NUCLEAR REACTORS

G. HETSRONIEmeritus Danciger Professor of EngineeringTechnion Haifa Israel

Contents1. History 2. Fission3. Nuclear reactor basics4. Classifications 5. Current technologies

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Enrico Fermi

Nuclear fission was first experimentally achieved by Enrico Fermi in 1934 when his team bombarded uranium with neutrons. In 1938, German chemists Otto Hahn and Fritz Strassmann, along with Austrian physicists Lise Meitner and Meitner's nephew, Otto Robert Frisch, conducted experiments with the products of neutron-bombarded uranium. They determined that the relatively tiny neutron split the nucleus of the massive uranium atoms into two roughly equal pieces. Leo Szilard recognized that if fission reactions released additional neutrons, a self-sustaining nuclear chain reaction could result. In the United States, where Fermi and Szilard had both emigrated, this led to the creation of the first man-made reactor, known as Chicago Pile-1, which achieved criticality on December 2, 1942. This work became part of the Manhattan Project.

Otto Hahn

Lise Meitner

Fritz Strassmann

NUCLEAR CHAIN REACTION

When a relatively large fissile atomic nucleus (usually uranium-235 or plutonium-239) absorbs a neutron it is likely to undergo nuclear fission. The original heavy nucleus splits into two or more lighter nuclei also releasing kinetic energy, gamma radiation and free neutrons; collectively known as fission products. A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on.

The nuclear chain reaction can be controlled by using neutron poisons and neutron moderators to change the portion of neutrons that will go on to cause more fissions. In nuclear engineering, a neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.

Nuclear reactor principle

As of 2005, nuclear power provided 6.3% of the world's energy and 15% of the world's electricity. As of 2007, there are 439nuclear power reactors in operation in the world, operating in 31countries. In 2007, nuclears share of global electricity generation dropped to 14%, because of earthquake in western Japan on 16 July 2007. The United States produces the most nuclear energy, with nuclear power providing 19% of the electricity it consumes, while France produces the highest percentage of its electrical energy from nuclear reactors78% as of 2006. In the European Union as a whole, nuclear energy provides 30% of the electricity.

Boiling water reactor (BWR) is the simplest of all facilities. Water absorbs heat from the reactions in the core and is directly driven to the turbines. After condensing the water is pumped back to the reactor core. CLASSIFICATIONS BY TYPE

Pressurized water reactor (PWR) uses a sealed system to prevent water circulating through the core from boiling due to high pressure. The heat from this system is removed by the water in pipes to the steam generator. CLASSIFICATIONS BY TYPE

Gas cooled reactor (GCR) uses CO2 gas to remove heat from the core. This is then piped through the steam generator where heat is removed from the gas and it can then be recirculated to the reactor. As usual steam generated is used to drive the turbine and generate electricity, condensed then recirculated. Graphite is used as a moderator to allow energy production by un-enriched uranium. CLASSIFICATIONS BY TYPE

BASIC PWR - STEAM CYCLE I. Primary circuit A. Reactor II. Secondary circuit B. Steam generator III. Tertiary circuit C. Turbine D. Condenser

PWR in Operation

A Primary circuit B. Secondary circuit C. Tertiary circuit 1. Reactor 2. Fuel assemblies 3. Control rods 4. Pressurizer5. Steam generator 6. Primary pump 7. Feedwater primary circuit 8. Feedwater secondary circuit 9. Steam secondary circuit 10. High pressure turbine 11. Low pressure turbine 12. Condenser 13. Feedwater pump 14. Generator 15. Exciter 16. Transformer 17. High voltage line 18. River 19. Intake cooling water 20. Cold cooling water 21. Warm cooling water 22. Cooling water 23. Upward airflow 24. Steam 25. Outlet cooling water

Diagram of the reactor

REACTOR COOLANT SYSTEM PHYSICAL ARRANGEMENT The PWR reactor coolant system (RCS) circulates water in a closed cycle, removing heat from the reactor core and internals and transferring it to a secondary (steam generating) system. The steam generators provide the interface between the reactor coolant (primary) system and the main steam (secondary) system. The steam generators are vertical U-tube heat exchangers with an integral economizer in with heat is transferred from the reactor coolant to the main steam system. Reactor coolant is prevented from mixing with the secondary steam by the steam generator tubes and the steam generator tube sheet, making the RCS a closed system thus forming a barrier to the release of radioactive materials from the core of the reactor to the containment building.

Reactor Vessel Assembly Arrangement

Fuel assembly

STEAM GENERATOR Nuclear Steam Supply System (NSSS) of KNSP uses two steam generators for transfer of heat from the RCS to the main steam system, One steam generator is located in each loop. PWR, inverted U-tube steam generator with an integral economizer which operates with the reactor coolant on the tube side and secondary coolant on the shell side.

HEAT EXCHANGERThis is a heat exchanger in which two water circuits meet each other: hot water under extremely high pressure in the primary circuit leaves the reactor and flows through thousands of U-shaped heat conducting tubes.

STEAM TURBINESteam turbine consists of a series of blades mounted on a shaft. As the steam jet is inflected, it puts pressure on these blades, making the shaft rotate.

Generator

Condenser and cooling towers

CLASSIFICATIONS BY TYPE

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