Neutron is neutral subatomic particle that is a constituent of every atomic nucleus except ordinary hydrogen.
It has no electric charge and a rest mass marginally greater than that of the proton but 1,838.68 times greater than that of the electron.
Neutrons and protons, commonly called nucleons, are bound together in the dense inner core of an atom, the nucleus, where they account for 99.9 percent of the atom’s mass.
Developments in high-energy particle physics in the 20th century revealed that neither the neutron nor the proton is a true elementary particle. Rather, they are composites of extremely small elementary particles called quarks.
b) Fission Vs Fusion
Both fission and fusion are nuclear processes by which atoms are alteredto create energy.
Fission is the division of one atom into two, and fusion is the combination of two lighter atoms into a larger one.
Thus, nuclear fission releases heat energy by splitting atoms whereas nuclear fusion refers to the “union of atomic nuclei to form heavier nuclei resulting in the release of enormous amounts of energy.”
c) Fertile Material Vs Fissile Material
Fertile is a term used to describe an isotope that is not itself fissile (it cannot simply undergo fission by thermal neutrons), but can be converted into a fissile material through irradiation in a nuclear reactor.
Uranium-238 and thorium-232 are known as fertile materials, and the production of fissile materials from them after capturing a neutron is known as breeding.
When these fertile materials capture neutrons, they are converted into fissile plutonium-239 and uranium-233, respectively.
d) Moderator
Material in the core which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.
e) Coolant
A fluid circulating through the core so as to transfer the heat from it.
The various primary heat transfer fluids (coolants) used in nuclear reactors are– gas, water, Helium, Carbon dioxide, Sodium, Lead, light metal, heavy metal and salt.
f) Criticality
A reactorachieves criticality (and is said to be critical) when each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions.
Nuclear Reactors
A nuclear reactor produces and controls the release of energy from splitting the atoms (controlled fission chain reaction) of certain elements.
Power reactors use the heat from fission to produce steam, which turns turbines to generate electricity.
In this respect they are similar to plants fueled by coal and natural gas.
Types of Nuclear Reactors
The speed of the neutrons in the chain reaction determines the reactor type, which in turn can be Thermal Neutron Reactors or Fast Neutron Reactors.
a) Thermal Neutron Reactors
Thermal reactors use slow neutrons to maintain the reaction.
These reactors require a moderator to reduce the speed of neutrons produced by fission.
Thermal reactors operate on the principle that uranium-235 undergoes fission more readily with slow neutrons than with fast ones.
Light water (H2O), heavy water (deuterium oxide, D2O), and carbon in the form of graphite are the most common moderators.
Since slow neutron reactors are highly efficient in producing fission in uranium-235, they usefuel assemblies containing either natural uranium (0.7% U-235) or slightly enriched uranium (0.9 to 2.0% U-235) fuel.
Rods composed of neutron-absorbing material such as cadmium or boron are inserted into the fuel assembly.
The position of these control rods in the reactor core determines the rate of the fission chain reaction.
The coolantis a liquid or gas that removes the heat from the core and produces steam to drive the turbines.
In reactors using either light water or heavy water, the coolant also serves as the moderator.
Reactors employing gaseous coolants (CO2 or He) use graphite as the moderator.
b) Boiling Water Reactor (BWR) Vs Pressurized Water Reactor (PWR)
The PWR and BWR use regular water as both its coolant and neutron moderator.
In a boiling-water reactor, the reactor core heats water, which turns directly into steam in the reactor vessel. The steam is used to power a turbine generator.
In a pressurized-water reactor, the reactor core heats water and keeps it under pressure to prevent the water from turning into steam.
This hot radioactive water flows through tubes in a steam generator to eventually bring the clean water present there to a boil and turn it into steam.
In the primary cooling circuit, the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type.
c) Light water reactors Vs Heavy water reactors
Light water reactors use low enriched uranium as fuel. Enriched fuel is required because natural water absorbs some of the neutrons, reducing the number of nuclear fissions.
Heavy water reactors use D2O (water molecules with deuterium atoms) as the coolant/moderator, allowing natural, unenriched uranium to be used as the fuel. This is possible because D2O absorbs fewer neutrons than H2O.
The heat transfer system is similar to that of the PWR, with the steam generator located within the containment structure.
The individual fuel assemblies of a heavy water reactor can be replaced without shutting down the reactor, thus eliminating the down time involved with refueling a light water reactor.
However, spent fuel produced by a heavy water reactor contains more plutonium and tritium than that from light water reactors.
This, coupled with the difficulty in monitoring a continuously fueled reactor, causes concerns about the proliferation of nuclear weapons.
Thus, heavy water is classified as a “sensitive material” because a nation possessing it can produce plutonium directly from natural uranium, eliminating the need for uranium enrichment.
The graphite moderated and gas cooled reactors have the same advantages as the heavy water reactors in that they can use natural uranium fuel and be fueled continuously.
If graphite or heavy water is used as moderator, it is possible to run a power reactor on natural instead of enriched uranium.
Because the light water absorbs neutrons as well as slowing them, it is less efficient as a moderator than heavy water or graphite.
d) Fast Neutron Reactors
Fast neutron reactors, also known as Fast Breeder Reactors (FBR), use high speed, unmoderated neutrons to sustain the chain reaction.
In contrast to thermal reactors, the neutrons in a Fast Neutron Reactor (or Fast Breeder Reactor, FBR) are not slowed by the presence of a moderator.
The coolant, usually a liquid sodium or lead, is a substance that does not slow or absorb neutrons.
It also has excellent heat transfer properties, which allow the reactor to be operated at lower pressures and higher temperatures than thermal reactors.
Because fast neutrons are not as efficient in producing fission as slow ones, FBRs require uranium oxide containing 20% U-235, plutonium oxide, or a mixture of these oxides, known as MOX, as fuel.
An FBR is configured and operated to produce more fuel than it consumes.
Fast neutrons are readily absorbed by fertile uranium-238, which then can undergo successive beta emissions to become fissile Pu-239.
Thorium-232 is another fertile isotope that can absorb neutrons and produce fissile uranium-233 by beta emissions.
These fissile isotopes can be reprocessed for nuclear reactor fuel or weapons.
Originally FBRs were thought to be a means of extending global uranium resources by producing fissile Pu-239 or U-233 as reactor fuel.
However, problems with reactor operations and material components combined with the discovery of new uranium deposits mean that FRBs are not economically competitive with existing thermal reactors.
FBR research has produced technical advances but the limiting factor continues to be the price of FBR-produced reactor fuel versus the cost of uranium fuel.
FBRs are more complex than other types of reactors and also raise concerns about the proliferation of plutonium for use in nuclear weapons.
Note: The Chernobyl reactor used a graphite moderator and water coolant without a containment structure.
Power Reactors Vs Production Reactors
While most reactors generate electric power, some can also produce plutonium for weapons and reactor fuel.
It is important to realize that while the U-235 in the fuel assembly of a thermal reactor is undergoing fission, some of the fertile U-238 present in the assembly is also absorbing neutrons to produce fissilePu-239.
Approximately one third of the energy produced by a thermal power reactor comes from fission of this plutonium.
Power reactors and those used to produce plutonium for weapons operate in different ways to achieve their goals.
Production reactors produce less energy and thus consume less fuel than power reactors.
The removal of fuel assemblies from a production reactor is timed to maximize the amount of plutonium in the spent fuel.
Fuel rods are removed from production reactors after only several months in order to recover the maximum amount of plutonium-239.
The fuel assemblies remain in the core of a power reactors for up to three years to maximize the energy produced.
However, it is possible to recover some plutonium from the spent fuel assemblies of a power reactor.
However, it is possible to recover some plutonium from the spent fuel assemblies of a power reactor.
Role of Zirconium in nuclear power generation
Zirconium is an important mineral for nuclear power, where it finds its main use.
The fuel pellets (usually about 1 cm diameter and 1.5 cm long) are typically arranged in a long zirconium alloy tube to form a fuel rod, the zirconium being hard, corrosion-resistant and transparent to neutrons.
Role of Gadolinium in nuclear power generation
Burnable poisons are often used in fuel or coolant. These are neutron absorbers which decay under neutron exposure, compensating for the progressive build up of neutron absorbers in the fuel as it is burned, and hence allowing higher fuel burn-up.
The best known is gadolinium, which is a vital ingredient of fuel in naval reactors where installing fresh fuel is very inconvenient, so reactors are designed to run more than a decade between refuellings.
India’s three-stage nuclear power programme
Homi Bhaba devised India’s three-stage nuclear power program in the 1954. It was formulated to provide energy security to India.
The main aim was to capitalize on India’ds vast thorium reserves while accounting for its low uranium reserves.
India has only about 2% of the global uranium reserves but 25% of the world’s thorium reserves.
Due to earlier trade bans and lack of indigenous uranium, India has uniquely been developing a nuclear fuel cycle to exploit its reserves of thorium.
a) Stage I – Pressurised Heavy Water Reactor
In the first stage of the programme, scarce natural uranium (UO2)fuelled Pressurised Heavy Water Reactors (PHWR) produce electricity while generating plutonium-239 (Pu-239)as by-product.
b) Stage II – Fast Breeder Reactor
The second stage envisages the use of Pu-239, obtained from the first stage reactor operation, as the fuel core (main fissile element) in Fast Breeder Reactors (FBR).
A blanket of U-238 surrounding the fuel core will undergo nuclear transmutation to produce fresh Pu-239 as more and more Pu-239 is consumed during the operation.
Besides a blanket of Thorium (Th-232) around the FBR core (Pu-239) also undergoes neutron capture reactions leading to the formation of U-233.
U-233, thus obtained, would be the nuclear reactor fuel for the third stage of India’s Nuclear Power Programme.
India has mastered the design and manufacturing of sodium cooled Fast Breeder Reactors (FBR).
Electricity generated by FBR would be a source of green energy as the waste from the first stage nuclear programme is reprocessed and used as fuel in FBR.
The spent fuel from this reactor can be fed back into the reactor core several times, till the spent fuel contains only short-lived fission products.
c) Stage III – Thorium Based Breeder Reactors
The third phase of India’s Nuclear Power Generation programme is breeder reactors, using U-233 as fuel which would be obtained from the second stage.
Besides, U-233 fueled breeder reactors, this stage will have a Th-232 blanket around the U-233 reactor resulting in the production of more and more U-233 fuel from the Th-232 blanket as more of the U-233 in the fuel core is consumed helping to sustain the long-term power generation fuel requirement.
India’s vast thorium deposits permit design and operation of U-233 fuelled breeder reactors.
The currently known Indian thorium reserves can easily meet the energy requirements during the next century and beyond.
However, eeventy years down the line, India is still stuck in the first stage. For the second stage, we need the fast breeder reactors.
A Prototype Fast Breeder Reactor (PFBR) of 500 MW capacity at Kalpakkam in Tamil Nadu, construction of which began way back in 2004, is yet to come on stream.
Advanced Heavy Water Reactor (AHWR)
Subsequent to the accidents at Three Mile Island in USA in year 1979 (TMI) followed by Chernobyl in 1986, passive safety systems became integral part in new reactors. This philosophy gave birth to the concept of Advanced Heavy Water Reactor (AHWR) being designed by Bhabha Atomic Research Centre (BARC).
The AHWR retained the pressure tube concept of PHWRs but adopted several passive systems in design making the reactor safe enough to deploy close to the population center.
Apart from this, AHWR targeted to produce significant power (more than 60 %) from thorium as a demonstration reactor for power from thorium.
