ML16224A748

Proposed 10 CFR Part 50 and Part 100 (Nonseismic) Rule Changes and Proposed Update of Source Term

ML16224A748
Person / Time
Issue date:	01/15/1992
From:	Ward D Advisory Committee on Reactor Safeguards
To:	Selin I Advisory Committee on Reactor Safeguards
References
D920115
Download: ML16224A748 (3)

Text

D920115 The Honorable Ivan Selin Chairman U.S. Nuclear Regulatory Commission Washington, D.C. 20555

SUBJECT:

PROPOSED 10 CFR PART 50 AND PART 100 (NONSEISMIC) RULE CHANGES AND PROPOSED UPDATE OF SOURCE TERM During the 381st meeting of the Advisory Committee on Reactor Safeguards, January 9-11, 1992, we discussed the NRC staff proposal to decouple nonseismic siting requirements from plant requirements through revisions to 10 CFR Part 50 and Part 100 and the proposal to update the fission product source term used for siting analyses.

Our Subcommittees on Safety Philosophy, Technology and Criteria; Severe Accidents; and Regulatory Policies and Practices discussed these matters during a joint meeting on January 7-8, 1992. During these meetings, we had the benefit of discussions with representa-tives of the NRC staff and industry and the documents referenced.

The staff has proposed separation of Part 100, "Reactor Site Criteria," from those requirements for plant design which more properly belong in Part 50. A two-stage program to accomplish this has been described. In Stage 1, radiological dose criteria and reference to fission product release quantities (the "source term")

from TID-14844 will be moved to 10 CFR 50.34 without other change.

Also, Part 100 will be augmented by adding to it certain quantita-tive criteria, without change, now specified in Regulatory Guide 4.7, "General Site Suitability Criteria for Nuclear Power Sta-tions."

In Stage 2, further changes will be made in Part 50 to update source term requirements by incorporating technical information about severe accidents developed since Part 100 was promulgated in 1962. Details of all changes to Part 50 have not been developed, but a preliminary description of the source term itself, and its derivation, was provided. This new source term will ultimately be described in a technical document which will be referenced in the regulations instead of TID-14844. The staff plans to issue this technical document in advance of further work on the Part 50 change so that it can be reviewed by industry and the reactor safety community. The present plan is to issue it for public comment in April 1992.

We were told that Stage 1 work is progressing on a schedule which should make the revised rules available prior to completion of the review of evolutionary plant applications and that schedules for Stage 2 work are compatible with those for passive plant reviews.

We believe the first stage proposal is reasonable and should proceed as the staff indicated. For Stage 2, development of a new surrogate source term is also proceeding along the right line.

However, a major part of the source term proposal, a description of mechanisms for depletion of the source term within containment by engineered safety features and natural processes, remains to be developed. Also, confirmation of details of the proposal through the public comment period is needed. Beyond that, we have major concerns about the Stage 2 program, not so much with what is being done as with what is not.

There is no plan for a Stage 2 upgrade of Part 100. Stage 1 merely provides a more logical arrangement and more completely codifies technical information on siting which was developed 30 years ago. The basis for key requirements such as the 0.4-mile radius for an exclusion zone, a 10-mile radius for an emergency planning zone, and a maximum population density for the low population zone has not been reexamined or justified with up-to-date information. We question the appropriateness of codifying Regulatory Guide 4.7 guidance.

While Stage 1 is acceptable in the interim, further work should be carried out to consider what other changes may be appropriate in light of the large amount of experience and information that has been accumulated since 1962. In particu-lar, the relation of these requirements to the Commission's Safety Goal Policy should be established.

There is no plan to incorporate meteorological requirements in Part 100. Enough is now known about meteorology to define, in advance, that certain sites would be unacceptable for a nuclear power plant.

While the source term is an important part of Part 50 require-ments for containment performance, it is not the most impor-tant part by any means. Far more significant to risk are characteristics which will govern whether a containment will continue to perform its function or fail during a severe accident and whether mitigation systems will operate effec-tively or not. The staff has done a good job in its prelimi-nary development of the fission product source term discussed above. This source term is, in essence, a surrogate for the spectrum of possible fission product releases defined in a way so that it can be of practical use in containment design.

This surrogate was developed using engineering judgment and the information about severe accidents and fission product releases developed over the last decade or so. The ACRS recommended in a May 17, 1991 report that criteria to accommo-date severe accident phenomena in containment design be developed. This report recommended use of an "energy source term" which would be a surrogate for the spectrum of severe accident challenges to containment integrity.

Another critical issue for designers, in an updated source term, is the timing of fission product releases and how that timing relates to the requirements for closure times of large containment isolation valves. In existing plants, such valves are required to close in about 5 seconds. Preliminary work described by the staff shows that closure times of 10-30 seconds could easily be justified. Further work may indicate

minimum closure times could be increased to 1 minute or more.

If required closure times can be increased enough, it may be possible to justify a class of valves for use in containment isolation which will be more generally reliable. This work should be pursued with that object in mind.

The staff should be complimented on the work it has begun to decouple siting and plant design criteria and to update source term requirements. However, the additional areas we have described should also be covered as the program progresses. The Committee will be interested in following this work as it develops.

Sincerely, David A. Ward Chairman

References:

1. Memorandum dated October 11, 1991 from Themis P. Speis, Nuclear Regulatory Commission, for Raymond F. Fraley, Advisory Committee on Reactor Safeguards,

Subject:

Proposed Revision of 10 CFR Part 100, Reactor Site Criteria, Revisions to 10 CFR Part 50, New Appendix B to 10 CFR Part 100 and Appendix S to 10 CFR Part 50, and Associated Regulatory Guides (Draft Predecisional)

2. Memorandum dated December 11, 1991 from Warren Minners, Nuclear Regulatory Commission, for Raymond F. Fraley, Advisory Committee on Reactor Safeguards,

Subject:

Revision of In-Containment Accident Source Terms for Nuclear Power Plants (Draft Predecisional)

3. Report dated May 17, 1991 from David A. Ward, Chairman, Advisory Committee on Reactor Safeguards, to Kenneth M. Carr, Chairman, Nuclear Regulatory Commission,

Subject:

Proposed Criteria to Accommodate Severe Accidents in Containment Design