核电英语300句doc UNIT I HIESTORY OF NUCLEAR POWERSOS 1. The discovery of nuclear fission in 1939 was an event of epochal significance because it o pened up the prospect of entirely new source of power 2. The worlds first self-susta ining nuclear fission chain was realized in the united states at the University of Chicago, 2k Wt CP-1, on December 2, 1942 3. A prototype of the submarine reactor(called STR Mark 1)started operation at Arco, Idaho, in March 1953and the first nuclear powered submarine commenced its sea trials in January I 955 4. The words first industry nuclear power plant (5Mw) was commenced in the U.S.S.R on Ju ne27.1954 5. The Shipping Port PWR, the first central-station nuclear power plant in the United States>, went to operation on December 2, 1957 6. A 20MW nuclear-power demonstration plant in Canada has put in operation since October 1 963 and the first CANdU power reactor unit at Douglas Point(200MW) reached full power o peration in 1968. 7. The first nuclear reactor(HWRR) in China went critical on June 13, 1958 and started pow er operation on September 23, 1958.< 8. The first atomic bomb in China was successfully exploded on October 16 and the first hydr ogen bomb in China on June 17, 1967 9. The first nuclear submarine in China commenced its sea trials on august 23. 1971 10. The 300 MWe QNPC, designed and constructed by China, was connected to the gird of el ectricity generation on December, 15, 1991 11. The Daya Bay Nuclear Power Station was connected to the gird on August 31, 1993 and started commercial operation on February 1, 1994 12. In addition to QNPC and Daya Bay Nuclear Power Station, other nuclear power plants are being constructed in China UNIT 2 DEMAND FOR ELECTRC POWER 1. During the present century, the world's consumption of energy has grown rapidly due to th e per capita increase in the use of energy for industry, agriculture and transportation
核电英语 300 句.doc UNIT 1 HIESTORY OF NUCLEAR POWER 1. The discovery of nuclear fission in 1939 was an event of epochal significance because it o pened up the prospect of entirely new source of power. 2. The world’s first self-sustaining nuclear fission chain was realized in the united states at the University of Chicago, 2kWt CP-1, on December 2, 1942. 3. A prototype of the submarine reactor (called STR Mark 1) started operation at Arco, Idaho, in March 1953and the first nuclear powered submarine commenced its sea trials in January 1 955. 4. The word’s first industry nuclear power plant (5MW) was commenced in the U.S.S.R on Ju ne 27, 1954. 5. The ShippingPort PWR, the first central-station nuclear power plant in the United States>, went to operation on December 2, 1957. 6. A 20MW nuclear-power demonstration plant in Canada has put in operation since October 1 963 and the first CANDU power reactor unit at Douglas Point (200MW) reached full power o peration in 1968. 7. The first nuclear reactor (HWRR) in China went critical on June 13, 1958 and started pow er operation on September 23, 1958.< 8. The first atomic bomb in China was successfully exploded on October 16 and t he first hydr ogen bomb in China on June 17, 1967. 9. The first nuclear submarine in China commenced its sea trials on August 23, 1971. 10. The 300 MWe QNPC, designed and constructed by China, was connected to the gird of el ectricity generation on December, 15, 1991. 11. The Daya Bay Nuclear Power Station was connected to the gird on August 31, 1993 and started commercial operation on February 1, 1994. 12. In addition to QNPC and Daya Bay Nuclear Power Station, other nuclear power plants are being constructed inChina. UNIT 2 DEMAND FOR ELECTRC POWER 1. During the present century, the world’s consumption of energy has grown rapidly due to th e per capita increase in the use of energy for industry, agriculture and transportation
2. It is of special interest, the larger and larger proportions of the energy used are in the for m of electrIc power 3. The generation of electricity requires primary energy sources and the increasing demand for electric power can be satisfied only if such primary sources are rapidly a vailable 4. The main energy sources for the generation of electricity have been the fossil fuels, ie,co l, natural gas, oil and hydroelectric (water) powe 5. The adverse environmental effects of strip mining and the burning of coal, as well as incr asing costs, are making coal less attractive for the generation of electricity 6. Although new reserves of oil and natural gas are being discovered, it appears that the worl dwide production of these fuels will start to decrease around the turn of the century 7. Coal and petroleum provide the essential raw materials for the production of chemicals, incl uding medicinal products, dyes, fibers, rubber and plastics 8. In the long run, the fossil fuels may prove to be more valuable in the respect of chemicals production than as primary sources of energy 9. The idea of making use of the suns energy is very attractive, but considerable research an d development will be required before electricity can be generated from solar energy on a co mmercial scale 10. Nuclear energy can be made availa ble either by the fission of heavy atom ic nuclei or the 11. The fusion process has been demonstrated, both in experiments and in the hydrogen bomb but is doubtful-that fusion energy can make any significant contribution to the power require ments before the end of the century 12. Nuclear fission has been established as a primary source of energy at costs that are compe titive with electricity from other sources UNT3 RADIOACIⅤY</ 5. For a given element, the num ber of protons present in the atomic nucleus is called the ato mic number of the element and the total num ber of nucleons, i.e., of protons and neutrons is called the mass num ber. <ov</ov
2. It is of special interest, the larger and larger proportions of the energy used are in the for m of electric power. 3. The generation of electricity requires primary energy sources and the increasing demand for electric power can be satisfied only if such primary sources are rapidly a vailable. 4. The main energy sources for the generation of electricity have been the fossil fuels, i.e., co al, natural gas, oil and hydroelectric (water) power. 5. The adverse environmental effects of strip mining and the burning of coal, as well as incre asing costs, are making coal less attractive for the generation of electricity. 6. Although new reserves of oil and natural gas are being discovered, it appears that the worl dwide production of these fuels will start to decrease around the turn of the centu ry. 7. Coal and petroleum provide the essential raw materials for the production of chemicals, incl uding medicinal products, dyes, fibers, rubber and plastics. 8. In the long run, the fossil fuels may prove to be more valuable in the respect of chemicals production than as primary sources of energy. 9. The idea of making use of the sun’s energy is very attractive, but considerable research an d development will be required before electricity can be generated from solar energy on a co mmercial scale. 10. Nuclear energy can be made available either by the fission of heavy atomic nuclei or the fusion of very light ones. 11. The fusion process has been demonstrated, both in experiments and in the hydrogen bomb; but is doubtful-that fusion energy can make any significant contribution to the power require ments before the end of the century. 12. Nuclear fission has been established as a primary source of energy at costs that are compe titive with electricity from other sources. UNIT 3 RADIOACITIVITY 2. Atomic nuclei are composed of two kinds of fundamental particles, namely, protons and neu trons. 3. The proton carries a single unit positive charge equal in magnitude to the electronic charge. 4. The neutron is very slightly heavier than the proton and is an electrically neutral particle. 5. For a given element, the number of protons present in the atomic nucleus is called the ato mic number of the element and the total number of nucleons, i.e., of protons and neutrons is called the mass number
6. The term nuclide is commonly used describe an atomic species whose nuclei have a specif ed composition, that is to say, a nuclide in nature is a species having given atomic and mass num bers <O 5. The control material commonly used in pressurized water reactor is alloy of 80(weight)perc ent silver. 15 percent indium and 5percent cadmium soexxos
6. The term nuclide is commonly used describe an atomic species whose nuclei have a specifi ed composition, that is to say, a nuclide in nature is a species having given atomic and mass numbers. 7. Such nuclides, having the same atomic number but different mass number, are called isotop e, e.g., three forms of uranium isotopes in nature with the atomic number 92 but mass numbe r 234, 235 and 238, respectively. 8. The unstable substances undergo spontaneous change, i.e., radioactive decay, at definite rates. 9. The radioactive decay is associated with the emission from the atomic nucleus of an electri cally charged particle, either a alpha particles, i.e., helium nucleus, or a beta particles, i.e., an electron. 10. In many instances of gamma rays, which are penetrating electromagnetic radiation of high energy, accompany the particle emission. 11. The most widely used method for representing the rate of radioactive decay is by means o f the half –life, which is defined as the time required for the number of radioactive nuclei to decay to half its initial value. 12. Since the number of nuclei (or their activity) decays to half its initial value in a half-life period, the number (or activity) will fall to one-fourth by the end of two half-life periods, and to less than 1 percent of its initial value after seven half-life periods. UNIT 6 REACTOR CONTROL 2. If the potentially unsafe conditions should arise, a protection system would automatically sh ut down the reactor. 3. An essential requirement of the control system is that it must be capable of introducing eno ugh negative reactivity to compensate for the build-in (excess) reactivity at initial startup of th e reactor. 4. Four general methods are possible for changing the neutron flux in a reactor, they involve t emporary addition or removal of (1) fuel, (2) moderator, (3) reflector, (4) a neutron absorber. 5. The control material commonly used in pressurized water reactor is alloy of 80(weight) perc ent silver, 15 percent indium and 5percent cadmium
6. The procedure most commonly employed, especially in power reactor, is the insertion or wit hdrawal of a material, such as boron or cadmium, having a large cross section for the absorpti of neutrons > 3. It should be noted that it is only with the fission nuclides that a self-sustaining fission chai n is possible. >> 4. Uranium-233, Uranium-235, Uranium-239, which will undergo fission with neutron of any e nergy, are referred to as fission nuclides. >> 5. Since fission of thorium-232 and uranium-238 is possible with sufficient fast neutron re knows as fissiona ble nuclides: moreover. since thorium-232 and uranium-238 can be convert ed into the fissile nuclides, uranium-233 and plutonium-239, respectively, they are also called issile nuclides 6. The fission of a single uranium-235 (or similar) is accompanied by the release of over 200 Mev of energy, with may be compared about 4eV released by the combustion of an atom of carbon-12.>>
6. The procedure most commonly employed, especially in power reactor, is the insertion or wit hdrawal of a material, such as boron or cadmium, having a large cross section for the absorpti on of neutrons. 7. When a reactor core is being assembled, the neutron absorbing control robs are fully inserte d, so the reactor is sub critical. 8. During startup, the control rods are withdraw slowly, thereby permitting a gradual increase i n the reactivity until the reactor becomes critical and then slightly supercritical. 9. The neutron flux is thus allowed to increase at a safe, controlled rate until its magnitude c orresponds to the desired operating power level of the reactor. 10. The rods are then inserted to the extent required to keep the system exactly critical, so th at the neutron flux and power level remain steady. 11. To shut the reactor son, the control rods would be reinserted in to the core, there by decr easing the reactivity neutron flux and the power output. 12.A control system consists of three control poops, i.e., “operator (manual) lo op”, “automatic loop” and “load loop”. UNIT 4 NUCLEAR FISSION> 2. After absorption of a neutron, a nucleus breaks into two lighter nuclei, called fission fragme nts, with the liberation of a considerable amount of energy and two or three neutrons; this phe nomenon is called nuclear fission.>> 3. It should be noted that it is only with the fission nuclides that a self-sustaining fission chai n is possible.>> 4. Uranium-233, Uranium-235, Uranium-239, which will undergo fission with neutron of any e nergy, are referred to as fission nuclides.>> 5. Since fission of thorium-232 and uranium-238 is possible with sufficient fast neutron, they a re knows as fissionable nuclides; moreover, since thorium-232 and uranium-238 can be convert ed into the fissile nuclides, uranium-233 and plutonium-239, respectively, they are also called f issile nuclides.>> 6. The fission of a single uranium-235 (or similar) is accompanied by the release of over 200 MeV of energy, with may be compared about 4eV released by the combustion of an atom of carbon-12.>>
7. The neutrons can strike other uranium atoms and cause additional fission and the continuing process of fissioning is known as a chain reactor.>> 8. Since two or three neutrons are liberated in each of fission whereas only one is required to mainta in a fission cha in. it would seem that once the fission reaction were initiated in n mass of fissile material, it would readily sustain itself>> 9. However, such is not the case because not all the neutrons produced in fission are available to carry on the fission chain, that is, some neutrons are lost in nonfission reactions(mainly r adioactive capture), whereas other neutrons escape from the system undergoing fission.>> 10. The minimum quantity of such material that is capable of sustaining a fission chain is call ed the critical mass UNIT 5 GENERAL FEATURES OF NECLEAR REACTORS> 3. The core contains the nuclear fuel, consisting of a fissile nuclide and usually a fertile material in addition.>> 4. The function of the moderator is to slow down the high-energy neutrons liberated in th e fi 5. The purpose of reflector is to decrease the loss of neutrons from the core by scattering back many of those which have escaped.>> 6. The heat generated in the reactor is removed by circulation of a suitable coolant, such as ordinary(light) water, heavy water, liquid sodium(or sodium-potassium alloy ), air and hel Ium etc.>> 7. The higher the temperature of the steam, the greater the efficiency for conversion into 8. If the energy released in the reactor is to be converted into electric power, the heat m ust be transferred from the coolant to a working fluid to produce steam. > 9. Reactor control, including startup, power operation and shutdown is generally by movin g control rods. >>
7. The neutrons can strike other uranium atoms and cause additional fission and the continuing process of fissioning is known as a chain reactor.>> 8. Since two or three neutrons are liberated in each of fission whereas only one is required to maintain a fission chain, it would seem that once the fission reaction were initiated in a give n mass of fissile material, it would readily sustain itself.>> 9. However, such is not the case because not all the neutrons produced in fission are available to carry on the fission chain, that is, some neutrons are lost in nonfission reactions (mainly r adioactive capture),whereas other neutrons escape from the system undergoing fission.>> 10. The minimum quantity of such material that is capable of sustaining a fission chain is call ed the critical mass. UNIT 5 GENERAL FEATURES OF NECLEAR REACTORS> 2. In outline, a reactor consists of an active core in which the fission chain is sustained a nd in which most of the energy of fission is released as heat.>> 3. The core contains the nuclear fuel, consisting of a fissile nuclide and usually a fertile material in addition.>> 4. The function of the moderator is to slow down the high-energy neutrons liberated in th e fission reactor.>> 5. The purpose of reflector is to decrease the loss of neutrons from the core by scattering back many of those which have escaped.>> 6. The heat generated in the reactor is removed by circulation of a suitable coolant, such as ordinary(light) water, heavy water, liquid sodium(or sodium-potassium alloy), air and hel ium etc.>> 7. The higher the temperature of the steam, the greater the efficiency for conversion into useful power.>> 8. If the energy released in the reactor is to be converted into electric power, the heat m ust be transferred from the coolant to a working fluid to produce steam.>> 9. Reactor control, including startup, power operation and shutdown is generally by movin g control rods.>>
10. In most commercial thermal reactors the fuel is either uranium (0.7% uranium-235 ),w ith heavy water or graphite as the moderator, or uranium containing 2-4 percent of the fis with ordinary water as the moderator.>> l1. Based on the purpose, the reactor can fall into experimental (or research)reactor, pro uction reactor, power reactor, dual purpose (power and production) reactor or nuclear heati 12. According to the type of coolant and moderator, reactor can be called pressurized wat r reactor, boiling water reactor, heavy water reactor (e.g. CANDU), graphite reactor, or li quid metal cooled reactor. UNIT 7 INSTRUMENTATION 2. Many instruments for the detection of nuclear radiation are dependent upon the behavio r in an electrical field of the ion-pairs formed by the ionizing particles in their passage th rou 3. Neutrons are unchanged particles and therefore cannot cause ionization directly, so they must interact with matter by means of a nuclear reactor which, in turn, will generate cha 4. The changed particles will cause ionization within a gas-filled detector and these ion pa irs will produce a voltage pulse or some mean level current when collected at the electro des of the detector.>> 5. Since the neutron flux covers a wide range(12 decades), no single instrument can prov ide a satisfactory indication of the neutron flux and hence three ranges, i.e., source range intermediate range and power range, of instrumentation are used to obtain accurate flux I evel measures. >> 6. BF3 gas generated filled detectors (proportional counter) are used in source range, com pensated ion chamber are in the intermediate range, and uncompensated ion chamber in th e power range in some nuclear power plant.>> 7. Since gamma radiation from fission and fission products in a reactor can be very inten se, the compensated ionization chambers are required in the intermediate range 8. The fission chamber is coated with a uranium compound and pulse produced by the fis sion fragments resulting from the interaction of neutrons with the uranium-235 are so larg
10. In most commercial thermal reactors the fuel is either uranium (0.7% uranium-235), w ith heavy water or graphite as the moderator, or uranium containing 2-4 percent of the fis sile isotope, with ordinary water as the moderator.>> 11. Based on the purpose, the reactor can fall into experimental (or research) reactor, prod uction reactor, power reactor, dual purpose (power and production) reactor or nuclear heati ng reactor.>> 12. According to the type of coolant and moderator, reactor can be called pressurized wat er reactor, boiling water reactor, heavy water reactor (e.g. CANDU), graphite reactor, or li quid metal cooled reactor. UNIT 7 INSTRUMENTATION> 2. Many instruments for the detection of nuclear radiation are dependent upon the behavio r in an electrical field of the ion-pairs formed by the ionizing particles in their passage th rough a gas.>> 3. Neutrons are unchanged particles and therefore cannot cause ionization directly, so they must interact with matter by means of a nuclear reactor which, in turn, will generate cha rged particles.>> 4. The changed particles will cause ionization within a gas-filled detector and these ion pa irs will produce a voltage pulse or some mean level current when collected at the electro des of the detector.>> 5. Since the neutron flux covers a wide range (12 decades), no single instrument can prov ide a satisfactory indication of the neutron flux and hence three ranges, i.e., source range, intermediate range and power range, of instrumentation are used to obtain accurate flux l evel measures.>> 6. BF3 gas generated filled detectors (proportional counter) are used in source range, com pensated ion chamber are in the intermediate range, and uncompensated ion chamber in th e power range in some nuclear power plant.>> 7. Since gamma radiation from fission and fission products in a reactor can be very inten se, the compensated ionization chambers are required in the intermediate range. 8. The fission chamber is coated with a uranium compound and pulse produced by the fis sion fragments resulting from the interaction of neutrons with the uranium-235 are so larg
e that there is no difficulty in discriminating even against"pile-up''pluses from gamma ra 9. Pressure, defined as force per unit area, is one of the measured and controlled properti 10. Typically application of Borden tube pressure sensors is locally mounted pump suction and discharge pressure gages. >> 11. Thermocouples are utilized as temperature sensors t core exits.>> 12. The hot and gold leg temperature detectors of Reactors Coolant System are Resistance Temperature Detectors(RTDs) unit 8 ENERGY REMOVAL 1. In practical, the maximum power level of a reactor is normally determined by the rate at which the energy(heat) can be removed.>> 2. In nuclear reactor operating at high neutron flux, such as those intended for central stat ion power or ship propulsion, the design of the core depends just as much on the heat re moval aspects as on nuclear consideration.>> 3. The term thermal-hydraulic design is commonly used to describe the effort involving th e integration of heat transfer and fluid mechanics principles to accomplish the desired rate of heat removal from the reactor fuel.>> 4. The temperature in a reactor could increase continuously until the reactor is destroyed i f the rate of heat removal were less than the rate of heat generation.>> 5. The rate of heat generation and heat removal must be proper balanced in a operating r 6. The maximum of permissible temperature must be definitely established to make sure t hat the cooling system is adequate under anticipated operating conditions.>> The temperature at any point in a reactor will be greater than that of the sink by amo unt equal to the sum of all the temperature drops along the heat-flow path.>> 8. The goal of reactor thermal-hydraulic design is to provide for the " optimum"transport of heat from the fuel to its conversion into useful energy, normally in a turbine. > 9. By"optimum"is meant a proper balance between many opposing parameters, such as coolant flow rate, temperature distribution in the core, materials, etc. >>
e that there is no difficulty in discriminating even against “pile-up” pluses from gamma ra ys.>> 9. Pressure, defined as force per unit area, is one of the measured and controlled properti es.>> 10. Typically application of Borden tube pressure sensors is locally mounted pump suction and discharge pressure gages.>> 11. Thermocouples are utilized as temperature sensors t core exits.>> 12.The hot and gold leg temperature detectors of Reactors Coolant System are Resistance Temperature Detectors (RTDs). UNIT 8 ENERGY REMOVAL 1. In practical, the maximum power level of a reactor is normally determined by the rate at which the energy (heat) can be removed.>> 2. In nuclear reactor operating at high neutron flux, such as those intended for central stat ion power or ship propulsion, the design of the core depends just as much on the heat re moval aspects as on nuclear consideration.>> 3. The term thermal-hydraulic design is commonly used to describe the effort involving th e integration of heat transfer and fluid mechanics principles to accomplish the desired rate of heat removal from the reactor fuel.>> 4. The temperature in a reactor could increase continuously until the reactor is destroyed i f the rate of heat removal were less than the rate of heat generation.>> 5. The rate of heat generation and heat removal must be proper balanced in a operating r eactor.>> 6. The maximum of permissible temperature must be definitely established to make sure t hat the cooling system is adequate under anticipated operating conditions.>> 7. The temperature at any point in a reactor will be greater than that of the sink by amo unt equal to the sum of all the temperature drops along the heat-flow path.>> 8. The goal of reactor thermal-hydraulic design is to provide for the “optimum” transport of heat from the fuel to its conversion into useful energy, normally in a turbine.>> 9. By “optimum” is meant a proper balance between many opposing parameters, such as coolant flow rate, temperature distribution in the core, materials, etc.>>
10. An important aspect of the thermal-hydraulic design is concerned with conditions that might arise from an accident. >> 11. Provision must be made in the design to accommodate deviations from normal operati ng conditions, such as following partial or complete loss in the coolant flow.> 12. Three general mechanisms are distinguished whereby heat is transferred from one point to another, namely, conduction, convection and radiation UNIT 9 REACTOR MATERIALS 1. A unique aspect of reactor environment is the presence of intense nuclear radiations of vari Ous 0 2. Mechanical properties, such as tensile strength, ductility, impart strength and creep, must be adequate for the operation conditions. </ov
10. An important aspect of the thermal-hydraulic design is concerned with conditions that might arise from an accident.>> 11. Provision must be made in the design to accommodate deviations from normal operati ng conditions, such as following partial or complete loss in the coolant flow.>> 12.Three general mechanisms are distinguished whereby heat is transferred from one point to another, namely, conduction, convection and radiation. UNIT 9 REACTOR MATERIALS 1. A unique a spect of reactor environment is the presence of intense nuclear radiations of vari ous types. 2. Mechanical properties, such as tensile strength, ductility, impart strength and creep, must be adequate for the operation conditions. 3. The material must be able of being fabricated or joined, e.g., by welding, into the required shape. 4. An important requirement for structural and cladding materials is that th ey have a small ads orption cross section for neutrons. 5. The alloys in common use as cladding material are zircaloy-2 and zircaloy-4, both of which have good mechanical properties and corrosion resistance. 6. Ordinary water is attractive as a moderator because of its low cost, its excellent slowing-do wn power
7. Water of high degree of purity, especially free from chloride ions, is necessary to minimize corrosion 8. The fuel material should be resistant to radiation damage that can lead to dimensional chan ges,e.g, by swelling, cracking, or creep. <oSx</o 9. The fuel material should have a high melting point and there should be no phase transform ations, which would be accompanied by density and other changes, below the melting point. <o </O 10. Uranium dioxide, ceram ic which is the most common fuel material in commerc ial power r eactors, has the advantage of high-temperature sta bility and adequate resistance to radiation. <o ②<O 1. The pellets are ground to specified dimensions and are loaded into thin zircaloy tubes whi h serve as cladding <o 12. The small annular gap between the fuel pellets and cladding conta ins helium gas to impro ve the heat transfer characteristic UNIT 10 REACTOR SAFETY<OO</oes 1. In general, the goals of reactor safety are to reduce the probability of an accident and to hi mit the extent of the radiological hazard oOx</oe
7. Water of high degree of purity, especially free from chloride ions, is necessary to minimize corrosion. 8. The fuel material should be resistant to radiation damage that can lead to dimensional chan ges, e.g., by swelling, cracking, or creep. 9. The fuel material should have a high melting point and there should be no phase transform ations, which would be accompanied by density and other changes, below the melting point. 10. Uranium dioxide, ceramic which is the most common fuel material in commerc ial power r eactors, has the advantage of high-temperature stability and adequate resistance to radiation. 11. The pellets are ground to specified dimensions and are loaded into thin zircaloy tubes whi ch serve as cladding. 12. The small annular gap between the fuel pellets and cladding contains helium gas to impro ve the heat transfer characteristic UNIT 10 REACTOR SAFETY 1. In general, the goals of reactor safety are to reduce the probability of an accident and to li mit the extent of the radiological hazard
2. Nuclear reactor systems are designed with a number of barriers to the release of radioactivit y, namely, fuel pellets, fuel-rod cladding, primary coolant boundary and conta inment. /Os 7. Plants are now being required to develop accident management programs, which should redu ce the likelihood of uncontrolled radioactivity releases during accident. </0 8. Finally, emergency planes are developed that include provisions for sheltering and evacuation n to further reduce potential doses to the pubice40z
2. Nuclear reactor systems are designed with a number of barriers to the release of radioactivit y, namely, fuel pellets, fuel-rod cladding, primary coolant boundary and containment. 3. The basic philosophy of the design of nuclear power plants has been d escribed as defense i n depth, expressed in terms of five levels of safety. 4. The first level of safety is to design of reactor and other components of the system so that they will operate with a high degree of reliability and the chances of a malfunction are very small. 5. The purpose of safety is to design the reactor and other components of the system so that they will operate with a high degree of reliability and the chances of a malfunction are very s mall. 6. The third level of safety is to provide engineered safety features, such as emergency core c ooling system, containment spray system, and emergency eclectic power. 7. Plants are now being required to develop accident management programs, which shou ld redu ce the likelihood of uncontrolled radioactivity releases during accident. 8. Finally, emergency planes are developed that include provisions for sheltering and evacuatio n to further reduce potential doses to the public
