Resolution of Generic Safety Issues: Issue 74: Reactor Coolant Activity Limits for Operating Reactors (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–35 )
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DESCRIPTION
Historical Background
This issue was raised519 by DSI in June 1983 and addresses the concern that several operating PWRs and BWRs either lack iodine coolant activity LCOs or have inadequate LCOs such that accidents involving the release of coolant cannot be satisfactorily precluded from causing offsite doses in excess of 10 CFR Part 100 guidelines.
Prior to 1974, limiting conditions of operation (LCO) for coolant activity were determined on a plant-by-plant basis, postulating accidents such that subsequent releases and exposures were an appropriately small fraction of the 10 CFR 100 guidelines. Gross activity limits were typically specified with an assumed isotope spectrum. The limiting accident was represented by the radiological consequences of a postulated SGTR in a PWR or a steam line break in a BWR.
Similarly, the allowable secondary activity in a PWR was limited by a postulated secondary coolant steam line break. Many plants licensed before 1974 only sample for gross activity and do not identify specific iodine isotope concentrations.
In May 1974, standard technical specifications (STS) were proposed for limiting the dose equivalent Iodine-131 coolant activity concentrations. The purpose was to establish uniform concentration limits for all plants. The basis for this uniform STS LCO is that the calculated exposure resulting from SGTR or steam line break accidents be below the 10 CFR 100 guidelines at the site having the worst meteorological characteristics. The STS limiting equilibrium 1-131 concentrations are 10-6 and 10-7 Ci/gm of water in the primary and secondary coolant, respectively, for PWRs, and 2 x 10-7 Ci/gm of water for BWRs.
All plants licensed since December 1974 have implemented the reactor coolant activity level STS. In addition, some plants licensed prior to 1974 have adopted the reactor coolant activity level STS. A review of the licensed plants in 1982 and a recent updating indicates that all plants have limits on coolant activity, but 10 PWRs have no LCO on reactor coolant iodine activity concentration and one PWR and twenty BWRs have LCOs that are higher than the STS LCO.
Since this generic issue was established, NRR, through the Steam Generator Tube Rupture Task Force (see Issue 66), has recommended that the STS reactor coolant activity level LCO be backfit to all remaining PWRs that have not yet adopted them. Because of considerations introduced by the presence of steam generators, it is appropriate to consider PWRs separately. Furthermore, this issue does not include any information or analysis on PWRs not included in Issue 66. Therefore, the PWR aspects of this generic issue are subsumed in Issue 66 and Issue 74 is reduced to consideration of backfitting the STS LCO for reactor coolant activity level for only those 20 BWRs that have not yet adopted them.
Safety Significance
The calculated radiological consequences for some accidents are dependent upon the magnitude of the reactor coolant iodine equivalent activity assumed in the dose calculation model. However, the magnitude of reactor coolant activity levels will have a significant effect for only non-core-melt accident consequences. Non-core-melt accidents are not major contributors to the expected public risk from nuclear power plant operation.
In addition to standard monitoring, sampling, and reporting requirements, STS promote uniform characteristics in shielding, personnel protection, and coolant cleanup system capacity.
PRIORITY DETERMINATION
A technical analysis of the proposed resolution for this issue was performed by PNL and is documented in Supplement 4 to NUREG/CR-800.64 As stated above, resolution of this issue is applicable to only the 20 operating BWRs that have not yet adopted the STS requirements for reactor coolant activity. These plants are assumed to have an average remaining lifetime of about 25 years.
Frequency/Consequence Estimate
Currently, BWRs operate with average I-131 concentrations of coolant of approximately 10-8 Ci/gm which is a factor of 20 below the STS LC0. Careful management of coolant activities, based primarily on the desire to control ORE, has resulted in these low levels being observed. Implementation of the STS LCOs at the 20 affected BWRs is, therefore, not expected to result in lower observed average equilibrium iodine concentrations. As a result, resolution of this issue is not expected to result in a public risk reduction for non-core-melt accidents which are assumed to occur when the reactor coolant activity is at the equilibrium condition. It should be noted that implementation of the STS LCOs would impose an increase in reactor coolant surveillance requirements and would, therefore, reduce the uncertainty in observed concentration levels. However, since observed average concentration levels are a factor of 20 lower than the STS
LCO limit, our conclusion that resolution of this issue would not be expected to result in a public risk reduction would not be altered by a reduced uncertainty in observed concentration levels.
Situations can be postulated where a plant could operate with iodine concentrations above the STS LCO due to the iodine spiking phenomenon. This situation has been addressed in the prioritization of Issue B-65, "Iodine Spiking." Resolution of Issue B-65 was also assumed to result in the imposition of new reactor coolant activity LCOs which would be derived from a better understanding of the iodine spiking phenomenon. It was assumed that the new LCOs which might be imposed would not allow for greater iodine concentration levels than those allowed by the STS LCOs. The public risk reduction thus afforded was estimated for both PWR and BWR plants. The public risk reduction estimated for BWR plants by limiting iodine spiking peak concentrations was less than 0.1 man-rem, again because the observed average activity levels in BWR plants are significantly lower than the STS LCO activity levels. We, therefore, conclude that resolution of this issue, i.e., backfitting the STS LCOs to the 20 BWRs that have not yet adopted them, would not be expected to result in a public risk reduction.
Cost Estimate
Industry Cost: Resolution and implementation of this issue were assumed to place the following requirements on the 20 affected BWRs. All the plants were assumed to process a TS change at a cost of one man-month of licensee staff effort per plant. The STS LCOs for BWR plants require sampling for dose equivalent iodine once every 31 days and gross activity sampling at least every 72 hours. Implementation of the STS LCOs would require additional sampling and isotopic analysis capability at some plants. Five of the affected BWRs were assumed to require additional sampling and analysis equipment estimated at $250,000 plus one man-month for installation and operational verification. At an assumed cost of $2,270/man-week, an industry cost for implementation of $1.54M is estimated. Imposition of the STS LCO is estimated to require about 44 additional samplings and analyses per year at each of the 20 affected BWRs. At 2 man-hours sample, the present worth of the total additional industry operating cost is estimated to be $2.5M. Adding the industry implementation and operating costs, a total industry cost of $4M is estimated.
NRC Cost: NRC impact is limited to the effort required to issue an order to the 20 affected BWRs to implement the STS LCO and the staff effort to audit the licensee TS changes. The estimated NRC cost ($25,000) is negligible in comparison to the estimated total industry cost ($4M).
Value/Impact Assessment
Since there is no perceived public risk reduction for the resolution of this issue, the value/impact score is 0 man-rem/$M.
Other Considerations
Resolution of this issue is assumed to require the installation of additional sampling and analysis at a quarter of the 20 affected BWRs and the gathering and analysis of about 44 additional reactor coolant samples per year at all of the affected plants. Since these activities are expected to be performed in a low level radiation field, additional ORE is anticipated. Assuming a 25 millirem/hr field, we calculated 5 man-rem ORE for installation of additional equipment and 1100 man-rem ORE for the gathering and analysis of additional reactor coolant samples over the remaining lifetime of the 20 affected plants.
On the other hand, imposition of STS LCOs for reactor equilibrium coolant activity levels would provide a means to limit reactor coolant activity levels during those infrequent instances in which severe fuel leaks develop. The limiting of reactor coolant activity levels for those instances of operation with "bad" fuel would reduce activity levels in the vicinity of the reactor coolant system and, therefore, would be expected to result in a reduction of ORE to the plant operating staff.
We have made a probabilistic estimate of the expected savings in ORE to plant personnel for the 20 affected BWRs over the next 25 years by backfitting STS coolant activity limits. In the analysis we have assumed that the average BWR coolant activity is 0.01 I-131 equivalent and that coolant activity levels could exceed the STS LCO (0.2 Ci/gm) by about a factor of 3 before other controls such as steam line activity level or plant stack activity level would require corrective action. Examination and evaluation of historical data on operator exposure at BWR plants lead us to the conclusion that about 20% of the annual exposure of plant personnel can be directly affected by reactor coolant activity levels, i.e., about 180 man-rem/RY of the average annual exposure of 900 man-rem/RY may be due to the current average reactor coolant activity level of 0.01 Ci/gm. When combined with a historical background which indicates somewhere between 2 to 6 instances of major BWR fuel leakage, we estimate that imposition of the BWR STS limit of 0.2 Ci/gm I-131 equivalent at the 20 affected plants could result in a total reduction in ORE of between 600 to 1700 man-rem over the remaining lifetime of the 20 plants. We also calculated the potential reduction in ORE from plant cleanup for mitigated LOCAs by limiting the reactor coolant I-131 activity level to no greater than 0.2 and estimated an average total ORE savings of 125 man-rem.
When the above increase and reductions in ORE are summed, we arrive at a conclusion that imposition of the BWR STS requirements on reactor coolant activity could result in a range of ORE change from a 375 man-rem increase to about a 725 man-rem reduction over the lifetime of the 20 affected BWRs.
CONCLUSION
The resolution and implementation of this generic issue is not expected to result in any appreciable offsite (public) risk reduction, but can result in additional costs for the licensees of the 20 affected BWRs. Estimates of ORE indicate an increase for additional coolant sampling, a very small projected averted ORE due to plant cleanup in the event of mitigated LOCAs with "bad" fuel, and, at best, a small decrease in ORE due to operation of the plant with lower peak coolant activity limits. The overall effect of resolution and implementation of this issue could range from a small ORE decrease to a small ORE increase which could very well negate each other. Thus, we do not view ORE as a significant consideration and recommend that the issue be DROPPED.
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