Potential exposures in nuclear installations
This survey was carried out from October 2006 to January 2007 via the EAN Forum. The aim was to determine if and how the operator of a nuclear installations takes into acount, in the dose planning for a job, the occupational radiological incidents and mishaps. The questionnaire contained the following questions:
- If this is taken into account, does a specific methodology exist?
- Have you examples of scenario (biological shielding defaults, ventilation defaults, unplanned dose rate, fire, etc.), which have been used for these kinds of prevision?
- Is that planning required by the authority?
- Is there a dose criteria, above which an incident has to be reported to the authority?
Answers from 6 different countries have been received.
All the activities which are dangerous from nuclear and radiation point of view (the list of this kind of activities are included in the technical specification of plant) shall fulfilled according the special order of the manager of ANPP and be agreed by the Armenian Nuclear Regulatory Authority (ANRA). There are requirements for daily, weekly dose limits. For each type of works on safety systems or decontamination works - working group, workplace preparing arrangements, collective dose limits, protective measures are defined. This document is called Dose order . The dose order is needed if the dose can exceed 0.2 mSv per person. If the dose exceeds the planned individual dose (2 mSv) for the given activity the operator must inform ANRA and present written explanations and future actions in order to prevent such violations.
There are requirements for both individual and collective doses in the Finnish YVL guides concerning the use of NPPs. These requirements give detailed instruction for using dose constraints/highest allowed dose which are below the values set in ICRP 60 (annual limit is 50 mSv and 100 mSv/ 5a).
The use of electronic dosimeters is mandatory. Due to that principle and because of the good planning of all works in a NPP there have been no accidents where dose limits have been exceeded.
If the collective radiation dose of personnel at one plant unit exceeds the value 2.5 man.Sv per one GW net electric power as an average of two consecutive years, the reasons for exceeding and the possibly needed measures aimed for improving radiation safety shall be reported for STUK.
There is a general requirement in regulatory guides that dose constraints shall be used. In practice NPPs have set dose-constraints (highest allowed dose) for one working day. The alarm is set in e-dosimeters to 2 mSv/day. There are also constraints for the individual doses during an outage (15 mSv/outage or dose even lower than that).
The planning of the works in NPP is also considered important: the documents on fuel reloading outages and planned extensive repair outages shall include e.g. the number of personnel participating in radiation protection, special arrangements during the outage concerning radiation protection and also an estimate of the collective radiation dose of workers caused by the outage work. If it is predicted while planning the work that the dose resulted will exceed 0.05 man.Sv or that there is a significant risk of internal radioactive contamination, a detailed document describing work plans and radiation protection actions shall be sent to STUK for information well before the work is initiated. If there are several tasks of this type, a combined document may be sent to STUK.
During refuelling and other corresponding extensive outages the daily report shall include also the events significant to radiation protection. This requirement means that even the incidents which are closed to happen are reported.
There are general requirements in the Greek Radiation Protection Regulations that dose constraints, which are below the values set in ICRP 60, shall be used. The suggested dose constraints for external and internal exposure are 5/10 and 3/10 of the annual dose limits, respectively.
Specifically, at the research reactor the dose constraint is set at 6 mSv, as defined in the research reactor radiation protection internal regulations document. Thus, job planning is performed according to the ALARA principle and taking into consideration that no individual dose will exceed the specified annual dose constraint.
The control of individual doses is achieved by all personnel wearing both photon and neutron TLD dosemeters. The photon and neutron dosemeters are issued and measured monthly and every two months, respectively. The measurement of the personal dosemeters is performed by GAECs laboratories and appropriate investigation levels for annual individual doses are defined at 4 mSv. Moreover, during special operations (planned exposures), direct reading electronic dosemeters are also used.
Advanced work planning is performed, including dose estimation, number of personnel participating and radiation protection arrangements. A document containing the work plan and radiation protection actions is prepared by the Radiation Protection Officer. Dose control information, including radiological incidents and bad practices, are reported to the Greek Atomic Energy Commission (GAEC), annually.
In any case of individual dose exceeding the set dose constraint the Reactor Manager along with the Radiation Protection Officer shall investigate the reasons leading to this exposure and possibly propose measures to improve radiation safety. The same applies for the case that a monthly dosemeter measurement indicates a potential excess of the dose constraint. We note that in case of an overexposure appropriate facilities enabling biological dosimetry are available within the institute. However, we stress that there have been no incidents where dose limits have been exceeded at the GRR-1 facility. Furthermore, there are requirements in the Greek Radiation Protection Regulations concerning doses or committed doses received during specially authorized exposures. The dose constraints in these cases are determined by GAEC.
(The information is from the Dutch NPP at Borssele)
When jobs are prepared, issues that are relevant for RP are always identified. The details are dependent on the risk and the collective and individual dose to be received. From routine jobs to special cases, these jobs are prepared (depending on dose and contamination risks) as follows:
- Job permits: always necessary in controlled areas; evaluated by the RP service and a ccompanied with specific measures,
- Job instruction from the RP service,
- Dose assessment form (when 1 mSv < coll. dose < 5 mSv or indiv.dose > 1 mSv), or ALARA-report (when coll. dose > 10 mSv, or extra dose due to maintenance/operation > 5 mSv/y, or indiv dose > 3 mSv),
- For refuelling periodes, a separe integral dose assessment needs to be made.
Methods that are used, are (next to experience and common sense):
- Check list with standard questions (for instance: is there a risk for contamination or release of gas etc.),
- Evaluation of "what-if" scenarios, or more systematic Risk Inventory & Evaluation (RI&E), and/or
- Written ALARA report.
Measures could be: shielding, personal protection measures, in advance cleaning and decontamination of systems, demarcation of work areas, extra step-over benches, local air extraction, plastic isolation tents, evacuation of non-necessary workers, provisions to evacuate (possibly wounded) workers from difficult to reach working areas etc.
Indiv. doses are reported to the authorities every 3 months. Reporting criteria are:
- Within 30 days: unforeseen dose ≥ 10mSv,
- Immediate reporting (< 8h): dose > limit (20 mSv),
- When the dose received is > 15 mSv per 3 months, the position of the dosimeter on the body and the circumstances of the exposure need also to be reported.
1. In your country, does the operator of a nuclear installation take into account, in the dose planning for a job, the occupational radiological incidents, mishaps and potential exposures, which could happen?
If the word "job" has to be understood as e.g. one of hundreds of jobs which have to be done during an outage in a nuclear power plant per year, then in some cases (not routine jobs, with a higher risk) the planning of a job or project has to include considerations about emergency measures.
2. If so, does a specific methodology exist?
No. On the other hand for big projects, like intalling nuclear facilities, there is a list of possible events for which the risks have to be analysed. The list of events is provided in our nuclear energy ordinance.
3. Have you example of scenarios (biological shielding defaults, ventilation defaults, unplanned dose rate, fire...), which have been used for these kinds of prevision?
For example: In the research reactor PROTEUS at the Paul Scherrer Institut an experiment was performed using a highly burned fuel sample (6.1014 Bq gamma/beta-emitter, 6.1013 Bq alpha-emitter). For some events like crash of the transportation cask, trop down or getting stuck of the sample inside the reactor or fire in the reactor hall, the consequences like exposure to the personal or emission of aerosols in the neighbourhood as well as protection measures had to be reported in the planning document.
4. Is that required by the authority?
Yes, if the job brings up a certain risk (e.g. the not routine use of high quantities of alpha emitters in a form which can be easly dispersed and incorporated) the planned project has to be reported (and in some cases to be approved).
5. Is there a dose criteria, above which an incident has to be reported to the authority?
No, for an potential (possible) incident during the planning phase.
Yes, if an incident happens and an incorporation can not be excluded, or the incorporation can be analysed and shows an E50-dose > 2mSv, the incident has to be reported.
In answer to the question about what is required by the authorities for potential exposures, then NII have set out their expectations within their Safety Assessment Principles. These are not really requirements, but at least they are publicly available on the NII's website. In the 1992 SAPs, Principle P43 is perhaps the most relevant.