Ethylene as a dosimeter at high pressures, temperatures and dose rates

IntcmationalJoumal ofApplied Radiationand Isotopes, 1966,Vol. 17,pp.64=47. PegamonPnss Ltd. Printed inNorthern Ireland

Ethylene as a Dosimeter at High Pressures, Temperatures and Dose Rates S. SRINIVASAN

and W. E. SMITH

Socony Mobil Oil Company, Inc., Central Research Division Laboratory, Princeton, New Jersey (Received 7 April 1966)

Primary hydrogen production from the radiolysis of ethylene has been used successfully as a dosimeter in the beam tube of a 5 MW nuclear reactor. C.P. grade ethylene, with an initial hydrogen content of 0.07%, is as satisfactory as research grade ethylene. G-value for hydrogen production f&,) is 1.2 f O.l-independent of pressures up to 200 pig and temperatures up to 120°C. P+st work has shown that Gas is also independent of dose rate in the region investigated. L’ETI-IYLENE

COMM-E DOSIMETRE AUX TENSIONS, ET TAUX DE DOSES ELEVES

TEMPERATURES

On a employt avec succb la production primaire de l’hydrogkne par la radiolyse de l’tthyltne comme dosim&re dans le tuyau de faisceau d’un rkacteur nu&aire de 5 MW. L’Cthyl*ne de qualit& C.P. ayant un contenu initial d’hydrogtne de 0,07x scrt tout au& bien que l’tthyltie de qualit@ ‘recherche”. La valeur G pour la production de l’hydrog&ne (Gz%) est 1,2 f O,l-indbpendante de la pression jusqu’lr 200 livres par pouce carrt selon jauge (14 kg/cma) et de la temperature jusqu’a 120°C. Le travail prtalable a montrC que Gap est indtpendant aussi du taux de dose dans le region recherchbe. 3THJIEH ICAK AOBHMETP l-IPll BbICOKHX aABJIEHBHX, TEMIIEPATYPAX H HOPMAX fiO3BPOBKH ChIbITIiOeII~OH8BO~CTBO BOAOpOAa H8 p~HOJIH%3aoTUJIeHa 6nno yCIleIIIH0&iCl?OJIb8OBaHOB HarIeCTBe AO8HMeTpa B KaHaJIe WIK BbIIIyCKa IIyYKa KgepHOrO peaKTOpa B 6 MBT. %fMHPeCKH ¶HCTbI8 aTHJIeH C IIepBOHaWJIbHbIM COAepXMHHeM BOAOpOR;a 0,07 IlpOJ& TaKJKe yAOBJ-leTBOPHTeJIeH, KaK EI HCCJleAOBaTeJIbCKHt COPT. Beaa=niaa G AJlfi lTpOH8BOACTBa BOAOpOAa (~,)COCTaBJIReT1,2 f O,1-He8aBHCHMOOTAaBJIeHElfBlIJIOTbAO 200 @yHTOB Ha KB~paTHblZt AIOtiM H TernepaTyp A0 120°C. Hpomnm pa6oTa nosaaana, PT~ CII, TaKxe rie ~~KECHT OT M~UIH~CTAA~~~IBM~~JI~A~~M~~~ o6sacTxi.

ATI-zYLEN

as

DOSISMESSER

BEI HOHEN DRUCKEN, DOSISLEISTUNGEN

TEMPERATuREN

UND

Pritirer Wasserstoff produziert durch Radiolyse von &hylen ist mit Erfolg als ein Dosismesser ih dem Strahlrohr eines 5 MW Kernreaktors verwendet worden. Eine handelsiibliche Qualitit von Athylen mit einem Anfangsgehalt an Wasserstoff von 0,070x ist genau so wirksam wir Athylen von einer Qua&&t fiir Forschungsarbeiten. Der G-Wert fiir die Wasserstofferzeugung (Gs,) betragt 1,2 f 0,l und ist unabhtigig von Driicken bis zu 14 ata und Temperaturen bis zu 120°C. Frtihere Arbeiten haben gezeigt dass Gal such von der Dosisleistung in dem untersuchten Bereich unabhangig ist.

1. INTRODUCTION THE purpose of this study was to determine radiation dose rates in our chemical reactor located at various positions along the axis of the beam tube in a 5 MW nuclear reactor. The design of the equipment permitted only con-

tinuous flow experiments. Hence, in searching for a dosimzter, our foremost criteria were that it should be operable in a flow experiment and amenable to rapid analysis. Our other requirements were that the dosimeter should have a high useful range, should be independent 643

644

S. Srinivasan and W. E. Smith

of dose rate and conversion (up to a reasonable degree), should be free of any radiation problems due to neutron activation, and should not react with the materials the equipment is made of. Unfortunately, many of the widely known chemical dosimeters do not meet all these requirements. The high-intensity mixed-radiation fields of a nuclear reactor dwarf the useful range of liquid dosimeters like Fricke’s and sodium formate, not to speak of the neutron activation. The cerous-ceric dosimeter is unsuitable because it is both limited in its range and is attacked by metals (e.g. ceric sulfate reacts with aluminum). Although calorimetric measurements can be done, they require elaborate experimental set-ups to insulate the equipment from the high y-heat of the surroundings. A gaseous dosimeter such as nitrous oxide has been used with success in nuclear reactors;(l*s) but the activation of nitrogen makes it very hard to handle in a continuous flow loop like ours where the irradiated gases return to the operating area shortly after admission. Moreover, the product analysis has to be done by using vacuum techniques and has been reported to take 20-30 min. We wanted to have a dosimeter that was easier to handle and faster and simpler to analyze than nitrous oxide. We then decided to try the ethylene dosimeter since it seemed to meet all our criteria. However, its adaptation to reactors posed some problems which have not hitherto been studied and these will be the subject of this paper. But first, a summary of what is available on the ethylene dosimeter in the literature seems appropriate. Radiolysis of ethylene has been studied quite extensively in the recent past(“ll). Hydrogen and acetylene are formed as primary unimolecular dissociation products. G-values for their are known to be formation (Gu, and Gc& 1.2 and 2.4 respectively, independent of dose ratew.7.10.11) conversion(@ and of pressure above 150 iorr.c7) The maximum pressure studied is 1000 torrt7) and almost all past work has been done around room temperature. On the basis of these studies, it has been suggested that ethylene can be used as a dosimeter by measuring either the hydrogen or the acetylene formed during radiolysis.(e) CsH, *

C,H,

f

Hs $1.

(1)

Since Gus is independent of conversion and since hydrogen is the principal impurity in C.P. grade ethylene, it appears that C.P. grade ethylene without further purification can be used as a dosimeter in place of the more expensive research grade ethylene. Such a replacement can be of much benefit in places like our nuclear reactor process loop where large quantities of ethylene have to be used. However, there are certain operational limitations to the adaptation of ethylene as a gas-phase dosimeter in beam tube reactors. In our own chemical reactor, the two most significant are :

(4

(b)

The necessity to flow ethylene through at high pressu.res (of the order of 200 psig) to get residence times long enough for measurable hydrogen production. The high temperatures encountered in the beam tube, depending on the dose rate position, in spite of full supply of cooling water.

The study was initiated to test the usability of ethylene under conditions listed above. C.P. grade ethylene was used in all experiments. The pressure employed was 200 psig and the temperature in the beam tube varied from 48 to 165°C. To help interpret the data, a series of ethylene radiolysis experiments with Co60 as the irradiation source were undertaken. Again, C.P. grade ethylene was used in these experiments. The pressure varied from 1000 torr to 200 psig, and the temperature from 25 to 120°C. In addition, several runs were made by using research grade ethylene at 1000 torr and 25°C The purpose was to with Coso gammas. reproduce some of the experiments reported in the literature and also to compare against the corresponding Gu, values in runs with C.P. grade ethylene. 2. EXPERIMENTAL 2.1 Nuclear reactor experiments The chemical reactor loop is housed in a beam tube of the 5 MW nuclear reactor at the Industrial Reactor Laboratories, Plainsboro, New Jersey. A prototype of the equipment used has been described elsewhere.02) The nuclear reactor itself is a swimming pool type with Usss MTR fuel elements. The spectra of

645

Ethykw as a absimeturat high @mum, temjeratures and dose rates prompt neutrons and y-rays can be taken very nearly as those of U s3s fission. At the maximum dose rate position of our reactor, the thermal and fast neutron fluxes are about loll n cm-s se& each. Matheson C.P. grade ethylene without any further purification was used in all experiments. The principal impurity was hydrogen (0.07 per cent in our samples). Ethylene was flown once through the reactor at 200 psig and on exit the gas passed through a trap at -80°C to condense any water and then through a chromatograph sampling valve. Steady-state temperature of the gas varied from 48 to 165°C depending on the dose rate position. The chromatograph was a Perkin-Elmer Model 154 fitted with a silica gel Argon was the carrier gas. The column. chromatograph was calibrated with samples of known hydrogen content; the accuracy of our hydrogen measurements was within & 1%. This analysis took only about 5 min and vacuum techniques were not necessary. 2.2 Coao ex@rimnts These experiments were conducted using about 4 x lo3 c of Coso to initiate the reaction. The container was a stainless-steel cylinder, 4.5 cm inside diameter x 33 cm long x 3 mm wall thickness and with a working volume of 510 ml. The exterior of the vessel was wound with electrical strip heaters and insulated with asbestos to minimize the heat losses. Temperatures were measured at the heating surface and in the gas phase by means of iron-constantan thermocouple probes. Temperature control was within f2”C. At the start of each run, the vessel was evacuated to pressures less than 1 torr and then filled with ethylene gas. Matheson C.P. grade ethylene was used. Operating conditions varied from 1000 torr, 25°C to 200 psig, 120%. Whenever deviations from the ideal gas law were considerable, moles of ethylene were calculated by using compressibility and fugacity data. At 1000 torr and 25°C several runs were also made using research grade ethylene. All other conditions, however, remained the same as the rest of the Coso experiments. Hydrogen was analyzed in all these experiments with the same equipment used in the reactor experiments.

3. RESULTS AND DISCUSSIONS The results of Coso y-radiolysis experiments with C.P. grade ethylene are shown in Table 1. Data with research grade ethylene are also shown in the same table with daggers. For the geometry of our irradiation vessel, the approximate equation (2)(13) for the energy loss of Compton electrons holds at all gas densities used. g = uNZB AX

(2)

where E = total energyoftheComptonelectron; x = linear path (cm) ; a = constant; N = number of atoms/cm3; Z = atomic number, and B = “stopping number”. As seen from Table 1, at 1000 torr and 25”C, GE, values obtained with C.P. grade ethylene (0.07 % H, present as impurity) are the same as with research grade ethylene-l .2 f 0.1. This is in accordance with the observation of YANG and GANT(~)that GH, in ethylene is independent of conversion. Their maximum conversion was 10 per cent in terms of ethylene disappearance and in our work the conversion ranged from 1.5 to 10 per cent. The energy absorbed by hydrogen at this level is so small that the decomposition of hydrogen into hydrogen atoms is insignificant. Besides the primary source of molecular hydrogen described by (I), a second one to consider is the recombination of atomic hydrogen which is also formed during radiolysis according to (3). W-b

+v+ C&f,

+ 2H qh

(3)

Although +I M Cs, atomic hydrogen combines very readily with ethylene to form ethyl radicals : H + C&HI + CsHs.

(4)

The rate constant for this reaction has been estimated byYAr&ti) to be 1.4 x 1O1lmole-1 ems se+ and the activation energy to be approximately 1 kcal rnole-l.(la) The high specific reaction rate of (4) makes the recombination of hydrogen atoms insignificant at our dose rates. Any contribution to the recombination of hydrogen atoms at high pressure by the third

646

S. Stinivasan and W. E. Smith TABLE 1. CoW ethylene exposures*

Pressure (psig)

Temperature (“C)

115 165 215 215 115 64.7 33.1 19.3 115 115 64.7 115 165 215 65 115 165 215 18.9 19.2 19.0 19.2 19.5 19.5

25 25 25 25 25 25 25 25 25 25 65 65 65 65 120 120 120 115 25 62 120 25 25 25

Exposure time (hr)

% Hydrogen produced (x 108)

69.5 93.9 89.3 40.5 43.3 43.8 40.7 21.6 41.9 19.3 19.2 22.0 16.8 17.2 17.0 19.2 17.1 23.0 25.3 17.0 17.0 18.8 16.0 15.9

19 27 28 12 11 10 10 4.6 10 5.0 4.8 6.1 4.6 4.6 4.6 4.9 4.6 6.0 7.4 4.9 4.8 5.1 4.3 4.3

% Ethylene consumed [G( -C&H,) = 451 7.3 9.8 9.4 4.2 4.5 4.5 4.2 2.2 4.3 2.0 22.2” 1:7 1.8 1.7 2.0 1.7 2.3 2.6 1.7 1.7 1.9 1.6 1.6

GHfl (&I) 1.1 1.1 1.2 1.2 1.1 1.0 1.1 0.9 1.1 1.1 1.1 1.2 1.2 1.2 1.2 1.1 1.1 1.1 1.3 1.2 1.2 1.2t 1.2t 1.2t

* Average dose rate 4.8 x 10” eV/g hr. Allexperiments were run with C.P. grade ethylene except t which were run with research grade material.

body effect is negligible. The third body in our case is ethylene itself. Even at 200 psig calculations show that for this recombination to be significant the hydrogen atom concentration should be in excess of lo17 atom per cm3 whereas the steady-state concentration of hydrogen atoms in our system is less than 10’ atom per cm3 at the highest dose rate. It is now clear that all the hydrogen we observe comes from (1). This is also substantiated by the fact that we observe constant Gn, over the entire temperature range studied. 3.1 LET e$ects Gamma ray and fast neutron spectra in our beam tube can be taken as equivalent to the corresponding Usss fission spectra. No direct experiment was done to determine the effect of LET on GHI. However, we conclude that there is no significant LET effect present in our studies for the following reasons:

(i) in our nuclear reactor experiments, the dose rate contribution to ethylene by the spectrum of fast neutrons was calculated according to the data of HURST et al.(17) At the maximum dose

rate

position, the fast neutron flux is for this position, a pessimistic calculation, assuming that all fast neutrons are

lo11 n cmW2 set-l;

of 10 MeV energy, gives a fast neutron dose rate of 5 x 10ls eV g-l see-l. This is a mere 3 per cent of the dose rate 1.6 x lOl* eV g-l set-l measured there. ii in our Coao y-experiments, the constancy of(G)n, over an eleven-fold change in ethylene density shows, although indirectly, that LET effects are negligible (Table 1). The dose rates at various positions in the beam tube reactor calculated from the results of this study ranged from 2.2 x 1017 to 1.6 x lOl* eV g-l se+, and the dose rate profile is shown in Fig. 1. The accuracy of our beam tube dose rate measurements is f 10 per cent.

Ethylene as a dosimeter at high pressures, t@eratures REACTOR POWER 5MWt 1

I ,

1

and dose rates

647

comes from the primary unimolecular decomposition step; the amount of molecular hydrogen produced by recombination steps is negligible. Acknowledgments-The authors express their appreciation to Professor F. W. LAMPE, Dr. J. R. WHITE and Mr. J. S. HICKS for their interest in the work and for their many helpful discussions.

REFERENCES

DISTANCE

FROM CORE

FACE. CMS

FIG.1 4.

CONCLUSION

The results of this study indicate that ethylene can be successfully used as a reliable dosimeter in the beam tubes of nuclear reactors. Grr, is 1.2 f 0. I-independent of conversion, pressure and temperature in the region investigated (hydrogen production up to 0.3 per cent, pressures up to 200 psig and temperatures up to 120%). It is also independent of dose rate. Such a dosimeter can use C.P. grade ethylene without any further purification and is simpler and faster than such other gaseous dosimeters as nitrous oxide. All the molecular hydrogen

S. Proceedings of the International Confcrcncc 1. DONDES on the Peace31 Uses of Atomic Energy 14,176 ( 1955). 2. HARTECK P. and DONDESS. NuIeonics 14, (3) 66 (1956). 3. FIEELDF. H., FRANKLINJ. L. and LNUPE F. W. J. Am. &em. sot. 79,2419 (1957). 4. LAMPEF. W. Rudiut. Res. 10,691 (1959). 5. YANG K. and MANNO P. J. J. phys. Chena., Ithaca 63, 752 (1959). 6. YANG K. and GANT P. L. J. jhys. Chem., Ithaca 65, 1861 (1961). 7. SAUERM. C. and DORFMANL. M. J.phys. Chem., Ithaca 66, 322 (1962). 8. AUSLOOSP. and &RDEN R. J. them. Phys. 36, 5 (1962). 9. ME~SELSG. G. and SWORSKIT. J. J. jhys. Chem., Ithaca 69,815 (1965). 10. MEISEL~G. G. J. Am. them. Sot. 87,950 (1965). 11. MEISELSG. G. J. them. Phys. 4!2,3237 (1965). W. E. 12. GUERNSEY E. O., SMW H. and Sm A.Z.Ch.E. Jl9, 744 (1963). 13. HEITLERW. Quantum i%ory of Radiation, 3rd edn., pp. 368-370. oxford University Press, London (1954). 14. SAUERM. C. and D~RPMAN L. M. J. them. Phys. 35, 497 (1961). 15. YANGK. J. Am. them. sot. 84, 719 (1962). 16. LAMPEF. W. Private communication (1965). R. H. and MILLS W. A. 17. HURSTG. S., RITCEIIE Proceedings of the First International Conference on the Peace@ Uses of Atomic Energy Geneva* Vol. 14, p. 220. United Nations, (1955).