Reactors, thermal

The two light water reactors (LWRs) are the pressurized water reactor (PWR) and the boiling water reactor (BWR). The systems are similar in that both employ light water as both moderator and coolant, which necessitates the use of fuel enriched to about 2.0 %-2.5 % due to the appreciable neutron absorption of the water in the core. They are also alike in that in both cases the core is enclosed within a single large steel pressure vessel (see Fig. 7,1). The distinction between the two designs is also illustrated in the figure. In the 

HEAVY WATER REACTOR ADVANCED GAS-COOLED REACTOR 

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The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. 

This reaction offers the advantage of a superior neutron yield of in a thermal reactor system. The abiHty to breed fissile from naturally occurring Th allows the world s thorium reserves to be added to its uranium reserves as a potential source of fission power. However, the Th/ U cycle is unlikely to be developed in the 1990s owing both to the more advanced state of the / Pu cycle and to the avadabiHty of uranium. Thorium is also used in the production of the cx-emitting radiotherapeutic agent, Bi, via the production of Th and subsequent decay through Ac (20). 

In Britain, a population of thermal reactors fuelled by metallic uranium have remained in use, side by side with more modern ones (to that extent. Lander et al. were not quite correct about the universal abandonment of metallic uranium). In 1956, Cottrell (who was then w orking for the Atomic Energy Authority) identified from first principles a mechanism which would cause metallic (ot) uranium to creep rapidly under small applied stress this was linked with the differential expansion of... 

Compartments, ANS, ENS Topical Meeting on Thermal Reactor Safety, I —. TN... 

The net yield of thermal neutrons from the fission of is higher than from that of and, furthermore, Th is a more effective neutron absorber than As a result, the breeding of is feasible even in thermal reactors. Unfortunately the use of the Th/ U cycle has been inhibited by reprocessing problems caused by the very high energy y-radiation of some of the daughter products. 

Many of the fission products formed in a nuclear reactor are themselves strong neutron absorbers (i.e. poisons ) and so will stop the chain reaction before all the (and Pu which has also been formed) has been consumed. If this wastage is to be avoided the irradiated fuel elements must be removed periodically and the fission products separated from the remaining uranium and the plutonijjm. Such reprocessing is of course inherent in the operation of fast-breeder reactors, but whether or not it is used for thermal reactors depends on economic and political factors. Reprocessing is currently undertaken in the UK, France and Russia but is not considered to be economic in the USA. 

Other variations of the dual-bed scheme exist as a combination of thermal oxidizing reactors and catalytic reducing reactors. The Questor company has developed a reactor with three zones the first zone is a thermal reactor with limited air to raise the temperature of the exhaust gas, the second zone is a catalytic bed of metallic screens to reduce NO, and the third zone is another thermal reactor where secondary air is injected to complete the oxidation of CO and hydrocarbons (45). 

Thermal oxide reprocessing plant, 6, 885 Thermal reactor fuels, 6,926 dissolution, 6,927 irradiated... 

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). 

In fast (neutron) reactors, the fission chain reaction is sustained by fast neutrons, unlike in thermal reactors. Thus, fast reactors require fuel that is relatively rich in fissile material highly enriched uranium (> 20%) or plutonium. As fast neutrons are desired, there is also the need to eliminate neutron moderators hence, certain liquid metals, such as sodium, are used for cooling instead of water. Fast reactors more deliberately use the 238U as well as the fissile 235U isotope used in most reactors. If designed to produce more plutonium than they consume, they are called fast-breeder reactors if they are net consumers of plutonium, they are called burners . 

Examples 9.4 through 9.7 illustrate the approach to problems dealing with noniso-thermal reactors. Chapter 19 extends this approach to multistage operations of solid catalyzed reactions. 

Plutonium is produced from natural uranium which is a mixture of nonfis-sionable uranium-238 (99.3%) and fissionable uranium-235(0.7%). The first synthesis of this element was in a cyclotron generating plutonium in microgram quantities. The isotope Pu-239 can be produced in much larger quantities in a nuclear reactor, either a conventional thermal reactor or a breeder type reactor by neutron bombardment of uranium- 238. The nuclear reactions are shown below. 

Continuous processes if photochemical (and thermal) reactors are installed in series. Such an arrangement could be useful in the production of previtamin D (PD), where tachysterol (T) produced as a secondary product (Figure 21) may be recovered by a subsequent triplet sensitized cis-trans isomerization [2, 3, 75-77]. 

The high-temperature gas-cooled reactor (HTGR) is a thermal reactor that produces desired steam conditions. Helium is used as the coolam. Graphite, with its superior high temperature properties, is used as the moderator and structural material. The fuel is a mixture of enriched uranium and thorium in the form of carbide particles clad with ceramic coatings. 

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