Resolution of Generic Safety Issues: Issue 135: Steam Generator and Steam Line Overfill (Rev. 3) ( NUREG-0933, Main Report with Supplements 1–34 )
Steam generator overfill and its consequences have received staff and industry attention because of the frequency and severity of overfill events. Over the years, a number of issues have been raised concerning steam generator overfill including Issue 66, "Steam Generator Requirements," and Issue 67, "Steam Generator Staff Actions." In order to provide an integrated work plan for the resolution of these issues, Issue 135 was initiated1075 and assigned a medium priority ranking based on the separate evaluation of Issue 67.7.0, "Improved Eddy Current Tests."
Resolution of Issue 135 was expected to provide a better understanding of steam generator and secondary steam integrity, including the effects of water hammer on secondary system components and piping as well as the resultant radiological consequences. The work scope for resolving this issue was divided into four tasks which called for the following staff actions:
Task 1: (a) survey the code requirements and industry practice for eddy current testing procedures; (b) assess the capability of existing methods to detect steam generator tube degradation; (c) review existing ASME Section II requirements on eddy current testing procedures and determine its adequacy for use as a standard for inspection of steam generator tubes; and (d) develop written recommendations for regulatory guidance and/or requirements, including possible endorsement of ASME Section II requirements on eddy current testing procedures for development of a draft regulatory guide.
Task 2: Review the results and conclusions of studies on SGTR and propose specific modifications to SRP11 Section 15.6.3 including tube integrity, operator action time, and offsite dose limits. Develop a regulatory analysis supporting the SRP11 changes including a risk analysis and a cost benefit of the proposed changes.
Task 3: Reassess the following concerns in Issue 67 for potential inclusion in an integrated resolution: reassessment of radiological consequences; reevaluation of design basis SGTR; supplemental tube inspections; integrity of steam generator tube sleeves; denting criteria; improved accident monitoring; reactor vessel inventory measurement; RCP trip; control room design review; EOPs; organizational responses; and RCS pressure control.
Task 4: Review the effects of water hammer, overfill, and water carryover on the secondary system and connecting systems and develop proposals for mitigating the consequences. Consider the effects of sagging due to water weight, operability of valves, and other components when subjected to two-phase flow of liquid.
The coordination of results of the different tasks was to provide a basis for the staff to develop a position on offsite dose, operator action time, and tube integrity. Water hammer mitigation studies were to be carried out to give the staff a better understanding for developing positions on water hammer in main steam lines and operability of valves and other components.
This issue was given a medium priority ranking and pursued by the staff. It was found that SGTR and steam line overfill events pose a relatively low public risk, as previously indicated in NUREG-0844681; comparable risk results for SGTR events were also published in NUREG-1150.1081 The staff technical findings were published in NUREG/CR-4893.1411 Thus, this issue was RESOLVED and no new requirements were established.1337 In an RES evaluation,1564 it was concluded that consideration of a 20-year license renewal period did not affect the resolution.