A LAYMAN'S BRIEF OVERVIEW
PRIMARY FEATURES OF NON-POWER REACTORS
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Designs: Non-power reactors are designed and constructed with thousands of variables. Therefore very few reactors are identical. The description below is generalized and shows only the main features of a non-power reactor with emphasis on open pool reactors. The personnel work area at the top is where operators maintain nuclear systems and conduct some experiments.
Tanks: Cores are immersed in water for radiation shielding, fuel cooling, and slowing down energetic neutrons. Water is an inexpensive, abundant, and optically clear material. Larger reactors are placed in closed tanks to control the argon found naturally in water and activated by core neutrons as well as the other activated trace elements and nitrogen-16. Medium sized reactors placed in open pools do not produce levels of activated elements that require strict control, only monitoring. Open pools range in height from 20' to 30' and diameter from 6' to 12'. Some pools are rectangular instead of cylindrical and they may be much larger, containing as much as 110,000 gallons of water. Most pools are built above floor level but some are all or partially below the lowest floor level.
Instrumentation and Systems: Typical reactors have 3 or more power monitoring detectors, two or more fuel temperature instruments, and 3 or more control rods. The nuclear systems consist of control systems, safety sytems, and auxiliary systems. Control systems monitor and control the power level and fuel temperature. Safety systems automatically shut down the fission reactions when safety limits on power level or fuel temperature are exceeded. Auxiliary systems conduct experiments, clean and cool the tank water, monitor personnel work area radiation levels, and other functions. Core cooling is accomplished either by natural currents induced by the hot fuel elements or in high power reactors by forced coolant flow through the core. Tank water is cooled in a primary heat exchanger located adjacent to the tank. The secondary coolant from the primary heat exchanger is typically cooled via cooling towers outside the reactor room. Small reactors may be cooled by running secondary cool water once through the primary heat exchanger and then disposing of the warmed secondary water.
Fuel: Normally low enriched uranium fuel consisting of less than 20% U-235 and 80% U-238 lightly dispersed in an inert matrix. High enriched fuels were fabricated to enhance fuel element lifetime, but these were recently phased out of non-military reactors for non-proliferation purposes. New replacement elements in high burnup reactors use high density low enriched elements. plate or rod fuel (under construction) General Atomic of La Jolla, CA manufactures TRIGA® reactor fuel elements in France for the majority of reactors in the US and around the world.
Experiment Devices: Sometimes very extensive experimental devices are located inside the core or directly adjacent to the core. These facilities usually facilitate the irradiation of samples. Samples may be manually or mechanically lowered into the core or pneumatically transported into the core. Large volumes adjacent to the core filled with carbon or heavy water are known as thermal columns and are used to slow down neutrons escaping the core. Sometimes experiment facilities are used to extract radiations from the core and tank for radiation studies/tests or sample activation. Radiation beams are extracted through vacuumed or helium filled tubes with neutron absorbing guide surfaces. Beams are almost always extracted at core level where experiments are conducted outside the thick concrete tank shield. A few have beams extracted upward at sharp angles. Some facilities have large controlled access rooms within the concrete tank shield.
Top View of Glowing Utah Reactor Core
A. Hexagonal top grid plate
B. Empty core grid positions filled with tank water
C. Fuel rod tops held in upper grid plate
D. External trapezoidal heavy water reflector and irradiator
E. Sample transfer tube into external slow neutron irradiator
F. Power level detectors
G. Start up power level detector
H. Fuel temperature instrumentation
J. Control rods
K. External fast neutron irradiator (cadmium lined, lead shielded)
L. Pneumatic sample transfer tubes into internal core region
The blue glow is the result of molecular interactions of a potential (electrical field) shock wave generated from energy released during the dispersal and slowing down of energetic beta particles (electrons and positrons) as they travel through water. The wavelength (this specific blue) is characteristic of the medium, in this case water. This radiation was named Cherenkov radiation after the Russian physicist who first explained the phenomenon.
Tour the University of Texas at Austin 1.0 Mwatt TRIGA nuclear reactor facility.
Tanks: Cores are immersed in water for radiation shielding, fuel cooling, and slowing down energetic neutrons. Water is an inexpensive, abundant, and optically clear material. Larger reactors are placed in closed tanks to control the argon found naturally in water and activated by core neutrons as well as the other activated trace elements and nitrogen-16. Medium sized reactors placed in open pools do not produce levels of activated elements that require strict control, only monitoring. Open pools range in height from 20' to 30' and diameter from 6' to 12'. Some pools are rectangular instead of cylindrical and they may be much larger, containing as much as 110,000 gallons of water. Most pools are built above floor level but some are all or partially below the lowest floor level.