Conditional clearance levels for the disposal, recycling and reuse of activated medical accelerator components

1.0 Background

Components of the head shielding, beamline and collimation system on medical accelerators will become activated through photo-neutron processes during the operation. The isotopes produced and their corresponding activities depend upon both the design and the composition of these components and the beam energy. The levels of activity are generally low, and most isotopes produced are short-lived. Consequently, this activation poses a negligible operational hazard for any activity other than highly invasive servicing of beamline components. However, the presence of radioactive material has regulatory implications for disposal of the accelerator at the end of its operational lifetime.

The dominant activation products, ⁶²Cu, ⁶⁴Cu and ⁵⁶Mn, all have half-lives of a half day or less. Consequently, these will decay to background levels within a few days following final shutdown and are not normally of concern from a disposal perspective. However, some longer- lived radionuclides will also be present. The most significant of these are ¹⁸¹W (T½ = 121.2 days) and ¹⁸⁵W (T½ = 75.1 days^{Footnote 1}). Neither ¹⁸¹W nor ¹⁸⁵W have radioactive progeny. Other than low-energy X-rays (6 to 65 keV), the decay of ¹⁸¹W results in very few (0.03% per decay) 136.3 keV gamma photons. The decay of ¹⁸⁵W results in the emission of very few (0.02% per decay) 125.4 keV gamma photons, ¹⁸⁵W being almost a pure-beta emitter. The mean energy of beta particles emitted by ¹⁸⁵W is 126.9 keV.

There are three possible disposition routes for activated medical accelerator components:

• return to the manufacturer (which is normally also a CNSC licensee)
• transfer of the accelerator or components thereof to an accelerator refurbishment company that does not hold a CNSC licence
• disposal of the accelerator, including activated components, via normal waste streams, including landfill disposal and metal recycling

The first of these disposition routes consists of a transfer from one licensee to another.

For the other disposition routes (transfer of components for refurbishment, and disposal/ recycling), the requirements of section 5.1 of the Nuclear Substances and Radiation Devices Regulations (NSRDR) must be satisfied:

Paragraph 5.1 (1) states that “A person may, without a licence, abandon or dispose of a radioactive nuclear substance if the activity or the activity concentration of the substance does not exceed

• (a) its exemption quantity;
• (b) its conditional clearance level; or
• (c) its unconditional clearance level. ”

The exemption quantities (a) and unconditional clearance levels (c) for a broad range of isotopes are listed in Schedules 1 and 2 respectively of those regulations. However, demonstrating that the isotopes and activities remaining in a medical accelerator are below these regulatory limits would require a detailed and highly complex theoretical analysis of the isotopes produced, and/or comprehensive analytical measurements to verify the activities. These types of analyses present very significant technological challenges and are generally unwarranted from an ALARA risk-benefit-cost perspective.

Consequently, the CNSC has reviewed and accepted a dose-rate-based method for demonstrating that components meet a conditional clearance level (CCL) per 5.1(1)(b). That is:
a conditional clearance level of 0.5 μSv/h measured at 5 cm from any accessible surface of any component of a medical accelerator; is acceptable and is consistent with the definition of a conditional clearance level as set out in the NSRDR.

The following information illustrates the basis for this CCL.

2.0 Conditional clearance levels for medical accelerator components

The Nuclear Substances and Radiation Devices Regulations (NSRDR) ^{Footnote 2} define a conditional clearance level as follows:

Conditional clearance level means an activity concentration that does not result in an effective dose

• greater than 1 mSv in a year due to a low probability event referred to in the IAEA Safety Standard RSG-1.7; or
• greater than 10 μSv in a year.

Exposure pathways

In order to establish CCLs for these types of isotopes, the potential exposure pathways to the public via external exposure or inhalation must be considered.

Table 1: Exposure pathways for conditional clearance of activated accelerator components

Receptors	Pathways
1. Disposition route: Transfer of accelerator components for refurbishment
1.1 Workers handling activated component	External exposure
1.2 Workers refurbishing components	External exposure Inhalation of re-suspended dust
2. Disposition route: Disposal of the activated component via normal waste streams and metal recycling
2.1 Workers handling activated component	External exposure
2.2 Workers at landfill	External exposure
2.3 Workers at foundry / recycling facility	External exposure Inhalation of re-suspended dust

Exposure of members of the public from releases to air from a recycling facility is not considered a credible exposure pathway. In metal recycling, tungsten is not released to air as a volatile compound. It predominantly partitions to the recycled metal product or to slag ^{Footnote 3}. In addition, tungsten radionuclides would be diluted with non-impacted metal scrap at the metal recycling facility, further reducing the potential for exposure.

Inhalation

The estimated amount of time a worker would require to incur a dose of 10 μSv due to inhalation of re-suspended dust from metal recycling processes is illustrated in table 2. The estimate assumes there is no dilution by non-impacted metal at the recycling facility, and that only radiologically contaminated tungsten (containing only ¹⁸¹W and ¹⁸⁵W) is handled. The results in table 2 show that the inhalation pathway is not of concern, as the dose from one entire working year of 2,000 hours would be less than 0.02 μSv.

Table 2: Exposure time to reach 10 μSv due to inhalation of ¹⁸¹W and ¹⁸⁵W

Radionuclide	Saturation activity Footnote 4	Dust loading	Air concentration	Dose per unit intake Footnote 5	Time to reach 10 µSv
	Bq/g	g/m3	Bq/m3	Sv/Bq	hours
181W	600	5.0 x 10-4	0.3	4.3 x 10-11	6.5 x 105
185W	600	5.0 x 10-4	0.3	2.2 x 10-10	1.3 x 105

External exposure

The effective dose from external exposure pathways conservatively assumes a distance of 30 cm from an activated component. The dose rate at 30 cm from a source ( {\dot{D}}_{30} ), when the dose rate at 5 cm from the same source ( {\dot{D}}_5 ), is known, was calculated using the following relation:

$${\dot{D}}_{30}={\frac{{\dot{D}}_5\times \left(\mathrm{5 }cm\right)}{{\left(\mathrm{30 }cm\right)}^2}}^2$$

From the above approach, when {\dot{D}}_5=0.5\text{μSv/hour, }{\dot{D}}_{30}=0.014\text{μSv/hour}

At a dose rate of 0.014 μSv/hour, the anticipated dose from working in very close proximity to the activated components from a single accelerator for one working week (40 hours) is therefore 0.56 μSv. On average, approximately 15 medical accelerators are decommissioned annually in Canada. Thus, even in the highly unlikely event that a single worker were required to handle or refurbish the activated components from all 15 accelerators, total exposure would still be less 10 μSv.