Moderators boron

Yttrium hydride or zirconium hydride may be used as a moderator. Boron carbide or alloys of rare earth elements (Eu, Gd, Hf) serve as absorbing elements in control drums. The beryllium reflector is sectional (at high temperatures, beryllium oxide maybe also employed). 

The moderator boron concentration is used for slow reactivity changes and for establishing the upper limit of a reactor power control range. It is also used for rapid shutdown by opening scram valves that let borated reactor pool water into the primary loop -at the coolant pump... 

Figure 7 shows x-ray diffraction patterns for moderately boron-doped ( 10 cm ) microcrystalline and nanocrystalline diamond thin... 

Figure 8 shows visible-Raman spectra for moderately boron-doped ( — 10 cm ) microcrystalline and nanocrystalline diamond thin films. The spectrum for the microcrystalline film consists of the one-phonon diamond line centered at 1333 cm The line width (FWHM) is ca. 10 cm and, to a first approximation, is inversely related to the phonon lifetime [123,130]. The line position is negligibly shifted from that for a reference... 

FIG. 9. Background cyclic voltammetric i-E curves for (a) glassy carbon, (b) a moderately boron-doped microcrystalline diamond film electrode, and (c) a heavily boron-doped microcrystalline diamond film electrode in 0.1 M HCIO4. Electrode area = 0.2 cm. Scan rate = 0.1 V/s. 

S. The computer could be used for the automatic control of moderator boron concentration from measurements of neutron flux and moderator height. This would assist in overriding xenon transients and minimize the amount of boron addition and removal. 

The reactor was taken critical with moderator boron concentrations ranging from 4.1-12.7 ppm and immersed fuel heights from 109-360 cm. A convenient Indicator of the accuracy of reactivity calculations is provided by the closeness of the predicted Keff to unity for states known to be just critical. Over the range of states... 

As has already been discussed the zero energy experiments have confirmed closely the predicted Influence of moderator boron and hence its predicted effectiveness as a control variable. 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Olefins are carbonylated in concentrated sulfuric acid at moderate temperatures (0—40°C) and low pressures with formic acid, which serves as the source of carbon monoxide (Koch-Haaf reaction) (187). Liquid hydrogen fluoride, preferably in the presence of boron trifluoride, is an equally good catalyst and solvent system (see Carboxylic acids). 

Moderate yields of acids and ketones can be obtained by paHadium-cataly2ed carbonylation of boronic acids and by carbonylation cross-coupling reactions (272,320,321). In an alternative procedure for the carbonylation reaction, potassium trialkylborohydride ia the presence of a catalytic amount of the free borane is utilized (322). FiaaHy, various tertiary alcohols including hindered and polycycHc stmctures become readily available by oxidation of the organoborane iatermediate produced after migration of three alkyl groups (312,313,323). 

Boron Removal. Boron [7440-42-8] is occasionaHy present in water suppHes at an unacceptable level. It cannot be removed with the standard anion-exchange resins unless the water is deionized. Selective removal is possible by using an anion exchanger functionalized with /V-methy1g1ucamine [6284-40-8]. This resin is in limited commercial supply. The borate form of conventional strong base anion exchangers is used in some nuclear reactors to adjust the concentration of boron in water used as a moderator. The resin releases boron as the water temperature rises. 

BWRs do not operate with dissolved boron like a PWR but use pure, demineralized water with a continuous water quality control system. The reactivity is controlled by the large number of control rods (>100) containing burnable neutron poisons, and by varying the flow rate through the reactor for normal, fine control. Two recirculation loops using variable speed recirculation pumps inject water into the jet pumps inside of the reactor vessel to increase the flow rate by several times over that in the recirculation loops. The steam bubble formation reduces the moderator density and... 

Boronic acids readily dehydrate at moderate temperatures (or over P4O10 at room temperature) to give trimeric cyclic anhydrides known as trialkyl(aryl)boroxines ... 

The Group 13 metals differ sharply from the non-metallic element boron both in their greater chemical reactivity at moderate temperatures and in their well-defined cationic chemistry for aqueous solutions. The absence of a range of... 

The dehydrogenation of 2,3-dihydro- and 2,5-dihydro-l//-l-benzazepines to 3//-l-benz-azepincs with heterocyclic enamines in the presence of boron trifluoride diethyl ether complex has been achieved in moderate yields (30-35%).241 In contrast, electrochemical oxidation of 2,5-dihydro-1 H- -benzazepines in buffered acetic acid solution furnishes initially 5//-l-benz-azepines in 35-45% yield.242... 

In the presence of boron trifluoride, the (substituted) 2-methyltetrahydroisoquinoline 22 was reported to be easily deprotonated29. As an intermediate, the formation of the carbanion 23 is assumed, which adds to 3,4-[methylenebis(oxy)]benzaldehydc (and related benzaldehydes) with moderate stereoselectivity. 

Studies have established that the partition between transition states 3 and 4 depends on the nature of the diol unit bound to boron and on the steric and electronic effects of the a-sub-stituent X23. The data shown below demonstrate that the reactions of2-(l-methyl-2-propenyl)-4,4,5,5-tetramethyl-l,3,2-dioxaborolane proceed with a moderate preference for transition state 3 with the C2 methyl group in an axial position. Selectivity diminishes with 2-(l-methyl-2-propenyl)-l,3,2-dioxaborolane and reverses with dimethyl (l-methyl-2-propenyl)boronale, suggesting that steric interactions (gauche interactions in the case of the tetramethyl-1,3,2-diox-aborolane) between X and the diol unit on boron are capable of destabilizing transition state 4 relative to 3. 

Relatively moderate diastcrcosclectivity is also obtained when the boron enolatc of (5S,6R)-4-(benzyloxyearbonyl)-5,6-diphenyl-2,3,5,6-tctra-hydro-4ff-l, 4-oxa/in-2-one is reacted with achiral aldehydes D. S. Reno, B. T. Lotz, M. J. Miller. Tetrahedron Lett. 31, 827 (1990). 

Thiazolinc 2 proves to be inert to organometallic attack. However, addition of organolithium and Grignard reagents to the C —N double bond of 2 can easily be achieved by previous activation with boron trifluoridc, providing /r[Pg.696]

A substance employed to slow down the neutrons in a nuclear reactor in order that they can be captured by the nuclei. Boron, heavy water (D20) and graphite are commonly used moderators. 

Boron is also obtained by the pyrolysis of diborane. Only moderate temperature is required (400-900°C) with a pressure up to 1 atm [6][7]... 

P-Keto esters have been prepared in moderate to high yields by treatment of aldehydes with diethyl diazoacetate in the presence of a catalytic amount of a Lewis acid such as SnCL, BF3, or GeCL. The reaction was successful for both aliphatic and aromatic aldehydes, but the former react more rapidly than the latter, and the difference is great enough to allow selective reactivity. In a similar process, aldehydes react with certain carbanions stabilized by boron, in the presence of (F3CC0)20 or NCS, to give ketones. 

The B atoms in the mono- and diaminoboranes (R2N)2BR and R2NBR2 show only moderate functions. Therefore, boron-metal interactions are relatively weak. 

Oxidation of chalcones with TTN has been studied in detail (95, 96), and it has been shown that the products obtained depend on the amount of reagent and the solvent employed. Oxidation with 1 equivalent of TTN in methanol, methanol-chloroform, or methanol-boron trifluoride leads to acetals of the type (XXXIV) (see also Scheme 21) in yields of 20-80%. When 3 equivalents of TTN are employed, however, and aqueous glyme containing a little perchloric acid used as solvent, the products are benzils. This remarkable transformation, which proceeds in yields varying from moderate to good (40-80%), involves three distinct oxidations by TTN, and these are outlined in Scheme 22. Each individual step in this reaction sequence has been investigated in detail, with the result that useful procedures have been developed for the oxidation of both deoxybenzoins and benzoins to benzils with TTN (96). 

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