Scientific basis for future management options of the Little Forest Legacy Site

Hefin Griffiths (ANSTO) presented.

ANSTO is the license holder and operator for the Little Forest legacy site (LFLS), formerly Little Forest burial ground (LFBG).

The LFBG was operational between 1960 and 1968, during which time LLW, including effluent sludge, laboratory wastes, and contaminated equipment from the Australian Atomic Energy Commission (AAEC) and other organizations were disposed of in unlined trenches. Disposal was by tumble tipping. There were 79 trenches in total that each measured 25m by 3m by 0.6m that were spaced 2.7m apart. In total, 1675 m³ of waste was disposed of in the trenches, equating to around 150 GBq of radioactivity. This included around 1100 kg beryllium and around 7 g of plutonium, both of which are viewed similarly by stakeholders. There was no consideration given to packaging or conditioning of wastes.

The LFLS is located to the south of Sydney, close to the Lucas Heights landfill I and II sites (Figure 4-1). There is a 1.6 km buffer zone around the LFLS, but different activities are carried out within this zone. For example, the area includes the current landfill site for the southern area of Sydney. A quarry that was used as a former landfill until 1987 is also present, along with a hazardous liquid waste disposal site. A human waste disposal area is also present. Therefore, even if the LFLS was completely remediated, there would remain existing hazards and unrestricted use of the site is therefore unlikely.

FIGURE 4-1.LOCATION OF THE LFLS(FORMERLY LFBG).LUCAS HEIGHTS I LANDFILL IS LOCATED TO THE RIGHT OF LFLS AND THE FORMER QUARRY LANDFILL AND LIQUID WASTE DISPOSAL SITES ARE LOCATED TO THE NORTH (NOT SHOWN ON FIGURE).

The trenches have been subject to significant scientific research since their closure and a number of papers have been published on the geology, hydrogeology and dose modelling for biota etc.

There were concerns locally when plutonium was found in the trenches. Only small quantities are present, but there was nonetheless concern and this triggered the need for remedial action, with a 1m soil cover being placed on top of the trenches. This remedial action was effective for over 20 years.

A trench model has been developed to see whether the processes resulting in the mobility of radionuclides could be modelled. A number of 40-gallon drums have been disposed of in the trenches and these have deteriorated and collapsed as a result of the weight of soil overlying them.

This has resulted in sink holes forming that allow for greater water ingress. However, the movement of water from the trenches is limited due to the presence of shales, resulting in a bathtub effect whereby mobilized radionuclides are brought to the surface (Figure 4-2). This is supported by several decades of water sampling that has shown that, with the exception of a tritium plume, there was little evidence of radionuclide movement in groundwater. The modelling of the processes in the trenches informs on options.

FIGURE 4-2.TRENCH MODEL SHOWING BATH TUBBING EFFECT AND THE MOBILIZATION OF RADIONUCLIDES TO THE SURFACE OF TRENCHES.

The plutonium that has been detected at the surface of trenches is at very low levels (a maximum of 0.8 Bq/g) and below thresholds for regulatory control in Australia. The plutonium is also very localized to particular trenches that ties with available disposal records. There are also clusters of Cs-137 and Sr-90 that are again localized, some being located around the area of an old receipt hut.

From the time that disposals started until they ceased in 1968, the site was not under any regulatory control; the Australian Nuclear Safety Act came much later. The lack of regulatory control subsequently created an issue both for ANSTO as the operator and ARPANSA the regulator.

The situation at the site in 2013 was that the site was recognized as a legacy and was managed by ANSTO. A management, monitoring and mitigation plan was in place and there was some evidence of tritium migration and limited migration of other radionuclides for which the mechanism was understood. There was no immediate threat to public health from the legacy site, but it was not to modern standards.

A change to the site license was issued in 2016, which recognized the LFBG as the first legacy site in Australia. A license condition was set, requiring ANSTO to develop a plan for the medium and long-term management of LFLS by June 2018. The submission of the plan will be after this date, but ARPANSA has been kept informed and is happy that work is progressing in the right direction.

Options for maintaining the isolation of the waste inventory from the environment and human intrusion for a sufficient period to demonstrate public and environmental safety following release from institutional control were identified. These included:

 Do nothing. This option was rejected since it would not meet regulatory requirements.

 Full exhumation, characterization and conditioning of the waste. The environmental and worker health and safety risks would be considerable for this option and there would also be large cost implications. The option may not, therefore, be justifiable.

 In-situ remediation. This could involve the installation of an engineered cover, surface bunds or other form of interception or subsurface barriers. Alternatively, a pump and treat approach could be employed or wastes could be grouted in situ.

In situ remediation options are being considered within a project that is being undertaken jointly with the University of Newcastle, Australia, the University of New South Wales and the University of Strathclyde. Key questions for the project are:

 What was put in the trenches?

 How can the trench contents be stabilized?

 What has come out of the trenches?

 How the input and export of water be controlled?

 What chemical and physical processes are occurring?

 What are the environmental and human impacts under various scenarios?

 What are the best remediation options?

The project includes six sub-projects.

Sub-project 1 is focused on in situ grouting options. The geology in the area of the LFLS is not porous; porosity is limited to the waste form itself. As such, in situ grouting could stabilize the waste and decrease the amount of water in the trenches by filling pore spaces. This could also help minimize any further subsidence. Grouting could be performed by pumping colloidal silica into the waste at very low pressure over time, to fill as many voids as possible. This may also result in the outside of the trenches also being encapsulated.

Test trenches are being dug next to the disposal trenches in sub-project 2 to allow the effectiveness of grouting to be investigated. Test trenches will also allow studies of rainfall events to be studied, the results of which could be used to improve hydrological models as well as supporting the testing of engineering solutions.

Sub-project 3 is focused on dose assessment and beryllium. Beryllium is a major hazard at the site and exposure to beryllium and mitigation of the inhalation pathway would be a major

consideration if exhumation is undertaken. A number of dose assessment scenarios are being evaluated, as detailed in

Table 4-1. RESRAD is being used for human dose modelling and the ERICA tool for non-human biota. Results of the dose modelling will inform on options selection.

Sub-project 4 is focused on records and the trench inventory. A detailed compilation of information on the trench contents is essential to assessing the status of the site and informing on the best remediation strategy. Within this sub-project, the available records on disposals will be converted

into a searchable electronic format to produce a reference inventory. Uncertainties around the inventory will be evaluated through uncertainty analysis.

The fifth sub-project is on radiochemistry. The objective is to develop a greater understanding of the distribution and mobility of radionuclides within the trenches.

The final sub-project is benchmarking against international, UK and Australian best-practices, with international perspectives helping to underpin decisions.

Legacy sites present a significant challenge and remediation options must be driven by science, whilst recognizing political and social drivers for decisions. Uncertainty cannot be an inhibitor to progress in addressing legacies. Uncertainties are inevitable and must be acknowledged and evaluated. The precautionary principle is therefore being applied to the LFLS, with conservative assumptions employed in the assessments that will underpin remediation option decisions.

Confidence will also be built in the assessments through sensitivity analyses.

TABLE 4-1.DOSE ASSESSMENT SCENARIOS FOR THE LFLS REMEDIATION PROJECT.

Scenario Receptors Timeframe (years)

Institutional Control (as is) Workers Trespassers Offsite Public Non-human biota

0, 100

Stabilization – Capped condition Workers Trespassers Offsite Public

Future onsite (adult, child) Non-human biota

0, 100, 200, 1000

Stabilization – in situ stabilization (e.g. grout) condition

Workers Trespassers Offsite Public

Future onsite (adult, child) Non-human biota

0, 100, 200, 1000

Fence interception – (e.g.

hydraulic controls/ passive barriers)

Workers Trespassers Offsite Public

Future onsite (adult, child) Non-human biota

0, 100, 200, 1000

Exhume and remove Workers

Trespassers Offsite Public

Future onsite (adult, child) Non-human biota

0 -10 (during removal), 100, 200, 1000

Legacy trenches exist in many countries, with shallow burial of wastes being a common practice in the 1950’s and 1960’s. There are a number of common problems with trench disposal legacy sites, including:

 lack of site-specific information (e.g. characteristics of buried wastes, flow paths and exposure pathways, and previous management interventions);

 unclear responsibility and ownership;

 limited availability of technical expertise;

 scientific uncertainty;

 societal issues; and,

 various constraints and limitations (e.g. financial, technical, legislative, etc.).

In addressing these sites, it should be recognized that no option will be perfect. Optimization will be required to achieve the best result in the prevailing and anticipated future circumstances.

4.2 An IAEA MODARIA II Working Group 1 Proposed Study to Better Define End