Rapid shutdown rods

Rapid shutdown rods " Additional shutdown rods ... 

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

Rapid shutdown rods " - No. of rods contributing to rapid shutdown within the first and second... 

No notable technical incident troubled the operation of the plant in 1996. Five unscheduled shutdowns, four of which resulted in the automatic activation of the protection systems with motorized control rod insertion (rapid shutdown), marked the operating periods of the reactor. These shutdowns, that were followed by a rapid restart, lasted only a total of about twenty days. 

The First or Rapid Shutdown System uses neutron absorbing rods which, when introduced in the core at high speed, produce the immediate stop of the fission chain. The absorbing material is a Ag-In-Cd alloy. 

The 87 horizontal control rods are hydraulically driven and can be extended through the active zone of the reactor Approximately half of them enter from each side of the reactor block The rod system provides both operating control and rapid shutdown functions of the reactor. This Is accomplished by having all rods water cooled and by having the Individual rod drives capable of rapid Insertion from any position as well as slower drive speeds for fine control of reactor flux... 

Normal shutdown of the GTR could be accomplished by withdrawing the moveable reflectors and/or inserting the safety rod(s). The GTR will likely also need a rapid shutdown function for reactor emergencies. The fomn of this shutdown function has not yet been determined and requires consideration of such aspects as public safety requirements, single-failure criteria and the desire for the reactor module to be as prototypical as possible to the flight unit. 

From the above paragraph it is obvious that the control rods must not be withdrawn coo rapidly, but another condition is imposed in restarting the reactor after a short shutdown. In this case the time available is limited by xenon growth (see Chap, 4) hence the rods must be withdrawn as quickly as possible. In.order to fulfill both these conditions, the control system has been designed to limit the "high" rate of rod withdrawal to an equivalent Afe/h of 0.1% per second. 

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. 

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. 

Although the postshutdown concentration of samarium can be a serious consideration in some reactor situations, the control of the xenon concentration is a problem of more general importance in all thermal reactors. This problem is especially serious when the I concentration at shutdown is relatively large. Under these circumstances the Xe concentration increases rapidly after shutdown and can reach appreciable magnitudes before decaying away. This buildup is important from the standpoint of control, since a large positive reactivity change must be available if it is to be required that there be a restart of the reactor around the time the maximum occurs. Thus special xenon-override control rods would be required in a mobile nuclear power plant if short-time startups were to be an essential operational feature. 

When in use as a compact critical assembly, emergency shutdown is provided by a rapid internal dump system which is ideally suited for the study of close packed cores where the provision of control rods involves an unacceptable perturbation of the core. 

Safety features. The reactor has two shutdown systems, the primary reactor shutdown system and the backup reactor shutdown system, either of which can stop the reactor rapidly independently of the other The primary reactor shutdown system employs rigid control rods, while the backup reactor shutdown system employs articulated rods to ensure insertion during earthquakes The backup reactor shutdown system is provided with a self-actuated shutdown mechanism to reduce the consequence of an anticipated transient without scram... 

Passive reactivity shutdown The plant control system (PCS) causes the reactor to follow load demand, and normally will maintain the core outlet sodium temperature within specified limits. If an emergency event develops too rapidly for the PCS to control it, then the safety-grade reactor protection system (RPS), located at the reactor module, will independently respond by causing a reactor scram (rapid insertion of the nine control rods). The RPS includes substantial internal diversity and redundancy and is expected to be Mghly... 

On June 28, 1980, Browns Ferry Unit 3, a BWR, reported that 76 of 185 control rods failed to insert fully into the core when a manual scram was initiated by the reactor operator. Fortunately, this occurred during a routine shutdown from about 35% power, rather than during the kind of reactor transient in which complete and rapid scram of all the rods might have been important. 

At the BOL hot standby, a large compressive stress is exerted on the cladding due to the high core pressure. As the core starts up, the fuel rod internal pressure increases rapidly due to the fuel rod heat up and the compressive stress is reduced accordingly. Here, the reactor shutdowns at the end of second and third cycles for fuel replacements are neglected. 

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