RE-USABLE SHIELDED TRANSPORT CONTAINERS 1 Design Considerations

Each RSTC will carry four 500 litre drums of immobilised ILW, or alternatively a single 3m³ drum or box which occupies the same space. The transport containers are therefore approximately cuboidal in shape. Although the family of RSTCs comprises four

Table 1

Summary of Nirex Standard Transport Packages

LID

Notes All dimensions and weights are nominal

[1] It may be possible to increase the maximum payload and gross weight of this package to 41t and 65t respectively

different wall thicknesses, all containers will have the same internal dimensions. The containers will weigh from around 15t for the thinnest-walled up to about 50t for the thickest.

The additional weight of the contents will range from 5t to 10t, depending on the nature of the waste.

Compliance with IAEA standards for Type B packages means, among other things, that the total transport package must be designed to withstand normal transport conditions and minor mishaps, and also to withstand transport accident conditions including impact, fire and water immersion, while sustaining no significant loss of either shielding or containment.

CHOCK

SHOCK ABSORBER

BODY

figure 2

Concept L re-usable shielded transport container - 70 mm Wall thickness.

LID

SHOCK ABSORBER \/

BODY

Concept L re-usable shielded transport container - 285 mm Wall thickness.

SHOCK ABSORBER

LOCKING KEY

BARREL SEAL

BODY

Figure 4

Concept N re-usable shielded transport container - 70 mm Wall thickness.

CASTELLATIONS

ACCESS COVER

SHOCK ABSORBER

LOCKING KEY

BARREL SEAL

BODY

Figure 5

Concept N re-usable shielded transport container - 285 mm Wall thickness.

The design of the RSTCs is based upon extensive UK experience with the transport of irradiated nuclear fuel from gas-cooled nuclear power stations. The flasks used for this purpose are of the same cuboidal shape as envisaged for the RSTCs. However, the design requirements for the Nirex transport containers are different from those for fuel flasks, in two important respects:

• The mass of the contents is much greater than in the irradiated fuel flasks.

This increases the stresses in any impact accident.

• The throughput of RSTCs at the repository will be greater than that of fuel flasks at a reprocessing plant, and the operations will be carried out without water shielding. Thus all processes such as the unfastening and removal of lids must be quick and simple, and preferably capable of remote operation.

The impact and fire accident scenarios have had a marked influence on the container designs. The main impact requirement is to maintain shielding and containment following a 9m drop test on to an unyielding target in any impact attitude [2]. The main design requirement associated with the 800°C, 30 minutes fire test [2] is to protect the O-ring seals, because the elastomeric material would degrade if the temperature exceeds certain limits for a significant length of time.

4.2 Design Concepts

The presently envisaged range of nominal wall thicknesses for RSTCs is 70mm, 145mm, 210mm and 285mm. These thicknesses are based on steel construction, and have been derived from consideration of the types and quantities of the conditioned waste streams requiring transport to the repository [3]. The descriptions which follow are specific to the 70mm and 285mm variants of each design concept; details of the 145mm and 210mm variants may be inferred by interpolation.

Concept L

The Concept L containers are shown in Figure 2 and 3. The design is similar to the existing irradiated fuel flasks, with a conventional top lid which is recessed into the body for added strength. The main impact protection is provided by massive steel shock absorbers at the top corners of the body. In the 70mm version, a separate wooden shock absorber is bolted to the outer surface of the lid to give additional protection against a direct top-down impact.

The lid is connected to the body by 24 radial chocks which engage in a continuous V-shaped groove around the top of the container body. Sealing is achieved by a separate Lid Sealing Member (LSM), a semi-flexible diaphragm which carries two concentric O-rings on the underside of its rim. Once the b'd has been placed in position, the O-rings are clamped against the mating surface on the container body. The LSM is attached to the lid for ease in routine handling, but the attachments are designed to break away under impact leaving the LSM free to move and flex independently. Inside the container, a layer of crushable aluminium honeycomb is provided underneath the LSM to help absorb the kinetic energy of the contents in top-down impacts.

The thermal behaviour of the container is controlled by an intumescent coating over the majority of the surface. In the event of a fire, this coating is formulated to char and swell to several times its original thickness, providing a very efficient layer of thermal insulation.

The lids of the 70mm and 145mm containers are not coated because the wooden shock absorber will itself provide a good thermal barrier.

Concept N

The Concept N containers are shown in Figures 4 and 5. It differs from Concept L by using a lid with a deep skirt which inserts inside the container body. The seal is well protected inside a vertical groove running around the bottom of the skirt. The two O-rings are attached to a rail on the container body, and when the lid in lowered into place the rail inserts into the groove in the skirt to form a seal. This 'barrel seal' requires no additional clamping and designed to be tolerant of vertical and lateral relative movements between the lid and the body. The lid is secured to the body by four large corner chocks at mid-height.

Impact and thermal protection in the 70mm and 145mm variants of Concept N is provided by bolted-on wooden shock absorbers, but this is not possible for the 210mm and 285mm thicknesses because of overall size constraints. These heavier containers rely on similar integral shock absorbers to those in Concept L, with an intumescent coating for thermal protection.

4.3 Assessment and Testing

The impact and the fire performance of the Concept L and Concept N containers have been assessed using finite element methods of calculation. These assessments indicate that either design concept will pass the regulatory impact and fire tests for Type B packages.

One third scale models of the 70mm and 285 mm variants of the Concept L and N containers have been manufactured and are currently undergoing a programme of drop testing to determine the performance of the containers during the regulatory impact tests. To date, both concepts have demonstrated an acceptable performance during drop tests.

The seal arrangements for Concept L and Concept N have also undergone further testing and development [4], particularly of their capability to remain leak-tight under mechanical deflections that might be produced by a severe impact (by comparison, the distortions induced by a fire accident would be much smaller). Experimental rigs were developed to simulate the Concept L and Concept N double O-ring seals, and to provide controlled radial and axial deflections from the normal geometry. Promising elastomer materials were identified and leakage tests were carried out at temperatures ranging from -40"C to -f200°C, in some cases using 0-rings which had been irradiated to simulate the dose that might be received during their operational lifetime. One material was identified which performed adequately across the entire temperature range. The indications were that the Concept L seal would be easier to manufacture and assemble in operational use than Concept N.

4.4 Material of Manufacture

Nirex is examining the feasibility of manufacturing these containers by means of casting instead of the more usual forging process, as this would bring advantages of lower cost and shorter manufacturing time. A programme of work has been carried out [5] to generate data to enable a decision to be made as to whether cast steel or ductile cast iron (DCI) could be used as the material of construction of the transport containers and to enable a choice of preferred material to be made for a subsequent programme of full-scale analysis and testing. Castings have been producied in both cast steel and DCI and subjected to a test programme involving non-destructive testing to locate and characterise flaws, mechancial property tests, including dynamic fracture toughness tests, welding and cladding tests and drop tests. Assessments of the results from this work is continuing.