Remediation activities at Fukushima Daiichi have produced large volumes of highly
radioactive adsorbents. These require immobilisation but are contaminated with Cs
which volatilises at the temperatures typical for common methods like vitrification. Here
a low temperature immobilisation process was developed.
Cs volatilisation studies were conducted for three commercial adsorbents: A-51 JHP,
IE-911 and IE-96. These were loaded with 8 wt.% Cs, and heated in air to establish
the highest temperature that avoided volatilisation. Any Cs evolved was measured using
atomic absorption spectroscopy, in the range 600-1000 C. Volatilisation started at 700ºC
in all samples, with a relative volatilisation of 1.67% ±0.15% and 0.097% ±0.0080 for
A-51 JHP and IE-96 respectively at 1000 C and 7.71% ± 0.66% at 900 C for IE-911
(which degraded at high temperature preventing examination at higher temperatures).
This established a maximum process temperature of 600 C.
Glass-Composite Material (GCM) waste forms were designed within this processing
envelope. Lead borosilicate (PBS) and a lead borate (PB) compositions were chosen
due to their low working temperatures. These were used to encapsulate Cs exchanged
chabazite and IE-96 by low temperature sintering. A 15 wt.% mass loss, observed
at 350 C, was confirmed, using evolved gas analysis mass spectroscopy, to be due to
adsorbed water. Its release led to trapped gases during sintering, resulting in closed
porosity in the final waste forms. A thermal treatment was developed to reduce closed
porosity in the final waste form with pre-treatment (600 C) of the adsorbent prior to
mixing, followed by degasification (350 C), sintering (either 400 C or 500 C) and annealing
(350 C and 450 C) produced dense waste forms for both PB and PBS respectively.
Wasteloadings of 50 wt.% (72.6 vol.%) in PBS GCM and 40 wt.% (65.2 vol.%) in PB
GCM were achieved. No influence of Cs on the sintering behaviour was found as Cs
remained sequestered in the adsorbent during sintering.