Radiation source rack for a pool-type cobalt-60 irradiator: the stainless frame of source modules and pencils, its hoist and guides, and how racks get stuck
Planar stainless frame of source modules and cobalt-60 pencils · raised from the storage pool by hoist · 14 stuck-rack reports in NRC's NUREG-1345
In a pool-type gamma irradiator the source is not a single object but a flat stainless frame, the rack or plaque, into which modules loaded with sealed cobalt-60 pencils are fitted. It hangs on cables, rises out of the pool into the cell for irradiation and drops back when the cell is entered. A buyer or rebuilder has to settle the frame size and module layout, where pencils of different activity go, and how the hoist, cables, guides and shroud keep the rack from jamming against product carriers.
Source racks (source plaques) of pool-type panoramic cobalt-60 irradiators: the stainless frame, source modules and pencil positions, the hoist and suspension cables that raise and lower it, guide cables or rails, protective shrouds around it, and the position sensing that tells the control system where it is. The complete irradiation plant with its cell, conveyor and controls is covered on the Gamma sterilization processing line page, and routine dosimeters and readers on the Dose monitoring system for gamma sterilisation page. The conveyor stations where product carriers are loaded and unloaded outside the cell are outside this page, as are the sealed source pencils as manufactured items and self-shielded or dry-storage irradiators.
CNFX editorial research · Sources checked October 13, 2026 · 3 sources
A planar frame of modules and pencils: the 6 ft × 8 ft × 2 in rack in Alpha Omega's T6-V irradiator
US patent 5,400,382, assigned to Alpha Omega Technologies (1995), describes the source rack of its T6-V automated irradiator as capable of holding hundreds of cobalt source pencils in modules that are assembled into a stainless frame. In one specification the rack is approximately 6 ft by 8 ft by 2 in, a flat radiating surface with product dwell units on both sides. [1]
Each pencil is described as a double-walled stainless-steel capsule about 18 in long containing up to 10,000 curies of cobalt-60, which has a half-life of about 5.26 years and emits gamma rays of 1.17 and 1.33 MeV per disintegration; the patent names caesium-137 as an alternative source. [1] Editorial calculation: 6 ft × 8 ft is about 1.83 m × 2.44 m, or 4.5 m² of radiating face, and 10,000 Ci is 3.7 × 10¹⁴ Bq.
The rack is stored in a pool of demineralised water that acts as the biological shield, and for use it is raised by a hydraulic hoist from below the floor slab to the in-air irradiation position, by remote control interlocked with the whole facility. The pool water is run through a demineraliser in a closed loop and surveyed for traces of radioactivity. [1]
Because the rack is planar, the patent's dwell units can be set at equal distances from it on either side, from as close as 15 cm outward, and it states that efficiency, the fraction of emitted energy absorbed by the product, ranges from 18 % to 35 % depending on pallet loading and product density. [1]
State the frame size, the number of module positions, the maximum installed activity the rack and pool are licensed for, and whether the rack is single or split into upper and lower halves.
02
Where the pencils sit: 14 pencils at 10 cm pitch, and a rearrangement that cut irradiation time by 10.7 %
A 2022 technical note in Nuclear Engineering and Technology by Hsingtzu Wu of Huazhong University of Science and Technology describes the arrangement it starts from: source pencils of cobalt-60 slugs are loaded in predetermined positions in source modules, which are then distributed over the source rack. Its model rack holds 14 pencils 10 cm apart, each 1 cm in diameter and 45 cm long at 3.7 × 10¹⁴ Bq, with every product position at least 20 cm from any pencil and positions at least 40 cm apart. [2]
The note keeps pencil size, the minimum pencil-to-product distance and the spacing between positions the same, and gains only by rearranging pencils and positions on the basis that radiation is emitted isotropically; each position is dwelt in for 1 minute. [2] It is a calculation, not a built rack, and it is limited to a single level.
Editorial calculation: the note's 3.7 × 10¹⁴ Bq per pencil is the same 10,000 Ci as the upper value in the Alpha Omega patent, so its 14-pencil model carries 140,000 Ci, a small fraction of a rack holding hundreds of pencils. [2][1]
Arrangement in the note
Irradiation positions
Result reported
14 pencils in one line at 10 cm pitch, shuffle-dwell at one level
About 8.7 % smaller area, 11.8 % more exposure, 10.7 % shorter irradiation time [2]
Ask for the module drawing with pencil positions, the activity of each pencil as loaded, and how replenishment pencils will be placed as the cobalt decays.
03
Hoist cables, guide cables and shrouds: the stuck racks and dropped plaques NRC reviewed in NUREG-1345
NRC's NUREG-1345, Review of Events at Large Pool-Type Irradiators (1989), groups equipment malfunctions such as improper source movement, source plaque problems, cable failures and conveyor failures, and counts 18 such events, more than in any other category it reports. Besides an earlier source-mechanism malfunction it lists 14 reports of the source rack becoming stuck, two of which led to fires. [3]
Across 13 events in which source movement was impeded, NUREG-1345 found product carriers interfering with the source plaque in six, and among its consequences two fires, two cases of individual pencils coming loose from the plaque and one distorted plaque. It counts seven events involving the source suspension cables in one place and six reports of cable problems in another, and for the RTI cable failure of 1981 found abrasion against the cable housing rather than fatigue. [3]
The fixes it records sit on the rack and its surroundings: after the Irvine event, a recommendation to modify the designs of the shroud, the upper plaque guides and the source hoist system, with the note that the cable should have been replaced instead of spliced; after the fires, inspection of product totes with deformed totes taken out of service; and after the 1981 cable failure, an improved maintenance programme. [3]
Event in NUREG-1345
What happened to the rack
Irvine, California, 1985
Plaque stuck exposed: the aluminium shroud had distorted and caught on the plaque frame; a design problem, with inadequate maintenance contributing [3]
Schaumberg, Illinois, 1986
Plaque stuck part-shielded because the cable had frayed and jammed; employees cut the cable and let the plaque free-fall into the pool [3]
Rockaway, New Jersey, 1988
Source cable broke; a module bent and pencils could only be removed with difficulty; all modules inspected, squared and reloaded [3]
North Canaan, Connecticut, 1981
Aluminium totes jammed into the rack; fire; top centre module and pencils found missing from the rack; totes redesigned and a shroud fitted around the rack [3]
Somerville, New Jersey, 1987
Control power lost with the plaque fully down; the cable drum kept turning and wound the plaque back to full-up until the motor stalled; hand-cranked down [3]
Ask how the rack is guided over its full travel, what shroud separates it from carriers, how cable wear is inspected and at what interval, and how rack position is sensed independently of the drum.
04
Data for an irradiator source rack
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Irradiator: [pool-type panoramic; cell and pool depth; maximum licensed activity]
Frame: [width × height × thickness; single or upper/lower halves; material]
Modules: [number; pencil positions per module; pencil type and length]
Loading: [initial activity; replenishment plan; pencil placement by activity]
Hoist: [hydraulic, pneumatic or electric; travel; speed; cables and drum]
Guides and shroud: [guide cables or rails; shroud material and clearance to carriers]
Position sensing: [up/down sensors; slack-cable detection; interlocks]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
How big is the source rack of a pool-type gamma irradiator?
In Alpha Omega's US patent 5,400,382 the rack of its T6-V irradiator is approximately 6 ft by 8 ft by 2 in, holding hundreds of cobalt-60 pencils in modules. [1]
How much cobalt-60 is in one source pencil?
The Alpha Omega patent gives up to 10,000 curies in a double-walled stainless capsule about 18 in long; the 2022 Wu note models pencils of 3.7 × 10¹⁴ Bq, 1 cm by 45 cm. [1][2]
Why do source racks get stuck?
In NUREG-1345, product carriers interfered with the source plaque in six of 13 impeded-movement events; frayed or broken cables and a distorted shroud caused others. [3]
Does rearranging pencils on the rack change throughput?
Wu's 2022 calculation for a 14-pencil shuffle-dwell layout reports 11.8 % more exposure and 10.7 % shorter irradiation time after rearranging pencils and positions. [2]
Sources and what still needs a direct check
Sources checked 2026-10-13. Alpha Omega Technologies' US patent on an automated pool-type irradiator (rack size, pencils, hoist), a 2022 technical note from Huazhong University of Science and Technology in Nuclear Engineering and Technology (pencil layout and spacing), and the U.S. Nuclear Regulatory Commission's NUREG-1345 review of events at large pool-type irradiators (stuck racks, cables, shrouds).
