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Resolution of Generic Safety Issues: Issue 1: Failures in Air-Monitoring, Air-Cleaning, and Ventilating Systems ( NUREG-0933, Main Report with Supplements 1–35 )


Historical Background

This issue is identified in Appendix D of NUREG-05724 and is one of the key observations made after the ACRS requested its members and consultants to make comprehensive reviews of all Licensee Event Reports (LERs) issued during the years 1976, 1977, and 1978.

Data collected for this 3-year period show that 14% of all LERs were related to failures in air-monitoring, air-cleaning, and ventilating systems. This translates into an average of over 350 LERs for each of the 3 years considered. Air-Monitoring equipment failures accounted for more than half of these failures in BWRs and more than one-third of these failures in PWRs. This disparity occurred because there are more air-cleaning and ventilating systems in BWRs than in PWRs.

Safety Significance

The proper operation of air-monitoring, air-cleaning, and ventilating systems is important for maintaining primary containment integrity and controlling the release of airborne particles and gases from plants. Proper ventilating system performance is also important to the operation of HPCI and RCIC systems in BWRs and waste gas processing systems in PWRs. The effects of ventilating system failures on HPCI and RCIC systems are addressed separately in NUREG-0572,4 Appendix D, Item XIV.

Proper air-monitoring equipment performance is essential for avoiding the buildup of hazardous concentrations of gases, e.g., hydrogen, and for limiting the impact of environmental releases.

Possible Solution

The overriding concern in this issue is the proper functioning of the stated systems under accident conditions. Although these systems would not prevent WASH-140016 accidents from occurring, their failure during these accidents would increase the risk of exposing offsite personnel to radioactive releases.

No technical solution was proposed in NUREG-0572.4 The analysis of this issue will be based on the possible solution that improving licensees' maintenance and testing programs for air-monitoring, air-cleaning, and ventilating systems will reduce failures in these systems at all plants. It is conservatively estimated that this possible solution will reduce the public risk associated with this issue by 50%.


Frequency Estimate

Since there are no available data on failures of the stated systems during accidents, the probabilities of all WASH-140016 accident categories listed in Table 3.1-1 will be considered in arriving at a frequency estimate.

In order for the air-monitoring, air-cleaning, and ventilating systems to lose their safety function during an accident, the following events would have to occur simultaneously: failure of air-monitoring equipment, failure of ventilating equipment, failure of air-cleaning equipment, and improper operator action. The probability of occurrence of this sequence of events is conservatively estimated to be 10-3/demand. Therefore, the probability of occurrence of these events, following any WASH-140016 type of accident, is the product of 10-3 and the corresponding accident probability. This product will be used as the frequency estimate (F) in the value/impact score calculation below. Thus, F = (10-3)(P), where P is the accident probability from WASH-1400,16 as shown in Table 3.1-1.

TABLE 3.1-1





PUBLIC DOSE, Q (man-rem) P x Q (man-rem/RY)
BWR 1 1 x 10-6 5.4 x 106 5.4
BWR 2 6 x 10-6 7.1 x 106 42.6
BWR 3 2 x 10-5 5.1 x 106 102.0
BWR 4 2 x 10-6 6.1 x 105 1.2
BWR 5 1 x 10-4 6.1 x 105 0.002
PWR 1 9 x 10-7 4.9 x 106 4.4
PWR 2 8 x 10-6 4.8 x 106 38.4
PWR 3 4 x 10-6 5.4 x 106 21.6
PWR 4 5 x 10-7 2.7 x 106 1.4
PWR 5 7 x 10-7 1.0 x 106 0.7
PWR 6 6 x 10-6 1.4 x 105 0.8
PWR 7 4 x 10-5 2.3 x 103 0.1
PWR 8 4 x 10-5 7.5 x 104 3.0
PWR 9 4 x 10-4 1.2 x 102 0.05

For BWRs: (P x Q) = 151.2 man-rem/RY

For PWRs: (P x Q) = 70.5 man-rem/RY

Consequence Estimate

During any type of accident, releases contained in a plant's air would be transported through the ventilating system to the air-cleaning system before discharge offsite. Credit will be taken for all releases associated with WASH-140016 type of accidents in deriving the consequence estimate for this issue. The public dose associated with these accidents is shown in Table 3.1-1.

For 95 PWRs in operation and under construction with an average remaining life of 30 years, the risk reduction is (95)(0.5)(10-3)(70.5)(30) man-rem or approximately 100 man-rem. For 48 BWRs in operation and under construction with an average remaining life of 30 years, the risk reduction is (48)(0.5)(10-3) x (151.2)(30) man-rem or approximately 109 man-rem. Therefore, the total risk reduction associated with this issue is (100 + 109) man-rem or 209 man-rem.

Cost Estimate

NRC Cost: It is estimated that 2 staff-years have been expended by the NRC so far in collecting and reviewing data relative to this issue. The possible solution used in this analysis would require an additional 0.5 staff-year to close out the issue. Based on a cost of $100,000/staff-year, the total estimated future NRC cost is $50,000 or $0.05M.

Industry Cost: An improved maintenance and testing program for air-monitoring, air-cleaning, and ventilating systems will require an estimated two additional technicians at each plant. It is assumed that the average remaining life of each operating plant is 30 years. Based on labor costs of $50,000/year for each technician, the total estimated industry cost per plant is $(30 x 2 x 50,000)M or $3M. Therefore, the total cost for the possible solution in 95 PWRs and 48 BWRs is $[0.05 + (143)(3)]M or $429M.

Value/Impact Assessment

Based on a public risk reduction of 209 man-rem, the value/impact score is given by,


Consideration of the following uncertainties will produce values of S that will not alter the conclusion reached on the issue.

(1) Assuming that the possible solution resulted in a reduction of all risk, the maximum attainable S value would be 1 man-rem/$M.

(2) Decreasing the consequence estimate would reduce the calculated S value.

(3) Increasing cost estimates by a factor of 10 would reduce the maximum value of S to 0.05 man-rem/$M. Conversely, decreasing cost estimates by a factor of 2 would increase S to a maximum value of 1 man-rem/$M.

Other Considerations

The safety significance of this issue is being addressed in several ongoing programs including:

(1) NUREG-0737,98 Item II.K.3(24): Confirmation of Adequacy of Space Cooling for HPCI and RCIC Systems

(2) Implementation of RETS under multiplant action A-02, a part of which is the evaluation of hydrogen control for waste gas systems.


Based upon the value/impact score above, this issue should be placed in the DROP category.


0004.NUREG-0572, "Review of Licensee Event Reports (1976–1978)," U.S. Nuclear Regulatory Commission, September 1979.
0016.WASH-1400 (NUREG-75/014), "Reactor Safety Study: An Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants," U.S. Atomic Energy Commission, October 1975.
0098.NUREG-0737, "Clarification of TMI Action Plan Requirements," U.S. Nuclear Regulatory Commission, November 1980, (Supplement 1) January 1983.