[Federal Register Volume 68, Number 177 (Friday, September 12, 2003)]
[Notices]
[Pages 53755-53758]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 03-23255]
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NUCLEAR REGULATORY COMMISSION
[Docket No. 50-285]
Omaha Public Power District, Fort Calhoun Station, Unit 1;
Exemption
1.0 Background
The Omaha Public Power District (the licensee) is the holder of
Facility Operating License No. DPR-40 which authorizes operation of the
Fort Calhoun Station, Unit 1 (FCS). The license provides, among other
things, that the facility is subject to all rules, regulations, and
orders of the U.S. Nuclear Regulatory Commission (NRC, the Commission)
now or hereafter in effect.
The facility consists of a pressurized water reactor located in
Washington County in Nebraska.
2.0 Request/Action
Title 10 of the Code of Federal Regulations (10 CFR), part 20,
section 20.1003 states that the definition of total effective dose
equivalent (TEDE) is the sum of the deep-dose equivalent (for external
exposures) and the committed effective dose equivalent (for internal
exposures). The proposed exemption would change the definition of TEDE
to mean the sum of the effective dose equivalent or the deep-dose
equivalent (for external exposures) and the committed effective dose
equivalent (for internal exposures). The licensee requests the
exemption because the current method of calculating TEDE, under certain
conditions (such as when there is a non-uniform exposure), can
significantly overestimate the dose received.
[[Page 53756]]
In summary, the licensee's application dated January 8, 2003,
requests an exemption from the 10 CFR 20.1003 definition of TEDE.
3.0 Discussion
Pursuant to 10 CFR 20.2301, the Commission may, upon application by
a licensee or upon its own initiative, grant exemptions from the
requirements of 10 CFR part 20 if it determines the exemptions are
authorized by law and would not result in undue hazard to life or
property.
The staff examined the licensee's rationale to support the
exemption request and concluded that the new method for calculating
TEDE, under certain conditions, is a more accurate means of estimating
worker radiation exposure and therefore would not result in undue
hazard to the workers. The basis for this follows.
4.0 Regulatory Evaluation
By letter dated January 8, 2003, the licensee requested an
exemption from the current definition, and the approval to use an
alternate definition of TEDE. The licensee requested that the
definition of TEDE, as used in 10 CFR 20.1003 (i.e., for the purpose of
complying with the dose recording requirements, dose reporting
requirements, or the dose limits), be changed to mean the sum of the
effective dose equivalent or the deep dose equivalent (for external
exposures), and the committed effective dose equivalent (for internal
exposures). The licensee also requested approval to use a method for
estimating the effective dose equivalent for external exposures
(EDEex) published by the Electric Power Research Institute
(EPRI) in Technical Report TR-101909, Volumes 1 and 2, and
Implementation Guide TR-109446. The effect of granting this request
would be to allow the licensee the option to control TEDE using
EDEex in those cases where it is a more accurate predictor
of the risk from occupational radiation exposure.
The radiation protection approach and dose limits contained in 10
CFR part 20 are based on the recommendations of the International
Commission on Radiation Protection (ICRP) in their 1977 publication No.
26 (ICRP 26). For stochastic effects, the ICRP-recommended dose
limitation is based on the principle that the risk should be equal,
whether the whole body is irradiated uniformly or whether there is non-
uniform irradiation (such as when radioactive materials are taken into
the body and, depending on their physical and chemical properties,
concentrate in certain tissues and organs). This condition will be met
if
[Sigma]T[omega]THT<=Hwb
,L
where WT is a weighting factor representing the
proportions of the stochastic risk resulting from tissue (T)
to the total risk, when the whole body is irradiated uniformly; HT
is the annual dose equivalent in tissue (T); and Hwb,L
is the recommended annual dose-equivalent limit for uniform irradiation
of the whole body, namely 5 rem (50 mSv). The
sum[Sigma][omega]TWTTHT is called effective dose
equivalent (EDE). The values for [omega]T are given in ICRP
26, for the various tissues (T), and are codified in 10 CFR
part 20.
For the purposes of implementing workplace controls, and due to the
difference in dosimetry, 10 CFR part 20 breaks this total EDE, or TEDE,
into two components: (1) dose resulting from radioactive sources
internal to the body, and (2) dose resulting from sources external to
the body. For radioactive material taken into the body, the
occupational dose limit is based on the resulting dose equivalent
integrated over 50 years (H50) of exposure such that
[Sigma]T[omega]TH50, T
<=Hwb,L.
This quantity
[Sigma]T[omega]TH50,T is
called the committed effective does equivalent (CEDE) in 10 CFR part
20.
Demonstrating compliance with the dose limits from internal
exposures is accomplished using direct measurements of concentrations
of radioactivity in the air in the work areas, or quantities of
radionuclides in the body, or quantities of radionuclides excreted from
the body, or a combination of these. Having determined the quantities
of radionuclides present or taken into the body, these can be compared
to secondary or tertiary limits (e.g., annual limits on intake or
derived air concentrations) listed in Appendix B to 10 CFR part 20.
These secondary and tertiary limits have been calculated using standard
assumptions of the physical and chemical forms of the radionuclides,
the standard physiological parameters from the Reference Man, and the
bio-kinetic models adopted in ICRP 26. Alternatively, the regulations
allow the licensee to adjust certain of these standard assumptions and
calculate CEDE directly, using appropriate models.
The normal practice for determining radiation dose from external
sources is to measure the radiation intensity at the surface of the
body with a monitoring device (dosimeter) calibrated to read in terms
of a tissue dose equivalent at a specified tissue depth. In 1991, when
10 CFR part 20 was revised to adopt the ICRP 26 recommendations on
limits and controls, there was little guidance on how to determine the
dose to the several tissues necessary to calculate EDEex. It
is impractical to separately monitor (or measure) the dose received by
the various organs and tissues that contribute to TEDE. As a practical,
conservative simplification, 10 CFR part 20 limits the dose from
external sources in terms of deep dose equivalent (DDE). The DDE is the
dose equivalent at a tissue depth of one centimeter, and is required
(by 10 CFR part 20.1201(c)) to be determined for the part of the body
receiving the highest exposure. The TEDE annual limit is met if
DDE + [Sigma]T[omega]TH50,T
<= 5 rem (50 mSv).
In addition to the annual limit on TEDE, 10 CFR part 20 provides a non-
stochastic annual limit of 50 rem (0.5 Sv) for each individual tissue
such that
DDE + H50, T<= 50 rem (0.50 Sv)
for all tissues except the skin and lens of the eye.
Using the highest DDE, to bound the individual tissue doses from
radioactive sources outside the body, generally results in a slightly
conservative estimate of EDEex from uniform exposures.
However, it can be overly conservative for non-uniform exposure
situations. Since many high-dose jobs at nuclear power plants are
performed under non-uniform exposure conditions, this can lead to a
significant overestimation of the actual TEDE dose, and the risk, to
the workers. To address this issue, the licensee has requested approval
to provide a more accurate dose assessment by replacing DDE with
EDEex when calculating TEDE from non-uniform exposures,
where the EDEex is determined with a method developed by
EPRI.
In developing this method, the EPRI investigators used mathematical
equations developed by Cristy and Eckerman to model standard, adult
human male and female subjects (phantoms). The Monte Carlo radiation
transport computer code MCNP was used to calculate the dose to
individual tissues modeled in the phantoms, and simulated dosimeter
readings, for a range of different exposure geometries. Dosimeters with
an isotropic response were modeled at several locations on the surface
of the phantoms. Both broad beam and point radiation sources (with
selected photon energies) were considered. Indicated doses (e.g.,
simulated dosimeter readings) and the actual EDEex (e.g.,
the sum of the products of the calculated phantom tissue doses and
their respective ICRP 26 weighting factors) were calculated for
[[Page 53757]]
photons incident on the phantoms from various locations. Empirical
algorithms were developed to relate the EDEex resulting from
the full range of exposure situations to the indicated doses that could
be measured at the surface of the body. Two algorithms were developed
to estimate EDEex from just two dosimeters worn on the trunk
of the whole body (front and back, respectively). The first algorithm
is a simple, non-weighted averaging of the front and back dosimeter
readings. The second algorithm weights the higher of the two dosimeter
readings.
5.0 Technical Evaluation
The staff reviewed the technical descriptions of the EPRI method
for estimating EDEex; the resulting data and conclusions
contained in Technical Report TR-101909, Volumes 1 and 2;
Implementation Guide TR-109446 and the supporting technical papers
published by the principal EPRI investigators. The staff also performed
independent calculations to verify a sampling of the results tabulated
in these documents.
The EPRI work indicates that a single dosimeter (calibrated to read
DDE), worn on the chest, provides a reasonably accurate estimate of
EDEex when the individual is exposed to a number of randomly
distributed radiation sources during the monitoring period. This is
consistent with current allowable dosimetry practices and requires no
special approval. The alternate definition of TEDE requested, would
allow the licensee the option to monitor worker dose with a single DDE
measurement as currently required, or to control TEDE using
EDEex (as determined by the EPRI two badge method) in
situations where monitoring the highest DDE would require moving, or
supplementing, the single badge.
The data presented in the EPRI reports indicate that the weighted
two-dosimeter algorithm provides a reasonably conservative estimate of
EDEex. However, the non-weighted algorithm does not always
give a conservative result. The licensee has stated that it will only
use the weighted two-dosimeter algorithm such that;
EDEex = \1/2\ (MAX + \1/2\ (Rfront +
Rback))
where Rfront is the reading of the dosimeter on the front of
the body, Rback is the reading of the dosimeter on the back
of the body, and MAX is the higher of the front or back dosimeter
readings.
Additional issues and limitations noted in the staff's review are
included in the following paragraphs.
Partial-body irradiations, that preferentially shield the
dosimeter, could bias the EPRI method results in the non-conservative
direction. The licensee has stated that they will ensure that the
dosimeters are worn so that at least one of the two badges ``sees'' the
source(s) of radiation. In other words, the radiological work will be
conducted, and the dosimeters worn in such a way, so that no shielding
material is present, between the radioactive source(s) and the whole
body, that would cast a shadow on the dosimeter(s) not cast over other
portions of the whole body.
Isotropic dosimeters (e.g., dosimeters that respond independently
of the angle of the incident radiation) are impractical and not widely
available commercially. Therefore, the licensee must implement the EPRI
method using dosimeters that will have an angular dependent response.
If the dosimeter reading decreases more rapidly than EDEex,
with increasing exposure angle, the resulting EDEex estimate
will be biased in the non-conservative direction. The EPRI principle
investigators have addressed this issue of angular dependance in their
published technical paper entitled ``A Study of the Angular Dependence
Problem In Effective Dose Equivalent Assessment'' (Health Physics
Volume 68. No. 2, February 1995, pp. 214-224). The licensee has stated
that the dosimeters used to estimate EDEex will have
demonstrated angular response characteristics at least as good as that
specified in this technical paper. In addition, the dosimeters will be
calibrated to indicate DDE at the monitored location, to ensure their
readings reflect electronic equilibrium conditions.
The EPRI method for estimating EDEex from two dosimeter
readings is not applicable to exposure situations where the sources of
radiation are nearer than 12 inches (30 cm) from the surface of the
body. Tables 5 thru 7 in EPRI TR-101909, Volume 2, provide calculated
EDEex values resulting from exposure to point sources in
contact with the torso of the body. However, the staff review
determined that the information provided in these tables does not bound
all of the pertinent point source exposure situations. The licensee has
stated that the use of EDEex, to determine compliance with
the TEDE limit, resulting from point sources (i.e., hot particles) on,
or near the surface of the body, is outside the scope of this request.
Table 8 in TR-101909, Volume 2, provides a summary of the
EDEex, and dosimeter (front and back) readings calculated
for parallel beams and point sources used to develop the EPRI
algorithms. However, the magnitude of the units for the parallel beam
dose factors listed are low by five orders of magnitude (e.g., ``E-15
rad-cm squared per photon'' instead of the correct ``E-10 rad-cm
squared per photon''). This error does not effect the conclusions drawn
from the data. However, the specific dose factors listed in Table 8
should not be used to calculate EDEex.
When EDE is used to calculate TEDE under the revised definition,
the requirement in 10 CFR part 20.1201(c), that DDE be determined for
the part of the body receiving the highest exposure, is not applicable.
However, when TEDE is calculated using the DDE (i.e., from a single
dosimeter reading), 10 CFR 20.1201(c) does apply.
The exemption applies only to the definition (and methods for
calculating) TEDE . It does not modify the dose limits for any
individual organ or tissue, or the methods for complying, specified in
10 CFR part 20 (i.e., 10 CFR 20.1201(a)(1)(ii), (a)(2) and 10 CFR
20.1208). The licensee is still required to provide surveys and
monitoring necessary to demonstrate compliance with these requirements.
6.0 Evaluation Summary
The staff concludes that calculating TEDE using EDEex as
proposed by the licensee in place of DDE provides a more accurate
estimate of the risk associated with the radiation exposures
experienced by radiation workers at a nuclear power plant.
Additionally, the staff finds that the proposal to limit TEDE such that
EDEex + CEDE <= 5 rem
is consistent with the basis for the limits in 10 CFR part 20.
Therefore, subject to the limitations noted above and agreed to by the
licensee, defining TEDE to mean the sum of EDEex or DDE (for
external exposures) and CEDE (for internal exposures), in lieu of the
current 10 CFR 20.1003 definition, is acceptable.
Additionally, the staff concludes that the methods for estimating
EDEex described in EPRI Technical Report TR-101909, Volumes
1 and 2, and Implementation Guide TR-109446 are based on sound
technical principles. The proposed EPRI weighted, two-dosimeter
algorithm provides an acceptably conservative estimate of
EDEex with a degree of certainty that is comparable to that
inherent in the methods allowed by 10 CFR part 20 for estimating CEDE.
Therefore, subject to the limitations noted above, using the EPRI
weighted, two-dosimeter algorithm so that
EDEex = \1/2\ (MAX + \1/2\ Rfront +
Rback))
[[Page 53758]]
for the purposes of demonstrating compliance with 10 CFR 20.1003 is
acceptable.
7.0 Conclusion
Accordingly, the Commission has determined that, pursuant to 10 CFR
20.2301, the exemption is authorized by law and would not result in
undue hazard to life or property. Therefore, the Commission hereby
grants Omaha Public Power District an exemption from the requirements
of 10 CFR 20.1003 for Fort Calhoun Station, Unit 1. The exemption
changes the definition of TEDE to mean the sum of EDEex or
DDE (for external exposures) and CEDE (for internal exposures). This
exemption is granted to allow the licensee the option to monitor worker
dose using EDEex based on the following conditions:
1. Only the EPRI weighted, two-dosimeter algorithm will be used
such that
EDEex = \1/2\ (MAX + \1/2\ Rfront +
Rback))
where Rfront is the reading of the dosimeter on the front of
the body, Rback is the reading of the dosimeter on the back
of the body, and MAX is the higher of the front or back dosimeter
readings.
2. The radiological work will be conducted and the dosimeters worn
in such a way, so that no shielding material is present between the
radioactive source(s) and the whole body, that would cast a shadow on
the dosimeter(s) and not over other portions of the whole body.
3. The dosimeters used to estimate EDEex will have
demonstrated angular response characteristics at least as good as that
specified in the technical paper entitled, ``A Study of the Angular
Dependence Problem In Effective Dose Equivalent Assessment'' (Health
Physics Volume 68. No. 2, February 1995, pp. 214-224). Also, the
dosimeters will be calibrated to indicate DDE at the monitored
location, to ensure their readings reflect electronic equilibrium
conditions.
4. The EPRI method for estimating EDEex from two
dosimeter readings is not applicable to exposure situations where the
sources of radiation are nearer than 12 inches (30 cm) from the surface
of the body.
Pursuant to 10 CFR 51.32, the Commission has determined that the
granting of this exemption will not have a significant effect on the
quality of the human environment(68 FR 52801).
This exemption is effective upon issuance.
Dated at Rockville, Maryland, this 8th day of September, 2003.
For the Nuclear Regulatory Commission.
Eric J. Leeds,
Acting Director, Division of Licensing Project Management, Office of
Nuclear Reactor Regulation.
[FR Doc. 03-23255 Filed 9-11-03; 8:45 am]
BILLING CODE 7590-01-P