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Specification of Safety-Critical Supervisory and Control System for AGR Nuclear Generating Station

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Added on  2019-09-20

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This specification outlines the requirements for a safety-critical supervisory and control system for an Advanced Gas-cooled Reactor (AGR) nuclear generating station. It includes details on the control system, reactor regulation, post-shutdown cooling, secondary shutdown, and user control.
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Detail of the task A safety-critical supervisory and control system is being specified for aging nuclear power plants. It must allow an Advanced Gas-cooled Reactor (AGR) nuclear generating station to be safely and efficiently controlled and monitored. A full description of an AGR nuclear plant is outside the scope of this specification; though a basic overview follows. Further background reading is available via a number of sources, including the IAEA’s Graphite Knowledge Base: While you should clarify and augment this specification with any real-world features from your own research, remember the following specification is a simplified description of an AGR plant, appropriate for the assignment weighting (20%) and duration. Be careful you don’t overcomplicate your problem! Nuclear power stations bear some resemblance to fossil fuel stations; they consume fuel to heat water, producing steam under pressure, which is used to spin aturbine, generating electricity. The key difference lies in how the fuel generates heat and how it is controlled: a nuclear fission reaction is sustained with a suitably enriched uranium fuel. Among other outputs, this produces immense heat. This energy heats the AGR’s cooling gas, pumped via a heat exchanger to heat water, which produces steam under pressure. The control system comprises a suitable computing platform, the embedded software you are specifying, along with numerous sensors (thermocouples, pressure switches, and position switches), and actuators (motors, air valves, and relays). Note, the following items lie outside the scope of this work:-Fuel removal/refuelling process; assume sufficient fuel is always loaded in the reactor. -Measuring nuclear fission; reactor core temperature is asufficient valid indicator of the intensity of fission. -The electricity generation itself; this system simply controls
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the nuclear reactor component. A typical control of the reactor will allow it to start up, reach and maintain criticality – where the reactor is in a stable chain reaction state, producing a regulated amount of heat. The reactor can also be shut down for operational (e.g. maintenance) or emergency reasons. Normal ongoing control is maintained by gradually inserting or removing a group of control rods from the central core of the reactor. These rods are made of materials that absorb neutrons, slowing or ceasing the reactor’s fission process. Therefore, when the control rods are fully inserted in thecore, the reactor will shut down. As control rods are gradually removed, neutron flux increases; the reactor begins its chain reaction and begins to fission. If the rods are removed further, eventually, an unsafe level of criticality would be achieved. Uncontrolled, the reactor would either sit idle or overheat and meltdown; so careful control is required. As the core continues to fission, it gradually uses up the fuel, and nuclear poisons are released into the reactor, both gradually slowing its reaction. As such, regular control rod adjustment is required to maintain safe criticality. The system should monitor core temperature to indicate the reactor state, and automatically set the position [0...100] of the rods, where 0 indicates the rods are fullyinserted, and 100 fully withdrawn. Reactor Regulation: At Low Core Temperature (< 639 ̊C), control rods should be withdrawn, while at High CoreTemperature (> 661 ̊C), control rods should be inserted. During start up and normal regulation, control rods should be moved a single step, and a period of 2s should elapse before another movement is made, to allow temperature detection. Note that in the case of a shutdown (emergency or otherwise), control rods can befully inserted without having to pause at each step. Post-Shutdown Cooling, Decay Heat, and Holddown: Once the system is shut down, the reactor
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still produces a significant amount of decay heat. This residual heat must be dissipated to avoid damage to thecore or fuel assemblies; itself a potentially very dangerous situation that could lead to meltdown. As such, the gas circulator pumps must continue running, even after shutdown, until the reactor has cooled. Secondary Shutdown (SSD): Along with failure to hold down, as above, if the reactor remains supercriticalfor any reason (e.g. control rods are removed too far or cannot be reinserted), or there is a coolant gas leak, there is a danger the reactor may continue to increase fission intensity, and overheat or meltdown, damaging its core. The Danger Core Temperature trip will activate shortly before the reactor’s maximum permissible temperature is reached to provide a sensor warning. Meltdowns, given the intense and residual heat involved, can deform either the control area of the reactor core or control rods, physically preventing a normal shutdown or holddown process (i.e. failure to contain shutdown heat). As such, the software must also control secondary shutdown (SSD) systems. The AGR has two SSD systems: SSD1) Nitrogen Injection: injects high-pressure nitrogen into the core. This is drawn from an external tank and will shut/holddown an intact reactor. An electronically-controlled discharge valve exists for this purpose. SSD2) Boron Beads: discharges boron glass beads into the core. Boron is a nuclear poison and causes fission to stop. They are propelled with high-pressure gas into the core; controlled via an electronic valve. Other Safety Trip Systems: In addition to the temperature sensors and control actuators discussed above, a number of other safety sensors are available tothe system.Low Gas Pressure – indicates cooling gas pressure has dropped. This likely indicates the reactor core has been breached. Given the loss of cooling, it should be shut down using control rod insertion, both
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