Safety requirements for structural systems can be deduced overall from the statutory requirement to prevent damage and from the safety goals to be complied with.

Specific requirements here are laid down in the nuclear rules, accident rules and KTA safety standards.

The storage building is required mainly to:

— provide shielding

— remove heat

— be designed for operating and exceptional loads

— provide protection against fire and lightning strike

— protect against the weather

— protect against third parties (sabotage).

4.3.2.1 Design criteria

Design criteria are governed by:

— Shielding

Most of the ionising radiation that fuel elements emit is shielded by their containers. The reinforced concrete building structure provides further shielding, keeping radiation levels within the limits laid down by the radiation protection regulations and protecting staff and the environment.

— Heat removal

The interim storage facility design is designed to remove the heat that the fuel elements give off as they decay, by way of natural convection. The air inlets and outlets required must be arranged and dimensioned to remove heat reliably.

— Building settlement

Building settlement due to the container loads involved must not compromise the structure or the operation of the cranes etc. Settlement is estimated technically at the planning phase, allowing for subsequent partial occupation levels, and is monitored in operation via recurrent settlement testing.

— Structural integrity

As with conventional structures, this requirement can be met via the rules of building design on the design of the roof and sealing the building externally, if groundwater conditions allow.

— Floor structure and decontaminatable coatings

The slab and ground in the storage area must have sufficient compression strength and wear resistance to take the containers put into storage. This is achieved by using a mechanically smoothed concrete surface with hardening agents mixed in. In the reception and maintenance area, the floor is given a decontaminatable coating as a precaution. In the loading and unloading zone in the reception area a shock-absorbent layer of so-called damper concrete can be included in the floor slab to protect containers and floor slab if a container is dropped from a height of 3 m, which cannot be ruled out.

— Durability

Interim storage facilities are designed to be permanent in accordance with conventional standards. If they are built properly of tried and tested reinforced concrete designs, they should last for their full working lives.

4.3.2.2 Building design

As we saw in Section 4.3.2, building structures in Germany fall into one of two different designs: WTI and STEAG. These designs differ from one another in particular in terms of their structural design.

WTI Design

The building is designed to withstand exceptional effects from outside, such as earthquakes and blast waves from explosions. They do not need to be designed to absorb aircraft impact, as the containers themselves are designed for this external event.

Exceptional events from inside are containers falling from a height of 0.25 m in the hall area and 3.00 m in the loading area. In the trans-shipment hall, so-called damper concrete is used in areas in which containers could fall, to absorb the energy released, enabling the loads involved to be transmitted without additional strengthening the floor slab at this point.

When floor slabs are occupied by CASTOR containers in blocks of eight, this gives a floor slab loading of 200kN/m2.

What is not typical, compared with similar lightweight hall constructions, however, is the roof construction; this has to be 55 cm of normal concrete to be radiation-proof. This high permanent load component means that this design has to have relatively high roof girder constructions.

STEAG Design

The greater roof slab and wall thicknesses of the solid STEAG design will at least protect against penetration from aircraft impact. Unlike the WTI design, such halls can also hold containers designed for a debris load of 21 at least, should roof sections fall in.

Temperature effects

The relatively high room temperatures of approx. 80 °C mean that the outer walls and roofs must be reinforced accordingly, to guard against a correspondingly high crack width, which must be demonstrated in many areas for centric forces as finally built (with the concrete at its full tensile strength).

The floor slabs are designed not merely for a high load per surface area of up to 200kN/m2, but also for hot spot temperature effects of approx. 120 °C immediately below the containers. This makes an additional consideration of the upper reinforcement of the floor slabs necessary. The hot areas cause concentric inherent stresses leading to cracking.

However, non-linear studies of floor slabs made at interim storage facilities have shown that no additional reinforcement is required because of the hot spot effect.