IRRADIATION FACILITIES AND DESIGN REQUIREMENTS All targets need facilities in order for them to be placed inside or near the

research reactor core during irradiation. For low activity requirements, existing beam tubes (radial, tangential), as well as reflector thimbles are often sufficient.

Their loading and discharge is performed on demand during reactor operation.

TABLE 1. NEUTRON ABSORPTION TARGETS^a

99Mo – medical —Low specific activity from MoO₃ targets;

—Specific activity directly proportional to available thermal neutron flux;

—5–15 d irradiation time at steady continuous power:

•~71% of saturation at 5 d irradiation;

•Product quality degrades with decay.

131I – medical —TeO2 targets:

•Purity is important;

• Exothermic reaction with Al if overheated;

—Acidity directly proportional to available thermal neutron flux;

—14–28 d irradiation at steady continuous power for saturation:

•90% of saturation at 28 d.

125I – medical —Enriched ¹²⁴Xe target:

•Enrichment requirement inversely proportional to available thermal neutron flux;

• Minimum enrichment ~5 wt% of ¹²⁴Xe;

• Cost increases exponentially with enrichment;

—Up to 1 week irradiation time at steady continuous power;

—Care needed in handling irradiated targets:

•Volatile Xe gas and I aerosols;

•Requires good vacuum and cryogenic processing;

•¹²⁶I impurities depend on irradiation time and flux (longer irradiation produces more ¹²⁶I);

•Gamma ray similar to ¹³⁷Cs (orders of magnitude higher activity);

•Activated capsules;

•I extraction requires high temperature (~650°C);

•High dose and contamination hazards.

192Ir – industrial —Requires high flux (>2 × 10¹⁴ n·cm^–2·s^–1);

—Very strong neutron absorption;

—Long irradiation times (several months);

—High level wastes generated.

60Co – industrial —Requires >5 × 10¹³ n·cm^–2·s^–1 for large sources;

—Very strong neutron absorption;

—Large quantity in target;

—Long irradiation times (5–7 a or more);

—High level wastes generated.

60Co – medical —Requires high flux (>2 × 10¹⁴ n·cm^–2·s^–1);

—High specific activity needed;

—Small quantity in target;

—Very strong neutron absorption;

—Long irradiation times (2–3 a or more);

—High level wastes generated.

a Table courtesy of A. Lee, AECL.

For high activity requirements, specially designed facilities exist in an enormous variety for manual, semi-automatic and fully automated loading. The design must satisfy many requirements and constraints. Typical issues to be considered during the design are:

— Adequate heat removal, as target samples, as well as sample containers are heated by the absorption of neutrons and gamma rays plus — in the case of fission targets — by the fission energy. This includes consideration of the specific heat of the sample itself and of its container. Sophisticated considerations of natural convection, bypass cooling or forced flow cooling may be necessary.

— Available excess reactivity during the entire fuel cycle of the research reactor, which often determines the number of facilities for radioisotope generation and their production frequency at a given plant.

TABLE 2. FISSION PRODUCT TARGETS^a

99Mo – medical —Typical 2–4 kW/g ²³⁵U specific power for large scale production;

—Typical 1–2 kW/g ²³⁵U specific power for small scale production;

—10–15 d irradiation time at steady power:

•Optimum is 13–14 d;

•~71% of saturation at 5 d irradiation;

•Need to account for decay during processing and delivery;

•Need to account for recovery efficiency;

•‘Just in time’ delivery;

•23% decay loss in 24 h;

•~78% decay loss in 6 d;

•Schedule matched to customer requirements.

133Xe – medical —Produced along with ⁹⁹Mo;

—Requires at least 7 d decay after processing to decay impurities:

•60% of product lost in decay;

•Shielded storage needed;

•Storage capacity depends on customer schedule.

131I – medical —Produced along with ⁹⁹Mo;

—Not available to be extracted from all target forms;

—Requires at least 12 d decay after processing to decay impurities:

•65% of product lost in decay;

•Shielded storage needed;

•Storage capacity depends on customer schedule.

a Table courtesy of A. Lee, AECL.

— Reactivity effects during loading and discharge, as net reactivity changes but also as reactivity changes per unit time, which the research reactor reactivity control system has been able to compensate.

— Flux perturbations caused by a target sample containing neutron absorbing material (activation target) depresses the flux locally and may even give rise to inclined flux distributions across the core.

— Flux perturbations caused by a target sample containing fissile material that enhances the flux locally and may lead to local power peaks at the nearby fuel assemblies. Also, new local sources of gamma rays may have to be considered.

— Careful consideration of sample container integrity under all impacts and stresses, to avoid inadvertent contamination.

— Adequate shielding during target handling.

— Disturbances to the operational parameters of the research reactor, such as both local and general power and flux levels, control rod positions and their impact on local neutron fluxes.

— Reactor operation requirements, such as long periods of stable power to obtain the desired specific activities for the target material under consid-eration. Often the ‘just in time’ delivery issue is a high priority.

— Careful consideration of the relevant facility infrastructure necessary to support all preparation, handling and supervision under the highest safety standards, as well as sufficient and experienced staff. Such infrastructure may include hot cells, gloveboxes, shielded handling flasks, lifting devices, remote welding, remote chemistry, etc.

— The waste issue and potentially higher personnel dose rates are two of the most serious obstacles to overcome when considering large scale radio-isotope production.

Not all the named constraints and requirements are directly linked to the design and construction of a facility for radioisotope production. However, all must be carefully considered when developing or modifying the application of a research reactor for radioisotope production. They will also be included in the safety considerations linked to a new radioisotope facility.

6. COMMERCIAL REQUIREMENTS

In addition to the technical and licensing requirements described above, a research reactor operator interested in commercial ventures must consider product quality and product purity requirements. Those demands, however, are

outside the irradiation facility as such. Nevertheless, Table 3 lists the commercial requirements, data and standards for six different radioisotopes.

7. EXAMPLES OF RADIOISOTOPE PRODUCING RESEARCH