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Detail of the taskA safety-critical supervisory and control system is beingspecified for aging nuclear power plants. It must allowan Advanced Gas-cooled Reactor (AGR) nucleargenerating station to be safely and efficiently controlledand monitored. A full description of an AGR nuclearplant is outside the scope of this specification; though abasic overview follows. Further background reading isavailable via a number of sources, including the IAEA’sGraphite Knowledge Base: you should clarify and augment this specificationwith any real-world features from your own research,remember the following specification is asimplifieddescription of an AGR plant, appropriate for theassignment weighting (20%) and duration. Be carefulyou don’t overcomplicate your problem!Nuclear power stations bear some resemblance to fossilfuel stations; they consume fuel to heat water,producing steam under pressure, which is used to spin aturbine, generating electricity. The key difference lies inhow the fuel generates heat and how it is controlled: anuclear fission reaction is sustained with a suitablyenriched uranium fuel. Among other outputs, thisproduces immense heat. This energy heats the AGR’scooling gas, pumped via a heat exchanger to heatwater, which produces steam under pressure.The control system comprises a suitable computingplatform, the embedded software you are specifying,along with numerous sensors (thermocouples, pressureswitches, and position switches), and actuators (motors,air valves, and relays). Note, the following items lieoutside the scope of this work:-Fuel removal/refuellingprocess; assume sufficient fuel is always loaded in thereactor.-Measuring nuclear fission; reactor core temperature is asufficient valid indicator of the intensity of fission. -Theelectricity generation itself; this system simply controls
the nuclear reactor component.A typical control of the reactor will allow it to start up,reach and maintaincriticality– where the reactor is in astablechain reactionstate, producing a regulatedamount of heat. The reactor can also be shut down foroperational (e.g. maintenance) or emergency reasons.Normal ongoing control is maintained by graduallyinserting or removing a group ofcontrol rodsfrom thecentral core of the reactor. These rods are made ofmaterials that absorb neutrons, slowing or ceasing thereactor’s fission process.Therefore, when the control rods are fully inserted in thecore, the reactor willshut down. As control rods aregradually removed, neutron flux increases; the reactorbegins itschain reactionand begins to fission. If therods are removed further, eventually, an unsafe level ofcriticalitywould be achieved. Uncontrolled, the reactorwould either sit idle or overheat andmeltdown; socareful control is required. As the core continues tofission, it gradually uses up the fuel, andnuclearpoisonsare released into the reactor, both graduallyslowing its reaction. As such, regular control rodadjustment is required to maintain safecriticality. Thesystem should monitor core temperature to indicate thereactor state, and automatically set the position[0...100] of the rods, where 0 indicates the rods are fullyinserted, and 100 fully withdrawn.Reactor Regulation:AtLow Core Temperature(< 639̊C), control rods should be withdrawn, while atHigh CoreTemperature(> 661 ̊C), control rods should be inserted.During start up and normal regulation, control rodsshould be moved a single step, and a period of 2sshould elapse before another movement is made, toallow temperature detection. Note that in the case of ashutdown (emergency or otherwise), control rods can befully inserted without having to pause at each step.Post-Shutdown Cooling, Decay Heat, andHolddown:Once the system isshut down, the reactor
still produces a significant amount ofdecay heat. Thisresidual heat must be dissipated to avoid damage to thecore or fuel assemblies; itself a potentially verydangerous situation that could lead tomeltdown. Assuch, the gas circulator pumps must continue running,even after shutdown, until the reactor has cooled.Secondary Shutdown (SSD):Along with failure tohold down, as above, if the reactor remainssupercriticalfor any reason (e.g. control rods are removed too far orcannot be reinserted), or there is a coolant gas leak,there is a danger the reactor may continue to increasefission intensity, and overheat ormeltdown, damagingits core. TheDanger Core Temperaturetrip will activateshortly before the reactor’s maximum permissibletemperature is reached to provide a sensor warning.Meltdowns, given the intense and residual heatinvolved, can deform either the control area of thereactor core or control rods, physically preventing anormalshutdownorholddownprocess (i.e. failure tocontain shutdown heat).As such, the software must also controlsecondaryshutdown (SSD) systems. The AGR has two SSDsystems: SSD1)Nitrogen Injection: injects high-pressurenitrogen into the core. This is drawn from an externaltank and willshut/holddownan intact reactor. Anelectronically-controlled discharge valve exists for thispurpose. SSD2)Boron Beads: discharges boron glassbeads into the core. Boron is anuclear poisonandcauses fission to stop. They are propelled with high-pressure gas into the core; controlled via an electronicvalve.Other Safety Trip Systems:In addition to thetemperature sensors and control actuators discussedabove, a number of other safety sensors are available tothe system.Low Gas Pressure– indicates cooling gaspressure has dropped. This likely indicates the reactorcore has been breached. Given the loss of cooling, itshould be shut down using control rod insertion, both
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