NATURAL CIRCULATION SYSTEMS: ADVANTAGES AND CHALLENGES

P.K. VijayanReactor Engineering Division,

Bhabha Atomic Research Centre, Mumbai, India

IAEA Course on Natural Circulation in Water-Cooled Nuclear Power Plants, ICTP, Trieste, Italy, 25-29 June, 2007 (Lecture : T-2)

COURSE ROADMAP

Opening Session

INTRODUCTIONS ADMINISTRATION COURSE ROADMAP

Introduction

GLOBAL NUCLEAR POWER ROLE OF N/C IN ADVANCED

DESIGNS ADVANTAGES AND

CHALLENGES

Local Transport Phenomena & Models

LOCAL MASS, MOMENTUM AND ENERGY TRANSPORT PHENOMENA

PREDICTIVE MODELS & CORRELATIONS

Integral System Phenomena & Models

SYSTEMS MASS, MOMENTUM AND ENERGY TRANSPORT PHENOMENA

N/C STABILITY AND NUMERICAL TECHNIQUES

STABILITY ANALYSIS TOOLS PASSIVE SAFETY SYSTEM DESIGN

Natural Circulation Experiments

INTEGRAL SYSTEMS TESTS SEPARATE EFFECTS TESTS TEST FACILITY SCALING METHODS

Reliability &Advanced Computational Methods

PASSIVE SYSTEM RELIABILITY CFD FOR NATURAL CIRCULATION

FLOWS

Opening Session

INTRODUCTIONS ADMINISTRATION COURSE ROADMAP

Introduction

GLOBAL NUCLEAR POWER ROLE OF N/C IN ADVANCED

DESIGNS ADVANTAGES AND

CHALLENGES

Local Transport Phenomena & Models

LOCAL MASS, MOMENTUM AND ENERGY TRANSPORT PHENOMENA

PREDICTIVE MODELS & CORRELATIONS

Integral System Phenomena & Models

SYSTEMS MASS, MOMENTUM AND ENERGY TRANSPORT PHENOMENA

N/C STABILITY AND NUMERICAL TECHNIQUES

STABILITY ANALYSIS TOOLS PASSIVE SAFETY SYSTEM DESIGN

Natural Circulation Experiments

INTEGRAL SYSTEMS TESTS SEPARATE EFFECTS TESTS TEST FACILITY SCALING METHODS

Reliability &Advanced Computational Methods

PASSIVE SYSTEM RELIABILITY CFD FOR NATURAL CIRCULATION

FLOWS

OUTLINE OF THE LECTURE T#2

- INTRODUCTION

- APPLICATIONS OF NCS

- WORKING PRINCIPLE OF NCS

- ADVANTAGES OF NCS

- CHALLENGES OF NCS

- CLASSIFICATION OF NCS

- TERMINOLOGIES USED IN NCS

- CLOSURE

Introduction to NCS

Fluid-filled systems with buoyancy-induced circulation caused by density gradients is known as NCS. The density gradient can be caused thermally (thermosyphon loops) or adiabatically by the injection of a low density fluid (adiabatic NCS).

A heat source and a heat sink connected by pipes form the essential hardware of thermosyphon loops

Heater

Cooler

Thermosyphon loop

Gas

Separator

MixerGas

Adiabatic NCL

A mixer and a separator connected by pipes form the essential hardware of adiabatic NCLs (e.g. gas lift pumps).

Circulation is established in the loop without any fluid moving machinery. High reliability as the driving mechanism is based on a natural physical law.

Both type of NCSs have extensive application in conventional and nuclear industries. In this talk we limit our attention to thermosyphon loops.

Application of NCS

Espresso Coffee Machine

Traditional applications: solar water heaters, geothermal power extraction, coffee machines, cooling of transformers, rotating machinery, nuclear reactors, etc.

Natural Circulation cooling in Cray Computer CPU Cooling in PCs

An Emerging field of application is the cooling of computer systems. NCSs for computer cooling can be made with very low inventory of working fluid. Also, the heat transport capability of such loops can be as low as a few hundred Watts.

WORKING PRINCIPLE OF NCS

Heater

cooler

H ρcρh

b a

Due to the difference in densities between the vertical legs, a pressure difference is created between stations a and b which is the cause of the flow.

At steady state the driving buoyancy force is balanced by the retarding frictional force thus providing a basis for the estimation of the flow rate.

2

2

2 A

RWgH hc

Natural circulation flow is enhanced by the loop height and the difference in densities

BRIEF HISTORY OF NCS

First large scale use of NC systems appears to have been in the automobile industry. Such systems were in use till the 1940s.

NCSs find extensive use in chemical and power generation industries

- Thermosyphon reboilers

- Natural circulation boilers (NCB)

NCBs of capacity up to 660 MWe are in operation today. NCBs are capable of similar thermal performance compared to ACBs, economically competitive with less operation and maintenance cost.

Even 900 /1000 MWe plants use 2 or 3 NCBs rather than an assisted circulation boiler (ACB).

Steam

Condensate

Thermosyphon Reboiler

NCB in FPP

Natural circulation in Nuclear Industry

-Current PWRs and PHWRs are designed to remove decay heat by NC in the event of a CLOP

-For existing PWRs as much as 20 % FP can be removed by single-phase NC

-District heating reactors upto 500 MWt based on single-phase NC are operational

-CAREM is a PWR being developed by Argentina

Single-phase NC

-High temperature reactors are required for enhancing thermal efficiency as well as for hydrogen production. Many of the high temperature reactor concepts prefer NC (e.g. LFR)

-NC reactor designs are being proposed even for SCRs (CANDU–X-NC and BF-500SKDI)

1 – reactor core2 – core barrel 3 – steam generators4 – guard tube block shroud5 – reactor pressure vessel6 – reactor closure head

The BF500 SKDI reactor

Natural circulation in Nuclear Industry – Contd.

NC based steam generators of rating around 1000 MWt are extensively used in PWRs, PHWRs and VVERs

NC is the preferred circulation mode for steam

generation in conventional as well as nuclear industry

Two-phase natural circulation

Two-phase NC based Reactors (NBWRs)

Humboldt Dodewaard VK-50 ESBWR VK-300 AHWRBay 3, USA Netherlands Russia GE Russia India

63 MWe 65 50 1380 250 300 1963-76 1969-97 Op. PAR Designed PLR

Dry

er

sect

ion

outlet Primary inlet

dow

nco

mer

Steam

Feed water

Level

Hea

t ex

chan

ge

sect

ion

NC SG

The Indian AHWR is the only one using the pressure tube concept.

Many new designs under development like the ESBWR, AHWR and VK-300 are also based on two-phase NC.

ADVANTAGES OF NATURAL CIRCULATION

Reduced cost due to the elimination of pumps- Capital, operating and maintenance costs are

lowImproved flow characteristics and distribution- NC flow increases with power- FC flow decreases with power in the two-phase region- NC can handle power maldistribution better than FC

Improved safety characteristics due to low power density and large coolant volumes - All safety issues associated with pump failure are eliminated - NC is based on a natural physical law and is not expected to fail - All transients become sluggish

- Large boil-off times

Simplicity in design and layout - Factory fabrication possible with better quality control

CHALLENGES FACING NCS

The NC based power reactors (Humbholdt Bay, Dodewaard and VK-50) are more or less of the same power rating and vintage.

Although, NCSs are well-proven in the fossil-fired power plants, their use in reactor systems has been limited.

Several innovative designs based on two-phase natural circulation are being pursued by several countries

Large scale deployment of NCRs are also hampered by certain characteristics of NCSs

CHALLENGES FACING NCSs – Contd.Low driving force

- Due to the use of tall risers NCRs are slender in structure, which may raise economic & seismic concerns (AHWR:40m; PHWR:35m)

- Low system pressure losses (a) Larger diameter of components leading to large coolant inventory (AHWR:330T; PHWR-220:70 T) (b) Elimination of mechanical separators can increase carry over and carry under. Increased carry under lowers both thermal as well as stability margin. Gravity separation may require large steam-water interface area and hence vessel. (c) Simplified piping and equipment layout

-Low mass flux ~400-1100 kg/m2s (2000-4000 kg/m2s) (a) Low channel power leading to larger core size for the same power rating (RPV of VK-300 VVER-1000; CV AHWR ~ PHWR-540) (b) Larger core size can lead to problems in reactor control (c) Loosely coupled regions in large core can induce zonal neutronic oscillations

- Low CHF - Core size optimization results in large exit quality - CHF enhancement techniques are desirable for power uprating

Instability Effects- NCSs are inherently less stable than forced CSs- Differences exist in the orificing to match power and flow- Conventional stabilizing techniques like inlet orificing does not always work.

Loop liquid continuity is desirable under all conditions to have smooth natural circulation flow at all powers (a) Inventory management procedures to accommodate the swell and shrinkage due to void formation and collapse required (b) Level control program required.

CHALLENGES FACING NCSs – Contd.

CHALLENGES FACING NCSs – Contd.

-Start-up with stagnant coolant at low pressure (a) Low pressure low flow (LPLF) regime is important (b) Identifying suitable thermal-hydraulic correlations could be an important design activity

-Specification of a start-up procedure- Avoidance of static and dynamic instabilities at low pressure- Minimize inventory transfer during heat-up- Selecting a suitable power and pressure raising procedure

-Specification of an operating procedure- The entire operating domain shall be stable- Selecting a power raising procedure to avoid instabilities- The complete operating procedure to be established

CLASSIFICATION OF NCSs

Bases for classification- State of the working fluid- Interaction with the surroundings- Shape of the loop,- Body force field- System inventory- Number of heated channels

State of the working fluid- Single-phase NC- Two-phase NC- Supercritical NC

- Single-phase NCS : Single-phase conditions prevail throughout- Decay heat removal in PWR, PHWR, VVER and FBR following a CLOP- District heating reactors (AST-500, NHR-200)- CAREM: 27 MWe- Cooling of canisters containing radioactive waste- Cooling of ventilation system filters following power failure

- Two-phase NCS: At least a part of the NCS experience two- phase conditions

- Diabatic (boiling/condensing) or adiabatic systems- Two-phase NC occurs following a CLOP in BWRs, or at reduced

inventory conditions in PWRs, PHWRs, etc.- Two-phase NC is capable of generating larger flows- Steam generation in conventional and nuclear industry- Operating experience exists with NC BWRs of smaller rating - Several new designs have been proposed

CLASSIFICATION OF NCSs - Contd.

- Supercritical NCS- Large change in the thermal expansion coefficient near

the critical point

- Larger efficiency (~ 40%, Peak flow rate at ~ 400oC)- Excellent heat transfer characteristics of SCW- CANDU-X and B-500 SKDI are some concepts

CLASSIFICATION OF NCSs - Contd.

Interaction with the surroundings- Closed loop NC (Only exchange energy with the surroundings)- Open-loop NCS (exchange energy as well as mass)

- Used in ship based reactors, research reactors, BWRs and NC SGs, thermosyphon reboilers, - NCSs for geothermal heat extraction, waste storage and surveillance facilities, ventilation systems etc.

CLASSIFICATION OF NCSs - Contd.

Based on the shape of the loop

heater

cooler

Rectangular

Cooler

Heater

Toroidal

Heater-2

Cooler-1 Cooler-2

Heater-1

Figure-of-eight

cool

er

heater

cool

er

U-tube

CLASSIFICATION OF NCSs - Contd.

Body Force Field

- Gravitational: - Stationary NCS

- Centrifugal: - Rotating NCS

A rotating closed loop NCS

Coo

led

sect

ion

Hea

ted

sect

ion

Radial limb

CLASSIFICATION OF NCSs - Contd.

Based on the System inventory- single-phase NCS- Two-phase NCS- Reflux condensation/boiling

Based on the number of heated channels- Single channel loops- Parallel channel loops

. Reactor systems, SGs, NCBs, etc.

Terminologies used in NCS

source

sink

Cold leg

Hot leg

dow

ncom

er

Feed

Steam

heat

er

riser

Hot leg, Cold leg, Riser, Downcomer

Terminologies used in NCS – Contd.

Feed

Steam

heat

er

Inlet orificing

Outlet orificing

Recirculation ratio, Inlet subcooling, inlet and exit orificing

rateflowsteamorFeed

rateflowmassLoopR

isatsub TTT

ifsub hhh

Terminologies used in NCS – Contd.Decay Ratio, Stability map, Islands of stability and instability

0 1000 2000 3000 4000 5000-2

-1

0

1

2

24

Orientation : HHHC

0 5000 10000 15000 20000 25000-3

-2

-1

0

1

2

3

P - mm of water column

Time - s

Orientation : HHHC

0 1000 2000 3000 4000 5000-3

-2

-1

0

1

2

3

P - mm of water column

Time - s

0 1000 2000 3000 4000 5000-3

-2

-1

0

1

2

3

Orientation: HHHC

P - mm of water column

Time - s

(d) Start-up from rest at 105 W(b) Start-up from rest at 90 W

(b) Start-up from rest at 80 W(a) Start-up from rest at 45 W

Orientation : HHHC

P -mm of water column

Time - s

A1 A2

DR= A2/A1 ; DR < 1 Stable; DR = 1 neutrally; DR > 1 unstable

stable unstable

stable

stable

power

Inlet subcooling

unstable stable

unstable

unstable

power

Inlet subcooling

Island of stability

Island of instability

CLOSURE

Brief History of NCS and their various applications- Thermosyphon reboilers, - NCB in fossil-fuelled power stations, - Nuclear SGs

The main advantages of NCSs are highlighted- Economics, Flow characteristics, Safety aspects, Simplicity

Challenges of NCS- Low driving force and its associated problems- Low CHF- Instability effects- Start-up and power raising procedure

Classification of NCS and common terminologies used in NCS

Thank you