50.59 – Why 10% is the criterion for determining “more than a minimal” increase

It’s given as guidance in NEI 96-07 without a basis, and 10% is not mentioned in the regulations at all. But the basis isn’t arbitrary. Here is the reason…

As you may know, 10 CFR 50.59 defines eight criteria against which facility or procedural changes (and tests and experiments) must be evaluated. If a change could have a significant enough impact on the plant’s licensing basis, then the plant must get the NRC’s permission before implementing the change (in the form of a license amendment).

These eight criteria identify if a change crosses that threshold. A plant must perform a 50.59 evaluation to evaluate design changes against the criteria to determine if they are met. If they are not (i.e., they screen out), then a license amendment (and NRC permission) is not required to proceed.

Quantifying “more than a minimal increase”

The first four criteria possess a qualitative limit defined as “more than a minimal increase.” For example, when considering a change’s impact on the consequences (dose rate) of an accident, it cannot “[r]esult in more than a minimal increase in the consequences of an accident previously evaluated in the final safety analysis report (as updated)” if it’s to be implemented without a licensing amendment.

“More than a minimal” is generally understood as an increase of greater than 10%. When considering this particular criteria [50.59(c)(2)(iii)], the NEI guidance explains:

An increase in consequences from a proposed activity is defined to be no more than minimal if the increase (1) is less than or equal to 10 percent of the difference between the current calculated dose value and the regulatory guideline value (10 CFR 100 or GDC 19, as applicable), and (2) the increased dose does not exceed the current SRP guideline value for the particular design basis event.

NEI 96-07, Revision 1, Section 4.3.3

You may be curious from where the 10 percent originates. It is not obvious in the NEI guidance. The 50.59 rules don’t mention it.

The answer is found in back issues of the Federal Register.

Three Options in the Proposed 50.59 Rule of 1998

The NRC issued a proposed 50.59 rule change in 1998 (Federal Register, Volume 63, No. 203). This is where they discussed the basis for the 10 percent. “More than a minimal” is a qualitative assessment, and this implies that a quantitative argument is not necessary to justify an evaluation.

But in some cases, such as when dealing with doses, quantitative analyses are required. There are clear numerical limits (such as what is found in Criterion 19 of Appendix A).

In fact, the NRC presented three different approaches for defining “more than a minimal.” They were:

  1. An increase of 0.5 rem in the calculated dose as a result of the change.
  2. A graduated approach with different increase allowances (10%, 5%, 1%) depending on how close the existing value is to the limit (less than 50%, less than 80%, or more than 80%).
  3. 10 percent of the remaining margin between current conditions and the acceptance limit.

Ultimately, the NRC decided upon the third option, even though it did not explicitly write this into the revised 50.59 rules. It published its reasoning in the October 4, 1999 issue of the Federal Register in which it issued the final rules.

The Commission will conclude that the requirements of the rule are met if the calculated doses from a change at a facility would be less than 10 percent of the remaining margin between current calculated dose values and acceptance values in the regulations.

This approach also accomplishes the intent of the second option. The greater the margin between the existing value and the limit, the greater an increase that a plant can make without needing NRC approval. The less existing margin there is, the less of an increase is permissible.

Under this approach, the threshold for what constitutes a minimal change varies as a licensee approaches the regulatory limit. The amount of change allowed would decrease as the limit is approach, and the limit could not be exceeded without prior NRC review. Specifically, it is no more than a minimal increase in consequences if the increase is less than or equal to the more limiting of either 10 percent of the difference between the existing calculated value and the regulatory guideline value (10 CFR part 100 or GDC 19 as applicable), or has reached the SRP [Standard Review Plan] guideline value for the particular design based event.

This conclusion made its way into the NEI 96-07 guidance, but without the explanatory material provided by the NRC. Reading the Federal Register back issues can sometimes aid in understanding why certain regulatory decisions were made. This can help you better understand the regulatory intent, which can help you develop better evaluations.

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