Determintation of Long-Lived Iodine-129 in Irradiated Graphite Fuel Systems GEORGE BUZZELLI General Atomic Division of General Dynamics Corp., John Jay Hopkins laboratory for Pure and Applied Science, San Diego, Calif.
b Iodine-1 29 is a long-lived fission product (tl 2 = 1.6 X 10' years) in irradiated fuel particles and in flssionproduct trap material and is readily determined using neutron activation analysis. Natural monoisotopic iodide carrier is added to the sample and radiochemically separated using a solvent extraction technique. The separated iodide is neutron activated together with an 1129 standard and these are subsequently counted for (tl 2 = 12.5 hours). The chemical yield of the carrieir is obtained via neutron activation of the iodide carrier. Fission is thus determined in irradiated fuel. The sensitivity of the method is at the nanogram level.
T
HE FATE of a fission element in irradiated fuel bodies is of considerable importanre in relation to fission-product inventory. Because of the high-temperature and long-residence operational lifetime design of some nuclear fuel systems, containment of the fuel and fission products is somewhat of an uncertainty. Fission iodine is particularly of interest, since halides by nature possess a corrosive habit and a tendency to form volatile compounds with the reactor fuel 1-onstituents. The commonly sought isotopes of radioiodine are too short-lived for practical radiochemical purposes. This is particularly true if the fuel has experienced a high burnup status or
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Figure 1. Gamma ray spectrum of neutron-irradiated natural iodine (carrier) showing the 25minute with 0.45 m.e.v. photon
chased sources was verified by colorimetric analysis. Iodine-129 has a half life of 1.6 X lo7 years, a fission yield of 0.8Yo, and Principal decays by beta emission (lOOyo8- = Isogamma rays 0.15 m.e.v.) to XelZQ, followed by tope Half life m.e.v. emission of a 40-k.e.v. gamma ( I ) . 1128 25 0 minutes 0 45 The thermal-neutron capture cross II2Q 1 6 X 107 years 0 040 section of measured relative to Co60 1130 12 56 hours 0 528, 0 66, (up to the cadmium cutoff of 0.4 e.v.) 0 409, 0 744, is 27 barns ( 2 ) . Iodine-130 (tliz = (1 15) 12.56 hours) decays by beta emission (8- = 1.02 and 0.597 m.e.v.) to stable Xe130. There are four principal gamma rays (see Table I) associated with this decay transition, which permit a degree of flexibility for scintillation counting. excessively long reactor residence time. Gamma spectra of these isotopes are Iodine-129 is a long-lived fission product shown in Figures 1 , 2 ,and 3. which conveniently lends itself t'o The method described herein has detection by neutron activation analysis. Sufficient quantities of IlZQ been applied to the analysis of fission iodine in irradiated fuel bodies and in are generated in 1 mg. of uranium for fragments from a fission-product' trapdetection even a t bhe lyo-burnup level. ping system. The fuel bodies are The separation technique described graphitic in nature, composed of here is a conventional iodine radiopyrolytic carbon-coated spherules of chemical method (3). Carrier iodine is UC2 and ThC2 dispersed in a graphite added to the specimen, which is pulmatrix. The trap material consists of verized in a basic medium. Exchange silver-impregnated charcoal. hnalytiwith carrier is accomplished by oxidacally, the graphite cermet fuel bodies tion to the iodate with hypochlorite. represent an ideal matrix for postirThe solution is acidified, and iodate is radiation studies, since the graphite reduced to iodine and ext,racted, as imposes no interference during subiodine. The cycle is repeated, finally sequent analysis. The procedure was reducing iodine to iodide with SO2 in modified for the analysis of the charcoal distilled water. The aqueous phase is withdrawn and transferred to a polyvial trap material. This method is thus adaptable to other types of fuel systems (polyethylene vial) for subsequent by virtue of the carrier addition and activation for yield and det,ermination of the 1 1 2 Q . separation technique. Yield of the iodide is first obtained using a brief irradiation utilizing the (n, y) capture reaction on 1'" to form 1'28 itl = 25 minutes). -4ftcr measurement of the 1128 activity, thc samples and standards are reirradiated for 1 hour in a neut,ron flux of 2 x 10l2 neutrons,/cm.2-second. The carrier activity of 25-minute II28 is allowed to decay. .kctivation of I'29 by thermal neutrons produces 1130 ( t l t z = 12.56 hours), which is c.onvenientlj- counted by gamma spectrometry. X standard solution activated simultaneously w L with the samples serves as a rcferencc CHANNEL NUMBER source. Iodine-129 can hp purchased from Oak Ridge Sational Laboratory Figure 2. Gamma ray spectrum of complete with an isotopic analysis. neutron-irradiated showing the The total iodine content of the I>uractivity of 12.56-hour isotope Il3"
Table 1.
Principal Gamma Activities of 1128, )Iz9, and llaO
VOL. 36, NO. 10, SEPTEMBER 1964
1973
EXPERIMENTAL
Reagents. hnalytical-grade reagents were used throughout this work. The hypochlorite solution was prepared by passing chlorine gas into a 61%' N a O H solut'ion. Iodine-129, carrier-free and used as the reference source, was obtained from ORNL Isotopes Development Center. An isotopic analysis furnished with the purchase of this element indicated 86.1Y0 f 0.7% I 1 2 g and 13.9% i 0.7YG The iodide content was verified by a st'andard sl~ectroyhot~ometric method. The isotopic composition was also verified by a combination of total iodine analysis and determination of the I i n isotope via neutron activation analysis. Dilutions of t'his isotope were made up to cont'ain 0.0100, 0.100, and 1.00 pg. of I12Qper ml., plus 2.00 mg. per ml. of I i n carrier. Iodide carrier solution, 2.00 mg. per ml., was made up using ammoniuni iodide and was standardized gravimetrically. .\I1 samples were counted wit8h an RCL-256-channel analyzer using a 3- by 3-inch NaI solid detector within a 4-inch thick lead shield. A lieckman Model Dt- Spectrophotometer was used for the colorimet'ric determinat,ion of the reference isotoiie solution. The General Atomic TRIG.4 reactor facilitv was used for the irradiation. Procedure. Pipet 2.00 mg. of iodide carrier into a 40-ml. centrifuge tube. d d d 1 ml. each of Cs, Zr. and Ce carriers (100pg./ml.) and I S Sa2CO3. Place the carbide fuel particle(s) in t'he tube and crush with a stirring rod. These particles can be suitably disintegrat,ed by this technique. Heat the tube and contents in a hot water bath. Add 3 ml. of the hypochlorite solution and let the mixture stand for about 30 minutes with continuous agitat'ion. Acidify the solution by slowly adding 3 ml. of concentrated nitric acid. Transfer the solution to a 60-nil. separatory funnel. Rinse t'he tube with additional acid solution and add this to the funnel. Add 3 ml. of 1.11 hydroxylamine, and extract the iodine into 15 ml. of CCI4. Drain the aqueous 100-ml. volumetric of other fis.sion
Table II. Results of Iodine Determinations Showing Relative Retention and Yield Frar- Carrier I119 , tion 1 ield. Sample pg retained c( 6.5 6 3A-1 0 010 63 2 3A-2 0 020 3.4-A 0 038 0 57 47 s 3.4-R 0 021 0 44 66 0 3A-C 0021 050 506 3A-Trap-T
3.4-Trap-;\I 3A-'rrap-f3
0 003 S.1) Y.1).
61 9 ,551 0 59 9
Riirnup analysis was not performed on these samples hPer gram of trap sample.
1974
ANALYTICAL CHEMISTRY
products. Wash the organic8 phase with 10 ml. of distilled water containing a drop of the hydroxylamine solution. Add this rinse solution to the volumet'ric flask. Shake the CCI, layer with 10 nil. of wat,er cont,aining a few drops of 1.V SaHPOpuntil both phases are colorless. Transfer the aqueous phase t o a clean separatory funnel. .4dd 1 nil. of 6Jf H S 0 3 and several drops of 1.11 ?;aN02 and extrart the iodine into I O ml. of xylene. Iliscard t,he aqueous 1,ha.e and wash the organic layer with 10 nil. of nat'er containing a trace each of nitric acid and SaNO?. Iliscard the n-ash solution. Reduce the iodine to iodide with SO2 gas bubbled into 5 ml. of deionized water. Shake this mixture until both phases are colorless. Withdraw the aqueous phase and collect it in a 4dram p o l y ~ i a l . S c p b the xylene with
