NUCLEAR POWER REACTORS
Dr. BC Choudhary Professor, Applied Science Department, NITTTR, Chandigarh-160019. Nuclear Reactors
Ever since, the world’s first nuclear reactor by Fermi & coworkers in USA, 1942 A variety of nuclear reactors have been built, primarily to meet increasing demand of energy.
Although, nuclear reactors are highly complex installations and great care has to be exercised in designing, they work on a very simple principle; Nuclear Fission
. The energy released in the process of fission is dissipated into the surrounding environment as heat;
. The heat generated in fission is removed by circulating a fluid, called ‘coolant’ around the fuel;
. This heat is subsequently used to generate high pressure and high temperature steam;
. The steam is fed to a turbine generator system to produce electricity. Reactor Classification
Reactors have been built for a wide range of uses, from power generation to testing new reactor components.
Design specifications vary widely depending upon the purpose for which the reactor is to be used.
One can classify reactors in different ways and some of the criteria used for classification are:
. Mean energy of neutrons causing the fission; . Material used in a reactor for fuel, moderator or coolant; . Geometrical structure; and . Purpose for which a reactor is meant. A. Mean Energy of Neutrons causing Fission
Classified broadly into Three Types: • Fast: Most of the fissions are induced by neutrons having energies of the order of few hundred keV, or in some special type as low as 10 keV.
• Intermediate: Mean energy of neutrons in the range 10-100eV. Intermediate reactors, sometimes also referred to as resonance reactors.
• Thermal: Most of the fissions are induced by neutrons of energy around 0.025 eV.
Extensively built for power generation and also for research purposes. B. Reactors according to the material used
Classified according to the materials used for fuel, moderator or coolant and their physical state,
Explicitly mention it by name along with the purpose it serves.
For Example: . A fission reactor in which graphite is used as moderator, carbon dioxide (a gas) as coolant and natural uranium as fuel is referred to as the ‘Graphite moderated - gas cooled - natural uranium fueled reactor’.
. Reactors with ordinary water both as moderator and coolant with enriched uranium as fuel are referred to as ‘Light water moderated and cooled, enriched uranium reactors’ or simply ‘Light water reactors (LWR)’ Continued…
Similarly one can have
. ‘Heavy water reactor’(HWR, PHWR),
. ‘Gas cooled heavy water moderated reactor’
. ‘Light water cooled heavy water moderated reactors’
C: Reactors by Structure
From the point of view of their structure, thermal reactors can be further classified as
. Homogeneous : Fuel and the moderator are in the same physical state or intimately mixed
. Heterogeneous : Fuel is in the from of rods or plates which are regularly dispersed in the moderator, i.e. fuel and moderator are geometrically separated.
Most of the present day reactors are of Heterogeneous type. D. Reactors According to Purpose Reactors can also be classified according to the purpose for which they are meant into the following categories:
. Power generation . Conversion of one material into another
Reactors built for basic research in various branches of science for testing new reactor designs or new reactor components, for producing radio-isotopes and for medical purpose (neutron therapy) are referred to as “Research reactors”
. Cirus, Apsara & Purnima at Trombay are examples of research reactors. Usually a single research reactor simultaneously serves many of these purpose. Other Classification Features
Reactors designed to convert a fertile isotope (232Th or 238U) into a fissile isotope (233U or 239Pu) “Convertors”.
If the amount of newly produced fissile isotope is more than what is burnt in maintaining the chain reaction, ‘Breeders’.
Reactors built to produce power are referred to as ‘Power reactors’.
Today most of nuclear power comes from ‘Thermal reactors’ based on the fission of 235U.
“Fast breeders” are expected to play an increasingly important role in future- one expects that they will provide a virtually unlimited source of energy. POWER REACTORS
Power Reactors
Thermal Fast
Graphite moderated Light water Heavy water Thermal Breeders Liquid metal Gas cooled gas cooled Reactors Reactors fast breeder fast breeder (GCR) (LWR) (PHWR) (LMFBR) (GCFB)
Pressurised Boiling water Molten salt Light water water reactors reactors breeder reactors breeder reactors (PWR) (BWR) (MSBR) (LWBR) Reactor Schematics
All nuclear reactors consist of following basic components:
. Reactor core, . Reflector, . Reactor vessel, . Radiation shield, . Structural materials, . Coolant loops and heat exchangers
In fast reactors, a blanket is also placed between the core and the reflector. Core : The central region of a reactor where fission takes place, resulting in the release of energy • In fast reactors it contains a nuclear fuel, a coolant, control rods and structural materials. • In thermal reactors a moderator is also present. . Usually fuel is in the form of a ceramic, i.e. either an oxide or a carbide or a nitride. In some cases, uranium in the metallic form is also used as fuel – fuel should have high melting point, high thermal conductivity, high resistance to radiation damage and chemically inert
Fuel rods are covered with some protective material known as ‘Cladding or Canning’- should be highly resistive to corrosion, a poor neutron absorber, high melting point and good mechanical strength
Zirconium, steel, aluminum, magnesium, nickel and some other similar materials have been proposed for this purpose. . Zirconium is the best and one generally uses Zirconium alloy, known as Zircalloy-2(ZR-2), in thermal power reactors. . In fast reactors, and sometimes in light water reactors, stainless steel is used. . Aluminium is used mainly in research reactors
A single cladded unit of fuel is known as the ‘fuel element’.
Several such fuel elements when put together constitute a “fuel assembly or a fuel bundle”.
Large number of such fuel assemblies are arranged in the form of a regular lattice.
The lattice is usually square or hexagonal in the form Arrangement of square/hexagonal reactor lattice in a core. To remove fission heat from the core, necessary to circulate a fluid (liquid or gas) through the reactor called coolant- should have high thermal capacity, low neutron absorption, good radiation & thermal stability and compatible with fuel & clad.
To slow down neutrons born in fission, a moderator is also present in the core of a thermal reactor - material of low mass number, large scattering cross- section and small absorption cross-section.
Commonly used moderators are heavy water, light water and graphite. Fuel-coolant-moderator arrangement Lithium & Beryllium used in some in reactor core molten salt breeder reactors.
In fast reactors, where no moderator is present, the same materials is used as coolant. To ensure safe operation of a reactor at desired power level, to start up and to shut down a reactor, fuel burn up and temperature effects, provision has to be made in the reactor core to control the multiplication factor.
Achieved by having control rods (or plates) in the core - made of some highly neutron absorbing materials such as Boron, Cadmium, Hafnium, Gadolinium or their alloys.
Control rods may sub-grouped as shim rods, regulating rods or safety rods, depending upon their function.
. To support fuel elements, for making coolant channels and for various other purposes, it is necessary to use some structural materials inside the core – similar properties as those of clad; stainless steel or Zircalloys. Blanket
. Fast reactors are generally compact, hence a significant fraction of neutrons in the core leaks out of the system.
To reduce this leakage and also to make proper use of the leaking neutrons, core in these reactors is surrounded by a region of fertile material (232Th or 238U) referred to as the ‘blanket’.
Also serves as an additional neutron reflector as well as a shield.
The neutrons absorbed in the blanket eventually leads to the production of fissile nuclei ; 233U or 239Pu. Reflector
A region of non-fissionable material put next to the core (or blanket if present) to return back the neutron escaping from the core In fast reactors the material chosen is one of high mass number and low absorption cross-section so that the mean energy of neutrons returned back from this region is not much different from that of neutrons entering it. . Ni, Cu and Mo are used as reflectors.
In thermal reactors any good moderating material can be used as reflector.
. In LWRs and PHWRs, H2O and D2O are themselves used as reflectors. . Graphite, Be or BeO may be used in gas cooled reactors. Reactor Vessel . The whole assembly is placed inside a vessel, called “Pressure or Reactor vessel” or “Calendria”
Usually stainless steel is used to make the reactor vessel. For PWRs, which are designed to operate at high pressure, the wall of the pressure vessel is several inches thick.
In BWR, the pressure is nearly one-half of that in a PWR so that in this case pressure vessel is thinner. Shielding . To protect the scientists and other personnel working around the reactor as well as the equipment placed around it from radiations emanating from the reactor core, the reactor vessel is encased inside thick concrete walls.
• In some cases alternate layers of heavy and light elements such as concrete and polyethylene or concrete and water are also used.
. Further to reduce the heating effect of nuclear radiations and hence to prevent radiation damage of the pressure vessel a ‘thermal shield’ usually made of stainless steel, is placed next to the reflector. Reactor Building
The entire structure, is placed inside a reactor building.
It is air tight and is maintained at a pressure slightly lower than the atmospheric pressure so that no air leaks out of the building, except through the ventilation channels.
. In the event of accident the building also helps to contain the radioactive materials and prevent their dispersal into the surroundings. Reactors inner view Outer shielding
Plant Safety Measures Ariel view of Nuclear Power plant Coolant Loops, Heat Exchangers & Electric Generators
. Heat generated due to fission inside the reactor core is removed by circulating a coolant through it. Usually, the coolant circulates in a closed loop, called the primary or reactor loop.
. The heated coolant carrying fission heat can become intensely radioactive when it comes out of the core. To prevent this radioactivity from spreading, it become necessary to introduce a secondary loop (may be closed or open).
. The primary fluid is made to give up its heat to the secondary fluid, usually water, in a heat exchanger. This results in the production of steam and for this reason, it is also called a ‘steam generator’.
. The high temperature and pressure steam from the steam generator expands in a turbine coupled to a large electric generator. The low pressure steam leaving the turbine is recondensed into liquid water in a steam condenser. The condensed water is then compressed and pumped back into the steam generator.
Efficiency of Reactor . Ratio of the electrical energy generated to the thermal energy produced, both measured in Megawatts; Plant efficiency or thermal efficiency denoted by ‘’ Electrical energy generated (W ) e Thermalenergy produced(Wth )
• Maximum value of ‘’ is ideal thermodynamic efficiency, given by
T : Temp.(K) of steam entering the turbine T2 1 1 T1 T2: Temp. at which heat is given off to the condenser
‘’ will be more when T1 is high and T2 is low.
In practice, T2 is more or less fixed, so to increase thermal efficiency one should produce steam at a temperature as high as possible. . In general, efficiency of nuclear plant (33%) is less than that of fossil plants (40%).
This is because the temperature of the nuclear fuel (also steam) is kept lower than that of the fossil fuel to avoid melt down.
. In HTGRs and FBRs, which use enriched uranium in ceramic form, thermal efficiency is raised to about 40%. Thermal Reactors
A survey of world’s reactors the choice of reactor types is limited and is governed by many local factors.
. USA favoured LWRs,
. UK concentrated on GCRs (due to lack of enrichment facilities)
. Canada focussed on PHWRs (CANDU)
. India also adopted PWRs, or PHWRs
At present, about 80% of the total nuclear power generated around the globe comes from LWRs, PWRs and BWRs.
Gas cooled reactors and HWRs account for the rest. Light Water Reactors (LWRs)
Light water has a very high scattering cross-section and is very efficient in slowing down neutrons. However, its absorption cross-section is also large and therefore if one wishes to use it as a moderator one has to use enriched fuel.
Development of LWRs has taken place along two lines
. Pressurized water reactors (PWRs): Coolant is maintained at high pressure, between 13.8 and 17.3 Mpa (2000-2500 psi) to prohibit boiling in the core.
. Boiling water reactors (BWRs): Coolant is allowed to boil in a controlled manner within the core itself.
More than 400 LWRs have been built and are in operation. Pressurized Water Reactors (PWRs)
. For higher efficiency in the production of electrical energy one requires high coolant (steam) temperatures.
To achieve this, coolant be maintained at high pressures. The efficiency increases with increasing pressure and attains saturation for a pressure of about 17.3 Mpa. . In PWRs, the optimum coolant pressure in the primary loop is kept at around 15.5 Mpa, whereas the steam pressure in the secondary loop is about 7.6 Mpa.
At this coolant pressure, the boiling point of water is 345oC, which cannot be raised by further increasing pressure.
Maintaining the coolant at high pressure introduces fabrication and design problems. . Firstly, the fabrication of the pressure vessel (the biggest reactor component) to sustain high pressures is very difficult. . Secondly one has to design special coolant pumps to circulate a large amount of coolant per second. Schematic representation of a PWR First built in 1957 in USA These reactors are very safe, present few corrosion problems and can be controlled easily. PWR is the most developed and sophisticated reactor concept today.
Only disadvantage is that they use enriched fuel and costly Schematic representation of a PWR structural materials Typical Design Specification for a PWR
Power 3800 MWth (1300MWe) 3 Power density 102 MWth /m Core height, Core diameter 4.17m, 3.37m Fuel, Average enrichment Enriched UO2 , 2.8 - 3.3 % Fuel pallet diameter 0.82cm, height 1.5cm Fuel-clad-thickness 0.06cm Fuel rod outer diameter 0.95cm Fuel assembly 1717 square array Fuel rods per assembly 264 No. of fuel assemblies 193 3 Fuel loading 115 10 kg Control rod elements per assembly 24 Moderator & Coolant H2O Coolant pressure 15.5 Mpa (2250 psi) o o Coolant inlet / outlet temperature 293 C / 329 C Coolant flow rate 18.3 Mg/s Steam Pressure 7.58 Mpa (1100 psi) Boiling Water Reactors (BWRs) . If water is made to boil, its heat removing capacity increases. This fact can be exploited if steam is generated inside the reactor core itself. . However, when boiling occurs at low pressures, fluctuations, such as bubble formation, lead to large uncontrollable instabilities in power level. But when pressure is raised, boiling becomes stable and the reactor can easily be controlled. . In such a reactor there is no need for separate heat exchangers. This minimizes loss of heat during its transfer from the core to the turbines.
A BWR, has a distinct advantage over a PWR, in that formation of steam in the reactor core provides an automatic reactor control. Schematic Representation of a BWR
Automatic Reactor Control • Steam is comparatively a poor moderator of neutrons than ordinary water • When volume fraction of steam is more, the value of multiplication factor will be less. • When power level rises, it tend to increase the volume fraction of steam with an automatic decrease in value of k. • This fact is used as an extra means of reactivity control under normal operating conditions of varying re- circulation flow rate. Schematic Representation of a BWR
Typical Design specification for a BWR
Proposed Reactor Indian Reactor
Power 1300MWe (3800 MWth) 200 MWe (660 MWth) Core height, Core diameter 3.76m, 4.18m 3.66m, 2.42m Fuel, Average enrichment Enriched UO2 , 1.9 – 2.6% Enriched UO2 , 2.2 –2.4 % Fuel pallet diameter 1.06cm 1.26cm Fuel-clad-thickness 0.086cm 0.081cm Fuel rod outer diameter 1.25cm 1.43cm Fuel assembly 88 square array 66 square array Fuel rods per assembly 62 36 No. of fuel assemblies 784 284 3 3 Fuel loading 168 10 kg 80 10 kg Control elements 193 69 Moderator & Coolant H2O H2O Coolant pressure 7.17 Mpa (1040 psi) 7.0 Mpa (1015 psi) o o o o Coolant inlet / outlet Temp. 216 C / 289 C 272 C / 286 C Coolant flow rate 14.3 Mg/s - • Table shows that the fuel and core configurations of two reactors are essentially same – in earlier BWRs each fuel assembly contained 36 rods & number of fuel assemblies 284, their number is 62 with number of fuel assemblies 784 in recent models.
• BWRs operate at comparatively lower pressure than the equivalent non-boiling systems so that the requirements for pressure vessel is somewhat less stringent i.e. thickness of the wall of the pressure vessel is somewhat less than PWR.
• Power density and specific power are also lower in BWR than in a PWR because the diameter of the BWR fuel rods is larger than that of PWR needed to improve the stability of a BWR. Developments in both PWR and BWR concepts have tended to bring the two closer and a system in which boiling occurs at still higher pressure is quite possible. Heavy Water Reactors – Pressurized Type
. LWRs require enriched uranium as fuel, which is very expensive. . Heavy water is the most efficient moderating material and if this is used one can attain a self-sustained chain reaction with natural uranium as fuel.
For this reason heavy water reactors have been developed by several countries, particularly those lacking enrichment facilities.
. Moderation by heavy water offers unique advantages in reactor control.
The average neutron life time in a heavy water reactor (0.1s) is nearly 100 times larger than that in a LWR. This means that the requirements concerning time of response of reactor control mechanisms are much less stringent in a PHWR than in LWR.
Hence, PHWRs are inherently more stable than LWRs. Schematic Representation of a PHWR
. LWR itself requires a large and expensive pressure vessel. For a similar design a PHWR need a still larger reactor vessel, because
the slowing down power of D2O is much less than that of H2O.
. To overcome this difficulty, the concept of ‘pressure tubes’ was introduced in CANDU (Canadian Deuterium Uranium) type reactors
Schematic Representation of a PHWR These reactors (PHWR) use heavy water as moderator as well as coolant but two are completely separated from each other. • The fuel bundle is contained in pressure tubes and to prevent boiling, the coolant is made to flow around and across the fuel bundles at high pressure. • The pressure tubes pass through a large tank “Calendria” and the coolant flows in opposite directions in any two adjacent tubes.
• The coolant channel is surrounded by an empty annular gap. The air in this gap thermally insulates the coolant from the moderator contained in the calendria. • The moderator is therefore at low temperature and at atmospheric pressure.
• To make optimum use of heavy water as moderator, Zirconium is used as the structural material. • The control rods are inserted in the moderator through penetrations in the calendria. . In addition to normal shut down systems, a back-up system is also provided.
In earlier PHWRs, the system could be shut down by rapidly draining heavy water into dump tank, however in larger cores of recent designs,Gadolinium nitrate solution is injected into the moderator. Advanced CANDU reactors, the core configurations are horizontal i.e. pressure tubes containing the fuel are horizontal allows “on-power refueling” (fueling without reactor shut down).
Since natural uranium is used as a fuel, the reactor requires refueling roughly one fuel channel every day for sustained full power operation.
Continuous refueling avoids reactivity burn up changes so that compensation by control system is small and system is inherently stable.
Typical Design specification for a CANDU type Advanced Reactor Advanced Reactor RAPP
Power 600MWe (2064MWth) 220 MWe (770 MWth) Core height, Core diameter 5.94m, 7.7m 5.07m, 6.05m Fuel, Average enrichment Natural UO2 Natural UO2 , Fuel rod diameter 1.31cm 1.52cm Fuel-clad-thickness 0.038cm 0.041cm Spacing between fuel elements 0.102cm 0.137cm Fuel rods per assembly 37 19 Length of fuel assembly 0.494m 0.495m Diameter of fuel bundle 0.102m 0.0826m 21.3 kg 15.2 kg UO2 per bundle Moderator & Coolant D2O D2O Coolant pressure 11.3 Mpa (1640 psi) 9.85 Mpa (1428 psi) o o o o Coolant inlet / outlet Temp. 266 C / 310 C 249 C / 293 C Coolant flow rate 7.7 Mg/s 3.04 Mg/s Total heavy water inventory 463,000 kg 420,000 kg Primary shutdown system 28 Cd rods Moderator level
Secondary shutdown system Gd(NO3)3 B2O3 Gas-Cooled Reactors
Countries like UK and France, which lacked enrichment facilities, adopted graphite moderated gas-cooled reactors for power production.
. In the earliest reactors of this type natural uranium rods cladded with Magnoy-an alloy of magnesium, aluminum and beryllium, were used as
fuel and CO2 gas, under a pressure of about 1.6 Mpa, was used as coolant. . The particular choice of a gas as a coolant is dictated by neutronic considerations for the given fuel-moderator combination.
Graphite is a poor moderator.This results in a large reactor cores. As a result, an appreciable fraction of neutrons is absorbed in the large mass of graphite present.
Also with natural uranium as fuel, one must use as coolant, a material which has either a low atom density (a gas) or very small microscopic absorption cross-section. Although these reactors have been operating successfully, their efficiency is rather low.
However, using enriched UO2 (with stainless steel clad) and higher coolant pressures and temperatures, it is now possible to raise their efficiency. A number of these Advanced Gas Reactors (AGR) have been built for power production in different parts of the world.
Further work on this type of reactors has resulted in the development of the coolant of High Temperature Gas Cooled Reactor (HTGR).
In HTGR, Helium rather than CO2 gas is used as coolant, the fuel is enriched (5%) UO2 and moderator is graphite. This fuel-coolant combination permits the system to operate at quite high temperatures even without raising the pressure much.
Higher primary coolant temperatures are attained leading to a higher thermal efficiency than that of LWRs.
In fact, the efficiency of these systems is nearly equal to the efficiency of the present day best fossil-fuel plants. High Temp Gas Cooled Reactor Current Power Reactor Types
Reactor Type Moderator Coolant Comments
Gas Cooled Reactor Graphite L. Water CO2 Coolant. Heat Exchangers (GCR or AGC) Primarily Built in UK
Pressurized Water Reactor L. Water L. Water >50% Reactors in 24 Countries (PWR) Water Pressure = 2000 psi
Boiling Water Reactor L. Water L. Water 2nd most common, >10% of World (BWR) Water Pressure = 1000 psi
Canadian Deuterium U. H. Water H.water Uses natural U fuel (<1% U235) (CANDU) - PHWR Can refuel while operating. Canada + a few foreign sales
Fast Breeder Reactor N.A. L. Sodium Complex. Produces more Pu239 (FBR) than U235 used. Expensive. Fear Electricity from Nuclear Fission
Nuclear power plants account ~20 percent of the worlds power. (T.N.)
Thank You
. For any query, you may contact me at
Dr. BC Choudhary Mobile: 94175 21382 Email: [email protected]