Am. oeatp. Hyg. Vol. 21, pp. 213-216. Pergunon Pren Ltd. 1973. Printed In Great Britain

RADIOLOGICAL SAFETY BY DESIGN—V. AN INSPECTOR'S VIEWPOINT WITH REFERENCE TO THE DESIGN OF GAMMA RADIOGRAPHIC EQUIPMENT* W. S. MURDOCH

Nuclear Installations Inspectorate, Health and Safety Executive, London Abstract—A brief review is given of the three basic designs of gamma radiographic equipment: the shutter, the torch and the remote control types. Analysis of failures of remote control cable projection equipment indicates that the main problem lies in the weakness of the cable connection. Suggestions are made for modifying the design of the equipment to eliminate the hazard associated with this weakness. It is concluded that one obstacle in radiological safety lies not in the lack of good designs, but in a resistance to change. INTRODUCTION

sources for gamma radiography are widely used in industry. The sources are housed in containers which provide shielding for the operator and enable the source to be transported, and the design permits controlled exposure of the source. This paper is concerned with designs of gamma radiographic equipment considered from the viewpoint of operator protection. There are three basic designs of equipment in use: the shutter type, the torch type and the remote control type (MURDOCH, 1976). SEALED

Shutter type In this type (Fig. 1) the source is exposed by rotating or removing part of the shielding. The radiation dose the operator receives is a function of the strength of the source and the number of times it is used. Since the source is well collimated and the method of operation simple, doses are normally small and incidents are few. The principal disadvantage is that the entire weight and bulk of the shielding has to be supported at the site of the exposure and this in many cases is not practicable. Torch type The description 'torch' refers to all hand held devices, where either some shielding is incorporated in the device or the handle is long enough to afford shielding by distance (Fig. 2). Generally this type is suitable only for relatively small sources in regular use. Again the dose the operator receives is a function of the strength of the source and the number of times it is used. Good access is essential so that the radiographer can quickly retreat to an area of low dose rate after removing the torch from the container and placing it in a 'castle' at the site for exposure. With certain types of torch and castle equipment it is important that the components are the correct dimensions and appropriately matched, otherwise the source in the exposed position * Crown Copyright. 213

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W. S. MURDOCH Operating handle

Shield mg

Thin window

Back-shielded source holder

Source (shielded position) FKJ. 1. Diagram showing principle of shutter type exposure container.

Removable handle

Storage for handle Spring loaded plunger

Shielding,

Source shown retracted

Shielded 'torch' assembly

Fio. 2. Diagram of a torch type of exposure container.

is improperly placed with respect to the shielding and this may result in up to a ten-fold increase in the exposure of the user. Improved protection can be provided by use of special castles (MURDOCH, 1975). Even with well adjusted equipment the shielded torch can be used regularly only with small sources of less than, say, 10 C\ iridium-192, owing to the proximity of the operator to the source. Projection type

The protection principle in this type of equipment is the use of distance between the operator and the exposed source, achieved in most types by using a long cable to transfer the source to the exposed position and restore it to the safe position (Fig. 3). Other factors being equal, operators of remote exposure equipment can be distinguished from those using other types by their relatively low dose exposure records.

Radiological safety by design Source shielded

215

Source exposed

Projection coble "it'e

stop

FKJ. 3. Diagram showing principle of operation of a cable operated projection container.

However, equipment failure combined with failure to monitor after exposure can result in serious excessive exposure of the operator. We have no precise information regarding the frequency of use of the three types. Perhaps 50 % of radiographic gamma sources are used in remote exposure types, 30 % in torches and 20 % in shutter types. Probably remote types are more frequently used now than they were, say 5 yr ago. The average size of source is increasing. Whereas even 5 yr ago a 30 Ci iridium-192 source was perhaps in use in an occasional firm, now 100 Ci sources may be found. What are the unseen problems arising from use of particular equipment? Which is the best system? The subject has been under review by HM Factory Inspectorate for some time. FAILURE ANALYSIS

All incidents of excessive exposure to ionizing radiations in factories are reportable to HM Factory Inspectorate. Valuable information is derived from the detailed investigations which are carried out in every case. Each incident is analysed to ascertain whether the failure was mainly due to management, operator or equipment. The contribution of human factors is obviously important; however, for the purpose of this paper the problems associated with equipment failures are examined, with specific reference to cable projection equipment. 'Autopsies' on a number of failed containers have shown that the majority of the failures arise from separation of one or other of the junctions between the flexible source holder or 'pigtail' and the projection cable. The failure is sudden, and will result in an acute excessive exposure unless a monitor or dose rate meter is used after every exposure to verify that the source is safely retracted before the exposure site is approached. Failure to carry out this elementary precaution has resulted in many serious over-exposures to industrial radiographers. However where the source is known to be 'out of control' arrangements can be made (with appropriate training and simple equipment) to restore the source to the shielded position without significant dose penalty to the operators. (MURDOCH, 1977).

SUGGESTED REMEDY

If the remote control unit does fail the operator cannot see where the source is within the head tube, and therefore does not know how to disconnect the tube and

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W. S. MURDOCH

get the source into shielding again. This is a situation which is better avoided, and it would seem possible to do so by minor alterations to the design. The projection cable junction is the main weak point and yet in almost all designs this joint travels to an inaccessible point in operation. If the pigtail was made long enough the junction need never enter the container and this would have several advantages : (a) if the junction failed, the available end could be used to restore the source to the container manually; (b) a more robust junction could be used, because it would no longer need to pass into the S-shaped channel; and (c) the reduced friction on the cable when it was pulled back after a failure would be more obvious and might be detectable by the operator, alerting him to the danger. Nevertheless, a monitor must be used to verify that all is well after every exposure. If this fault area could be moved away from the source, the remaining weak point would be the connection between source and pigtail. This joint comes under strain at the end of the wind out traverse and again when the source is returned to the shielding. Provisions of a tapered constriction on the inside of the head tube so that it engages the cable stop at the end of the outward traverse would reduce potential damage to this vital junction. COMMENT

There is clearly a need for a reduction in the present level of incidents arising from equipment failure. None of the available equipment incorporates the suggested safety features, which would be a step in the right direction. Good designs are only effective when they are translated into hardware. Designs have been suggested; it may be that the real problem is resistance to change. REFERENCES MURDOCH, W. S. (1975) Investigation of dose rates around a manual extraction (Torch type) radiographic exposure container in the exposed position. Proceedings of the 3rd European Congress of IRPA, Amsterdam, May 1975. MURDOCH, W. S. (1976) Br. J. non-destr. Test. 18,144. MURDOCH, W. S. (1977) Proceedings IAEA Symposium on the Handling of Radiation Accidents, Vienna, March 1977. STI/PUB/463. International Atomic Energy Agency, Vienna. DISCUSSION W. H. WALTON (Institute of Occupational Medicine): What do you mean by an 'Incident' ? An overdose, or mechanical handling difficulties ? MR MURDOCH : These are incidents reported under the requirements of the Ionising Radiation (.Sealed Sources) Regulations, 1969 where there has been an apparent dose in excess of that permitted.

Radiological safety by design--V. An inspector's viewpoint with reference to the design of gamma radiographic equipment.

Am. oeatp. Hyg. Vol. 21, pp. 213-216. Pergunon Pren Ltd. 1973. Printed In Great Britain RADIOLOGICAL SAFETY BY DESIGN—V. AN INSPECTOR'S VIEWPOINT WIT...
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