Resolution of Generic Safety Issues: Issue 140: Fission Product Removal Systems ( NUREG-0933, Main Report with Supplements 1–34 )
The issue, as originally proposed,1161 addressed the concern for fission product removal by the containment sprays and suppression pools in PWRs and BWRs, respectively; however, it was expanded to include PWR plants that use ice condenser systems for fission product cleanup. Thus, the original title of the issue, "Fission Product Removal by Containment Sprays or Pools," was shortened to be consistent with the addition of the ice condenser systems as part of this issue. Specifically the issue called for SRP11 changes to: (1) eliminate overly conservative assumptions in estimating the effectiveness of post-accident fission product removal by containment spray systems; (2) specify the pH range for recirculating solutions; and (3) resolve an inconsistency in regulatory guidance by giving credit to pressure suppression pools as post-accident fission product cleanup systems in BWRs. Although considered to be part of the Severe Accident Program, this issue was raised to ensure continuity in its resolution following the NRC reorganization in April 1987.
In PWR plants, sodium hydroxide or sodium tetraborate is added to the containment spray or ice condenser solutions. These chemical additions are required to improve the iodine scrubbing effectiveness of the water/ice solu‚tions and thereby counter a rapid release of large amounts of iodine vapor pos‚tulated to result from a severe accident (core-melt). However, considerably smaller quantities of elemental iodine are now estimated to be released from the reactor coolant system much later after an accident.
Consequently, the need for the above chemical additives no longer exists during the injection phase of the spray, but the pH of the containment sump must be above 7 prior to, and during, the recirculation phase to prevent regeneration of iodine from the containment sump.
BWR plants with suppression pool type containments also have a Standby Gas Treatment System (SGTS) which treats leakage from the containment and removes iodine through adsorption in the charcoal filters. No credit was previously given for retention of iodine in the suppression pools.
Relaxation of the chemical additive requirements for PWR containment spray solutions in dry containment designs and ice beds in ice condenser containment designs is not expected to result in significant changes in public risk. For BWRs, any credit given for the suppression scrubbing action should not relax the BWR containment leak rate or change a safety grade system to a non-safety grade system.
Revisions to the SRP11 would eliminate overly conservative assumptions used to estimate the effectiveness of chemical additives to the containment sprays and ice beds. The affected SRP11 Sections are 6.5.2 "Containment Spray as a Fission Product Cleanup System," and 6.5.4 "Ice Condenser as a Fission Product Cleanup System." The revisions will affect future PWR plants.
In addition, a new SRP11 Section 6.5.5, "Pressure Suppression Pools as Fission Product Cleanup Systems," would provide guidance and review procedures or acceptance criteria for the suppression pool retention capabilities and would allow relaxation of maintenance and surveillance of the BWR SGTS and less frequent replacement of charcoal media. The new SRP Section will affect all BWR plants with suppression pool containment designs.
The SRP11 revisions discussed above could reduce: (1) the cost of maintenance (including the replacement of chemicals); (2) the cost surveillance; and (3) the hazard to personnel and equipment from chemical spills or spray initiation. The new SRP11 Section that would provide allowances for suppression pool retention could result in reduced costs for maintenance (including replacement of charcoal) and surveillance of systems such as the SGTS. All SRP changes became effective on January 25, 1989.1162 Thus, the technical concerns of this issue were addressed in the Severe Accident Program and Issue 140 was DROPPED from further consideration as a new and separate issue.