Before the building of the pre-stressed concrete pressure vessel at Schmehausen and the pre­stressed concrete containment at Gundremmingen, pre-stressing was only used as a general rule in Germany in wide-spanned precast girders in the turbine buildings and other special support structures, such as the instrument room inside the reactor building at Kriimmel.

3.2.2 Reactor building

In terms of structural particularities, it is the reactor building that poses the highest requirements. In what follows, we will limit ourselves to looking at reactor buildings in light water reactors, PWRs and BWRs. The different functional requirements involved here also mean that the shapes of the buildings themselves differ, rectangular buildings being preferred for BWRs and curved building structures with circular footprints (cylindrical or spherical) for PWRs.

A Convoy type reactor building is shown in Figure 4.3. This consists of the spherical reinforced concrete shell typical of many PWRs, with very thick walls (h = 1.80 m)

System components

1. Reactor pressure vessel

2. Steam generator

3. Circulation pump

4. Main coolant lines

5. Pressuriser

Structural components

6. Fuel pool

7. Containment

8. Outer reinforced concrete shell

9. Annular space

^9.60

Fig. 4.4 Reactor building with fuel element storage and safety building of a PWR-EPR type [22]

designed to withstand an aircraft crash. This encloses a steel safety container as containment, which maintains integrity even in an anomaly.

The further development of the Convoy power plant model as part of the Franco — German partnership led to the EPR, a Generation Ш+ reactor, as is currently being built in Finland and France. The main features of the EPR reactor building are as follows (Figure 4.4):

— There is a clear structural separation between the building complexes of the nuclear island (reactor building, fuel element storage building, safety systems building etc.) and those of the conventional island (turbine building, etc., which is why it is also often called the ‘turbine island’).

— There is a common baseplate for the relevant buildings on the nuclear island, to make it easier in the event of an earthquake to manage the induced shocks acting on the building structures and mechanical components and avoid individual buildings shifting in relation to one another.

— The double-shelled outer wall structure of the reactor building consists of an outer reinforced concrete wall 1.80m thick, an air gap of 1.30 m and an inner pre-stressed concrete wall 1.30 m thick. The inner wall is of pre-stressed concrete design, with an additional steel liner 6 mm thick on the inside to ensure that the containment does not lose its integrity even in an extreme accident (internal pressure approx. 0.5 MPa at temperatures of approx. 150 °C)

The so-called ‘double containment concept’ described above has established itself worldwide as far as the layout of the reactor building is concerned. What this means is that external influences, such as earthquakes, aircraft impact, pressure waves etc., can be absorbed by a reinforced concrete structure of a suitable thickness (APC shell). The
integrity of the reactor building to contain radioactive substances is maintained by a separate integrity barrier, which constitutes the actual containment or safety enclosure of the reactor building. As well as maintaining integrity, however, the containment must also contain the internal pressures resulting from operations and accidents, plus high thermal stresses.

There are a number of containment concepts, depending on what kind of reactor is involved:

— Reactor containment of steel (e. g. Convoy PWR models)

— Pre-stressed concrete containments without liners (in French N4 reactors, for example, but note that this concept has not proved itself, as integrity requirements cannot be met long term)

— Pre-stressed concrete containment with steel liner (e. g. EPR, approx. 6 mm thick)

— Non-pre-stressed steel containment with steel liner (e. g. KERENA, approx. 10 mm thick)

This list in itself makes it clear that a combination of pre-stressed and reinforced concrete and its associated steel liner is extremely important as an integrity barrier.

Regulatory authorities worldwide are demanding increasingly that plant technology provides passive safety systems and robust design. Whether this should also apply to structural engineering, even using pre-stressing with composite construction, raises ‘argumentation problems’ as far as this robustness requirement is concerned. The pre­stressing, which is usually extremely high, must be maintained over the very long period of more than 80 years. Monitoring pre-stressing with composite construction is difficult, and pre-stressed members cannot be replaced in practice.

The very high pre-stressing also has other drawbacks, as it devolves creep and shrinkage onto the steel liner and other steel components, such as pipe mountings and locks.

More recent containment developments, like AREVA’s KERENA containment, thus omit the pre-stressing, preferring instead to use thicker steel liners and suitable composite construction elements as structural elements in a composite construction with the concrete.

In terms of building construction, making the reactor building roof structure is particularly important, as it has to be extremely thick to withstand the impact of an aircraft. Once the reactor pressure vessel is installed, the interior work begins soon afterwards, so that supporting the shell inwardly is no longer possible in most cases. So the hemispherical roof structure with cylindrical reactor buildings, which can be precast separately, was developed — this was done also because curved structures are much better at withstanding the stresses of an aircraft impact once the membrane strength cuts in than flat surfaces.

On the other hand, doubly curved load-bearing structures are more expensive and take longer to construct, which is why the roof structures of Gundremmingen B and C reactor buildings were made with precast, wedge-shaped laid precast segments with locally cast concrete added (Figure 4.5).

Fig. 4.5 Gundremmingen B and C reactor building roof structure, with precast wedge-shaped laid precast segments with cast concrete added locally [17]