Nuclear power reactors

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 The Magnox design has been superseded by the Advanced Gas-cooled Reactor (AGR) which, with higher steam temperature and pressure, operates at much higher efficiency.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.

In electrical power production, there is no fundamental difference between fossil fuel and nuclear systems. 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

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 pressurized water 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 24 life-time uranium requirement of a LWR is 4000 to 6000 tons of natural U3O8.

Pressurised Water Reactor

Thermal Reactor-Water Moderated

Indicative data for а reactor of 1200 MW(е) size: Uranium enrichment3.2 % U235. Coolant outlet temperature 324 °С. Pressure 2250 psi. Thermalefficiency 32 %. Core dimensions З.0 m dia× 3.7 m high.

Developed by the United States and the Soviet Union as а compactreactor for marine propulsion, this is the most widely used type of reactor.Over one hundred and ninety reactors are now operating in nuclearpower stations and there are more than 350 PWR powered naval vesselsin 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 pressurevessel containing the light water moderator, which is also the coolant.Heat extraction: The light water in the pressure vessel at high pressureis heated by the core. It is pumped to а steam generator where itboils water in а separate circuit the steam drives а turbine coupled to anelectric generator.

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Вариант 4

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Boiling-Water Reactor

The BWR is conceptually different than the PWR in that steam is actuallyallowed to form in the core. The pressure is maintained at about1000 psi, at which pressure water boils at 545 °F. Formation of steamchanges 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 secondarysystem is necessary for producing steam to drive the turbogenerators.The fluid exiting from the core passes through steam separators justabove the core. About 13 %, by weight, of this fluid is steam, the remainderbeing liquid that is recirculated. The steam from the separators exits thereactor vessel after passing through dryers which remove most of the remainingliquid. After passing through the turbogenerator, the steam is condensedand returned to the reactor as feedwater, thus completing the cycle.As a result, the reactor vessel (and contents) is basically the entire steamsupply system, in contrast to the PWR NSSS.

The core of a BWR is not much different than that of a PWR, exceptfor its size. The fuel rods have a slightly larger diameter, and a typicalassembly is a square array of 49 (or 64) rods in a channel that is onlyopen 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 ofjet pumps at the wall of the reactor vessel. Some water is actually withdrawnfrom the vessel by pumps which recirculate it for use in drivingthe 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. Thereactor 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 iscontained in the vessel. A 1000 MWe BWR vessel is about 72 ft highand 21 ft in diameter. The walls are 6 to 7 in thick steel.

The reactor vessel and primary piping are contained in a steel pressurevessel (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 otherabnormal conditions, valves close the steam lines which pass throughthe dry well to the turbogenerator. Any steam released from the reactorwould force fluid down into the suppression pool, where steam wouldbe condensed, thus relieving the pressure in the dry well. The latter isdesigned to withstand the transient pressures to which it would be subjectedunder accident conditions.

There are two types of ECCS available. As soon as the water leveldrops below a preset minimum, a high-pressure injection system drivenby steam turbines is activated. Backing this up are low-pressure electrically-driven core-spray and coolant-injection systems, which would becomeoperative after failure of the high-pressure system and subsequentdepressurization (through pressure relief valves) into the dry well anddowncomer arrangement. The low-pressure systems are sized to handlethe reactor decay heat without damage to the core.

The steel dry well, and the reinforced concrete structure immediatelysurrounding it, are enclosed by a secondary containment building. Gasexhausting from this building passes through multiple filtration systemsfor trapping volatile radioactive species. More recent BWR systemshave another leak-tight containment structure between the primary containmentand the reactor building.

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

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Обозначение проблемы

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Выводы (ваше отношение и решение проблемы)

Заключение.

Вариант 5

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