Nuclear Graphite Research Group University of Manchester, UK Robert Worth Nuclear Graphite Research Group University of Manchester, UK Characterisation and Thermal Treatment of Irradiated PGA Graphite with Investigation into 3H and 14C Behaviour Lorraine McDermott, Greg Black, Abbie Jones, Paul Mummery, Barry Marsden, Anthony Wickham 14th International Nuclear Graphite Specialists Meeting Seattle, USA 15th-18th September, 2013

Overview Irradiated Graphite in the UK Thermal Treatment at Manchester Future Research Conclusions

Irradiated Graphite in the UK

UK Graphite legacy Graphite has been used in nuclear power plants worldwide Historically, the UK has constructed many graphite-moderated reactors These include power production, plutonium production and research reactors Some still operational Graphite contributes to a significant UK waste legacy The majority of this graphite waste is ILW Consequently, dismantling and management of radioactive graphite waste is an important issue in the UK More than 100 NPP worldwide ~96,000 te in UK, ~250,000 te worldwide 44 graphite reactors in UK

Why treat graphite? There is no current disposal route for irradiated graphite in the UK Geological Disposal Facility (GDF)? Treatment of irradiated graphite could allow reduction in the volume of ILW (cost-saving) Utilise GDF space Allow disposal in current near-surface facilities This could be achieved by preferential removal of radioisotopes, such as tritium and carbon-14 Goal: Maximise radioisotope removal with minimal weight loss

Carbon-14 formation There are two dominant mechanisms by which 14C is produced in irradiated graphite in a reactor environment: (1) 13C (n,γ) 14C (2) 14N (n,p) 14C

Historically difficult to determine nitrogen content of graphite Nitrogen sensitivity ~50ppm ~10ppm Historically difficult to determine nitrogen content of graphite

Thermal treatment at manchester

Thermal treatment A program of thermal treatment work has been conducted at the University of Manchester as part of the collaborative European project ‘CARBOWASTE’ My own research is a continuation of this thermal treatment research: Investigation of dependent variables, including temperature, time and oxygen Investigation of 14C and 3H behaviour Comparison of current world data to UK-irradiated graphite Optimisation of the process Using pre- and post-treatment characterisation techniques

Isotopic inventory determination Thermal oxidation has been used as a method for 3H and 14C determination Graphite samples are placed in a ceramic combustion boat in a Carbolite® MTT Furnace A suitable cover gas flows past the sample and the temperature is raised A copper oxide catalyst promotes further oxidation of any gasified 3H and 14C . Analysis done in duplicate

Isotopic inventory determination HTO and 14CO2 are subsequently trapped in the bubbler system for analysis using liquid scintillation counting (LSC) Bubblers have a trapping efficiency of 98% Analysis done in duplicate

Isotopic inventory determination Analysis done in duplicate Typical determined radioisotope content in Oldbury Magnox installed graphite: Isotope Activity (Bq/g) 3H ~37400 14C ~63700

Isotopic validation How do we know we are capturing all of the 3H and 14C? Regular recovery checks are performed – a known quantity of 3H and 14C labelled sucrose standards are put through the furnace 3H recovery in the range of 88 – 98 % 14C recovery in the range of 85 – 94 % LSC quenched standard analysis to ensure LSC efficiency LSC 3H quenched LSC standards 99%. 14C quenched LSC standards 100%.

Thermal Treatment Experimental programme A thermal treatment programme has been designed to determine the effects of time, temperature and oxygen on 3H and 14C release The following experimental conditions have been applied to samples machined from installed sets retrieved from the Oldbury Magnox power station: Argon 1% Oxygen in Argon Time Temperature 4 5 6 7 8 600oC  -- 700oC 800oC 900oC Time Temperature 4 5 6 7 8 600oC  700oC 800oC 900oC -- Comparison of i-graphite samples pre and post treatment, A = 800°C 1% O2/Ar for 5 hours, B = 700°C 1% O2/Ar for 5 hours, C = 700°C in Argon gas for 5 hours and D = untreated sample Sample D in figure 23 is the untreated material, it can clearly been seen in the photograph that the effects of a 1% O2 environment at 800°C has had on the material structure. All samples analysed at 800°C in 1% O2 gas become heavily eroded and had a powered surface texture. This affected post thermal treatment analysis handling and unfortunately impacted on the tests that were able to carry out on these samples.

Thermal Treatment Experimental programme Issues with the integrity of the samples post-treatment: A B C D Comparison of i-graphite samples pre and post treatment, A = 800°C 1% O2/Ar for 5 hours, B = 700°C 1% O2/Ar for 5 hours, C = 700°C in Argon gas for 5 hours and D = untreated sample Sample D in figure 23 is the untreated material, it can clearly been seen in the photograph that the effects of a 1% O2 environment at 800°C has had on the material structure. All samples analysed at 800°C in 1% O2 gas become heavily eroded and had a powered surface texture. This affected post thermal treatment analysis handling and unfortunately impacted on the tests that were able to carry out on these samples. A = 800°C in 1% O2/Ar for 5 hours B = 700°C in 1% O2/Ar for 5 hours C = 700°C in Argon gas for 5 hours D = untreated sample

AUToradiography Hotspots

Time Dependence Tritium, 3H Carbon-14, 14C The effects of increase in temperature is clearly seen in this graph – this shows the dependence of temperature with tritium migration

Weight Loss Tritium, 3H Carbon-14, 14C

Pre-Treatment Activity (kBq/g) Post-Treatment Activity (kBq/g) Gamma spectrometry Sample Pre-Treatment Activity (kBq/g) Post-Treatment Activity (kBq/g) Percent Loss OM1 8.824 8.473 3.99% OM14 6.055 5.132 15.26% OM18 10.093 9.561 5.26% OM21 6.951 4.689 32.53% Cobalt-60:

Future Research

Future work - phd ‘Characterisation and Thermal Treatment of Irradiated PGA Graphite with Investigation into 3H and 14C Behaviour’ Full optimisation of thermal treatment of irradiated Oldbury Magnox reactor graphite with respect to the sensitivity of: Goal: Maximise radioisotope removal with minimal sample weight loss Temperature 600 - 900oC Time 3 - 9 hours Oxygen content of gas 0.5 - 2% oxygen in argon

Pre- & post- treatment analysis Weight Loss 4 d.p. Balance Metrology Digital Micrometer Porosity Helium-pycnometry Surface Area Tristar BET Laser Confocal Microscopy To try and determine: Amount of weight loss during treatment The typical location of the radioisotopes before removal

Pre- & post- treatment analysis Radioactive Content Liquid Scintillation Counting Gamma-spectrometry Autoradiography To determine: Amount of radioisotope loss during treatment Identification of ‘hotspots’ of radioactivity, which might influence the results

Visual representation Laser Confocal Microscopy (LCF)

Visual representation LCF Height Mapping

Visual representation LCF 3D Image

Conclusions It has been demonstrated that thermal treatment in an oxidising atmosphere is a potential means of removing 3H and 14C radioisotopes from irradiated graphite The current data suggests that this treatment technique may be suitable for removing up to ~80% 3H and ~55% 14C from Oldbury Magnox reactor graphite Further work will be required to optimise this thermal treatment process and to determine the mobility and origin of these radioisotopes

acknowledgments The authors are pleased to acknowledge EPSRC funding under agreement EP/P113315 A portion of this work was carried out as part of the CARBOWASTE Program: Treatment and Disposal of Irradiated Graphite and Other Carbonaceous Waste, Grant Agreement Number FP7-211333

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