OBSERVE – A comprehensive dose rate and temperature measurement programme for long-term interim storage

The dose rate and temperature measurement programme OBSERVE as a method of monitoring the condition of casks during extended interim storage is key part of the research programme of BGZ.

The distribution of dose rates and temperature across the cask surface is influenced by the arrangement and characteristics of the radioactive contents and cask components. Measurements are routinely carried out prior to the placement of casks in storage. The aim is to verify compliance with the relevant dose rate and temperature limits for storage in the corresponding storage facility. These measurements are performed at designated representative measuring points on the cask surface, in order to obtain reliable information about the mean or maximum values.

In contrast to this, the OBSERVE programme, initiated by BGZ, is designed to measure the dose rate and temperature over the entire surface of selected casks. In the first stage of the project, sensitivity studies were carried out to check the extent to which dose rate and temperature measurements are suitable for drawing conclusions about the condition of cask components and the inventory. The sensitivity analyses have demonstrated that, to a certain extent, both changes in the fuel and shielding arrangement can be detected by close-meshed dose rate measurements on the cask surface. It can therefore already be stated that the OBSERVE measurement programme is a suitable addition to existing ageing management measures and is able to assess the cask’s condition by means of measurement.

During the second project phase, a measuring device will be developed capturing values at the entire cask surface automatically in accordance with ALARA principles. For this purpose, the results of the sensitivity analyses will be utilized to ascertain the requirements for the measurement programme and the measuring device. This includes the definition of a suitable measurement location within the storage facility and the consideration of background radiation effects.

In order to evaluate the measurement results, a comparison will be made with the pre-calculation values. These pre-calculations must consider the actual inventory at the time of measurement as well as the cask configuration. The results of the comparison and the respective distribution are used to conclude about the actual shielding and heat dissipation properties. Assuming the measured values are within the expected range, this confirms that no unexpected changes have occurred. In this way, the measurement programme makes an active contribution to ageing management. Furthermore, safety-related assessments of the long-term behaviour of the cask components and the inventory can be used as direct input for safety analysis of future approvals.