One important key issue remaining to be studied however is the effect of a large scale - length of the heated part about 10 times the outer diameter of the gallery - thermal load on the behaviour of Boom Clay. Indeed the impact on the Boom Clay of the thermal load generated by the waste is important since it will significantly affect the temperature and the stress profiles on the whole thickness of the Boom Clay layer in the short term after the disposal. The early THM perturbation might well be the most severe transient that the repository system will undergo on a large spatial scale and in a relatively short period of time.

In the frame of retrieveability of the waste the combined impact on the gallery lining of stresses induced by the host rock and thermal effects induced by the heat emitting waste still have to be studied.

Therefore, one of the PRACLAY in-situ experiments that will realised is the so called PRACLAY heater test wich is a large scale heater test aiming to verify the performance of Boom Clay submitted to a thermal load at large scale.

The large scale heater test has to demonstrate that Boom Clay will behave as predicted. It will be important to verify that fracturation and desaturation remain acceptable and that the decrease of effective stress due to the increase of pore water pressure will not lead to the liquefaction of Boom Clay. A long term (more than 10 years) large scale heater test would be representative of the real case. However after 5 years heating around 2012, preliminar conclusions could already be drawn indicating weather or not Boom Clay is suitable for hosting heat-emitting radioactive waste.

Additionally, the heater test could be used also to study the behaviour of some EBS components , to study the oxydation around excavation and to realise a migration experiment with heavy water in order to investigate migration under a thermal load (representative of an early container failure scenario), or after thermal loading (heated clay).