DESIGN PROVISIONS FOR SODIUM INVENTORY CONTROL IN FBR

B. Anoop, S. Athmalingam, S. Raghupathy & P. Puthiyavinayagam

Indira Gandhi Centre for Atomic Research

Department of Atomic Energy

Kalpakkam-603102, India

First IAEA Workshop on Challenges for Coolants in Fast Spectrum System : Chemistry and Materials5 – 7 July 2017

CONTENTS

Sodium systems and inventory requirement

Regulation of inventory

Prevention and mitigation of sodium leak

INTRODUCTION

Sodium is the most widely used coolant in fast breeder reactors due to

Availability

thermal properties

neutron economy

Thermal power generated in the core during reactor operation to be transferred toultimate heat sinks with/without generation of electric power by means of coolantcircuits.

Control and maintenance of sodium inventories is required for safe, efficient andeconomic operation of the nuclear reactor

The quantity of radioactive and non-radioactive sodium in the plant has to beregularly monitored and recorded for accountability.

Decommissioning activities of the reactor are planned and designed, based on theactive and inactive sodium inventories in the plant.

COOLANT SYSTEMS FOR FBR

Usually there will be a primary coolant circuit in sodium cooled Fast BreederReactors which transfers thermal energy to an intermediate sodium coolant circuit.Intermediate circuit exchanges heat to the steam water system or ultimate heat sink.

Primary circuit can be pool type or loop type

Number of loops in each coolant circuit varies based on operational convenience,safety approach, space and economic considerations.

There can be a dedicated decay heat removal circuit or the decay heat removalforms part of normal systems

Illustrations are based on Prototype Fast Breeder Reactor (PFBR) which is a pool type reactor.

HEAT TRANSPORT SYSTEMS AND SODIUM INVENTORIES

Nuclear heat in

the core

Primary sodium circuit

Secondarysodium circuit

Steam water circuit

Water / air

Electric power

Dedicated decay heat

removal sodium circuit

Air

Sodium inventory in pool type reactor

Primary sodium Secondary sodium DHR sodium

PRIMARY SODIUM IN POOL TYPE REACTOR

Intermediate Heat Exchanger

Cold pool

Pump

Hot pool

PRIMARY SODIUM CIRCUIT – SODIUM INVENTORY

For a pool type reactor : reactor core, intermediate heat exchanger (IHX), decay heatexchanger (DHX) and primary sodium pumps are immersed in primary sodium filled inmain vessel.

Height of the primary sodium in main vessel is decided by the height of inletwindows of the heat exchangers with margin to avoid gas entrainment.

Diameter of main vessel is decided by that required to accommodate all thecomponents of reactor assembly and heat transport systems.

Hence, sodium inventory in main vessel is decided by its diameter and height ofsodium pool.

For a loop type reactor heat exchangers and pumps are external to the reactor vesseland connected with piping. Sodium inventory is decided by volume of vessels, heatexchangers and piping.

Variations in bulk density of sodium under various operating conditions areconsidered in filling height of sodium in main vessel.

Sodium inventory in purification systems are added in total inventory filled.

SECONDARY SODIUM CIRCUIT – SODIUM INVENTORY

Sodium inventory in secondary sodium loop is governed by

Number of loops

Heat transfer capacity of the loop

Position of components in the loop

Inventory in auxiliary circuits like fill and drain, purification and SG Tube leakdetection

Sodium inventory in secondary circuit is decided by that filled incomponents and piping of these circuits and sodium level maintained in theconnected secondary sodium storage tanks to which sodium is drained.

Storage tank Purification loop

SG leak detection

Feed water in

Steam out

Steam Generator

Surge Tank

Pump

Intermediate heat exchanger

Primary sodium

Leak off *

Overflow

EM Pump

SECONDARY SODIUM CIRCUIT

* : Leak off line required only if pump is at lower elevation as in the case of PFBR

DECAY HEAT REMOVAL CIRCUIT – SODIUM INVENTORY

There can be a dedicated decay heat removal (DHR) system or decay heatremoval can be performed by normal heat removal system

Multiple loops can be provided to enhance availability of the system

Capacity of each loop is decided by the maximum decay power to beremoved from the reactor assuming unavailability of one loop

Sodium inventory is governed by

Heat transfer capacity of the loop

Elevation difference between thermal centres in the loop in case of natural circulationloop

Inventory in auxiliary circuits like fill and drain & purification

Sodium inventory in DHR circuit is decided by that filled in components andpiping of the loops

Storage tank

Purification loop

Air out

Air Heat exchanger

Expansion Tank

Decay heat exchanger

Primary sodium

EM Pump

DECAY HEAT REMOVAL CIRCUIT

Air in

INVENTORY REGULATION : PRIMARY CIRCUIT

For pool type reactor : Continuous and discontinuous level probes in hot pooland cold pool. Three important levels in hot pool are

High level : To detect leak of secondary sodium / DHR loop sodium to primary sodium

Fill level : Used for initial filling of sodium in main vessel

Low level : Top of IHX window - Sodium level required for heat transfer

Low level in secondary sodium storage tank or DHR expansion tank will giveearly indication for leak through IHX or DHX.

Cold pool level probes gives alarm if level is below fuel subassembly top.

For loop type reactor, individual levels in reactor vessel, pump tank andstorage tank are monitored.

For secondary sodium loop of fixed volume, the highest level of sodium is the surgetank and overflow line is provided in surge tank to send excess sodium in the loop tothe storage tank.

Storage tank and the loop in connected during operation.

During operation of the secondary sodium circuit, there will be a continuous loss ofinventory from the loop to the storage tanks due to leak of flow through secondarysodium pump labyrinths if the pump is at lower elevation in the cold leg and smallsampling flow through SG leak detection system.

To make up the losses, sodium is continuously fed into the secondary sodium loopby an electromagnetic (EM) pump from the storage tank

For an effective control of inventory in secondary sodium loop, the EM pumpinjection rate is sufficiently excess to the total expected drain rate from the systemconsidering all states of operation and the excess sodium injected into the loop isreturned to the storage tank though the overflow pipeline of surge tank connecting tothe storage tank.

A small quantity of sodium remains in the secondary storage tank to enable sodiumcirculation through the fill and drain lines

SECONDARY SODIUM CIRCUIT – DYNAMIC BALANCING OF INVENTORY

Storage tank Purification loop

SG leak detection, q3

Feed water in

Steam out

Steam Generator

Surge Tank

Pump

Intermediate heat exchanger

Primary sodium

Q = q1+q2+q3

Leak off, q2

Overflow, q1

EM Pump

SECONDARY SODIUM CIRCUIT – DYNAMIC BALANCING OF INVENTORY

INVENTORY REGULATION : SECONDARY CIRCUIT

Mutual inductance type continuous level probes and discontinuous levelswitches are provided in the system tanks.

Surge tank is at the highest elevation in the system and level switches forhigh, normal and low levels are provided. In addition to the alarmannunciation, the high and low levels in surge tank will trip the secondarysodium pump.

Low and fill level switches are provided in storage tank and low level switchtrips secondary sodium main pump and the EM pump.

Any overall leak from the system is detected by continuous level probe in thestorage tank when sodium is below the set point.

Low and high level are provided in the pump tank. The discontinuous levelswitches give interlock to trip the sodium main pump.

INVENTORY REGULATION : DHR CIRCUIT

The sodium is filled in the loop up to the specified level in the expansion tank of the loop, which is situated at the highest elevation.

Once filled, the loop is isolated from the storage tank and sodium inventory in the loop is maintained.

Mutual inductance type continuous and discontinuous level probes in the expansion tank and storage tank. Discontinuous level switches indicate high level, fill level and low levels.

Need to prevent loss of sodium in the circuits

To ensure safety of the reactor by adequate

removal thermal power from the core

Plant outages resulting in revenue loss

Primary sodium is active due to Na-24, Na-22, corrosion and fission products. Leak of sodium will lead to additional

Man-Rem consumption.

Leak into atmospheric air generates sodium aerosols

constituted by sodium hydroxide and sodium carbonate. These are corrosive and harmful to health.

Sodium hydroxide has a Threshold Limit Value of 2.0

mg/m3 in air.

PREVENTION OF SODIUM LEAK

PROVISIONS TO PREVENT SODIUM LEAK

Stringent design and manufacturing procedures are followed for sodium systems of a fast breeder reactor.

Sodium systems important for safety are designed and manufactured according tohighest safety class as per stringent nuclear codes. Systems undergo detailed designanalysis.

Material of construction selected for sodium systems are based on wide experience andmaximum expected service conditions

Stringent specification for fabrication and erection of sodium systems with proper qualitycontrol, qualification of fabrication procedure, all welded construction, volumetric andsurface inspections, integrity testing, helium leak testing etc.

ENGINEERED FEATURES FOR MITIGATION OF LOSS OF INVENTORY

In a pool type reactor full inventory of primary sodium is in the reactor vesseland the core is always submerged in primary coolant.

A safety vessel around main vessel can be provided to contain the primarysodium in case of a leak for both pool type and loop type reactors.

The gap between safety vessel and main vessel is restricted such that even incase of a main vessel leak, the limited volume between main vessel & safetyvessel restricts fall in sodium level such that sodium circulation through heatexchangers are ensured.

For a loop type reactor, siphon break arrangement is provided in the pipingconnected to primary sodium in the reactor vessel. This will prevent drainingof sodium below a safe level in the reactor vessel.

ENGINEERED FEATURES FOR MITIGATION OF LOSS OF INVENTORY

Most of the equipment and piping of the auxiliary circuits are kept at higherelevations such that sodium in the loop can be drained to the main vessel orstorage tanks in case of a leak incident. Syphon break arrangement to beprovided for any piping leading to lower elevations than the sodium level inthe main vessel.

In case of sodium leak in the system, the EM pump is tripped, argon isinjected into the system to drain to sodium from the loop.

In order to prevent sodium fire in reactor building, all the sodium pipelines inthe building are kept in guard pipe or steel cabins filled with nitrogen gas

Sodium piping without guard pipe are provided with leak collection trays formitigation of sodium fire after a leak incident

Provision to isolate the part of particular loop from balance system and todump sodium from the loop

LEAK DETECTION

System to be designed for an early detection and mitigation of sodium leakand loss of inventory.

Leak before break approach is followed in the design of large piping andcomponents to enable leak detection before catastrophic failure of the system

Sodium leak detectors required to be placed external to all sodium systempiping and components which gives alarm in the main control room as well aslocal control centres seeking urgent operator action

Diverse and redundant leak detectors based on different principles are used .

Spark plug leak detector

Mutual inductance leak detector

Wire type leak detector

MUTUAL INDUCTANCE TYPE LEAK DETECTORS

ASSEMBLY FOR MI LEAK DETECTOR FOR GUARD PIPE

SECONDARY

S.S. FORMER

SECONDARY

PRIMARY COIL

COIL LEADS

COIL

PRIMARY

DOUBLE ENVELOPE

SODIUM PIPE

Py. & Sy. Windings

FIG.6: MUTUAL INDUCTANCE TYPE LEAK DETECTOR

2 METERS

WIRE TYPE LEAK DETECTORS

CERAMIC BEAD NICKEL WIRE

SS Binding

SS Pipe

Na Wire Type Leak

Detector

Heater Wire

Glass Wool

Insulation

Al. Cladding

Thermocouple

LEAK DETECTION

Sodium aerosol detector are provided near to the piping and components insteam generator buildings for leak detection.

Sodium leak in steam generators tubes will result in sodium water reaction.Continuous sampling of sodium at the outlet of all the Steam Generators isdone to detect hydrogen evolved due to any leak in SG tube bundle. Nickeltube detectors and Electrochemical hydrogen meters are used hydrogen insodium detection. Thermal conductivity detectors are used in cover gas forhydrogen detection at lower temperature.

Indirect methods like rise / fall in sodium level in the tanks will also help inleak detection.

Sodium leaking due to valve passing into storage tank for a normallyisolated loop will be detected by level rise in the tank.

SG TUBE LEAK DETECTOR – HYDROGEN IN SODIUM

ELECTRONICS

MILLI TORR

GAUGESTD H2

LEAK

SO

RP

TIO

N

PU

MP

SPUTTER

ION

PUMP

MHEATER

ELECTRO

CHEMICAL

HYDROGEN

METER

ECHM

ELECTRO-

NICS

EC

HM

MAKE UP FOR LOSS OF INVENTORY

Any significant sodium loss in the system due to leak will be replenishedfrom the reserve sodium storage.

Sodium loss due to that sticking on the components taken out formaintenance and spent subassemblies immersed in primary sodium to bereplenished periodically.

Additional quantity of sodium is filled in the DHR loop considering the loss of inventory through seat leakage of isolation valves to storage tank. The additional quantity is for operation between two fuel handling campaigns including the DHR period.

Make up is done by means of EM pumps or pressurization method from the storage tanks.

SUMMARY

Quantity of sodium in heat transport circuits decided by thermal power andsystem configuration

Effective heat removal from the core can be impaired due loss of sodiumfrom the heat transport sodium circuits which affects safety in operation ofthe reactor.

Redundant and diverse instrumentation have to be provided for monitoringsodium level and for leak detection

Engineered safety features for prevention and mitigation of leak

The sodium inventory control and accountability of radioactive and non-radioactive sodium is necessary for planning and design of decommissioningactivities of the plant after the service life.

THANK YOU FOR KIND ATTENTION