U.S. Nuclear Regulatory Commission

Consideration of Measurement Uncertainty When Measuring Radiation Levels Approaching Regulatory Limits

HPPOS-223 PDR-9111220129

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

Applicability: All