Radiation - shielding material

A radiation shielding element especially for use in the temporary shielding of ancillary equipment or apparatus in nuclear-powered steam raising installations, said element comprising a flexible radiation-shielding sheet laminate comprising a flexible inner layer of plastic material, preferably silicone, filled with particles of radiation shielding material such as lead, said layer being interposed between two flexible skins of silicone elastomer.

This invention relates to radiation shielding material primarily intended 
for shielding sources of high energy shortwave electromagnetic radiation, 
in particular shortwave X-rays such as gamma rays such as are found in 
association with nuclear-powered steam raising installations and the like. 
In such installations, ancillary equipment and apparatus such as valves, 
pumps and pipes of the steam generating circuit, located in areas to which 
human access may be required, e.g. for routine maintenance, overhaul and 
repairs, can become contaminated with high energy radiation such as gamma 
radiation (including beta-gamma radiation) and it is therefore desirable 
to provide shielding to protect the operatives who have to enter and work 
in it. 
One method of achieving such temporary shielding is by the use of sheets or 
tiles of radiation-shielding material which are hung over or wrapped round 
the equipment from which operatives are to be protected or which are 
formed into temporary housings round the equipment e.g. by hanging sheets 
from overhead fixings. 
In many instances, the equipment is in a confined space and/or in a 
location which can only be approached with difficulty through tortuous 
routes and it is therefore necessary for the tiles or sheets to be 
flexible so that for example, they can be manoeuvred round sharp comers or 
down narrow passages and/or wrapped round equipment such as pipes. 
While lead is well known to be an excellent shield against such radiation, 
it is unsuitable for such purposes because it is too heavy and 
insufficiently flexible; moreover if the sheets become contaminated, they 
have to be disposed of, which is expensive. Thus, attention has focused on 
elastomeric or rubbery plastics materials filled with radiation-shielding 
particles, e.g. of lead. However, it has been found that if sufficient 
particles are incorporated to achieve the required shielding, the product 
is frequently not strong enough to support its own weight and also tends 
to tear or split when it is flexed. On the other hand, if the mount of 
particles is adjusted so as to obtain a product which is self supporting 
and/or has adequate flexibility, its shielding capability is inadequate or 
is only adequate if the material is used in a thickness which creates 
problems due to its bulk and also restricts the ability of the material to 
flex. Moreover, where as is generally the case, the radiation-shielding 
particles are metallic, placing these materials in contact against 
metallic components of the equipment to be shielded is not recommended 
because of the risk of electrolytic action between the particles and the 
component, especially in the presence of water vapor, moisture or steam. 
In an attempt to overcome the problem of inadequate strength in the highly 
filled plastic sheets, there have been many proposals for laminating 
lead-filled plastic materials between protective sheets such as of plastic 
impregnated cloth, e.g. for use in hospitals for protection against 
X-rays; see, for example, GB-A-851479, 954594, 1122776 and 2118410. 
However, these materials are not suitable for the applications with which 
the present invention is especially concerned, particularly the temporary 
shielding of ancillary equipment and apparatus in nuclear installations. 
One reason for this is that much of the equipment in nuclear installations 
which is to be shielded is made of stainless steel and there are severe 
restraints on what materials may be brought into contact therewith. For 
example, materials such as PVC or rubber which are likely to yield halide 
or sulphide ions are generally considered unsuitable. Another reason is 
that because the equipment to be shielded is frequently in confined 
spaces, inflammable materials or materials such as polyurethane or 
polyamides which are likely to yield toxic fumes if burned are also 
banned. Thus, for these and other reasons, such laminates have not proved 
acceptable to any significant degree in practice and the art has sought 
alternative solutions. 
One solution which is in use is to employ tiles or sheets of lead-filled 
plastic in protective plastic bags. However, this, too, has not proved 
entirely satisfactory. Firstly, the tiles or sheets themselves are not 
very flexible and cannot easily be wrapped round pipes when in their bags, 
and if they are hung over the pipes it is necessary, in order to ensure 
that the laminate does not accidentally slip off, for the length of 
laminate hanging down each side of the pipe to be not much less than the 
length of laminate extending along the pipe. Secondly, when the bags are 
hung vertically, the sheets tend to slump inside, thus reducing the 
protection. Counteracting this effect requires the use of more layers or 
thicker sheets, thus still further increasing both bulk and weight, and 
further exacerbating the problem. Thirdly, the bags can collect water and 
must then be disposed of because of the risk of contamination if there is 
a steam or water leak. Fourthly, the most suitable material found for the 
bags is PVC which is a potential source of chloride ions. 
The present invention aims to reduce or obviate these problems. 
According to the present invention, there is provided a radiation-shielding 
element comprising a flexible radiation-shielding dense sheet laminate 
comprising a flexible inner layer of plastic material filled with 
particles of radiation shielding material such as lead, said layer being 
interposed between two flexible skins of silicone elastomer. A preferred 
class of silicone for the flexible outer skins is that used for the 
production of molds. 
It is to be understood that the term sheet, as used herein, also covers 
small elements such as tiles and elongated elements such as strips, as 
well as sheet products. 
The laminate is flexible and, with suitable choice of the plastic material 
for the inner layer and within the preferred thickness range of 5 to 20 
mm, may readily be twisted or bent for transfer along tortuous routes and 
curved to conform with the external profiles of pipes and the like even 
pipes having diameters of 25 mm or less. 
Moreover, it has been found that silicone elastomers with the desired 
physical properties are readily available which are free or substantially 
free of components capable of yielding unacceptable ions such as halide or 
sulphide; in particular, the Content of each of fluoride, chloride and 
sulfide is less, and generally substantially less, than 200 mg/kg. Thus, 
the laminates can be placed in direct contact with the equipment to be 
shielded and the use of a protective bag is not required. This, in 
combination with the flexibility of the laminate, reduces the space 
required to achieve the desired protection against radiation, a factor 
which is important where the equipment is in a confined space. Further, 
any moisture which collects on the surface of the laminate is readily 
removable by wiping, particularly where the silicone is chosen to be 
hydrophobic, thus rendering decontamination simple. 
Another advantage for the laminates of the present invention is obtained if 
a silicone material which exhibits surface tackiness is employed since 
this reduces the risk of the laminate slipping where, for example, it is 
hung over a pipe. In fact, it has been found that laminates of the 
invention may safely be laid over a pipe with the long axis of the 
laminate parallel to the pipe axis even at length/width ratios for the 
laminate of 3:1 or more. 
With suitable choice of the plastic material for the inner layer, the 
laminates of the invention are remarkably tough and resilient; for example 
laminates of the invention which are as much as 9 mm thick may be bent 
round a radius as small as 12-13 mm without tearing or splitting. They are 
also resistant to damage from knocks, unaffected by moisture, able to 
tolerate temperatures in and beyond the range likely to be encountered in 
normal use, and do not yield toxic or noxious fumes on combustion. 
In a preferred embodiment, some or all of the edges of the laminate are 
clad with silicone so that the filled plastic layer is encapsulated. 
While any plastic material may be employed for the filled plastics layer 
provided that it is compatible with and bonds well to the silicone polymer 
of the outer, i.e. skin layers and the layer has the desired flexibility, 
it is preferred that this, too, is silicone elastomer since this avoids 
any problems of incompatibility between this layer and the skin layers, 
and ensures a strong bond between them. Other materials that may be 
employed include polyolefins, polyamides, polyesters, vinyl polymers, 
polyurethanes, and the like. 
While any material known to be an efficient absorber of high energy 
shortwave electromagnetic radiation, and in particular gamma rays, may be 
used for the particles with which the inner layer is filled provided the 
particles can be incorporated in the plastic material of the inner layer 
and do not adversely affect it, e.g. are inert to it, the preferred 
material is lead. Other suitable materials will be known to those skilled 
in the art. In general, it will be preferred to include as high a 
proportion of the particles of radiation shielding material in the inner 
layer as possible consistent with achieving the desired flexibility and 
obtaining a coherent sheet. In general, however, the limiting factor is 
the volume of particles that can be mixed into the resin. For lead 
particles and silicone elastomer, a preferred concentration of the 
particles is in the range 50 to 95% by weight, more preferably 75 to 95% 
based on total weight of lead particles and resin. Below 50%, the 
protection is poor and above 95% there is difficulty in incorporating the 
particles into the resin. Other radiation-shielding materials and/or other 
polymeric materials may lead to different ranges of optimum concentration 
but these can readily be determined by simple experiment. 
It will be understood that the radiation-shielding efficiency of the 
laminate will depend inter alia on the concentration of 
radiation-shielding particles in the inner layer of the laminate and the 
thickness of that layer. On the other hand, the flexibility of the 
laminate will tend to decrease with increase in its overall thickness. 
Also, increase in thicknesses of the inner layer relative to the sum of 
the thickness of the two skin layers will increase the radiation-shielding 
efficiency expressed as a function of thickness. It is therefore desirable 
for the thickness of the skin layers to be as small as possible 
commensurate with providing the desired properties in the laminate. In 
general, we have found that thicknesses as small as 1 to 2 mm are adequate 
for the skin layers and even thinner layers may be satisfactory in some 
cases. Of course, thicker layers may also be used but little additional 
advantage is likely to be gained thereby. 
The overall thickness of the laminates is controlled by the desired level 
of radiation protection on the one hand and weight and flexibility on the 
other. Preferred thicknesses are in the range 5 to 20 mm, more preferably 
8 to 16 mm. It will be understood that in some circumstances, such as 
shielding of small diameter pipes, the desired protection is best achieved 
by employing several layers of a thin laminate, e.g. by winding a strip of 
the laminate round the pipe two or more times, rather than one layer of 
thicker laminate. 
Preferred lengths and widths for the sheets of the invention are 
length: 30cm to 120cm, more preferably 60 to 100cm. 
width: 15cm to 50cm, more preferably 20 to 35cm. 
This is not to say, however, that dimensions outside these ranges may not 
be found acceptable in special circumstances. If a laminate is not 
sufficiently thick to provide the desired level of protection, this may be 
resolved by using two or more layers of laminate. In this case, lateral 
locations of the sheets of one layer should be staggered relative to those 
of the next layer so that the spaces between adjacent sheets of the first 
layer are covered by sheets of the next layer. 
The laminates of the invention may be formed by depositing a layer of 
silicone elastomer to the desired thickness in a suitably shaped mould to 
provide the first skin layer and then causing or allowing it to partially 
cure so that it is no longer fluid but still noticeably tacky. A preformed 
inner layer or the composition to form the inner layer may then be 
deposited on this first layer. In a preferred embodiment, this composition 
is obtained by mixing the radiation-shielding particles with a liquid 
curable elastomeric resin material, e.g. silicone elastomer, pouring the 
mixture into the mold to the desired thickness and causing or allowing it 
to partially cure so that it is no longer fluid but still noticeably 
tacky. Thereafter, a further layer of silicone elastomer is deposited to 
the desired thickness to provide the second skin layer and the whole is 
cured. The laminate may then be removed from the mould. The partial curing 
steps and the final curing may be accelerated by heating. 
Where it is desired to encapsulate the inner layer of filled plastic 
material, the side walls of the mould may be coated with silicone 
elastomer prior to depositing the three layers or after depositing the 
first layer and before depositing the second and third layers. 
Alternatively the edges of the laminate may be coated with silicone 
elastomer composition after the laminate has been removed from the mold. 
If desired, one or more other layers may be included in the laminate, e.g. 
to extend the protection afforded by the laminate and/or to modify its 
physical and/or surface properties. 
Fillers and/or other additives other than the radiation-shielding particles 
may be included in the inner layer, if desired, and one or both of the 
outer silicone layers may include fillers or other additives, e.g. 
pigments. It may even be acceptable to include small quantities of 
radiation-shielding particles in one or both of the outer layers; however 
this is not advisable where the layer is intended to come into contact 
with the equipment it is shielding where that equipment is metallic, 
especially stainless steel. 
Reinforcement, e.g. in the form of fibrous material, e.g. carbon or glass 
fibre, may be included in the laminate e.g. as chopped fibres, rovings or 
woven or unwoven webs. 
While the laminates of the invention have been developed primarily to solve 
the problems of finding an acceptable radiation screening material for 
high energy shortwave electromagnetic radiation such as gamma rays which 
typically have a quantum energy of at least 0.3 MeV and more particularly 
at least 1 MeV, and especially for the temporary screening of ancillary 
plant and apparatus of nuclear powered installations such as for steam 
raising in order to protect operatives from exposure to such radiation, 
they may also find use in other applications where the same or similar 
radiation emissions are encountered, e.g. as in hospitals, medical 
research and experimental laboratories. They may also find application in 
providing shielding for lower energy longer wavelength X-rays, e.g. having 
a quantum energy somewhat below 1MeV, for example in the range 1 keV-1 MeV 
and/or having a wavelength somewhat greater than 0.1 or 1 .ANG., e.g. up 
to 10 .ANG. or 100 .ANG..

The invention is now illustrated by the following Example 
EXAMPLE 
357.3 g of the base component of the silicone elastomer system marketed by 
Dow Corning as Silastic E, 3.9 g of yellow pigment (WS 15414A from West 
and Senior Limited of Manchester England), and 35.7 g of curing agent for 
the base were mixed together and poured into a 914.4 mm .times.30.5 mm 
.times.9 mm high mold to form a layer about 1.25 mm thick and partially 
cured by heating. 
The side walls of the mold were then coated with a pre-mixed thixotropic 
composition of 26.5 g of the same silicone base, 0.3 g of the pigment, 2.7 
g of the curing agent and 0.5 g of amorphous silica and this coating was 
then heated to partially cure it. 
The composition for the inner layer was formed by mixing together 914.4 g 
of the silicone base, 91.4 g of the curing agent and 8097.2 g of 80-200 
mesh lead particles and this composition was poured on to the partially 
cured first layer in the mold and heated to achieve partial cure. 
Finally, a top layer was formed from an identical composition to that of 
the bottom layer and any excess was removed by doctor knife. The whole 
laminate was then heated to cure the top layer and complete the cure of 
the inner layer, bottom layer and side layers. 
The 9 mm thick laminate so obtained could readily be wrapped round a 25.4 
mm diameter pipe without any sign of tearing or splitting and could 
support its own weight. The faces of the laminate were slightly tacky 
which enabled it to be draped over pipes and other equipment with a 
reduced risk of slipping. 
The attenuation of the laminate was measured using an iridium 192 isotope 
and found to be equivalent to about 5 mm of lead; however, the weight of 
the laminate is substantially less than the weight of a corresponding 
sheet made from 5 mm lead. Using an RO2 radiation dose meter with a 37GBq 
Cs 137 source, the attenuation at a dose rate of 370 micro Sv/hr was found 
to be 28.3%. The attenuation of a collimated Co 60 source of mean energy 
1.25 MeV was measured at 21% at dose rates of 500 .mu.Gyh.sup.-1 and 
50.mu.Gyh.sup.-1. 
A sample of the outer skin of the laminate was analysed for fluorine, 
chlorine and sulphur and found to contain 67.7, 24.05 and 73.3 mg/kg, 
respectively. The nitrogen content of the molding was negligible. 
In similar manner, and using the same quantities of materials for the outer 
layer and side wall coating but a proportionately larger quantity of 
material for the inner layer, a 15 mm thick tile was obtained. 
While, in the above Examples, a thixotropic composition was used to coat 
the sides of the mold, it has been found that the thixotropic agent may be 
omitted.