U.S. Nuclear Regulatory Commission
Consideration of Measurement Uncertainty When Measuring Radiation Levels Approaching Regulatory Limits
Title: Consideration of Measurement Uncertainty When
Measuring Radiation Levels Approaching Regulatory Limits
See the memorandum from J. W. N. Hickey and L. J.
Cunningham to M. R. Knapp (and others) dated August 3,
1990. The memo states that as with any regulation, limits
must be given as exact, precise values. The method of
demonstrating compliance with these limits is usually left
to the regulated person. Any method that provides a
reasonable demonstration of compliance will be accepted.
The NMSS and NRR Offices became aware of a letter
transmitting a notice of violation that appeared to send an
incorrect message to licensees. The incorrect message was
that licensees must consider inherent uncertainties when
measuring radiation levels approaching regulatory limits
and must establish procedural limits that are less than the
regulatory limits by an amount that equals (or exceeds) the
"instrument error." That message is incorrect.
The following statement was made by the NRC in response to
a petition for rule making with regard to limits for
surface radiation levels of packages prepared for transport
(44 FR 22233, April 13, 1979): "As with any regulation,
the (safety) limits must be given as exact, precise values.
The methods of demonstrating compliance with these limits
are usually left to the regulated person. Any method which
provides a reasonable demonstration of compliance will be
accepted. In most cases, exact measured values are not
required." This statement is still valid.
All measurements are inherently imprecise and inaccurate to
some degree. Inevitably, there will be cases involving
transportation of radioactive materials in which a valid
measurement by the shipper shows a radiation level below
the limit and a valid measurement by the receiver shows a
radiation level above the limit. Without evidence that the
shipper's measurement is invalid, there is no reason to
assume that the shipper's measurement is incorrect and,
consequently, that the shipper had inadequate control over
shipping of packages.
The NRC position is that the result of a valid measurement
obtained by a method that provides a reasonable
demonstration of compliance or of noncompliance should be
accepted and that the uncertainty inherent in that measured
value need not be considered in determining compliance or
non-compliance with a regulatory limit. Thus, only the
measured value (and not the sum of the measured value and
its uncertainty) need be less than the value of the limit
to demonstrate compliance with the limit. Conversely, only
the measured value (and not the measured value less its
uncertainty) need be greater than the value of the limit to
demonstrate non-compliance with the limit.
Regulatory references: None
Subject codes: 6.6, 7.1, 12.7