Aconsortium of four Italian companies operating in the geoenvironmental, nuclear, geoengineering and geo-mechanical sectors has proposed the construction of underground nuclear power plants and associated repositories for lowintermediate and high level radioactive wastes, to three countries: Chile, Armenia, and Saudi Arabia. The CERNS consortium is composed of ELA (Energies, Large & Alternative), Rocksoil, Nucem, and SrS (Services of Research and Development).

This kind of project can be carried out by applying the ‘Super-safe and Simply/Easily Decommissionable Nuclear Power Plant’ (SUSE-NPP) scheme to any existing commercial reactor. The SUSE-NPP scheme was patented in the USA and China in 2014 and issued from the European Patent Office in Munich.

“Underground nuclear power plants can be built as conventional nuclear power plants sited above ground, but placed in caverns,” explains Pietro Lunardi, owner of Rocksoil. “Nowadays we have many tunnelling technologies, that allow major excavation up to 50m in diameter.

“For example, the CERN Neutrinos to Gran Sasso project is based in a tunnel under the Gran Sasso Mountain in Italy at a depth of 1,450m.”

To prevent any risk of contaminating the groundwater, it is of utmost importance to design in waterproofing and carry out grout injections into the cavern walls if the rock is fractured. These injections can achieve a pressure of 200 or 300 bar and, outside them, it requires building drainage tunnels to carry groundwater away from the structure.

Spent nuclear fuel can be placed in caverns, isolated from direct human involvement. The radioactive waste is fully shielded so that it can’t get into the groundwater. “However,” says Lunardi, “everything is dependent on ground conditions. For example, we don’t need to do any injections if we encounter ground such as clay.”

PRELIMINARY PROJECTS IN CHILE, ARMENIA AND SAUDI ARABIA

There are 5,000km of mountain ranges in Chile and the entire country is one of the most active tectonic boundaries of the world, where the Nazca lithospheric oceanic plate subducts the South American continental plate. It shows clear geological deformation caused by the movement of the tectonic plates. From here, the Andes emerge and run along the eastern margin of the South American Plate.

“In such situations, we expect to find many kinds of geological conditions, including volcanoes,” Lunardi says. Chile is also a country with very high seismicity due to fractured rocks and the huge amount of frictional resistances, which come together with the deformations produced by the collision between the Nazca and the South American plates. Lunardi says: “Earthquakes can’t put underground nuclear power plants at risk because underground works are not affected by shock waves, so this is not a concern.” In the event of distruption to the electricity supply above ground, the reactor would not be affected because it runs independently.

“Based at roughly 300m below ground, the nuclear power plant creates heat to convert water into steam, which activates turbines. These turbines can be placed above or below ground, but it’s better to put them underground for safety reasons, for example to prevent terror attacks.”

Saudi Arabia’s geology is mainly young limestone, which can be good for this type of underground work, says Lunardi. Geologist and professor Sergio D’Offizi, member of the CERNS Consortium, explains that in Armenia there is essentially volcanic ground. “Of course, to find the most appropriate location there is a need to carry out Geognostic surveys to check if there is any groundwater,” D’Offizi says.

Lunardi adds: “As Armenia has an existing power plant above ground, the nuclear waste can be lowered underground in the established location to join that of the new power plant. To move the nuclear waste underground, it is necessary to excavate shafts, which could have diameters of 20 to 25m. After that, we need to provide a network of underground works to produce energy and to store waste.

“Fitted in metal or concrete casks, nuclear waste can be lowered down the shaft and then these casks can be placed into the caverns, which are always accessible to operators. “Once the project is approved we can establish everything by carrying out in-depth analysis. As a starting point, a pre- feasibility analysis can be done on a preliminary design, the main aim of which is to show the adaptability of the existing nuclear power plant to an underground version.”

LOGISTICS AND ACCESS TO UNDERGROUND SITES

Both reactors and radioactive waste can be placed within caverns or even under the seabed, and they can be connected to the surface by an inclined main access entrance and vertical access shafts.

“The purpose of the shafts is to lower components of the reactor pressure vessel, which can be mounted underground inside the caverns. Shafts can be adapted to the dimensions of these components,” Lunardi says.

“For example, if these components span 8m diameter we will need shafts with a diameter of 20m. Access tunnels with a diameter of 10-15m will serve the main tunnel.

“These access tunnels are available to operators to monitor and move any relevant materials into the reactor. Caverns can span up to 50m diameter to contain the reactor.”

GROUND STABILISATION TO BUILD UNDERGROUND CAVERNS

Lunardi explains that these kind of caverns need to be excavated starting from the crowns and then the walls have to be reinforced, requiring heavy rockbolting using bolts with a length of 20-25m – depending from the type of rocks and caverns.

“We can find ourselves excavating caverns with heights of 50 or 70m for these sorts of projects. And lifespan requirements can be high. We carried out rockbolting in stainless steel to be long lasting at the CERN project,” Lunardi says.

“This kind of rockbolting can be injected with high-strength mortar. When rocks are in good condition, reinforced rockbolts are useful to avoid a plasticisation.

“Again, we also need to look at waterproofing injection to avoid a leakage of groundwater into the caverns.”

SETTLEMENT

These caverns are usually equipped with sliding micrometres, which measure displacement and deformation profiles. By drilling a hole in the crown or in the walls of the cavern, sliding micrometres can give information on the behaviour of ground whether or not is moving or plasticising.

“This is important to evaluate if it’s necessary to intensify the level of rockbolting. During construction we can evaluate that by checking the results of sliding micrometres, which provide measuring anchors metre by metre. This kind of extensometer system measures how these anchors move towards the face.

“When you open [excavate] a cavern, the tensile stress changes and tension is concentrated on the face. This tensile stress can damage the rock, which is no longer in elastic deformation but elastoplastic, and in turn, increases its volume. The increase of volume doesn’t go through the rock but into the empty space, where we have excavated causing some convergence.

“We can control this subsidence with special steel strips, which measure how much the contraction of excavation tends to be closed. This kind of analysis to check the stability is not new at all, because it has already carried out for hydroelectric power plants.”

ADVANTAGES

D’Offizi explains that underground nuclear power plants can be the ideal solution for several reasons:

¦ To deal with possible accidents such as a core meltdown by simply making a water pool at ground level just over the reactor; or providing water from natural rivers, lakes or the sea to cool the core, dropping this water down by gravity and eventually recovering the heated water on the surface by natural circulation. That was not possible at Fukushima where the tsunami that overwhelmed the nuclear power plant caused the emergency pumps for cooling the core to fail;

¦ To reduce radioactive emissions into the environment to zero in the event of severe internal accidents;

¦ To protect the site from a potential terrorist attack or extreme natural events such as seismic and volcanic events, tropical storms, tsunamis, flooding;

¦ To facilitate the site for the nuclear power plants even in proximity of urban areas. It’s also possible to reduce the cost for building and managing expensive long transmission electric lines;

¦ To reduce land consumption;

¦ To reduce the cost of the final decommissioning of the nuclear reactor, which at the end of its operational life can remain in the same cavern. The cavern becomes its final repository.

“Two scientists, Andrei Sakharov and Edward Teller, have expressed their positive opinion of underground nuclear power plants,” Lunardi says.

“In his memoirs, written four years after the accident at Chernobyl, Sakharov wrote: ‘The solution I prefer is to build nuclear reactors at depth underground so that even in the worst case accidents no radioactive substances are released in the atmosphere.’”

Teller also said that for the containment of radioactive substances in the event of an accident, his suggestion was to place nuclear reactors below ground at depths from 30 to 90m.

UNDERGROUND REPOSITORY

As mentioned, these caverns will serve during plant operation and also, after their final shutdown, as repositories for radioactive waste – one final repository for low to medium activity waste, and an interim one for high activity and long life radioactive waste.

If possible, they could be placed under the seabed near the shoreline, as has been the case in Sweden, where a 63,000cu.m waste repository (SFR), owned by the Swedish Nuclear Fuel and Waste management (SBK) is sited.

The SFR has five different rock chambers for different types of waste.

The most active waste is disposed of in a concrete silo surrounded by a clay bluffer and the other four chambers consist of a cavern for Low-Level Waste (LLW), two caverns for concrete tanks with dewatered ion exchange resins and a cavern for Intermediate-Level Waste (ILW).

The CERN Consortium has noted that the entire Swedish repository is located in crystalline bedrock at 60m under the bed of the Baltic Sea.

The entrance is at the Forsmark harbour, close to the nuclear power plant and two tunnels lead to the nuclear waste disposal area.

“As foreseen by the SUSE-NPP technology, pools for cooling the spent fuel could be placed underground to achieve the maximum possible safety against eventual war or terrorist attacks,” the CERNS Consortium says.

In addition to its existing research the group says, the possibility of using seawater to be eventually moved through natural circulation for plant condenser cooling should be analysed. “Seawater in closed circuit could be also analysed as option for decay heat removal.

“Emergency conditions could be discussed to ensure the cooling of the nuclear installation, in the case of power failure or the malfunction of the pump and of the primary circuit for extraction of heat from the core.

“Special plant solutions based on amounts of sea water sinking from the surface by gravity could also be analysed as a way of tackling extreme failures. Both the reactor and the repositories, and a centre for characterisation (to identify the level of radioactivity), treatment and conditioning of radioactive waste will be built underground.

“This is possible by using new mining technologies that allow the excavation of large volumes of rock quickly, easily and at a very low cost”.