Ultimate shutdown system

Number of IHTS Loops Safety Shutdown Heat Removal Ultimate Shutdown System Fuel Handling Seismic Design... 

The primary shutdown system is backed up by an ultimate shutdown system (USS). These control rods use magnetic latches, which can be actuated by either the reactor protection system (RPS) or automatically when the latch temperature exceeds the magnetic curie point temperature of the latch. 

The summary of reactivity feedbacks resulting from Doppler effect, uniform radial expansion, and various sodium voidings in the equilibrium core and during the startup cycle are given in Table XX-4. This table also includes the reactivity worths of the control rod system, the GEM (gas expansion module), and the USS (ultimate shutdown system). The gas expansion module is a device that introduces negative reactivity through gas expansion under temperature increase. Its position in the core is shown in Fig. XX-7 no additional details are provided. The USS is explained further in this section. 

The KALIMER adopts the ultimate shutdown system (USS) as an alternative means of shutting down the reactor. The design concept is a self-actuated shutdown system (SASS) located in the centre of the core. 

The core consists of driver fuel assemblies, internal blanket assemblies, radial blanket assemblies, control rods, ultimate shutdown system (USS) assembly, gas expansion modules (OEMs), reflector assemblies, B4C shield assemblies, shield assemblies, and in-vessel storages (IVSs). There are no upper or lower axial blankets surrounding the core. A fission gas plenum is located above the fuel slug and sodium bond. The bottom of each fuel pin is a solid rod end plug for axial shielding. The reflector assemblies contain solid Inconel-600 rods. The control assemblies use a sliding bundle and a dashpot assembly within the same outer assembly structure as the other assembly types. 

The ultimate shutdown system (USS) located at the center of the core is a self-actuated shutdown system. The USS is actuated passively when the temperature of the primary sodium reaches the Curie point. The USS drops neutron absorbers by gravity as a means to bring the reactor to cold critical conditions in the event of a complete failure of the normal scram system and after the inherent reactivity feedbacks have brought the core to a safe, but critical state at an elevated temperature. 

The reactor refueling system (RRS) provides the means of receiving, storing, transporting, and handling reactor core assemblies. Fuel, control, shield, reflector, and ultimate shutdown assemblies are handled by the RRS for all core configuration operations. 

Back-up shutdown system A single ultimate shutdown rod ... 

The reactor is a pool type (integral type) as all primary components are installed inside the reactor vessel (RV). Major primary components are the IHX, primary EM pumps, moveable reflectors which form a primary reactivity control system, the ultimate shutdown rod which is a back-up shutdown system, radial shielding assemblies, core support plate, coolant inlet modules and fuel subassemblies. 

In addition to the inherent safety features, there are two independent systems for reactor shutdown. The primary shutdown system provides for a drop of several sectors of the reflector, and the back-up shutdown system provides for insertion of the ultimate shutdown rod, located as a central subassembly on a stand-by in a fully out condition. 

Ultimate safety systems are, by definition, only required in extremis. When they are required, they really must work - that is so obvious it should not have to be said. And yet, at Chernobyl (the reactor shutdown system). Deepwater Horizon (the blow out preventer) and Fukushima (the tsunami barrier), the ultimate safety system in each case was woefully inadequate and unable to prevent disaster when called for. 

Space needs to be provided for the auxiliaries, including the lube oil and seal systems, lube oil cooler, intercoolers, and pulsation dampeners. A control panel or console is usually provided as part of the local console. This panel contains instmments that provide the necessary information for start-up and shutdown, and should also include warning and trouble lights. Access must be provided for motor repair and ultimate replacement needs to be considered. If a steam turbine is used, a surface condenser is probably required with a vacuum system to increase the efficiency. AH these additional systems need to be considered in the layout and spacing. In addition, room for pulsation dampeners required between stages has to be included. Aftercoolers may also be required with knockout dmms. Reference 8 describes the requirements of compressor layouts and provides many useful piping hints. 

Where feed lines have short pipe runs, where hot wells or FW tanks are of small volume, or when FW is too cold, there often is insufficient time for full DO scavenging to take place, even when using catalyzed scavengers. The inevitable result of this lack of contact time is the formation of oxygen-induced corrosion products, which by various secondary mechanisms may settle out to form permanent deposits within the boiler system. These deposits may develop in several forms (e.g., where DO removal is particularly poor, they often appear as reddish tubercles of hematite covering sites where pitting corrosion is active). Active pitting corrosion combined with the presence of waterside deposits ultimately may lead to tube failure in a boiler or other item of system equipment and result in a system shutdown. 

Control systems will play a key role in future distributed plants ]139,145]. As a rule of thumb, plants will be smaller and simpler, but the control systems will be much more advanced, of a standard not known today. Plant personnel for operation and managing will ultimately no longer be required, except for start-up, shutdown, and services. This is a shift from a regulatory to a servo role, supported by a sophisticated sequence control. Control is needed for safety issues, operability, and product quality control. Sensors have a central role to provide the information needed for control and modeling and simulation is needed for process models. 

The ultimate objective for fast reactors has always been to maximise the utilisation of the natural uranium resource and in common with the main development programmes world wide, EFR has pursued the sodium coolant technology. The safety approach recognises the differing requirements of a sodium cooled fast reactor core compared to the established water and gas cooled thermal reactors which has resulted in a different balance between prevention and mitigation with consequences for the shutdown, decay heat removal and containment systems. 

Gases. The chlorine and hydrogen generated in the cells ultimately depend on some form of gas mover to force them through the process. Design of the piping system must cover the failure of the gas mover or any other part of the downstream process. Each gas line therefore should have some means of relief that at least allows an orderly shutdown of the cells. The appropriate sections of Chapter 9 cover these relief systems. 

A key feature of LEADIR-PS, shared with the Modular High Temperature Gas-Cooled Reactor (MHTGR) under development by General Atomics, is that radionuclide releases are prevented by retention of the radionuclides within the fuel particles under all design basis events without operator action or the use of active systems. Thus, the control of radionuclide releases is achieved primarily by reliance on the inherent characteristics of the coolant, core materials, and fuel. Specifically, the geometry and size of the reactor core, its power density, coolant, and reactor vessel have been selected to allow for decay heat removal from the core to the ultimate heat sink through the natural processes of radiation, conduction and convection, while the negative temperature coefficients of the fuel and moderator assure reactor shutdown. 

The SSWS cools the Component Cooling Water System (CCWS) through the Component Cooling Water Heat Exchangers and rejects the heat to the ultimate heat sink during normal, transient, and accident conditions. The CCWS in turn provides cooling water to those safety-related components necessary to achieve a safe reactor shutdown, as well as to various non-safety reactor auxiliary components. 

---