Tritium speciation in nuclear decommissioning materials


Kim, Dae Ji (2009) Tritium speciation in nuclear decommissioning materials. University of Southampton, Faculty of Engineering Science and Mathematics, School of Ocean and Earth Science, Doctoral Thesis , 196pp.

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Description/Abstract

Tritium is a by-product of civil nuclear reactors, military nuclear applications, fusion programmes and
radiopharmaceutical production. It commonly occurs, though not exclusively, as tritiated water (HTO)
or organically-bound tritium (OBT) in the environment but may exist as other forms in nuclear-related
construction and fabrication materials. During the lifetime of nuclear sites (especially those involving
heavy water) tritium becomes variably incorporated into the fabric of the buildings. When nuclear
decommissioning works and environmental assessments are undertaken it is necessary to accurately
evaluate tritium activities in a wide range of materials prior to any waste sentencing. Of the various
materials comprising UK radioactive wastes, concrete and metal account for approximately 20% of
the total weight of low level waste (LLW) and 12% and 35% of the total weight of intermediate level
waste (ILW).
Proper sampling and storage of samples are significant factors in achieving accurate tritium activities.
The degree of loss of 3H and cross-contamination can be significantly reduced by storing samples in
an air/water tight container in a freezer (-18°C). The potential for tritium contamination is dependent
on the 3H form. Most 3H loss originates from tritiated water which is easily exchanged with
atmospheric hydrogen in the form of water vapour at room temperature. However, the loss of more
strongly bound 3H, produced in-situ in materials by neutron activation, is not significant even at room
temperature. Such tritium is tightly retained in materials and does not readily exchange with water or
diffuse.
In nuclear reactor environments tritium may be produced via several neutron-induced reactions,
2H(n,g)3H, 6Li(n,a)3H, 10B(n,2a)3H and ternary fission (fission yield <0.01%). It may also exist as
tritiated water (HTO) that is able to migrate readily and can adsorb onto various construction materials
such as structural concrete. In such locations it exists as a weakly-bound form that can be lost at
ambient temperatures. Bioshield concretes present a special case and systematic analysis of a
sequence of sub-samples taken from a bioshield core (from UKAEA Winfrith) has identified a
strongly-bound form of 3H in addition to the weakly bound form. The strongly bound 3H in concrete is
held more strongly in mineral lattices and requires a temperature of >850°C to achieve quantitative
recovery. This more strongly retained tritium originates from neutron capture of trace lithium (6Li and
potentially 10B) distributed throughout minerals in the concrete. The highest proportion of strongly
bound 3H was observed in the core sections closest to the core. Weakly bound tritium is associated
with water loss from hydrated mineral components.
Tritium is retained in metals by absorption by free water, hydrated surface oxidation layer, H ingress
into bulk metal and also as lattice-bound tritium produced via in-situ neutron activation. Away from
the possible influence of neutrons, the main 3H contamination to metals arises from absorption and
diffusion via atmospheric exposure to the HTO. Here contamination is mainly confined to the metal
surface layer. The tritium penetration rate into metal surfaces is controlled by the metal type and its
surface condition. Where metals are exposed to a significant neutron flux and contain 6Li, 7Li and 10B
then in situ 3H production will occur which may propagate beyond the surface layer. In such cases
tritium may exist in two forms namely a weakly bound HTO form and a non-HTO strongly bound
form. The HTO form is readily lost at moderate temperatures (~120°C) whereas the non-HTO
requires up to 850°C for complete extraction.

Item Type: Thesis (Doctoral)
Subjects: Q Science > QD Chemistry
G Geography. Anthropology. Recreation > GC Oceanography
Divisions: University Structure - Pre August 2011 > School of Ocean & Earth Science (SOC/SOES)
ePrint ID: 72145
Date Deposited: 22 Jan 2010
Last Modified: 27 Mar 2014 18:51
URI: http://eprints.soton.ac.uk/id/eprint/72145

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