Nuclear Reactors, Moderators and Coolants

Several types of nuclear reactors have already been mentioned in the previous section with respect to the use of nuclear fuel and the manufacture of fuel elements. The various types of nuclear reactors are distinguished on the basis of the following aspects  

Power reactors have also been developed and installed for ship propulsion, for instance in submarines (e.g. Nautilus , USA) or icebreakers (e.g. Lenin , Russia). 

World-wide the production of energy by nuclear power amounts to about 470 GWe (1995) and increases by about 4% per year, although the problems with respect to the storage of the radioactive waste (fission products and actinides) are not yet solved in a satisfactory way. 

Reactor type Percentage I uch ) Canning Moderator Coolant Coolant temp. [ Ci Coolant pressure [MPaj Power iMW,i Burn-up IMWd per kg) 

Pressurized heavy-water reactor (PHWR) % 5 UO2 pellets (natural U) Zircaloy DnO D2O 280- 310 8 11 700 -800 8-10 

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The recognition in 1940 that deuterium as heavy water [7789-20-0] has nuclear properties that make it a highly desirable moderator and coolant for nuclear reactors (qv) (8,9) fueled by uranium (qv) of natural isotopic composition stimulated the development of industrial processes for the manufacture of heavy water. Between 1940 and 1945 four heavy water production plants were operated by the United States Government, one in Canada at Trail,... 

The nuclear plants now operating in the U.S. are light water reactors, which use water as both a moderator and coolant. These are sometimes called Generation II reactors. In these Generation II Pressurized Water Reactors, the water circulates through the core where it is heated by the nuclear chain reaction. The hot water is turned into steam at a steam generator and the steam is used by a turbine generator to produce electric power. 

Besides the fuel elements, the other main components in a nuclear reactor are the moderator and the coolant. Moderator and coolant may be identical. 

Table 21.1 compares some of the common properties of H2O with those of D2O. Deuterium oxide, or heavy water as it is commonly called, is used in some nuclear reactors as a coolant and a moderator of nuclear reactions (see Chapter 23). D2O can be... 

Table 1.2 Examples of nuclear power reactors with separate fuel, moderator, and coolant...

In the CANDU heavy-water reactor the dominant source of tritium is the deuterium activation reaction of Eq. (8.53). The data given in Prob. 3.3 for the Douglas Point Nuclear Power Station provide a basis for estimating the rate of production of tritium in the heavy-water moderator and coolant ... 

There are several different kinds of nuclear reactors that use diverse types of fuel (uranium compounds) that are placed in different sorts of fuel elements and these in turn are arranged in varions fuel assemblies. A detailed survey of these is beyond the scope of this book however an elementary understanding of nuclear fuel and its characterization is highly relevant. Figure 1.14 depicts the main nuclear reactor, types of fuel, moderator, and coolants in service. 

In nuclear technology, graphite is used as a moderator to slow down the neutrons released during fission. This is necessary for continuous fission to be maintained. The Chernobyl accident happened with a reactor using graphite as moderator and ordinary water as coolant This combination caused instability. Better stability is achieved in reactors using a graphite moderator and pressurized COj as the coolant However, the most common reactor type nowadays uses pressurized water as both moderator and coolant. 

Over the more than 40 years since the first nuclear fission reactor was constructed numerous designs of reactor have been evolved by variation of the basic parameters such as fuel type, moderator, and coolant. One possible classification is by intended use, e.g., research, plutonium production, electricity generation, or propulsion units for submarines or surface ships. In this chapter we will concentrate on power reactors, both on account of their practical importance and because of the complexity in engineering design introduced by the need to convert the energy released by nuclear fission into a mechanical or electrical output. Many of the characteristics of the various reactor types have been touched on in earlier chapters, but the objective in the present chapter is to provide a systematic summary of the main classifications of reactor prior to the more detailed descriptions to be given in the following chapters. 

LIGHT-WATER REACTOR (LWR). A nuclear reactor that uses ordinary water (as opposed to heavy water) as the moderator and coolant. Low-enriched uranium (LEU) is commonly used as the fuel in such reactors. 

There have been three notorious accidents at nuclear power stations. The first occurred at the Three Mile Island (TMl) generating plant near Middleton, Pennsylvania, in 1979. The TMI reactor is of the light-water type, in which water is used as the moderator and coolant. In this accident, some coolant (also the moderator) was lost the chain reaction in the reactor stopped because there were too few slow neutrons. However, radioactive decay of the fission fragments continued, causing the fuel rods to get very hot. A partial meltdown resulted and, in turn, caused a fracture in one of the reactors. The fracture permitted the venting of a small amount of radioactive steam into the atmosphere. The reactor is now sealed, but electronic robots have discovered substantial damage to the fuel rods. 

A number of pool, also called swimming pool, reactors have been built at educational institutions and research laboratories. The core in these reactors is located at the bottom of a large pool of water, 6 m deep, suspended from a bridge. The water serves as moderator, coolant, and shield. An example is the Lord nuclear reactor at the University of Michigan, started in 1957. The core is composed of fuel elements, each having 18 aluminum-clad plates of 20% enriched uranium. It operates at 2 MW, giving a thermal flux of 3 x 10 (cm -s). The reactor operates almost continuously, using a variety of beam tubes, for research purposes. 

Heavy water (p. 39) is now manufactured on the multikilotonne scale for use both as a coolant and neutron-moderator in nuclear reactors its absorption cross-section for neutrons is much less than for normal water [Pg.623]

The question of the compatibility of metals and alloys with carbon and carbonaceous gases has assumed considerable importance in connection with the development of the gas-cooled nuclear reactor in which graphite is used as a moderator and a constituent of the fuel element, and carbon dioxide as the coolant. Tests of up to 1 000 h on a series of metals and nickel-containing alloys under pressure contact with graphite at 1 010°C" showed that only copper was more resistant than nickel to diffusion of carbon and that the high-nickel alloys were superior to those of lower nickel content. The more complex nickel-chromium alloys containing titanium, niobium and aluminium were better than the basic nickel-chromium materials. 

Boric acid [B(OH)3] is employed in primary coolant systems as a soluble, core reactivity controlling agent (moderator). It has a high capture cross-section for neutrons and is typically present to the extent of perhaps 300 to 1,000 ppm (down from perhaps 500 to 2,500 ppm 25 years ago), depending on nuclear reactor plant design and the equilibrium concentration reached with lithium hydroxide. However, boric acid may be present to a maximum extent of 1,200 ppm product in hot power nuclear operations. 

Nuclear reactors are classified by their neutron energy level (thermal or fast reactors), by their coolant (water, gas, liquid metal) and by their neutron moderator (light water, heavy water, graphite). Most existing plants are thermal reactors using pressurised (PWR) or boiling water (BWR) as a coolant and moderator PWR and BWR together represent more than 80% of the commercial nuclear reactors today, of which PWR accounts for 60% alone (Olah et al., 2006). 

The essential ingredients for producing heat in a thermal fission nuclear reactor are the fuel and a moderator. A heat transport system with its coolant is necessary to convey the heat from the reactor to boilers where steam is produced to drive the turbogenerator. The natural materials available for fuel and moderator are uranium ore and water natural uranium extracted from the ore comprises the fissionable isotope uranium-235 and water contains hydrogen which is a good moderator. (Table I)... 

Because early Canadian reactors used heavy water, and because it is also fundamentally the most efficient moderator, Canada naturally adopted the heavy water reactor for the development of a nuclear power system. By using heavy water both as moderator and as coolant, and by refuelling with the reactor at power, it was possible to develop the CANDU system to operate efficiently and economically with natural uranium fuel. This in turn resulted in the simplest possible fuel cycle. 

In a typical experiment, reduction of the water s volume from 2400 L to 83 mL yields 99% pure D20. Large amounts of D20 (about 160 tons per year in the United States) are manufactured by this method for use as a coolant and a moderator in nuclear reactors. 

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