The curious case of STAR and Moata

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Strange bedfellows: The curious case of STAR and Moata

A.M. Smith

^⇑

, V.A. Levchenko, G. Malone

Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC NSW 2232, Australia

a r t i c l e i n f o

Article history:

Received 12 July 2011

Received in revised form 12 December 2011 Available online 25 January 2012 Keywords:

AMS

Accelerator mass spectrometry

14C Radiocarbon C-14 Nuclear reactor Argonaut Moata STAR ANTARES Reactor graphite

a b s t r a c t

The 2 MV tandem accelerator named ‘STAR’ was installed at ANSTO in 2003 and commissioned in 2004. It is used for ion beam analysis (IBA) and for radiocarbon measurements by accelerator mass spectrometry (AMS). Convenient space for the accelerator was found in the same building occupied by the decommis- sioned Argonaut-class nuclear reactor ‘Moata’; the name derives from the aboriginal word for ‘ﬁre stick’

or ‘gentle ﬁre’, appropriate for a 100 kW research reactor. This reactor operated between 1961 and 1995.

In 2007 ANSTO’s Engineering Division assembled a team to dismantle and remove the reactor structure, along with its 12.1 tonnes of graphite reflector. The removal and remediation was completed in Novem- ber 2010 and has won the team a number of prestigious awards. The entire operation was conducted inside a negatively-pressurised double-walled vinyl tent. An air curtain was positioned around the reactor core. The exhaust air from the tent passed through 2-stage HEPA filters before venting through an external stack. Neither ANSTO staff nor contractors received any significant radiation dose during the operation.

Given the sensitivity of STAR for detection of¹⁴C/¹²C (10¹⁶) and the numerous routes for production of¹⁴C in the reactor such as¹³C(n,c)¹⁴C,¹⁴N(n, p)¹⁴C and¹⁷O(n,a)¹⁴C there was the potential to directly contaminate the STAR environment with¹⁴C. Furthermore, there was concern that reactor-¹⁴C could ﬁnd its way from this building into the building where the radiocarbon sample preparation laboratories are located. This necessitated restrictions on staff movement between the buildings.

We report on¹⁴C control measurements made during and after the operation. These involved direct measurements on the reactor graphite and concrete bioshield, blank targets that were exposed in the building, swipe samples taken inside the tent and around the building and aerosol samples that were collected inside the building throughout the operation.

1. Introduction

In the late 1950s an Argonaut [Argonne Nuclear Assembly for University Training] class low-power research reactor was constructed at ANSTO in building 22 (seeFig. 1for location). It was commissioned in 1961 at a maximum thermal power of 10 kW and was upgraded in 1972 to 100 kW. This reactor was given the name ‘Moata’, which is an aboriginal word for ‘fire stick’ or ‘gentle fire’. At first it was used to train scientists in reactor control and neutron physics and to accumulate experimental nuclear data on fuel/moderator systems. Later it was used for neutron activation analysis, boron neutron capture therapy research and for radiogra- phy. Moata first went critical at 5:50 am on 10th April 1961 and ended operations on 31st May 1995 because it was found to be uneconomical to operate. It was permanently shut down and the fuel removed in 1995.

In August 1996 the ANSTO Board decided that the reactor should be taken irreversibly out of service. In order to implement

this decision, the Moata Decommissioning Project Management Team was set up to plan and manage the decommissioning pro- cess, guided and directed as necessary by the Moata Decommis- sioning Advisory Panel. In February 1997 the coolant was drained and the cooling system removed. In May 1998 the control system was removed and in June 2000 a Facility License was issued autho- rising ANSTO to possess, control and decommission a nuclear installation.

In 2000 ANSTO tendered for the supply of a new tandem accelerator to increase the throughput of radiocarbon measurements and to replace the ageing single-ended 3 MV Van der Graaff accelerator, which was used for ion beam analysis science since 1964.

High Voltage Engineering Europa was awarded the contract in December 2000 for a 2 MV Tandetron we named STAR (Small Tandem for Applied Research). In September 2001 the decision was made to locate STAR in building 22, the same location as the 3 MV machine and the Moata reactor. In September 2002 the STAR accelerator was delivered and in July 2004 it passed the acceptance tests: STAR ofﬁcially opened on 24th Jan 2005.Fig. 2was taken in August 2003 while STAR acceptance tests were underway and shows its proximity to Moata.

doi:10.1016/j.nimb.2012.01.027

⇑Corresponding author. Tel.: +61 2 97179054; fax: +61 2 917173257.

E-mail address:[email protected](A.M. Smith).

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1.1. Physical construction of Moata

Moata was an ‘‘Argonaut’’ type of reactor, similar to others built overseas [1,2]. These reactors used highly enriched fuel (HEU,90%²³⁵U). A sectional view is shownFig. 3. The core cavity measured 1.21.51.6 m and contained two parallel, open- topped rectangular aluminium tanks, joined and supported at the base by a common pipeline along which the primary light

water moderator/coolant was circulated with a similar arrange- ment at the top for return of coolant. Each core tank was divided by vertical aluminium spacer plates into six locations for fuel assemblies. The HEU fuel assemblies consisted of 12 aluminium clad fuel plates each of which contained about 22 g of ²³⁵U in an aluminium/uranium alloy. Almost the entire remainder of the core cavity volume was occupied by an assembly of reﬂector graphite blocks.

Fig. 1.ANSTO site map, showing the locations of building 22, which housed both STAR and Moata, building 53, the ANTARES accelerator facility, and building 16, which houses the AMS chemistry laboratories.

Fig. 2.Inside building 22 showing the southern wall with Moata in the eastern end and STAR in the foreground. This photograph was taken in August 2003 while STAR acceptance tests were underway. The fuel, cooling system and control system had been removed from Moata by this time.

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Control was provided by four neutron-absorbing rods, adjacent to the core tanks. Complete shutdown was achievable by dumping the water through a valve. The low maximum power level of the reactor avoided excessive radiation levels after reactor shutdown. This en- abled experimental equipment to be rearranged and reactor components to be maintained without the need for remote operations.

The reactor core was contained within a monolithic concrete bioshield, approximately 5.8 m wide, 6.4 m long and 3.3 m high.

The bioshield contained seven major penetrations, four of which were on the sides with the other three on the top face. The side penetrations consisted of concrete doors on rail with centrally located beam ports. The central top face penetration was ﬁtted with three layers of massive shield blocks removable by the crane and containing beam and fuel element access ports. Other penetrations were cast in the bioshield to provide access for safety rod drives, primary water supply, neutron source movement control, irradiation target movement and electric cables. The eastern and western faces of the reactor and the inner 300 mm of the bioshield surrounding the core cavity were composed of concrete impregnated with steel shots to provide an effective density of 5100 kg m³. The remainder of the Moata structure was castin situin pours of standard concrete of 2300 kg m³grade. A 25 mm thick lead slab was mounted across the entire inner face of the eastern side penetration to provide the extra radiation protection. Each of the major bioshield side penetrations contained cavities ﬁlled with assemblies of graphite thermal blocks.

1.2. Decommissioning and dismantling of Moata

In October 2007 planning commenced for the decommissioning and dismantling of Moata with a project budget of AU$4 M. The project was divided into three phases: ‘Planning and Pre-dismantling’,

‘Dismantling’ and ‘Post Dismantling and Site Remediation’. A

summary of the overall dismantling project can be found in[3]and [4]. All aspects of the plan were designed in accordance with good waste management, environmental and safety practices and were submitted for approval by ANSTO’s nuclear regulator, the Australian Radiation Protection and Nuclear Safety Agency (ARPAN- SA). These plans drew upon a report[1]which assessed the radioactive material inventory arising from the irradiation of reactor materials with thermal neutrons over its operating history; it did not include resonance reactions. This report was concerned with high energy beta and gamma emitters, speciﬁcally⁶⁰Co,¹⁵²Eu and¹⁵⁴Eu. It did not consider low energy beta and soft X-ray emitters, for example

3H,¹⁴C,³⁶Cl and⁵⁵Fe, which could also be expected to be present.

Analytical assessment of the radioactive material inventory was difficult for two reasons. Firstly, the compositional data of the reactor components, particularly for the activated trace elements, was not available. Secondly, the irradiation history was complex, given the power upgrade from 10 to 100 kW and the manner in which the reactor was operated over its lifetime (4519 separate irradiation runs at power levels over a range of 0.1 W–100 kW for periods between 5 min and 48 h). A consideration of the neutron flux levels, based upon measured flux levels, lead to the conclusion that all material within a sphere of 2.2 m radius, centered on the mid point of the internal reflector graphite, had been irradiated with a flux level in excess of 810⁶n cm²s¹. The report concluded with the recommendation that the principal reactor components stainless steel, mild steel, aluminium, graphite, concrete and lead should be sampled and subjected to activity analysis for a fuller assessment of the total radioactive inventory. These measurements were undertaken but are not reported on here. In the next section we consider the production of¹⁴C in Moata reactor graphite and concrete and present the results of measurements that were undertaken to asses the levels. In addition to ¹⁴C, other AMS radioisotopes can be produced in reactors, notably¹⁰Be and³⁶Cl.

Fig. 3.Sectional view of Moata.

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No measurements of ¹⁰Be were undertaken, although concrete samples were measured for³⁶Cl which was below the detection limit of 0.33 Bq/kg concrete.

In order to safely contain the release of radionuclides during the dismantling operation, a negatively-pressurised, double-walled vinyl tent was erected over the reactor and construction site. Addi- tionally, an inner tent was positioned around the top of the reactor and an air curtain was provided around the reactor bioshield. There were approximately nine air changes per hour for the main tent and 30 changes per hour for the inner tent. The exhaust air from the tent passed through 2-stage HEPA filters before venting through an external stack. The first phase of dismantling involved removing the internal components of the reactor and unloading the graphite. All this material was placed in specially constructed boxes for storage at the ANSTO waste facility. The sec- ond phase involved dismantling the concrete bioshield and reactor sub-floor. This was achieved by diamond-wire sawing the majority of the bioshield into manageable blocks with the remaining heavy density concrete mechanically broken for removal. Water was used to contain concrete dust generated during dismantling. The bulk of the bioshield had sufficiently low activation it could be released to landfill while the remainder is stored on site. Normal activities continued in building 22, including accelerator mass spectrometry, over the duration of the work. Neither ANSTO staff nor contractors received any significant radiation dose during the operation. More detail on the engineering aspects of the project may be found in [3,4].Table 1lists the events during the dismantling operation that were considered significant for the possible release of ¹⁴C into building 22.

1.3. Production of¹⁴C in Moata

14C is produced in all nuclear reactors due to absorption of thermal neutrons by carbon, nitrogen, or oxygen. These elements may be present as components of the fuel, moderator, or structural hardware or they may be present as impurities. Due to its long half life and copious production,¹⁴C represents a signiﬁcant challenge for nuclear waste disposal[5]. The relevant nuclear reactions are [6]:

14Nðn;pÞ¹⁴C ð1Þ

13Cðn;

c

Þ¹⁴C ð2Þ

17Oðn;

a

Þ¹⁴C ð3Þ

15Nðn;dÞ¹⁴C ð4Þ

16Oðn;³HeÞ¹⁴C ð5Þ

Reactions(4) and (5)are highly endothermic and were assigned a cross section of 0 for thermal reactors in[6]. For an Argonaut class reactor, the ﬁrst three reactions are important for the production of¹⁴C in graphite and concrete, with reaction(1)expected to dominate due to the large neutron cross section and the abundance of¹⁴N. Activation estimates were undertaken for the reactor of the University of Strasbourg (RUS), a 100 kW Argonaut reactor[7]. The methodology involved activation of graphite and concrete samples to determine their chemical composition, calculation of expected activation and an experimental program to measure the activation in selected samples. Measured¹⁴C activities in a graphite sample taken from the reﬂector were found to be three times greater than those calculated. The authors concluded that uncertainty in the impurity level on N in the graphite was the explanation. Hou reported [8] on methods developed for the rapid analysis of ¹⁴C and³H in reactor graphite and concrete. Test samples were taken

from the Danish research reactors DR-2 and DR-3 and measured for these nuclides. The highest graphite activities were from samples taken by scraping the graphite surface samples whereas the lowest were from samples that had the surface removed.

1.3.1.¹⁴C production in graphite

Moata contained 12.1 tonnes of nuclear grade graphite with seven distinct assemblies of modular pieces, accessible and removable from outside. The graphite was of 4⁰⁰4⁰⁰ section in varying lengths. Volume averaged neutron dose to these regions varied by over five orders of magnitude with the inner reflector (2958 kg) receiving the greatest dose with a volume averaged flux of 4.610¹¹n cm²s¹[1].

In early October 2008 some graphite stringers were removed for activity measurement. Further stringers were removed in March 2009 and test cores were taken from the concrete bioshield at about the same time. At ﬁrst these measurements were only concerned with the gamma emitters⁶⁰Co,¹⁵²Eu and¹⁵⁴Eu.¹⁴C decays with the emission of a beta particle with a maximum energy 156 keV which is quickly absorbed. Table 2lists the expected ranges for 150 and 175 keV electrons in common materials[9]. Only specially designed detectors, such as the pancake Geiger-Müller monitor, are able measure¹⁴C and even then only surface measurements can be made. An assessment of reactor ¹⁴C inventory requires a bulk technique. Due to the risk of contaminating laboratories and equipment with¹⁴C we were reluctant to undertake the processing and measurements at ANSTO.

Table 3lists powdered graphite samples taken from two sec- tions of graphite from the reﬂector, one of length 1.55 m and the other of length 0.28 m. The ﬁrst six samples were taken from the 1.55 m graphite block and the last (sample B) was taken from the 0.28 m graphite block. Both samples A and B were taken by boring a small hole on the side of each block near the end with the highest dose rate reading and enlarging this hole with a conical boring tool to a maximum depth of about 1 cm. Another tool, a three edged reamer, was used to scrape across the entire end face to obtain the surface samples. This tool was also used to bore into the Table 1

The timing of events during the dismantling operation that were considered signiﬁcant for the possible release of¹⁴C into building 22. The times at which the swipe and aerosol samples were collected are also listed.

2/10/08 Removal of two graphite blocks for preliminary measurement Mid 04/

09

Main tent fully erected

22/04/09 Commenced aerosol ﬁlter sampling inside B22

13/07/09 Swipe samples QF5–6 collected on Perspex ion source cover 17/07/09 Removal of graphite commenced

30/07/09 Removal of all graphite completed 08/09 Inner tent on bioshield removed

14/01/10 Swipe samples QF39–40 collected inside main tent 11/02/10 Commenced coring and diamond-wire sawing of external

inactive (releasable) concrete End 05/

10

Completion of removal of external inactive concrete Early 06/

10

Commencement of demolition of the active heavy density concrete

18/06/10 Demolition completed to ﬂoor level

26/07/10 Swipe samples QF95–102 collected inside main tent 7/08/10 Demolition of reactor subﬂoor zone completed 18/09/10 Removal of graphite stringers stored external to the tent 21/09/10 Local ventilation system and internal ﬁxtures removed 25/09/10 Tent internal lining removed

26/09/10 Tent outer cover removed

30/09/10 Tent structural steel frame removal completed

8/10/10 Ventilation power off and all apertures sealed with plastic 16/10/10 Ceased aerosol ﬁlter sampling inside B22

23/10/10 Completion of removal of extraction system and ﬁlters 14/12/10 Swipe samples QF126–149 collected inside building 22

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graphite to a depth of32 mm for the deep samples; the number of holes for each sample is listed in the table. These samples were sent to the Risø National Laboratory for Sustainable Energy, Denmark, for analysis.

Two aliquots (about 0.1 g) of each sample were weighed on a cellulose pad and mixed with cellulose powder. Samples were then combusted in a sample oxidizer and the CO2collected in CarboSorb solution. After mixing a scintillation cocktail, the¹⁴C activity was measured by liquid scintillation counting. A¹⁴C standard was prepared from a Packard (Perkin–Elmer)¹⁴C standard, along with four blank samples. For details of the method refer to[8]. The results are shown inTable 3, where the listed speciﬁc activity is the average for the two aliquots. The listed percent Modern Carbon (pMC) is derived from the speciﬁc activity of the Modern Reference Stan- dard (100 pMC, ¹⁴C/¹²C = 1.210¹²) which is 0.226 ± 0.001 Bq/

g; it has not been normalised to the (unknown) d¹³C of the graphite.

The specific activity of¹⁴C in the 2958 kg of inner reflector graphite was calculated, using a volume-averaged neutron flux of 4.610¹¹n cm²s¹ and the same irradiation periods as in [1]

and considering contributions from reactions (1)–(3). For (1), a concentration of 40 ppm for nitrogen, an isotopic abundance of 99.63% and a¹⁴N cross section of 1.83 Barns gave an inventory of 3.710⁸Bq. For(2), an isotopic abundance of 1.11% and a ¹³C cross section of 0.0014 Barns gave an inventory of 8.410⁷Bq.

For(3)a concentration of 100 ppm for oxygen, an isotopic abundance of 0.04% and a¹⁷O cross section of 0.24 Barns gave an inventory of 4.010⁴Bq. Together, these gave a speciﬁc activity of 1.510²Bq/g of carbon or 6.810⁴pMC, in good agreement with

the measured values for the deep samples at the ‘hot’ end of the 1.55 m graphite block inTable 2. The surface sample at this end measured signiﬁcantly higher than the cored material. Similarly the surface sample at the ‘cool’ end showed elevated activity compared with the deeper sample at this end.

Higher activity on the surface of the graphite was also found in [5,8,10]. Takahashi et al.[10]found that N₂was adsorbed onto the graphite surface from the surrounding air gaps (78% N) leading to enhanced surface production of¹⁴C. Hou et al. reported levels as high as 1.20 ± 0.0610⁶Bq/g (5.30 ± 0.2710⁸pMC) for surface samples taken from the reﬂector graphite of the Danish DR3 reactor[8]. Whilst we measured¹⁴C activities orders of magnitude less than this, it is clear that dismantling the reactor graphite with levels of more than 850 times ‘modern’ posed a serious threat for contamination of the radiocarbon laboratories and the STAR accelerator, where the detection limit for radiocarbon AMS is of the order of¹⁴C/¹²C10¹⁶.

1.3.2.¹⁴C production in concrete

The eastern and western faces of Moata and the inner 30 cm of the bioshield were composed of concrete impregnated with chopped steel fragments to provide an effective density of 5100 kg m³. The remainder of the bioshield was castin situwith standard concrete of 2300 kg m³. Concrete is composed of cement and other cementitious materials such as ﬂy ash, sand and course aggregates. As such the concentration of trace elements that may be activated by exposure to neutrons is quite variable and has not been speciﬁcally analysed for Moata concrete.

Three samples of pulverised concrete were sent to the Risø National Laboratory for analysis of¹⁴C and³H, along with other radionuclides, using the methods described in[8]. These samples were taken from a 50 mm diameter concrete core at mean distances of 5, 20 and 40 cm from the 30 cm thick high density concrete surrounding the graphite core. For¹⁴C, these samples re- turned 38.9 ± 28.2, 37.0 ± 24.8 and 55.6 ± 50.4 Bq/kg of concrete, respectively, close to the reported detection limit of 42.9 Bq/kg.

These levels were sufﬁciently low that dismantling of the concrete bioshield was not judged to be a threat to radiocarbon operations.

For comparison, the maximum¹⁴C activity in DR2 concrete was found to be 12.35 ± 1.01 Bq/g[8]at a distance of 108.5 cm from the reactor core.

2.¹⁴C precautions and monitoring 2.1. Precautions

The tent and ventilation system were expected to be effective in containing dust and particles produced during the dismantling operation. There was some concern over the removal of a number of graphite blocks for inspection prior to erection of the tent structure. It should, however, be realised that reactor graphite had been moved in and out of the reactor over the course of its lifetime so there was already an uncertain reactor graphite legacy within the building. Of more concern was the possible release of¹⁴C-bearing Table 2

Range of 150 and 175 keV electrons in selected materials from[9]. The full range is calculated from the continuous slowing down approximation (CSDA).

Material Density Energy CSDA range Range

[g/cm³] [keV] [g/cm²] [mm]

Graphite 2.227 150 3.158E-02 0.142

175 4.062E-02 0.182

Aluminium 2.699 150 3.659E-02 0.136

175 4.693E-02 0.174

Tissue-equiv plastic 1.127 150 2.792E-02 0.248

175 3.592E-02 0.319

Adipose tissue 0.920 150 2.734E-02 0.297

175 3.517E-02 0.382

Dry air @ 1 atm 0.001 150 3.193E-02 265

175 4.103E-02 340

Bone-equiv plastic 1.450 150 3.023E-02 0.208

175 3.884E-02 0.268

Mylar 1.400 150 3.026E-02 0.216

175 3.890E-02 0.278

Perspex 1.190 150 2.894E-02 0.243

175 3.721E-02 0.313

Teﬂon 2.200 150 3.392E-02 0.154

175 4.357E-02 0.198

PVC 1.300 150 3.219E-02 0.248

175 4.134E-02 0.318

Water 1.000 150 2.817E-02 0.282

175 3.622E-02 0.362

Table 3

Measured¹⁴C activity of Moata reﬂector graphite. The ﬁrst six samples were taken from a 1.55 m graphite block from the under the core and the last (sample B) was taken from the 0.28 m graphite block above the core.

Sample Sample location Speciﬁc activity [Bq/g] pMC

1 Hot end surface 194.8 ± 38.1 8.62E + 04 ± 1.68E + 04

2 Hot end deep, four holes 160.1 ± 58.5 7.08E + 04 ± 2.59E + 04

A Hot end 1 cm depth 145.9 ± 32.2 6.46E + 04 ± 1.43E + 04

3 Middle deep, three holes 10.3 ± 1.5 4.56E + 03 ± 6.42E + 02

4 Cool end surface 1.1 ± 0.4 4.87E + 02 ± 1.68E + 02

5 Cool end deep, two holes <0.2 <8.80E + 01

B Hot end 1 cm depth 29.8 ± 5.3 1.32E + 04 ± 2.36E + 03

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dust coating the inner side of the tent when it was removed following dismantling. Additionally, several graphite stringers were found stored in a service duct outside the tent and had to be specially removed. This was accomplished by bagging each piece separately as it was removed and cleaning the duct area with a HEPA-ﬁltered vacuum cleaner immediately afterwards.

As a precaution, all loading and unloading of the STAR sample wheels was done in the nearby building 53 and the wheels were transported in sealed plastic containers. These containers were opened for the minimum time needed to place the wheels under vacuum in the ion source. Care was taken to minimise exposure of the ion source components when ion source maintenance was required. Additionally, staff members were not permitted access into building 16, the location of the radiocarbon sample preparation laboratories, if they had been in building 22 that day.

2.1.1. Monitoring

2.1.1.1. Exposure of¹⁴C-depleted graphite near ion source: ‘Canary’

cathodes. As an interim measure, on 15/07/09 we began leaving¹⁴C depleted (‘canary’) cathodes exposed to the air in building 22 as a crude check on the¹⁴C level of the dust in the building. A set of 12 canary cathodes was prepared from ‘blank’ spectroscopic graphite which typically measures 0.015–0.020 pMC on STAR. The cathodes were individually exposed on open horizontal surfaces of STAR for periods of 24–48 h a few days prior to the scheduled AMS radiocarbon run. The practice of checking with canary cathodes lasted for approximately 6 weeks over three AMS radiocarbon runs, until the ﬁne particle aerosol monitoring program began in building 22. None of the 12 canary cathodes exhibited an enhanced¹⁴C as the surface was sputtered when compared to unexposed blank cathodes. Indeed, the background¹⁴C level as measured in STAR did not change over the entire dismantling operation.

2.1.1.2. Aerosol monitoring inside B22.ANSTO’sQuality Safety Envi- ronment and Radiation Protectionstaff set up aerosol monitoring stations inside and outside the tent. These were ThermoScientific Minialarm 7–10 provide with glass filters in a RUH-20 filter unit.

Two of these filters exposed between mid March and early April 2009 along with two new filters were combusted and measured for¹⁴C at ANSTO. The measured¹⁴C/¹²C suggested that levels above modern may have been detected, however the variable carbon content of these filters made interpretation difficult.

Rather than trying to solve this problem, it was decided that a program of aerosol monitoring outside the tent but inside building 22 was a better approach. ANSTO has developed aerosol sampling units that collect the ﬁne fraction (<2.5

l

m) on Teflon filters as part of an aerosol monitoring network[11]. These automated units are based on a cyclone system with a PM2.5 cut off for flow rates of 22 L/min. In April 2009 one of these units was set up on the first floor balcony in building 22 in the approximate location of the pho- tographer forFig. 2. Quartz filters of 25 mm diameter, pre-baked in oxygen, were used which provided a sufficiently low carbon content (8

l

g per filter). Quartz filters for AMS analysis of¹⁴C were exposed over Monday–Tuesday and Thursday–Saturday at a flow rate of 19 L/min and Teflon filters for routine aerosol analysis were undertaken on Wednesdays and Sundays. Regular aerosol sampling commenced mid July 2009 with the preliminary dismantling phase and continued throughout the entire dismantling operation until the removal of extraction system and filters in October 2010.

All together, 109 quartz aerosol ﬁlter samples were collected over this period. Of these, 43 were selected for radiocarbon measurement, with 30 prepared and measured at Accium Biosciences and 13 prepared and measured at ANSTO.

The 30 quartz ﬁlters sent to Accium BioSciences were combusted in a sealed quartz tube at 900°C with CuO to form CO2

which was then transferred to a septa-sealed vial for graphitisation,

as described in[12]. This technique prevents the spread of¹⁴C from

‘hot’ samples but means that the carbon sample mass is not volu- metrically determined. Accium BioSciences does offer the option of sample mass determination, using an isotope dilution technique [13]however this requires knowledge of thed¹³C of the sample.

Due to the uncertain composition and origin of the aerosol it was decided that knowledge of the carbon mass was unlikely to be help- ful, so the samples were directly measured for pMC and ford¹³C.

Carbon masses were estimated on the basis of the sample¹³C current in comparison with the currents of standards of known mass.

Three samples failed to produce measurable¹³C current. Thirteen quartz ﬁlters were prepared and measured at ANSTO following the method described in[14], which include volumetric determination of the carbon mass.d¹³C was available from the STAR accelerator for ﬁve of these samples.

The results for the quartz ﬁlter aerosol samples are shown in Table 4. With the exception of QF1, collected just after the main tent was fully erected, all samples measured less than 90 pMC, with a mean of 68 pMC. The aerosol is a complex mix of materials and the carbonaceous component is likely to comprise biogenic, fossil and mineral sources along with possible contributions from reactor graphite and concrete. The meand¹³C was28‰, with a minimum value of 37‰, indicating a signiﬁcant contribution from an isotopically light carbon source. Possible sources include leaves, which typically haved¹³C of27‰, or carbonates, which can be as light as60‰. Nuclear graphite is made by baking petro- leum pitch and particulated coke in a mould and typically hasd¹³C in the range27 to28‰[15]. No correlation can be found be- tweend¹³C and the events listed inTable 1or the day of the week the sample was collected. There is no obvious explanation for the anomalously high activity measured for QF1.

2.1.1.3. Swipe samples inside tent during dismantling. In order to assess the ¹⁴C level of the dust generated during the dismantling operation, swipe samples were taken on the inner surface of the North wall of the tent on 14/01/10, following removal of the graphite, and again on the North and South walls on 26/07/10, following removal of the concrete bioshield. The same oxygen-baked quartz ﬁlters used in the aerosol sampler were also used as tent swipes.

The region on each wall was sub-divided into four approximately A4 sized areas, arranged in a 22 grid in landscape orientation, with a mean height of3 m from the ﬂoor. These samples were sent to Accium Biosciences for measurement and the results are shown inTable 5. The swipe code identiﬁes each area, for instance

‘NBL’ is the bottom left area on the North wall and ‘STR’ is the top right area on the South wall. Of the first set of four swipes taken on the North wall on 14/01/10, two did not produce sufficient¹³C current for measurement, whereas the two adjacent samples did. This demonstrates that the dust loading on the vinyl tent walls was inhomogeneous. The final set of eight swipe samples taken on 26/07/10 visually contained concrete dust; in the case of the North wall samples most of this had been deposited following the first swipe on 14/10/10. Two of the eight samples were estimated to contain less than 20

l

g of carbon but the¹⁴C activity was measurable. No elemental analysis was done on the material picked up by the swipes so it is not possible to apportion sources for the carbon content. Like the aerosol filter samples, mostd¹³C was significantly lighter than 25‰; this may indicate a significant contribution from reactor graphite or carbonate. The high activities (up to 23 times ‘modern’) measured for some of the swipes certainly rein- forced the fact that dismantling Moata posed a significant¹⁴C contamination risk for AMS operations.

After all dismantling and remediation work was completed, the entire tent inner surface was cleaned down with damp pads to help limit the spread of dust. As a further precaution, the HEPA ﬁltered

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ventilation system continued to operate while the tent was col- lapsed. After removal of the tent the ﬂoor was repainted.

2.1.1.4. Swipe samples throughout B22 following site remediation. The same oxygen-baked quartz ﬁlters used in the aerosol sampler were also used as building swipes. Two swipe samples

(QF5 and QF6) were taken on the Perspex cover of the STAR AMS ion source on 13/07/09, just prior to the removal of the graphite.

These were prepared and measured at ANSTO to be 81.5 ± 0.7 and 71.6 ± 1.1 pMC, respectively. These were the only swipe samples taken outside of the tent prior to completion of the dismantling operation. Following site remediation, 24 swipe samples Table 4

Quartz filter aerosol sample results. Samples with the laboratory code prefix 1179 were prepared and measured at Accium Biosciences while samples with the prefix OZ were prepared and measured at ANSTO. Samples QF28, 81, and 93 did not give any carbon current. Samples QF47, 79, and 82 gave low currents. Carbon masses in italics were estimated from the¹³C current.

Aerosol ﬁlter Start date End date Lab code Carbon mass [lg] d¹³C [‰] pMC 1rerror

QF1 22/04/09 OZL727U2 62 142.0 1.4

QF2 26/04/09 OZL727U3 47 43.4 0.9

QF3 29/04/09 OZL727U4 59 50.7 0.8

QF4 3/05/09 OZL727U5 105 87.8 0.7

QF7 13/07/09 14/07/09 OZL727U8 86 44.4 0.6

QF8 16/07/09 18/07/09 OZL727U9 127 25.0 49.6 0.6

QF9 20/07/09 21/07/09 OZL727U10 133 21.4 47.3 0.6

QF10 23/07/09 25/07/09 OZL727U11 132 24.3 69.9 0.8

QF11 27/07/09 28/07/09 OZL727U12 63 74.2 0.6

QF12 29/07/09 31/07/09 OZL727U13 80 66.8 0.5

QF13 3/08/09 4/08/09 OZL727U14 71 51.6 0.5

QF14 6/08/09 8/08/09 OZL727U15 170 24.3 62.3 0.7

QF15 10/08/09 11/08/09 OZL727U16 240 24.0 52.4 0.6

QF16 13/08/09 15/08/09 1179-044-00 22 30.1 85.3 0.6

QF17 17/08/09 18/08/09 1179-045-00 10 27.3 86.6 0.6

QF21 31/08/09 1/09/09 1179-046-00 71 35.9 90.0 0.5

QF22 3/09/09 5/09/09 1179-047-00 178 24.6 80.5 0.3

QF28 24/09/09 26/09/09 1179-048-00 0

QF45 8/02/10 9/02/10 1179-049-00 58 35.9 45.6 0.3

QF46 11/02/10 13/02/10 1179-050-00 119 26.7 51.3 0.3

QF47 15/02/10 16/02/10 1179-051-00 0 25.0 61.9 1.0

QF60 1/04/10 3/04/10 1179-052-00 81 36.0 57.6 0.4

QF76 27/05/10 29/05/10 1179-053-00 109 29.7 64.7 0.4

QF78 4/06/10 6/06/10 1179-054-00 34 32.6 79.0 1.0

QF79 7/06/10 8/06/10 1179-055-00 0 25.0 63.2 0.8

QF80 10/06/10 12/06/10 1179-056-00 60 36.7 80.4 0.5

QF81 14/06/10 15/06/10 1179-057-00 0

QF82 17/06/10 19/06/10 1179-058-00 0 30.4 81.8 0.8

QF83 21/06/10 22/06/10 1179-059-00 0 26.4 79.9 0.8

QF84 24/06/10 26/06/10 1179-060-00 157 22.8 56.5 0.4

QF92 22/07/10 24/07/10 1179-061-00 49 27.8 89.3 0.8

QF93 26/07/10 27/10/10 1179-062-00 0

QF105 9/08/10 10/09/10 1179-063-00 163 31.1 68.5 0.3

QF116 13/09/10 14/09/10 1179-064-00 71 25.0 77.4 0.4

QF117 16/09/10 18/09/10 1179-065-00 100 27.4 78.1 0.6

QF118 20/09/10 21/09/10 1179-066-00 120 31.7 60.3 0.3

QF119 23/09/10 25/09/10 1179-067-00 163 23.4 61.5 0.3

QF120 27/09/10 28/09/10 1179-068-00 122 29.8 83.5 0.4

QF121 30/09/10 2/10/10 1179-069-00 96 30.8 74.2 0.5

QF122 4/10/10 5/10/10 1179-070-00 6 34.0 56.9 0.6

QF123 7/10/10 9/10/10 1179-071-00 195 22.5 13.5 0.1

QF124 11/10/10 12/10/10 1179-072-00 88 29.7 76.6 0.4

QF125 14/10/10 16/10/10 1179-073-00 12 26.7 66.6 0.6

Table 5

Swipe samples taken inside the tent. A region on the North wall and a region on the South wall were chosen. Each region was sub-divided into four approximately A4 sized areas, arranged in a 22 grid in landscape orientation. The mean height was3 m from the ﬂoor. The swipe code identiﬁes each area, for instance ‘NBL’ is the bottom left area on the North wall and ‘STR’ is the top right area on the South wall. The same areas on the North wall were swiped on 14/01/10 and again on 26/07/10. Masses are estimated from the¹³C current in comparison with that produced by standards during the run.

Filter Tent location code Swipe date Accium code Carbon mass [lg] d¹³C [‰] pMC 1rerror

QF39 NTL 14/01/10 1179-001-00 <20

QF40 NBL 14/01/10 1179-009-00 <20

QF41 NTR 14/01/10 1179-002-00 200 22.6 925.8 2.5

QF42 NBR 14/01/10 1179-010-00 100 21.2 356.8 1.5

QF95 NTL 26/07/10 1179-011-00 20–50 20.5 302.4 3.6

QF96 NBL 26/07/10 1179-040-00 70 25.1 829.2 2.3

QF97 NTR 26/07/10 1179-041-00 <20 394.1 7.5

QF98 NBR 26/07/10 1179-012-00 100–200 28.1 211.6 1.0

QF99 STL 26/07/10 1179-013-00 100–200 29.9 1341.5 2.2

QF100 SBL 26/07/10 1179-042-00 <20 31.0 598.9 2.0

QF101 STR 26/07/10 1179-043-00 150 31.4 2012.3 5.5

QF102 SBR 26/07/10 1179-014-00 100–200 26.0 2258.9 3.7

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were taken throughout building 22 on both the ground and first floor levels to check on¹⁴C levels. Horizontal surfaces were chosen where there was an obvious dust build up. After collection, all swipe filters were kept frozen in Petri dishes to minimise the growth of moulds and fungi. Later they were dried in a vacuum oven at 80°C for 24 h inside their partially opened Petri dishes and were transferred to multiple zip-lock plastic bags for shipment to Accium Biosciences; no desiccant was added to the packaging.

The results of the measurements are listed in Table 6, along with the mass increase for each ﬁlter following the swipe. Again, carbon sample masses are estimated from the¹³C current each sample produced in the accelerator. Apportioning sources to the carbonaceous component on the swipes is even more difﬁcult for these building swipes because such a wide range of materials have accumulated in building 22 over many decades. Typically less than 10% of the material picked up by the swipe was carbon and there is a wide spread ind¹³C of this carbon. Of the 24 post-remediation swipes, six samples measured less than 100 pMC while four were near modern. The remaining 12 all measured less than 250 pMC.

No particular pattern in distribution could be discerned for these more active swipes and in some cases sub-modern levels were measured for nearby swipes.

The fact that any of the post-remediation swipes measured sig- niﬁcantly above modern pointed to the presence of reactor-generated¹⁴C in the building. Based on the measurements made on the concrete and graphite samples this is most likely from activated graphite. The two graphite reﬂector blocks (Table 3) had¹⁴C activities ranging from approximately modern to over 850 times modern. There would have been less activation in the outermost thermal graphite although it is likely that high levels could have accumulated on the surfaces of some of the blocks from adsorption of N2. Any ‘hot’¹⁴C-material picked up by the swipes has been

‘diluted’ to some unknown extent by ‘dead’ to ‘modern’ carbon that was also collected on the swipe. The hot materials may have been present as a discrete number of ﬂecks or as a more ﬁnely divided aerosol. It should be noted that deposition of this reactor-generated¹⁴C could have pre-dated the Moata dismantling operation and may have arisen from reactor operations over the period

1961–2008. Swipe samples QF5 and QF6, collected prior to removal of the graphite, gave the lowest pMC of all the swipe samples. Swipe sample QF143 was also taken from the Perspex cover of the STAR AMS ion source, but following tent removal and site remediation. It measured somewhat higher at100 pMC, suggest- ing some slight elevation of activity.

Given that none of the building swipes exhibited exceptionally high¹⁴C levels and that they were considered to be representative samples of the dust that had accumulated within building 22, it was judged that there was no obvious threat of¹⁴C contamination.

Several cleaning campaigns have since been undertaken to remove as much dust and dirt as possible throughout the building. Normal AMS operations have since resumed and background levels for the radiocarbon laboratory and for the STAR accelerator remain unchanged.

3. Conclusions

14C levels in representative samples of the Moata concrete bioshield and graphite reflector were measured. The results suggested that the¹⁴C levels in the concrete were low, but that the graphite surrounding the reactor core had levels of the order of 850 times modern that posed severe risk of affecting AMS operations. Mea- surement of dust accumulated on the inside of the tent during the dismantling operation confirmed this, with¹⁴C levels as high as 23 times modern found. The measures taken during the Moata dismantling operation to control the spread of radionuclides, including¹⁴C, have been effective. This was demonstrated by the low¹⁴C activities measured by a fine-fraction aerosol monitoring program conducted external to the tent throughout the dismantling operation.

Swipe samples following site remediation clearly revealed some reactor-produced¹⁴C within the building. This¹⁴C may have pre- ceded the dismantling operation as a legacy from decades of reactor operation. The 24 swipes were considered to give a good representation of the¹⁴C activity of the accumulated dust in the building. It is probable that this dust contained irradiated reactor graphite along with dead to modern carbon from other sources.

Table 6

Building swipe sample results. QF5 and QF6 were taken on 13/07/09. The remaining swipes were all taken on 14/12/10 following removal of the tent and remediation of the site. Samples with the laboratory code preﬁx 1179 were prepared and measured at Accium Biosciences while samples with the preﬁx OZ were prepared and measured at ANSTO. Carbon masses in italics were estimated from the¹³C current.

Filter DMass [mg] Laboratory code Carbon mass [lg] d¹³C [‰] pMC 1rerror

QF5 L727U6 230 20.8 81.53 0.74

QF6 L727U7 36 71.62 1.06

QF126 1.43 1179-016-00 154 22.6 127.23 0.43

QF127 3.06 1179-017-00 128 22.2 138.35 0.46

QF128 2.09 1179-018-00 173 18.1 112.43 0.46

QF129 2.07 1179-019-00 182 16.0 92.10 0.47

QF130 2.18 1179-020-00 167 22.8 97.72 0.44

QF131 2.00 1179-021-00 181 18.4 144.82 0.39

QF132 1.65 1179-022-00 165 22.3 178.63 0.58

QF133 3.00 1179-023-00 151 17.4 198.97 0.90

QF134 2.34 1179-024-00 162 25.8 97.17 0.34

QF135 0 1179-025-00 <20 115.55 1.38

QF136 3.06 1179-026-00 186 19.2 104.44 0.37

QF137 1.83 1179-027-00 149 20.3 102.65 0.47

QF138 1.36 1179-028-00 144 24.8 98.05 0.46

QF139 2.18 1179-029-00 173 20.7 123.48 0.49

QF140 1.11 1179-030-00 <20 87.36 1.03

QF141 2.67 1179-031-00 95 34.9 120.76 0.58

QF142 1.84 1179-032-00 146 17.7 100.17 0.35

QF143 1.48 1179-033-00 117 26.4 99.82 0.44

QF144 2.43 1179-034-00 139 28.2 249.38 1.33

QF145 0.43 1179-035-00 76 34.2 119.75 0.58

QF146 1.95 1179-036-00 163 26.0 126.51 0.41

QF147 1.94 1179-037-00 174 26.8 101.01 0.34

QF148 3.52 1179-038-00 160 21.6 284.75 0.67

QF149 1.75 1179-039-00 39 36.1 114.42 0.86

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Overall¹⁴C/¹²C levels were judged to be sufﬁciently low that normal AMS operations could resume once the building had been thoroughly cleaned of dust and dirt. Background levels for the radiocarbon laboratory and for the STAR accelerator have not been affected by the dismantling and removal of Moata.

Acknowledgements

AMS and Accelerator Science staff acknowledges the excellent work of ANSTO Engineering in containing reactor-produced radionuclides and in keeping staff, contractors and equipment uncon- taminated. Thanks are due to Eduard Stelcer, Institute for Environmental Research, ANSTO, for assistance with aerosol monitoring in building 22. Thanks are also due to Geoff Parsons, Waste Processing, ANSTO, for assistance with the graphite sampling.

Finally, we would like to thank Dr. Ugo Zoppi, Accium Biosciences, for measurement of the aerosol and swipe samples and Dr. Xiaolin Hou, Risø National Laboratory, Denmark, for measurement of the graphite and concrete samples.

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