I
Shell Development Co., Emeryville, Calif.
The Effect of Nuclear Radiation on Jet Fuels Auxiliary hydrocarbon fuels in a nuclear-powered aircraft must be stable at various radiation dosages. Here are some of the preliminary studies on fuels available at this time. The optimum fuel for these applications will be developed from such data
Om
configuration for a nuclear powered aircraft includes conventional hydrocarbon-burning engines to provide auxiliary power for take-off and high speed performance. Fuels for such service should have high radiation tolerance as well as good thermal stability. The effects of irradiation with electrons, y-rays, and, in part, neutrons, on the properties of several fuels, separated fractions, and model compounds have been determined. The changes in physical and chemical properties and in some cases thermal stability have been noted, and attempts have been made to account for these on the basis of the reactivity of hydrocarbon types present. As it is thought that hydrocarbons react by free radical and ion molecule mechanisms, the effect of addition of aromatic compounds, which could act as acceptors for such reactive moities, has been assessed. On the basis of the present, admittedly incomplete, investigation and barring a nonlinear dosage effect, it is not difficult to obtain fuels that can satisfactorily withstand 10%of gamma radiation. From the standpoint of gamma radiation stability and thermal stability, saturates appear to be the best choice as auxiliary fuels for nuclear-powered aircraft. Naphthenes constitute a preferred class of saturates, because of the inherent radiation stability of the ring C-C bonds (4, 9) and the possibility of strain-relieving cis-trans isomerization-for example, in the Decalins. Naphthenes also possess high heat values
on a volume basis, and although their smoke points are much lower than alkanes, they are far superior to aromatics (5). Aromatics, as they occur in jet fuels, have not been characterized by high thermal stabilities as measured in the heat exchanger test. Aromatics by themselves possess superior radiation stability but, because of their marked radical scavenging properties, they are frequently highly reactive in dilute mixtures with saturates found in jet fuels. However, the effects of such mixtures on thermal stability have not been ascertained. The apparently satisfactory situation with respect to gamma radiation does not hold for neutron irradiations. The failure of three jet fuels at comparatively low dosages despite chemical and physical changes of only one tenth the order of magnitude of the gamma-irradiated samples remains unexplained. As this could be a function of total dosage, a detailed study of the effects of dosage on radiation stability will be presented in a subsequent paper. Experimental
Radiation Sources. Gamma irradiations were used. This is not generally regarded as important, because the gross similarity in the types of changes induced by neutrons, x-rays, and electrons has been demonstrated, particularly by Charlesby (2) and Collins and Calkins ( 3 ) . The latter have summarized the
basis for the equal energy-equal damage concept. The samples were irradiated by Phillips Petroleum Co. at Idaho Falls using spent fuel elements and a nominal dose of 1 X 10% in most experiments. For large samples (1400 ml.) an approximate correction for attenuation due to the sample thickness and the 0.216inch thick aluminum canister gave a calculated dose of 8.5 X IO'r. The bulk samples were irradiated in air. Smaller (20-ml.) samples were irradiated under both nitrogen and air, in 1ounce glass vials, with four tiers of samples to the canister and seven samples per tier. No attenuation correction was attempted in this case. Three jet fuels (40-gallon samples) were also irradiated in the Convair G T R reactor, in 55-gallon steel drums in an air environment. Nominal dose was 5 to 7 X 10% y plus 3 to 4 X IOl4 thermal neutrons per sq. cm. Materials. The fuels included an East Coast paraffinic kerosine (99), a JP-4 fuel (107) from San Joaquin crude, the saturates of which are predominantly one-ring naphthenes, and a JP-5 fuel (104) from Los Angeles Basin crude, also predominantly naphthenic but more polycyclic in nature. The 107 saturates were obtained chromatographically. The Decalin was purifed by distillation and chromatography. I t showed no impurities by GLC and contained 61% cis, 39% trans. The aromatics used for radiation protection studies were C.P. grade and included benzene, naphthaVOL. 52, NO. 1
*
JANUARY 1960
47
~
~~~~
These Materials Were Investigated in Greatest Detail Boiling Range,
Fuel 99 104
107
107 saturates Decalin
O
F.
3 94-49 1 365-498 171-464 171-464 366-3 82
%S 0.002 0.33 0.081
None None
lene, biphenyl, phenanthrene, anthracene, quinoline, 8-quinolinol, 4-methyl2,G-di-tert-butylphenol (Ionol), toluidine, and indole. Procedure. Carbon, hydrogen, sulfur, density, gross heat of combustion, bromine number, and maleic anhydride values (MAV) were determined by conventional methods before and after irradiation. Infrared and ultraviolet absorption spectra were similarly compared. Thermal stability, which involves high temperature oxidation stability and is perhaps the most important single property evaluated, was measured by the Shell high-temperature stability test. This involves heating the airsaturated fuel to 450' F. in an aluminum coil and measuring the filter-plugging tendency over a 5-micron stainless steel sintered filter at 40 ml. per minute fuel flow rate under recycle conditions. Deposits on the preheater coil and filter are weighed to give additional information on thermal stability (8). This property could be determined only for the 1400-ml. and larger samples. Thermal stability of the 40-gallon fuel samples was also evaluated in the CFR coker test ( 7 ) . Oxidation stability was measured by determining gum by the Chromatogum procedure (6) on samples aged 1 week at 70' C. under 1-atm. 0 2 .
Discussion of Results Effects of Gamma Irradiation. Some properties of the jet fuels before and after irradiation to 108r are given in Table I. At this level of irradiation a maximum of about 5 to 10% of the molecules could undergo a primary reaction, assuming that one bond broke for each 28 e.v. of energy introduced and that the average molecular weight is about 200. Accordingly, one would not expect gross changes in properties that can be affected directly by radiation. This is borne out by the data. Carbon content is up slightly (on a weight basis), hydrogen content is slightly down in general, and sulfur and heat of combustion do not change. The bromine number increases substantially, indicating that dehydrogenation is important. A concomitant increase in density occurs. When Decalin and the saturates isolated chromatographically from 107, were irradiated the changes were of the same order of magnitude and in the
48
Conon. of Component Types, Vol. % NaphParaffins thenes Olefins Aromatics 65.0 34.0 Trace 1 24.8 19.9 23.6
55.6 64.1 76.4 100
0
18.5 15 0 0
1.1 1.0
0
0
same direction as before. However, at 10%-drastic changes in Decalin occur: The loss of hydrogen is 12-fold greater than at 10% and oxidation is significant, as shown by increase in peroxides and lack of closure of C H. Bromine number and maleic anhydride value also increase markedly. The magnitude of MAV indicates that a substantial proportion of the olefins formed are being reattacked by the radiation to form dienes. As shown in Figure 1, the oxidation stabilities are in line with composition changes-i.e., formation of olefins and dienes, evidenced by bromine number, MAV, and infrared observations. Soluble gum is formed (20 to 50 mg. per dl.) but no insoluble gum. Unirradiated samples similarly aged formed less than 2 mg. per dl. of soluble gum. The irradiation atmosphere does not affect soluble gum content significantly. These results agree with previous research on the relation between gas oil composition and oxidation stability (in the absence of irradiation), which showed that mixtures conzisting only of olefins and saturates formed soluble gum exclusively.
+
Table
I.
Only one fuel suffered damage at 10*r as judged by the thermal stability test-JP-4 fuel 107. Both JP-5 and kerosine improved substantially with respect to filter plugging. Because 107 contains more aromatics and sulfur compounds than 99, its greater susceptibility might be attributed to these factors, except that 104 is also high in the same components. JP-4 differs from both the other fuels in possessing a greater proportion of components boiling below 365' F.; this may be the reason for its poorer stability, but it has not been established. The rate of filter deposition was reduced by irradiation only in the case of kerosine. Effects of Neutron Irradiation. Exposure of a fuel to neutrons inevitably also involves exposure to major gamma irradiation, because of the mixed nature of the radiation generated by a nuclear reactor. In this case the gamma dosage amounts to about 90% of the total radiation, which was 5 to 7 X 106r gamma and 3 to 4 X 10l4 neutrons per sq. cm. Changes in chemical and physical properties are in line with the lower radiation level involved, which is equivalent (on an energy basis) to about 8 X 10%- gamma. The JP-4 fuel is apparently the only one to lose appreciable hydrogen (Table 11). The sulfur content has decreased somewhat in every case. If these losses are a direct effect of radiation (doubtful at this low dosage), they could he thought to arise from the reaction S32(n,p) P32. This cannot be the case, because fast neutrons (2 m.e.v. or greater) are required for this reaction
108r Gamma Radiation Does Not Effect Gross Changes Properties (Idaho Falls, aerobic conditions)
Fuel
C, TVt.
__
H, wt. %
Nu loz2 - , L4toms/ cc.
Wt. %
d '2
G./100 G. < I
Br No., S,
99control Irrad.
85.27 85.31
14.74 14.70
6.92 6.90
0.002 0.002
0.7841 0.7864
104 control
86.28 86.31
13.35 13.35
6.66 6.69
0.33 0.322
0.8333 0.8378
10
86.12 86.33
13.85 13.54
6.56 6.57
0.081 0.092
0.7917 0.8108
2 7
Irrad. 107control Irrad.
Table II.
7
Gross Heat of Combustion, Cal./G. 11,194 11,170
4
Effect of Neutron Irradiation on Fuel Properties
(SPT No. 2, Convair) T h e outstanding chemical change is increase in peroxide Fuel 99 control SPT" 104 control
SPT 107 control SPT
INDUSTRIALAND ENGINEERINGCHEMISTRY
a
Wt. %
diol G./Cc.
Heat of Combustion, Cal./G.
1.23
0.002 0.001
0.7843 0.7848
11,181 d= 19 11,192 t 3
13.36 13.36
0.05 0.21
0.334 0.312
0.8334 0.8335
13.81 13.75
0.02 0.99
0.082 0.076
0.7909 0.7957
C, Wt. %
H. Xt. %
85.21 85.16
14.74 14.74
86.29 86.29 86.09 86.11
Peroxide, Meq./lOOG.
