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Radiation Damage to PlasticsShawn RobinsonColorado School of MinesNuclear Materials

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IntroductionRadiation damage is an important issue in the design and maintenance of nuclear

facilities. Nuclear radiation such as alpha, beta, gamma, and neutron each have their unique

effect on materials. Plastics known as polymers show drastic changes in material properties after

irradiation. The polymer now acts as a brittle material with significantly reduced strength.

Polymers do not have an arranged lattice structure like metals. The material structure of

polymers is damaged by ionizing radiation. After studying the damage mechanisms of ionizing

radiation, radiation damage to plastics materials can be understood.

Radiation DamageParticles in the emitted radiation impact at high energies when compared to the bonds in

the crystal lattice of a material. These particles slam into the crystal lattice, knocking off or

displacing atoms. Radiation damage continues over the lifetime of a material in a nuclear facility,

and as a result, bulk defect in materials can arise with varying severity. It is important to know

the impact of alpha, neutron, gamma, and beta particles on various types of materials and their

resulting bulk defects in the crystal lattice. This is crucial to the design of the nuclear facility

when material considerations are made. Awareness of how the radiation damage affects the

performance of the materials ensures reliably and safety in the system. Each type of particle

leaves its own type of radiation damage on a given material so it is good to have an

understanding of what each particle’s effect on a lattice is individually. Once this is understood,

the expected defects in a material would be a random combination of the various types possible,

since constant bombardment of random particles in the nuclear reaction.

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Ionizing RadiationGamma, beta, and alpha particles interact with the electron cloud of a material, departing

their energy to the electron in an atom which can strip the electrons off an atom, leaving them as

charged ions in a high energy state. [1] This is what is called ionizing radiation, and its effect on

atoms in a lattice of a material can cause some covalent or ionic bonds to break since the atoms

are stripped of their electrons, changing their charge. The strength of a material is dependent on

its chemical bonds, which rely on either the sharing of electrons or the pair of like charges.

Therefore, ionizing radiation can cause serious impact to crystal lattice.

Radiation Damage to PlasticsIonizing radiation has a great effect on covalent bonds, which comprise most polymers.

The ionizing radiation reconfigures the covalent bonds by breaking the current chains or creating

different chains. [1] This weakening of the polymer’s bonds compromises its structural integrity.

In knowing the type of damage to expect in a radioactive environment, we can predict the

problems that may arise in material performance of plastics. After the polymer chain is broken, it

re-links to any available bond yielding a shorter or longer chain than it was originally. In the

event of shorter bond chains, the plastic becomes soft and weaker, possibly becoming liquefied

with increasing radiation. Teflon, Plexiglas, Lucite and butyl are mentioned as plastics most

likely to fail by these means. [1] On the other hand, when bonds in the plastic recombine into

longer chains, the resulting plastic become harder and more brittle. Brittle failure is common in

plastics such as polyethylene, polystyrene, silicone, neoprene and natural rubber.

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Figure 1a 2a Plastic Glass Composite [4] Figure 2a 2b Plastic Glass Composite after irradiation [4]

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Figures 1-2 are of a samples of a commercial grade plastic scanned before and after

radiation damage was iduced. In the study, commecial plastics were assesed for the ability to be

used as structural materials for low level waste disposal. The sample composite was made by

mixing glass fibers into the plastic to increase the structural and chemical stability of the plastic.

[4] The image on the left shows the composite mixture before irradiation and the right is after the

material expirenced a slightly higher radiation dose than would be expected in the lifetime of a

structual material used for low level nuclear waste disposal. After irradiation the effects of the

radiation damage become visually apparent. Cross-linking and gasification of the plastic causes a

volume reduction of the material as the specific gravity is increased[4]. Once the a significant

portion of the gas has diffused out of the matarial, the bonds holding the polymer chains are

weakened, leaving a weak support material for the glass rods. Ionizing radiation still damages

plastic more significantly. Attempts to mix the plastic with support materials only acted as

localization points for the radiation damage, as the glass fibers were not easilty ionized or

damaged by the incoming radiation.

Radiation damage to Rubber

Rubber materials are also susceptible to brittle failure. Rubber relies on long bond chains

to remain flexible, with the amount of cross linking between the chains determining the stiffness

or tension available to it [1]. Natural rubber is more resistant to having its bonds broken down by

radiation damage than artificial plastics [1]. In all cases, the tensile strength of the rubber is

reduced as it becomes brittle or liquefied. In an actual system, seals and O-rings in air and D2O

systems of containment failed due to radiation and temperature damage. This part malfunction

can cause this major subsystem to fail. Upon scheduled outages, valves and rings are inspected

and replaced on an as needed/ call-up frequency to avoid failure of these system components.

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Plastics undergo a more drastic change when exposed to radiation damage when compared to

metals because the actual chemical bonds in the plastic are changed or displaced, resulting in a

new degraded material.

Discussion.

In a study at Oakridge National lab, various commercial plastics were irradiated. This

study was a follow up study on commercial plastics where changes after radiation and oxygen

aging were studied. The plastics and elastomers were put into the maximum flux region of the

ORNL graphite reactor to study the effects of various types of radiation damage. Fast and

thermal neutrons were tested to determine the type of damage modes on the plastic. Gamma

damage was the most significant to the study, so by wrapping the experiment in cadmium, more

n-gamma reactions were produced when the cadmium was hit with a neutron, thus increasing the

effect of ionizing radiation on the plastic being tested. An attempt to “alloy” or mix the plastics

with different types of rubber was made to mitigate the damage done to the material.[2] This

attempt failed due to the dependency on the bond chains which were broken regardless of the

plastic or rubber used. In application, failures such as these have been seen in the insulation on

some power and instrument cables, where the plastic became so brittle it just fell off the cables.

Terminal blocks have been seen decomposing in junction boxes where radiation fields were high.

[1] It is very important to be aware of this possibility when making material of plastics since they

might not be able to perform regular maintenance because of their location, and you’d want to

ensure the long lifetime and function of a plastic component, whatever the function may be. In

the text there is a useful chart to show the expected impact form radiation damage on plastics as

a function of the type of plastic and the total absorbed dose in Mrad. This can be used to

determine which plastic is best for commissioning for a task depending on the expected

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replacement time and damage incurred over the lifetime of that part. By knowing the radiation

fields around the facility, you could know which materials are at risk on certain timescales. From

the chart most plastics start to experience some moderate damage after absorbing 10 Mrads, and

severe damage around 800-1000 Mrad.

Testing.Polymers have low neutron cross sections, so most of the damaging in the plastic comes

from ionizing gammas that break the covalent bonds [1]. Broken bonds were expected to

decrease tensile strength after radiation exposure. Using cadmium experiment covers converted

some of the thermal neutrons into gamma radiation [3]. More damage to the polymer can be

Figure 3 Chart of radiation exposure to polymers [1]

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expected with increasing amounts of gamma radiation. Commercial plastics were irradiated to a

determined exposure time. They were tested for some of their material properties, such as

hardness and tensile strength. Once maximum stress-strain curves were plotted as a function of

exposure time, the effect of the damage became apparent. With this data it is better understood

what the effects of nuclear radiation will be on the final strength of various commercial plastics.

With this knowledge, a design engineer can determine how much the radiation is going to

degrade function of the part, and determine a modified design of this plastic part. If redesign is

not possible a removal/ replacement plan can be made to retrieve a part before failure at a

predicted time. [2] These experiments were very useful because they used ASTM procedure for

material testing, which most US engineers can understand, and make their own design

adjustments using standard stress strain formulas. If such a part were retrieved from service for

testing and material analysis, it would mostly likely disintegrate from embrittlement, or liquefy

before the test was complete, although chemical analysis could probably still be done.

Material using standard “dog-bone” and sample coupons for the irradiated polymers

gives a template for comparing results between many experiments. Using the test from many

Figure 4 Plastic test samples [2]

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studies on the effects of stress on these damaged materials gives a comparison data, and after

cross checking, the tensile test data was comparable between polymers of various studies.

Figure 5 reveals the tensile stress data for a natural rubber sample irradiated to varying

severity. The un-irradiated curve characterizes a polymer, having a decent stress value at failure,

as well as a high percent elongation or stain value. After irradiation, the polymer has less than

50% of its usual maximum stress and strain values. Stress curves of the irradiated polymer

appears as brittle material or weak ceramic. This results from the cross linking that stiffens the

polymer after experienced ionizing radiation. [5]

Effects on Material propertiesRadiation damage can cause a volume change in a plastic material. [2] Ionizing radiation

breaks the hydrocarbon chains releasing hydrogen. Now, the gaseous hydrogen diffuses through

the plastic, leaving shorter chains with more cross linking between hydrocarbon chains. This

results in a volume shrinkage in the polymer, since gas was released from the plastic. [2].

Figure 5 Tensile strength of irradiated plastics [2]

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After irradiation of the various commercial polymers, radiation embrittlement was

observed for most of the plastics and natural rubber [4]. In this process the long covalent bond

chains within the plastic were cut by the radiation so that their length is shortened. The severed

bonds recombined horizontally instead of vertically. This formed a new polymer and stiffened

the material as a bulk defect. The horizontal cross-linking of the bonds chains resulted in less

give and stretch than if oriented vertically. As a result, the plastic was stripped of its elastic

characteristics. The remaining material is brittle. Commercial grade rubbers, butyl rubber and

Thiokol underwent a liquid transformation as a result of radiation damage. In these elastomers,

the horizontal bonds and responsible for the stiffness and “rubbery” characteristic that returns

and retains rubber in its formed shape [1]. Radiation damages these bonds cutting the amount of

cross-linking between the bonds, which softens up the material. Over time, the amount of bonds

is reduced to the point that the rubber becomes liquid-like and cannot return to its original shape.

Two plastics Hycar and Silastic saw an initial increase in tensile strength due to radiation

hardening [1][4]. After continued radiation damage the tensile strength is reduced. These

commercial plastic can experience damage till the point where they can reach less than 1% of

their original elongation, and behave more as a brittle ceramic material than as plastic/ elastomer.

[2]

Figure 6 polymer properties after irradiation [2]

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Fig 7 serves as a visual to the effects of gas diffusion in plastic-cloth filters used in D2O

systems. The hydrogen in the hydro-carbon bonds holding the plastic together are released after

gamma radiation. The lose hydrogen diffuses out of the material, causing a volume reduction in

the material that is left. Hydrogen acts as the spacing in polymer chains. When released, the

material is likely to crumble since the material left is made up of mostly carbon and oxygen.

Color and translucence of the polymer are also affected by radiation damage. UV rays excite the

polymer chains, tinging the plastic from yellow to black as the radiation is induced [5]. The

effect of the UV radiation was not as significant as that of gamma, but since there was a color

change, there is evidence of a chemical reaction in the material.

Figure 7 Plastic cloth filter volume reduction [2]

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Conclusion.Nuclear radiation significantly affects the material properties of commercial grade

plastics. Radiation damage can be occurred over the lifetime of a part. Among the various types

of radiation, gamma rays cause the most material damage. The rays ionize hydrocarbon bond

within a polymer. Hydrogen diffuses through the plastic, reducing the volume of the polymer.

The broken bonds in the plastic recombine wherever possible. This change is volume and

orientation of the bonds is “cross linking”. Cross-linking is the main defect from radiation

damage in polymers. This now brittle material has reduced maximum tensile strength and

elongation values. A good plastic material for a radiation environment would have to most

resistance to cross-linking and shrinkage. By having a higher ionization energy between atoms,

some plastics can be more resistant to radiation damage than others.

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References[1] D. Dawson, R. Fleck. A. Wadham, Radiation Damage to Materials, course 228, module 4,

Obj. 4.5 b, p 8, P. Bird. WNTD, (June 1993).

[2] C. D. Bopp, 0. Sisman, RADIATION STABILITY OF PLASTICS AND ELASTOMERS,

ORNL-1373, Oak Ridge National Lab, Tennessee, (1954)

[3] H. BERGER, “Track-etch radiography: alpha, proton, and neutron,” Argonne National

Laboratory Material Science Division (Apr 1973).

[4] H. Bonin, M. Walker, V. Bui, APPLICATION OF POLYMERS FOR THE LONG-TERM

STORAGE AND DISPOSAL OF LOW- AND INTERMEDIATE-LEVEL RADIOACTIVE

WASTE, Royal Military College of Canada, Department of Chemistry and Chemical

Engineering P.O. Box 17000, Station FORCES, Kingston, Ontario K7K 7B4, Canada, (April

2003)

[5] I. Rosman, K. Zimmer, Damage to Plastic Scintillators by ionizing radiation, pg 56-62, (May

1956)