Spectrographic determination of boron in nuclear graphite

The use of graphite as a power roactor moderator ncccssitntcs ;L study of ;malyticai methods for the determination of very low conccntrationa of those elements which show a high neutron cross section. Amongst these elements is boron, which is usually present in q-aphite and for whicli normaI l~~lrific~Ltion processes are ineffective, The chemical clctermination of boron at the concentrations which do not affect the reactor performance rcquircs clifficult and time-consuming techniques. The purpose of this work was to clcvclop ;I method for the spectrographic rletcrmination of the boron content of nuclear graphite at concentrations between 0.x and I.0 p.p.m. Such a mctliocl had to lx, morcovcr, rapid and economical, i.e. properly developed for low-cost routine analysis on thousands of samples. It was ckxxnccl aclvisablc, clue to cost problems, to eliniinritc the tisf! of boron-free gtxphite for the electroclcs. For this rcason, among the methods in the literuture we excluded the possibility of adapting the methods of 17!eLI,~lAN AN I) ~LLENIJURC’, RUSSMANN~, SHUGAS~:~, Ct2xt*roN4 or I,EITJ_A ANI] MARKEI.“. Some of these methocls were also cscfudccl because the sensitivity indicated by their authors did not meet our rquircmcnts. The ncldition of substances as internal standards ad their accurate misture -- as far as graphite is conccrnecl - arc rather tcclious operations (its 12CssaritS.‘iuz points out). Therefore the possibility was studier1 of using the background as an internal standarcl with considerable time saving in the preparation of analysis samples. The sclcction of background as internal stanclrtrcl proved to be quite satisfactory and effectively ~tllowecl the USCof the sample itself without any ll~~n~lin~ other than shaping.

A “ Hilger” mcclium quartz spectrograph (type E +_$3; dispersion at zgoo A appros. I) A/mm) wets cmployecl for the cletcrtninations. For a rapid consumption of the sample, an arc source of $30 V c1.c. and dclivcring ;L 20-A maximum output, was constructccl. For the direct satnplc support, metallic (brass) electrocle holders were udoptecl. Ilford ordinary plates were used and were clevelopecl by Ilforcl fD2.

For the preliminary work WC considered the possibility of andysing both block and potvclcr samples. Of course it is easy to obtain cylindrical sticks from blocks for the

SPECTROGRAPHY

OF H IN

NUCLEAR GRAPHITE

9=

formation of the two electrodes. On the other hand, powder samples require working up and moulding. For this purpose we adapted the method used by GARTON~ to prepare graphite pellets suitable for insertion on the end of his boron-free graphite electrodes. The method was as follows: to 5 parts of finely powdered sample (approximately xoo mesh), one part of bakclite solution was added (we used the SUPERROL product of Monti Sr Martini Co. which blank tests showed to be boron-free). The substances were mixed thoroughly and accurately and the misture was dried at 110~. The sample was subjected at 150~ to a pressure of r4,ooo p.s.i. Rectangular rods which were 5 mm wide and 20-25 mm high were obtained. Such electrodes were used with a 17-A arc. In order to obtain sufficiently intense boron lines, a rather high sample consumption and relevant esposure time were required. On the other hand it was necessary to prevent the background from reaching prohibitive values; for this purpose the spectrograph was illuminated by an intermediate image of the arc from which the electrode image was removccl by masking. The clectrocle spacing was maintained during esposure at 4 mm and the slit width was 5 ,IL. 13cfore esposure, electrodes were arced for a few seconds in order to eliminate any traces of volatile and easily inflammable bakclite residues. The arc was then re-lighted for 3 min for the csposurc. Electrode masking allowed such a long esposure time without escessivc background values around the two boron lines. Plate calibration was made by the two-step filter methocl on an iron spectrum. As the boron analytical line, we used HI 2497.73, on which no interference has been noted by the elements present as impurities in nuclear ~raphitc. As background value the minimum blackening value bctwccn I3 2396.7S and I3 2497.73 lines was cmploycd. RESUI-TS The calibration curve was plotted by starting with two analyzed powclcred graphite samples containing 1.05 ancl 0.10 p.p.m. of boron respectively. Ry mising the first with Ringsdorff R1VD graphite powder (boron-free), two other samples were obtained containing 0.525 and 0.210 p.p.m. of boron. Once the clectroclcs had been obtained and arced as described above, the calibration curve was detcrminccl (Fig. I). The abscissa values indicate the boron concentration in the initial sample, i.c. before bakelite mising. Boron lines proved to be easily measurable ior concentrations rang’ing from 0.2 to 1.0 p.p.m.; between 0.x and 0.2 p.p,m., insufficient blackening against the background allows only a scmicluantitntive estimation. The points shown on the calibration curve arc the average of three relatively consistent values. When a series of electrode couples containing 0.525 p,p.m. of boron were arced twelve times in the same plate, the reproducibility of the method proved to be aoh = 9.8, which was quite suitable for our purposes, especially if the data are used as the average of 3 or 4 determinations. Different values for samples of the same Concentration, were found only when the results were obtained with electrodes which were considerably different in geometric size from couple to couple, or when the same couple was used for more than one esposure. For samples directly cut from blocks of the same boron content, the line-to-backAnal.

CJGm.

Ada,

25, (1961)

90-_1)2

ground ratio was found to depend on material aggregation and other uncontrollable fact.ors. For this reason, the direct arcing of solid samples (cut from blocks) was impracticable because it would have been necessary to prepare as many calibration curves as the types of graphite to be tested.

incrtl~ocl described is of practical ~intl economic value for the analysis of a larp2 number of l~owcleretl or pul\w-izablc samplcs. ‘I’ho precision of the mcthotl is suffic:ien t for t hc concentration range under consideration. ‘he time requirecl for xnalysis of at least IO samples, each being annlyzccl three times and tlie calibration cur\ws

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A much more detailed account of tllis work will be publish1 in Itaiinn in MalnlIitvgilr Ilrrlicw~.

Qi3 1.0 p.p.m.B

curve for boron in gr;lphitc

A spcctrogmphic rncthod is tlcscrilxxl for the dctcrmination of borcln (o.z- x .o p.p.m.) in nuclear Nicithcr horon-fwc graphite clcctroclcs Iqxphite and for the estimation of 0.1-0.2 l>.l>.m. of bcJrcJn. nor adtlitirmal forcijin subst;mccs as internal stnnclarcl m-c rcquirccl. 130th tlic clcctroclcs consist nf sample Iwxidcr, properly llnlcclitc?-l,rr,ccss~~l, ‘I’hc stantlartl ckwiatioii is loo/u,. Rl%UMIT Unc mbthoclc SpcCtrographiC~Uc cst clbcritc pour ie dosage tlu bow ctalis ic pxphitc nucldairc, pour clcs tcncurs de 0.2 ZLI .o p.p.tn. Ixs CICIIX dlcctrodcs sont c~JIIStitll&!S tic 1’bch;lntillon ?I ~Llli~l~SC~ cotnprim6c ct durcic par “l~illcdlis;~tir~ll”.

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