Pressurized water reactors control

A variety of nuclear reactor designs is possible using different combinations of components and process features for different purposes (see Nuclear REACTORS, reactor types). Two versions of the lightwater reactors were favored the pressurized water reactor (PWR) and the boiling water reactor (BWR). Each requites enrichment of uranium in U. To assure safety, careful control of coolant conditions is requited (see Nuclearreactors, water CHEMISTRY OF LIGHTWATER REACTORS NuCLEAR REACTORS, SAFETY IN NUCLEAR FACILITIES). 

Fig. 1. Pressurized water reactor (PWR) coolant system having U-tube steam generators typical of the 3—4 loops in nuclear power plants. PWR plants having once-through steam generators contain two reactor coolant pump-steam generator loops. CVCS = chemical and volume-control system.

All over the world, 432 nuclear power reactors are under operation and more than 36 GW of electricity could be produced as of December 31, 2001. There are several types of reactors such as boiling water reactor (BWR), pressurized water reactor (PWR), Canada deuterium uranium (CANDU), and others. In these reactors, light water is normally used not only as a coolant, but also as a moderator. On the contrary, in CANDU reactors, heavy water is taken. It is widely known that the quality control of coolant water, the so-called water chemistry, is inevitably important for keeping the integrity of the plant. 

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. 

Fig. 15. Control element assembly and fuel for contemporary pressurized water reactor...

Fig. 16. Standard pattern of control assemblies in contemporary pressurized water reactor core. The pattern provides morc-than-sufficient control for self-generated plutonium recycle. For complete open-market plutonium recycle, 4-element control assemblies are added in positions marked S...

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.)...

The Pressurized Water Reactor (PWR) reload core optimization problem, though easily stated, is far from easily solved. The designer s task is to identify the arrangement of fresh and partially burnt fuel (fissile material) and burnable poisons (BPs) (control material) within the core which optimizes the performance of the reactor over that operating cycle (until it again requires refueling), while ensuring that various operational (safety) constraints are always satisfied. 

In addition to understanding the interaction of radiation with water, the nuclear industry must obviously also take into account the excess production of molecular hydrogen and hydrogen peroxide, and control this excess in order to avoid explosive conditions and corrosion of the water circuitries. Due to the working conditions of the current reactors (T > 310 °C, P > 100 atm in Pressurized Water Reactor, PWR), it is mandatory to predict the evolution ofthe chemistry when submitted to high temperature and pressure. Nevertheless, a few experiments have shown that the linear Arrhenius law model is not applicable at temperatures above 250 °C. Hydrogen production overestimates have been necessary in... 

As motioned in Chapter 19, the name implies that a pressurized water reactor is cooled by hot high pressure water, either H2O (PWR, VVER) or DjO (PHWR). In the PWR and VVER types the coolant is also us as moderator whereas a separate D2O containing moderator tank is normally used in the PHWR type. These power reactor types have several things in common primary — secondary coolant circuits separated by heat exchangers (steam generators), a pressurizer to adjust primary system pressure and often diemical shim control for adjustment of the excess reactivity with fresh fuel. 

The control effectiveness of such alloys in water-moderated reactors can approach that of hafnium and is the control material commonly used in pressurized-water reactors. The alloys (generally 80% silver, 15% indium, 5% cadmium) can be readily fabricated and have adequate strength at water-reactor temperatures. The control material is enclosed in a stainless steel tube to protect it from corrosion by the high-temperature water. 

Advantages Highly effective neutron absorber. Control effectiveness in water-moderated reactors is close to hafnium. Used in pressurized-water reactors. Easily fabricated and adequate strength... 

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,... 

In the 1970s, there was a series of unanticipated operational events that occurred in commercial operating pressurized water reactors (PWRs) in the USA (NRC, 2012).These events resulted in pressures and temperatures in the RPV that were outside the P-TUmits specified for normal operation. The conditions associated with these unanticipated events could be placed into two categories. Rrst, there were approximately 30 transient events where the pressure in the RPV exceeded the allowable pressure at relatively low temperature. These events were isothermal pressure transients that generally occurred at temperatures below approximately 93 C (200°F) during reactor start-up. In many instances, the transient pressures were several times the allowable pressure. Typically, the transients occurred while the reactor coolant system was filled with water and were a result of operators failing to follow appropriate procedures to control and prevent... 

The 1255 MW(th)/400 MW(e) Consolidated Nuclear Steam System (CNSS) is an integral pressurized-water reactor (PWR) with the reactor core and steam generators located within the reactor pressure vessel (see Figure 1). Reactor coolant system pressure Is controlled via an electrically heated pressurizer connected to the reactor vessel through four surge lines. 

Marchl, T., Riess, R. (b) Reducing the cobalt inventory in Siemens PWRs. Paper presented at the EPRI Radiation Field Control Seminar, Seattle, Wash., USA, 1993 Marchl, T., Riess, R., Odar, S. Possibilities to influence personnel exposures illustrated at recent pressurized water reactors. VGB Kraftwerkstechnik 72, 561-563 (1992)... 

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