Safety shutdown rods

Rapid shutdown rods Additional shutdown rods  

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Plant Safety (shutdown) rods Regulating rods Rapid shutdown rods Additional shutdown rods "... 

Safety (shutdown) rods - No. of safety (shut down) rods... 

The reactor vessel surrounds the core and a combination of fixed and movable reflectors surround the vessel. The movable reflector is segmented and used to maintain reactivity at the desired operating temperature over life. Instrumentation to monitor power and temperature is used to determine when to move reflector segments during reactor startup and to compensate for uranium burn-up during operation. The reactor uses at least one safety shutdown rod. Safety rods are only used during transport and launch and would be withdrawn from the core prior to initial criticality. 

In normal conditions of operation, therefore, a nuclear power reactor system can respond to the requirements of the electrical power grid system, and the rate at which power can be increased will be dictated by the mechanical limits of the components, rather than by the reactor physics of the core. However, all reactors have systems by which shutdown rods can be—and in many cases are— automatically forced into the core to avoid circumstances that could endanger the plant ( Scram —said to have been formed from the term Safety Control Rod Axe Man at the first man-made reactor, Stagg Field, Chicago). 

Figure 1-1. Drawing of the CPI pile. Scram - this term means fast shutdown of a reactoK various explanations have been proposed for its origin. The most credited one assumes that it derives from the abbreviated name of the CPI safety rod which could be actuated by an axe. In the original design sketches of the pile, the position of the operator of the axe was indicated by SCRAM, the abbreviation of Safety Control Rod Ax Man. The designated operator was the physicist Norman Hilberry, subsequently Director of the Argonne Laboratory. His colleagues used the name Mister Scram. The drawing is courtesy of Prof. Raymond Murray.

Control absorber rods or blades are associated with the fuel assemblies and their drive mechanisms are located on the RPV top closure head. Safety shutdown is achieved by control rod drop. A boric acid injection system is usually provided as an additional shutdown system for use in emergency situations. 

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. 

Additional shutdown systems Two independent passive safety shutdown systems 28 shut-off rods and 6 liquid poison injection circuits... 

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. 

The following preliminary list of accidents or potential accidents has been identified. Consistent with the philosophy of avoiding reliance upon active safety systems, for the analysis it is assumed that automatic insertion of the shutdown rods does not occur that is, there is an assumed failure to scram in each accident. Analyses show that unacceptable system temperatures and core damage will be averted by strictly passive means without scram. 

Control All control and safety rods are B.C packed in steel tubes six safety rods and six shutdown rods two coarse control rods and one fine... 

No single failure in the shutdown system shall be capable of preventing the system from fulfilling its safety function when required (e.g. with the most reactive shutdown rod stuck in the out position). 

Reactivity control First shutdown system Safety control rods Second shutdown system Boron injection... 

Control of the core is affected by movable control rods which contain neutron absorbers soluble neutron absorbers ia the coolant, called chemical shim fixed burnable neutron absorbers and the intrinsic feature of negative reactivity coefficients. Gross changes ia fission reaction rates, as well as start-up and shutdown of the fission reactions, are effected by the control rods. In a typical PWR, ca 90 control rods are used. These, iaserted from the top of the core, contain strong neutron absorbers such as boron, cadmium, or hafnium, and are made up of a cadmium—iadium—silver alloy, clad ia stainless steel. The movement of the control rods is governed remotely by an operator ia the control room. Safety circuitry automatically iaserts the rods ia the event of an abnormal power or reactivity transient. 

Safety features at a nuclear power plant include automatic shutdown of the fission process by insertion of control rods, emergency water cooling for the cote in case of pipeline breakage, and a concrete containment shell. It is impossible for a reactor to have a nuclear explosion because the fuel enrichment in a reactor is intentionally limited to about 3% uranium-235, while almost 100% pure uranium-235 is required for a bomb. The worst accident at a PWR would be a steam explosion, which could contaminate the inside of the containment shell. 

The safety demonstration tests in the HTTR are conducted to demonstrate an inherent safety feature, that is an excellent feature in Shutdown of the HTGRs, as well as to obtain the core and plant transient data for validation of safety analsis codes and for establishment of safety design and evaluation technologies of the HTGRs. The safety demonstration tests consist of Reactivity insertion test - control rod withdrawal test and Coolant flow reduction test as shown in Figure 6. In the control rod withdrawal test, a central pair of control rods is withdrawn and a reactivity insertion event is simulated. In the gas-circulators trip test, primary coolant flow rate is reduced to 67% and 33% of rated flow rate by running down one and two out of three gas-circulators at the Primary Pressurized Water Cooler without a reactor scram, respectively. 

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. 

Standardized protection actions include shutdown of the reactor, of the main cooling system (which is not required for keeping the reactor within safe limits) and of the reformer plant. The two (required) shutdown systems consist of in total 18 absorber rods which are moved in the side reflector. Diversity is given by the employment of different propulsion systems. In particular, the active cooling of the core by the main cooling system is not required because fuel temperatures remain within the safety limits. Only for reactor vessel protection purposes, the surface cooling system in the reactor is necessary which alone is able to account for heat removal. 

The safety concept considers two nuclear shutdown systems, a set of six reflector rods for reactor scram and power control and a KLAK system of small absorber balls for cold and long-term shutdown. Decay heat removal is made via the heat exchanger, an auxiliary cooling system, and the panel cooling system inside the concrete cavern, or, in case of a failure of these systems, passively by heat transfer via the surface of the reactor vessel. 

The number and location of the top entry control rods and the diverse reserve shutdown control have been specified to assure that the reactor thermal power is controlled both for normal and off-normal conditions. The radial thickness of the active core annulus was specified on the basis of assuring that the control rod worths of the reflector-located rods would meet all shutdown and operating control worth requirements. The choice of reflector control, coupled with the choice of a control system withdrawal sequence and safety classification was made to assure that the control rod integrity is maintained during passive decay heat removal. 

The six control rods located in the central reflector are not "safety-related" and are inserted only from hot-shutdown or low-power conditions to achieve a cold shutdown. Boronated graphite pellets housed in hoppers above the core provide a reserve shutdown capability. Upon actuation, these pellets drop into channels in selected columns of the active core to provide reactor shutdown in the event that the control rods are inoperable, or if necessary, to provide additional shutdown margin over what may be provided by the control rods located in the hexagonal side reflector. 

SRDC-2 is similar to SRDC-1, except that the reactor is shut down with the reserve shutdown rather than with the control rods. The initial phase of this condition is similar to the initial phase of DBE-2 and the later phases to SRDC-1. As stated in the discussions of those two cases, no damage or any safety consequences will occur. 

The ex-vessel neutron detection equipment consists of fission chamber neutron detectors mounted in six equally spaced vertical wells located just outside the reactor vessel as illustrated in Figure 4.3-4. The signals from these detectors are supplied to the nuclear instrumentation cabinet and Safety Protection Subsystem equipment located primarily in the reactor building. These data are used by the automatic control systems to operate the control rod drives or the reserve shutdown equipment, thereby changing the neutron flux levels within the reactor core. 

The outer control rod drives (CRDs) shall be designed to permit the Safety Protection Subsystem to interrupt the power supply to the drives when reactor trip levels are reached, causing the control rods to drop by gravity into the core. The inner control rod drives shall operate in a similar manner but are tripped from the Investment Protection Subsystem. This trip command shall override all other commands. The reserve shutdown material shall be stored in hoppers above the core and released to fall into the core upon receipt of a signal from the Safety Protection Subsystem. 

The "safety-related" design conditions (SRDCs) are discussed in detail in Chapter 15. As in the DBEs, the NCSS performs its detection and shutdown functions early on, before conditions progress to a point where the capability of the NCSS equipment to perform its "safety-related" functions is threatened and none of the SRDCs cause a significant rise in the temperature of the control rod drives prior to their being tripped. The same is true of the reserve shutdown control equipment. Also, these two sets of equipment are not affected by pressure changes or other changes in environment that occur prior to their being called upon to perform. 

Another feature of the MTB control arrangement is the safety circuits which protect against higher-than-normal power levels. Unsafe levels causW a reduction in current to the electromagnets and subsequent release of the control rods. The time required for.release of the rods had been measured as approximately 15 msec before installation in the Mock>Up. During operation the time interval between the "scram signal and shutdown of the reactor was observed to be approximately 30 nsec. 

The control rod system provides for automatic control of the required reactor power level and its period reactor startup manual regulation of the power level and distribution to compensate for changes in reactivity due to burn-up and refuelling automatic regulation of the radial-azimuthal power distribution automatic rapid power reduction to predetermined levels when certain plant parameters exceed preset limits automatic and manual emergency shutdown under accident conditions. A special unit selects 24 uniformly distributed rods from the total available in the core as safety rods. These are the first rods to be withdrawn to their upper cut-off limit when the reactor is started up. In the event of a loss of power, the control rods are disconnected from their drives and fall into the core under gravity at a speed of about 0-4 m/s, regulated by water flow resistance. 

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