1

M.P.S. FernandoDy Chief Engineer

NUCLEAR POWER CORPORATION INDIA LIMITED

2

RadioactivityRadioactivity

• Emission of particles or waves from the nucleus of an atom

• Types of radiation common in nuclear Engineering

- Alpha Particles - Beta Particles - Gamma Rays n - Neutrons

3

Decay

Ray (0.31 Mev)

10745Rh

Ground State

10746Pa

Beta Decay Followed by Gamma Ray Emission

Beta Decay Followed by Gamma Ray Emission

4

Exponential Radioactive DecayExponential Radioactive Decay

5

Radioactive decayRadioactive decay• It is a fundamental law of Nature

– Number of random events occurring is proportional to the total number of active elements in the sample.

693.0

2

2log

2

1

)0()(

dt

dN

Samplein Number time

Events ofNumber

21

21

Te

etNtN

N

T

t

21T Half Life of a decay process of radioactive

isotopes is the time taken for the total number in the sample to reduced to one half of its initial value

6

Interaction of Neutron With Nuclei

Interaction of Neutron With Nuclei

Neutrons Nuclei

Scattering

Absorption

Elastic Scattering

Inelastic Scattering

Radiative Capture

Nuclear Transmutation

Induced Fission

7

• The three major particles emitted by spontaneous radioactive decay are alpha, beta and gamma.

• Alpha particles are doubly charged helium nuclei, which move slowly when they are emitted. They are emitted from large nuclei such as U-235, U-238 or Thorium.

• Beta particles are electrons. At the time the are emitted they are generally traveling at a speed greater than 90% of the speed of light. They are emitted from a nucleus with too many neutrons. A neutron in the nucleus changes to a proton and a beta particle is emitted.

• Gamma usually accompanies alpha or beta decay. They are photons of electromagnetic energy that travel at the speed of light.

• Alpha and beta particles are directly ionizing radiations.They leave a trail of ionized atoms in their wake.

• Gamma rays are indirectly ionizing radiation, and interact with atoms to generate ions. The three gamma interactions are Compton effect, photoelectric effect and pair production.

• Beta and alpha can be shielded by placing material between the source of the radiation between the source and a person.

• Gamma is the most difficult to shield. The effectiveness of a material in shielding gamma is referred to as a half value layer the thickness of material required to reduce the gamma energy by one-half.

8

Elastic ScatteringElastic Scattering

9

In Elastic ScatteringIn Elastic Scattering

10

TransmutationTransmutation

Neutron Oxygen Nitrogen Proton

11

• Radiative Capture

• Nuclear Transmutation

• Nuclear Fission

Examples of Neutron AbsorptionExamples of Neutron Absorption

Fuel)in (reaction + U n U

Adjusters)in (reaction + Co n Co

Moderator)in (reaction H n H

23992

10

23892

6027

10

5927

31

10

21

N+ H n+ O

He+ H n+ Li

He+ Li n+ B

167

11

10

168

42

31

10

63

42

73

10

105

+n+2Sr+ XeUnU 10

9038

14454

*23692

10

23592

12

s/s2

13

us

Mass Defect and Binding Energy

14

Binding Energy per NucleonBinding Energy per Nucleon

15

16

17

1322 HHH Deutron BE=2.23 MevTotal =4.46 Mev

Tritium BE=8.48 MevNet =8.48-4.46 Mev=4.02 Mev

FUSION REACTIONS

Atleast one heavier,more stable nuclei is produced from two lighter, less stable nuclei

18

23592U

23692U

9536Kr

13956Ba

NUCLEAR FISSIONNUCLEAR FISSION

19

FISSION CHAIN REACTIONFISSION CHAIN REACTION

20

U235

Pu239

Formation of Fission ProductsFormation of Fission Products

21

• The fission products (fission fragments) are nuclides of roughly half the mass of uranium

• Not always the same in every fission. Great number of different fission products, each produced in a certain percentage of the fissions.

• Most fission-product nuclides are “neutron rich”; decay typically by - or -disintegration, are radioactive, with various half-lives.

• To prevent the release of radioactivity, therefore, they used fuel is safely stored and contained.

PRODUCTS OF FISSIONPRODUCTS OF FISSION

22

23

Incident Beamof Radiation

=area presentedto neutron by nucleus

I

dX

I-dI

1

N

)(

xN

oeII

dxNIdI

Outgoing BeamThin Slab

24

25

Schematic View of Cross-sections

Schematic View of Cross-sections

26

Energy From FissionEnergy From Fission

• Energy released per fission ~ 200 MeV [~ 3.2*10-11 J].

• 85% as kinetic energy of fission fragments, and 15% as kinetic energy of other particles.– The energy is quickly converted to heat;– The heat is used to make steam by boiling

water,– The steam turns a turbine and generates

electricity

27

Approximate Distribution of Fission Energy Release

Approximate Distribution of Fission Energy Release

• Kinetic energy of fission fragments 164 Mev

• Kinetic energy secondary neutrons 5 Mev

• Energy of Prompt rays 6 Mev

• Beta particles gradually released from

Fission products (FPs) 8 Mev

• Gamma ray energy released from FPs 6 Mev

• Neutrinos (energy escapes from reactor) 11 Mev200 Mev

28

Atoms in 1 kg of U-235

1 kg of U-235 Consumed

29

• A fissile material is composed of nuclides for which fission is possible with neutrons of any energy level (even thermal neutrons).

• Thermal neutrons have very low kinetic energy levels because they are roughly in equilibrium with the thermal motion of surrounding materials

• EXAMPLES

– Uranium-235, Uranium-233, and– Plutonium-239.

FISSILE MATERIALFISSILE MATERIALFISSILE MATERIALFISSILE MATERIAL

30

FERTILE MATERIALFERTILE MATERIALFERTILE MATERIALFERTILE MATERIAL

Fertile materials are materials that can undergo transmutation to become fissile materials.

27.4 d

22.2 min

2.3 d

23.5 min

2.4 104 yr1.6 105 yr

31

NATURAL URANIUMNATURAL URANIUM• Natural uranium mined from the earth contains the

isotopes uranium-238, uranium-235 and uranium-234.

• The majority (99.2745%) of all the atoms in natural

uranium are uranium-238.

• Most of the remaining atoms (0.72%) are uranium-235, and a slight trace (0.0055%) are uranium-234.

• Although all isotopes of uranium have similar chemical properties, each of the isotopes has significantly different nuclear properties.

32

K pf reproduction factor fast fission factor

Neutron produced in fission in one generation

Neutron absorption and leakage in preceding generationeffectiveK

resonance escape probabilityp thermal utilisation factorf

effective fast thermalK pf P P

Neutron Multiplication Factor

Neutron Multiplication Factor

33

States of the Reactor CoreStates of the Reactor Core

• Sub-critical keff<1 Neutron flux decreases

• Critical keff=1 Neutron flux constant

• Supercritical keff>1 Neutron flux increases

– Reactivity is defined as the deviation of the reactor core from critical condition,

eff

eff

k

1 - k ρ mk , pcm, cent, dollar

34

Thermal Fissions

Fast Fissions

Absorption in Fuel

Absorption in non-fuel

components

Resonance Capture

Escape Resonance

Capture

thermal neutrons

N pf

N Thermal Neutrons

fast neutronsN

neutronsN p

fast neutronsN

N pf

KN

35

Homogeneous System

Fuel and moderator thoroughly mixed1 for Natural Uranium Fuel= 1.32

For Criticality Kinfinity=1 requires

77.032.1

1 pf

Vary ratio of moderator to Fuel atomsMax pf obtained is 0.55 graphite, 0.78 D2O

Natural U Homogeneous system cannot be critical

36

Practical Reactor

Fuel ConsumptionFission Poison AccumulationK should be 1.1 to 1.2

Required

Graphitefor 18.255.0

2.12.1

ODfor 54.178.0

2.12.1 2

pf

pf

Increases with enrichment

37

Heterogenous ReactorHeterogenous Reactor

Location of Fuel FixedReplacement of Spent Fuel EasierEase of Heat RemovalFast Fission factor increasesResonance Escape probability increasesOver moderated / Under Moderated(f)

1.102

1.32 x 0.9 x 0.9 x 1.03

K

fpK

38

Critical MassCritical Mass• Because leakage of neutrons out of

reactor increases as size of reactor decreases, reactor must have a minimum size

• Below minimum size (critical mass), leakage is too high and keff cannot possibly be equal to 1.

• Critical mass depends on– shape of the reactor– composition of the fuel– other materials in the reactor.

• Shape for which critical mass is least, is shape with smallest surface-to-volume ratio: a sphere.

39

40

WHAT IS CRITICALITY ?WHAT IS CRITICALITY ?NET RATE OF

NEUTRON

PRODUCTION

RATE OF LOSS OF NEUTRONS RATE OF PRODUCTION

DUE TOOF NEUTRONS

LEAKAGE ,BY FISSIONS

ABSORPTION

PRODUCTION = ABSORPTION + LEAKAGE

41

CRITICAL REACTOR

• REACTOR CAN BE CRITICAL AT ANY POWER LEVEL

- FULL POWER ( 800 MW )- VERY LOW POWER ( 10-6 FP )

• IN A CRITICAL REACTOR POWER LEVEL IS STEADY

42

43

44

Average Number of Elastic Collisionsto Thermalise neutrons

Average Number of Elastic Collisionsto Thermalise neutrons

45

Bare and Reflected Core

Bare and Reflected Core

46Reflect

or

Reflector

Core

Bare

Reflected

47

• Keffective should be always 1 while reactor operating at any steady power level

• If reactor operates for some period, the fissile content reduces U235

• Number of Fission reactions decrease and hence Keffective

• To restore back Keffective to 1, Reactor Regulating System withdraws adjuster rods, thereby reducing neutron absorption.

• To restore back adjuster rod position to the control range, U235 content is increased by replacement of fresh fuel

Reactor Control

48

KT

etPP

etnn

Kn

dt

dn

Kt

Kt

)0(

)0(

49

If reactivity was 0.5 mk

KT

sec 25

10

0005.0

001.0T

If reactivity was 2 mk

sec 5.02

1

002.0

001.0T

Effective regulation and protection becomes difficult

50

s 0923.0 neutrons all of life Average

s) (15 neutrons delayed all of Life average theis

)(

K

KT

If reactivity was 0.5 mk

sec 2005

1000

0005.0

1.0T

If reactivity was 2 mk

sec 502

100

002.0

1.0T

Effective regulation and protection becomes difficult

is the fraction delayed neutrons (0.65 %)

51

Production of Delayed Neutron from Br-87Production of Delayed Neutron from Br-87

52

53

54

Delayed Photoneutrons in D2O

55

Xenon Transient

0 20 40 60 80 100 120 140 160 180

Time (Hr)

0

20

40

60

80

100

120

Re

ac

tiv

ity

(m

k)

Xenon(mk)

Reactor is restarted

Reactor is shut down

Poison outTime

Poison Override Time

56

• 1 Fission = 200 Mev

• 1 Fission =200 x 1.602 10-13 W-sec

• 1 Watt = 3.12 x 1010 Fissions/sec

• 800 MW = 2.496 x 1019 Fissions/sec

• U-235 is getting depleted, hence refuelling is necessary

57

• Neutron Flux = n.v where n is the number of neutrons per

unit volume and v is their speed

• Macroscopic cross-section (cm-1) = n. where is the microscopic cross-

section

• Reaction Rate (neutrons.cm-3.s-1)– R = .

• Irradiation / Fluence (neutrons.cm-2) = .t where t is the time spent by the

material in that neutron flux

Nuclear TermsNuclear Terms

scm

neutrons2

58

59

Reactor PrinciplesReactor Principles

• Neutron Chain reaction for Power• Control of Neutrons• Moderation of neutrons• Removal of Energy

60

Reactor ClassificationReactor Classification• Energy of the neutrons that induce fission

– Thermal neutron reactors– Fast neutron reactors

• Arrangement of components in the core– Homogeneous– Heterogeneous

• Purpose– Research reactors– Material testing reactors– Power reactors– Propulsion reactors

1st Reactor CP-1 built by Enrico Fermi,Chicago, USA, 1942

61

Components of a Nuclear Reactor

Components of a Nuclear Reactor

Component

Function Materials

Fuel Fission reactions to produce energy

Coolant Remove heat from the reactor coreH2O, D2O, paraffin, Air,

Na, He

ModeratorSlow down and thermalise fast fission neutrons

H2O, D2O, graphite

Reflector Reduce neutron leakageSame as moderator materials

ShieldingProtect personnel from ionizing radiation

Concrete, Steel, lead, H2O

Control Rods Control criticality and power maneuver Cd, B, SS, Gd

Structurescontain fuel and physical support of core

Al, SS, Zr, Concrete

PuU, U, 23994

23592

23392

62

63

Reactor TypesReactor Types

1. MAGNOX Reactor2. Advanced Gas-Cooled Reactor3. High Temperature Gas Cooled Reactor4. Pressurised Water Reactor5. Boiling Water Reactor6. Pressurised Heavy Water Reactor7. Steam Generating Heavy Water

Reactor8. Fast Breeder Reactor

64

  BWR PWR PHWR LMFBR

Mwe 1100 1100 508 1200

Efficiency (%) 33 33 30 40

Assembly Geometry 8 x 8 17 x 17 CylindricalHexagonal

Array

  9 x 9      

Assembly Length (m) 3.8 3.7 0.5 1

Number of Assemblies 590 180 4680(12/ch) 360

Core Height (m) 3.8 3.7 5.95 1

Mass of Fuel / Assembly (kg) 270 600 37 80

Total Mass of Fuel in core (kg) 138000 90000-100000 105000 29000

Burnup (MWD/TeU) 45000 45000 8000 100000

Fuel Replaced every Year 1/4 1/3 continous Varied

Enriched (%) 2.5 3.5 0.711 ~20

Power density (KW/ft) 54 100 12 280

Linear Heat Rate (KW/m) 19 17 26 29

65

66

67

Characteristics of VVER-1000

Characteristics of VVER-1000

• Vertical core: Height = 355 cm; Dia = 316 cm

• Slightly enriched (~ 2.5 %) U-235 (SEU) as fuel• Light water (H2O) as Coolant and Moderator

• Large Core – Potential local critical masses

• Similar to western PWRs

• Hexagonal geometry (163 FA 311 fuel pins)