three stage indian nuclear programme - nptelnptel.ac.in/courses/103106101/module - 1/lecture -...

NPTEL Chemical Engineering Nuclear Reactor Technology

Joint Initiative of IITs and IISc Funded by MHRD Page 1 of 12

Three Stage Indian Nuclear Programme K.S. Rajan

Professor, School of Chemical & Biotechnology

SASTRA University



Table of Contents 1 NUCLEAR FISSION .................................................................................................................. 3

1.1 ENERGY RELEASED DURING FISSION ................................................................................................ 4 1.1.1 Reaction cross sections ................................................................................................... 4

2 INTRODUCTION TO REACTOR SYSTEM ................................................................................... 5 2.1 FUEL ........................................................................................................................................ 6 2.2 MODERATOR ............................................................................................................................. 6 2.3 COOLANT .................................................................................................................................. 7 2.4 CONTROL MATERIALS .................................................................................................................. 7 2.5 OTHER COMPONENTS .................................................................................................................. 7

3 THREE-STAGE INDIAN NUCLEAR PROGRAMME ...................................................................... 7 3.1 STAGE I .................................................................................................................................. 8 3.2 STAGE II ................................................................................................................................. 9 3.3 STAGE III .............................................................................................................................. 10

4 REFERENCES/ADDITIONAL READING .................................................................................... 12



In this lecture, we shall discuss probability of occurrence of various nuclear reactions, followed by discussion on the components of a nuclear reactor and the three-stage nuclear programme of India.

At this end of this lecture, the learners will be able to

(i) identify more probable of the reactions based on information on cross sections (ii) list the essential components of a nuclear reactor (iii) list the materials for reactor components (iv) understand the fuel cycle involved in the Indian nuclear programme

1 Nuclear fission Most of the energy produced from nuclear power plants is derived from nuclear fission. The target nucleus must be fissile, meaning the material must be capable of sustaining a nuclear chain reaction with slow neutrons. The common example of naturally available fissile material is Uranium-235.

Uranium-235 is not available in pure form in nature. It is present along with Uranium-238, which comprises 99.3 % (by mass).

In a fission reaction, upon neutron irradiation of target nucleus, an excited nucleus is produced which divides into two main fragments along with the release of several neutrons. The following animation illustrates events that occur during a fission reaction

Fig. 1. Animation of fission

Note: Can be viewed only in Acrobat Reader 9.0 and above



The fission reaction for 235U as the target can be written as:

n + 235U 236U* 144Ba + 89Kr + 3n

The fission fragments possess high kinetic energy, which is converted to heat. The quantum of heat liberated during fission will be discussed later.

1.1 Energy released during fission The energy during the relaxation of 236U* to the ground state 236U can be determined by calculating the mass defect

Mass defect = M(236U*)-M(236U) = 236.052588-236.045562 = 0.007026 amu ~ 6.56 MeV

This energy causes the nucleus to be distorted to a dumb-bell shape, before being broken into fragments (Barium, Krypton and 3 neutrons). During the formation of fragments, they acquire high kinetic energy due to the strong electrostatic force that was holding nuclei intact. These fission fragments carry about 166 MeV of total 200 MeV released due to fission of one U-235 nucleus. We shall discuss the source of remaining energy (~34 MeV) in the later modules.

1.1.1 Reaction cross sections As discussed above, when a neutron interacts with a nucleus, one or more of the above four reactions (elastic scattering, inelastic scattering, neutron capture, fission) occur. The probability of occurrence of each of these reactions is defined as the reaction cross section ().

An easy way of understanding reaction cross section is considering the same as the effective cross-sectional area presented by the target atom (target nucleus) to the incident particle (neutron) for a given interaction. Cross sections are expressed in cm2 or in barn (1 barn =10-24 cm2).

Statistically speaking, let P(x) be the probability of a particular neutron reaction, x be the path length and N the number of nuclei per unit volume, then

=!(!)!!!

(1)

The probability of a neutron reaction, P(x) is the ratio of number of neutrons undergoing a particular reaction to the number of incident neutrons. P(x) can be determined experimentally.

The reaction cross sections are greater or lower than the geometric cross section, depending on the probability of occurrence of that reaction P(x).



The total cross section (T) is the sum of cross sections of the individual reactions.

T = el+ in+ + f (2)

el, in, and f are cross sections for elastic scattering, inelastic scattering, neutron capture and fission

Tables 1 and 2 give typical cross section values for thermal and fast neutrons for two major competing neutron reactions (neutron capture and fission) in some common isotopes used in nuclear reactors

Table 1. Cross sections for some nuclei for thermal neutrons (Kinetic energy = 0.025 eV)

Nucleus Cross sections (b) f T a

U-235 95 586 700 681 Pu-239 270 752 1028 1022 U-238 2.73 11.8e-6 12.2 2.7

Table 2. Cross sections for some nuclei for fast neutrons (Kinetic energy = 1 MeV)

Nucleus Cross sections (b) f a

U-235 0.11 1.2 2.7

What do we infer from Tables 1 and 2?

The relative values of and f determine the suitability of a nucleus (or an element) for appropriate use in a nuclear reactor.

When f/ is greater than for a material, it implies that during neutron bombardment, the probability of occurrence of fission occurring during this interaction is more than the probability of neutron capture. Hence the nuclei with higher f/ are suitable as fuel.

When el/ is greater than for a material, it implies during neutron bombardment, the probability of neutron moderation is more than the probability of neutron capture. Hence the nuclei with higher el/ are suitable as moderators.

2 Introduction to reactor system A nuclear reactor used for the purpose of power generation or for the production of radio-isotopes is defined as a system wherein a nuclear fission is carried out as a controlled



chain reaction. Nuclear reactors that utilize thermal neutrons to initiate and sustain fission are called thermal reactors and those that utilize fast neutrons are called fast reactors.

The most important components of a nuclear reactor are (i) fuel (ii) moderator (iii) coolant and (iv) control material

2.1 Fuel Analogous to the role of fuel in any heating application, the fuel in a nuclear reactor is the source of energy and hence it is the material containing a fissile nucleus. Materials that undergo nuclear fission when bombarded with thermal neutrons are called fissile material and their respective nuclei called fissile nuclei. As discussed few moments before, nuclei with higher f/ are suitable as fuel. From table 1, one may observe that the nuclei U-235, Pu-239 and U-233 have higher f/ ratio. Hence they can be used as fuel in nuclear reactor. Of these three, U-235 is the only fissile nucleus that is available in nature, while Pu-239 and U-233 are produced in nuclear reactors.

Hence most of the nuclear reactors for power generation use uranium as fuel. U-235 is not present in nature in pure form. It is present as a mixture with U-238, the major component present to the extent of 99.3 %. In other words the isotopic abundance of natural U-235 in a mixture of U-235 and U-238 is only 0.7 %. In most reactors the fuel is in oxide form as UO2.

Pu-239 is used as fuel in Fast Reactors while U-233 is proposed as fuel in Advanced Heavy Water Reactors.

2.2 Moderator Moderation with respect to nuclear reactors implies slowing down of neutrons. In other terms, neutrons with higher kinetic energy (fast neutrons) are allowed to undergo elastic scattering with a light nucleus resulting in reduction in speed and change in direction of neutrons. As a result the neutron energy is brought down to thermal levels (0.025 eV). The importance of moderation in a reactor can be understood looking at the first rows of Table 1 and Table 2 (Lecture -2). Comparing the fission cross sections of U-235 with fast neutrons and thermal neutrons, it is clear that the probability of fission in U-235 with thermal neutrons is more than with fast neutrons. Hence the neutron energies must be brought down or in other terms, the neutron needs to be slowed down.

The most widely used moderators are the one that contain light nuclei like hydrogen, deuterium, carbon etc. Hence water (H2O, also called light water), heavy water (D2O) and graphite are used as moderators.



2.3 Coolant The purpose of coolant is to remove the heat liberated during fission and to utilize the same for steam generation. The coolant supply to a nuclear reactor must be continued even if the chain reaction has ceased and reactor shut down. This is to ensure the removal of decay heat and to avoid the melting of reactor core.

Water and heavy water are the most common liquid coolants in thermal reactors. Liquid sodium is used as coolant in fast reactors. Helium and CO2 are used as gaseous coolants in Gas-cooled reactors.

2.4 Control materials Sustenance of a controlled fission is the key for generating electricity from nuclear reactors. Control materials are essential for regulating the power of a reactor, apart from bringing about rapid shut down when required. These are the materials that have high tendency to absorb neutrons through neutron capture. In other terms, these are the materials that possess high . Boron and Cadmium have high capture cross sections and hence are used as control materials in the form of rods or plates.

2.5 Other components Apart the above four important components of a nuclear reactor, other components include structural components and control systems.

3 Three-stage Indian Nuclear Programme Dr. Homi Jehangir Bhabha is the father of Indian Nuclear programme, which has been designed taking into account of resources of Uranium and Thorium. The availability of uranium is limited in India. However, 1/3rd of the worlds Thorium resources are in India. Hence Thorium utilization is an important part of the programme. The conversion of fertile material to fissile material, to harness nuclear energy from the later forms an integral part of the programme.

The Indian nuclear programme comprises three stages as follows:

Stage I: Use of pressurized heavy water reactor fuelled by natural uranium

Stage II: Fast breeder reactors that utilize Plutonium-based fuel

Stage III: Advanced nuclear power systems for Thorium utilization

The three-stage programme links the fuel cycles of each stage that enables multiplication of the potential of nuclear fuel many folds. These stages are executed in parallel (concurrently) to satisfy the fuel requirement.



3.1 Stage I The first stage utilizes natural uranium dioxide as fuel matrix, in Pressurized Heavy Water Reactor (PHWR) for generation of electricity. Heavy water is used both as coolant and moderator in PHWRs.

The use of PHWR technology in the first stage of programme has the following advantages:

(i) Optimum utilization of limited uranium resources (ii) Higher yield of Plutonium that can be used as fuel in the second stage (iii) Indigenous technology evolved through R & D and operating experience

Fig. 2. Animation showing First stage of Indian Nuclear Programme Note: Can be viewed only in Acrobat Reader 9.0 and above

There are 12 PHWRs in operation in India.

They are

(i) Rajasthan Atomic Power Station (RAPS -1, 2, 3 &4) Four PHWRs (ii) Madras Atomic Power Station (MAPS- 1 &2) Two PHWRs (iii) Kakrapur Atomic Power Station (KAPS 1 &2) Two PHWRs (iv) Kaiga Atomic Power Station (Kaiga 1 &2) Two PHWRs (v) Tarapur Atomic Power Plant (TAPP 3 & 4) Two PHWRs



In PHWRs, Pu-239 is produced due to nuclear transmutation of U-238 caused by neutron bombardment. This serves as fuel for Fast Breeder Reactors, being built as part of second stage of the programme.

India is self-sufficient in all aspects of PHWRs. The capacity factors of PHWRs operating in India are high (~80 %).

Indias own operating experience of PHWRs and operating experience outside India, along with the need for indigenization and enhanced safety features resulted in progressive improvement in the design of PHWRs. Research & Development carried out by BARC units provide support for the implementation of Indian Nuclear Programme.

The standard sizes of PHWRs in operation in India are 220 MWe and 500 MWe. The total installed capacity of all PHWRs in operation in India is ~ 2720 MWe.

As a part of the I-stage, two Light Water Reactors (LWR) are in operation in Tarapur (1 & 2). In fact, these two LWRs were the first two nuclear reactors for power generation in India.

Two Light Water Reactors (LWR) of VVER type have been built in Koodangulam (Tamilnadu) with Russian collaboration, with a capacity of 1000 MWe each.

A closed fuel cycle is followed where the U-238 and Pu-239 are separated from the spent fuel and recycled. The radioactive fission products are disposed off safely.

3.2 Stage II The objective of this stage is to utilize the Pu-239 obtained from the first stage. A Fast Breeder Test Reactor was built at Indira Gandhi Centre for Atomic Research, Kalpakkam and attained criticality in 1985. This 40 MWt uses indigenously developed (U+Pu) carbide fuels. Based on the data obtained from this test reactor, a 500 MWe Prototype Fast Breeder Reactor (PFBR) was designed which is being set up in Kalpakkam.

Breeder reactors are the ones that produce more fissile nuclei than they consume. The fissile nucleus in PFBR is Pu-239. A blanket of U-238 is provided surrounding the fissile core. This blanket is used to produce Pu-239 through nuclear transmutation of U-238. Another blanket of Th-232 is provided that produces U-233 during neutron irradiation through nuclear transmutation. U-233 is also a fissile nucleus and is the fuel proposed for use in the third stage.

Thorium-based fuel along with a small amount of Pu-based fuel is likely to be used in Advanced Heavy Water Reactor (AHWR).



Fig. 3. Animation showing Second stage of Indian Nuclear Programme Note: Can be viewed only in Acrobat Reader 9.0 and above

3.3 Stage III The major goal of the third stage is the utilization of Thorium. U-233 can be obtained from fast breeder reactors through nuclear transmutation of Th-232. Hence, the third stage proposes to use U-233 as the fuel in breeder reactors. Already, Th-232 has been



introduced in Indian PHWRs in a limited way. Effective utilization of large reserves of Thorium is the objective of all the initiatives in the third stage.

India has built the only reactor in the world (Kalpakkam Mini Reactor, KAMINI) that uses U-233 as the fuel. This fuel (U-233) was obtained from Th-232 irradiation in the existing reactors. This reactor has helped our scientists and engineers in gaining considerable experience on the thorium irradiation.

U-233/Th-232 (U-233 core and Th-232 blanket) based breeder reactor utilizing thermal neutrons are under development. This type of reactors will produce more and more U-233 when the reactors begin to be operated.

Fig. 4. Animation showing Third stage of Indian Nuclear Programme Note: Can be viewed only in Acrobat Reader 9.0 and above

The other goals of this stage are

(i) To deploy nuclear power to larger scale in the country At present, Nuclear Energy contributes about 3 % (4780 MW) of total

estimated supply of 1.78 lakh MW in the country. Nuclear Power Corporation Limited (NPCIL) has an ambitious plan to generate 20000 MW and 63000 MW of nuclear power by the years 2020 and 2032 respectively

(ii) To achieve good economic performance This is aimed at bridging the price gap that exists in the cost of one unit of

electricity generated by nuclear energy and those by other sources (iii) To attain higher levels of safety



(iv) To provide adaptability of nuclear energy to applications other than power generation like desalination, high temperature processing etc.

4 References/Additional Reading 1. Nuclear Energy: An Introduction to the Concepts, Systems, and Applications of

Nuclear Processes, 5/e, R.L. Murray, Butterworth Heinemann, 2000 (Chapter 11) 2. Nuclear Energy, 2/e, D. Bodansky, Springer-Verlag, 2004. (Chapter 5) 3. www.barc.gov.in 4. www.igcar.gov.in

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