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Author: Stephen Fennell (Radiological Protection Institute of Ireland - RPII)


In 2002 Ireland submitted its national report to the meeting of the Oslo-Paris (OSPAR) ad-hoc working group on Radioactive Substances (RSC) describing how it intended to implement its strategy with regard to the discharge of radioactive substances to the marine environment. In assisting the Department of the Environment, Heritage and Local Government (DEHLG) in preparing Ireland’s submission to OSPAR the Radiological Protection Institute of Ireland (RPII) carried out a comprehensive review of the use of unsealed radioactive sources across all sectors throughout Ireland. The discharge of iodine-131 through patient excreta, arising as a result of activities administered to patients undergoing thyroid ablation treatments, was identified as one of the contributors to the total activity of unsealed radionuclides discharged to the marine environment from Ireland each year. In reviewing the use of unsealed radioactive sources in the medical sector the RPII determined that it would need to review its own regulatory requirements in relation to the installation of sewage holding tanks in hospitals. These tanks would take waste from the iodine ablation suites and store it for a number of weeks to allow for the decay of iodine-131 prior to discharge to the sewers. This action was subsequently included in the set of intermediate goals Ireland would take to implement the OSPAR strategy.

issue26-holdingtank.pngFigure 1. Holding tank in a hospital in Luxembourg

Iodine Ablation Therapy in Ireland

Treatment for thyroid cancer using iodine ablation therapy is currently carried out in Ireland at four hospitals located on the east and south coasts of the country. Patients undergoing ablation therapy are administered between 3 and 7.4 GBq of iodine-131 and are kept isolated as in-patients in dedicated iodine suites for up to six days. During the course of their stay approximately 80% of the administered activity is excreted in urine. In the three hospitals located on the east coast, waste from patients goes directly to the hospital’s main sewer for eventual discharge into the Irish Sea, while waste from patients based in the hospital located in the south of the country is piped to a small 1000 litre delay and decay holding tank, where it is allowed to decay on average for three weeks (approximately three half lives) before being discharged to the hospital’s main sewer. When the RPII originally considered the licence applications for these facilities the licensees would have been required to undertake a risk assessment of the potential doses to critical groups such as hospital plumbers, sewer workers, sewage treatment plant workers, fishermen etc. For each of these critical groups the application was assessed against an annual dose constraint of 300 µSv/yr for non-occupationally exposed workers. In all cases the doses to these groups were considerably below the dose constraint and hence consideration of further optimisation such as the installation of holding tanks was not required by the RPII. The incorporation of a holding tank in the hospital located in the south was a decision taken locally.

Consultancy Project

In June 2007 the RPII contracted the UK consultancy firm Enviros Ltd to assist it in evaluating the need to install iodine holding tanks in both existing and future iodine ablation facilities. The evaluation reviewed existing practices in Ireland in relation to iodine-131 ablation discharges to the sewers and made recommendations for an RPII regulatory policy, based on international best practice and forecasts of future activity. As part of the contract, Enviros Ltd was tasked with undertaking an analysis of the following items:

  • A summary of international advice (e.g. ICRP, IAEA, EC) on best practice in relation to iodine ablation discharges;
  • A summary of current practices relating to the provision of holding tanks in a selection of other EU countries;
  • A review of current practices in Ireland;
  • Through discussion with relevant parties and by reviewing existing literature, provide an overview of the likely future demand for iodine ablation therapies in Ireland and any implications these would have on doses to workers and members of the public and on discharges to, and concentrations in, the environment;
  • An evaluation of the merits and demerits of utilising holding tanks in an Irish context including consideration of:
    • Installation: building requirements, cost, retrofitting, green field, maintenance and upkeep requirements;
    • Impact on radiation doses to particular groups, including patients, medical staff, hospital maintenance staff, other staff likely to be affected and the public;
    • Impact on discharges to the environment and environmental concentrations.


The final report, which is available on the RPII’s website (www.rpii.ie), provides a summary of current ablation practices throughout Ireland. It notes that in 2006 91 ablation therapies were carried out using a total of 435 GBq I-131. Through discussions with relevant staff at each facility, and the Office of the National Plan for Radiation Oncology, the authors of the report estimate that over the next 5-10 years demand for ablation therapies will increase by approximately 50%. This expected increase is in part attributed to better diagnosis and increased referrals for thyroid ablation, as well as general population increase and an overall increase in the population age.

For each facility typical doses to critical groups were calculated using models developed by the former National Radiation Protection Board (NRPB) (UK) and Environment Agency (UK) through a consideration of the amount of iodine-131 administered over the course of the year and the flow rates at the relevant sewage treatment plants. The report finds that the potentially most exposed critical group is on‑site hospital plumbers who may have to deal with a blocked sewage pipe exiting the ablation suite; in these cases the exposure arises from a one-off event rather than over the course of a year. Typical doses to plumbers dealing with such an incident are estimated to be in the range of 50-70 µSv per incident.

Other critical groups considered in the analysis include sewage workers working in man accessible pipes, workers at sewage treatment plants and coastal fishing families. After the hospital plumber the next exposed member of the public is a sewer worker who receives a dose estimated to be less than 4 µSv per year. For the projected future numbers of ablation treatments the dose to sewer workers is estimated to be less than 6 µSv per year. For all other members of the public the doses for current and projected workloads are estimated to be less than 3 µSv per year.

The report finds that there is no consistent approach to the regulation of radioactive discharges to sewers across Europe. A summary of the different practices throughout the Member States as reported in EC [1999][1] and updated as a result of consultation responses obtained during this project, is provided in Table 1. The authors point out that projected doses that are at, or close to, 10 µSv per year are generally considered to not require further reduction unless it is clear that Best Available Techniques (BAT) are not being applied. The report also notes that overall, the risk-based approach taken in Ireland to the regulation of these activities is consistent with IAEA and ICRP recommendations and is also consistent with approaches in Great Britain and Northern Ireland.

Table 1. Approaches to the management of patient excreta by EC Member States as of April 2008

Management approach
Direct Discharge
Delay and Decay

In Denmark there is no limit for the total activity that can be discharge (that is controlled by limits for purchase and use). However, dilution of I‑131 discharges to 0.1 MBq/l is required at the point where the hospital drain meets the municipal sewer

Discharge limits from institutions do not apply to patient excreta that may be freely discharged to sewer as long as discharges at any one time do not exceed 100 MBq and that over the course of a year does not exceed 100 GBq.

Effluents eliminated by patients in protected rooms (iodine dose > 740 MBq) are normally collected via bi-sectional toilets. Effluents from ordinary sanitary installations in the nuclear medicine unit are usually linked to a septic tank. Due to the length of time the material stays in the septic tank and the brief half-life of the radionuclides, volume activity in the collector is greatly reduced before release into the sewage network.

All facilities required to have holding tanks installed and discharges from facilities must remain below a limit of 5 Bq/l at the point of discharge into the public waste water network.
Direct discharge to sewer allowed, provided that the waste is readily dispersible in water and the maximum concentration of radioactive substances is not greater than 3.7 MBq/l. For I‑131 thyroid post-operatory therapy waste decay storage prior to discharge to sewer is required to meet this criterion.
Republic of Ireland
Both direct discharge to sewer and use of holding tanks are currently employed. Hospitals are authorised on activity administered not discharged.
Northern Ireland

Decay storage is used, although not a regulatory requirement. Activity concentration limit of 80 kBq/l prior to discharge to sewer.

Waste is retained in holding tanks for between 30 and 60 days prior to discharge to sewer. Two tanks are used, one being filled as the other is left to decay prior to discharge.

All new treatment facilities are required to install holding tanks, with patient excreta being held for a minimum of 210 days prior to discharge. Activity concentrations of I‑131 in discharges from the holding tanks to sewer should remain below 5 Bq/l.
Clearance levels are used to determine disposal routes. Where activities are above clearance levels waste should be stored for decay.

Free release to sewer the preferred option. Decision based on direct measurements at a large hospital. External radiation exposure to sewer worker of about 2 µSv calculated on basis of 50 GBq I‑131per year direct release to sewer.
The Netherland

Radioactive waste with radionuclides with half-lives below 100 days should be stored for up to 2 years to allow for decay. No specific mention is made of requirements for patient excreta.
Great Britain
Direct discharge to sewer allowed, but sites required to demonstrate BPM and that the critical group dose constraint of 300 µSv y-1 is not exceeded. Consideration being given to use of delay tanks for new facilities undertaking treatment of large numbers of patients with I‑131.

The authors make a number of recommendations in relation to Ireland’s approach for ablation waste management including:

  • The benefit (on the grounds of radiological protection) of retrofitting of tanks into existing facilities is grossly disproportionate to the financial cost incurred and to the logistical issues involved. Nonetheless, appropriate work control systems should be in place to minimise any potential incidents of plumber exposure;
  • Fitting of delay and decay tanks into a new facility is advantageous, particularly if only one or two facilities are established. This is particularly true where multiple ablation suites may occur in the same facility and more than one patient may be undergoing treatment at one time. However, the final requirements should be assessed on a site by site basis in line with the EC guidelines for demonstrating BAT.
  • Where delay and decay tanks are installed a multi-tank vacuum system has sufficient advantages that it could represent BAT. Using such a system a factor of 500 to 1000 reduction in activity through decay is achievable. This is considered sufficient to ensure that all possible exposure scenarios would not lead to a dose of 10 µSv being exceeded.


Following a detailed review of the recommendations made in the Enviros report the RPII formally adopted a regulatory position on its requirements for iodine holding tanks in the context of both existing and new thyroid ablation facilities as follows:

  1. In the case of existing iodine ablation facilities, licensees will not be required to retro-fit iodine holding tanks.
  2. Licensees with existing ablation facilities will be required to undertake both on and off site monitoring to validate the assumptions and calculations used in their risk assessments when first applying for a licence for ablation therapies.
  3. Licence applications for new ablation facilities will continue to be assessed on a case by case basis to determine whether holding tanks are required. Each licence application must be supported by a risk assessment which estimates the likely doses that would be received by critical groups (hospital plumbers, sewer pipe workers, sewage treatment plant workers, public etc) as a result of the discharges of excreta from patients having undergone ablation therapies.
In early 2009 the RPII advised the Irish Government of the findings from the review project that had been completed, and of its new licensing requirements for thyroid ablation facilities. It recommended to the Government it consider these in the context of taking a national decision on the requirements for iodine holding tanks for both existing and new ablation facilities.

In September 2009 the Department of the Environment, Heritage and Local Government formally adopted the RPII’s regulatory position as national policy for Ireland agreeing that the RPII’s licensing requirements were consistent with Ireland’s commitments to OSPAR.

[1] Management of radioactive waste arising from medical establishments in the European Union. Proceedings of a Workshop, Brussels, 16-17 February 1999, EUR 19254.