Воробева Нуцлеар Реацтор Тыпес (Леарн то реад бы реадинг) 2010

naval reactor started up in March 1953 in Idaho, and the first nuclearpowered submarine, USS Nautilus, was launched in 1954. In 1959 both USA and USSR launched their first nuclear-powered surface vessels.

The Mark 1 reactor led to the US Atomic Energy Commission building the 90 MWe Shippingport demonstration PWR reactor in Pennsylvania, which started up in 1957 and operated until 1982.

Since the USA had a virtual monopoly on uranium enrichment in the West, British development took a different tack and resulted in a series of reactors fuelled by natural uranium metal, moderated by graphite, and gas-cooled. The first of these 50 MWe Magnox types, Calder Hall-1, started up in 1956 and ran until 2003. However, after 1963 (and 26 units) no more were commenced. Britain next embraced the Advanced Gas-Cooled Reactor (using enriched oxide fuel) before conceding the pragmatic virtues of the PWR design.

UNIT III

NUCLEAR POWER REACTORS

• Answer the question before reading each paragraph and then tell if your answers are correct.

Calder Hall

21

1.What is the main difference between fossil fueled stations and nuclear ones?

The main difference between coal and oil-fired power stations and nuclear ones is the source of heat.

2.What reactions produce energy in different stations?

With coal and oil, the heat is produced by burning the fuel - a chemical reaction. In nuclear stations the heat comes from energy released when the nucleus of a heavy atom (uranium or plutonium) is split - a nuclear reaction. This produces several million times the energy of a chemical reaction weight for weight.

3.Which of the two reactions are more competitive?

Although our present day 'thermal' reactors can use only a small percentage of their uranium fuel, this still yields tens of thousands of times as much energy as a chemical reaction.

4.What are the examples of improving efficiency during the first 30 years of nuclear power?

So, the volumes of nuclear fuel to be mined, processed and transported, and of the resultant waste products are all much smaller than in the case of fossil fuels. The first 30 years of nuclear power have seen considerable developments in reactor design. The early stations in Britain, the Magnox type were improved versions of Calder Hall. The Magnox design has been superseded by the Advanced Gas-cooled Reactor (AGR) which, with higher steam temperature and pressure, operates at much higher efficiency.

Parallel development of water-cooled reactors, mainly in the United States, produced the Pressurized Water Reactor (PWR). Much research and development has gone into the fast reactor, which can use uranium far more effectively. A commercial demonstration fast reactor, featuring a common European design, is expected to be built by the turn of the century.

READING 3-A

In electrical power production, there is no fundamental difference between fossil fuel and nuclear systems (See the picture). Heat generated from splitting atoms or burning coal, oil or gas is used to boil water, make steam, drive turbines and generate electricity.

This book is basically concentrated on the nuclear power reactor

22

types. However in some detail questions of safety, economics, waste management, transport of nuclear materials, radiation, and avoiding weapons proliferation will be also addressed.

•Find the only difference in the two schemes above.

READING 3-B

The reactors considered here are the pressurized-water reactor, the boiling water reactor, the heavy water reactor and the gas-cooled reactor. All the types considered are thermal reactors, that is, the neutrons are moderated to make use of the higher fission cross-sections at low neutron energy.

The predominant commercial reactor is the light-water reactor (LWR). The light-water reactor is found in two types, the pressurizedwater reactor (PWR) and the boiling water reactor (BWR). These reactors use ordinary water as the moderator and coolant. Natural uranium cannot be used as the fuel in a LWR. All the fuel is, indeed, uranium. But the concentration of the fissile U235 has to be increased from its natural 0.7 % to almost 3 % in order for such a reactor to operate. The

23

life-time uranium requirement of a LWR is 4000 to 6000 tons of natural

U3O8.

Pressurized-Water Reactor

The core of a pressurized-water reactor (PWR) consists of a large number of square fuel assemblies or bundles. Each of these contains about 200 fuel pins closely held together in a matrix with no outer sheath. A full-sized (i.e. 1000 MWe) PWR may contain nearly 40,000 fuel pins, containing about 110 tons of UO2. The power generation density in the core is about 98 kW/liter. All assemblies have provision for the passage of control rods through the bundle. These rods are manipulated by drives at the top of the reactor. Additional control is available by addition of neutron absorbers (such as boric acid) to the coolant. The reactor vessel itself may be 40 ft high by 14 ft in diameter, made of steel 8 in, or more thick. The top head is removable for refueling.

The coolant enters the reactor vessel near the top of the core and constrained by a "core barrel" between the vessel and the core, flows to the bottom of the core, then up through the core itself and out to the steam generators from which it is recirculated by large pumps. Maintaining the pressure at about 2250 psi prevents the formation of steam in this "primary” system. Instead, steam is raised in a "secondary" system by allowing heat to flow from the high pressure primary coolant to the lower pressure secondary fluid. This heat transfer occurs through the walls of large numbers of tubes through which the primary coolant circulates in the team generators. After passing through separators to remove water droplets, the steam leaves the steam generator for the production of electricity. After condensation, it returns as liquid to the steam generators. The overall thermal efficiency of a PWR is about 32 %.

A large PWR has four external circuits, each with its own steam generator and pump. Since maintenance of the pressure near the design value la crucial (to avoid the formation of steam, on the one hand, and rupture of the primary circuit, on the other), a PWR system also includes a "pressurizer", connected to the "hot" leg of one of the steam generator circuits. The pressurizer is filled partly by water and partly by steam, it initiates condensation or vaporization as needed to keep the pressure within specified operating limits.

All of these components of the "primary" system - the reactor vessel, steam generators, pumps, and pressurizer - are in the containment building. This structure is steel-lined, reinforced concrete, designed to with-

24

stand the overpressures expected if all the primary coolant were released in an accident. Sprays or other means are available for condensing steam (thereby reducing pressure) and for removing any radioactive material released into the containment atmosphere.

Various features are available for limitation of abnormalities. These include introduction of control to shut down rapidly (i.e. to "trip” or “scram") the chain reaction, and systems for continuation of core cooling thereafter. In case much of the primary coolant is lost through a pipe-break or other leak, one of a number of redundant emergency core cooling systems (ECCS) becomes operative. The first is an accumulator which automatically injects water into the system to provide continued cooling of the core. In addition, there are independently-powered systems for actively pumping stored water into the primary system. These systems rely on redundancy of components to assure their availability in emergencies. This is also true of other devices. For example, each of the four primary coolant pumps has a capacity large enough to provide by itself for removal of decay heat after shutdown of the reactor.

Schematic Pressurized -Water Reactor

The primary reactor system is enclosed in a steel-lined concrete containment building. Steam generated within the building flows to the tur- bine-generator system (outside the building), after which it is condensed and returned to the steam generator.

25

Exercises

Ex. 1. Answer the following questions.

1.What reactors are referred to as thermal reactors?

2.What are the essential features of the LWR?

3.What is the life-time uranium requirement?

4.What is the core structure of the PWR?

5.In what way is heat removed? Describe the flow diagram of PWR heat removal.

6.What are the components of the primary circuit?

7.What prevents the formation of steam in the primary circuit?

8.What is the flow path of steam in the secondary circuit?

9.How is the pressure maintained within specified, operating lim-

its?

10.What is the function of the containment building?

11.In what way is the core controlled?

12.What features are available to limit abnormalities?

13.In what case does the ECCS become operative?

14.What devices are referred to as ECCS?

15.What do letters LOCA stand for?

Ex. 2. Translate. Pay attention to the underlined words.

1. Each assembly contains about 200 .fuel pins. 2. A full-sized (i.e. 1000 MWe) PWR may contain nearly 40,000 fuel pins, containing about 110 tons of UO2. 3. The fuel pins are closely held together in a matrix. 4. The pressuriser is filled partly by water and partly by steam. 5. Sprays or other means are available for condensing steam thereby reducing pressure. 6. Instead, steam is raised in a secondary system. 7. In addition, there are independently powered systems. 8. Since maintenance of the pressure near the design value is crucial, a pressuriser is included in a PWR system. 9. In case much of the primary coolant is lost, one of a number of emergency core cooling systems becomes operative.

Ex. 3. Give the 3 forms:

26

Ex. 4. Condense the following sentences using, the Participle instead of the underlined clauses according to the models.

1. The reactor vessel which is made of steel may be 40 ft high. 2. The coolant which is contained by a core barrel between the vessel and the core flows to the bottom of the vessel. 3. The steam which first leaves the steam generator, after condensation returns back to it. 4. The device which initiates condensation is filled partly with water and partly with steam. 5. Various features which limit the abnormalities are available. 6. ECCS which becomes operative is one of a number of redundant emergency core cooling systems. 7. The first is an accumulator which automatically injects water into the system. 8. In addition, there are systems, which are independently powered to pump the water which is stored into the primary system. 9. A FWR system which includes a pressurizer is connected to the hot leg of one of the steam generator circuits. 10. The type of the reactor which is being considered now is of great importance. 11. The types of reactors which are existing today use uranium as fuel.

Ex. 5. Translate. Pay attention to ing-forms.

1. Changing other reactor parameters we can improve uranium utilization. 2. Changing reactor parameters can improve uranium utilization. 3. Maintaining the pressure at about 2500 psi prevents the formation of steam in the primary system. 4. Having established the need for control, we can ask whether such control can be implemented. 5. After passing through separators the steam leaves the steam generator. 6. All the commercial plants now being designed are light-water reactors. 7. In many reactors there are both “control” and “shutdown” rods, the latter being designed for rapid shutdown. 8. The total number of neutrons being produced depends on the volume of the core. 9. U233 is not currently being

27

produced in any large quantity. 10. The fusion program is proceeding rapidly. 11. So far we have been discussing the theoretical approach. 12. Each neutron is capable of producing three or four fissile nuclei. 13. It is worth considering what is meant by multiplication factor. 14. Neutron absorption by a fissile nucleus results in breaking up the nucleus. 15. Additional thermal energy results from stopping or absorbing the neutrons and gamma rays given off during fission.

READING 3-C

The primary interest in nuclear reactors arises from their potential for serving as heat sources for plants that generate electrical power. Electricity can be generated in a number of ways, but the conventional method by which it is generated is to use thermal energy (heat), to produce steam, which drives a turbine-generator system. This technique may be thought of as employing two basic systems: a steam supply system, which uses heat from the combustion of fossil fuels or from nuclear reactions to boil water, and an electrical generating system, which uses the resulting steam to produce electricity. In principle, even the sun may serve as the heat source for the steam supply system, but for the near future fossil-fueled boilers and nuclear reactors will be the central components in large electrical generating plants. In recent years, a growing portion of such generating capacity has been provided by nuclear power plants.

The nuclear power plants of this century depend on a particular type of nuclear reaction, fission, for the generation of heat. Fission is the splitting of a heavy nucleus, the center of an atom such as uranium, into two or more principal fragments, as well as lighter pieces, such as neutrons. Neutrons are, in fact, one of the two basic components of nuclei (the other is the proton), and, as noted, they are released during fission, thereby becoming available to induce subsequent fission events. Under suitable conditions, a "chain" reaction of fission events may be sustained. The energy released from the fission reactions provides the heat, part of which is ultimately converted into electricity.

However, dependence on a fission chain reaction does introduce some special aspects to the reactor. The first arises from the fact that a nuclear reactor depends on a chain reaction. Maintaining a constant power level requires that the chain reaction be controlled so that, on the average, each fission causes only one subsequent fission. The second

28

feature of a nuclear reactor is that the products of reactor operation are highly radioactive. As a result, reactor design is aimed at limiting the probability of release of these products.

Ex. 6.

На большинстве ядерных электростанций сейчас используются реакторы с водой под давлением. Основная характеристика такого реактора состоит в том, что пар образуется в теплообменнике, называемом парогенератором, и этот пар приводит в движение турбину. В герметичной оболочке заключена система первого контура. Она состоит из корпуса реактора, двух или более петель теплоносителя первого контура. Каждая петля включает в себя трубопровод, насосы и парогенератор. Системы аварийного охлаждения активной зоны также заключены в эту герметичную оболочку.

READING 3-D

PWR

29

Pressurised Water Reactor Thermal Reactor-Water Moderated

Indicative data for а reactor of 1200 MW(е) size: Uranium enrichment 3.2 % U235. Coolant outlet temperature 324 °С. Pressure 2250 psi. Thermal efficiency 32 %. Core dimensions З.0 m dia ×3.7 m high.

Developed by the United States and the Soviet Union as а compact reactor for marine propulsion, this is the most widely used type of reactor. Over one hundred and ninety reactors are now operating in nuclear power stations and there are more than 350 PWR powered naval vessels in service. More than 20 countries have PWRs and Britain is constructing its first at Sizewell.

Fuel: Uranium dioxide clad in an alloy of zirconium (Zircaloy). Moderator: Light water (ordinary water, Н20)

Core layout: Fuel pins, arranged in clusters, are placed inside а pressure vessel containing the light water moderator, which is also the coolant. Heat extraction: The light water in the pressure vessel at high pressure is heated by the core. It is pumped to а steam generator where it boils water in а separate circuit the steam drives а turbine coupled to an electric generator.

UNIT IV

BOILING-WATER REACTORS

READING 4-A

Boiling-Water Reactor

The BWR is conceptually different than the PWR in that steam is actually allowed to form in the core. The pressure is maintained at about 1000 psi, at which pressure water boils at 545 °F. Formation of steam changes the density of the water and thus of the moderator. As a result, neutrons will be better moderated and will be more likely to induce fission.

The other important result of allowing the coolant to boil is that no secondary system is necessary for producing steam to drive the turbogenerators. The fluid exiting from the core passes through steam separators just above the core. About 13 %, by weight, of this fluid is steam, the remainder being liquid that is recirculated. The steam from the separators exits the

30