French very lows

According to French regulation, all waste produced in part of a nuclear facility where contamination or activation is possible must be managed with appropriate traceability, whatever its actual activity is. As a large proportion of this waste does not require specific containment because its activity is sufficiently low, it would not be appropriate to dispose of it in the Centre de l’Aube disposal facility for low and intermediate level short-lived waste. When decommissioning programmes started on first generation power reactors and the first French reprocessing plant, Marcoule, the need for an adequate disposal site for this very low-level waste (VLLW) in France became more pressing. A suitable repository was therefore designed and built.

Forecasting waste inventory

In the framework of a working group Andra and the three major French radioactive waste producers – EDF, the French Atomic Energy Com­mission (CEA), and Areva – prepared a provisional inventory of waste that would arise within the next 30 years, by taking into account the major decommissioning activities planned.

In particular it took into consideration EDF’s 2001 decision to dismantle all of its reactors that have ceased commercial operation, over a period of 25 years. The facilities concerned included the eight first-generation power plants, which were built during the 1950s and 1960s and ceased commercial operation between 1973 and 1994, namely: Brennilis (heavy water technology prototype reactor); the six natural uranium gas graphite reactors at Chinon, Saint-Laurent des Eaux and Bugey; and the first French PWR reactor Chooz A. The fast breeder reactor Creys-Malville, shutdown in 1985 in accordance with a government decision, was also to be totally dismantled.

Another important programme considered was the decommissioning of the UP1 reprocessing plant in Marcoule. Operations at the plant were suspended in late 1997 after it had reprocessed nearly 20,000t of fuel from gas-cooled and research reactors since it was commissioned in 1958. Plant decommissioning is expected to end around 2030.

In total, therefore, delivery of some 800,000t of waste was expected. In order to take into account uncertainties and the amount of additional incoming waste from other producers, the waste capacity of the disposal facility was set at 650,000m3, which corresponds to about 1,000,000t of waste.

A provisional radiological inventory was drawn up, as well as a chemical inventory of toxic elements, such as chromium, lead, nickel, cadmium, arsenic and mercury.

Repository design

The repository was designed to comply with the regulations governing disposal facilities for non-radioactive hazardous waste. By applying this design principle, it is possible to accommodate both VLLW and toxic chemicals in the same repository. Containment relies on the properties of the clay layer in which the disposal trenches are built.

Waste is disposed of on a plastic impermeable membrane settled in large trenches excavated in a clay layer. Emplacement is performed under a mobile roof to protect against rainwater. Trenches are filled with sand then covered by a plastic membrane and a clay layer. An inspection hole is used to check that there is no water seepage around the waste.

Except in the case of hazardous waste, which is subject to acceptance criteria, there are no specific waste-containment requirements. Hence, the conditioning of packages only serves to facilitate handling operations and to prevent the dispersion of labile contamination in the disposal trenches during loading. As the case may be, some waste, such as bulky items, may be disposed of without any form of preliminary packaging.

Repository siting

Andra selected a suitable site in the village of Morvilliers, Aube, 2km from the Centre de l’Aube Disposal Facility, which allows synergies between both operations. An area of 45ha was surveyed and a clay layer varying between 15 and 25m in thickness was characterised during two reconnaissance campaigns in 1999 and 2000. The clay was found to have good properties in terms of permeability and homogeneity.

Due to the radiological inventory of the repository, it is not considered a nuclear facility and is therefore not subject to the nuclear site (INB) licensing requirements. Instead it must comply with regulations for classified facilities for the protection of the environment (Installations classe´es pour la protection de l’environnement). Therefore it was the local prefecture that reviewed the application for the repository and delivered authorisation. This procedure gave rise to two public inquiries in 2001 and 2002. The first was related to the public use of the facility and with the clearing of land. The second inquiry, which took place in June and July of 2002, considered the construction and operation of the facility itself.

Construction on the very-low-level disposal facility started in August 2002 with site clearance, and excavation began in January the following year. The first waste packages were deposited in October 2003 in disposal cells measuring 80m long, 25m wide and 6m deep. Treatment units (presses, solidification, inerting) were commissioned at the end of 2004.

Safety approach

The safety approach selected for the VLLW disposal facility is consistent with the safety approach adopted for the Centre de l’Aube. Doses are compared with value limits set by Andra and consistent with the regulations or the proposed objectives of international organisations, such as the International Atomic Energy Agency (IAEA) or the International Commission on Radiological Protection (ICRP). More specifically, Andra has adopted a maximum value of 0.25mSv/y for public exposure and 5mSv/y for workers in normal conditions.

Water transfer studies were used to determine the acceptable character of the inventory considered for the disposal facility. More particularly, they helped to define radiological capacities for 25 radionuclides, as reflected in the order authorising the operation of the repository.

Air transfer scenarios were investigated to cover not only the operating phase of the facility (eg accidental fire scenario), but also its long-term impacts (eg on construction of homes or roads on the site). In all cases, the radiological impacts were much lower than the 0.25mSv/y limit set by Andra for the public.

A second approach was performed to determine the maximum admissible waste content, that would not only ensure compliance with the maximum doses limits set by Andra for the different scenarios under investigation, but also protect the workers under both normal and accident conditions. Andra transposed that maximum content into a requirement on the average specific mass of a “waste batch” delivered by the producer.

That activity is expressed in the form of an index as follows: where Ami is the average specific mass of waste batch for radionuclide i and Classi is the activity class for that radionuclide. Andra has defined four activity classes and the classes of some radionuclides are shown in Table 2. The reference activity is therefore 10Bq/g for 60Co and 137Cs, 100Bq/g for uranium isotopes and 1000Bq/g for 14C.

The IRAS index for the entire batch must not exceed 1, but variations of up to 10 are allowed for a single package of the batch. The control of this index at the time any waste batch is accepted ensures the acceptability of the batch in relation to its impact, knowing that the average impact remains very low with the required radiological capacity.

Operation

As mentioned earlier, there are no waste packaging requirements for VLLW and therefore only waste that could be dispersed during handling is packaged, for instance in big bags. Waste packaging requirements have been established to standardise handling equipment as much as possible. However as handling techniques are simple due to the low dose rates, pretty much any type of container can be used.

Waste is emplaced in a cell while it is covered by a mobile roof. When a disposal cell is filled, it is covered with a high-density polyethylene (HDPE) membrane. Then the mobile roof can be removed and located above the next cell to be constructed.

Operation is performed in another cell during these construction works. Therefore two mobile roofs are required.

The simplicity of handling techniques enables the disposal of large items of waste. For decommissioning activities such an option can prevent costly cutting work. It can be also beneficial for some large waste, pieces that would usually be destined for disposal at the low-level waste repository in Centre de l’Aube. This can be decontaminated in order to take advantage of simpler handling techniques.

Unfortunately for very large waste items it is not generally possible to use the access ramp to the disposal cell and the height of the cell is too small for a crane; so it is necessary to wait until the level of waste in the cell is at ground level before disposal can take place. The implementation of a specific cell with walls supporting a crane is currently being investigated for waste items above 23t.

As of the end of December 2007 the very low level waste facility at Morvilliers contained some 89,100m3 of very-low-level waste packages. Its capacity of 650,000m3 should be sufficient to support France’s VLLW needs until 2030.