Nucler Energy

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Nucler plant

Atoms ⇒molecule

Atom = positively charged nucleus (nucleaons) + negatively charged electrons.


                             neutrons (electrically neutral)
 Nucleaons     ↗
           ↘ protons (positively charged)
 

                       electron
                        proton
                       neutron

The atom is electrically neutral as a whole.

 One atom may be transformed into another by losing or acquiring some of the subparticles (nucleons + electrons). Such reactions result in a change in mass Δm and therefore release (or absorb) large quantities of energy ΔE, according to Einstein’s law 

                              2

          ΔE = Δm.C 

 

                                                                                                                   8

C = speed of light in vacuum = 2.997925×10  m/s Neutron mass, mn = 1.008665 amu

Fission

 Unlike fusion, which involves nuclei of similar electric charge and therefore requires high kinetic energies, fission can be caused by the neutron, which, being electrically neutral, can strike and fission the positively charged nucleus at high, moderate, or low speeds without being repulsed. Fission can be caused by other particles, but neutrons are the only practical ones that result in a sustained reaction because two 2 92U 235 92 0 54 38 0 or three neutrons are usually released for each one absorbed in fission. These keep the reactions going.

Scattering: 

When neutrons travel through matter, they collide with nuclei and are decelerated, mainly by the lighter nuclei, thus giving up some of their energy with each successive collision. This process is called scattering. Neutrons are classified into three categories according to energy: (i) Fast > 105 eV 20,000 km/s (ii) Intermediate (iii) Slow < 1 eV 2200 m/s (thermal)

Parts of a nuclear reactor 

(i) Nuclear fuel: Fuel of a nuclear reactor should be fissionable material which can be defined as an element or isotope whose nuclei can be caused to undergo nuclear fission by nuclear bombardment and to produce a fission chain reaction.

92U 233 , 92U 235 , 94Pu239 

(ii) Moderator: In the chain reaction the neutrons are fast moving neutrons. These fast moving neutrons are far less effective in causing the fission of 92U 235 and try to escape from the reactor. To improve the utilization of these neutrons their speeds are reduced. It is done by colliding them with the nuclei of other material which is lighter, does not capture the neutrons but scatters them. Such material is called

(iii) Control rods: A nuclear reactor contains as much fuel as is sufficient to operate a large power plant for several months. The consumption of this fuel and the power level of the reactor depends upon its neutron flux in the reactor core. The energy produced in the reactor due to fission of nuclear fuel during chain reaction is so much that if it is not controlled properly the entire core and surrounding structure may melt and radioactive fission products may come out. To control the power of the reactor control rods are used. These rods can be moved in and out of the holes in the reactor core assembly. Their insertion absorbs more neutrons and damps down the reaction and their withdrawal absorbs less neutrons. Thus power of reaction is controlled by shifting control rods. Boron or Cadmium in the form of cylinder or sheets are used as control rods.

(iv) Reflector:

 The neutrons produced during the fission process will be partly absorbed by the fuel rods, moderator, coolant or structural material etc. Neutrons left unabsorbed will try to leave the reactor core never to return to it and will be lost. Such losses should be minimized. It is done by surrounding the reactor core by a material called reflector which will send the neutrons back into the core. Generally the reflector is made up of graphite and beryllium. 

Reflector properties: 

 Low absorption 

 High reflection 

 Radiation stability 

 Resistance to oxidation

 (v) Reactor vessel: 

It is a strong walled container housing the core of the power reactor. It contains moderator, reflector, thermal shielding and control rods. (vi) Biological shielding: Shielding the radioactive zones in the reactor from possible radiation hazard is essential to protect the operating men from the harmful effects. During fission of nuclear fuel, alpha particles, beta particles, deadly gamma rays and neutrons are produced. Out of these neutrons and gamma rays are of main significance. A protection must be provided against them. Thick layers of lead or concrete are provided all round the reactor for stopping the gamma rays. Thick layers of metals or plastics are sufficient to stop the alpha and beta particles. (vii) Coolant: Coolant flows through and around the reactor core. It is used to transfer the large amount of heat produced in the reactor due to fission of the nuclear fuel during chain reaction. The coolant either transfers its heat to another medium or if the coolant used is water it takes the heat and gets converted into steam (BWR) in the reactor which is directly sent to the turbine.

  Stable under thermal condition 

 Low melting point and high boiling point 

 High heat transfer coefficient 

 Radioactivity induced in coolant by the neutron bombardment should be nil. 

 

H2O, D2O, Air, CO2, H2, He, Liquid metal (Na). 

Greater density Higher specific heats 

             ⇓ 

 Less pumping power

Methods: 

(i) Use of control rods: Control rods provide the ability to change the amount of neutron absorption. The control rods are operated by control-rod drives that can move them in and out of the core around a power equilibrium position which is usually a partially inserted position. 11

 (ii) Use of chemical shim in addition to control rods: Chemical shim is the use of a soluble absorber, usually boric acid (H3BO3), in the moderator coolant. Boric acid strongly absorbs neutrons in proportion to the number of boron atoms and thus inhibits neutron multiplication. The concentration of this absorber in the moderator coolant is decreased slowly during the core lifetime to overcome the effect of fuel depletion.

 (iii) Use of reflectors: These are mechanically operated devices, situated just outside the core. The reflectors are swung away or toward or are axially moved with respect to the core to increase or decrease power.

 (iv) Use of movable fuel rods.

Waste disposal 

Wastes from atomic energy installations are radioactive, create radioactive hazard and require strong control to ensure that radioactivity is not released into the atmosphere to avoid atmospheric pollution. 

Wastes form: (i) Liquid (ii) Gas (iii) Solid

Liquid wastes: 

Two ways of disposal 

(i) Dilution: Liquid wastes are diluted with large quantities of water and then released into the ground. This method suffers from the drawback that there is a chance of contamination of underground water if the dilution factor is not adequate.

 (ii) Concentration to small volumes and storage: When the dilution of radioactive liquid wastes is not desirable 12 due to amount or nature of isotopes, the liquid wastes are concentrated to small volumes and stored in underground tanks. 

Gaseous wastes:

 Gaseous wastes can most easily result in atmospheric pollution. Gaseous wastes are generally diluted with air, passed through filters and then released to atmosphere through large stacks.

 Solid wastes: 

Solid wastes consist of scrap material or discarded objects contaminated with radioactive matter. These wastes if combustible are burnt and the radioactive matter is mixed with concrete, drummed and shipped for burial. Noncombustible solid wastes, are always buried deep in the ground. 

Thermal reactors

 i) Pressurized-water-reactor (PWR)

 ii) Boiling-water-reactor (BWR) 

iii) Gas-cooled-reactor (GCR)

 iv) Heavy-water-reactor (PHWR)

 

Pressurized water reactor (PWR) In a PWR, the coolant pressure is higher than the saturation pressure corresponding to the maximum coolant temperature in the reactor, so that no coolant boiling takes place.

 Two loops: i) Coolant loop (primary loop) ii) Water-steam loop (working fluid loop)

The coolant picks up reactor heat and transfers it to the working fluid in the steam generator (heat exchanger). The steam is then used in a Rankine type cycle to generate electricity.

Pressurizer 

In PWR primary loops, the coolant is maintained at a pressure around 155 bar greater than the saturation pressure corresponding to the maximum coolant temperature in the reactor. This avoids bulk boiling of the coolant and keeps it in the liquid phase throughout the loop. Because liquids are practically incompressible, small changes of volume caused by changes in coolant temperatures because of normal load changes cause severe or oscillatory pressure changes. These may be quite unsafe when the pressures increase. They cause flashing into steam and consequent disruption of the reactor nuclear characteristics and possible burnout of the reactor fuel elements. They cause cavitation when the pressures decrease. For these reasons it is necessary to provide a surge chamber that will accommodate coolant (also moderator) volume changes while maintaining pressure within acceptable limits.

 Such a chamber is called a pressurizer. 

 Vapor pressurizer 

 Gas pressurizer

Vapor pressurizer

  A small boiler

  Liquid in the pressurizer is the same as the primary coolant. 

 The pressure in the pressurizer is same as that of the primary coolant at the junction between the pressurizer and the hot leg of the primary loop.

  The pressurizer temperature is higher than the primary coolant temperature.

Gas pressurizer 

 A large volume of gas situated above the primary coolant at the junction between the pressurizer and the hot leg of the primary loop. 

 The gas not miscible with the coolant.

  Limited to use in low-pressure systems. 

Boiling-water reactor (BWR) In the boiling water reactor, the coolant boils in the same compartment in which the fuel is located. The reactor pressure is maintained at about 70 bar. Because water and vapor coexist in the core, a BWR produces saturated steam. 

The coolant serves the triple function of

  coolant  moderator  working fluid

 Slightly subcooled liquid enters the reactor core at the bottom. When it reaches the top of the core, it has been converted into a very wet mixture of water and steam. The steam separates from the water, flows to the turbine, does work, is condensed by the condenser, and is then pumped back to the reactor by the feed water pump. 

The saturated liquid that separates from the vapor at the top of the reactor (internal) or in a steam separator (external) flows downward via downcomers within or outside the reactor and mixes with the return condensate.

Fast-Breeder Reactor Liquid metal fast breeder reactor (LMFBR) Fast reactors are those whose neutrons are not slowed down by a moderator. Because sodium and other liquid metals suffer from high induced radioactivities, and are generally chemically active, intermediate coolant loops are used between the primary radioactive coolant and the steam cycle. The intermediate coolant is usually also a liquid metal, often Na or NaK. The intermediate loop guards against reactions between the radioactive primary coolant and water.

  Sodium and potassium both have low absorption crosssections.

                                                        o                                             o

 Sodium melts at 98 C and boils at 883 C. 

 Sodium has higher specific heats than most metals. 

 Sodium has a high thermal conductivity and it is cheap. 

 Sodium need not be pressurized.