NUCLEAR REACTORS FOR RESEARC

[Music] patient at hospitals universities and industrial plants in current use is this low-power research reactor which was designed and built at North American Aviation's Atomic Energy Research Department under contract with the Atomic Energy Commission it is used for nuclear research and perform functional tests on components of different reactors under development a reactor of this type operated at a university could be invaluable for biological and medical research as a source of very short half-life isotopes chemists can conduct experiments which directly utilize nuclear radiation physicists can analyze the properties of neutrons or use the neutron beam to study the basic properties of matter and reactor engineering students can be trained with the latest equipment available in the field of nuclear science this compact solution type reactor was constructed for an Atomic Energy Commission research contractor the core is a stainless-steel spherical tank one foot in diameter which serves as the container for an enriched uranium sulfate solution a spillover tank is incorporated to catch any solution which would overflow in the event of an accidental power surge surrounding the core is a neutron reflector of stacked graphite blocks the assembly is enclosed in a steel housing 1/4 inch thick and five feet in diameter the control system consists of four rods made of stainless steel and compressed boron carbide powder boron is an extremely effective absorber of neutrons two of these rods function as control rods and two as shutdown rods the uranium solution is poured a little at a time into a mixing chamber and then drained by gravity into the core as the control rods are moved away from the core fewer neutrons are absorbed and a uranium fission chain reaction is built up the control rods are adjustable to permit operation at the desired power level here in simplified form is a demonstration of the nuclear fission process the collision of a neutron with a uranium nucleus splits the nucleus into small fragments and releases two or three additional neutrons some of these neutrons collide with other uranium nuclei and cause further fission in the reactor this fission process is maintained as a self-sustaining chain reaction with the production of excess neutrons being the chief objective one of the byproducts of the fission process is heat to maintain safe operating temperatures water is circulated through a cooling coil other byproducts radioactive gases and decomposed water flow to a gas handling system the radioactive gases are stored for eventual disposal and the decomposed water is catalytically recombined and returned to the core the shutdown rods are used to stop the reactor if its power level should become too high during operation each shutdown rod is held away from the core by an electromagnet to shut down the reactor power to the magnet is cut and a falling weight returns the rod to the core with the shutdown rods in neutrons are absorbed so rapidly that the power level is reduced almost to zero ionization chambers located below the reactor housing receive radiation which is proportional to the power level at the operating console reactor power level is shown by Rick and indicators which are controlled by the ionization chambers a layer of lead five inches thick provides part of the shielding needed to safeguard personnel from radiation an outer wall a three-foot thick concrete blocks completes the shielding various Neutron exposure facilities are available and are shown in this overhead view of the reactor a thermal column permits Neutron and gamma irradiation of large samples it is especially valuable for biological studies beam hole openings provide a neutron beam of uniform density for basic physics research a channel through the sphere permits exposure in the region of maximum Neutron flux pneumatic tubes allow samples to be exposed for very brief periods of time and radio isotope production tubes permit exposure for extended periods of time a unique safety feature is demonstrated by this full-scale plastic model colored water simulates the uranium solution within the reactor core in the event of an extremely sudden rise in power and consequent increase in operating temperature bubbles would form so rapidly that part of the solution would be pushed into the spillover tank this would render the reactor subcritical thereby stopping the reaction a lift pump would then return the solution slowly to the core and the reaction could start again this system would provide safe shutdown in an extreme emergency it is never used during normal operation the construction of a nuclear reactor is an exacting procedure the first item to be built is the steel housing seven layers of graphite blocks are stacked inside this housing to form the lower part of the neutron reflector these blocks were machined from high-grade commercial graphite which meets the special purity requirements or applications since small particles of dirt or foreign matter could reduce the efficiency of the reflector all graphite blocks are vacuum cleaned prior to installation the complete reflector weighs over five tons because each block is machine to such close tolerance this phase of reactor construction is like working a king-sized jigsaw puzzle in the laboratory the reactor core undergoes a series of checks static pressure tests are made at 300 pounds per square inch and the volumetric capacity is calibrated next the assembly is evacuated and connected to a helium sensitive leak detector a nozzle directs helium under pressure against all welds and joints in the event of leakage the gas would be drawn through the sphere and into the leak detector following these tests the core is transferred to the assembly area for installation in the reactor housing the small pipes connect the core to the gas handling and cooling systems although this reactor will usually be operated near a power level of 100 watts the cooling system capacity is sufficient to permit operation at 2,000 watts three additional layers of graphite blocks are stacked above the core and a steel lid will cover the top of the housing meanwhile the gas handling system is given a final checkout connections are tightened flow meters tested and continuity checks are made following this inspection the reactor control console and all associated units are crated and shipped after arriving at its destination the reactor is installed the lead shielding secured a 9 foot high concrete radiation shield constructed and all components are checked for leakage the reactor is complete in every detail except the most important the loading with its ultimate source of energy uranium this was supplied by the Atomic Energy Commission in the form of a urinal sulfate solution the amount required to make the reactor critical was predetermined by reactor theory analysis small portions of the calculated critical amount are transferred to plastic bottles and weighed the contents of each will be added to the reactor one at a time during the loading sequence the stainless steel mixing chamber feeds to the reactor core a carefully determined quantity of distilled water has first been added to the core so the total amount of solution will be at the desired level when criticality is reached each succeeding portion of the uranium solution is added to the mixing chamber one at a time the solution is transferred to the sphere through a remotely operated valve this remote operation protects the technician from any possible radiation during loading the loading cycle is repeated until enough uranium solution has been added to the sphere to obtain the critical condition after each loading cycle several neutron sensitive foils are exposed near the reactor core the radioactivity induced in the foils increases as the loading progresses this increase in activity is measured and it's reciprocal plotted against the amount of uranium added this makes possible an accurate experimental prediction of the final quantity of uranium to be added to the core this completes the loading sequence and the reactor is now critical with the final checkout of all electrical and mechanical components the reactor is ready for operation from the control console the operator monitors and controls all phases of reactor performance he checks all instruments each is functioning he checks the safety circuits coolant water on gas pump on the reactor is clear for the startup first the control rod system is energized next the shutdown rods are moved away from the core power level increases slightly but remains well below the critical level the operator starts the mechanism which withdraws the course control rod he watches the rods position on the coarse rod dial the power level now builds up steadily counting rape meters show the increased Neutron activity in approximately five minutes the reactor reaches its operating level the control rods are adjusted for a steady stage of operation gas evolution meters show the flow rates of gases within the system the reactor is switched to automatic control and the operator is free to maintain a periodic check of all instruments throughout the run and record whatever specialized conditions are required for the experiment being conducted research nuclear reactors have numerous academic medical and industrial applications with the entry of private industry into the field of Atomic Energy Research a variety of nuclear reactors have been designed and developed to meet the needs of medicine industry and science [Music]