Experiences in Neutron Dosimetry

Transcription

1 Experiences in Neutron Dosimetry Nasser B. Rashidifard M.S., CHP NECHPS Annual Technical Symposium May 29 th, 2014 Westford, Massachusetts Radiation Safety & Control Services, Inc.

2 Why is it Important to Characterize? New Dosimetry Vendors New Dosimetry Types Dry Fuel Storage Campaigns Independent Spent Fuel Storage Installations Extended Power Uprates Modification of shielding

3 Why is it Important to Characterize? Every Dosimeter type has a different neutron energy response curve. You need to use the correct value to ensure the most accurate (or reasonably conservative) dose to personnel or public. Most sites can have a variety of fields all having very different spectrums. Sites will typically use a vendor recommended neutron correction factor. These values have a sound technical basis but site specific characteristics may make them not representative. ANI Bulletin Neutron Monitoring

4 Where to Start What activities contribute the most to the site s neutron dose? What new activities are going on? What activities are no longer being performed? What activities may be expected in the future? Can measurements occur at the location?

5 Collecting Data Neutron measurements done using a Tissue Equivalent Proportional Counter (TEPC). Dosimeters irradiated on ANSI N13.11(2009) 30 cm x 30 cm x 15 cm Phantoms. Active area of 10x10 cm to prevent edge effects. Performed in a uniform field in parallel. Performed in series for smaller unchanging fields. Ensure enough dose on the dosimeters.

6 Pressurized Water Reactor Have the Highest neutron dose of the industry. Surveys in containment while at 100% power. Online Maintenance. Routine Operations walkdowns. Variety of neutron fields.

7 Pressurized Water Reactor Values can vary greatly even within one containment. Measurements were performed at a variety of Location Dosimeter Dose Rate (mrem/hr) TEPC Dose Rate (mrem/hr) TEPC/ Dosimeter locations: Outside Bioshield Instrument Penetrations Refueling floor Upper elevations Up to a factor of 10 difference

8 Boiling Water Reactor Can be measured outside the Drywell Hatch or in piping penetration rooms No measurements inside. Lower neutron dose rates that inside PWRs Positioning is critical due to high potential of streaming

9 Boiling Water Reactor Can very greatly since every measurement can represent a different area outside the drywell. Some measurements are through penetrations providing less thermalized fields. Sites Dosimeter Dose Rate (mrem/hr) TEPC Dose Rate (mrem/hr) TEPC/ Dosimeter

10 Spent Fuel Movements Measurements collected after draining of the inner canister during the vacuum drying phase. Typical irradiation is hours. Setup is generally on middle levels of scaffolding due to uniformity of fields and accessibility.

11 Spent Fuel Movements Each operation has its own shielding setup that can effect the spectrum. Water jackets and Polyethylene wraps can change the neutron spectrum Sites Dosimeter Dose Rate (mrem/hr) TEPC Dose Rate (mrem/hr) TEPC/ Dosimeter change the neutron

12 Independent Spent Fuel Storage Installation (ISFSI) Low dose rates Typically highly thermalized neutrons Require long integration times on dosimetry Additional shielding added after fuel loading can make access to fields difficult.

13 Independent Spent Fuel Storage Installation (ISFSI) Due to the highly thermalized fields certain dosimeters can over respond by a significant amount. Even though the dose to personnel is low, correction factor selection is important to meet dose estimates. Sites Dosimeter Dose Rate (mrem/hr) TEPC Dose Rate (mrem/hr) TEPC/ Dosimeter

14 Example 1 An operating BWR that is actively performing spent fuel transfers for dry storage. Using CR-39 dosimetry with a neutron correction factor of 0.68 based on BWR energy spectrum from their NVLAP accredited vendor for the whole site. Area Original NCFs Used Actual NCFs calculated BWR SFC The result of this is that the site was under estimating the dose of the spent fuels group by approximately 20%. This is due to the higher average neutron energy of the spent fuel. The site issued two sets of dosimetry, one for the routine site work and another for the spent fuels work.

15 Example 2 ISFSI site was conducting site repairs over the course of a week using a local contractor. The workers were considered escorted non-rad workers since the pre-job dose estimate was well below 100 mrem and they would not be handling any radioactive materials. General area surveys were less than 2 mrem/hr. There was no need to issue dosimetry but spare badges were rush ordered for the workers anyway as good idea

16 Example 2 (cont.) An investigation was conducted to find the cause by gathering and reviewing the following information: Area surveys Reconstructing the activities of every worker Interviewing all escorts and site staff Dosimetry storage Neutron correction factors

17 Don t Forget Survey Meters! Accurate surveys of areas with large or high dose rate neutron fields. Particularly applicable to PWR containments, spent fuel operations, ISFSI installations, or Cyclotrons where neutrons are almost as large as the photon components. Instrument response of neutron survey meters can vary up to a factor of 4 or more. Important for establishing boundaries, ALARA planning, occupational or public dose estimates.

18 Remball and Time Keeping Some sites still use Remballs and time keeping to assign dose. During a neutron study at an operating PWR survey instruments were evaluated to their response in a variety of areas. Since the neutron dose rates are fairly high this factor represents a significant portion of their dose. Effect work planning and dose estimates when using time keeping methods. Location Normalized Response to measured DE

19 Its not easy Physical Concerns High Heat Low Light Trip Hazards Foreign Material Exclusion Engineering holds for accident analysis Long hours Radiological Concerns High Dose Jobs Dose and Dose Rate Alarms Potential for accidental high exposures Contamination Activation of equipment

20 Questions?