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Electromechanical engineering aspects of irradiator design
Journal of Food Engineering 3 (1984)
Electromechanical
265-284
Engineering Aspects of Irradiator Design
J.C. Etienne and R. Buyle Institut
National
des RadioBlCments,
B-6220,
Fleurus,
Belgium
ABSTRACT IRE, Institut National des Radioelements at Fleurus, has been irradiating foodstuffs since 1979. The steadily-increasing demands of the food industry led IRE to design and install a second, different type of irradiator. Selection criteria for choosing between the different alternatives or possibilities are given based on the primary consideration that a contract food irradiator must be able to provide a service in accordance with the requirements of his customers. The principal components - the radiation source geometry, the transport system and the control systems - are described. The choice of the major electromechanical components is discussed taking into account their susceptibility to radiation damage.
INTRODUCTION The IRE decided in 1975 on the construction of an industrial-scale irradiation plant. Construction started in 1977 and the plant was operational in October 1978. Two cobalt-60 irradiators were built. The first, GAMMIR I, with a total design capacity of 1.5 MCi, has an actual cobalt-60 activity of about 300000 Ci and is used for the continuous radiosterilization of medical supplies. The second, GAMMIR II, has a maximum source strength of O-5 MCi and was built as a research irradiator, which allowed the necessary experience in food irradiation technology to be gained generally and especially in plant engineering, safety precautions and the training of operating personnel. During the years 1979-83, the Mediris Department (at the time engaged in the irradiation of medical supplies) of the IRE coordinated 265 Journal of Food Engineering 0260-8774/84/$03.00 Publishers
Ltd, England,
1984. Printed
in Great Britain
- @ Elsevier
Applied
Science
J. C. Etienne, R. Buyle
266
an extensive programme of research, development and promotion in the field of food irradiation. These activities led to a series of ministerial approvals authorising the irradiation of foodstuffs. Table 1 lists the irradiated food materials cleared in Belgium as of mid-1984. Ministerial approvals authorising the commercial application of the irradiation process had allowed IRE to irradiate increasing quantities of foodstuffs, first on a research basis and then on an industrial scale. Table 2 shows quantities of food items irradiated at IRE installations annually since 1978.
2. DESCRIPTION OF ORIGINAL PLANT GAMMIR II, as it was originally designed and constructed, three main parts (see Fig. 1):
consisted of
TABLE 1
List of Clearancesin Belgium, Mid-1984 (from ‘ArrdtBs ministbriels publies Moniteurs Belges’ of 16.07.1980, Product
Potatoes Strawberries Onions Garlic Shallots Animal feeds Paprika Pepper Herbs and spices (78 different kinds) Dried vegetables Gum arabic
16.10.1980
Purpose oj irradiation
Sprout inhibition Radurization Sprout inhibition Sprout inhibition Sprout inhibition
and 29.09.1983) Dose
Date of
lkGyl
approval
0.15
July 1980 July 1980 October 1980 October 1980 October 1980
3
Decontamination Decontamination
0.15 0.15 0.15 60, max. 10, max. 10, max.
Decontamination Decontamination Decontamination
10, max. 10, max. 7, max.
October 1980 October 1980 September September September
1983 1983 1983
Other products for which clearance has been sought: chicken, crustacea, papain, frogs legs, dried beef and pork plasma, gelatine, dried egg products, herb teas.
Electromechanical
engineering aspects of irradiator design
267
TABLE 2 Quantities of Foodstuffs Irradiated by IRE Since 1978 Year
Toons
1978 1979 1980 1981 1982 1983
0 92 162 633 1427 1820
Fig. 1.
Original GAMMTR I1 plant.
(i) The building, which is constructed of standard density concrete, houses the product-handling equipment and source storage pool. One access to the irradiation chamber is through a large concrete door. It was also originally intended to install, subsequently, a lead rotating door through which to load and unload the irradiation room automatically; this place was initially blocked with a concrete plug.
268
J. C. Etienne,
R. Buyle
(ii) The product handling system which consisted of a simple conveyor moving 15 aluminium boxes each of 100 litres capacity around a circular radiation source. The products were loaded manually into the aluminium boxes inside the cell. (iii) The radiation source rack which consisted of a series of tubes each containing one cobalt pencil. An elevating mechanism was provided for raising the radiation source out of the 5.5 m deep water-filled storage pool into the irradiating position and for returning it to the safe position at the bottom of the pool. The safe position permitted access to the irradiation chamber for loading the conveyor and for maintenance of the electric, pneumatic and mechanical equipment. This irradiator was used, inter alia, to irradiate foodstuffs but it was not adequate to meet the increasing demands of the food industry. At the end of 198 1, it was decided to modify the handling system in order to increase its capacity.
3. FACTORS 3.1.
TO BE CONSIDERED
Physics of gamma radiation
3. I. I. Radionuclide source availability Gamma irradiation installations use either cobalt-60 or caesium-137 as the radiation source. Cobalt-60 is currently used more widely than caesium- 13 7. Cobalt-60 is produced in research reactors and CANDU nuclear power stations. Atomic Energy of Canada Ltd (AECL), produce a large fraction of the cobalt-60 used in irradiators throughout the world. 20 MCi are currently being produced per year by the Pickering reactors (Ontario). A demand of about two or three times this in 10 years is projected. Because of this expectation, serious consideration is being given to commercial production at the Bruce and Darlington reactor complexes. Each site will be capable of producing about 15 MCi year-‘. Caesium-137 comes from the reprocessing of spent nuclear fuel. In future years the availability of caesium-137 from reprocessed nuclear power plant wastes will depend on the efficiency of the process for producing caesium in a usable form. Recent projections indicate that a
Electromechanical
engineering aspects of irradiator design
269
total of about 700-1000 MCi year-’ will be available by 1990 from the different reprocessing plants. 3.1.2. Cobalt-60 characteristics Cobalt-60 is produced by irradiating cobalt-59 in nuclear reactors. It decays to stable nickel with the emission of beta particles and y-rays as shown in Fig. 2(a). Cobalt-60 has a half-life of 5.26 years and the specific activity of industrial cobalt used in y-sources is about 60-100 Ci g-l of metallic cobalt or cobalt oxide. The radiation energy is generated at the rate of 15 kW MCi-’ and is converted to heat by absorption of the emitted radiation either within the source (lo-30%) or in the material irradiated and the irradiator structure. The half-life of cobalt-60 requires that the irradiator owner adds some 12.3% of the original activity every year to maintain a constant capacity. 3.1.3. Caesium-I 3 7 characteristics Caesium-137 is recovered from nuclear fission product waste. It decays with the emission of &particles and y-rays to stable barium as shown in Fig. 2(b). Caesium-137 has a conveniently long half-life (about 30 years) and a reasonably good specific activity in the chloride form (25 Ci g-‘) for use in y-sources. Radiation energy at the rate of O-12 W g-i caesium chloride (4.84 kW MCi-‘) is generated and converted to
(5263years)
12% par-’ 15kW MCI-’
12% 5years“
Metallic cobalt Cobalt oxide 60-100 Ci g”
Caeslum chloride
(a) Fig. 2.
(30 yuars)
4.64 kW MCI-’
-25 Ci g“ (b)
Decay schemes for (a) cobalt-60 and (b) caesium-137.
J. C. Etienne,
270
R. Buyle
heat by absorption within the source (c. 55%), in the irradiated material and in the irradiator structure. The slow decay of caesium-137 results in a relatively constant y field intensity in an irradiator. The relatively long half-life requires that the irradiator owner adds 12% of the original activity only every 5 years to maintain a reasonably constant flux field in the radiation zone. 3.1.4. IRE’s choice IRE manufactures high-specific-activity cobalt-60 sources for cobaltotherapy and industrial radiography, but not the large, relatively lowspecific-activity industrial cobalt-60 sealed sources. Up to now, IRE has bought its industrial sources from the Commissariat B 1’Energie Atomique (CEA) of France. Figure 3 shows the COP-4 source. This source can have a strength of approximately 20 000 Ci cobalt-60 and was chosen for the loading of IRE irradiators which were designed and constructed by Sulzer F&es, Winterthur, Switzerland. The source racks of the two irradiators were nevertheless designed in order to be able to use either AECL’s or CEA’s pencils. The use of caesium-137 was considered but not adopted as it requires more sophisticated control systems because of the high radiotoxicity of this radionuclide.
Fig. 3.
COP - 4.S2
source - classification IS0 CEA, France.)
DIS 2919
E63446.
(Courtesy
Electromechanical
Fig. 4.
engineering aspects of irradiator design
(Courtesy DOE/USDA/AIBS (1982) and AECL (1983).)
Figure 4 shows AECL’s cobalt-60 capsule. 3.2.
271
pencil and Handford’s
caesium-137
Absorbed dose: overdose ratio efficiency
Cobalt-60 and caesium-137 emit p- and y-radiation. Beta radiation electrons travelling at high speeds - can be stopped by a sheet of aluminium a few millimetres thick and are therefore stopped by the wall of the source container. Gamma radiation consists of high energy photons which are very penetrating. This is the type of radiation that is used in the r-irradiation process. The energy levels of the y-irradiation from cobalt-60 and caesium-137 are too low to induce radioactivity in the material being irradiated. A flux field radiates from a radiation source in all directions and in straight lines. The intensity of the generated radiation depends on two factors: the activity (measured in curies or in becquerels) and the configuration of the irradiation source. The ‘dose’ of y-radiation absorbed by a product depends on the radiation intensity and the product density and decreases exponentially with distance as the photons pass through the material. Figure 5(a) shows how the dose absorbed decreases in intensity through the product in a direction away from the source.
272
Fig. 5.
J. C. Etienne,
R. Buyle
Relationship of absorbed dose: (a) to thickness. T, and density, p; (b) one side/two pass irradiation; (c> two sides/two pass irradiation.
The ratio of maximum-to-minimum absorbed dose is a function of three factors: pack density, pack thickness and conveyor and source configuration (Figs 5(a)-S(c)). The maximum-to-minimum absorbed dose ratio or overdose ratio is defined as the ratio of maximum-to-minimum dose in a product pack. In order to achieve at least the minimum irradiation dose throughout the product pack, it is necessary to deliver more than this dose to certain parts of the pack. If the desired minimum dose should be 0.66 Mrad then the maximum irradiation dose will be 1 Mrad if the overdose ratio in a particular operation is 1.5.
Electromechanical
engineering aspects of irradiator design
273
Overdose ratios may vary from one pack to another if it is necessary to handle products of different pack densities. Irradiation efficiency is defined as the fraction of the total amount of y-radiation emitted from the radiation source that is absorbed in the product. This must be based on the minimum pack dose and the socalled curie content of the radiation source. The curie content of a radiation source is greater than the curie output by the amount of the emitted radiation absorbed in the radiation source itself. The irradiation efficiency depends on the manner in which products surround the radiation source and must be specified as a function of product pack density.
3.3.
Operating personnel protection
The safety of the operating personnel is governed by standards and laws determined by official committees and health ministries. The permissible dose has been fixed by the CIPR (Commission Internationale de Protection contres les Rayonnements) to: - 500 mrems year- ’ for industrial workers and the public. - 5000 mrems year- ’ for specialised nuclear workers. Commercial irradiators installed generally in industrial countries are permitted to produce no more than 0.25 mrem h-i at the external face of the irradiation chamber. Operating personnel could thus potentially receive over 50 weeks of 40 h each, a maximum annual dose of 500 mrems. In practice, they receive much less. (Man is already exposed to natural radioactivity which is estimated at 130 mrems year- 1 for countries such as Belgium and Great Britain, but could be more than 1000-5000 mrems year-’ in countries such as India or Bolivia.) This requirement determines the thickness of the irradiator walls, the depth of the water storage pool, the geometry and location of various inserts required for mechanical and electrical components and arrangements for access. A safe environment for operating personnel must be complemented by safety precautions to prevent access to the irradiation zone during irradiation or when the radiation source has not been lowered properly to its safe position. These aspects are discussed below.
274
J. C. Etienne,
R. Buyle
3.4. Processing factors To determine known:
the optimum
irradiator
design, the following
must be
(a) Type of irradiator: - In-line irradiator - this type irradiates a well-defined product (weight, shape, pack, density and dose are generally known explicitly). - Contract irradiator - this type irradiates many different products. Each product must be irradiated in accordance with specific legislation and customer requirements. (b) The minimum absorbed dose, the tolerated maximum absorbed dose and the overdose ratio which the product or the food can tolerate without significant radiation degradation effects. Table 3 shows the three principal dose ranges and their applications. (cl The plant capacity: - Weekly and annual product throughputs. - Anticipated growth. - Plant operation with regard to labour availability and cost.
Radiation
TABLE 3 Dose Required for Various Objectives
Low dose (< I kGy, 100 krad) Sprout inhibition Insect disinfestation Ripening delay
Onions, potatoes, etc. Fruits Avocados
Medium dose (I-10 kGy, 100-1000 krad) Reduction of microbial load Deep-frozen shrimps, spices, dried vegetables Extension of shelf life I High dose (1 O-50 kGy, 1000-5000 Commercial sterilization
krad) Animal feeds: food for consumption by patients who require a sterile diet as an essential part of their treatment
E~ectr#~gc~a~i~al
engineering aspects of irradiator design
275
4. IRE’S APPROACH Figure 6 illustrates the range of packs a contract irradiator must be prepared to handle. Table 4 shows the quantities of different foodstuffs IRE irradiated in 1982 and the radiation dose range required for each food item. Taking into account the limitations imposed by the existing concrete structure, the following were selected: 1. Use of carriers of smaller area than the co~esponding source area in order to avoid the need to reload and thus to reduce to the minimum labour requirements for loading and unloading the products. 2. The dimensions of the carriers were chosen to provide: - Good uniformity and dist~bution of dose. - An acceptable maximum-to-minimum absorbed dose ratio for high density products. - A capacity of 250 kg per carrier.
Fig. 6.
J. C. Etienne, R. Buyle
276
TABLE4 Quantities
of Food Items Irradiated by IRE in 1982 and Associated Doses (from Lacroix, 1983) Quantities
Product
Food for laboratory Ingredients Spices Frozen foods Enzymes
animals
140 678 281 315 13
tonnes tonnes tonnes tonnes tonnes
Minimum dose
kGy
Mad
10-50 6 5-10 2-4 10
l-5 0.6 05-l 0.2-0.4 1
1427 tonnes
Total
3. Choice of a batch-type irradiator to provide a high degree of flexibility in the organising of the irradiation programme (diversified densities, high diversity of doses, etc.). 4. Automatic loading and unloading of the irradiation room. 5. Possibility of unloading and loading another set of carriers whilst one is undergoing irradiation.
5. DESCRIPTION
OF THE NEW PLANT
The new GAMMIR II plant is shown in Fig. 7. It incorporates a 2m high source rack (6) and aluminium carriers containing 1.6 m high product stacks. Each carrier has a volume of O-5 m3. The carriers circulate on a closed loop within the cell (1). They are conveyed horizontally by overhead trolleys driven by pawls attached to a driving chain. The source-pass mechanism was designed and constructed to process 12 product carriers which are loaded into the irradiation chamber batch-wise in a double row on each side of the source rack. The movement of the carriers around the radiation source is automatic so that each carrier occupies each carrier position for an equal interval of time. The interval time setting, which of course depends on the installed cobalt-60 activity. the density of the product
Electromechanical
engineering aspects of irradiator design
271
Fig. 7. GAMMIR II - new plant. 1, Internal conveyor; 2, entrance for product into the radiation cell; 3, feed and discharge device; 4A, input storage conveyor; 4B, output storage conveyor; 5, cell entrance door; 6, source rack.
and the radiation dose required, is regulated by the master timer on the control console. The carriers are loaded with product at an external loading station before being transferred to the input storage conveyor (4A). The input storage conveyor accommodates 12 product carriers awaiting automatic entry into the irradiation chamber. Following irradiation, the carriers containing irradiated product are returned automatically to the output storage conveyor (4B) before being unloaded at the unloading station. After unloading, the empty carriers are automatically transferred to the loading station for reloading with unirradiated product. Loading and unloading are performed manually with the carriers in a vertical position (Figs. 8 and 9). The discharging and charging of the irrradiation chamber are also performed automatically with a transfer trolley (3) which transfers the carriers between the internal conveyor and the external conveyor through a door (2) which is, of course, closed and bolted during the irradiation cycle. Operating for 7000 h year-’ and allowing 20 min to load and unload each batch, the irradiator output is about 2350 tonne-Mrad year-’ with a 300 000 Ci cobalt-60 source (see Table 5). The design of the irradiator permits wide flexibility in the dose delivered to the product:
278
J. C. Etienne, R. Buyle
Fig. 8.
Fig. 9.
Loading station.
Electromechanical
engineering aspects of irradiator design
219
TABLE 5 Typical Irradiation Programme for a Week with a 300 000 Ci Cobalt-60 Source Products
Deep-frozen foods Dried vegetables Spices Animal feeds
Tonnes
Dose
batch-’
(key)
Batches week-’
Equivalent (tonne-M& year-‘)
2040 1800 2700 2160 1200
3-5 6 10 25 40
9 5 3 4 1
367.2 270.0 405.2 1080.0 240.0 Total: 2350.0
for example, at a product density of 0.5 g cmm3 with a 1 MCi radiation source, a dose as low as 0.10 Mrad is possible. Figure 10 shows an internal view of the irradiation chamber with the source rack and different carriers.
6. RADIATION RESISTANCE OF MATERIALS AND COMPONENTS The most important chemical changes which occur during the irradiation of polymers are crosslinking and degradation. When a polymer is irradiated, both these processes occur, usually competitively but at different rates. The presence of oxygen and water affects the rate at which such crosslinking and degradation occurs. Chemical transformations occurring on irradiation of polymers result in considerable changes in their mechanical properties and can also affect their electrical properties, both of which are of the utmost importance in irradiator engineering. Variations in temperature and humidity may cause water to condense on the irradiated component. The absorption of this condensed water depends on the component size, porosity and other physical properties. The radiolysis of the absorbed water may lead to the formation of
J. C. Etienne,
280
Fig. 10.
Internal
R. Buyle
view of the irradiation chamber different carriers.
with
the source
rack and
in gaseous form, initiate stresses inside the material which accelerate the degradation process. In order to obtain a satisfactory operating performance, the selection of electrical insulating materials, electric limit switches and pneumatic cylinders must take into account the above-mentioned degradations. Figure f 1 shows the different components selected for the IRE irradiation chamber: hydrogen
and oxygen
(a) The normal microswitch.
which,
electrical limit switch was replaced by a Honeywell This switch was modified as follows:
Electromechanical
engineering aspects of irradiator design
281
Fig. It.
- The electric cable was replaced by a mineral-insulated wiring cable. - The small conductors were insulated with mineral beads. -- The O-ring was removed and left out. (b) The standard pneumatic actuator was modified in order to avoid degradation of the ‘Perbunan’ rings which were replaced by metallic seals. Metals and ceramics are not may cause some operating bearings in the radiation impregnated self-lubricating steel ball-bearings.
affected by radiation; nevertheless, corrosion problems. For example, carbon steel ballchamber have to be replaced by graphitebushings or specially-manufactured stainless
7. RADIATION PROTECTION EQUIPMENT Gamma-level guards are used for measuring local radiation levels and for monitoring spaces where radiation could conceivably exceed permissible levels. Figure 12 shows the location of the three gamma-level guards.
J. C. Etienne,
282
R. Buyle
Fig. 12. Location of the different y-level guards. Front view (RHS): (a) Built-in hooter; (b) acknowledgement of hooter; (c) trouble indicator; (d) indicator for operation of acknowledgement key; (e) + (f) 1st and 2nd alarm stages, respectively, with checking of setting; (,g) indicating instrument. Geiger-Miiller The electronics such as:
(GM) counter tubes are used as radiation detectors. connected to the GM must include various functions
(9 count indication; (ii) (iii) (iv) (v)
danger indication (excess count); normal operation indication; two alarm thresholds: continuous checking system to verify proper functioning.
The GM counter (A) is connected to indicating instruments on the control console in the control area in order to avoid damage to the electronics by radiation. The GM counter tubes (B) and (C) and their associated indicating instruments are located on the external wall of the irradiation chamber near the access doors so that the radiation level is automatically checked when the operator opens a door. These three monitors are connected to: (i) the door locks in such a when the radiation at ~ one of the two monitors ~ the monitor (A) is not in this case, the first calibrated source.
way that the doors cannot
be opened
(B) or (C) is above a certain level: between the two alarm thresholds alarm threshold is overridden with a
Electromechanical
engineering aspects of irradiator design
283
(ii) the source lifting mechanism in such a way that the radiation source is automatically lowered into the storage pool when the radiation measured by either of the two monitors (B) or (C) is above the second alarm threshold. Moreover, only authorised personnel carrying a portable radiation monitor are permitted to enter the irradiation chamber. Such an instrument is always available in the control room of the irradiator along with a calibrated test source.
8. REGULATIONS These regulations
AND OTHER SAFETY
CONTROLS
include:
- general operating instructions; - special instructions for the daily operators; - special instructions for the maintenance technicians. Safety control
also includes many other safety features
such as:
(i) a delay timer located in the irradiation chamber compelling the operator to ensure that no one is in the room prior to start-up; (ii) one single operating or master key which is made available only to the authorised operating personnel; (iii) any timing or other irregularities in the automatic operations cause the irradiation process to be interrupted by returning the source to the safe storage position; (iv) emergency ‘pull’ connected to the control system located all round the walls inside the irradiation chamber for emergency return of the radiation source to the storage pool or to prevent start-up when operated manually.
9. PLANT MAINTENANCE Downtime is generally limited to repairing malfunctions of pneumatic cylinders or electric limit switches. The cause of each shutdown is recorded in a log book which is periodically reviewed. Through the use of this information, preventive maintenance procedures can be scheduled. Even though operational downtime is normally minimal, there are areas
284
1. C. Etienne, R. Buyle
which need continuous attention. such as those with high radiation fields. Preventive maintenance is divided into three frequencies - monthly, semi-annual and annual. - Monthly: preventive maintenance includes a general inspection of the facility. During this downtime, the plant is cleaned and general lubrication carried out. --- Annual and semi-annual: lnaint~l~allce includes the replacement of specific components, inspection and servicing of the monitoring instruments and checking of the electronic components of the control desk.
10. CONCLUSION The economics of this batch proved favourable. The versatility of the unit is In 12 months, only minor enced with this plant, despite and safety.
irradiator
for contract
irradiation
have
considerable. operational problems have been experitoday’s high demands for productivity
REFERENCES AECL (1983). 3rd Gamma Processing Seminar. Ottawa, 6-9 June. DOE/USDA/AIBS (1982). Workshop OH Low Dose Irradiation Treatment of Agricultural Commodities, Washington DC. 19-21 April. Lxroix. J.-P. (1983). Belgiatz National Report in the Field of Food lrradiat~o~~, IRE, Fleurus. (14rk A~?~zual~eeti~~g of the ~~ropea~zSociety of~~elear~et~ods i/l Agric~dtrtre, Madrid. 5-9 September.)
Electromechanical
265-284
Engineering Aspects of Irradiator Design
J.C. Etienne and R. Buyle Institut
National
des RadioBlCments,
B-6220,
Fleurus,
Belgium
ABSTRACT IRE, Institut National des Radioelements at Fleurus, has been irradiating foodstuffs since 1979. The steadily-increasing demands of the food industry led IRE to design and install a second, different type of irradiator. Selection criteria for choosing between the different alternatives or possibilities are given based on the primary consideration that a contract food irradiator must be able to provide a service in accordance with the requirements of his customers. The principal components - the radiation source geometry, the transport system and the control systems - are described. The choice of the major electromechanical components is discussed taking into account their susceptibility to radiation damage.
INTRODUCTION The IRE decided in 1975 on the construction of an industrial-scale irradiation plant. Construction started in 1977 and the plant was operational in October 1978. Two cobalt-60 irradiators were built. The first, GAMMIR I, with a total design capacity of 1.5 MCi, has an actual cobalt-60 activity of about 300000 Ci and is used for the continuous radiosterilization of medical supplies. The second, GAMMIR II, has a maximum source strength of O-5 MCi and was built as a research irradiator, which allowed the necessary experience in food irradiation technology to be gained generally and especially in plant engineering, safety precautions and the training of operating personnel. During the years 1979-83, the Mediris Department (at the time engaged in the irradiation of medical supplies) of the IRE coordinated 265 Journal of Food Engineering 0260-8774/84/$03.00 Publishers
Ltd, England,
1984. Printed
in Great Britain
- @ Elsevier
Applied
Science
J. C. Etienne, R. Buyle
266
an extensive programme of research, development and promotion in the field of food irradiation. These activities led to a series of ministerial approvals authorising the irradiation of foodstuffs. Table 1 lists the irradiated food materials cleared in Belgium as of mid-1984. Ministerial approvals authorising the commercial application of the irradiation process had allowed IRE to irradiate increasing quantities of foodstuffs, first on a research basis and then on an industrial scale. Table 2 shows quantities of food items irradiated at IRE installations annually since 1978.
2. DESCRIPTION OF ORIGINAL PLANT GAMMIR II, as it was originally designed and constructed, three main parts (see Fig. 1):
consisted of
TABLE 1
List of Clearancesin Belgium, Mid-1984 (from ‘ArrdtBs ministbriels publies Moniteurs Belges’ of 16.07.1980, Product
Potatoes Strawberries Onions Garlic Shallots Animal feeds Paprika Pepper Herbs and spices (78 different kinds) Dried vegetables Gum arabic
16.10.1980
Purpose oj irradiation
Sprout inhibition Radurization Sprout inhibition Sprout inhibition Sprout inhibition
and 29.09.1983) Dose
Date of
lkGyl
approval
0.15
July 1980 July 1980 October 1980 October 1980 October 1980
3
Decontamination Decontamination
0.15 0.15 0.15 60, max. 10, max. 10, max.
Decontamination Decontamination Decontamination
10, max. 10, max. 7, max.
October 1980 October 1980 September September September
1983 1983 1983
Other products for which clearance has been sought: chicken, crustacea, papain, frogs legs, dried beef and pork plasma, gelatine, dried egg products, herb teas.
Electromechanical
engineering aspects of irradiator design
267
TABLE 2 Quantities of Foodstuffs Irradiated by IRE Since 1978 Year
Toons
1978 1979 1980 1981 1982 1983
0 92 162 633 1427 1820
Fig. 1.
Original GAMMTR I1 plant.
(i) The building, which is constructed of standard density concrete, houses the product-handling equipment and source storage pool. One access to the irradiation chamber is through a large concrete door. It was also originally intended to install, subsequently, a lead rotating door through which to load and unload the irradiation room automatically; this place was initially blocked with a concrete plug.
268
J. C. Etienne,
R. Buyle
(ii) The product handling system which consisted of a simple conveyor moving 15 aluminium boxes each of 100 litres capacity around a circular radiation source. The products were loaded manually into the aluminium boxes inside the cell. (iii) The radiation source rack which consisted of a series of tubes each containing one cobalt pencil. An elevating mechanism was provided for raising the radiation source out of the 5.5 m deep water-filled storage pool into the irradiating position and for returning it to the safe position at the bottom of the pool. The safe position permitted access to the irradiation chamber for loading the conveyor and for maintenance of the electric, pneumatic and mechanical equipment. This irradiator was used, inter alia, to irradiate foodstuffs but it was not adequate to meet the increasing demands of the food industry. At the end of 198 1, it was decided to modify the handling system in order to increase its capacity.
3. FACTORS 3.1.
TO BE CONSIDERED
Physics of gamma radiation
3. I. I. Radionuclide source availability Gamma irradiation installations use either cobalt-60 or caesium-137 as the radiation source. Cobalt-60 is currently used more widely than caesium- 13 7. Cobalt-60 is produced in research reactors and CANDU nuclear power stations. Atomic Energy of Canada Ltd (AECL), produce a large fraction of the cobalt-60 used in irradiators throughout the world. 20 MCi are currently being produced per year by the Pickering reactors (Ontario). A demand of about two or three times this in 10 years is projected. Because of this expectation, serious consideration is being given to commercial production at the Bruce and Darlington reactor complexes. Each site will be capable of producing about 15 MCi year-‘. Caesium-137 comes from the reprocessing of spent nuclear fuel. In future years the availability of caesium-137 from reprocessed nuclear power plant wastes will depend on the efficiency of the process for producing caesium in a usable form. Recent projections indicate that a
Electromechanical
engineering aspects of irradiator design
269
total of about 700-1000 MCi year-’ will be available by 1990 from the different reprocessing plants. 3.1.2. Cobalt-60 characteristics Cobalt-60 is produced by irradiating cobalt-59 in nuclear reactors. It decays to stable nickel with the emission of beta particles and y-rays as shown in Fig. 2(a). Cobalt-60 has a half-life of 5.26 years and the specific activity of industrial cobalt used in y-sources is about 60-100 Ci g-l of metallic cobalt or cobalt oxide. The radiation energy is generated at the rate of 15 kW MCi-’ and is converted to heat by absorption of the emitted radiation either within the source (lo-30%) or in the material irradiated and the irradiator structure. The half-life of cobalt-60 requires that the irradiator owner adds some 12.3% of the original activity every year to maintain a constant capacity. 3.1.3. Caesium-I 3 7 characteristics Caesium-137 is recovered from nuclear fission product waste. It decays with the emission of &particles and y-rays to stable barium as shown in Fig. 2(b). Caesium-137 has a conveniently long half-life (about 30 years) and a reasonably good specific activity in the chloride form (25 Ci g-‘) for use in y-sources. Radiation energy at the rate of O-12 W g-i caesium chloride (4.84 kW MCi-‘) is generated and converted to
(5263years)
12% par-’ 15kW MCI-’
12% 5years“
Metallic cobalt Cobalt oxide 60-100 Ci g”
Caeslum chloride
(a) Fig. 2.
(30 yuars)
4.64 kW MCI-’
-25 Ci g“ (b)
Decay schemes for (a) cobalt-60 and (b) caesium-137.
J. C. Etienne,
270
R. Buyle
heat by absorption within the source (c. 55%), in the irradiated material and in the irradiator structure. The slow decay of caesium-137 results in a relatively constant y field intensity in an irradiator. The relatively long half-life requires that the irradiator owner adds 12% of the original activity only every 5 years to maintain a reasonably constant flux field in the radiation zone. 3.1.4. IRE’s choice IRE manufactures high-specific-activity cobalt-60 sources for cobaltotherapy and industrial radiography, but not the large, relatively lowspecific-activity industrial cobalt-60 sealed sources. Up to now, IRE has bought its industrial sources from the Commissariat B 1’Energie Atomique (CEA) of France. Figure 3 shows the COP-4 source. This source can have a strength of approximately 20 000 Ci cobalt-60 and was chosen for the loading of IRE irradiators which were designed and constructed by Sulzer F&es, Winterthur, Switzerland. The source racks of the two irradiators were nevertheless designed in order to be able to use either AECL’s or CEA’s pencils. The use of caesium-137 was considered but not adopted as it requires more sophisticated control systems because of the high radiotoxicity of this radionuclide.
Fig. 3.
COP - 4.S2
source - classification IS0 CEA, France.)
DIS 2919
E63446.
(Courtesy
Electromechanical
Fig. 4.
engineering aspects of irradiator design
(Courtesy DOE/USDA/AIBS (1982) and AECL (1983).)
Figure 4 shows AECL’s cobalt-60 capsule. 3.2.
271
pencil and Handford’s
caesium-137
Absorbed dose: overdose ratio efficiency
Cobalt-60 and caesium-137 emit p- and y-radiation. Beta radiation electrons travelling at high speeds - can be stopped by a sheet of aluminium a few millimetres thick and are therefore stopped by the wall of the source container. Gamma radiation consists of high energy photons which are very penetrating. This is the type of radiation that is used in the r-irradiation process. The energy levels of the y-irradiation from cobalt-60 and caesium-137 are too low to induce radioactivity in the material being irradiated. A flux field radiates from a radiation source in all directions and in straight lines. The intensity of the generated radiation depends on two factors: the activity (measured in curies or in becquerels) and the configuration of the irradiation source. The ‘dose’ of y-radiation absorbed by a product depends on the radiation intensity and the product density and decreases exponentially with distance as the photons pass through the material. Figure 5(a) shows how the dose absorbed decreases in intensity through the product in a direction away from the source.
272
Fig. 5.
J. C. Etienne,
R. Buyle
Relationship of absorbed dose: (a) to thickness. T, and density, p; (b) one side/two pass irradiation; (c> two sides/two pass irradiation.
The ratio of maximum-to-minimum absorbed dose is a function of three factors: pack density, pack thickness and conveyor and source configuration (Figs 5(a)-S(c)). The maximum-to-minimum absorbed dose ratio or overdose ratio is defined as the ratio of maximum-to-minimum dose in a product pack. In order to achieve at least the minimum irradiation dose throughout the product pack, it is necessary to deliver more than this dose to certain parts of the pack. If the desired minimum dose should be 0.66 Mrad then the maximum irradiation dose will be 1 Mrad if the overdose ratio in a particular operation is 1.5.
Electromechanical
engineering aspects of irradiator design
273
Overdose ratios may vary from one pack to another if it is necessary to handle products of different pack densities. Irradiation efficiency is defined as the fraction of the total amount of y-radiation emitted from the radiation source that is absorbed in the product. This must be based on the minimum pack dose and the socalled curie content of the radiation source. The curie content of a radiation source is greater than the curie output by the amount of the emitted radiation absorbed in the radiation source itself. The irradiation efficiency depends on the manner in which products surround the radiation source and must be specified as a function of product pack density.
3.3.
Operating personnel protection
The safety of the operating personnel is governed by standards and laws determined by official committees and health ministries. The permissible dose has been fixed by the CIPR (Commission Internationale de Protection contres les Rayonnements) to: - 500 mrems year- ’ for industrial workers and the public. - 5000 mrems year- ’ for specialised nuclear workers. Commercial irradiators installed generally in industrial countries are permitted to produce no more than 0.25 mrem h-i at the external face of the irradiation chamber. Operating personnel could thus potentially receive over 50 weeks of 40 h each, a maximum annual dose of 500 mrems. In practice, they receive much less. (Man is already exposed to natural radioactivity which is estimated at 130 mrems year- 1 for countries such as Belgium and Great Britain, but could be more than 1000-5000 mrems year-’ in countries such as India or Bolivia.) This requirement determines the thickness of the irradiator walls, the depth of the water storage pool, the geometry and location of various inserts required for mechanical and electrical components and arrangements for access. A safe environment for operating personnel must be complemented by safety precautions to prevent access to the irradiation zone during irradiation or when the radiation source has not been lowered properly to its safe position. These aspects are discussed below.
274
J. C. Etienne,
R. Buyle
3.4. Processing factors To determine known:
the optimum
irradiator
design, the following
must be
(a) Type of irradiator: - In-line irradiator - this type irradiates a well-defined product (weight, shape, pack, density and dose are generally known explicitly). - Contract irradiator - this type irradiates many different products. Each product must be irradiated in accordance with specific legislation and customer requirements. (b) The minimum absorbed dose, the tolerated maximum absorbed dose and the overdose ratio which the product or the food can tolerate without significant radiation degradation effects. Table 3 shows the three principal dose ranges and their applications. (cl The plant capacity: - Weekly and annual product throughputs. - Anticipated growth. - Plant operation with regard to labour availability and cost.
Radiation
TABLE 3 Dose Required for Various Objectives
Low dose (< I kGy, 100 krad) Sprout inhibition Insect disinfestation Ripening delay
Onions, potatoes, etc. Fruits Avocados
Medium dose (I-10 kGy, 100-1000 krad) Reduction of microbial load Deep-frozen shrimps, spices, dried vegetables Extension of shelf life I High dose (1 O-50 kGy, 1000-5000 Commercial sterilization
krad) Animal feeds: food for consumption by patients who require a sterile diet as an essential part of their treatment
E~ectr#~gc~a~i~al
engineering aspects of irradiator design
275
4. IRE’S APPROACH Figure 6 illustrates the range of packs a contract irradiator must be prepared to handle. Table 4 shows the quantities of different foodstuffs IRE irradiated in 1982 and the radiation dose range required for each food item. Taking into account the limitations imposed by the existing concrete structure, the following were selected: 1. Use of carriers of smaller area than the co~esponding source area in order to avoid the need to reload and thus to reduce to the minimum labour requirements for loading and unloading the products. 2. The dimensions of the carriers were chosen to provide: - Good uniformity and dist~bution of dose. - An acceptable maximum-to-minimum absorbed dose ratio for high density products. - A capacity of 250 kg per carrier.
Fig. 6.
J. C. Etienne, R. Buyle
276
TABLE4 Quantities
of Food Items Irradiated by IRE in 1982 and Associated Doses (from Lacroix, 1983) Quantities
Product
Food for laboratory Ingredients Spices Frozen foods Enzymes
animals
140 678 281 315 13
tonnes tonnes tonnes tonnes tonnes
Minimum dose
kGy
Mad
10-50 6 5-10 2-4 10
l-5 0.6 05-l 0.2-0.4 1
1427 tonnes
Total
3. Choice of a batch-type irradiator to provide a high degree of flexibility in the organising of the irradiation programme (diversified densities, high diversity of doses, etc.). 4. Automatic loading and unloading of the irradiation room. 5. Possibility of unloading and loading another set of carriers whilst one is undergoing irradiation.
5. DESCRIPTION
OF THE NEW PLANT
The new GAMMIR II plant is shown in Fig. 7. It incorporates a 2m high source rack (6) and aluminium carriers containing 1.6 m high product stacks. Each carrier has a volume of O-5 m3. The carriers circulate on a closed loop within the cell (1). They are conveyed horizontally by overhead trolleys driven by pawls attached to a driving chain. The source-pass mechanism was designed and constructed to process 12 product carriers which are loaded into the irradiation chamber batch-wise in a double row on each side of the source rack. The movement of the carriers around the radiation source is automatic so that each carrier occupies each carrier position for an equal interval of time. The interval time setting, which of course depends on the installed cobalt-60 activity. the density of the product
Electromechanical
engineering aspects of irradiator design
271
Fig. 7. GAMMIR II - new plant. 1, Internal conveyor; 2, entrance for product into the radiation cell; 3, feed and discharge device; 4A, input storage conveyor; 4B, output storage conveyor; 5, cell entrance door; 6, source rack.
and the radiation dose required, is regulated by the master timer on the control console. The carriers are loaded with product at an external loading station before being transferred to the input storage conveyor (4A). The input storage conveyor accommodates 12 product carriers awaiting automatic entry into the irradiation chamber. Following irradiation, the carriers containing irradiated product are returned automatically to the output storage conveyor (4B) before being unloaded at the unloading station. After unloading, the empty carriers are automatically transferred to the loading station for reloading with unirradiated product. Loading and unloading are performed manually with the carriers in a vertical position (Figs. 8 and 9). The discharging and charging of the irrradiation chamber are also performed automatically with a transfer trolley (3) which transfers the carriers between the internal conveyor and the external conveyor through a door (2) which is, of course, closed and bolted during the irradiation cycle. Operating for 7000 h year-’ and allowing 20 min to load and unload each batch, the irradiator output is about 2350 tonne-Mrad year-’ with a 300 000 Ci cobalt-60 source (see Table 5). The design of the irradiator permits wide flexibility in the dose delivered to the product:
278
J. C. Etienne, R. Buyle
Fig. 8.
Fig. 9.
Loading station.
Electromechanical
engineering aspects of irradiator design
219
TABLE 5 Typical Irradiation Programme for a Week with a 300 000 Ci Cobalt-60 Source Products
Deep-frozen foods Dried vegetables Spices Animal feeds
Tonnes
Dose
batch-’
(key)
Batches week-’
Equivalent (tonne-M& year-‘)
2040 1800 2700 2160 1200
3-5 6 10 25 40
9 5 3 4 1
367.2 270.0 405.2 1080.0 240.0 Total: 2350.0
for example, at a product density of 0.5 g cmm3 with a 1 MCi radiation source, a dose as low as 0.10 Mrad is possible. Figure 10 shows an internal view of the irradiation chamber with the source rack and different carriers.
6. RADIATION RESISTANCE OF MATERIALS AND COMPONENTS The most important chemical changes which occur during the irradiation of polymers are crosslinking and degradation. When a polymer is irradiated, both these processes occur, usually competitively but at different rates. The presence of oxygen and water affects the rate at which such crosslinking and degradation occurs. Chemical transformations occurring on irradiation of polymers result in considerable changes in their mechanical properties and can also affect their electrical properties, both of which are of the utmost importance in irradiator engineering. Variations in temperature and humidity may cause water to condense on the irradiated component. The absorption of this condensed water depends on the component size, porosity and other physical properties. The radiolysis of the absorbed water may lead to the formation of
J. C. Etienne,
280
Fig. 10.
Internal
R. Buyle
view of the irradiation chamber different carriers.
with
the source
rack and
in gaseous form, initiate stresses inside the material which accelerate the degradation process. In order to obtain a satisfactory operating performance, the selection of electrical insulating materials, electric limit switches and pneumatic cylinders must take into account the above-mentioned degradations. Figure f 1 shows the different components selected for the IRE irradiation chamber: hydrogen
and oxygen
(a) The normal microswitch.
which,
electrical limit switch was replaced by a Honeywell This switch was modified as follows:
Electromechanical
engineering aspects of irradiator design
281
Fig. It.
- The electric cable was replaced by a mineral-insulated wiring cable. - The small conductors were insulated with mineral beads. -- The O-ring was removed and left out. (b) The standard pneumatic actuator was modified in order to avoid degradation of the ‘Perbunan’ rings which were replaced by metallic seals. Metals and ceramics are not may cause some operating bearings in the radiation impregnated self-lubricating steel ball-bearings.
affected by radiation; nevertheless, corrosion problems. For example, carbon steel ballchamber have to be replaced by graphitebushings or specially-manufactured stainless
7. RADIATION PROTECTION EQUIPMENT Gamma-level guards are used for measuring local radiation levels and for monitoring spaces where radiation could conceivably exceed permissible levels. Figure 12 shows the location of the three gamma-level guards.
J. C. Etienne,
282
R. Buyle
Fig. 12. Location of the different y-level guards. Front view (RHS): (a) Built-in hooter; (b) acknowledgement of hooter; (c) trouble indicator; (d) indicator for operation of acknowledgement key; (e) + (f) 1st and 2nd alarm stages, respectively, with checking of setting; (,g) indicating instrument. Geiger-Miiller The electronics such as:
(GM) counter tubes are used as radiation detectors. connected to the GM must include various functions
(9 count indication; (ii) (iii) (iv) (v)
danger indication (excess count); normal operation indication; two alarm thresholds: continuous checking system to verify proper functioning.
The GM counter (A) is connected to indicating instruments on the control console in the control area in order to avoid damage to the electronics by radiation. The GM counter tubes (B) and (C) and their associated indicating instruments are located on the external wall of the irradiation chamber near the access doors so that the radiation level is automatically checked when the operator opens a door. These three monitors are connected to: (i) the door locks in such a when the radiation at ~ one of the two monitors ~ the monitor (A) is not in this case, the first calibrated source.
way that the doors cannot
be opened
(B) or (C) is above a certain level: between the two alarm thresholds alarm threshold is overridden with a
Electromechanical
engineering aspects of irradiator design
283
(ii) the source lifting mechanism in such a way that the radiation source is automatically lowered into the storage pool when the radiation measured by either of the two monitors (B) or (C) is above the second alarm threshold. Moreover, only authorised personnel carrying a portable radiation monitor are permitted to enter the irradiation chamber. Such an instrument is always available in the control room of the irradiator along with a calibrated test source.
8. REGULATIONS These regulations
AND OTHER SAFETY
CONTROLS
include:
- general operating instructions; - special instructions for the daily operators; - special instructions for the maintenance technicians. Safety control
also includes many other safety features
such as:
(i) a delay timer located in the irradiation chamber compelling the operator to ensure that no one is in the room prior to start-up; (ii) one single operating or master key which is made available only to the authorised operating personnel; (iii) any timing or other irregularities in the automatic operations cause the irradiation process to be interrupted by returning the source to the safe storage position; (iv) emergency ‘pull’ connected to the control system located all round the walls inside the irradiation chamber for emergency return of the radiation source to the storage pool or to prevent start-up when operated manually.
9. PLANT MAINTENANCE Downtime is generally limited to repairing malfunctions of pneumatic cylinders or electric limit switches. The cause of each shutdown is recorded in a log book which is periodically reviewed. Through the use of this information, preventive maintenance procedures can be scheduled. Even though operational downtime is normally minimal, there are areas
284
1. C. Etienne, R. Buyle
which need continuous attention. such as those with high radiation fields. Preventive maintenance is divided into three frequencies - monthly, semi-annual and annual. - Monthly: preventive maintenance includes a general inspection of the facility. During this downtime, the plant is cleaned and general lubrication carried out. --- Annual and semi-annual: lnaint~l~allce includes the replacement of specific components, inspection and servicing of the monitoring instruments and checking of the electronic components of the control desk.
10. CONCLUSION The economics of this batch proved favourable. The versatility of the unit is In 12 months, only minor enced with this plant, despite and safety.
irradiator
for contract
irradiation
have
considerable. operational problems have been experitoday’s high demands for productivity
REFERENCES AECL (1983). 3rd Gamma Processing Seminar. Ottawa, 6-9 June. DOE/USDA/AIBS (1982). Workshop OH Low Dose Irradiation Treatment of Agricultural Commodities, Washington DC. 19-21 April. Lxroix. J.-P. (1983). Belgiatz National Report in the Field of Food lrradiat~o~~, IRE, Fleurus. (14rk A~?~zual~eeti~~g of the ~~ropea~zSociety of~~elear~et~ods i/l Agric~dtrtre, Madrid. 5-9 September.)