Nuclear Reactors

A nuclear reactor produces and controls the release of energy from splitting the atoms of certain elements. In a nuclear power reactor, the energy released is used as heat to make steam to generate electricity. (In a research reactor the main purpose is to utilise the actual neutrons produced in the core. In most naval reactors, steam drives a turbine directly for propulsion.) The principles for using nuclear power to produce electricity are the same for most types of reactor. The energy released from continuous fission of the atoms of the fuel is harnessed as heat in either a gas or water, and is used to produce steam. The steam is used to drive the turbines which produce electricity (as in most fossil fuel plants). There are several components common to most types of reactors: Fuel. Usually pellets of uranium oxide (UO2) arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core. Moderator. This is material which slows down the neutrons released from fission so that they cause more fission. It may be water, heavy water, or graphite. Control rods. These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. (Secondary shutdown systems involve adding other neutron absorbers, usually as a fluid, to the system.) Coolant. A liquid or gas circulating through the core so as to transfer the heat from it. Pressure vessel or pressure tubes. Either a robust steel vessel containing the reactor core and moderator, or a series of tubes holding the fuel and conveying the coolant through the moderator. Steam generator. Part of the cooling system where the heat from the reactor is used to make steam for the turbine. Containment. The structure around the reactor core which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation or any malfunction inside. It is typically a metre-thick concrete and steel structure. There are several different types of reactors as indicated in the following table.

1.PRESSURIZED WATER REACTORS (PWR) The PWR was developed initially from the system used to power the USA's nuclear submarines. Ordinary water used both as the coolant and moderator in a common circuit. The fuel is 2-4% enriched uranium oxide clad in zirconium alloy. The system is highly pressurised to achieve a high coolant temperature. LWR's use a massive steel pressure vessel to hold the complete reactor core. In order to refuel, the reactor must be shutdown, cooled, and depressurised. The lid is then removed from the pressure vessel. Fuelling is carried out at intervals of 12-18 months with a required shutdown period of several weeks.

The Pressurised Water Reactor (PWR) has 3 separate cooling systems. Only 1 is expected to have radioactivity - the Reactor Coolant System. The Reactor heats the water that passes upward past the fuel assemblies from a temperature of about 300 C to a temperature of about 334 C. Boiling, other than minor bubbles called nucleate boiling, is not allowed to occur. Pressure is maintained by a Pressuriser connected to the Reactor Coolant System. Pressure is maintained at approximately 15 MPa through a heater and spray system in the pressuriser. The water from the reactor is pumped to the steam generator and passes through tubes. The Reactor Cooling System is expected to be the only one with radioactive materials in it. Typically PWRs have 2, 3, or 4 reactor cooling system loops inside the containment. In a Secondary Cooling System (which includes the Main Steam System and the Condensate-Feedwater Systems), cooler water is pumped from the Feedwater System and passes on the outside of those steam generator tubes, is heated and converted to steam. The steam then passes through the Main Steam Line to the Turbine, which is connected to and turns the Generator. The steam from the Turbine condenses in a Condenser. The condensed water is then pumped by Condensate Pumps through Low Pressure Feedwater Heaters, then to the Feedwater Pumps, then to High Pressure Feedwater Heaters, then to the Steam Generators. The diagram above simplifies the process by only showing the condenser, a pump, and the steam generator. The condenser is maintained at a vacuum using either vacuum pumps or air ejectors. Cooling of the steam is provided by Condenser Cooling Water pumped through the condenser by Circulating Water Pumps, which take a suction from water supplied from the ocean, sea, lake, river, or Cooling Tower. The first commercial PWR plant in the United States was Shipping Pennsylvania.

2.BOLING WATER REACTORs (BWR) The BWR is a light water reactor and is similar in many respects to the PWR. The basic difference is that the coolant is permitted to boil in the core of the BWR. The fuel is 2-3% enriched uranium clad in zirconium alloy. Like the PWR, the reactor is fuelled off-load at intervals of 12-18 months. The core of the BWR is less compact than a PWR. Therefore a larger pressure vessel is required but it is thinner walled because of the lower operating pressure. In a Candu and a PWR the heat from the reactor is passed through a boiler to generate steam which goes to the turbine that drives the electrical generator. This is called an indirect cycle. In a BWR the steam generated in the reactor core is taken straight to the turbine, without the use of an intermediate boiler. This is known a direct cycle. Water is circulated through the Reactor Core picking up heat as the water moves past the fuel assemblies. The water eventually is heated enough to convert to steam. Steam separators remove water from the steam in the upper part of the reactor. The steam then passes through the Main Steam Lines to the Turbine-Generators. The steam typically goes first to a smaller High Pressure (HP) Turbine, then passes to Moisture Separators (not shown), then to the 2 or 3 larger Low Pressure (LP) Turbines. The turbines are connected to each other and to the Generator by a long shaft (not one piece). The Generator produces the electricity, typically at about 20,000 volts AC. This electrical power is then distributed to a Generator Transformer, which steps up the voltage to either 230,000 or 345,000 volts. Then the power is distributed to a switchyard or substation where the power is then sent offsite. The steam, after passing through the turbines, then condenses in the Condenser, which is at a vacuum and is cooled by ocean, sea, lake, or river water. The condensed steam then is pumped to Low Pressure Feedwater Heaters. The water then passes to the Feedwater Pumps, which in turn, pump the water to the reactor and start the cycle all over again. The BWR is unique in that the Control Rods used to shutdown the reactor and maintain a uniform power distribution across the reactor, are inserted from the bottom by a highpressure hydraulically operated system. The BWR also has a Torus or a Suppression Pool. The torus or suppression pool is used to remove heat released if an event occurs in

which large quantities of steam are released from the reactor or the Reactor Recirculation System, used to circulate water through the reactor.

3.PRESSURIZED HEAVY WATER REACTORs (PHWR), CANDU The Canadian-designed Candu-PHW reactor is fuelled with natural uranium oxide clad in zirconium alloy. To avoid the use of a large pressure vessel, the CANDU utilizes the pressure tube concept. The fuel is loaded into horizontal zirconium alloy pressure tubes that pass through a large tank - the calendria - filled with heavy water moderator in alow pressure circuit. This tank is penetrated by several hundred horizontal tubes, which contain the fuel. Heavy water, at high pressure, is used as the fuel coolant. The use of pressure tubes in the reactor core allows the primary coolant system to be pressured without the need for a massive pressure vessel. Coolant does not boil. The Candu system is fuelled on-load: a pair of remotely operated machines, one at each end of the reactor, simultaneously insert fresh fuel while removing used fuel bundles. Oload fuelling results in optimum reactor flux patterns and efficient utilization of fuel; in addition it contributes to Candu's high capacity factor. Because the Candu system uses neutrons more efficiently than light water reactors, it requires less uranium for a given electrical output.

A complete coolant circuit involves two fuel tubes and two circulation loops. The heavy water enters the reactor at a temperature of about 266 C and exits at 310 C. It passes from the reactor to a header, that is, a junction chamber for the coolant tubes, and then to an inverted U-tube steam generator where steam is produced and carried to the turbines. The coolant then returns to the reactor, passing in the opposite direction through an adjacent fuel tube, where it is heated again before flowing to a second steam generator. The pressuriser performs the same function in the CANDU system as it does in the PWR. Although most of the heat from the fuel is carriedaway by the heavy water coolant, some energy is deposited in the heavy water moderator. This is removed by the moderator coolant loop.

4.LIQUID METAL FAST BREEDER REACTOR (LMFBR) The world's reserves of U235 are not adequate to support indefinitely the needs of a growing nuclear power industry based only on burner or converter reactors. Breeder reactors are capable of satisfying the electrical energy needs of the world for thousands of years. The fundamental principles underlying the fast breeder reactor concept were discovered before the end of World War 2, and the potential impact of breeder reactors on future energy supplies was immediately recognised. The world's first nucleargenerated electricity came from an LMFBR. The LMFBR is the only breeder, which has reached a stage of practical commercialisation anywhere in the world.

The LMFBR operates on the uranium-plutonium fuel cycle. This means that the reactor is fuelled with bred isotopes of plutonium in the core or driver, and the blanket is natural or depleted uranium. Assemblies of fuel elements are placed inside a tank containing the liquid sodium coolant. The core is surrounded by a "blanket" of depleted uranium (dioxide) in stainless cans. The sodium is heated by the core and pumped through an intermediate heat exchanger where it heats sodium in a separate secondary circuit. The sodium in the secondary circuit transfers its heat to water in a steam generator; the steam drivers a turbine coupled to an electric generator. There is no moderator, so the core and the blanket contain only fuel rods and coolant. Sodium has been universally chosen as the coolant for the modern LMFBR. Sodium is also an excellent heat transfer agent, so that an LMFBR can be operated at high power density. Furthermore, because sodium has such a high boiling point, reactor coolant loops can be operated at high temperature and this leads to high plant efficiency. Finally sodium, unlike water, is not corrosive to many structural materials. Sodium also has some undesirable characteristics. Its melting point is much higher than room temperature, so the entire coolant system must be kept heated at all times to prevent the sodium from solidifying. Sodium is highly reactive chemically. All LMFBRs utilise two sodium loops: the primary reactor loop carrying radioactive sodium, and an intermediate sodium loop containing nonradiactive sodium. The detailed manner in which the intermediate sodium loop is arranged divides LMFBRs into two categories: the loop-type LMFBR and the pool-type LMFBR.

5.HIGH TEMPERATURE GAS COOLED REACTOR (HTGR) One of the great advantage of gas cooled reactors is their high thermal efficiency. The plant produces superheated steam at approximately 540 C and 16 MPa, and operates at an overall efficiency of about 40 percent, as high as the most efficient fossil fuel plant available today. At startup, the HTGR is fuelled with a mixture of thorium and highly enriched uranium, but, in time, the U-235 converted from the thorium replaces some of the U-235. The reactor does not breed, however, so that some U-235 must always be present. The equilibrium core contains U-235, fertile Th-232, and all the recycled U-233. Fuel for the HTGR is in the form of small uranium and thorium dicarbides, which specially coated to prevent the release of fission products. These particles are cast into rods using a carbonaceous binder. The rods are then inserted into holes in hexagonal graphite blocks, and the blocks are arranged in a cylindrical array to form the core. Additional holes through the graphite provide passages for the coolant gas while others hold channels for control rods.

Helium flows downward through the core, then through the steam generators, and is pumped back to the core through the circulation blowers. All of these components are located within prestressed concrete vessel. A unique feature of the HTGR is the very high temperature of the circulating helium. Such helium can be used directly in a gas turbine to drive an electrical generator, thus eliminating the need for an intermediate steam cycle. There are many advantages to such a system. Firstly, gas turbines and their associated cycle components are considerably more compact then comparable steam cycle equipment. Secondly, the temperature of the reject heat is so high that this energy can be used itself in a number of practical applications. The HTGR can provide high-temperature heat required in many chemical processes, such as the glassification of coal and desalination of sea water, among others.

6.ADVANCED GAS COOLED REACTOR (AGR) The AGR was developed in Britain as a successor to the Magnox system. It was designed to allow higher fuel and coolant temperatures, thus improving the steam conditions. The AGR uses graphite as moderator and carbondioxide as heat transfer medium. Its uranium dioxide fuel is clad in stainless steel and is in the form of a cluster of small diameter rods, permitting relatively higher power levels to be achieved. This allows the size of the reactor core to be the smaller than that of the Magnox reactor, but necessitates the use of 2-3% enriched uranium fuel. In order to obtain maximum availability of the plant, onload refuelling was adopted for the AGR. Clusters of fuel elements are joined together end-to-end in a stringer, placed in vertical holes in the graphite. Carbondioxide gas is heated by passing over the fuel in the core. It transfers its heat to water in a steam generator; the steam drives a turbine coupled to an electric generator. Because of the high temperatures, the AGR stations are of the single-pressure type. They are also characterised by prestressed concrete vessels, double containment of all access penetrations, and provision for refuelling the reactors on load for high availability.

7.MAGNOX REACTOR Magnox reactors have a low power density-0.1 to 0.5MW(e)/cu.m - compared to 30 MW(e)/cu.m. for PWR's. These results from using natural uranium as fuel and graphite as the moderator, which requires heterogeneous reactor designs with a very large carbonto-U-235 ratio, typically 12000 to 18000 (compared to 3000 in HTGR(s). Furthermore, the maximum specific power is limited to 4 to % MW(t)/tonne of maximum permissible cladding and fuel temperatures and because of limitations on the transfer surface area. Magnesium alloy Magnox (0.8% aluminium, 0.002 to 0.50% beryllium, 0.008% calcium, and 0.006% iron additions) is used in the United Kingdom and a Mg-Zr alloy in France. Because of carbondioxide oxidation and melting of the cladding at 645 C, the maximum cladding temperature is limited to 500 C or less. A relatively high coolant pressure and high pumping power (as a ratio of the thermal power) as well as extended surfaces on the fuel element, are required in order to obtain an acceptable exit gas temperature of 375 to 415 C. Graphite corrosion by carbondioxide could be reduced by hydrogen or methane injection in the coolant. Finally, the fuel burnup is limited to values on the order of 5 to 6 MWd/kg (compared to 30 MWd/kg in PWRs and 100 MWd/kg in HTGRs) because of the metallic uranium under irradiation, and also because of the reactivity limitations with natural uranium fuel.

The International Nuclear Event Scale Level, Descriptor

For prompt communication of safety significance Off-Site Impact On-Site Impact Defence-inExamples Depth

Degradation 7 Major Accident

Major Release: Widespread health and environmental effects

Chernobyl, Ukraine, 1986

6 Serious Accident

Significant Release: Full implementation of local emergency plans

-

Limited Release: 5 Partial Accident Severe core implementation of with Off-Site damage local emergency Risks plans

Windscale, UK, 1957 (military). Three Mile Island, USA, 1979.

4 Accident Mainly in Installation either of:

Minor Release: Public exposure of the order of prescribed limits

Partial core damage. Acute health effects to workers

Saint-Laurent, France, 1980 (fuel rupture in reactor). Tokai-mura, Japan, 1999 (criticality in fuel plant for an experimental reactor).

3 Serious Incident any of:

Very Small Release: Public exposure at a fraction of prescribed limits

Near Accident. Major Loss of contamination, Defence-inOverexposure of Depth workers provisions

Vandellos, Spain, 1989 (turbine fire, no radioactive contamination). Davis-Besse, USA, 2002 (severe corosion)

2 Incident 1 Anomaly

nil

nil

nil

Incidents with potential safety consequences

nil

Deviations from authorised functional domains

0 nil nil Below Scale Source: International Atomic Energy Agency

No safety significance