Developing the DN30

ENRICHED URANIUM HEXAFLUORIDE (UF6) HAS been transported since the birth of the industry in the ‘30B’ cylinder, a standardised steel vessel with a valve for filling and emptying. Although the robust design allows an internal pressure of up to 1.38MPa, these cylinders require additional protection against normal (NCT) and hypothetical accident conditions of transport (ACT/HAC) to ensure the radioactive material is contained and safe against criticality. This additional protection is provided by an outer protective structural packaging (PSP) which acts as a shock absorber during drop tests and as thermal protection in fire tests.

Several packaging designs have been used. The early designs were gradually taken out of order due to technical or licensing issues some decades ago and replaced by improved designs. Further improvement will be needed to meet new licensing requirements including new transport regulations coming into force in 2021. DAHER Nuclear Technologies (DNT) started development of the DN30 package, comprising the 30B cylinder and the DN30 PSP.

Initially, the project was considered a manageable and straightforward task as the basic transport technology has been in place for a long time. However, design and licensing turned out to be a long and winding road. One major difficulty was modelling UF6 with a surrogate material during the drop tests: the high density of UF6 (almost five times that of water) had to be modelled and the properties of the very inhomogeneous solid UF6 simulated. A further surprise was the outcome of the fire test: the fire-retardant material started to produce heat at some point during the test which resulted in temperatures of the 30B cylinder far above the acceptable limit. Layers of intumescent and insulating material had to be added to the design to pass the test. DNT resolved all issues and licences for the DN30 package were granted by France’s Nuclear Safety Authority (ASN) and the US Nuclear Regulatory Commission(NRC).

Four patents were granted for the new and enhanced safety features. A fleet of DN30 PSPs is being manufactured.

Further work will cover UF6 with an enrichment up to 10wt.% U-235 and the transport of heels of reprocessed uranium, where the dose rates are too high to allow timely transport after emptying.

Design of the DN30 package

The DN30 package consists of the 30B cylinder (according to the standards)1,2 and of the DN30 PSP.

The top and bottom halves of the DN30 are stainless- steel shells, connected by three closure devices at each side. A gasket on the step-joint prevents water ingress. It has two holders for high security seals. The lifting interfaces permit safe handling: lifting lugs for the top half; lifting lugs at the feet; and forklift pockets (see Figure 1). The tie-down interfaces permit the safe stowing of the DN30. They are compatible with existing PSP designs and allow transport of four DN30 packages on a 20ft flat-rack (industry standard).

As well as the energy-absorbing closed-cell foam enclosed by the shells it has a thermal insulation system which ensures that the temperature of the 30B cylinder and its content remains below the acceptable limits and therefore keeps the latter from building up unacceptable pressure in case of a fire. This system comprises an insulation layer between the inner shell and the foam, an intumescent layer covering the inner shells which expands at a certain temperature, and thermal plugs at the outer shells which melt at a given temperature and allow hot gases produced by pyrolysis of the foam to escape.

The DN30 package includes innovative safety features to maintain containment. The valve-protecting device guarantees that there is no contact between the valve and any part of the DN30 package other than its initial point of attachment. The same applies to the plug-protecting device, which also permits the hex head and the socket head plug design to be installed in the 30B cylinder. Two rotation- preventing devices ensure that the 30B cylinder does not rotate on its axis within the DN30 PSP at any time and does not render other safety features inoperable (see Figure 2).

Contents of the package

The DN30 package is licensed by the French competent authority and approval certificates are being validated in countries where UF6 is transported. In the USA, an NRC Certificate of Compliance for a type AF package is available. The French and US certificates are valid for five years and will be renewed in 2023 and 2024, respectively.

For all these certificates, the number of cylinders on board a vessel is not restricted by the sum of the CSIs but only by the loading capacity of the vessel. Using the DN30 package allows more 30B cylinders in each vessel.

The type AF certificates allow shipment of commercial grade UF6 with up to 5wt.% U-235 enrichment at all filling ratios from heels (a few kg) to full (2277kg UF6). Cylinders containing heels may be refilled without washing.

French AF approval also allows the shipment of commercial grade UF6 with traces of cross contamination with reprocessed UF6.

The type IF certificate allows the shipment of enriched UF6 from reprocessed uranium, and also UF6 with specifications exceeding ASTM C996 standard, eg up to 60ppb U-232.

Cylinders containing heels of reprocessed UF6 must be shipped with a type B(U)F certificate.

In principle, this covers contents defined by type AF and type IF certificates and also cylinders containing heels of reprocessed uranium whose concentration of decay products breaches LSA-II. As the dose rate of these heels may be quite high, to minimise storage time a simple criterion is specified in the certificate of package approval to allow quick shipment after emptying.

Licensing in France

When DNT presented the design to German and the French authorities in 2008 it was agreed that France should be its country of origin and Germany’s BAM should perform drop and fire tests.

A conceptional safety report was established and refined. The drop test programme comprised five sequences of a 1.2m drop simulating NCT, a subsequent 9m drop, and a final 1m drop onto a steel bar, both simulating ACT.

Unexpectedly there were three drop test campaigns with design changes in between. After the first campaign in 2011/2012 the phenolic foam had to be changed to polyurethane foam and the manufacturing process optimised to ensure stable mechanical properties. The second campaign showed that the surrogate material for UF6 plays an important role in the behaviour of the package. The free moving steel balls used caused more damage than could have been anticipated from the UF6 content. The third campaign showed the DN30 package met the requirements of transport regulations.

For the fire test, an empty DN30 prototype pre-damaged by two consecutive drop test sequences was used to maximise the temperatures. Unexpected design modifications included adding insulating layers far beyond then-existing techniques. Fire test of the final design of the DN30 was successful. The temperatures of the 30B cylinder and content were well below acceptable limits.

The French authority issued certificates on the basis of mechanical, thermal, shielding, containment and criticality safety analysis. Validations for these certificates were applied for in countries where UF6 is transported: Belgium, Brazil, Canada, Germany, the Netherlands, Russia, South Korea, Sweden, the UK and the USA.

Licensing in the USA

Thanks to the modular structure of the safety analysis report, most parts of the report used for licensing in France applied to the certificate of compliance necessary for domestic transport of commercial grade UF6 in the USA. Only the summary report had to be “translated” from European regulations ADR to the US regulations 10 CFR 71 and 49 CFR 173.

Previous testing of the package meant this was completed within one year and the certificate of compliance for a type AF package was issued by the NRC.

Manufacturing

Manufacturing started immediately after the certificates of package approval were issued. First, a detailed manufacturing and test sequence plan with all related instructions and protocols (for the visual, dimensional, penetration, leak and overloading tests) was established in compliance with the requirements of the certificates. Next, prototypes were manufactured to verify dimensions before and after assembling as well as the welding procedures. Finally, series manufacturing aimed to reach a stable output of 20-30 units per month.

Further developments

The US National Nuclear Security Administration is evaluating commercial nuclear fuel fabrication enrichments above 5wt.% U-235. Currently, neither primary packaging nor adequate transport solutions are available.

So DNT has developed a package comprising:

• A modified 30B cylinder with criticality safety control features, designated 30B 10;
• The DN30 PSP as used for UF6 with max. 5 wt.% enrichment without any modifications.

This offers:

• The same operating conditions as the 30B cylinder;
• High capacity — 80% of the capacity of a 30B cylinder;
• Conformity to existing infrastructure on enrichment and fabrication sites;
• Use of existing DN30 PSPs for the transport of UF6 with higher enrichments;
• Similar procedures for cylinder cleaning and recertification.

DNT is working on the safety analysis report and expects to file a licence application with the US NRC in mid 2020.

The concept of the DN30-10 can also be used for higher enrichments than 10wt.% U-235. The concept is also valid for up to 20wt.% U-235. This meets the needs of small modular reactors.

When 30B cylinders containing UF6 of RepU are emptied, the decay products, especially U-232, are concentrated in the heels. As U-232 produces nuclides with a very high gamma intensity, dose rates in a cylinder empty except for heels are, somewhat counter-intuitively, much higher than the full cylinder before emptying. Transport of 30B cylinders containing heels of RepU might be delayed by required storage times of several years.

To reduce this, DNT developed the DN30H to be used for the transport of 30B cylinders containing heels of RepU by adding shielding. This adds weight but the empty 30B cylinder is lighter than a full cylinder, so the mechanical safety of the DN30H package can be derived from the safety of the original DN30 package.

References: 1. ISO 7195, Nuclear Energy – Packaging of uranium hexafluoride (UF6) for transport 2 ANSI N14.1 Uranium Hexafluoride – Packaging for Transport

Author information: Franz Hilbert, Chief operating officer at DAHER Nuclear Technologies GmbH; Maik Hennebach, Head of design and development at DAHER Nuclear Technologies GmbH; Yara van Wijk, Project Manager at DAHER Nuclear Technologies GmbH

Main Image: DN30 package overview