Considerations about German proposal relative to LSA III leaching test requirements French Nuclear Safety Authority- 12April 2013
Abstract
It is true that the release and inhalation scenario defined in the advisory text in 601.2to support the LSA III leaching test does not look robust.There are however many other instances in the TS-R-1 Regulations where the acceptance criteria are not precisely justified. IAEA has recently undertaken a programme of actions to make an inventory of them.
As for the LSA features, we have found unresolved issues with the current applications of LSA II and III definitions and requirements. The main issue of concern is that industry uses the LSA concept for contaminated items where activity is distributed over surfaces instead of throughout masses. This use does not seem consistent with the safety objective of a maximum intake dose of 50 mSv for an inhaled quantity of 10 mg. The risks in exceeding the reference dose by a factor 10000 are presented in the attached commentnote. Questions related to parameter values and accident scenarios selected in the German paper are also presented. In particular it was found that the external irradiation consequences should be considered.
In this context wewould recommendto keep the current regulatory requirements and even to try to reduce the possible impact of heterogeneitiesthat may be present in the LSA materials.
- Comments about some parameters published and used
In the paperpresented by GRS at the PATRAM’10 conference,no heterogeneity factor has been considered above 10. In practice, a very wide range of specific activities may be encountered within a material transported as LSA. This is not officially permitted by the regulations but not excluded either. The advisory text provides examples (see section 2 below).
The accident scenarios that have been considered are mainly mechanical. The influence of other scenarios with rain, wind, fire and extinguishing with water after mechanical impact should be checked. The dispersion capacity of a water fire hose is not studied. Scenarios with dispersion of radioactivity under a combination of fire and water have a significant probability; the Langres accident[1] in 1999 is an example. In addition a lot of events have been recorded in France where industrial packages have been found containing unexpected water (coming from rain penetrated before shipment or from non efficient or omitted draining or drying of internal fluids).
For transport accidents, a downwind distance of 50 to 100 m is the reference distance considered while the Q-system assumes the more conservative distance of 1 m. Air turbulence considerations do not seem robust for miscellaneous release scenarios.
When a solid radioactive material may be separated by leaching, the activity concentration may be higher in the liquid phase and is possibly increased in case of precipitation.
No clear justification is given for the release times of 5 minutes in air and 1 week in a river respectively considered in the paper.
Only the activity intake by inhalation or consumption of drinking water has been assessed in the German paper. The dosimetric consequences by direct external irradiation from the radioactive releases have not been assessed; the dose rate at 3 m from the radioactive material should have been checkedreferring to the acceptable dose rate of 10 mSv/h and taking into account possible relocation, after accident, of soluble or leachable material.
- Impact of the heterogeneity factor
Thehomogeneity “level” of LSA II and III materials is not precisely defined in the TS-R-1 requirements. The terms “which by its nature has a limited specific activity” raise no difficulty while the terms “for which limits of estimated average specific activity apply”, “in which the activity is distributed throughout”, “distributed throughout a collection of solid objects” and “essentially uniformly distributed in a solid binding agent” do not actually support the concept of a maximum local quantity of radioactivity.
In para. 226.7 of TS-G-1.1 it is mentioned that “LSA-II materials are recognized as being clearly not uniformly distributed”.
The main issue is to show with an acceptable confidencethat small quantities in the range of a few milligrams and which can be released in aerosol form would have an activity not more than 10-6 A2when complying with TS-R-1 rules applicable to LSA-II.
Then the local concentration should not exceed about 10-3 A2/g considering a tolerance of a factor 10 (as suggested in TS-G-1.1 para 226.14)[2]
As suggested in para. 226.15 avolume of 0.2 m3of LSA II materials could represent the elementary volume for homogeneity check. Then considering that this volume may correspond to a mass of 100 kg, the suggested allowed heterogeneous volume of 0.2 m3 may contain as much as 100 x 103 x 10-4 x 10 = 100 A2. Then the question is whether this upper threshold of 100 A2 in 200 litresis consistent withan upper activity concentration of 10-6 A2 in 10 mg in practice.
It is commonly admitted that contaminated and activated equipments may be candidate for LSA classification (see 226.19) subject to average specific activity limit. Contamination is most often concentrated in particles; this is called “spot” contamination and investigations performed in 1998 showed several examples of such hot particles found on spent fuel transport packages and vehicles. Para. 226.7 mentions as an example “many items of activated equipments from the decommissioning of nuclear plants”. Then considering that the activity comes from “cruds” or fuel oxide debris, it could be considered a local maximum activity concentration equal to that of spent fuel pellets, i.e. about 5 A2/g for UO2 irradiated fuel and more than 20 A2/g for mixed oxide fuel. This is 10000 times more than the desired value of 10-3 A2/g.
- Discussion
About heterogeneity
One single UO2 particle of diameter 17 mand specific activity of 5 A2/g (spent fuel) isenough to deliver a dose of 50 mSv when inhaled. This particle weighs less than 10-5 mg; it may be airborne. Contaminated equipment from reactor decommissioning can contain many such particles.
Even if probability for one of these particles to become inhaled by a person after a handling or transport accident is low and could perhaps be reasonably excluded, the number of these particles that can be present in the reference volume of 0.2 m3 with a total activity of 1 to 100 A2 is so important (in the range of 106 to 108) that such an event can hardly be excluded when the containment is assumed lost in accident.
Preliminary investigations have also shown that possible heterogeneities in the specific activity of the contents of Type A packages may warrant revisiting the presently applied Q-system. In this system (see TS-G-1.1 appendix 1) the assumed release fraction of 10-2 to10-3 of the contents dispersed in airborne form could represent a much higher fraction in terms of activity.
About leaching
In case of partially or fully soluble material, a significant portion of the total activity of the material could be released outside the package. For LSA III combustible solids, the package total activity is limited to 100 A2 (conveyance limit); for LSA III non-combustible solids there is no activity limit. If the leaching test were deleted, activities much higher than A2 could be released from the package in liquid form and lead to unacceptable dose rates if the shielding material embedded in the LSA III material is no longer acting after dissolution.Note that the value A2 corresponds to the maximum activity that can be released in an accident as considered in the Q-system to assure a dose rate below 100 mSv/h at 1m which is close to the criterion of 10 mSv/h at 3 m from unshielded LSA III material. The current regulatory criterion of 0.1 A2 for leached activity is preventing such occurrences.
- Recommendations
The set of following recommendations could be :
- Keep the insolubility criterion for LSA III.
- Limit the use of LSA categories to materials intrinsically homogeneous down to the mg leveli.e. where the specific activity is known not to exceed ca. 10-4 A2/g in any mg or 10 mg. Examples of such materials: reprocessed uranium oxide, activated metal after decontamination…
- Assure that the possible maximum size airborne particle with maximum possible specific activity would not lead to a dose above 0.1 mSv when inhaled.
- Exclude the use of LSA categories for contaminated equipments or wastes coming from facilities where contaminants may have high specific activity.
- Recommendation for transitional conditions
As a kind of transitional condition, it could be proposed that the use of LSA categories for materials that do not meet recommendations in section 4 should be restricted to Type A quantities in elementary volumes where the average specific activity is determined and checked for compliance to the criterion. A tolerance factor of 10 should not be allowed.
- Conclusion
The LSA definitions are confusing and do not guarantee a fixed reference safety level. Activity uptake in an accident could be much higher than expected. Due to heterogeneities of local specific activitiesof LSA materials in current shipments the reference activity uptake of 10-6 A2 in case of accident might be exceededby a factor 10000. Evaluation of the probability of such releases has not been done. It is not possible to confirm that these probabilities are negligible, if it cannot be guaranteed that some kind of highly contaminated products can be excluded.
The LSA III solubility criterion which provides a mean to limit the activity intake and the external irradiation should be kept.
Moreover it could be envisaged to review the current regulatory requirements to address the risks of exceeding the activity intake objective when the radioactive material contains concentrated activity particles.
[1]A truck entered in collision and was subjected to a 2 to 3 hour fire on the French A31 highway. Contamination was spread in the truck wreck and around the scene.
[2] Note that a radiological consequence of 500 mSv instead of 50 mSv would however raise a serious issue