I
BARRY 1. TARMY, WILLIAM BARTOK, and PETER J. LUCCHESI Esso Research and Engineering Co., Linden, N. J .
The Methane Dosimeter This chemical device can quickly and accurately measure the radiation absorption of vapor phase hydrocarbon systems
for energy absorption measurements fall into three main categories-calorimetric, electrical, and chemical. Descriptions of techniques in each categor); are available in the literature ( 4 ) . The first two of these classes of techniques. while of a more basic nature. include devices that are either difficult to use. time-consuming, or not suitable for use in chemical reactors. Generally, chemical techniques are more practical, and many dosimeters of this type have already been developed (4). They are especial1:- useful because they can evaluate directly the extent and nature of chemical reactions. Dosimeters are based on the principle that the absorption of a given amount of radiation energy results in a specific yield of an easily measured product. However, most of these dosimeters involve liquid phase systems-for example. that based on the oxidation of the iron(I1) ion lironjI1)-iron(II1) dosimeter]. \Vhile excellent for many iiscs. they cannot accurately measure the energy absorbed in vapor phase chemical reactions where radiation energy can be deposited within the reactor vessel by secondary electrons coming from its solid walls or lost to the walls because of the lesser absorptive capacity of the less dense medium. While vapor phase dosimeters have been developed, they are considered too difficult to use because of complex analytical problems (5). .4nother disadvantage of the chemical dosimeters presently employed is that they act differently under different types of radiation. For example, aqueous dosimeters [e.g. iron (11)-iron (I II), cerium( I I) -cerium(I I I) ] have greater yields from fast neutrons than are obtained from a n equivalent amount of gammaray energy ( 6 ) . Thus, it is advantageous to carry out dosimetry in the same container used in the chemical study with a substance of the same density and composed of the METHODS
~
For measurement of mixed gammaneutron irradiation of hydrocarbon systems this method i s especially useful. In less complex irradiation systems (such as those involving gamma or electron radiation), dosimetry methods have been developed with accuracies in the range of 5 to 10%. Except for a few special circumstances, however, this has not been the case for dosimetry in nuclear reactors. This situation is far more complex because of the variety of interactions with matter for the whole energy spectra of neutrons and gamma photons. Not only are there interactions with the reaction vessel and its contents but also with the graphite, uranium, and other materials located nearby within the nuclear reactor. These effects can result in severe spatial and time-dependent changes within the system. One of the major purposes of dosimetry i s the evaluation of the extent and nature of chemical reactions, Since the various radiation species react b y different mechanisms, it i s advantageous to carry out the dosimetry in the same container used in the chemical study with a substance of the same density and composed of the same elements. This i s especially advantageous when attempting to correct for such different mechanisms b y calculation. One of the main techniques used i s the calculation of energy absorption from ionization chamber data and neutron flux measurements. There are many such techniques of varying degrees of complexity presented in the literature. All, however, are relatively inaccurate because of their use of unperturbed flux measurements and the inherent inaccuracies of their approximations.
same elements. This report describes a chemical dosimetry technique that satisfies these conditions. The method is based on the measurement of the yield of hydrogen produced by the radiolysis of methane. Lampe (70) has shown that a t low conversions the same radiation yield of hydrogen (5.7 molecules per 100 e.v.) is produced by 1.7-m.e.v. electrons as that reported by Lind ( 7 7 ) using 6-m.e.v. alpha particles. Furthermore. the 100e.v. yield is independent of pressure and radiation intensity. As the amount of reaction will depend only upon the total radiation dose. the dose rate for very low conversions will be related to the hydrogen production and the time of irradiation by
R
=
1.06 X 1010 H/B
where R is the dose rate in rads per hour, is the irradiation time in hours, and H is the mole fraction of hydrogen measured in the products of irradiation. Measurements were made at dose rates ranging from 0.1 to 28.6 megarads per hour using gamma, electron, and nuclear reactor radiation sources. The gamma radiation results agreed with independent iron( I I ) -iron (I I I) measurements within 5%. For the mixed neutron-gamma radiation from a nuclear reactor, satisfactory agreement to about 25% was found with independent calorimetric measurements. The technique has been found to be particularly useful in the case of electron irradiation in which the radiation dose depends on the electron density of the system. The method was found to be accurate and rapid in both batch and flow operations. This dosimetric technique may be used over a very wide range of radiation and experimental conditions. I t offers unique advantages in measuring the actual energy absorbed in hydrocarbon gases undergoing radiolysis directly in the reaction vessel. I t can be also used in
e
VOL. 53, NO. 2
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147
CHROMATOGRAPHY COLUMN 13X MOLECULAR SIEVE
THERMAL CONDUCTIVITY DETECTOR CELL
! I
t Es:
*
1
S A M P L E COIL
WAY FLUSH IN G VALVE
SAMPLE CHARGE CARRIER VACUUM
VENT
MANOMETER
FOUR WAY SAMPLING VALVE
A back flushing system was used in the dosimeter assembly to remove heavier components quickly from the gas chromatographic column Adsorbent-1 3 X Molecular Sieve Column-1 0 feet of 0.25-inch tubing Carrier gas-nitrogen Carrier gas pressure-920 rnm. Hg Operating temperature-35' C. Sample charge-20 cc. at 700 mm. Hg Thermal conductivity detector-4.1 volts (reverse polarity)
heterogeneous radiation chemistry-for example, in the study of catalytic reactions of hydrocarbon gases, where chemical dosimeters based on polymerization reactions (acetylene polymerization) are inadequate.
employed in the pneumatic tube of the nuclear reactor. For these systems, the accuracy of measurements was checked by alternate dosimetry methods. For thc cobalt-60 source, the iron(I1)iron(II1) dosimetry was used as standard, while a specially designed calorimeter (7) was used as an independent check on the measurements obtained in the pneumatic tube. This calorimeter consisted of a vacuum-jacketed quartz cell with an open capillary extension. I t operated in the same manner as a maximum reading thermometer. The other measurements were made to study dosimetry in flow systems: a 120-cc. vessel was used for the electron irradiation of hydrocarbons at high pressure and temperature, while the flow system for in-pile studies was a 500-cc. tubular vessel located in the nuclear reactor. Because of the local heat generation. the temperature for the experiments in the nuclear reactor was controlled by means of vaporizing liquid nitrogen.
Experimental
Radiation Sources and Dosimeter Systems. Four radiation types were used in this study: a cobalt-60 gamma source of 2200 curies: electrons from a 2.1-m.e.v. van de Graaf generator, and the mixed neutron-gamma radiation in two different locations of differing intensities within the 24-megawatt Brookhaven graphite-moderated nuclear reactor. The systems have been described in the literature (7, 2, 8 ) . The studirs in the cobalt-60 gamma source and in a pneumatic type system of the nuclear reactor were done in static experiments: 600-cc. stainless steel vessels were used in the cobalt-60 gamma studies, and 10-cc. cylindrical bombs made of 61s aluminum were Table I. HP
Concn., hlole 70 0.11 0.09 0.07 0.05 0.03 0.00
148
Calibration of Vapor Phase Chromatograph Was M a d e with the Same Standard Sample Ratio HP to CHa Peaks, H2 Peak Height, M m . Hz/CHI X 1/160 1st day 134 98 84 53 28 -5
2nd day
141 105 92 60 34
...
3rd day
1st day
2nd day
...
6.64 4.85 4.16 2.62 1.38 0.25
6.87 4.92 4.34 2.82 1.61
...
92 57 33
...
INDUSTRIAL AND ENGINEERING CHEMISTRY
...
3rdday
... ...
4.16 2.56 1.47
...
Determination of Hydrogen Concentration. The usefulness of this technique depends on the ease of measuring the concentration of hydrogen. Because low conversion levels are desirable, the hydrogen produced must be measured in concentrations as low as 0.01 mole %. This was accomplished using a gas chromatographic column composed of 13X Molecular Sieve adsorbent in a Perkin-Elmer Model 154 Vapor Fractometer. This column gave good resolution between air (the reference peak), hydrogen, methane, and higher hydrocarbons. Because of the good resolution, however, the desorption of methane takes about 10 minutes and higher hydrocarbons much longer. A back flushing system (shown in the flow diagram a t left) \vas therefore installed for quickly removing the heavier components. I n this ivay, the column can be cleared in about 10 minutes, and the hydrogen concentration is measured in about 20 minutes. Calibration of Fractometer. The principles of gas chromatography are well coverted in the literature ( 9 ) . The desorption of the adsorbent in the chromatographic column results in the formation of a characteristic voltagetime curve in which the peaks represent the components of the mixture and the peak heights correspond to their concentrarions. A typical curve is shotvn (below). Because the output voltage of the detector had to be calibrated to determine the concentration of hydrogen in large amounts of methane. samples prepared by mixing kno\vn quantities of prepurified hydrogen gas \vith 283 liters of Matheson research grade (99.6Yc pure), hydrogen-free methane were run as standards without further purification. Sitrogen, ethane, and propane \\'ere the impurities in the methane, which contained no unsaturates. NO AMPLIFICATION METHANE
16X AMPLIFICATION
HYDROGEN
AIR
-TIME
-
Typical chromatograph pattern shows peaks representing mixture components. Peak heights correspond to their concentrations
M E T H A N E DOSIMETER Table II.
Methane Dosimeter Results Are Accurate for Gamma Radiations from 2200-Curie Co60Source
Pressure.
Irradiation Time,
Temp., O c'.
.Itin.
Hr.
24 24 24 24 24 54 54
1
63.6 63.6 63.6 41.2 41.1 45.8 17.8
1
2.4 2.4 35 39 59
T h e measurement of lo\v hydrogen concentration was quite sensitive to small changes in operating conditions, so that the calibrations of the output voltage of the thermal conductivity detector were checked repeatedly. Calibrations xvere made either using the hydrogen peak itself as a direct measure of the hydrogen concentration, or alternately using the ratio of the hydrogen peak to the methane peak. T h e first method is faster, while the second one is less subject to external effects. Both methods were used in this study. T h e calibration table (Table 1:) shoivs readings on different days, Lvith the same standard samples.
Results Gamma Radiation Dosimetry. T h e dosimetry technique could be best evaluated by means of cobalt-60 radiation. T h e actual radiation dosage had been determined with the greatest accuracy (*.57c) previously ( 7 ) for the system, using iron(I1)-iron(II1) dosimetry and independent calculations based on the known strength of the cobalt source and the geometry of the system. T h e results for a series of seven experiments at dose rates of 0.10 and 0.76 megarad per hour and ranging in temperatures from 24" to 55" C., pressures from 1 to 39 arm., and irradiation times from 17.8 to 63.6 hours are given in Table 11. T h e standard deviation of the results with the methane dosimeter was 5'3--i.e.: Lvithin the accuracy of the iron (I I ) -iron (I I I) dosimeter. As expected, no measurable differences were observed as a result of changes in pressure, temperature, or time of irradiation. For paraffins of higher molecular weight than methane, radiation initiates a free-radical chain reaction a t temperatures higher than about 180' C. while initiating a relatively temperatureindependent nonchain process below this temperature level (72). I n such systems, the radiation yields increase a t higher temperatures and decrease with increasing radiation intensities. Thus, the upper limit of the temperature range for
H? (~oIlcil. Mole %
0.062 0.060 0.060 0.042 0.296 0.344 0.131
Dose Rate, AIegaradiHr. Measured Actual 0.10 0.10 0.10 0.11 0.76 0.79 0.78
0.10 0.10 0.10
0.10 0.78 0.78 0.78
the operation of the methane dosimeter may be influenced by the same factors. T o study this effect, experiments \Yere carried out at 204" C. using a dose rate of 0.10 megarad per hour. About 50% more hydrogen was produced than would be predicted from the radiation yields at low temperatures (Table 111). Control measurements made under identical conditions, but without radiation, shoived no hydrogen production. Consequently! there was no thermal contribution to the reaction. Furthermore, mass spectrometric analyses of the gas, while unable to yield a quantitative picture of the product distribution, did show that such unsaturates as ethylene and propylene were produced in addition to the saturated paraffins obtained at lower temperatures. Thus. at these temperatures the higher paraffins produced by radiation ma): in turn crack via a radiation-induced reaction. Mixed Neutron-Gamma Radiation Dosimetry. Xuclear reactors present by far the greatest difficulties in dosime:ry. This problem is directly due to the diversity of radiation types and intensities and to their numerous interactions. Frequently it is difficult to determine the radiation dose even within a factor of 2. T h e methane dosimeter was used to measure the energy absorption in a pneumatic tube within the Brookhaven nuclear reactor. This sample position had a neutron flux at full power of 3.9 X 10l2 neutrons'sq. cm.-second and a cadmium ratio of 240. T h e pneumatic system was used because it presents a method of both rapidly introducing and recovering a sample from the reactor and allows the use of the quartz calorimeter in the same location. A dose rate of 3.9 + 0.2 megarads per hour was measured \vith the calorimeter, within the 95y0 confidence limits. IVith the methane dosimeter the energy absorption rate in the same location \vas measured to be 3.1 k 0.6 megarads per hour, differing by about 257, from the mean calorimetric value. These results are summarized in Tables I V and V. There are several possible reasons for
Table Ill. Radiation Yields of Hydrogen Are Higher than Expected at High Temperatures Radiation source: Co60 gamma source Dose rate: 0.10 megaradlhr. Temperature: 2 0 4 ' C.
Presjure,
Irradiation
l-inIe,
.Itm. 39 11 11
Hr. 24 48 65
H? Concn.. AIole % .Ictual Predicted 0.034 0.084 0.097
0.021 0.047 0.061
the discrepancy. First, results for the independent development of the calorimeter showed it to be accurate within about 15%. Secondly. the geometry of the calorimeter and the absorptivity of its material of construction (quartz) differ from those of the aluminum bombs used for methane dosimetr). Consequently, the radiation spectrum may be somewhat different in the two cases and hence give different results. This, of course, would be avoided by measuring dose rates bvith methane in the chemical reaction vessel itself. Dosimetry measurements !Yere also made in a flow system located within the nuclear reactor in a position inaccessible to measurement by other techniques. In this location the thermal neutron flux at full power was 4.8 X lo1? neutrons' sq. cm.-second with a cadmium ratio of only 3.8. T h e experiments were performed progressively as the nuclear reactor was shut down by a series of successive reductions in the power output of the reactor. T h e results show that the dose rate \vas directly proportional to the operating polver of the reactor (page 150). These results could not be predicted by means of other methods ( 3 , 4 ) available for estimating radiation dosage in graphite-moderated nuclear reactors. Such methods are inadequate because
Table IV. Calorimetric Method Was Used to Check Dosimetry in Pneumatic Tube of Nuclear Reactor Quartz calorimeter using a-methylnaphthalene medium Temperature: 30" C.
Do-e Iiradiation Time, JIin.
8 8 6
5
Temp. Rise, O C. 2.00 1.93 1.48 1.29
Rate, lIegarad/Hr. 3.9 3.8 3.9 4.0
M e a n = 3.9 Standard deviation = 0.10
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Table VI. Good Precision Was Also Obtained with Methane Dosimeter for Electron Irradiation van d e G r a a f voltage: 2.1 m.e.v. Temperature: 1 2 1 C.
Irradiation Current, Time, pa. Min.
Collector
Dose
Pressure 60 60 60 125 125 125 200 200 200
Rate,
H2 Concn., Megarad/ XIole % Hr.
25.1 25.1 25.1 9.4 9.5 9.5 9.6 9.6 9.5
= 21
atm.
0.041 0.043 0.046 0.025 0.025 0.025 0.040 0.040 0.040
10.4 10.8 11.6 16.7 16.6 16.6 25.0 25.0 24.6
Pressure = 42 atm. 200 200 200
Nuclear radiation intensity decreased linearly with decreasing reactor power
they disregard transient effects as well as the effect of other materials in the environment and the differences in the neutron energy operation. For example, one method ( 3 ) predicts that the dose rate in rads per hour is approximately four times the square root of the thermal neutron flux expressed in neutrons per square centimeter-second. For the pneumatic tube, this would yield a dose rate of 8 megarads per hour, or twice the actual value, while for the flow unit a dose rate of 9 megarads per hour or one quarter of the actual rate a t full power would be calculated. The large discrepancy in the latter case is caused by the increased gamma radiation generated by the absorption of neutrons in the stainless steel walls of the vessel and in the insulating structure surrounding it. Furthermore, these steady state calculation methods predict that on lowering the power level the energy absorption rate should be proportional to the square root of the power level, when actually the dose rate decreased directly with the decreasing power. These results again emphasize the importance of performing the measurements in the same reaction vessel. Electron Irradiation Dosimetry. The difficulty in measuring the energy absorption of electrons is primarily due to the fact that they do not penetrate matter easily. This is especially true in the gas phase where the amount of absorption will depend to a great extent on the electron density of the gas and the geometry of the system. In addition,
1 50
as gases must be contained, some of the energy must be expended in penetrating the wail. Thus, only a portion of the energy of the electron beam is actually absorbed in the radiation zone. Consequently, measuring the total electron current a t the operating. voltage is not sufficient. The methane dosimeter, however, can give accurate results by operating a t such a pressure that the electron density is the same as that of the system to be studied. Then, the electron current can be calibrated for these conditions in terms of energy absorption within the system. As shown in Table VI, excellent reproducibility was obtained using this approach. The variations of electron density directly affect
Table V. Comparable Results Were Obtained with Methane Dosimeter in Pneumatic Tube of Nuclear Reactor Temperature: 30’ C. Ir-
Dose
radiation HZPeak Time, Ratio
INDUSTRIAL AND ENGINEERING CHEMISTRY
Hr.
X lo3
0.5 0.5 1.0 1.0 1.0 1.0 1.0
2.5 3.5 9.8 10.0 6.7 7.5 7.0
Rate H2 Concn., Megarad/ Mole % Hr. 0.014 0.016 0.032 0.033 0.025 0.027 0.026
Mean Standard deviation
3.0 3.4 3.4 3.5 2.7 2.9 2.8 = 3. I = 0.15
9.5 9.5 9.5
0.043 0.043 0.043
27.8 27.8 27.8
the dose rate with electron irradiations, For example, increasing the density by doubling the pressure from 21 to 42 atm. resulted in a 10% increase in dose rate from 25.0 to 27.8 megarads per hour. Acknowledgment
The authors acknowledge the assistance of James. L. Carter in performing the experiments and express their appreciation to Esso Research and Engineering Co. for permitting the publication of this work. Literature Cited (1) Black. J. F.. Kunc, J. F., Clark, G. B.. Intern. J . Appl. Radiation and Isotopes 1,
256 (1956). (2) Brookhaven National Laboratories, Upton, N. Y . , “Research Reactor
Facilitv-Irradiation Services and Radio:sotopes,” 1956. (3) Calkins, V. P., Chem. Eric. Progr. Sqmposium Ser. 50, No. 12, 28 (1954). (4) Hine, G. J., Brownwell, G. L., eds., “Radiation Dosimetry,” Academic Press, New York. 1956. (3) Ibid., p. 361. ( 6 ) Ibid.. p. 408. (7) Houston, R. W.. Industrial Reactor Laboratory, Inc., Plainsboro, N. J.. private communication, Oct. 24, 1960. (8) Kaplan, I., “Nuclear Physics,” Addison-Wesley Publishing - Co., . Cambridge, Mass., 1955. (9) Keulemans, A. I. M., “Gas Chromatoarauhv.” 2nd ed., Reinhold. New York: 1959: (10) Lamue. F. W.. J . Am. Chem. SOC.79, ‘ 1055 ( l j 5 7 ) . (11) Lind, S. C., Bardwell. D. C., Perry, J. H., Ibid., 48, 1556 (1926). (12) Lucchesi, P. J., Tarmy, B. L., Long, R . B.. Baeder, D. L., Longwell, J. P., IND.ENG.CHEM.50, 879 (1958). I
RECEIVED for review March 10, 1960 ’ACCEPTED November 14, 1960
