodinserts

Topreflector

fixed

Verticalexpansion andpressure gap

Centrereflector

Tie rodlocators


Core Lateral and Seismic Restraints

• The individual columns of outer side reflector supported by closed rings

• Two materials SS-316 L and CFRC are used in the construction to compensate for differential thermal expansion between CB and CSC

• Allow for small vertical movements between columns by means of sliding and not allowing relative movements of individual columns due to temperature differences.

• Reduce formation of gaps and reduces leakage flow• Reduces the breathing loads during thermal cycling• Protrusions from the metallic portions provide for connection

between the CSC and core barrel during a seismic event.


Lateral and Seismic Restraint Design

OSR Block Dowel

Inlet flow riser

channel

OSR Block

Metallic strap

link

Seismic

support

snubber

CFRC

Racetrack

strap

Strap link pin

Recess in the

OSR block to

capture the

strap


THE RSA PROJECT RELEASE STATUS

• The South Africa government has designated PBMR a National Strategic Project;

• Project being restructured;

• Approach to Key Licensing Issues agreed by December 2004;

• Safety Analysis Report, Rev. 2 being prepared in format of Reg. Guide 1.70 with a target date of January 2006 for handover to the Regulator;

• Appeals to Environmental Impact Assessment positive Record of Decision being dispositioned (December 2004)


THE RSA PROJECT RELEASE STATUS

• Site preparation to start in the second quarter of 2007;

• Construction to start in April 2007

• Required to meet 2010 first fuel load date.

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