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Experimental study of consistency degradation of different greases in mixed neutron and gamma radiation

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Experimental study of consistency degradation of different greases in mixed neutron and gamma radiation

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Many of the moving components in accelerator and target environments require lubrication. Lubricants in such environments are exposed to high fluxes of secondary radiation, which originates from beam interactions with the target and from beam losses. The secondary radiation is a mix of components, which can include significant fractions of neutrons. Lubricants are radiation-sensitive polymeric materials. The radiation-induced modifications of their structure reduce their service lifetime and impose additional facility maintenance, which is complicated by the environmental radioactivity. The study of the lubricants radiation resistance is therefore necessary for the construction of new generation accelerators and target systems. Nevertheless, data collected in mixed radiation fields are scarce. Nine commercial greases were irradiated at a TRIGA Mark II Research Reactor to serve for the construction of new accelerator projects like the European Spallation Source (ESS) at Lund (Sweden) and Selective Production of Exotic Species (SPES) at Legnaro, (Italy). Mixed neutron and gamma doses ranging from 0.1 MGy to 9.0 MGy were delivered to the greases. For an experimental quantification of their degradation, consistency was measured. Two of the greases remained stable, while the others became fluid. Post-irradiation examinations evidence the cleavage of the polymeric structure as the dominant radiation effect. Dose and fluence limits for the use of each product are presented. Apart from the scientific significance, the results represent an original and useful reference in selecting radiation resistant greases for accelerator and target applications. Keywords: Materials science, Aerospace engineering, Industrial engineering, Nuclear engineering, Materials chemistry, Mechanical engineering, Nuclear reactor irradiation, Polymer degradation, Neutron damage, Lubricating grease, Radiation effect, Mixed radiation field
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Heliyon 5 (2019) e02489
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Heliyon
www.elsevier.com/locate/heliyon
Experimental study of consistency degradation of different greases in
mixed neutron and gamma radiation
Matteo Ferrari a,b,, Aldo Zenoni a,b,, Monika Hartl c, Yongjoong Lee c, Alberto Andrighetto d,
Alberto Monetti d, Andrea Salvini e, Fabio Zelaschi e
aDipartimento di Ingegneria Meccanica e Industriale, Università degli Studi di Brescia, Via Branze 38, I-25123 Brescia, Italy
bIstituto Nazionale di Fisica Nucleare, Via Bassi 6, I-27100 Pavia, Italy
cEuropean Spallation Source, Odarslövsvägen 113 SE-22484 Lund, Sweden
dLaboratori Nazionali di Legnaro dell’INFN, Viale dell’Università 2, I-35020 Legnaro (PD), Italy
eLaboratorio Energia Nucleare Applicata LENA, Università degli Studi di Pavia, Via Aselli 41, I-27100 Pavia, Italy
A R T I C L E I N F O A B S T R A C T
Keywords:
Materials science
Aerospace engineering
Industrial engineering
Nuclear engineering
Materials chemistry
Mechanical engineering
Nuclear reactor irradiation
Polymer degradation
Neutron damage
Lubricating grease
Radiation effect
Mixed radiation field
Many of the moving components in accelerator and target environments require lubrication. Lubricants in such
environments are exposed to high fluxes of secondary radiation, which originates from beam interactions with
the target and from beam losses. The secondary radiation is a mix of components, which can include significant
fractions of neutrons. Lubricants are radiation-sensitive polymeric materials. The radiation-induced modifications
of their structure reduce their service lifetime and impose additional facility maintenance, which is complicated
by the environmental radioactivity. The study of the lubricants radiation resistance is therefore necessary for the
construction of new generation accelerators and target systems. Nevertheless, data collected in mixed radiation
fields are scarce. Nine commercial greases were irradiated at a TRIGA Mark II Research Reactor to serve for
the construction of new accelerator projects like the European Spallation Source (ESS) at Lund (Sweden) and
Selective Production of Exotic Species (SPES) at Legnaro, (Italy). Mixed neutron and gamma doses ranging from
0.1 MGy to 9.0 MGy were delivered to the greases. For an experimental quantification of their degradation,
consistency was measured. Two of the greases remained stable, while the others became fluid. Post-irradiation
examinations evidence the cleavage of the polymeric structure as the dominant radiation effect. Dose and fluence
limits for the use of each product are presented. Apart from the scientific significance, the results represent an
original and useful reference in selecting radiation resistant greases for accelerator and target applications.
1. Introduction
Polymeric materials used in high power accelerator and target envi-
ronments are exposed to mixed radiation fields. Neutrons and gamma
can represent significant components of these fields. The absorbed dose
due to different radiations causes structural modifications of these mate-
rials. Radiations affect polymers mainly through the basic mechanisms
of cleavage and cross-linking of macromolecular chains [1, 2, 3]. There-
fore, significant radiation-induced modifications of their physical and
mechanical properties can be induced, which could lead to a premature
failure of the components.
The European Spallation Source (ESS) project is under construction
in Lund, Sweden, aiming at becoming the brightest neutron source in
the world [4]. The Selective Production of Exotic Species (SPES) facility
*Corresponding authors at: Dipartimento di Ingegneria Meccanica e Industriale, Università degli Studi di Brescia, Via Branze 38, I-25123 Brescia, Italy.
E-mail addresses: matteo.ferrari@unibs.it (M. Ferrari), aldo.zenoni@unibs.it (A. Zenoni).
is being built in Legnaro, Italy aiming at producing intense Radioactive
Ion Beams via fission reactions occurring in its target [5, 6]. Both of
their targets will produce intense mixed fields of neutrons and gamma
radiation. Elastomeric O-rings, lubricating oils and greases are neces-
sarily employed in their target areas. As they are exposed to secondary
neutrons and gammas, their modifications must be evaluated for safe
and reliable facility operations. Unfortunately, radiation damage data
on polymers in neutron fields and in mixed radiation fields are scarce
in the literature.
Most of the available data originate from technical reports produced
by scientific organisations and by companies manufacturing products
certified to be radiation-resistant. The Yellow Reports from CERN are
one of the widely used references concerning the radiation resistance
of polymeric materials. Results on seven types of elastomeric O-rings
https://doi.org/10.1016/j.heliyon.2019.e02489
Received 18 February 2019; Received in revised form 10 April 2019; Accepted 27 August 2019
2405-8440/©2019 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/li-
censes/by-nc-nd/4.0/).
M. Ferrari et al. Heliyon 5 (2019) e02489
available on the CERN store shelves and two types of lubricating oils
irradiated up to several MGy of gamma dose are reported in Ref. [7].
Greases were not analysed.
In an ITER report [8], two commercial elastomeric gaskets and four
greases declared by the producer as radiation-hard were tested up to
high gamma doses from 10 MGy to 100 MGy. A recent work [9]iden-
tifies O-rings, lubricants and cable insulators among the most critical
components in the ITER design. Radiation effects on oils and greases
evaluated in the viewpoint of their chemical composition are reported
in Refs. [1]and [10]. Neutron and gamma irradiation data on twelve
solid lubricants are reported in [11].
The available literature on the radiation effects in polymers is not
satisfactory for the construction of new accelerators and target facili-
ties. In fact, most of the studies were performed more than a decade
ago. Considering the rapid evolution of the industrial product develop-
ment, most of the tested products are now outdated. Furthermore, most
of the literature and data available by manufacturers lack details about
the irradiation conditions such as radiation type, dose rate, presence
of oxygen and linear energy transfer (LET), which influence radiation
effects in polymers [12, 13]. Lastly, most of the studies used gamma
sources, although accelerator and target environments generate mixed
radiation fields. In some of them neutrons represent the dominant com-
ponent.
Gamma sources have been preferred to neutron sources for materi-
als testing because of their wider availability and because the irradiated
samples maintain a much lower residual activation. It has been believed
that the radiation-induced damage depends on the total amount of ab-
sorbed dose only, irrespectively of the type of radiation [7, 14, 15].
Accordingly, the polymers radiation resistance has been expressed in
terms of gamma dose and directly translated to neutron and mixed field
dose. Nevertheless, the equivalence of gamma and neutron doses has
not been theoretically verified nor experimentally validated.
Contrary to the assumed equivalence between gamma and neutron
doses, it is reported in Ref. [1]that the effects of mixed reactor radia-
tion in polymers might differ from that of gamma alone. The data here
presented challenge this long believed equivalence as well. In a UKAEA
report [16], the need for establishing more precisely the relative effects
of similar doses of neutrons and gammas is addressed. In a seminal
study performed at ORNL, the radiation damage induced in elastomers
by reactor radiation is compared with the damage produced by differ-
ent gamma sources. It was reported that similar damages are induced
by doses of the two radiations that can differ of a factor two [17]. In
a more recent work, the understanding of radiation effects in polymers
and their dependence on the radiation typology are clearly stated as one
of the most important problems to be solved [18]. According to Riva-
ton and Arnold, fast neutrons effects on organic materials are not fully
understood despite their relevance in the nuclear engineering and new
investigations are needed [19]. The full comprehension of the effects
of intense neutron fields in polymers is reported as a requirement for
their application in nuclear technology by Bonin et al. [20]. The need
for research on fast neutron effects on polymers is reported by Seguchi
et al. [3]. They report as well the lack of such studies in the literature,
which is dominated by gamma radiation studies.
For the mentioned reasons, radiation damage on polymeric materi-
als has to be further investigated. In particular, mixed radiation data are
necessary for the design and construction of new facilities producing in-
tense neutron fields. We dedicated a previous study to the development
of a methodology for testing elastomeric O-rings irradiated in neutron
and gamma mixed fields [21]. A following study was dedicated to the
experimental determination of the end-of-life conditions of a specific
EPDM O-ring to be installed in a gate valve of the SPES facility. It rep-
resents an example of the design of a component which operates in a
mixed neutron and gamma radiation environment [22]. The manage-
ment of the SPES facility as an intense neutron source is detailed in
[23]. The problem of the radiation resistance of lubricants is briefly
mentioned therein.
In this framework, the present study is dedicated to lubricating
greases. A methodology for greases irradiation in a mixed neutron
and gamma reactor field was developed. Post-irradiation examinations
based on consistency evaluations were completed. The chosen defini-
tion of dose thresholds allowed the selected products to be compara-
tively evaluated.
In Section 2.1 a description of the commercial products selected for
this study can be found as well as their most relevant properties. Seven
of the most important grease producers in the world have been con-
sidered. In Section 2.2 a description of the mixed neutron and gamma
irradiation facility of a TRIGA Mark II nuclear research reactor used
for sample irradiation is reported. In Section 2.3 a description of the
dose components absorbed by the different greases in the neutron and
gamma irradiation mixed field is detailed. The methodology devel-
oped to irradiate and test the grease evolution with dose is reported
in Section 2.4. Consistency is chosen as the most significant indicator of
radiation-induced grease deterioration, as reported in Section 2.5. Re-
sults on nine greases irradiated in a range between 0.1 MGy and 9MGy
of absorbed dose are shown in Section 3. Experimental evidences of
radiation-induced cleavage of the grease structure are reported in Sec-
tion 4.1 whereas considerations on the grease radiation resistance as a
function of their chemical compositions are reported in Section 4.2. In
the discussion section, the greases are compared based on their consis-
tency. Dose and particle fluence thresholds are defined to estimate the
usability of each grease, as reported in Section 4.3. Consistency results
are compared with the radiation resistance declarations of the produc-
ers in Section 4.4. The possibility of using the present results to foresee
the end-of-life grease conditions in specific applications is discussed in
Section 4.5. Conclusions are drawn in Section 5.
2. Product selection and testing methodology
Greases are semi-fluid to solid systems originating from the dis-
persion of a thickener in a liquid oil [1]. A standard grease contains
about 85% base oil, 10% thickener and 5% other additives. Greases
are complex multi-phase systems whose chemical and physical proper-
ties originate from the coexistence and interaction of their components
[24].
The radiation resistance of base oils is reported to predominantly
depend on their aromatic content. A higher content of aromatic struc-
tures is associated to a higher stability of the oil viscosity as a function
of the absorbed dose. For this reason, greases based on aromatic and
polyether oils are considered more radiation resistant than mineral oil
based ones [1][10]. However, the experimental data here presented
challenge this general statement.
The academic literature on the radiation resistance of lubricating
greases is very limited, as reported by R.M. Mortier et al. [24]. For this
reason, in the present paper all the available information provided by
the producers is used as reference too. Radiation effects in greases are
complicated by the interaction of the base oil with the thickener and
the additives and the overall effect is expected to depend not only on
the effects on its bulk components, but on their complex interaction as
well.
Since the available information on the grease radiation resistance is
so limited, a wide and qualified spectrum of products to be tested was
necessary. A market research has been completed to select: i) experi-
enced companies certifying a quality control on the grease production;
ii) greases differing in the chemical compositions of their components;
iii) both greases declared as radiation resistant by the producer and
generic ones; iv) greases having different features: vacuum compatibil-
ity, extreme pressure resistance and extreme duration.
These criteria lead to a selection of high-quality greases suitable for
different applications in accelerator and target environments.
2
M. Ferrari et al. Heliyon 5 (2019) e02489
2.1. Selected lubricating greases
Nine commercial greases were selected from the market (see Ta-
ble 1). Indications about their chemical composition are provided by
the producer and reported in data sheets. Consistency, whose value is
reported as a NLGI class, is one of the most relevant properties charac-
terizing the grease [24].
The nine selected greases and their major properties are listed be-
low:
•AFB-LF is produced by THK (Japan). It is a general-purpose grease
for bearing applications, manufactured using a refined mineral oil
and a Li based thickener. It features extreme pressure resistance
and high mechanical stability.
•Apiezon M is produced by M&I Materials (United Kingdom).1It
is developed for vacuum use and it is declared as radiation resis-
tant up to about 1MGy of dose delivered using a 4MeV electron
beam. Apiezon M is manufactured with a hydrocarbon oil and it
is halogen free. This is an advantage for applications in radia-
tion environment, because halogens can evolve acids. Apiezon M
is manufactured without a thickener. For this reason, it is more
properly referred as a very viscous fluid rather than a grease.
•FAG Arcanol LOAD 220 is produced by Schaeffler (Germany). It is
developed for ball and roller bearings. It is manufactured using a
mineral oil and a mixed thickener. It features high load resistance
and declared durable performance.
• Grizzlygrease No. 1 is produced by Lubricant Consultant GmbH
Lubcon (Germany). It is manufactured using a mineral oil and
a Li/Ca special soap. It is developed for gears application and
declared as resistant to gamma radiation up to a dose level of
1.2 MGy.
•Klüberlub BE 41-542 is produced by Klüber Lubrication (Germany).
It is manufactured using a mineral oil and a special lithium soap.
It is defined as heavy-duty product, developed for high-load rolling
bearings and featuring extreme pressure (EP) resistance.
•Krytox 240 AC is produced by Chemours Company (USA).2Krytox
fluoropolymers are high-performance lubricants initially developed
for the aerospace industry. Now they have a broader range of
applications, from automotive to electronics. Krytox is a perfluo-
ropolyether (PFPE) whose polymer chain is completely saturated
with fluorine. According to the producer declarations, it contains
carbon, oxygen and fluorine only. Krytox 240 AC grease is com-
pounded using polytetrafluoroethylene (PTFE) as thickener. It is
selected in the present work in view of its claimed outstanding
performance and of its hydrogen-free composition. The grease and
its base oil were irradiated using a
60Co gamma source and then
tested by the producer. A progressive grease softening is reported
as a function of the absorbed dose. Reported consistency variations
are generally not exceeding a 10%-15% compared to the unirradi-
ated samples. The radiation stability of the base oil was evaluated
in a nuclear reactor, up to a maximum value of 1MGy.
•Petamo GHY 133 N is produced by Klüber Lubrication. It is manu-
factured using a mineral oil and a polyurea thickener. It is declared
as a long-term and high-temperature grease for rolling bearings.
• RG-42R-1 is produced by MORESCO Corporation (Japan). It is
declared as radiation resistant up to high gamma doses. Tests
are performed by the producer on its base oil only. It is consid-
ered as representative of the whole grease in a simplistic way.
Absorbed dose levels up to 30 MGy are delivered to the oil us-
ing a
60Co gamma source with a dose rate of 0.01 MGy/h. The
viscosity evolution of the oil as a function of the dose is eval-
uated. A dose threshold of 15 MGy is determined accordingly.
1Apiezon®is a registered trademark.
2ChemoursTM and KrytoxTM are trademarks.
Table 1
The lubricating greases selected from the market for irradiation and testing
are listed. The composition of the base oil and of the thickener is specified.
Consistency expressed in NLGI grade is reported. Radiation resistance, when
declared by the producer, is reported. The information originates from the data
sheets of the products provided by the suppliers.
Producer and product Base oil Thick. Cons.
NLGI
Rad-hardness
AFB-LF Mineral Li 2Not declared
Apiezon M Hydrocarbon 1MGy,electr.
FAG Arcanol LOAD 220 Mineral Mixed 1-2 Not declared
Grizzlygrease No.1 Mineral Li/Ca 01.2 MGy, 𝛾
Klüberlub BE 41-542 Mineral Li 2Not declared
Krytox 240 AC PFPE PTFE 21MGy,reactor
RG-42R-1 PPE PC 115 MGy, 𝛾
Petamo GHY 133 N Mineral Polyurea 2Not declared
Turmopolgrease 2 Polyglicol Li 2Not declared
Table 2
Elemental composition of the selected greases in weight percent is obtained
with CHN analyses. The percentage of the mass not measured by the CHN anal-
ysis is reported as well. The measurement errors are contained in the second
decimal figure of the quoted results.
Product C
%
H
%
N
%
Others
%
AFB-LF 82.75 14.13 0.00 3.12
Apiezon M 86.19 13.70 0.11 0.00
FAG Arcanol LOAD 220 80.57 13.15 0.25 6.03
Grizzlygrease No.1 79.23 11.63 0.18 8.96
Klüberlub BE 41-542 82.50 12.29 0.58 4.63
Krytox 240 AC 21.03 0.00 1.01 77.96
Petamo GHY 133 N 84.26 12.42 1.23 2.09
RG-42R-1 78.50 9.67 0.00 11.83
Turmopolgrease 2 62.97 10.71 0.00 26.32
The radiation resistance of the grease is supposed to be equal
to the radiation resistance of the oil by the producer. Most of
the MORESCO radiation resistant products including RG-42R-1 are
manufactured using a polyphenylether base oil and a polycarbon-
ate thickener.
•Turmopolgrease 2 produced by Lubricant Consultant GmbH Lub-
con is a grease manufactured with a polyglicol oil and a lithium
based soap. It features a very long lubrication service compared to
conventional products with the same composition.
The number of greases declared as radiation resistance on the world
market is very limited.
As described in details in Paragraph 2.2, the dose absorbed by
greases in a fast neutron field highly depends on its light element com-
position. In particular, it is proportional to the hydrogen content. For
this reason, the mass fraction of some relevant light elements composing
the greases is measured by CHN chemical analysis. The percent content
of carbon, hydrogen and nitrogen in each product is measured and re-
ported in Table 2.
The hydrogen concentration for the selected products ranges from
0.00% to 14.13%, depending on their chemical composition. Krytox
240 AC is hydrogen-free, as declared by the producer. It is completely
saturated with fluorine, whose amount is about 70% of the total mass.
The hydrogen mass percentage of MORESCO RG-42R-1 product is
9.67%. It is compatible with the chemical composition of its oil, which
is polyphenylether based. The hydrogen mass concentration of min-
eral oil based products ranges from 11.63% and 14.22%. The results
of these analyses are used to simulate the composition of the materials
for dosimetry calculations
3
M. Ferrari et al. Heliyon 5 (2019) e02489
Fig. 1. The in-core Central Thimble irradiation facility of the TRIGA Mark II nuclear reactor of the University of Pavia. Left: a picture of the reactor core. The
aluminum pipe of the Central Thimble, reaching the middle of the reactor core, can be seen. Right: a horizontal cross-section of the reactor core as modelled with
the simulation program MCNP5. The Central Thimble facility is indicated.
Table 3
Irradiation conditions of the Central Thimble facility of the TRIGA
Mark II nuclear reactor of the University of Pavia. Parameters refer to
the nominal working power of 250 kW. Fluxes are calculated using a
reactor model realized with the simulation Monte Carlo code MCNP5.
The fast neutron flux refers to neutrons having energy higher than 0.5
MeV. The gamma flux considers the fission prompt gammas and the
photons from radiative capture of neutrons on the reactor materials.
Since the gammas produced by the fission product decay are not sim-
ulated by MCNP5, the gamma flux is underestimated.
Parameter Value
Neutron flux 1.72 1013 neutron/(cm2s)
Fast neutron flux 3.80 1012 neutron/(cm2s)
Gamma flux 1.65 1013 photon/(cm2s)
Temperature range 50 C–70C
Atmosphere Air at atmospheric pressure
2.2. Irradiation conditions, fluxes and spectra
The TRIGA Mark II nuclear reactor of the University of Pavia, Italy
is equipped with irradiation facilities for research. The Central Thimble
(see Fig. 1) is an in-core facility reaching the centre of the fuel elements.
Radiation components in the Central Thimble originate from sev-
eral reactions occurring in the reactor core. Nuclear fission is the main
source of neutrons and photons. Most of the gammas are produced by
fission reactions and by the decay of radioactive fission products. They
are referred to as prompt gammas and delayed gammas respectively. In
addition, a minor gamma component is due to radiative capture reac-
tions in the materials constituting the reactor [25].
In the present work, radiation fields and dose components are cal-
culated with a simulation of the reactor [26]realized with the Monte
Carlo code MCNP5 [27]. In a previous experimental campaign, the Cen-
tral Thimble neutron spectrum was measured. The calculated neutron
spectrum agrees with the measured one [28]. Accordingly, the calcu-
lated neutron spectrum can be considered a reliable representation of
the measured one. On the contrary, the calculated gamma flux is un-
derestimated because MCNP5 does not account for the fission products
decay. Based on existing measurements on similar TRIGA Mark II reac-
tors, the missing gamma component can be estimated. It can represent
up to 30% of the total gamma component [29]. The systematic error
on the gamma dose is evaluated accordingly. Table 3summarizes the
Central Thimble calculated parameters.
Fig. 2shows the calculated neutron energy spectrum in the Central
Thimble. The spectrum is normalized to the nominal reactor work-
ing power of 250 kW, which is associated to a source intensity of
1.9 1016 n/s. Two main components are present. The fast component
refers here to neutrons with energy higher than 0.5 MeV. Fast neu-
trons originate from fission reactions. Their distribution in the Central
Fig. 2. The neutron energy spectrum in the Central Thimble facility calculated
with MCNP5. The spectrum is normalized to the nominal reactor power of 250
kW. The fast neutron component and the thermal-epithermal one are dominant.
Thimble facility is comparable to a typical fission spectrum. An im-
portant component of slower neutrons in the thermal and epithermal
energy range is present as well. They originate from the moderation
of fast neutrons in the reactor moderator and structures. The neutron
dose absorbed by the irradiated greases depends on the specific neutron
spectrum.
2.3. Dosimetry calculations
The neutron and photon dose components absorbed by each irra-
diated grease are calculated using MCNP5. The grease composition in
the simulation is based on the CHN analyses results (see Table 2). Addi-
tional assumptions are made to model the missing composition fraction.
For Krytox 240 AC, which is a fluorinated material, the missing mass
is assumed to be fluorine (70.4%) and oxygen (7.56%). These assump-
tions are based on the base oil and thickener chemical compositions, as
declared by the producer. For all the other greases, the remaining mass
is assumed to be oxygen. Metallic traces are present in the products in
most cases. However, their relevance in the overall dosimetry calcula-
tion is negligible. As a first approximation, metallic traces are neglected.
The systematic error introduced by these hypotheses in the dosimetry
calculations is estimated by changing the assumed compositions in dif-
ferent ways. However, since the neutron dose is mostly related to the
hydrogen content, whose amount is measured, the error due to the as-
sumptions on the composition is not larger than few percent.
4
M. Ferrari et al. Heliyon 5 (2019) e02489
Fig. 3. Dose rates calculated with MCNP5 for greases irradiated in the Central Thimble facility at the nominal working power of 250 kW. Greases are ordered
from left to right according to the increasing hydrogen content, ranging from 0.00% (Krytox 240 AC) to 14.13% (AFB-LF). The neutron dose rate component is in
dark grey, the gamma one in pale grey. The overall systematic error is estimated to be lower than 15%. The statistical error on the simulated results is negligible
compared to the systematic one.
The dose absorbed in a neutron and gamma mixed field highly
depends on the composition of the irradiated materials and on the ra-
diation energy spectrum. For this reason, the dosimetry considerations
here reported specifically refer to the selected greases irradiated in the
Central Thimble. The calculated total dose rates for the greases are re-
ported in Fig. 3. The dose rate is separated in its two main neutron and
gamma components.
Except for Krytox 240 AC, whose hydrogen content is negligible,
for the other greases the total dose rate ranges from 0.76 MGy/h to
0.94 MGy/h. The difference depends on the neutron contribution, rang-
ing from 0.50 MGy/h to 0.67 MGy/h. It is proportional to the hydrogen
mass content, ranging from 9.67% to 14.13%. The neutron components
dominate, representing 65%-71% of the total. The gamma component
is about 0.26 MGy/h for all the greases. It is roughly irrespective of the
specific material composition.
Because of its hydrogen-free composition, Krytox 240 AC behave dif-
ferently in this neutron field. Its total dose rate is 0.28 MGy/h, about
three times lower compared to the other greases. This is due to the neu-
tron component, which is more than one order of magnitude lower.
The gamma component dominates, representing about 93% of the to-
tal. Products containing halogens are usually not employed in radiation
environments because they can produce acids during irradiation. Never-
theless, fluorinated products are interesting for applications in neutron
fields in view of their low hydrogen content. This is the reason why a
fluorinated product has been selected for the present study.
The neutron dose component is proportional to the hydrogen con-
tent for all the greases. This is because the main energy release mecha-
nism is the elastic scattering of fast neutron on light nuclei contained in
the grease. The average energy transferred to a nucleus in this process
increases when the mass of the target nucleus decreases. For this rea-
son, the process is mostly effective on hydrogen. Moreover, the average
energy transferred via elastic scattering is proportional to the incom-
ing neutron energy. For this reason, the average energy transferred by
fast neutrons is several orders of magnitude higher than the one trans-
ferred by thermal neutrons by the same mechanism[30]. High-energy
secondary protons originating from neutron scattering on hydrogen de-
liver a large fraction of the total dose.
The systematic error on the total absorbed dose is estimated to be
lower than 15%. It depends on the agreement between the measure-
ments and the simulated model. However, the total absorbed dose is
underestimated because of the fraction of the gamma component that
is not accounted for by the code. The statistical error on the simulated
results is negligible compared to the systematic one.
In the present work, the total amount of absorbed dose is chosen
as the parameter to which the grease consistency evolution is referred.
In addition, radiation effects in greases are referred as well to the total
neutron and gamma particle fluence to which the samples are exposed
during irradiation. Table 4shows the dose values absorbed by the irra-
diated greases and the total neutron and gamma fluence. Those values
depend on the particle fluxes in the irradiation facility, as reported in
Table 3and on the exposure time only, irrespective of the grease compo-
sition. In fact the irradiated grease samples do not significantly perturb
the particle fluxes in the facility because of their small size.
2.4. Experimental methodology
The nine greases were irradiated for exposure times ranging between
10 minutes and 10 hours. The most stable greases were selected for 5h
and 10 h long irradiations. The irradiations are realized in the Cen-
tral Thimble facility at the nominal reactor power of 250 kW. Table 4
reports the chosen irradiation times and the corresponding absorbed
doses. Absorbed doses range from about 0.1 MGy to about 9 MGy. The
samples are irradiated in air at atmospheric pressure.
A single irradiation was completed on Krytox 240 AC grease. An
irradiation time of 3h and 30 minutes was selected in view of its fluo-
rinated composition. Additional irradiations were not possible because
of the limited amount of available product, whose commercial price is
extremely high compared to the other ones.
Qualitative preliminary tests were performed on irradiated greases
to design a safe an efficient set-up. One test evaluates the radiation-
induced grease mobility. Small plastic containers were filled with dif-
5
M. Ferrari et al. Heliyon 5 (2019) e02489
Table 4
Doses absorbed by the selected greases in MGy. Corresponding irradiation times in the Central Thimble facility of the
TRIGA Mark II reactor are reported. Krytox 240 AC is not reported here. Since its dose rate in the facility is much lower
compared to the other ones, a single irradiation time of 3hours and 30 minutes was chosen, corresponding to a total
dose of 0.99 MGy. Total neutron and gamma fluence to which the samples are exposed is reported as well as a function
of the irradiation time.
Irradiation time 10 min 30 min 1 h 2 h 5 h 10 h
Product
AFB-LF (MGy) 0.47 0.94 1.88
Apiezon M (MGy) 0.16 0.47 0.94 1.88 4.70
FAG Arcanol LOAD 220 (MGy) 0.15 0.46 0.91 1.83 — —
Grizzlygrease No.1 (MGy) 0.14 0.42 0.85 1.70
Klüberlub BE 41-542 (MGy) 0.15 0.44 0.88 1.75 4.39
Petamo GHY 133 N (MGy) 0.15 0.44 0.89 1.78 4.44 8.89
RG-42R-1 (MGy) 0.38 0.76 1.52 3.80 7.60
Turmopolgrease 2 (MGy) 0.13 0.41 0.81 1.63
Neutron fluence (1016 par cm−2) 1.03 3.1 6.2 12.4 31.0 62.0
Gamma fluence (1016 par cm−2) 0.99 3.0 5.9 11.9 29.7 59.4
Fig. 4. Left: a syringe filled with grease, broken due to the radiation-induced gas
pressure. Right: a section view of the syringe after irradiation. The radiation-
induced grease mobility can be observed. The scattering of the grease inside the
syringe volume is due to the production of gas bubbles.
ferent amounts of grease ranging from 20% to 80% of the total volume.
An important radiation-induced gas evolution is observed, promoting a
high grease mobility in the containers (see Fig. 4). The developed pres-
sure due to the gas production in the set-up can reach critical levels,
sometimes damaging and breaking the container.
Radioactive nuclides are produced in the irradiated greases due to
neutron capture reactions. Because of their residual activation, irradi-
ated greases must be handled by qualified operators according to the
existing radiation protection regulation. Seven days after irradiation
the contact dose rate ranges between 0.1 μSv/h and 0.3 μSv/h for all
the greases. This is comparable with the natural radiation background,
therefore the grease samples can be safely tested after irradiation.
Based on the preliminary tests, a 20 mL disposable plastic syringe is
chosen as irradiation set-up. A maximum of 10 mL of grease is irradi-
ated, to avoid critical gas evolution and excessive mobility. The grease
is distributed as a layer on the inner syringe surface. The syringe is
placed in a cylindrical aluminum container routinely used for sample
irradiation. Fig. 5shows the set-up.
2.5. Consistency tests
Consistency is widely considered as the most important property of
a lubricating grease and marks the difference between liquid oils and
greases [24]. The presence of the thickener gives the grease a solid
character and a rigidity commonly referred to as consistency. Consis-
Fig. 5. The set-up used to irradiate grease samples in the Central Thimble facil-
ity. A disposable plastic 20 mL syringe is filled with about 10 mL of grease. The
syringe is placed in an aluminum container.
tency is measured for both irradiated and unirradiated grease samples
according to ASTM D217-02 and ASTM D1403-02 standards. Consis-
tency is therein defined as the degree of resistance to movement under
stress. It is measured with a penetrometer equipped with a cone whose
tip penetrates the flat grease surface (see Fig. 6). The cone is free to
fall into the grease under gravity for 5seconds. Consistency measures
the cone penetration in tenths of millimeters. A semi-automatic preci-
sion penetrometer by S.D.M. Apparecchi Scientifici s.r.l. equipped with
a one-quarter scale cone is used in this work. The one-quarter scale
equipment as described in ASTM D1403 allows a consistency test to be
completed on a sample of about 5 mL only. This is an advantage con-
sidering the limited available space in the irradiation facility and the
need to reduce as much as possible the amount of produced radioactive
waste.
According to the standard, the grease must be worked using a ma-
nipulator before being tested (see Fig. 7). The grease is subjected to
60 full double strokes in one minute. This manipulation is necessary
to perform a worked penetration test. The consistency measured after
manipulation is referred to as worked consistency.
The National Lubricating Grease Institute (NLGI) classifies greases
based on their worked consistency. NLGI grades are universally ac-
cepted and used by producers and final users. NLGI grades range from
000 to 6. Grade 000 indicates a soft and almost fluid material con-
dition, corresponding to high penetrations (445-475 mm/10). Grade 6
indicates a hard solid-like material condition, corresponding to low pen-
etrations (85-115 mm/10). For the greases selected in the present study,
a 10% consistency variation approximately corresponds to a variation
of one grade.
6
M. Ferrari et al. Heliyon 5 (2019) e02489
Fig. 6. The semi-automatic penetrometer by S.D.M. Srl used for consistency measurements. Right: the penetration cone positioned over the flat surface of the grease,
before the measurement. The one-quarter scale equipment is used.
Fig. 7. The one-quarter scale equipment used to manipulate the grease be-
fore the measurement of consistency. The grease is subjected to 60 full double
strokes in about one minute.
The one-quarter scale equipment used in this work can measure
NLGI grades ranging from 0 to 4. As reported. Measured values lower
than 0 in NLGI class will be here referred to as out-of-scale. Measured
values higher than 4 were not observed.
3. Results
Consistency variations of the irradiated samples relative to the unir-
radiated ones are reported as a function of the total absorbed dose (see
Fig. 8). The relative consistency values are calculated as follows:
𝐶𝑟𝑒𝑙(𝐷)= 𝐶(𝐷)
𝐶0
(1)
C(D) is the consistency value (in mm/10) of the grease irradiated at
a total dose D. 𝐶0is the consistency value (in mm/10) of the unir-
radiated grease. The error associated to 𝐶𝑟𝑒𝑙(𝐷)is estimated to be
lower than 5%, basing on repeated measurements on unirradiated sam-
ples.
Six out of the nine tested greases experience severe radiation-
induced consistency increase as a function of the dose. They become
almost fluid between 0.4 MGy and 5MGy. The condition and appear-
ance of these grease samples after irradiation are completely different
from the unirradiated ones. The consistency increase is remarkable, ex-
ceeding the range of the instrument. These exceeding values are anyway
used in Equation 1to estimate a relative consistency variation. They are
associated to a fluid grease state and they are in the present paper re-
ferred to as out-of-scale values. They are marked by dashed lines in
Fig. 8.
Post-irradiation consistency results can be analysed with reference
to their percentage deviation from their unirradiated samples consis-
tency, recalling that a 10% consistency variation is approximately asso-
ciated to a change in NLGI grade.
Three groups of products can be distinguished according to their
consistency modification with absorbed dose:
1. Huge consistency increases are reported for AFB-LF, Grizzlygrease
No.1, Krytox 240 AC and Turmopolgrease 2 at dose values lower
than 1MGy.
AFB-LF consistency increases of 25% at 0.94 MGy. It becomes
fluid at 1.9 MGy, showing a 46% increase in consistency.
Grizzlygrease No.1 consistency increases of 25% at 0.4 MGy.
This value is out-of-scale and the grease becomes fluid
(see Fig. 9). The grease remains steadily fluid up to about
1.7 MGy.
Krytox 240 AC becomes fluid at 0.97 MGy, showing a 45% con-
sistency increase. Acid gases evolved during irradiation
due to the fluorine content led to the corrosion of the
aluminum container (see Fig. 10).
Turmopolgrease 2 consistency increases of 37% at 0.8 MGy. It
becomes fluid at 1.6 MGy of dose, showing a 60% con-
sistency increase.
2. Apiezon M, FAG Arcanol LOAD 220 and Klüberlub BE 41-542 re-
main rather stable up to 1.0 MGy. Their consistency remarkably
increases between 1.5 MGy and 5MGy.
FAG Arcanol LOAD 220 is stable up to 0.9 MGy. It abruptly be-
comes fluid at 1.8 MGy, showing a 40% consistency in-
crease.
Klüberlub BE 41-542 exhibits a progressive consistency increase
as a function of the dose. Consistency value increases
of 7% at 0.9 MGy and of 10% at 1.8 MGy of dose. It
becomes fluid at 4.4 MGy, showing a 30% consistency
increase.
7
M. Ferrari et al. Heliyon 5 (2019) e02489
Fig. 8. Relative consistency value 𝐶𝑟𝑒𝑙 (𝐷)as a function of the total absorbed dose for the nine selected greases. Dashed lines indicate consistency values exceeding
the maximum instrument range, corresponding to an almost fluid material state. High consistency values are associated to softer greases, which correspond to lower
NLGI consistency grades.
Fig. 9. Post-irradiation consistency test of Grizzlygrease No.1. Top left: grease
irradiated at 0.15 MGy. Consistency is comparable to the unirradiated one. Bot-
tom left: Grizzlygrease No.1 irradiated at 0.45 MGy. A remarkable consistency
increase can be observed. The measured consistency value is out of the instru-
ment scale. The irradiated grease becomes fluid. Right: the grease irradiated at
0.45 MGy after penetration measurement. The grease drips from the penetrom-
eter tip.
Apiezon M does not exhibit significant consistency variations up
to 1.9 MGy. It abruptly becomes fluid at 4.7 MGy, show-
ing a 70% consistency increase.
3. Petamo GHY 133 Nand RG-42R-1 do not exhibit significant con-
sistency variations up to the maximum investigated dose values of
8.9 MGy and 7.6 MGy respectively.
The colour of some greases is darkened by irradiation. It is particu-
larly evident for Petamo GHY 133 N (see Fig. 11), whose colour turns
from yellowish (unirradiated sample) to black at about 0.4 MGy.
Fig. 10. Left: Krytox 240 AC sample irradiated at 0.97 MGy (3 h and 30 min).
Right: the aluminum set-up used for irradiation. The stopper has been corroded
by the acid gases evolved.
4. Discussion
4.1. Cleavage observation in the grease structure
Greases are complex multi-phase systems. For this reason, the con-
sistency evolution with dose is expected to be complex too. Radiation
effects in greases result from the interaction of several damage mecha-
nisms. Two main effects characterize the interaction of radiation with
polymers at the structural level: cleavage and cross-linking of the poly-
meric chains. As reported by R.O. Bolt and J.G. Carrol [1], the first
radiation effect expected in greases is the damage of the gelling struc-
ture of the thickener, causing a consistency increase. Radiation can
induce fractures in the soap fibres, which become unable to maintain
a gelling structure. At higher dose levels, a consistency decrease is ex-
pected because of the increased viscosity of its base oil due to polymeric
chains cross-linking. The former behaviour was observed for most of the
investigated products, whereas the latter was not observed in the con-
sidered irradiation conditions.
According to the literature, the presence of aromatic groups in the
base oil should determine the radiation-resistance of a grease. However,
the consistency results don’t allow the grease chemical structure and its
radiation resistance to be easily correlated. The collected data confirm
the complexity of the radiation interaction with multi-phase systems.
8
M. Ferrari et al. Heliyon 5 (2019) e02489
Fig. 11. Petamo GHY 133 N grease samples. From left to right: an unirradiated sample, a sample irradiated at 0.15 MGy (10 min irradiation) and a sample irradiated
at 0.44 MGy (30 min irradiation). A progressive grease darkening can be observed, from yellowish to completely black. Samples irradiated at higher dose levels
have the same colour of the one irradiated at 0.44 MGy.
The tested greases feature either a stable consistency or a con-
sistency increase with dose. Seven out of the nine irradiated greases
become fluid and drip from the testing instrument under gravity (See
Fig. 9). By definition, a grease should remain in place as a solid body
under gravity [24]. Grease fluidization is an indicator of severe mate-
rial damage, probably related to a complete damage of the thickener
structure. The thickener is a radiation-sensitive component and for this
reason it is relevant in determining the overall radiation-resistance of
the grease.
Apiezon M behaviour is discussed separately because it is manufac-
tured without a thickener. For this reason, its fluidization at 4.7 MGy is
not related to the thickener disruption and might be related to the oil
structure cleavage.
The qualitative preliminary tests provide indirect information on the
grease radiation effects too. Grease mobility is highly promoted by ir-
radiation for all the tested products. The intense gas evolution could
originate from the cleavage of the polymeric chains. The gas evolved
during irradiation leads to the formation of bubbles, which modify the
distribution of the grease in the set-up. Acid gases are evolved by Krytox
240 AC because of its fluorinated composition.
In conclusion, cleavage dominates as damage mechanism for the
greases here analyzed. Further understanding of the mechanisms of
radiation interaction at the structural scale is not possible based on con-
sistency results only. Greases are complicated systems, whose chemical
and physical characteristics cannot be easily modelled. The lack of aca-
demic base research into the grease fundamentals evidences the need
for further investigation [24].
4.2. Correlation between radiation effects and chemical composition
According to the literature, the grease radiation stability should
be predominantly determined by its base oil chemical composition.
For example, polyphenylether based products are reported to be more
radiation-resistant than mineral oil based ones [1, 10]. The outstand-
ing stability of RG-42R-1 complies with this consideration. However,
there are reasons to believe that radiation stability depends on a more
complex interaction between the chemical nature of the base oil, the
thickener and the additives. This is supported by some of the results
here reported.
Five of the tested greases (AFB-LF, FAG Arcanol LOAD 220, Griz-
zlygrease No.1, Klüberlub BE 41-542 and Petamo GHY 133 N) are
manufactured with mineral oils but different thickeners. Despite the
similar composition of their base oil, they show very different be-
haviour, maybe thanks to the presence of different thickeners. Petamo
GHY 133 N has a stable consistency up to about 9MGy, whereas the
others become fluid below 5MGy.
Petamo GHY 133 N and RG-42R-1 have a comparable stability with
dose, despite their different chemical composition. The former is real-
Table 5
Thresholds for the tested greases associated to the functional endpoint defini-
tion, corresponding to a 10% variation from the original consistency. Thresh-
olds are determined using a linear interpolation. When variations are lower
than 10% for all the tested dose values, consistency variation at the maximum
dose is reported.
Product Dose
MGy
Cons. var
%
Fluence (n)
par cm−2
Fluence (𝛾)
par cm−2
Turmopolgrease 2 0.1 +10% 7.6 1015 7.3 1015
Krytox 240 AC 0.2 +10% 4.1 1016 4.0 1016
Grizzlygrease No. 1 0.3 +10% 2.2 1016 2.1 1016
AFB-LF 0.4 +10% 2.7 1016 2.6 1016
FAG Arcanol LOAD 220 1.1 +10% 7.5 1016 7.2 1016
Klüberlub BE 41-542 1.8 +10 % 1.2 1017 1.2 1017
Apiezon M 2.3 +10% 1.5 1017 1.5 1017
RG-42R-1 7.6 +5% 6.2 1017 5.9 1017
Petamo GHY 133 N 8.9 3% 6.2 1017 5.9 1017
ized with a mineral oil and a polyurea thickener, while the latter is
realized with polyphenylether oil and a polycarbonate thickener.
Therefore, it is concluded that the chemical composition of the oil
alone is not representative of the grease radiation sensitivity. Specific
additives are expected to influence radiation stability as well. For those
reasons, general conclusions based on the chemical composition can be
hardly drawn and specific products must be individually tested.
4.3. Thresholds of radiation damage
The concept of radiation damage threshold in greases is scarcely de-
fined. However, it is of utmost importance in the engineering viewpoint
and for end-users like the ESS and the SPES facilities. In the present
work, two endpoints are defined to evaluate the radiation-resistance of
the tested products.
A 10% deviation from the non-irradiated consistency is considered
in the present study as a functional endpoint. Consistency can be ex-
pressed as well in NLGI grades, ranging from 000 (soft) to 6 (hard). The
NLGI grade is a relevant parameter for the grease specific application.
A deviation of one grade from the original consistency grade approx-
imately corresponds to a 10% consistency variation. Table 5contains
the thresholds associated to this functional endpoint. Thresholds are
expressed in terms of total absorbed dose and in terms of exposure to
the total neutron and gamma fluence in the mixed irradiation field of
the facility.
A severe structural endpoint is associated to the radiation-induced
fluidization of the grease. It corresponds to consistency values being
out of the maximum penetrometer range. This endpoint is evidenced
by dashed lines in Fig. 8and marks an end of life condition for the
irradiated grease.
The defined endpoints provide useful dose and particle fluence
thresholds to compare the products radiation-resistance. Petamo GHY
9
M. Ferrari et al. Heliyon 5 (2019) e02489
133 N and RG-42R-1 are identified as the most radiation resistant prod-
ucts comparatively. In fact, their consistency deviation from the original
value is lower than 10% for all the investigated dose values. For these
products, the consistency variation at the maximum absorbed dose is
reported in Table 5. Fluorinated products, despite the lower amount
of neutron dose absorbed in comparison with hydrogenated ones, are
not recommended for application in radioactive environments. In fact,
the acid gases evolved during irradiation could damage the surrounding
metallic components.
4.4. Comparison with producers’ declarations
Consistency results obtained in the present work can be compared
with the radiation resistance declarations of the producers. The results
achieved on Grizzlygrease No.1 are particularly intriguing. Its consis-
tency abruptly increases at 0.4 MGy of mixed neutron and gamma dose.
This value is much lower than the 1.2 MGy gamma dose threshold de-
clared by the producer. The equivalence of the effects of gamma dose
and neutron dose in polymers has been longly believed. However, the
differences reported for Grizzlygrease No.1 could be attributed to the
different irradiation condition adopted in the tests, in particular the use
of pure gamma or mixed radiation fields.
In addition, it is interesting to note that the two most stable greases
have different producer declarations concerning the radiation resis-
tance. RG-42R-1 is declared by the producer as radiation resistant up
to 15 MGy of gamma dose, while Petamo GHY 133 N has no radiation
resistance declaration.
It can be concluded that the products lacking a radiation resistance
declaration do not necessarily feature scarce radiation resistance. In
fact, only a limited number of companies in the world tested their
products with radiation. As here reported, some greases featuring excel-
lent performance but not tested with radiation show excellent radiation
stability. On the contrary, products having a declared dose threshold
do not necessarily have a comparable radiation resistance when irradi-
ated at same doses in different radiation fields. This evidence supports
the need for further testing on specific products, especially when it is
needed to know their radiation resistance in specific irradiation condi-
tions. The producer’s declaration based on gamma irradiation cannot
be considered as satisfactory for the use of a product in a different radi-
ation field.
4.5. Predictive ability of the results
In the present study, greases are irradiated in a neutron and gamma
mixed reactor field. The chosen irradiation conditions are significant for
applications in facilities that will produce similar radiation fields, as the
SPES and the ESS systems. However, the radiation fields in operation
will differ from the one used in the present study by several parameters.
Firstly, the dose rate expected in operation will be in general orders of
magnitude lower than the one used for testing in the reactor. Tests in
the reactor are necessarily accelerated. Greases have been irradiated in
reactor up to several MGy of total dose, corresponding to neutron and
gamma fluences of the order of magnitude of 1017 particle cm−2. Similar
exposures will be cumulated in much longer operation times, ranging
from weeks to decades in the SPES and in the ESS facilities. Testing
conditions can not easily replicate so long exposure times, especially in
case of extensive experimental campaigns which count several products
irradiated at various dose levels. To complete irradiation in a reason-
able time, quicker radiation damage tests, referred to as accelerated,
are necessary.
Moreover, greases are tested in air atmosphere at atmospheric pres-
sure, while in some applications they will work in vacuum or in absence
of oxygen. The temperature in the irradiation facility is higher that the
temperature expected for most applications. The expected neutron spec-
trum in operation will differ from the reactor one. Neutrons with higher
energy will be produced by spallation neutron targets and by facilities
for the production of radioactive ion beams.
Damage mechanisms in polymers are expected to depend on several
parameters as the radiation fields, the dose rate, the oxygen diffusion
in the material and the temperature, in a synergistic way. For this rea-
son, the present results can not be used to exactly predict the expected
grease behaviour in specific operating conditions. Nevertheless, the con-
sistency behaviour of several different commercial greases irradiated in
the same conditions in a neutron and gamma mixed field over a total
absorbed dose of about 10 MGy are here reported. The set of collected
data is unique in the present literature.
5. Summary and conclusions
The present work approaches the problem of radiation effects on
lubricating greases from an experimental viewpoint. A methodology
for grease irradiation in a neutron and gamma mixed field and post-
irradiation examination is developed. Nine commercially available
products are selected from some of the most important producers in the
world. Grease samples are irradiated in an in-core facility of a research
nuclear reactor at absorbed doses ranging between 0.1 MGy and 9MGy.
Dosimetry in the neutron and gamma mixed field is calculated. Consis-
tency is tested after irradiation as the most significant functional pa-
rameter characterizing a grease. Extreme radiation-induced consistency
modifications are observed for seven out of the nine tested products,
leading to the grease fluidization. Two out of the nine tested products
exhibit stable consistency as a function of the dose.
It can be concluded that the cleavage of the grease structure is the
dominant process in the chosen irradiation conditions. This is evidenced
by the grease fluidization and by the evolution of gas, which depends
on the thickener disruption and on the production of fragments respec-
tively.
Some of the greases having base oils with a similar chemical com-
position show very different radiation effects. For this reason, it is not
possible to directly correlate the chemical structure of greases with their
radiation resistance. The present study contributes to provide experi-
mental evidences of cleavage as the dominant process induced by radi-
ation on the grease structure up to the tested values of absorbed dose.