Formation of Volatile Radioiodine Compounds in Sodium Pool Burning*

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 12[11] pp. 717~721 (November 1975). 717

Formation of Volatile Radioiodine Compounds

in Sodium Pool Burning*

Susumu KITANI, Junichi TAKADA, Gunji NISHIO and Tetsuo SHIRATORI

Division of Nuclear Fuel Research, Japan Atomic Energy Research Institute**

Received April 10, 1975

In sodium pool burning occurring in the case of an LMFBR accident, some radioiodine in the sodium coolant may be transported into the gas phase and act in common with sodium oxide aerosol. If some iodine is converted to volatile compounds, the radioactivity may remain for many hours in the gas phase of the reactor containment. The present work was carried out in an attempt to throw more light on these circum- stances. Reactor-grade sodium with sodium iodide tagged with 131I in an amount of about 1 ppm was burned by heating in a closed vessel containing ,air. Most of the iodine released into the gas phase took the form of aerosol, but some amount remained in vaporous state. It was determined by Maypacks and radio-gaschromatography that the volatile radioiodine compounds were of organic form. The proportion of organic iodide referred to total airborne iodine in the present experiments falls within the experimental data on what will be formed in a loss of coolant accident involving a light water reactor. It is concluded that volatile iodine formation would present similar aspects under accidental conditions affecting both LWR and FBR, though the mechanisms governing the two cases would be different. KEYWORDS: accidents, LMFBR type reactors, sodium pool burning, volatile radioiodine, organic iodine compounds, methyl iodides, sodium oxide aerosol, radio-gaschromatography, iodine 131, sodium radioactive aerosols

I. INTRODUCTION only 15% of the oxidized sodium was released. The total fraction of the 131I which either In design basis accidents of a sodium cooled adhered to the wall or settled on the floor fast breeder reactor, one form of serious ac- after dispersing in the chamber differed from cident is a sodium fire following the leakage that of the sodium. This difference in behavior of the primary sodium coolant. In this type between iodine and sodium might be ascriba- of accident, a large quantity of sodium oxide ble to that in the particle size distribution of aerosol containing 24Na and fission products aerosols. may be dispersed due to combustion of sodium On the other hand, Baurmash et al.(2) have in the reactor containment. reported that radioiodine released from sodi- Morewitz at al(1) made sodium burning um pool burning decreases its concentration experiments in a chamber measuring about in the chamber at the same rate as airborne 1 m3 in volume. In one run of their experi- * This work was performed in part under con. ments, 13.6 g of sodium containing 10 ppm of tract between the Power Reactor and Nuclear sodium iodide tagged with 131I was burned. Fuel Development Corporation and the Japan About 40% of, the sodium iodide was released Atomic Energy Research Institute. into the chamber in the form of aerosol, while ** Tokai-mura , lbaraki-ken.

55 718 J. Nucl. Sci. Technol.,

sodium oxide. In the same series of their presence of hydrocarbon in the atmosphere, experiment, sodium oxide aerosol was released which might serve as source of carbon to in air containing molecular iodine vapor. A contribute to the formation of volatile iodine major part of the iodine vapor was adsorbed compounds. on sodium oxide aerosol, but a small part Runs 9 to 15 were aimed at complete prior persisted as vapor and passed through par- mixing of sodium iodide in the liquid sodium. ticulate filter. The sodium iodide was dissolved in liquid The present work was undertaken to study sodium at 350-C, allowing 3.5 hr for mixing, the formation of volatile radioiodine in sodi- and then the sodium was transferred to a um pool burning. In the experiment, most of separate crucible through a heated pipe of the iodine released in the air took the form stainless steel under argon gas cover. Uni- of aerosol state, while some amount became form distribution of iodine in the sodium was volatile and remained suspended in the cham- ascertained by observing the transfer of 131I ber for many hours. The volatile iodine radioactivity from one crucible to the other. compounds were found upon examination to After the transfer, some sodium carbonate be in the form of organic iodides. originating from the iodine reagent was found to remain as residue in the first crucible. The second crucible filled with the sample II. EXPERIMENTAL was then transferred from argon gas cover The sodium used in the experiment was to the heating pot, where it was exposed to reactor-grade sodium purified by passage room air for ignition, which took place in through cold trap, which reduced the carbon about 15 min. A thermocouple set in the impurity in the sodium to less than 50 ppm. sodium measured the temperature during the In each run, a solution of carrier-free 131I heating and burning. containing about 0.5 mg of Na2CO3-NaHC3* Airborne substances released into the was mixed with the solution containing a chamber were sampled and analyzed by given quantity of Nal. An aliquot of the Maypacks, which consisted of two packs of solution was evaporated to dryness in a nickel particulate filter (membrane filter), one pack crucible (35 mm in dia. x,10 mm high) by heat- of five sheets of copper gauze-momentarily ing over 1 hr at 120-C in argon atmosphere. immersed in hydrogen chloride solution before In this process NaHCO3 decomposes thermally use-to trap molecular iodine vapor and three to Na2CO3. A given weight of the sodium packs of activated charcoal (coconut shell— metal sampled in a glass tube was then heated each bed 1 cm thick) to trap volatile iodine and placed in the crucible in argon gas at- compounds. mosphere. Another aliquot of the solution The radioactivity trapped by the Maypacks was used for determining the quantities of was found distributed mainly on the particu- iodine collected on several iodine traps by late filter. To measure the quantity of sodium comparison with their specific activities. collected on the particulate filter and copper In Runs 1 to 8 the crucible placed in a gauze, the filters were dipped in a dilute shallow heating pot was heated directly in aqueous solution of potassium chloride and air at a rate of 25dc/min until ignition of the the concentration of sodium in the solution sodium sample in the crucible. The whole was determined by atomic absorption spectro- assembly was set in an experimental chamber scopy(3). made of stainless steel, 2 m high and about Radio-gaschromatography was applied to 1 m3 in volume. Usually, the air in the chamber identify the chemical form of volatile radio- contained water vapor with 60~70% of rela- iodine compounds trapped on the activated tive humidity at room temperature. In Runs charcoal(4)(5). The volatile radioiodine contain- 7 and 8 some kerosene vapor was introduced * Na2CO3-NaHCO3 was added by the manufactur - into the chamber in advance of sodium igni- er as stabilizer in the carrier-free radioiodine tion, in order to confirm the influence of the reagent.

56 Vol. 12, No. 11 (Nov . 1975) 719 ed in the air was sampled in a glass tube III. RESULTS cooled in liquid nitrogen after the air was passed through a particulate filter and a The experimental conditions and results magnesium perchlorate column to remove are summarized in Table 1. Heated in air, the particulate substances and water vapor, re- sodium ignited spontaneously at a tempera- spectively. This sampling was performed 20 ture above 300-C, and sodium oxide aerosol hr after sodium ignition. The content of the was released together with radioiodine. The glass sampler was carried by a 25 cm3/min progress of a typical run is presented in flow of helium into a chromatography column Fig. 1, where the change of aerosol concen- (4 min I.D. and 5m long) packed with celite tration is plotted against lapse of time after (60~80 mesh) coated with DOP (15%), and ignition. The maximum aerosol concentra- heated from 30- to 140-C at a rate of 4dc tions of both sodium and iodine appeared just /min. before termination of the combustion, which

Table 1 Summary of experimental data

continued for 12~15 min after ignition. It is seen that the aerosol concentrations of both sodium and iodine decrease with the same half-life. The changes of volatile iodine compounds trapped on the activated charcoal and on the copper gauze are also shown in the same figure. It is observed that the iodine trapped on the copper gauze decreases its concentration gradually with lapse of time, but that trapped on activated charcoal remains suspended in the chamber for many hours. After about 1,000 min from termination of combustion, the aerosol had almost disappear- ed in the chamber, leaving only the volatile iodine compounds. The chemical form of the volatile iodine compounds was determined by radio-gaschro- Fig. 1 Behavior of airborne materials released in chamber during so• matography and found to be organic iodides dium pool burning (Run 4) mainly methyl, and some ethyl, n-propyl, -

57 720 J. Nucl. Sci. Technol., n-butyl and methylene forms, as indicated in of formation did not become exceptionally Fig. 2. large in spite of the relatively high concentration of hydrocarbon. From this result, it can be considered that methane present in the air would have no influence on the formation of the organic iodides. It may hence be surmised that the carbon required for forming alkyl radical may already have been present in the original sodium. And, the difference observed in the organic iodide formation between Runs 1 to 6 and Runs 9 to 15 might then be explained by the removal of carbonate contained in the stabilizer of the carrier- Fig. 2 Development of volatile iodine compounds by radio-gaschroma- free iodine (see preceding footnote). tography (Run 4) It is known that sodium, even when purified by cold trapping, contains several kinds of carbon ; elemental, carbide, carbonate, cyanide IV. DISCUSSION and alkyl. Sodium also contains hydrogen in In pool burning of sodium containing radio- atomic, hydryde and alkyl forms. But the iodine, most part of the iodine released into mechanism of methane evolution in sodium the air presents a behavior similar to the by heating is still unknown. Dutina et al.(9) sodium oxide aerosol, while a part remains surmised that the sodium carbonate in sodium in vaporous state, as shown in Fig. 1. The would be transformed at high temperature concentration of the sodium and iodine in into sodium oxide and elemental carbon, aerosol state decreases by gravitational settl- and that reaction between the elemental ing on the floor and adherence to the wall ; carbon and sodium hydride might directly the time required to halve the initial concen- generate methane, although thermodynamic tration was 2570 min. It is known that this considerations would deny strong possibility half-life depends on such factors as the initial of this reaction. Another possible mechanism aerosol concentration and particle size distri- of methane generation may be reaction be- bution, as well as the shape and size of the tween elemental carbon in the sodium and chamber")—"). hydrogen gas which would be formed by re- The life pattern of the airborne iodine action between sodium and water vapor, which trapped in the copper gauze differed somewhat could be favored by the reducing atmosphere from those trapped on the particulate filter prevailing in the zone of sodium combustion. and on the activated charcoal. As indicated Several chemical processes would be asso- in Table 1 the copper gauze did not trap any ciated in a complex manner to the formation matter on some runs. The slowly decreasing of methyl iodide in sodium pool burning, such rate of concentration with time shown by the as the condition of high temperature in the zone of combustion, the reducing atmosphere gauze-trapped iodine (Fig. 1) suggests that this iodine is not in molecular form. due to the strong reducibility of sodium, the The iodine compounds trapped on the thermal decomposition of sodium iodide, the charcoal packs proved mainly to be methyl evolution of methane in or on sodium, reac- iodide as indicated in Fig. 2. From Table 1, tion between methane and iodine (atomic or it is seen that the proportion of organic iodides molecule) and the escape of methyl iodide was 1% or less of the total airborne iodine from zones of high temperature field toward in Runs 9 to 15, while in Runs 1 to 6 it cooler regions induced by its volatility. A averaged about 4%. The influence of hydro- thermodynamical explanation of the forma- carbon on the organic iodide formation was tion of methyl iodide by the chemical reactions examined in Runs 7 and 8. The percentage mentioned above was tried, but no mechanism

58 Vol. 12, No. 11 (Nov. 1975) 721 was found which was plausible from consid- experiment differs in condition from the cases erations of thermoequilibrium. It might be surveyed by Postma at al. in respect of organic noted in this connection that the reactions so iodide formation, the volatile iodine can be far considered are transient, and that the considered to have formed under similar con- possibility of other kinds of reaction, such ditions. as between sodium alkyl and sodium iodide, should further be studied for explaining the ACKNOWLEDGMENT organic iodide formation. The authors are grateful to Dr. E. Tachi- The proportion of organic iodide in air- kawa, Dr. H. Katsuta and Mr. M. Saeki in the borne iodine sampled from actual reactor Japan Atomic Energy Research Institute for containment vessels has been examined by their valuable discussions. The authors also many workers from safety considerations re- express their indebtedness to Mr. T. Ishihara, lative to light water reactors, and the resulting former head of the Nuclear Fuel Research data have been summarized by Postma at al. (10) According to this review Division, JAERI, for his continuous encour- , the proportion agement. is a function of the initial iodine concentration released into the containment. The -REFERENCES proportion of organic iodide observed in the (1) MOREWITZ, H.A., KOONTZ, R.L., NELSON, C.T., present study is compatible with Postma's et al.: Int. Conf. Safety of Rapid Neutron data as seen from Fig. 3. While the present Reactor, Aix. En. provence, (1967). (2) BAURMASH, L., NELSON, C.T., GRANGER, J., et al.: CONF-700816, 373 (1970). (3) MURATA, M., KINANI, S.: J. Nucl. Sci. Tech- nol., 9[10], 622 (1972). (4) NUMAKURA, K., SAEKI, M., TACHIKAWA, E.: ibid., 10[6], 367 (1973). (5) SAEKI, M., NUMAKURA, K., TACHIKAWA, E.: Int. J. Appl. Radiat. Isotop., 25, 407 (1974). (6) KOONTZ, R.L., NELSON, C.T., BAURMASH, L.: SM-110/19, (1968). (7) KOONTZ, R.L., HUBNER, R.S., GREENFIELD, M.A. : AI-AEC-12837, (1969). (8) KITANI, S., MATSUI, H., UNO, S., et al.: J. Nucl. Sci. Technol., 10[9], 566 (1973). (9) DUTINA, D., SimPsoN, J.L., YOUNG, R.S.: ANL- Fig. 3 Organic iodide formation in loss- 7520, Pt. 1, 488 (1968). of-coolant accident of LWR and (10) POSTMA, A.K., ZAVADOSKI, R.W. : WASH-1233, sodium pool burning in LMFBR (1972).