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Interim storage facilities for nuclear fuels: Checkpoint safety

The term safety refers to the necessary precautions to be taken according to the state of the art of science and technology against damage that may result from the storage of nuclear fuels in an interim storage facility. This is about radiological safety in the interim storage facility’s normal operation, in case of incidents during the operation and in case of natural events, whereas the protection against impacts of third parties such as terrorist attacks or acts of sabotage is termed security.

For each licence for an interim storage facility and for each modification to a licence, the BfE verifies that the safety requirements according to the state of the art of science and technology are complied with. The primary objective is to prevent an impermissible radiation exposure inside and outside the interim storage facility.

Some limit values set out in the Radiation Protection Ordinance

  • Persons working in the interim storage facility: 20 mSv per year (§5 StrlSchV, para. 1)
  • Normal population: 1 mSv per year (§46 StrlSchV, para. 1); 50 mSv for all incidents for which an interim storage facility is examined (§49 StrlSchV, para. 1).

In these cases the principle of minimisation is imperative: The radiation exposure must "be kept as low as possible in the light of the state of the art of science and technology and taking into account all circumstances of the individual case, also below the limit values". (§6 StrlSchV, para. 2)

Minimisation according to the state of the art of science and technology

Scientific findings define what radiation exposure is impermissible: The Radiation Protection Ordinance (§6 StrlSchV, para. 2) prescribes that each radiation exposure associated with the interim storage of nuclear fuels “be kept as low as possible in the light of the state of the art of science and technology and taking into account all circumstances of the individual case, also below the limit values”. The limit values set out in the Radiation Protection Ordinance are thus only a maximum value but not the standard for assessment.

The Atomic Energy Act – the legal basis for the licensing of interim storage facilities for nuclear fuels – demands accordingly that the safety of the storage comply with the state of the art of science and technology (§6 AtG, para. 2). This is in particular represented by the nuclear rules and regulations. It contains numerous provisions such as technical regulations, acknowledged detection and calculation methods, safety management regulations, provisions for organisation and quality management systems.

Audit objectives

In order to fulfil the primary objective of preventing an impermissible radiation exposure, the BfE examines four partial objectives:

  1. Enclosure of the radioactive materials: In the storage casks, solid radioactive materials are stored that emit small quantities of gases. The storage casks must be such that neither the solid nor the gaseous radioactive materials can be released in normal conditions. Precautions must be taken for incidents occurring in the interim storage facility or e.g. for the event of an earthquake that no casks can be damaged so seriously that this will lead to impermissible radiation exposure.
  2. Criticality safety: No nuclear chain reaction must start in the storage casks. Regarding the storage of spent fuel elements this is ensured, among others, by the assembly of the fuel elements in the casks and the assembly of the casks among each other.
  3. Heat removal: Spent nuclear fuels produce very much heat. This heat must be dissipated. Outdoors, the outdoor air is sufficient for that. In an interim storage facility, care must be taken by appropriate ventilation that the heat emitted by the casks is dissipated outside. Furthermore, the casks must be placed with sufficient distance to prevent them from heating each other.
  4. Shielding of radioactive radiation: Gamma and neutron radiation can penetrate solid materials. The steel of the casks and special materials used inside the casks must ensure that the radiation is shielded and that as little radiation as possible leaves the casks. Complete shielding cannot be achieved, however. Therefore the operator of an interim storage facility must take additional measures to keep the radiation exposure to persons inside and outside the interim storage facility as low as possible. These measures may include, for example, building materials with shielding effect or access restrictions to the areas where the casks are kept.

The storage cask

The storage casks are key for the safety of the interim storage facilities. Although all storage casks are also licensed as transport casks, they are entirely re-examined for the storage in an interim storage facility. The reason for this is that in interim storage the prevailing conditions are different from those in transportation.

Here is one example: For a cask to be used for transportation, shock absorbers are permanently attached to the casks. In the interim storage facility, on the other hand, the casks are moved without shock absorbers. Therefore, for storage in an interim storage facility, evidence must be furnished again that the casks provide the necessary protection, even in case of a crash, or that technical measures are taken to prevent a crash.

The test procedure

Who applies for a licence for an interim storage facility, has to provide complete evidence as to the safety of the storage facility to the BfE. This includes in particular an exact description of the storage casks, the storage building, the technical equipment, the planned monitoring measures as well as calculations or experimental examinations with regard to the radiological consequences of the storage facility’s operation and to all events to be tested that may occur during the operation.

The application documents are at first checked by the BfE to see if they are complete and plausible. Experts commissioned by the BfE then carry out a complete examination of the evidence submitted. For example, calculations made by the applicant are recalculated or the results are reviewed with other methods of calculation or test procedures. If questions remain unanswered or documents are lacking, the BfE calls on the applicant to submit additional evidence or documents.

All evidence must be conservative. This means that one does not assume realistic or very probable conditions but overestimates the risk deliberately. The applied methods of calculation or furnishing of evidence must comply with the state of the art of science and technology.

For all aspects associated with the storage facility’s safety the BfE determines whether the limit values set out in the Radiation Protection Ordinance are complied with taking into account the principle of minimisation and the state of the art of science and technology. Only after all these requirements are fulfilled, will the test procedure for the checkpoint "safety" conclude with a positive result.

State of 2016.09.20

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