BREEDER REACTORS

Dr. BC Choudhary

Applied Science Department

NITTTR, Chandigarh.

Nuclear Power Plants

Presently most of the reactors in operation are all essentially

“Burner Reactors” which use only the uranium 235U.

This isotope is present to the extent of less than 1% in naturally

occurring uranium.

A very important aspect of NPPs efficient utilization and

management of nuclear fuel.

In the nuclear reactors naturally occurring uranium is used as

nuclear fuel.

A nuclear power plant needs about 4 tones of fuel (in the form of

uranium oxide) for every Megawatt of electricity and this

requirement will be met by the reasonably assured resources.

Evident that a difficult situation will arise if the present design

technology continues to be used.

All reactors working in present day nuclear power plants are

essentially burner reactors waste the abundant U238

isotope present in naturally occurring uranium.

A nuclear reactor utilizing only U235 as fuel is called a “Burner

Reactor”.

Naturally occurring uranium contains three isotopes, U234, U235

and U238 with relative percentage of these isotopes as

Of these isotopes, only U235 undergoes spontaneous fission

when subjected to bombardment by slow neutrons. It is in

fact the only naturally occurring “Fissile material”.

U234 0.006 percent

U235 0.711 percent

U238 99.283 percent.

Nuclear Fuel Materials

Uranium is the principal fuel material for nuclear reactors. Many

other materials are essential to make use of Uranium as fuel in

reactor core.

Uranium and Thorium Nuclear fuels

Zirconium Zircalloy (Zr, Sn, Cr, Fe, Ni) - cladding

Beryllium Neutron source, moderator, reflector

Heavy Water or Graphite Moderators

Niobium-Tantalum Structural material and electronic components

Rare Earths (Na, Cd,…) Control rods and nuclear fire extinguishers

Heluim Cover gas

Uranium fuel is loaded once in a year and fuel capacity is 61 tones

for a 235 MWe PHWR.

Principal fuel operations in

NPPs

Uranium Ores

Source material for Uranium-Plutonium cycle is raw

Uranium ore chiefly in the form of primary minerals,

Pitch blende Uraninite

These are oxides of variable

composition ranging from UO2

to U3O8 and are generally

represented by aUO2 . bUO3

Ratio b/a varies from 2 to 0.

Also called

“Yellow cake”

Uranium Enrichment- Centrifuge

Processing of Uranium

Nuclear Fuel Cycles

Uranium-Plutonium Fuel Cycle

Operation of nuclear fuel starting from

excavation of the ore to the final disposal

of nuclear waste via fabrication of fuel

elements, in core utilization and

reprocessing.

Basic materials for nuclear reactors

Uranium & Thorium

In present day reactors a large

amount of 238U is always present

and 239Pu is produced as bye-

product.

However, in a given time the total number of 239Pu nuclei

produced is less than the number of 235U nuclei consumed.

Uranium-Plutonium Breeding

Thorium- Uranium Cycle

Source material for Thorium-Uranium cycle is “Monazite Ore”

Most abundant deposits of monazite ore bearing sands are located

in India, USA and Brazil. Large quantities of these also occur in

Canada, Australia and Turkey.

In its raw form, the ore is essentially a mixture of rare earth

phosphates and contains nearly 1-5% of thorium dioxide (ThO2).

As in case of Uranium, Thorium is first concentrated to 5-8%

using solvent extraction or ion exchange processes.

Using standard techniques, rare earths are separated and purified

thorium is precipitated as oxalate, which on heating yields ThO2.

If ThO2 is heated in presence of HF, one obtains ThF4 , which

may be reduced to metallic thorium by heating with calcium.

None of the isotopes of Thorium occurring in nature are fissile.

Thermal neutron capture cross-section for Th232 is nearly three

times that of U238, while fast fission cross-section of Th232 is

less than that of U238.

Fuel elements are made using Thorium mixed with fissile isotope

of U233 or (even U235 ) but, for making the reactor critical the

enrichment required is more in this case than in case of

Uranium.

This makes this cycle more costlier.

However, the basic aim of using this fuel cycle is that the value of

(breeding ratio) in thermal energy region is more in 233U than for 235U by about 10% possible to achieve a higher conversion

ratio, which would lead to a more efficient fuel utilization.

Thorium-Uranium Fuel Cycle

The essential difference between the Th-U and U-Pu cycles is

that the former is based on the utilization of 232Th, whereas the

later utilizes 238U as the basic fertile isotopes.

Comparison of physical and chemical properties of Th and U

ceramics indicates that the Th-U cycle is more advantageous.

Monazite ores contain about 5% of Thorium

Thorium Vs Uranium Reactors

Production of Fissile Isotopes

Reactors based on above principle are called “Breeder reactors”

Although U238 is not a fissile material, it is a

“Fertile material”

Can be converted by neutron bombardment

into a fissile material, Plutonium-239

(Pu239).

Similarly naturally occurring Thorium-232

(Th232) is also a fertile material. It can be

converted into U233 which is a fissile material.

The neutrons generated by fission reaction of

U235 can be used in converting fertile material to

a fissile material.

A breeder reactor working on the U238 to Pu239 cycle utilizes

naturally occurring uranium almost completely and thereby helps

to extend the supply of uranium by a factor of at least 100.

Similarly a breeder reactor working on the Th232 to U233 cycle

helps in utilizing the vast Thorium resources of the world.

Breeder Reactors

For India, Th232 to U233 cycle is of particular

significance, because of fairly large deposits of

Thorium in the “Monazite beach” sands in Kerala.

Country Tonnes % of total

Australia

USA

Turkey

India

Venezuela

Brazil

Norway

Egypt

Russia

Greenland

Canada

South Africa

Other countries

World total

489,000

400,000

344,000

319,000

300,000

302,000

132,000

100,000

75,000

54,000

44,000

18,000

33,000

2,610,000

19

15

13

12

12

12

5

4

3

2

2

1

1

Estimated World Thorium Resources

(Reasonably assured and inferred Th resources recoverable)

Thorium Potentials in India

Tuesday, August 21, 2007

India's 30% Thorium resource

base can fuel for next 2500 years

of Electricity

India is trying hard to get a n-deal

when it's a superpower in Thorium

and stands 2nd in Thorium deposit

with which it can be self sufficient

for next 2500 years.

Thorium Potential in India

Types of Breeder Reactor

Breeder reactors are classified on the basis of the energy of

neutrons used.

In U238 to Pu239 cycle, fast neutrons released in fission are

directly used to sustain the fission chain reaction, whereas in

case of Th232 to U233 cycle, neutrons are being slowed down

to energy 0.0235eV (Thermal Neutrons) for better yield.

• Reactors designed to achieve breeding using Pu239 as fuel with

fast neutrons are called fast breeder reactors (FBRs) and

• Reactors designed to achieve breeding with U233 as fuel and slow

neutrons are known as thermal breeder reactors (BRs).

Fast Breeder Reactors (FBRs)

FBRs provide the key to the full utilization of the country’s

Uranium resources and considerable efforts have been made to put

the concept of the fast breeder in practice.

Quite few Experimental Breeder Reactors have been built by now

and many more are under construction in different countries .

At high energies fission cross-sections of fissile isotopes are only

a few barns (almost 100 times less than in thermal energy region),

much more fuel is needed to sustain a chain reaction in these

reactors critical mass is much greater in a fast reactor.

The capture cross-sections for fertile materials are of the same

order as the fission cross-sections of fissile isotopes in the high

energy region. Hence for criticality the ratio of fissile fuel to

fertile material in a fast reactor need to be relatively high

requires highly enriched fuel (15-25%).

Design studies carried out so far indicate preferences for Liquid-

metal fast breeder reactors (LMFBRs) and Gas-cooled fast

breeder reactors (GCFBRs) over other fast breeder concepts.

• Although these two reactor types differ in details, the basic

constraints physics considerations and design specifications

in designing these are essentially the same.

• Use of enriched fuel and absence of any moderating materials

results in very compact cores power density in these reactors is

quite high.

• In view of this, one requires an extremely efficient heat transfer

system need for finely divided core so that a large heat transfer

area becomes available.

• One must use a non-moderating coolant with very good heat

transfer properties with FBRs.

Fast Breeder Test Reactors (FBTR)

GCFBRs: Also of considerable interest and are being developed

particularly in Europe.

In these systems gaseous Helium is used as primary slowing down effect

on neutrons, the average energy of neutrons is comparatively high and this

leads to larger breeding potential.

In India, a fast breeder test

reactor of LMFBR type

built at Kalpakkam (TN)

Critical on October 18, 1985

Uses mixed Uranium –

Plutonium Carbide as fuel and

liquid Sodium as coolant.

General Schematic of a FBTR

LMFBR- Schematic

FBTR at Indira Gandhi Centre for Atomic

Research (IGAR), Kalpakkam (TN)

Schematic Flow Diagram

Reactor Core

Fuel used in FBTR is a mixture of plutonium carbide and

natural uranium carbide (PuC:UC = 70:30)

Sintered pellets of mixed carbide

stacked inside a stainless steel

tube 5.1 mm diameter and 531

mm length.

Pellets are kept in position in the

tube by a spacer tube and a spring.

FBTR CHARACTERISTICS

Three stages of Indian Nuclear Power Programme

Kalpakkam has the unique distinction of being the only place in the

world, where all the three fissile isotopes viz., U-235 [MAPS], Pu-239

[FBTR] & U-233 [KAMINI] are used as fuel in reactors.

Thermal Breeder Reactor (BRs)

At present nuclear power comes mostly from thermal

reactors.

Their breeding potential is rather low.

Investigations on experimental thermal breeders indicate

that this disadvantage should be somewhat offset by the

fact that the initial fissile fuel required to start a thermal

breeder will be nearly one third of that required for a FBR.

For complete utilization of available Uranium and Thorium

resources the fast and thermal breeders seem to be

essentially complementary.

Classification of BRs

Broadly classified into two groups depending upon the

physical state of the fuel used;

Solid-fuel Reactors: CANDU-Th and LWBRs fall in this group.

Based on very reliable and well proven technologies.

Fluid-fuel Reactors: MSBR and HWBRs fall in this category.

Have advantage of enhanced neutron economy and hence higher breeding potential.

They are not fully developed technologies

Breeding in thermal reactors can only be marginal.

Enhanced breeding, needs extra care to increase fast fissions in

the fertile isotope and/or minimize neutron loss by leakage and

absorption in structural materials, coolant & fission products etc.

In all thermal breeders, efforts are made to reduce neutron

leakage

To reduce neutron absorption by fission products and 233Pa in

particular, they are removed as quickly as possible.

In solid fuel reactors, this is achieved by adopting a ‘seed-blanket’

design, whereas in fluid-fuel reactors the fuel is continuously removed

from the core and is processed in a chemical plant integrated with reactor.

To reduce parasitic absorption, one uses materials with poor

affinity for thermal neutrons.

For this reasons, no control rods, adjusters, burnable or chemical poisons

are used in reactor core.

In a LWBR, reactor control is done by moving the core/fuel whereas in a

MSBR, movable graphite rods are used to obtain local adjustment.

In HWBRs and CANDU-Th reactors done by using D2O

reflectors, whereas in MSBRs and LWBRs fertile blankets are

used.

• Of the four types of thermal breeders, the HWBRs and the

CANDU-Th reactor concepts are heavily based on PHWR

knowhow.

• Discussion of typical experimental LWBRs and MSBRs only.

Light Water Breeder Reactos (LWBRs)

LWBR concept evolved in the US, where a demonstration core

has been installed in the 90MWe PWR at Shippingport,

Pennsylvania.

The breeding performance of such reactors is expected to be good

because the fraction of neutrons lost by leakage is quite small.

Fuel in the LWBR consists of a mixture of 232Th and 233U.

Core is made of hexagonal assemblies surrounded by a

reflector (breeding) blanket.

Cross-sectional view of a LWR core and reflector

blanket

• Each fuel assembly is divided into a

control hexagonal region, called the

‘seed region’.

• It consist of thin movable fuel rods

with varying proportion of 233U (0 to

6%)

• The ‘seed’ is surrounded by the so

called ‘blanket region’ of wider

stationary rods.

• Proportion of 233U varies from 0-3%

The fuel rods are clad with Zircalloy.

• The extensions of fuel rods at the top and bottom contain Thorium

dioxide and make up the axial breeder blanket.

• The outer breeder blanket and reflector also contain ThO2 rods.

• Water under pressure serves as the coolant and flows upward

through the core and blanket region.

• As such, the seed is smaller (in volume) than the blanket but large

enough, so that most of the neutrons originating in this region

cause further fission in it.

As a modification of this design, a close packed heavy water seed-

blanket breeder has been developed with much greater breeding

potential and much lower specific fuel loading.

Compared with PHWRs, it require much less heavy water

inventory.

Super Critical Water Cooled Reactor

Molten Salt Breeder Reactor (MSBRs)

MSBR concept developed at Oak Ridge National Laboratory

(USA); An experimental reactor with power rating of 7.4 MWth

operated there from June 1956 to December 1969.

Schematic of a MSBR

In MSBR, fuel consists

of a fluid salt which

contains both fissile and

fertile materials.

Experience gained with

this system led to the

conceptual design of a

1000 MWe MSBR plant.

It’s breeding performance is comparable to that of a fast breeder.

Best choice of the fuel salt is 7LiF-BeF2-ThF4 - 233UF4. Graphite is used as the moderator.

The core of MSBR is made of a regular array of bare graphite

bars. In this reactor concept, no conventional coolant is used.

The fluid salt serves both as the fuel and the coolant.

A basic problem in the development of fluid fuel reactor is the

corrosion caused by fluoride salts. To contain the molten salt, a

nickel based alloy has been developed. It can withstand

temperatures upto 815oC.

To achieve good breeding in a MSBR, the fission products and

other neutron absorbing heavy elements like 233Pa, 135Xe and 86Kr

etc. are removed continuously and a chemical processing unit is

integrated with reactor operation for this purpose.

Molten Salt Breeder Reactor

Reactors working on both breeder cycles have been build.

However, major efforts has been on liquid-metal cooled

fast breeder reactors working on U238 to Pu239 cycle.

About twenty years ago it appeared that breeder reactors

would be in commercial operation in the coming years.

However, fears of nuclear accidents, difficulties associated

with radioactive waste disposal and the possibility of

plutonium being misused for weapons have caused the

breeder development programme in some countries to be

slowed down.

* * * * *

Summary

Thank You

Dr. BC Choudhary

Mobile: 94175 21382

Email: bakhshish@yahoo.com

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