Chapter 03 _ Reseach Nuclear Power Reactors

8/2/2019 Chapter 03 _ Reseach Nuclear Power Reactors

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Nuclear Energy: Peaceful Ways of Serving Humanity

Edited by Dr. Mir F. Ali Page: 1

03. Research Nuclear Power Reactors

Many research reactors were built in the 1960s and 1970s. 1975 saw the peak number ofoperating research reactors with 373 in 55 countries.

These reactors are primarily designed to produce neutrons, activate radioactive or otherionizing radiation sources for scientific, medical, engineering or other research purposesincluding teaching and training. Many of them are located on university campuses.

According to IAEA, no new research nuclear reactors were added to the list of more than 240operation research power reactors around the world in 2009. Many of these reactors are usedfor materials testing and the production of isotopes for medicine and industry. As olderreactors are retired and replaced by fewer more multipurpose reactors, the number ofoperational research reactors is expected to drop to between 100 and 150 by 2020.

Thefigure 3-1presented above illustrates that Russia has the highest number of researchreactors, followed by USA, Japan, France, Germany and China. Many developing countries alsohave research reactors, including Algeria, Bangladesh, Colombia, Ghana, Jamaica, Libya,Thailand and Vietnam. The trends reveal that even though many research reactors are under-utilized and many older ones will be shut down and subsequently undergo decommissioning;the need for research reactors is not waning. Presently, seven new research reactors are underconstruction and nine more are planned. Some of these new reactors are innovative reactorsdesigned to produce high neutron fluxes and will be either multipurpose reactors or dedicatedto specific needs.
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Nuclear Energy: Peaceful Ways to Serve Humanity


These reactors are relatively smaller than power reactors whose primary function is to produceheat to generate electricity. Their power is designated in megawatts or kilowatts thermal(MWth or MWt), but a common practice is to use MW or KW for megawatts or kilowatts. Mostofthese reactorsrange up to 100 MW, compared with 3,000 MW (ie.1000 MWe) for a typicalpower reactor. These reactors operate at lower temperatures. They need far less fuel, and far

less fission products build up as the fuel is used. On the other hand, their fuel requires morehighly enriched uranium, typically up to 20 percent U-235 (Uranium), although some olderones use 93 percent U-235. They also have a very high power density in the core, which requiresspecial design features. Like power reactors, the core needs cooling, and usually a moderator isrequired to slow down the neutrons and enhance fission. As neutron production is their mainfunction, most research reactorsalso need a reflector to reduce neutron loss from the core.1. TYPES OF RESEARCH NUCLEAR REACTORS:Because of a wide range of research covered by these reactors, a much wider array of designsare used for research reactors whereas 80 percent of the worlds nuclear plants are of two

similar types. They also have different operating modes, producing energy that may be steadyor pulsed. The common designs for research nuclear reactors are divided into the followingthree categories:

1.1 The Pool Type Research Nuclear Reactors:
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A common design is the pool type reactor where the core is a cluster of fuel elements sitting ina large pool of water. Between the fuel elements are control rods and empty channels forexperiments. In one particular design (Material Testing Reactor), a fuel element comprisesseveral curved aluminium-clad fuel plates in a vertical box. The water moderates and cools thereactor, and graphite or beryllium is generally used for the reflector, although other materials

may also be used. Apertures to access the neutron beams are set in the wall of the pool.

The swimming pool reactoris very simpleand initially more than 40such reactors were built inthe United States alone. The core is often made up of what are called Materials Testing Reactor(MTR) type fuel elements; aluminium clad, curved plates of fuel arranged in long rectangularboxes, which are arranged between grid plates to form the core. Several positions in the grid arenot occupied by fuel elements, but by control rods, beryllium reflectors, or experimentalcapsules. Cooling may be by natural convection of the pool water, although this is augmentedfor operation at higher power by pumping pool water through the core. This design led to thetank-in-pool reactor, similar to the open-pool type but with the core contained in an

aluminium tank. The cooling (light) water is pumped through the core, but the pressure withinthe tank is only moderately elevated above that in the open pool. The pressurization beingmostly due to the pressure drop across the core of the pumped coolant water flow. Again, in theUnited States, aluminium clad fuel plates are usual.

1.2 The Tank Type Research Nuclear Reactor:This type of research reactors is similar except that cooling is more active.
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1.3 The TRIGA Type Research Nuclear Reactor:The core of this type of research nuclear reactor consists of 60-100 cylindrical fuel elementsabout imperial equivalent for 36 mm diameter with aluminium cladding enclosing a mixture ofuranium fuel and a zirconium hydride moderator.

It sits in a pool of water and generally uses graphite or beryllium as a reflector. This kind ofreactor can safely be pulsed to very high power levels (e.g., 25,000 MW) for fractions of asecond. Its fuel gives the TRIGA a very strong negative temperature coefficient, and the rapidincrease in power is quickly cut short by a negative reactivity effect of the hydride moderator.

Perhaps the most interesting reactor design of the common types, from a technical and safetyperspective, is the TRIGA, developed in the 1950s by General Atomic. Its unique fuel and coredesign concept has a very large and very prompt negative temperature coefficient; it is being ahomogenized mixture of fuel and hydrogenous moderator in the form of uranium-zirconium

hydride. This provides prompt negative feedback because there is no delay between fuel andmoderator temperature variations. This is in addition to the usual prompt Doppler Effect inU238in reduced enrichment fuels. Beyond these effects, erbium can be added as a burnablepoison and adds even more prompt negative temperature coefficient because it has a strongresonance: Absorption at about 0.5 eV.


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The fuel/moderator/poison has a design operating temperature of up to 750 C (1382 F) degreesand a safety limits of 1150 C (2102 F) degree, obviously much higher than aluminudfuelmixtures. It is formed into rods clad with stainless steel (Incoloy800).With this combinationof design features very large reactivity insertions can be tolerated, and many TRIGA researchnuclear reactors are routinely and safely operated as pulsed reactors with peak power levels,

during a few millisecond pulse, of up YO 10 GW.

Cooling is by natural convection of light water for power levels up to two MW. At higher powerlevels forced flow is used, but the high fuel temperature tolerance and negative reactivitycoefficients mean that pony motors are not needed for shutdown cooling following a loss of theprimary coolant Dumps.

Other designs are moderated by heavy water or graphite. A few are fast reactors that require nomoderator and can use a mixture of uranium and plutonium as fuel. Homogenous type reactorshave a core comprising a solution of uranium salts as a liquid contained in a tank about 300

mm diameter. The simple design made them popular early on, but only five are now operating.

The IAEA has classified broadly research nuclear reactors into several categories. They include60 critical assemblies (usually zero power), 23 test reactors, 37 training facilities, 2 prototypesand even 1 producing electricity. However, most (160) are largely for research, although somemay also produce radioisotopes. As expensive scientific facilities, they tend to be multi-purpose,and many have been operating for more than 30 years.Russia has the most research nuclear reactors (62), followed by USA (54), Japan (18), France(15), Germany (14) and China (13). Many small and developing countries also have researchnuclear reactors, including Bangladesh, Algeria, Colombia, Ghana, Jamaica, Libya, Thailand andVietnam. About 20 more reactors are planned or under construction, and 361 have been shut

down or decommissioned, about half of these in USA.

2. THE USE OF RESEARCH NUCLEAR REACTORS:Research nuclear reactors have a wide range of uses, including analysis and testing of materials,and production of radioisotopes. Their capabilities are applied in many fields within the nuclearindustry as well as in fusion research, environmental science, advanced materials development,drug design and nuclear medicine.

Using neutron activation analysis it is possible to measure minute quantities of an element.Atoms in a sample are made radioactive by exposure to neutrons in a reactor. The characteristic

radiation each element emits can then be detected.Neutron beams are uniquely suited to studying the structure and dynamics of materials at theatomic level. Neutron scattering is, used to examine samples under different conditions such asvariations in vacuum pressure, high temperature, low temperature and magnetic field,essentially under real-world conditions.


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Chapter 03 _ Reseach Nuclear Power Reactors