Nuclear Power PlantPower Plant Engineering

TopicsModerators and their importanceHomogeneous Acqeous Reactor (HAR) Safety measures for nuclear power plants

MODERATORSIt is a device that reduces the energy of

neutron evolved during fission in order to maintain the chain reaction.

It also reduces the quantity of fuel required to maintain the chain reaction.

Graphite, Heavy Water and Beryllium are generally used as moderator.

Enriched Uranium does not require moderator.

Properties of ModeratorHigh Thermal ConductivityLarge quantities in pure formHigh melting point (solid) and low melting

point (liquid)Resistance to corrosionStable under heat and radiationAble to slow down neutrons

MODERATING RATIOIt is the ratio of Moderating Power to the

Macroscopic Neutron Capture Coefficient.Higher the value of MR, more it is suitable

for the usage.

Material Moderating Ratio

Beryllium 160

Carbon 170

Heavy Water 12000

Ordinary Water 72

What is HAR???Aqueous homogeneous reactors (AHR) are a

type of nuclear reactor in which soluble nuclear salts (usually uranium sulfate or uranium nitrate) are dissolved in water. The fuel is mixed with the coolant and the moderator, thus the name "homogeneous" ('of the same physical state') The water can be either heavy water or ordinary (light) water, both of which need to be very pure.

Homogeneous reactor experimentThe first aqueous homogeneous reactor built at Oak

Ridge National Laboratory went critical October 1952. The design power level of one megawatt (MW) was attained in February 1953. The reactor's high-pressure steam twirled a small turbine that generated 150 kilowatts(kW) of electricity, an accomplishment that earned its operators the honorary title "Oak Ridge Power Company." However AEC was committed to development of solid-fuel reactors cooled with water and laboratory demonstrations of other reactor types, regardless of their success, did not alter its course.

Why HAR???Initial studies of homogeneous reactors took place

toward the close of World War II. It pained chemists to see precisely fabricated solid-fuel elements of heterogeneous reactors eventually dissolved in acids to remove fission products—the "ashes" of a nuclear reaction. Chemical engineers hoped to design liquid-fuel reactors that would dispense with the costly destruction and processing of solid fuel elements. The formation of gas bubbles in liquid fuels and the corrosive attack on materials (in uranylsulfate base solutions), however, presented daunting design and materials challenges.

ADVANTAGES  A heavy water aqueous homogeneous reactor can

achieve criticality (turn on) with natural uranium dissolved as uranium sulfate. Thus, no enriched uranium is needed for this reactor. The heavy water versions have the lowest specific fuel requirements (least amount of nuclear fuel is required to start them). Even in light water versions less than 1 pound (454 grams) of plutonium-239 or uranium-233 is needed for operation. Neutron economy in the heavy water versions is the highest of all reactor designs.

Their self-controlling features and ability to handle very large increases in reactivity make them unique among reactors, and possibly safest. At Santa Susana, California, Atomics International performed a series of tests titled The Kinetic Energy Experiments. In the late 1940s, control rods were loaded on springs and then flung out of the reactor in milliseconds. Reactor power shot up from ~100 watts to over ~1,000,000 watts with no problems observed.

KSTR (Kema suspensie test reactor)From 1974 till 1979 the KEMA (Keuring

van Elektrotechnische Materialen Arnhem) operated a aqueous homogeneous reactor, called Kema suspensie test reactor (KSTR) on their site in Arnhem (the Netherlands). The reactor was build in cooperation with experts from ORNL (Oak Ridge National Laboratory) because of their experience with the homogeneous reactor experiment. The reactor consisted of a reactor vessel (ø310 mm, content 18,3 liter), manufactured by Werkspoor in Utrecht. The fuel was a mixture of 14% UO2 (highly enriched, 90% U235) and 86% ThO2 in a concentration of 400 g/l. The uranium (6766 grams, containing 6082 grams of U235) was delivered by NUKEM. The fuel grains (ø 5μm) were designed by KEMA via a unique so-called sol-gel process, which also attracted attention from the industry. The reactor operated at a temperature of 255°C, a pressure of 60 bar and a maximum power of 1000 kW.

Aqueous homogeneous reactors were sometimes called "water boilers" (not to be confused with boiling water reactors), as the water inside seems to boil, but in fact this bubbling is due to the production of hydrogen and oxygen as radiation and fission particles dissociate the water into its constituent gases, a process called radiolysis. AHRs were widely used as research reactors as they are self-controlling, have very high neutron fluxes, and were easy to manage. As of April 2006, only five AHRs were operating according to the IAEA Research Reactor database.

SAFETY MEASURES FOR NUCLEAR POWER PLANTS

A nuclear reactor produces α, β and ϒ particles which can disturb the normal functioning of living organisms.

Nuclear power plants involve radiation leaks, health hazard to workers and community, and negative effect on surrounding forests.

SAFETY MEASURES FOR NUCLEAR POWER PLANTS

LocationQuality of constructionWaste water purificationExtensive ventilation systemExclusion zoneSafety systemPeriodical checksWaste disposal

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