Principal shutdown systems

DFR(UK) 12 bottom-entry fuel rods and 3 top entry boron shut-off rods 

BOR-60 (Russian Federation) safety and regulating control rods 

CEFR (China) 5 CR (primary) and 23safety rods (secondary) 

PFR(UK) 5 control and 5 shut-off rods held by 2 guard lines 

CRBRP (USA) two independent and diverse systems primary system has 9 rods secondary system has 6 rods 

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On-power refuelling provides the principal means for controlling reactivity in the CANDU 6. Additional reactivity control, independent of the safety shutdown systems, is achieved through use of reactivity control mechanisms. These include light-water zone compartments, absorber rods, and adjuster rods all are located between fiiel channels within the low pressure heavy water moderator and do not penetrate the heat transport system pressure boundary. The reactor is controlled by the dual redundant computer control system. The overall station control system is described in Section 5.7.2.3. 

Principal role in achieving safety (high-integrity shutdown systems)... 

In 1982, the Research Center Jiilich presented the conceptual design of a 50 MW(th) nuclear process heat plant with a pebble-bed HTGR, named AVR-II, for which a safety-related study has been conducted [29]. Its characteristic features are a slim steel pressure vessel, no separate decay heat removal system, shutdown and control system via reflector rods, surface cooling system, and a simplified containment. The safety of the reactor is principally based on passive system feamres. 

Transfer system. The principal system used to transfer fuel and other components to and from the reactor vessel is the closed loop ex-vessel machine (CLEM) shown in Fig. A.2. The CLEM loads all components into the reactor vessel and removes all components from the reactor vessel and operates only when the reactor is shutdown. Under most conditions, CLEM moves a core component pot (CCP) that contains the fresh or SNF fuel to and from the reactor vessel. In a fast reactor, the core power density is very high thus, there is significant decay heat in each SNF assembly immediately after reactor shutdown. To prevent fuel failure from overheating, the SNF is kept in sodium at all times to ensure effective cooling. This is accomplished by transferring each fuel assembly in its own pot of sodium—the CCP. CLEM is also used to transfer a variety of other components within the reactor containment. 

The Unifonn Listing of C. L, P. and R Reactor Systenu. Acronyms, and Fungtional Classifications (ReC 4 33) provides a current and complete tabulation of system titles, acronyms, and associated fiinctional classifications for principal structures and components. The functional classification NS, CP, PS, or GS) of stiuctures, systems, and components m L and P-Reactors are provided for Cold Shutdown. The report also provides fiinaional descriptions of each sy em during Cold Shutdown and the classification justifications. 

The principal natural phenomena that influence transient operation are the temperature coefficients of the moderator and fuel and the buildup or depletion of certain fission products. Reactivity balancing may occur through the effects of natural phenomena or the operation of the reactor control system using the RCCs or chemical "shim." A change in the temperature of moderator or fuel (e.g., due to an increase or decrease in steam demand) will add or remove reactivity (respectively) and cause the power level to change (increase or decrease, respectively) xmtil the reactivity change is balanced out. RCC assemblies are used to follow fairly large load transients, such as load-follow operation, and for startup and shutdown. 

The other principal contributor is a spuriously actuated pressurizer PORV. In this scenario, fire-related component damage in the control room includes control power for a number of safety systems. Full credit was given for independence of the remote shutdown panel from the control room except in the case of PORV block valves. Discussions with utility personnel indicated that control power for these valves was not independently routed. 

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