Additional shutdown rods

Rapid shutdown rods Additional shutdown rods  

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

JOYO (J an) none (4inMK-l) 6 (2inMK-l) none 6 (4inMK-l) none 

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

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The reactor can be shut down purely by dropping the absorbers into the reflector bore holes. Additional shutdown rods, which have to be inserted into the pebble bed using external energy, are not necessary. 

Rapid shutdown rods " Additional shutdown rods ... 

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. 

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. 

The suitability of the LEADIR-PS concept to reactors of larger output was assessed, and a reactor ou ut limit of about 1000 MWjh established. Reactors with outputs above about 600 MWth require an annular core configuration (similar to MHTGR). The central reflector blocks provide additional heat capacity to accommodate postulated accident conditions, and locations for the control and shutdown rods necessary for reactor control and shutdown. 

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. 

Fully automated reactor start-up can be achieved by the LRM, yet another passive device incorporated in the RAPID concept. Figure XVII-5 shows the LRM basic concept. LRM is similar to LIM however, Li is reserved in the active core part prior to reactor start-up. The LRM is placed in the active core region where the local coolant void worth is positive, as is also the case with LEMs and LIMs. The RAPID is equipped with an LRM bundle in which 9 LRMs and an additional B4C rod are assembled. The reactivity worth of the LRM bundle is +3.45, once each LRM includes a 95% enriched Li enclosed in a 20mm-diameter envelope. A B4C rod is used to ensure the shutdown margin (-0.5 ). An automated reactor start-up can be achieved by gradually increasing the primary coolant temperature with the primary pump circulation. The freeze seals of LRMs melt at the hot standby temperature (380°C), and Li is released from the lower level (active core level) to the upper level to achieve positive reactivity addition. An almost constant reactivity insertion rate is ensured by the LRMs because the liquid poison, driven by the gas pressure in the bottom chamber, flows through a very small orifice. It would take almost 14 hours for the liquid poison to move into the top chamber completely. A Sn-Bi-Pb alloy is used as the freeze seal material to ensure the reactor start-up at 380°C. 

Secondary reactivity control system - a system, additional to the control or shutdown rods, which is designed to shutdown the reactor or keep it deeply and permanently subcritical after being shut down by the control rods and subsequent cooling down. Data providers should choose the appropriate option from the multiple-choice menu H3BO3 injection, nitrogen... 

Depending on design, shutdown rod drives for research reactors should be operated in a proper sequence so as to minimize the rate of reactivity addition. 

Analyses to determine the structural adequacy of the penetration support structure and the control rod components are performed. In addition, qualification tests on control rod and reserve shutdown assembly prototypes in a test rig, which simulates the penetration support structure and the reactor core, are undertaken to demonstrate the required performance in a seismic environment. 

Nominal Reactivity Control Worths The calculation of control rod and reserve shutdown control (RSC) worths under both hot and cold conditions have been performed for both the initial cycle BOG conditions and the equilibrium cycle EOC condition. In addition, the worth of all 30 control rods has been calculated for other times in cycle for both the initial core and an equilibrium reload cycle to determine how the total control rod bank worth is expected to change over the cycle. Other specific rod pattern control worths for hot conditions for the selected withdrawal of groups of three rods each in the outer bank of control rods were analyzed to define the maximum group worth for use in the transients analyzed in Chapter 15. These calculations were only performed for the EOC equilibrium core loading since that cycle condition yields the minimum temperature coefficient of reactivity and the maximum rod group reactivity worth for a rod group withdrawal transient. No reduction in control rod poison worth due to burnup has been assumed in this or other EOC rod worth calculations discussed below, although this effect would be minimal. 

In spite of these changes, the reactor was left with the possibility of a fast positive power coefficient, with a graphite temperature that was too high, and without a secondary shutdown system. In addition, it is now clear that the speed of insertion of the control rods under emergency conditions was totally inadequate. 

The accident is analysed for various locations of the steam line break (an5rwhere along its length, for example before or after the isolation valve/s, inside or outside the container, etc.). Various initial operating conditions (full power or hot shutdown), as well as various additional malfunctions (loss of the external power supplies, highest worth control rod fully extracted, etc.) are possible. Some of these situations, in fact, are the worst for potential fuel damage, others for the primary over-pressure or for external radiological consequences. 

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. 

Reactivity Insertion (incl ATWS) PROTECTION LEVEL Strong negative temperature coefficient, avoiding a high heatup of fuel (L) Provision of two diverse neutron control systems, control rods and shutdown pellets (R) Provision of startup control rods m addition to operatmg control rods (L)... 

All cores were composed of 24 Unit cells (6 x 4.6 in. in cross section and 38 in. long) in a honeycomb assembly constructed of rectangular aluminum tubes. The assemblies were reflected bn all sides with 7 in. of BeO. Shutdown of the facility was obtained by two large vertical rods that extended through both the core and reflector. In addition, six BF, fuses were incorporated in each core. A diagram of the core face is shown in Fig. 1. Each unit cell contained BeO, polyethylene, fuel (fully enriched uranium in aluminum) and boron stainless steel. The fuel inventory in all cores was 46.1 kg of U/235 and the BeO/U ratio was... 

The movable boron-carbide (B4C) control rods are sufficient to provide reactivity control from the cold shutdown condition to the full load condition. Additional reactivity control in the form of solid burnable poison is used only to provide reactivity compensation for fuel burnup or depletion effects. 

PRISM has multiple and diverse means for reactivity control and shutdown. Each PRISM reactor has nine control rods (Section 6.4.5.4.4), controlled by a triply redundant reactivity controller, which is part of a highly reliable PCS. The reactor can be reliably shutdown by PCS-directed rod run-in. In addition, PRISM has an RPS (Section 6.4.4.4) which is totally separate from the PCS and provides two diverse means for scramming each of the nine control rods. 

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