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Transport of NORM

Niobium and tantalum raw materials can contain low levels of thorium (Th) and uranium (U), which leads to these raw materials being treated as naturally occurring radioactive materials (NORM).

Due to the Th and U being locked within the mineral matrix, it is not possible to separate the Th/U easily from these minerals.  Only pyrochlore is typically treated near the minesite, with an aluminothermic reduction process removing the Th/U to a waste slag.

For columbite, tantalite, tin slag and other Ta/Nb raw materials, the extraction of the valuable niobium and tantalum content and removal of the Th/U to waste, requires specialised digestion in e.g. hot HF/H2SO4 acid which is conducted by chemical processors located far from the minesites.  Transport is therefore required, almost always involving an element of maritime transport, to bring the raw materials from the mines to the processors.

International transport is governed by a number of regulations and agreements, which include an exemption level for how much Th and U may be transported without it being of regulatory concern.  Materials containing Th and U above this exemption level are transported as Class 7 Dangerous Goods, which means a number of requirements come into effect that ensure their safe transport.

Among some sectors of the public and some authorities there is a negative perception towards any material subject to Class 7 regulation, regardless of the actual nature of the material, its characteristics and its uses to society.  The resulting compliance burden of international and/or local regulatory requirements can lead to carriers reaching the sober commercial decision that the transport is more effort than it is worth, resulting in denial of shipment for such Class 7 goods.

These problems prompted the T.I.C. in 2003 to question whether the existing exemption level for Th and U is appropriate to the risk posed by the transport of these materials.  It prompted a detailed risk assessment of the transport of tantalum raw materials conducted by the external consultants SENES, with the final report being published in 2007, which later became part of a Coordinated Research Programme by the International Atomic Energy Agency on the appropriate regulation of NORM transport, which resulted in the report TECDOC-1728 published in 2010.

Summary of T.I.C. Risk Assessment

Throughout this article the figures quoted in Bq/g refer to the relevant (parent) nuclide only, in accordance with the values listed in the IAEA regulations SSR-6 of 2012.  The conversion factors applied are as follows:

  • 1% ThO2 = 35.6 Bq/g   (1% Th = 40.6 Bq/g)
  • 1% U3O8 = 104 Bq/g   (1% U = 123 Bq/g)

As part of its commitment to the tantalum and niobium industries the T.I.C. commissioned a detailed investifation into the transport of tantalum raw materials, specifically tantalite and tin slag. The main objectives of this study were to determine the radiological characteristics of these materials and to evaluate the potential radiological exposures associated with normal transport and in the event of an accidental spill. The study was carried out by SENES Consultants Limited (SENES) with support from a number of T.I.C. member companies for the analysis and gamma radiation surveys.

 

Main conclusions

Radioactive equilibrium in the uranium (U-238) and thorium (Th-232) decay series was found to be a reasonable assumption for tantalum raw materials for dose assessment purposes.

A range of about a factor of 10 in radioactivity concentrations was measured in 67 shipments of tantalite and tin slag, with an average activity concentration (U-238 + Th-232 combined) of about 20 Bq/g for tantalite and about 25 Bq/g for slag.  The majority (78%) of tantalite shipments and 45% of slag shipments had concentrations exceeding 10 Bq/g.

Exposure scenarios that considered both duration and location of exposure were established for several types of transport workers and for members of the public.  In addition, exposures to facility workers during loading and unloading were assessed in this study to provide perspective on potential exposures from tantalum raw materials. Based on an evaluation of potential exposure pathways, exposure to gamma radiation was determined to be the only significant exposure pathway.

Doses from exposure to spilled materials due to potential accidents were calculated and determined not to be a regulatory concern, as the resulting doses were less than 10 μSv/y.

An assessment of potential dose rates around the transport containers was conducted using the range of measured radioactivity concentrations and modelling of the associated gamma radiation doses using the MicroShield model.  The modelling approach overestimated the measured dose rates, primarily due to the assumption that the transport containers always carried full loads whereas in practice the loading pattern varied.

Calculated doses to transport workers and members of the public based on the distribution of measured concentrations are shown in Table 1.

Table 1 - Summary of Doses Calculated for Normal (Non-Accidental) Transport Activities

Exposure Scenario

Mean Dose a, b (mSv/y)

Slag

Tantalite

Transport Worker – Truck Driver

0.24

0.16

Transport Worker – Dockworker

0.032

0.02

Transport Worker – Seaman

0.0041

0.0026

Transport Worker – Trainman

0.019

0.012

Public – Living Adjacent to Road

0.00038

0.00024

Public – Living Adjacent to Rail

0.00017

0.0001

a           Mean annual dose from shipments of tantalum raw materials analyzed in this study.

b           For perspective, doses to facility workers (who are not considered transport workers) were 0.49 and 0.31 mSv/y from slag and tantalite, respectively.

 

Doses to members of the public from the transport of these materials were found to be insignificant, that is, much less than 10 μSv/y.

The calculated doses to transport workers were well within the internationally accepted dose limit of 1 mSv/y for non-radiation workers.  If it were assumed that the tantalum raw materials considered in this study reliably represent the likely range of tantalum raw materials in general, then the expected (mean) dose to the most exposed group of transport workers would be about 0.24 mSv/y to truck drivers from the transport of slag (Table 1).  Thus, there is considerable allowance for truck drivers who transport tantalum raw materials to transport other materials containing elevated levels of naturally occurring radioactivity without exceeding a cumulative annual dose of 1 mSv.

On the basis of the analyses of doses arising from the transport of tantalum raw materials described in the report there is no apparent reason with regards to radiological dose for an exemption value as restrictive as the current value of 10 Bq/g for these materials.  Even in the absence of an exemption value, as shown in Table 1 no one would be expected to receive a dose above 1 mSv/y arising from the transport of tantalum raw materials.

Notwithstanding the low doses from the transport of tantalum raw materials, specific numerical exemption values are required for the practical, real-world application of the transport regulations. Moreover, the radioactivity levels of future shipments of tantalum raw materials may differ from the distribution of levels from which the conclusions of this study were derived.  If it were conservatively assumed that the radioactivity levels of all future exempted shipments of tantalum raw materials were at a selected specific exemption value, the resultant annual doses due to exempted shipments under actual transport situations would be overestimated. The calculated doses to transport workers under this assumption are summarized in Table 2 for various potential exemption values.

 

Table 2 - Summary of Annual Doses (mSv/y) to Transport Workers at Various Potential Exemption Values

Potential Exemption Value (Bq/g)

Material

Annual Dose (mSv/y) a

Truck Driver

Dock Worker

Seaman

Trainman

10

Tantalite

0.12

0.016

0.0021

0.0097

Slag

0.12

0.016

0.0021

0.0098

30

Tantalite

0.31

0.040

0.0052

0.024

Slag

0.37

0.049

0.0062

0.029

50

Tantalite

0.48

0.062

0.0080

0.038

Slag

0.54

0.071

0.0090

0.043

70

Tantalite

0.65

0.084

0.011

0.051

Slag

0.71

0.093

0.012

0.056

90

Tantalite

0.82

0.11

0.014

0.064

Slag

0.89

0.12

0.015

0.070

100

Tantalite

0.91

0.12

0.015

0.071

Slag

0.97

0.13

0.016

0.077

a           Assumes maximum load (1.5 tiers) of material and all loads at specific exemption value.

 

To account for the possibility of other transport-related exposures, an annual dose constraint of 0.3 mSv/y, for example, might be considered.  Considering the conservatism in the dose calculations summarized in Table 2, an exemption value of 30 Bq/g (U-238 + Th-232) would result in doses that would be unlikely to exceed 0.3 mSv/y to the most exposed transport workers. Thus, on the basis of a 0.3 mSv/y reference dose, an exemption value of at least 30 Bq/g is considered appropriate for the transport of tantalum raw materials.

Irrespective of the exemption value selected, the radiological dose assessments described in the report provide assurance to the tantalum industry and to its shippers that the doses arising from the transport of tantalum raw materials are low and well within international norms for both transport workers and members of the public.  Using conservative assumptions and on the basis of a 0.3 mSv/y reference dose, an exemption value of at least 30 Bq/g is considered appropriate for the transport of tantalum raw materials.

Further reading:

  • Transport Saves Lives – a brochure calling for action to sustain the transport of radioactive material (T.I.C. 2012)
  • Transport for Life - a call for action to sustain the transport of radioactive material (T.I.C. 2011)

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