Pressurized water reactors control rods

The control element assemblies consist of an assembly of 4. 8, or 12 fingers approximately 0.8-inch (2-centimeter) outside diameter and arranged as shown in Fig. 14. The use of cruciform control rods, as in boiling water and early pressurized water reactors, necessitates large water gaps between the fuel assemblies to ensure that the control rods will scram (prompt shutdown) satisfactorily. These gaps cause peaking of the power in fuel rods adjacent to the water channel compared to fuel rods some distance from the channel. 

As an example, a fuel element of the type used in a pressurized water reactor (PWR) is shown in Fig. 11.11. The fuel element has 16 x 16 positions for 236 fuel rods and 20 control rods. 

The actual fuel elements in pressurized water reactors consist of individual fuel rods and control rod tubes mounted in a self-supporting construction of spacers fitted with a top and feet. Fuel elements for boiling water reactors, by comparison, have no control rod tubes, the fuel element zirkaloy claddings being used to guide the control rods and the coolant. 

Figure 12. Pressurized water reactor rod-cluster control assembly. (After Ref. 11.)...

Many years ago, two pressurized water reactors were built, with the lower support plate of the eore subdivided into two plates, about three metres apart in the vertical direction and connected by an external row of round rods in traction (tie rods - TR) and by internal guide tubes for the control rod followers (cruciform) containing fuel rods, as illustrated in Figure 12-1. 

AR360 1.77 Assumptions used for evaluating a control rod ejection accident for pressurized water reactors,... 

I) Concrete containment 2) Containment steel shell 3) Polar crane 4) Reactor pressure vessel S) Control rod drive mechanism 6) Spent fuel pool 7) Refuelling machine 8) Steam generator 9) Pressurizer 10) Pressurizer relief tank 11) Main coolant pump 12) Main steam line 13) Feedwater line 14) Concrete shield IS) Accumulator 16) Personnel lock 17) Mate rials lock 18) Lifting gantry 19) Fresh fuel assembly storage 20) Borated water storage tank 21) Residual heat cooler 22) Component cooler 23) Safety injection pump (By courtesy of Siemens/KWU)... 

The following example apphcable to pressurized water reactors (PWRs) may further illustrate the approach described. One of the SFs relevant for Levels 1-3 of defence in depth is prevention of unacceptable reactivity transients. This SF can be challenged by insertion of positive reactivity. Several mechanisms lead to such a challenge, including control rod ejection, control rod withdrawal, control rod drop or misalignment, erroneous startup of a circulation loop, release of absorber deposits in the reactor core, incorrect refuelling operations or inadvertent boron dilution. For each of these mechanisms there are a number of provisions to prevent its occurrence. For example, control rod withdrawal can be prevented or its consequences mitigated by ... 

Fig. 9.3. Sectional view of the Sequoyah pressurized water reactor (courtesy of Nuclear Engineering International). A, Control rod drive head adaptors B, instrumentation ports C, thermal sleeves D, upper support plate E, support column F, control rod drive shaft G, control rod guide tube H, internals support ledge J, inlet nozzle K, outlet nozzle L, upper core plate M, baffle and former N, fuel assemblies O, reactor vessel P, thermal shield Q, access port R, lower core plate S, core support T, diffuser plate U, lower support column V, radial supports W, instrumentation thimble guides.

The different reactivity control systems in a nuclear power plant allow keeping at any time the control of the nuclear fission reactions in the core power steering, safe reactor shutdown, wear compensation of the fuel. They are also part of the neutron protection of the out-of-core components. These systems can take various forms gas (such as helium 3 in some experimental reactors), liquid (soluble boron in pressurized water reactor (PWR) coolant to balance the reactivity evolution of the reactor), and most of the time solid (Table 15.1). In a reactor, they are most often combined [e.g., in PWR with Ag-In-Cd (AIC) plus boron carbide control rods and with boron present both as soluble boron and as boron carbide]. In all cases those materials incorporate neutron-absorbing nuclides, unlike the fuel which is a medium generally multiplier... 

Each pressurized water reactor manufactured by Combustion Engineering or by Babcock and Wilcox must have a diverse scram system from the sensor output to interruption of power to the control rods. This scram system must be designed to perform its function in a reliable manner and be independent from the existing reactor trip system (from sensor output to interruption of power to the control rods). 

For a standard pressurized water reactor (PWR), the assembly contains 264 rods (geometry 17 x 17 with 24 guide tubes to accommodate control rods containing absorbent materials, and another central one, for nuclear detectors). A PWR contains 157 to 193 of such assemblies these are regularly renewed (by a third or a quarter) as and when the fuel is exhausted. 

Nuclear Boiler Assembly. This assembly consists of the equipment and instrumentation necessary to produce, contain, and control the steam required by the turbine-generator. The principal components of the nuclear boiler are (1) reactor vessel and internals—reactor pressure vessel, jet pumps for reactor water circulation, steam separators and dryers, and core support structure (2) reactor water recirculation system—pumps, valves, and piping used in providing and controlling core flow (3) main steam lines—main steam safety and relief valves, piping, and pipe supports from reactor pressure vessel up to and including the isolation valves outside of the primary containment barrier (4) control rod drive system—control rods, control rod drive mechanisms and hydraulic system for insertion and withdrawal of the control rods and (5) nuclear fuel and in-core instrumentation,... 

Small amounts of silver are used annually in such diverse applications as a backing for mirrors, and in control rods for pressurized water nuclear reactors Miscellaneous uses like this account for only a small fraction of total silver consumption. 

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