9. Inherently Safe Reactor Designs

For longer fuel cycles the number can be increased

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Unformatted text preview: an be increased. Fig. 13: The Safe Integral Reactor (SIR) flow diagram. The power level control is designed to be by boron content and coolant temperature, without the need for the control rods. Boron dilution makes possible a 2 percent per minute load increase. The gadolinia shim the coolant temperature coefficient of reactivity is kept strongly negative, a desirable neutronic safety feature. This negative coefficient in combination with once through steam generation makes possible excellent load following properties. Fast load steps of up to 25 percent of full power are possible without boron dilution. The system can withstand grid disturbances such as short circuits of 250 ms duration. The plant startup from cold conditions is about the same as that for standard light water reactors. Let us consider the response of the plant to a loss of auxiliary power accident, or a station blackout during days. The following events would occur: The recirculation pumps are starved of electric power and they trip, or stop functioning. Because of the drop in pressure, connection is established through the lower and upper locks between the primary circuit and the coolant pool. The boron injection from the pool water into the core shuts the reactor immediately. Natural circulation is established naturally leading to core cooling even though the circulating pumps are not operational. Hot water collects in the upper part of the pool and is cooled by natural circulation to an external atmospheric pool. Thus is any short-term damage to the core is averted. For the longer term, thrre scenarios can be envisioned: 1. Water is refilled in the external pool within about three days at 4 m3/hr, by using off site vehicles and personnel. This process can last indefinetely if needed at all. 2. The plant is left alone. In this case the depressurization valves open when the atmospheric pool has boiled dry. The pool water boils off through the depressurization valves. The core starts to be uncovered after about 20 days. Boric acid congestion of the core occurs after one week, unless more volume of water is made available under the core. 3. The plant is left alone. The depressurization valves remain closed. The internal water pool heats up and boils off through the relief valves. The core starts getting uncovered within 12 days. A gain of a sufficient period of time for adding any needed water. The protection using passive measures is effective against conceivable accidents caused by equipment malfunction and operator errors, as well as against unforeseen events such as earthquakes, sabotage or military attack. Fig. 14: The Integral Safe Reactor pressure vessel containing the steam generators. THE SIR, SAFE INTEGRAL REACTOR Combustion Engineering, now a unit of Switzerland's ABB Combustion Engineering Nuclear Systems, developed the Safe Integral Reactor (SIR) concept, jointly with Rolls Royce and Associates, Ltd., Stone and Webster Engineering Corp., and the UK Atomic Energy Authority. The basic design philosophy of the SIR is adopted from naval reactor designs where the heat exchangers and the associated piping are enclosed together with the reactor core within the same reactor vessel. In this way as shown in Fig. 13, the possibility of coolant leakage from the piping is eliminated, since any leakage would be contained within the reactor vessel's space as shown in Fig. 14. This eliminates the traditional reactor coolant loop primary system piping associated with more conventional PWRs. The reactor core, the pressuriser, and the steam generators are contained in a single reactor pressure vessel. The reactor coolant pumps are mounted on the side of the reactor vessel. Integral type coolant circuits have been used in the Advanced Gas cooled Reactors (AGRs) in the UK, and in the type of reactor commonly used for ship propulsion. In this type of coolant circuit, the primary circulating coolant is contained within the vessel, eliminating the need to circulate the primary coolant through the connecting pipe work to the steam generator, where coolant leakage is most likely to occur in the loop design approach to the primary circuits. A safety depressurization system is provided to divert steam to a suppression tank. This tank also feeds an emergency coolant injection system. The condensate storage tank as well as a secondary condensing pool are sources of extra emergency coolant. The SIR reactor is 325 Mwe. Its size is limited by the practical construction and transportation requirements of the pressure vessel. It contains a pressure-suppression containment and 12 cylindrical once-through steam generators, only 11 of which are needed to reach full power. The safety systems are primarily passive relying on natural circulation and a large heat capacity rather than active AC power and equipment. Reactor control is maintained by the use of control rods and burnable poisons, with the traditional PWR borin shim being eliminated for the sake of simplicity and corrosion protection. 3. THE INTEGRAL FAST REACTOR (IFR) DES...
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