Nuclear Challenges

The Challenge of Removing Residual Sodium from Redundant Nuclear Facilities

The safe removal of residual sodium from nuclear facilities presents particular challenges.  The first is geometry.  The coolant systems for sodium cooled fast reactors are large, complex, and closed. They include pipes and vessels of varying geometries, often with complex internal structures that can retain sodium and hinder access to reactant gases. In most, if not all cases, the eventual draining and removal of sodium was not addressed during the reactor design.  The reactor design focused on operational safety and efficiency, rather than the challenges of decommissioning. Consequently, it is not possible to fully drain the sodium coolant, and some sodium metal will remain in pipes and vessels. The amount, location and depth of these residuals cannot be reliability predicted or measured.  Estimates of the amount of sodium remaining can vary greatly. This presents challenges for the sodium removal process because an unstable and unsafe situation can occur during treatment and subsequent flooding with waters.

In some cases, such as France’s Superphènix reactor, which had a relatively simple internal structure, robotic camera systems were developed to allow inspection of the major elements and visually confirm the location of the residual sodium and then cut holes to drain it. These robots cannot enter all spaces.  Loop-type reactors with a more complicated design are less suited to such robotic inspection and management.

If the amount and depth of the sodium remaining cannot be determined, of if there are blocked pipes with restricted gas flow, it is essential that the sodium removal process proceeds to completion, without stalling, until all metal sodium is safely reacted. Any metallic sodium remaining after the chemical conversion phase is complete will create an unsafe situation when the reactor is later flooded with water.  This is the root cause of the accident at EBR-II that destroyed part of the secondary coolant loop of the reactor when metallic sodium that remained after carbonation came into contact with the acidified flush water.

Another challenge is the requirement to preserve the integrity of the coolant systems during cleaning, particularly for the primary circuit which may contain radioactive material. Some removal technologies, such as WVN, create large stresses that can rupture piping systems (pages 137-139 of the actual document) due to pressure excursions from sudden water/sodium reactions. 

A third challenge is that all implementation steps for sodium removal are subject to regulatory approval, and must be systematically reviewed, planned, proven, approved and controlled. This requires confidence in the track record and experience of using a particular sodium removal technology. It is more difficult to construct a reliable safety case if that experience includes unexpected violent incidents, such as with carbonation or WVN.  

The available sodium removal technologies have different abilities to deal with these three challenges.  The limitations of each technology must also be considered (example is the depth limitation of the carbonation process).  In the table below, projects that experienced unexpected violent incidents leading to system damage and projects that have not resulted in a reactor cleaned of its residual sodium are not classified as “successful”.  Only two technologies are proven to be successful for major nuclear projects:

  • In situ treatment with CEI-SHS™ at the Fermi-1 and SEFOR reactors, and the FFTF-CRCTA test facility.
  • Physical removal of all deep sodium deposits by cutting up plus robotic inspection and drilling, followed by carbonation of remaining thin films at the Superphènix reactor.
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