United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-18: Compliance with 10 CFR Part 20 for Airborne Thorium

                                   UNITED STATES
                           NUCLEAR REGULATORY COMMISSION
                               WASHINGTON D.C. 20555

                                  March 25, 1996



All material licensees authorized to possess and use thorium in unsealed form.


This notice is provided to alert recipients to radiological problems that may
be encountered in using thorium in unsealed form.  These problems were
identified by U.S. Nuclear Regulatory Commission (NRC) inspectors, during
inspections of the approximately 120 licensees authorized to use unsealed
thorium, some of which are engaged in processing and manufacturing activities
that pose a potential for generating significant airborne radioactive
contamination.  It is expected that recipients will review the information for
applicability to their facilities and consider actions, as appropriate, to
avoid similar problems.  However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written
response is required. 

Description of Circumstances

NRC inspections at facilities using thorium in unsealed form revealed a number
of programmatic weaknesses in the control and monitoring of airborne thorium
hazards at an unexpectedly high proportion of these facilities.  One of the
areas of weakness frequently encountered was worker intake monitoring programs
that did not appear capable of adequately quantifying intakes for purposes of
demonstrating compliance with the requirements of 10 CFR Part 20, particularly
the annual limits on intake (ALI).  A second area of concern was the frequent
lack of adequate licensee efforts to maintain exposures as low as reasonably
achievable (ALARA), as required by 10 CFR 20.1101(c).  NRC inspectors
repeatedly observed intakes and resulting organ doses that appeared to be
unnecessary, or avoidable, in view of the potential to reduce them by
implementation of relatively simple ALARA measures.  Some of the intakes in
these cases were evaluated and produced organ doses in the 0.2 to 0.3 Sv 
(20 - 30 rem) range in a year.  Such high doses, representing a substantial
fraction of the maximum permissible organ doses, cannot be viewed as
acceptable unless justified by a thorough ALARA analysis.  In most of the
observed cases, however, an adequate ALARA assessment had not been performed.

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Demonstration of compliance with dose limits to members of the public, from
airborne thorium, was also found, in some cases, to have been less than
adequate.  In some cases, the licensees were found to have no adequate 
monitoring systems for their airborne effluents, and in others the methods
used to quantify these effluents did not possess sufficient sensitivity to
enable demonstration of compliance. 

In response to the regulatory violations noted above, NRC issued Confirmatory
Action Letters (CALs) to a number of licensees, confirming commitments to
taking specific actions to correct these deficiencies.  Notices of Violation
and other enforcement actions were also taken by NRC, in some cases.  These
actions, as well as extensive discussions with licensees, to alert them to the
problems, have resulted in substantial improvements in most licensees�


The programs that licensees should develop for control of airborne hazards
arising from the use of unsealed thorium do not differ in any basic respect
from those needed in the case of programs to control the hazards from any
airborne radioactive material.  Facilities using thorium, however, must make
allowances for certain constraints imposed by the nature of the thorium decay
chain.  The major constraint is the difficulty of measuring thorium-232 
(Th-232) in the body after an intake using bioassay methods, either in vivo,
such as whole body counting, or in vitro, such as urine analysis.  This is
caused, in part, by the relatively low ALI for Th-232, which is 37 Bq (1 nCi)
for class W, and 111 Bq (3 nCi) for class Y aerosols, as well as the type of
radiation emissions from the thorium decay chain, which are mostly alpha and
beta radiations, with only relatively low-intensity gamma radiations.

The difficulties regarding the use of bioassay methods were increased after
implementation of the revised 10 CFR Part 20, which became mandatory for all
licensees on January 1, 1994.  Intakes of Th-232 by inhalation before the 
Part 20 revisions were limited to 520 MPC-hours per quarter, where MPC was the
maximum permissible concentration tabulated in the old Appendix B to 10 CFR
Part 20.  This was equivalent to an intake of about 700 Bq (19 nCi) per
quarter for both the soluble and insoluble forms of thorium, or about 2800 Bq
(75 nCi) per year.  The revised Part 20 lowered that limit to ALIs of about 
40 Bq (1 nCi) and 100 Bq (3 nCi) for classes W and Y aerosols, respectively. 
Therefore, bioassay methods that may have been capable of detecting intakes
that were a small fraction of the allowable limits in the old Part 20 were no
longer capable of the same performance under the revised Part 20 limits, and
could therefore not serve the same monitoring functions in a routine airborne
radioactivity control program as they did previously.                    

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Although bioassay techniques are still useful in assessing relatively large
intakes, they are not capable of providing routine monitoring for intakes
substantially below the ALI.  The air monitoring program therefore usually
must assume a much greater importance at facilities using unsealed thorium
than for other radionuclides.  Facilities using thorium need to rely on
accurate air sampling to estimate intakes that cannot be detected by bioassay
techniques, which, in effect includes all intakes other than those that
approach or exceed the ALI.  Because of this reliance on air sampling to show
compliance and assess internal doses, the air sampling program must be 
carefully designed to provide accurate intake estimates for all occupationally
exposed workers, as well as members of the public who may be exposed to
airborne thorium as a result of licensed operations.  However, appropriate
bioassay procedures should be established and available for use in assessing
accidental or suspected high exposures, and for use in cases where adequate
air sampling was inadvertently not provided.  In this latter case, bioassay
would provide an upper limit on the magnitude of any intake that may have
occurred, even though it may not be capable of quantifying intakes below an

Air Sampling

The major deficiencies noted in air sampling programs at some of the inspected
facilities included programs that did not provide samples that are representa-
tive of the intake by each exposed worker, monitoring frequencies that were
far too low to be capable of detecting changes in air concentrations over
time, and counting techniques that did not possess adequate sensitivity for
their intended purpose.

One of the factors that led to non-representative samples was the excessive
reliance on general area air sampling to monitor worker intakes in that area. 
Studies have repeatedly shown that air concentrations in a work area can vary
by several orders of magnitude over distances of only a few feet, and a
general area sample is most likely to grossly underestimate the intake of a
worker involved in activities that generate aerosols.  With rare exception,
the most reliable method of assessing worker intakes is by use of personal air
samplers.  In the case of effluent sampling, the method chosen should be
capable of obtaining a representative sample from the exhaust duct or other
outlet.  For aerosols, this usually means use of isokinetic sampling methods,
and licensees should determine, for their particular case, whether such
sampling methods are needed.

The choice of method of analysis should also be given careful consideration. 
This includes choice of the filter medium to use in the air sampler, air flow
rates, as well as choice of counting techniques.  These factors should be .                                                               IN 96-18
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selected to ensure that the desired monitoring sensitivity, expressed as a
lower limit of detection (LLD), is achieved.  A good guide as to the appro-
priate LLD to use in any application is that it should not exceed 10 percent
of the value to which compliance is to be demonstrated.


Licensees are required, by 10 CFR 20.1101(b), to demonstrate that the doses
received by their workers, or by members of the public, as a result of their
activities, are ALARA.  The most effective method to maintain internal doses
ALARA is usually to contain the radioactive material and prevent it from
entering the air in the work space.  Other methods might be use of wet pro-
cesses, which have the effect of preventing or minimizing the generation of
aerosols, or use of other engineering controls, depending on the details of
the aerosol-generating process and the configuration of the workplace.       
Regardless of the choice of engineering controls, their use must include
periodic maintenance to ensure continued effectiveness, as well as periodic
checks to ensure that the systems remain effective.

If engineering controls fail to maintain airborne concentrations at suffi-
ciently low levels, then other methods may be used, such as limiting stay
times, or restricting access to the contaminated areas.  Alternatively,
respirators may be used to limit intakes during periods when other measures
are not sufficiently effective.  It should be noted, however, that 10 CFR 
Part 20 specifies that respirators are to be used only when other methods of
control of intake fail to achieve the desired result or are impractical.

The above discussion on air sampling and ALARA is not exhaustive, and only
highlights some of the most frequently encountered problems.  Licensees should
thoroughly evaluate their operations, and design and implement programs that
would properly protect the workers, minimize intakes, and show compliance with
applicable regulations.  These evaluations are not one-time efforts, but
should be ongoing and integral parts of the overall radiation protection
program on site.

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This information notice requires no specific action or written response.  If
you have any questions about this matter, please call one of the technical
contacts listed below or the appropriate regional office.

                                            signed by

                                      Donald A. Cool, Director
                                      Division of Industrial and
                                        Medical Nuclear Safety
                                      Office of Nuclear Material Safety
                                        and Safeguards

Technical contacts:  Sheri Arredondo, Region I
                      (610) 337-5342

                      Sami Sherbini, NMSS
                      (301) 415-7902
Page Last Reviewed/Updated Thursday, November 21, 2013