Object

There are currently three operating test and research reactors in The Netherlands: the HFR and LFR, both located in Petten, and the HOR, located in Delft. The waste has a couple of characteristics that distinguishes it from the other waste streams:

1. The fuel elements are generally made from metallic fuel meat (an aluminium uranium alloy) with aluminium cladding.
2. The uranium in the spent fuel element is highly enriched.

Facility design

The repository is assumed to be constructed in a rock salt or a clay formation with conventional mine techniques. Each high level waste container is placed in an individual horizontal borehole drilled in the side walls of the galleries. Each borehole is 4.3 m deep. After placement of a canister in a borehole the borehole is back-filled with 3 plugs, each 1 m long. If required the canister can be retrieved as displayed in Figure 1.

Figure 1: Retrieval of a HLW canister

Criticality

Spent fuel from nuclear reactors has to be stored and eventually disposed of in such a way that it always remains sub-critical (i.e. k_eff is less than 1). The potential for critical conditions is of particular importance for the spent fuel from test and research reactors, because of:

1. The amount of ²³⁵U present.
2. The other materials present
3. The geometry of the system.

Critical conditions would not occur in a repository in a rock salt formation, due to the high absorption cross section of chlorine for thermal neutrons.
In a repository in clay it was shown that, depending on e.g. the geometry, critical conditions could occur should water from the clay pores ingress into the container.

Long term safety

A preliminary performance assessment has been carried out for disposal in both salt and clay formations. This assessment was limited to carrying out scooping calculations for a limited number of scenarios. The general conclusion is that for all four scenarios analysed the estimated dose rates are very low in comparison to dose limits and to the doses to individuals from natural sources of radiation.

Corrosion

In a disposal facility conditions may (or will) occur in which the integrity of the disposal cell and container are threatened by mechanical and corrosive forces. Under these circumstances the package may eventually fail, and the waste is directly exposed to the repository environment. The corrosion or leaching rate of vitrified waste or UOx fuel is quite low. However, the corrosion rate of metallic fuel (UAlx) is much larger. The corrosion behaviour of aluminium and Al-type cladding has been studied for interim storage and repository conditions. It follows that for all wet repository conditions, the Al-cladding does not function as a durable barrier on a geological time scale.

Non-proliferation safeguards

Full safeguards measures will be needed for a geological repository in which the spent fuel from the test and research reactors in the Netherlands is disposed. These safeguards measures are intended to ensure that the repository is constructed in accordance with the design that has been reviewed by IAEA inspectors and that nothing has been altered which will facilitate the diversion of nuclear material, either immediately or at some later date.

Acknowledgement

The PASTA project was conducted as part of the CORA programme, instigated and financed by the Dutch Ministry of Economical Affairs.

Figure 2: Criticality and geometry

Figure 3: Scenario analysis, top: dose rate, bottom: individual doserate

Figure 4a: Diffusion scenario (clay)

Figure 4b: Diffusion scenario (salt)