Building work begins on the Habog store

Covra operates the only waste management facility in the Netherlands at Borssele in the south west of the country. This comprises a low-level waste treatment facility (AVG) and a low-level waste storage facility (LOG).

The organisation is now building a new facility, known as Habog, for the storage of fuel and high and medium-level waste. The centre will store spent fuel from the Netherlands’ two reactors, after it has been reprocessed, and waste from two high enriched uranium (HEU) fuel reactors and a research centre.

Safety is the main issue in Habog’s design. In particular the facility must comply with the rules of the American Standard ANSI-ANS 57-9, with minor deviations for specific aspects. Two barriers are required between the radioactive materials and the environment.

The design elements to be considered are:

•Flooding up to 2.71m above the finished ground floor (ie 9.96m above sea level).

•Earthquake (a horizontal free-field peak acceleration of 1m/sec2).

•Aircraft crash (the General Dynamics F16-A Falcon Fighter is the design aircraft, considering direct and indirect effects).

•Pressure wave resulting from an external explosion (a pressure of 0.3bar).

•Whirlwinds (a maximum wind velocity of 125m/sec).

There are also a number of important operational issues. An allowance has to be made for the existing AVG facility on site, for example, and for all the types of waste that it receives the facility will need different handling facilities and tools.

Specific site studies have been completed for each of the events to be considered. For flood protection, a waterproof concrete was used for the storage compartments. Even if the building is flooded, operations can continue on the first floor of the building.

For stability during an earthquake, equipment was designed according to a spectrum of ground acceleration drawn up for each floor level of the building. This seismic design applies to equipment, ensuring the integrity of at least one containment barrier.

An aircraft crash could cause a cooling airflow disturbance, clogging the air inlet by up to 95%, or damaging the exhaust stacks. At Habog, ten perpendicular impact points on the building were considered, and a spectrum calculated for each.

An explosion close to the facility could lead to external overpressure. The building itself and the metallic containment barrier for heat-producing waste have been built to withstand the pressure wave that this could cause, and a gas detection system at the site boundary closes all inlets and ventilation ducts to prevent an internal explosion.

STORAGE PACKAGES

The facility will store packages in the form of vitrified waste canisters and spent highly-enriched uranium (HEU) fuel canisters. These involve both heat-producing and non heat-producing waste packages. The latter can be stored in bunkers, releasing decay heat by conduction through the walls and the slab of the bunkers. But the heat-producing waste will need efficient cooling during the first 100 years to guarantee the integrity of the first containment barrier.

The cooling process has been chosen according to the maximum allowable temperatures in normal and abnormal conditions. Heat-producing packages will be stored in vertical wells cooled by air that circulates between the external wall of the storage well and a double jacket. This process has been used in the EVSE in La Hague, and usually requires a horizontal plate.

But the decay heat from HEU fuel is very different from that of vitrified waste and it had to be examined in more detail. Thermoaeraulic studies were therefore carried out to see if these two types of canisters could be stored in the same vault . The concern was that heat radiation from vitrified canisters would increase the temperature of HEU fuel. Natural convection was verified in a wind tunnel under different conditions, taking into account the exact site layout.

INTERMEDIATE LEVEL WASTE

Any segregation between the different types of waste stored in the bunkers would be a drawback for operational flexibility, but may be required because of differing temperature limits in the storage rooms. Another reason for segregation is the high radiation level produced by hulls and end pieces. This radiation may produce radiolysis of bitumen and gas production in the bituminous waste drums. The swelling of bitumen due to radiation from hulls and end pieces was assessed, and it was concluded that hulls and end pieces must be stored in separate bunkers from bituminous waste. So in Bunker 1, there are compacted hulls and end pieces. Bunker 2 houses other types of waste, including cemented technical waste, BNFL process waste and bituminous waste. Bunker 3 is spare.

FOUNDATION DESIGN

Soil in the south-western part of the Netherlands mainly consists of soft layers and all buildings have to be piled. The extreme nature of the design events (earthquake and aircraft crash) to be considered means that the piles would be complex and very expensive. What is more, piles are designed for vertical and not the horizontal loads that would occur in these cases.

In collaboration with HBKC, Covra found an easier foundation method that could save time and money before work started. The soil was excavated to 6m below ground level and replaced by high-grade sand. Then a pre-load mountain, 8m high and the same weight as Habog, was placed on the site to compact deeper layers.of soil. After six months of continuous monitoring, it was removed. The pre-loading meant that it was not necessary to put the building on piles.

The Habog facility will provide safe storage for at least 100 years. Vitrified waste and fuel canisters are stored in two identical vaults, each containing wells for vitrified waste and for HEU fuels. Together, these two vaults provide five separate spare wells for vitrified canisters and three spare wells for overpacked canisters. A third vault, identical to the other two, is also available. Non-heat producing waste is stored in two separate identical bunkers. A third is available as a spare..

A hot cell is provided for overpacking and opening canisters, and for decontamination. Austenitic stainless steel with molybdenum is used for the storage wells, to reduce susceptibility to pitting corrosion, taking into account the salt content of the air due to the proximity of the sea. Electrostatic filters are installed at air inlets.

The storage wells can be removed and replaced, because they hang from the slab through sleeves and are guided at the bottom. This means that they can be removed vertically after the canisters have been relocated in the spare vault and new wells can be installed.