1959,Vol.6, pp. 41-42. PergamonPressLtd. Printed
Nuclear Sources of Radiation Energy S. JEFFERSON Technological Irradiation Group, Isotope Division, Wantage Radiation Laboratory, Want.age, Berks Radioactive cobalt is the most readily available source of y-rays. 100-500 c are desirable for experimental work, but 50,000 c upwards are needed for treating bulk materials. The efficiency of utilization of the radiation depends upon the treated material occupying a maximum of the solid angle surrounding the source. Conveyors can be arranged to move packages around a source inside a concrete emplacement. Spent fuel elements form a useful source ofy-activity up to about 100 days from withdrawal from a high flux reactor. Radioactive caesium separated from fission products is another useful source of y-rays having a half-life of about 27 years. THE practical considerations involved in r a d i a t i o n processing are the a m o u n t s o f material to be treated, the r a n g e o f dosage, a n d the a c c o m m o d a t i o n o f a p p a r a t u s n e e d e d d u r i n g the irradiation. F o r e x p e r i m e n t a l work o n r a d i a t i o n effects a source o f 100-500 c is desirable. C o b a l t which has b e e n i r r a d i a t e d in a r e a c t o r to b e c o m e in p a r t radioactive Co 80 is the most readily available type o f source. T h e cobalt c a n be housed in a c o m p a c t lead shield but, w i t h o u t extra shielding, this a r r a n g e m e n t c a n only conveniently a c c o m m o d a t e a n i r r a d i a t i o n space h a v i n g a b o u t 500 c m 3 capacity. It is generally more c o n v e n i e n t to build a concrete shield o f large e n o u g h dimensions to allow the e x p e r i m e n t a l staff to enter the r a d i a t i o n c h a m b e r for the purpose o f setting u p a p p a r a t u s associated with the irradiation. I n such a c h a m b e r one wall c a n be m a d e thick e n o u g h to act as a store for the radioactive material. As a n alternative the radioactive material c a n be stored in a lead shield inside the i r r a d i a t i o n c h a m b e r o f a concrete shield. A r r a n g e m e n t s must be m a d e so that the radioactive source c a n n o t be b r o u g h t into the r a d i a t i o n c h a m b e r until a n u m b e r o f interlocks a n d safeguards h a v e b e e n b r o u g h t into operation. F o r treating bulk materials a source size o f 50,000 c u p w a r d s o f radioactive material
is needed. T h e a m o u n t depends m a i n l y u p o n three factors; the t h r o u g h p u t o f material being processed, the total dose n e e d e d for the process and the efficiency o f utilization o f the radiation. As a n example, a source o f 100,000 c o f Co e° can process 250 lb o f material per h o u r with a dose o f 2 M r a d s , the efficiency being 40 per cent. D o u b l i n g the dose would n a t u r a l l y halve the throughput. T h e time t a k e n to absorb a dose o f 1 M r a d in efficient a r r a n g e m e n t o f Co ~° is 5 to 10 hr. W h e n food is being processed it is i m p o r t a n t t h a t the i n a c t i v a t i o n dose for vegetative organisms (100,000 rads) should be given in a short e n o u g h time to p r e v e n t serious multiplication. T h e o p t i m u m t e m p e r a t u r e for multiplication is in the region o f 37°C where the d o u b l i n g time is a b o u t 20 min. At r o o m t e m p e r a t u r e the rate is r e d u c e d to less t h a n a q u a r t e r and so the d o u b l i n g time becomes m o r e t h a n 1 hr. A l t h o u g h spores are m o r e resistant to r a d i a tion, they are d o r m a n t a n d so the delay in acquiring the larger inactivation dose is not a problem. I n designing the process h a n d l i n g equipm e n t a r o u n d the radioactive source the m a x i m u m fraction o f the solid angle r o u n d the source must be occupied. This is because the r a d i a t i o n leaves the source in all directions a n d a n y which reaches the shield 41
unimpeded is wasted. I f the packages are small, not only should they occupy as much as possible of the solid angle around the source, but they should also be arranged in a second or even a third layer so that the radiation which penetrates the first layer without giving up all its energy is able partially to treat the next layer. Packages going through such an arrangement should traverse each layer in turn and also pass each side of the source so that the greatest uniformity of dose deposition is achieved. The Co e° so far used in the United Kingdom has been irradiated in the Windscale piles but it is planned in the future to introduce cobalt into the Central Electricity Generating Board reactors during the early part of their operating lives. During this time there are spare neutrons which would otherwise be absorbed in steel rods or wedges and so would not form a useful radioactive source. The fission products formed during the burn-up of uranium fuel contain gammaemitting radioisotopes and it is possible to use the fuel elements as a source of gamma rays 2 or 3 days after they have been withdrawn from a reactor. An experimental arrangement of this kind has been built at Harwell to make use of the spent fuel elements from the two high flux reactors D I D O and P L U T O . For industrial purposes the use of spent fuel elements is not
very convenient because the average rate of decay of the fission products is rapid at first and so they have to be replenished at short intervals. In addition, to maintain a steady g a m m a flux in any one place it is necessary to rearrange the fuel elements as they decay. However, spent fuel elements do provide us with our first opportunity of having several millions of curies of g a m m a activity. After the radioactivity of the spent fuel elements has decayed for about 100 days, they are then subjected to a chemical separation treatment primarily to recover the unused uranium. An extension of the plant can provide separated Cs 13~ and Sr 9°. Cs 137 is a useful source of gamma energy having a half-life of about 27 years. The gamma energy from Cs 137, however, is less penetrating than that of Co ~° and so is not a direct replacement for it; the lower energy also means that for a given dose rate more caesium is required. A plant is already under consideration for separating Cs 137 at the rate of about 1,500,000 c/annum commencing at the end of 1960. The gaseous and volatile fission products from high temperature gas-cooled reactors can be used as sources of gamma radiation. There are plans for concentrating these fission products, which are mostly short-lived, on carbon beds. O n account of the very short half-lives involved this form of source must be situated close to a reactor.