Воробьева И.А., Смирнова С.Н. Nuclear reactor types

reactor vessel after passing through dryers which remove most of the remaining liquid. After passing through the turbogenerator, the steam is condensed and returned to the reactor as feedwater, thus completing the cycle. As a result, the reactor vessel (and contents) is basically the entire steam supply system, in contrast to the PWR NSSS.

The core of a BWR is not much different than that of a PWR, except for its size. The fuel rods have a slightly larger diameter, and a typical assembly is a square array of 49 (or 64) rods in a channel that is only open at the top and bottom. A large BWR has around 40,000 fuel rods, containing a total of about 200 tons of UO.

Water is forced to circulate upward through the core by the action of jet pumps at the wall of the reactor vessel. Some water is actually withdrawn from the vessel by pumps which recirculate it for use in driving the jet pumps.

The reactor is typically controlled by the cruciform control element. This element actually contains numerous boron-carbide-filled rods. These rods serve for both reactivity control and power flattening. The reactor is also controlled by the recirculation rate.

The reactor vessel is larger than that of a PWR of comparable power, both because of lower power density and because more equipment is contained in the vessel. A 1000 MWe BWR vessel is about 72 ft high and 21 ft in diameter. The walls are 6 to 7 in thick steel.

The reactor vessel and primary piping are contained in a steel pressure vessel (the "dry well") which constitutes the reactor's primary containment. This is connected through very large piping to a series of "downcomer" pipes which open into a pool of water in the large torus. Should the water level in the reactor become too low, or under other abnormal conditions, valves close the steam lines which pass through the dry well to the turbogenerator. Any steam released from the reactor would force fluid down into the suppression pool, where steam would be condensed, thus relieving the pressure in the dry well. The latter is designed to withstand the transient pressures to which it would be subjected under accident conditions.

There are two types of ECCS available. As soon as the water level drops below a preset minimum, a high-pressure injection system driven by steam turbines is activated. Backing this up are low-pressure electri- cally-driven core-spray and coolant-injection systems, which would become operative after failure of the high-pressure system and subsequent

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depressurization (through pressure relief valves) into the dry well and downcomer arrangement. The low-pressure systems are sized to handle the reactor decay heat without damage to the core.

The steel dry well, and the reinforced concrete structure immediately surrounding it, are enclosed by a secondary containment building. Gas exhausting from this building passes through multiple filtration systems for trapping volatile radioactive species. More recent BWR systems have another leak-tight containment structure between the primary containment and the reactor building.

Several types of reactors that use boiling water in pressure tubes have been considered, designed or built. The principal reactor type constructed in the Soviet Union uses a boiling-water pressure tube design, but with carbon moderator.

Exercises

Ex. 1. Answer the following questions:

1.What principle is the boiling-water reactor based on?

2.What is the flow path of the coolant in the BWR?

3.What is the core configuration?

4.What is the vessel like?

5.What are the two types of ECCS? How do they operate?

6.In what way is control provided?

7.What is the containment system of the BWR?

8.Describe the scheme of the BWR.

BWR

Boiling Water Reactor

Thermal Reactor-Water Moderated

Indicative data for а reactor of 600 MW(e) size:

Uranium enrichment 2.6 % U 235. Coolant outlet temperature 286 °С. Pressure 1050 psi. Thermal efficiency 32 %. Core dimensions 3.7 m dia × 3,7 m high.

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coolant has to be lost for a long period before the fuel is damaged 3. The core of a BWR is not much different than that of a PWR, except for its size. 4. Another important feature is that no secondary system is available. 5. The reactor vessel is larger than that of a PWR of comparable power, both because of lower power density and because more equipment is contained in the vessel. 6. Organic materials can operate at higher t°, thereby improving the thermal efficiency of the power plant. 7. As soon as the water level drops below a preset minimum, a highpressure injection system is activated. 8. One neutron, on the average, is needed to continue the chain reaction, one must convert a fertile nucleus to one that is fissile.

Ex. 4. Give the 3 forms:

Ex. 5. Analyze the sentences from the text. Translate them.

1.Neutrons are better moderated and are likely to induce fission.

2.About 13 %, by weight, of this fluid is steam, the remainder being liquid. 3. The moderator and coolant systems are actually separate, the moderator filling a large low-pressure vessel. 4. Keeping a reactor operating at constant power level requires maintenance of balance between neutron production and absorption. 5. After passing through the turbogenerator, the steam is condensed and returned to the reactor as feedwater, thus completing the cycle. 6. Should the water level in the reactor become too low, valves close the steam-lines. 7. Steam would be condensed in the suppression pool, thus relieving the pressure in the dry well. 8. The latter is designed to withstand the transient pressures. 9. Backing this system up are low-pressure systems. 10. Backing up this system low-pressure systems would become operative in emergency. 11. Surrounding the steel dry well is the reinforced concrete structure.

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Ex. 6. Translate. Pay attention to -ed forms.

1 .The solution of the problem required concentrated efforts of many scientists. 2. The data required were analyzed la our lab. 3. The chain reaction can be shut down rapidly when required. 4. It is required that, on the average, for every neutron absorbed or lost, precisely one be produced. 5. Required for the emergency shutdown system is a supply of light water. 6. The technique applied increased the accuracy. 7. When applied to nuclei, this concept is theoretical. 8. When this approach is applied to simpler cases, it should be modified. 9. The laboratory installed modernized equipment. 10. The equipment installed modernized the laboratory. 11. Much information can be obtained by raising the energy of the nucleus to an excited level. 12. The target nucleus excited into a high-energy state emitted electromagnetic radiation, as it returned to its ground state. 13. Some nuclei do not fission at once when excited. 14. The structural component designed is inherently involved. 15. The reactor accident involved the release of radioactivity. 16. The accident involved resulted in the release of radioactivity.

Ex.7. Condense the following sentences using the Participle instead of the underlined clauses according to the model.

Mode: We made use of the results which were published recently. We made use of the results published recently.

1 .We attended the conference which was devoted to nuclear steam supply system. 2. I commented on the paper which is concerned with the various back-up systems of the reactor. 3. I mentioned the reactor which is referred to as the BWR. 4. Dr. Smith presented new results which were obtained on the phenomenon of power flattening. 5. The specialists considered the advantages of the technique which was used for reactor control. 6. The paper emphasizes the difficulties which were met with under accidental conditions. 7. He cited the data which were presented for scram. 8. The electrons of the beam which are little affected by the electrons in the atoms are scattered by the nuclei. 9. A brief discussion which was followed by development of equations deals with heat and mass.

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Ex. 8. Give a free translation.

Во многих странах нашел распространение кипящий тип водоводяного реактора. Вода в нем обычно находится при давлении около 70 кг/см2. Она не только нагревается до кипения, но и частично испаряется. В такой установке обычно нет парогенератора. Давление в реакторе примерно такое же, как и в трубопроводе. Однако при эксплуатации кипящего реактора паровая турбина становится радиоактивной. Последнее усложняет и удорожает установку. Габариты корпуса кипящего реактора значительно больше, чем у ВВЭР. Это также приводит к трудностям при их изготовлении и транспортировке.

Разрабатываются кипящие реакторы с корпусом из предварительно напряженного бетона. Такой корпус практически не испытывает механических напряжений. Внезапное разрушение такого корпуса невозможно. Все оборудование может быть размещено внутри корпуса. Таким образом, рассматриваемый реактор является наиболее безопасным по отношению к окружающей среде и населению. Он наиболее подходит для теплоснабжения больших городов.

Plants using ordinary water as the reactor coolant may operate in two distinct fashions: steam generated in the reactor coolant may be used directly to drive the turbo generator, or the coolant may recirculate raising steam in a secondary system. The second approach is that used in

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“pressurized-water reactors” and in reactors using other than a lightwater coolant. The first, and more direct, approach is used in “boilingwater reactors”. In pressurized-water reactors, gross boiling in the primary coolant is prevented by maintenance of the system pressure at roughly 2250 pounds per square inch, about twice the pressure in a boil- ing-water reactor.

READING 4-B

Safety

From the outset, safety of nuclear reactors has been a very high priority in their design and engineering. About one third of the cost of a typical reactor is due to safety systems and structures. The Chernobyl accident in 1986 was a reminder of the importance of this, whereas the Three Mile Island accident in 1979 showed that conventional safety systems work.

At Chernobyl in Ukraine 30 people were killed (mostly by high levels of radiation) and many more injured or adversely affected. This reactor lacked the basic engineering provisions necessary for licensing in most parts of the world (other reactors of that kind still operating have been significantly modified). At Three Mile Island in the USA with a similarly serious malfunction, the effects were contained and no-one suffered any harm or injury.

• While discussing the “Safety” make use of the following vocabulary:

From the outset; safety of nuclear reactors; a very high priority; design and engineering; is due to; safety systems and structures; a reminder; conventional safety systems; high levels of radiation; injured or adversely affected; lacked the basic engineering provisions; licensing;significantly modified; malfunction; suffered any harm or injury.

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UNIT V

HEAVY WATER REACTORS

READING 5-A

Pressurized Heavy-Water Reactor (CANDU)

We will here describe the reactor currently available in Canada. The term "CANDU" applies specifically to the fact that this reactor is a deu- terium-moderated uranium-fueled device.

Instead of moderating neutrons with “"light” water, containing hydrogen with an atomic mass of 1, this design uses “heavy” water (containing deuterium, mass 2), which absorbs fewer neutrons than light water does.

The fuel of a CANDU is very similar to that of an LWR in that fuel pins containing uranium dioxide pellets are arranged into bundles which can be inserted into the reactor fuel channels.

Each of these channels is a pressure tube, containing a single line of fuel bundles arranged end-to-end. The core actually consists of a large number of these tubes, all of which pass through a lattice of tubes which are part of a “Calandria” which contains the moderator. The pressurized coolant is pumped through the pressure tubes and boilers (steam generators) much like the coolant of a PWR, except that the CANDU coolant’s sealed off from the moderating fluid, which is maintained at a much lower pressure and temperature than the coolant. The lower pressure removes the necessity of fabricating a single large pressure vessel. At the boilers, steam is raised in a light water system which drives the turbogenerators. Both the moderator and coolant fluids of current CANDU are heavy water; a possible variant uses light water as the coolant.

One result of the individual pressure tube structure of the CANDU is that the reactor can be refueled without shutdown. A single channel can be opened and partially refueled by two refueling machines. The fact that the reactor is refueled only a little at a time results in continuous (on-line) refueling, as opposed to the abrupt once-yearly loading. The latter scheme causes abrupt increases in the fissile load, thus increasing the neutron multiplication factor. In this case substantial amounts of control are required. A reactor such as the CANDU does not need this much control since the fissile load remains essentially constant and more neutrons are available for useful purposes such as conversion of

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fertile material to fissile. Decreased control thus improves conversion. A basic feature of the CANDU, permitted by the use of a moderator

that absorbs few neutrons, is the use of natural uranium as the fuel. Uranium fed into the CANDU contains only 0.7 % 235U and this is reduced to about 0. 2% by the time a bundle is removed. (The irradiated bundle also contains plutonium, as a result of neutron capture by 238U.) This removes the need for costly enrichment of uranium to the 2 to 4 % 235U content needed by LWR’s. However, an equivalent cost is production of heavy water by separation from ordinary water, of which 0,016 % is heavy. The lower fuel enrichment means that excess fissile material is not available in the fuel. As a result, the design must be based on a minimum of control and on an associated continuous refueling procedure, as just described.

The pressure maintained in the primary coolant system, again by a pressurizer as in a PWR, is 1500 psi, and the coolant is allowed to rise to 590 °F, below the boiling point for that pressure. The moderator, on the other hand, is maintained at approximately atmospheric pressure, under a cover of helium gas. Operating temperatures are in the range 110 — 160 °F. As a result, the calandria, which has a basically cylindrical shape, 25 ft in length, with an outer diameter of about 25 ft, is constructed of much thinner steel than the pressure vessels for light-water reactors. The tubes through which the fuel pressure tubes pass are made of zircaloy.

Control of the reactor is maintained by several systems including a number of variable neutron absorbers (actually compartments into which a specified amount of light water can be pumped) and a number of adjusting rods (typically consisting of an absorber such as stainless steel, although these have sometimes been booster rods, consisting of highly enriched fuel). Shutdown capability exists in the form of gravityoperated absorbing rods, backed up by a system for injecting poisons into the moderator. It is also possible to dump the moderator out of the calandria into a storage tank.

The CANDU has an additional cooling system (also heavy water) of sufficient capacity to cool the reactor after shutdown. Connected to this shutdown system is an emergency-core-cooling supply of light water which may be injected should the basic system fail.

The calandria is actually in a vault that is filled with light water, which serves as shielding and helps to maintain the calandria at a constant temperature. The vault is itself in a sealed containment building and in some

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designs can be maintained at a negative pressure with respect to the external surroundings. A spray system exists within the containment building. The overall thermal efficiency of a CANDU system is about 29 %, significantly lower than that of most commercial nuclear power plants.

The primary coolant flow pattern is relatively simple: coolant from a primary pump passes through a distribution header to the individual tubes, goes once through the header at the stem generator, and through the U-tube steam generator to the primary pump. The flow rate (600 MWe CANDU) is about 60 million pounds per hour.

CANDU

Thermal Reactor

Heavy Water Moderated

Indicative data for а reactor of 600 MW(e) size:

Uranium enrichment 0.7 % U235 (natural). Coolant outlet temperature 305 °С. Pressure 1285 psi. Thermal efficiency 30 %. Core dimensions 7.1 m dia × 5.9 m high.

То avoid the need for enriched uranium Canada designed this heavy water reactor. CANDUs have been exported to India, Pakistan, Argentina, Korea and Romania. There are 26 reactors in use generating some 15000 MW(e) А further 16 reactors are under construction.

Fuel: Uranium dioxide in Zircaloy cans

Moderator: Heavy water (D20) which allows natural uranium fuel to be used.

Core layout: Each cluster of fuel elements is in а separate pressure tube; the pressure tubes are in а tank of heavy water.

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