Leaching of 14C in repository conditions. Transport and speciation

Long-term exposure of materials in a repository may result in significant alterations in material structures. The activated decommissioning wastes including the reactor pressure vessel and its internals made of stainless steel, as well as, spent fuel claddings and reactor graphite contain nitrogen as impurity. 14C is generated as a result of nuclear reactions involving nitrogen (14N) or 13C atoms in these materials. 14C is one of the most important radionuclides when considering safe disposal of nuclear waste due to its long half-life.

Both organic and inorganic carbons have been identified in leaching experiments with carbon containing steel, although clearly higher proportion is released as small organic molecules. The origin of these compounds and their reaction mechanisms is not fully understood. The specific iron-water experiments suggest that both the carbide carbon and the reduction of aqueous CO2 could be the sources of hydrocarbons in solution.

Carbon analyses are highly sensitive to filtration which may cause incorrect results by either releasing or sorbing organic compounds. The filters should be cleaned by soaking in double distilled water (DDW) before use or at least to discard the initial filtration volumes. The best materials for DOC analysis are hydrophilic polyethersulphone and hydrophilic polypropylene membrane filters which induced least interference.

In this literature research, it has become clear that there is very little conclusive knowledge and evidence on the form and fate of 14C in irradiated steel materials. A need for careful assessment of the different routes of 14C production in neutron-activated materials and its chemical form and transport is required.

More information:
The paper is available online: www.vtt.fi/inf/pdf/technology/2014/T157.pdf

Provided by VTT Technical Research Centre of Finland