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Is Mark I shell failure really important? — Part one
Nuclear FJl~neeringand Design 121 (1990) 441-446 North-Holland
441
IS MARK I SHELL FAILURE REALLY IMPORTANT?. - PART O N E
Herschel S P E C T E R New York Power Authority, 123 Main Street, White Plains, New York 10601, USA
and
Peter B I E N I A R Z Risk Management Associates, 2309 Dietz Farm Road NW, Albuquerque, New Mexico 87107, USA Received 15 May 1989
This two part paper discusses Mark I shell failure from two perspective& In Part One a general overview of nuclear health consequem~= and risks is provided. Comparis~ between calculated risks and the NRC's safety goals are offered. Use is made of ps,emat PRA rmults to calculate risks as well as shnple~ techniques that do not rely on more real~tic calculations of source terms. These ~n'mlar teehniqucs for estimating nuclear risks also minimiT~the use of PRA methodoloS~. In Part Two the results of specific Mark I accident sequences is analyzed. Source term& azcid~t ~ containment pressure transisat plots, and a discussion of the relevance of this information to the Mark I shell failure isane is provided. Of particular interest is the mitigation potential of extended use of the automatic deprmsur~tion s y ~ m during station blackout conditions.
1. h m m l m i m
2. Overview of nadear lmwer [Ihmt risks
A principal concern for nuclear power plants with Mark I containments is the possibility that in a severe accident molten core material will penetrate the reactor vessel, spread across the drywell floor, and then attack the drywell shell. This high temperature attack might lead to shell failure and releases of radioactive material into the environment. It may be some time before additional experiments and further development of our analytical capabilities can describe the drywell shell failure process with much greater certainty. What safety actions should be taken in the meanwhile? That depends on how well we can answer the question: "Is M a r k I shell failure really important?" The above q u ~ t i o n will be addressed at bQ~ gener~ and specific levels in this two part paper. In the first part, a discussion is provided which gives an overview of nuclear power plant risks. In part two a set of Mark I analyses that bear on this specific issue is given.
In one fundamental respect we already have the answer to the above question. Even if Mark I shell failure is assumed to occur each time there is a core melt, nuclear health risks are expected to be quite small. Furthermore, this can be shown without reliance on smaller, but more realistic, source terms and with minimum use of PRA. Assuming a large, WASH-1400 type [1], release of radioactive material into the envirommmt, virtually no early fatalities would be expected, provided an efficient emergency response was taken. A highly efficient emergency response, described in a recent N U M A R C report [2], is the "Graded Response". This response would use a combination of prompt ~ a z u a t i o a of ~ two mile O,r ~ i n n ~ z o ~ ~ ~ p!aut and sheltering, as
appropriate, ~
in the ~
Phan~. Z 0 ~
(EPZ). Some downwind sheltered people might be relocated later if radiation m o n i t o r i ~ showed that they were in areas of u n a c c ~ t a b l y high dose rates (fig. 1).
0 0 2 9 - 5 4 9 3 / 9 0 / $ 0 3 . 5 0 © 1990 - Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d )
442
H. Specter, P. Bieniarz / Is Mark I shell failure really important? - Part 1
Table 1 EPRI analysis of Indian Point Impacts of different emergency responses PWR-2 release/six hours of exposure
EVACUATE[m
I
\
i0 MILES
Emergency response
Mean number of calculated early fatalities
Mean number of calculated early injuries
1. Normal activities
466.00
1550.00
22.50
465.00
0.22
126.00
2. All sheltering (0-10 Miles) 3. Graded response (10 mph evacuation of inner two mile zone beginning at time of release, sheltering downwind beyond two miles)
PHASE #1:
NOTIFYPUBLIC TO EVACUATEAN INNER ZONEOF ABOUT A Z-NILE RADIUSAND TO SHELTERIN THE REST OF THE IO-MELE EPZ, AS APPROPRIATE.
PHASE #2:
MONITORHADIATTON LEVELS.
PHASE #3:
NOTIFYSHELTEREDPEOPLEIN PLUMEPATHWAYTO RELOCATE IF REASUREDRADIATION LEVELSARE TOO HTEH.
Fig. 1. Graded response strategy.
Analyses by the N R C [3], D O E [4] and E P R I [5] show that if the Graded Response were fully utilized, essentially zero early fatalities would be expected, even at our most populated sites. The E P R I effort analyzed various emergency responses at the nation's most populated site, Indian Point, using a very large source term. Referring to table 1, a total lack of response (normal activities) for six hours after a severe release of radioactive material might be considered an extraordinary failure in the emergency response process. Even in this extreme situation, less than 0.2% of the approximately 270000 people within this EPZ might become early fatalities. This limited impact is largely due to the inherent risk reduction associated with plume dilution as the radioactive plume moves away from its point of release a n d the fact that only a portion of the EPZ would be downwind from the plant. N o t e that taking shelter provides a 20 fold reduction in the number of calculated early fatalities relative to normal activities. Still further improvements are obtained when the G r a d e d Response is used. Assuming a total, prompt evacuation at 10 mph of the inner 2 miles and 6 hours of sheltering in the two-to-ten mile outer
zone, another 100-fold reduction in the early fatality risk is observed. In summary, finite plume size and natural dilution effects limit the number of early fatalities to about 0;2% of the EPZ population. Graded Response can further reduce this percentage approximately 2000 fold, that is, to an effectively zero early fatality risk. Thus the N R C ' s early fatality safety goal * can be met at any site and with any L W R design, even assuming severe releases, by taking simple protective actions, i.e., prompt evacuation in the immediate area accompanied by sheltering downwind of the plant. This conclusion is reached independent of core melt or containment failure frequency, and without reliance on P R A ' s or more realistic source terms. At many sites there are very few people within two miles of the plant. Such sites are, in effect, " p r e evacuated" and have largely accomplished the first phase of a Graded Response. The graded response assumes total evacuation of the inner zone. Recent evaluations of actual evacuations indicate a 99.5% participation level. Much of the residual early fatality risk is associated with that small percentage of the population which does not take protective actions. By first concentrating the emergency resources on the inner zone in a G r a d e d Response, it is probable that the percentage of the public that does not participate would be further reduced. * Both the early and latent fatality safety goals are set at limiting nuclear power plant risks to one part in a thousand of corresponding background non,nuclear risks.
H. Specter, P. Bieniarz / Is Mark I shell failure really important? - Part I
A t this time the Graded Response has not been gene~lly implemented at U . S . n u c l e a r sites, although some emeagency plans utilize similar approaches. The Graded Response has been favorably reviewed by the ACRS and the N R C staff. In addition to the use of an efficient emergency response, there are a number of other ways in which early health effects can be minimized. Small reductions in the amount of radiation a person is exposed to would bring about large reductions in the mortality rate. Earlier studies show that, assuming minimal medical treatment, a 400 tad exposure results in an 80~$ chance of a mortality within 60 days of exposure. If the exposure is one half of this value, 200 rads, there is about a 0.5~ chance of causing an early fatality [6]. A two fold reduction in exposure results in about an 160 fold reduction in mortality rates. More recent studies do not display quite as strong a relationship between dose and mortality rate. Two principal ways in which radiation exposure can be minimized is by reducing the dose rate and by reducing the time of exposure. Dose rate itself is largely driven by the concentration of the radionuclides in the vicinity of the receiver. If the receiver is at some distance from the point of release, then the diffusion of the radioactive plume through the atmosphere will rapidly decrease radioactive concentrations. The normal diffusion process, coupled with haman sensitivity to small reductions in exposure, is the fundamental reason why the early fatality risk is geographically very short ranged. If the radioactive plume is buoyant, concentrations within a few miles of the plant will also be reduced, as will early health effects. Many well known consequence analyses, e.g., N U R E G / C R - 2 2 3 9 [7] (generally referred to as the Sandia Sitting Report) assumed zero plume internal energy upon release from the containment, thereby overstating nearby concentrations and the calculated early health consequences. More recent analyses [8] account for plume buoyancy. Accounting for buoyancy is especially important in early containment failure accident sequences. Such sequences minimize the time available for source term reduction and offsite protective actions. However, a characteristic of many early release scenarios is that they are very energetic, e.g., they contain enough energy to overpressurize the containment. Therefore they are quite likely to produce buoyant plumes. Chernobyl was an extreme example of plume buoyancy. No offsite radiation induced early fatalities or injuries were reported at Ch~nobyi. A third mechanism that affects the concentration of radionuclides in the plume is source term reduction.
443
Many natural chemical and physical removal processes rapidly reduce the amount of ~ radioactive material in the containment. Studies conducted by Kaiser [9] show that once the average core release fraction (of iodine, cesium and tellurium) falls below - 0.1, the conditional mean number of early fatalities is very small or zero. Based on earlier source terms studies in large, dry PWR containments, very low concentrations of airborne radionuclides occur if containment integrity can be maintained for about 8 to 10 hours after core. melt. It is expected that all containments types will limit releases to below the - 0.1 early fatality threshold, should post core melt containment integrity be maintained for about 10 hours. Other mechanisms exist that also can lower early health effects. If the duration of the plume release from the containment is long, then wind shifts are likely to occur and average downwind exposures will be lower. Even though a larger area would be affected, lower total early fatality consequences are usually expected. Various past consequence analyses have assumed rapid or "puff type" releases, rather than prolonged releases. "Puff" releases would be more justified with early containment failure sequences, but these are likely to be buoyant. If a radioactive plume traverses over a populated area, it may experience a "thermal island" effect. Such plume heating from the populated area itself can cause reductions in the plume concentration. The heat that rises from city areas may result in shifting plume stability one or more categories [10]. For example, consider a highly concentrated plume identified as a Pasquill category F. A shift from Pasquill category F to category D at a point 3 km downwind from the plant would increase the plume cross section area by about a factor of 13, with a corresponding reduction in concentration. If a narrow, highly concentrated plume should strike a tall building, its concentration would be lowered. The calculated full width of a Pasquill category F plume is only 3 m, even 3 km from the release point. The 3 m width is narrower than the sides of most buildin~. The effects of thermal islands and tall buildings are not accounted for in present consequence analyses. Concentrations can also increase. The ground concentration of radioactive material will be higher if the plume passes through an area where there is rain. This can result in local hot spots. These can be readily identified by radiation surveys, and people can be relocated if necessary. Thus higher dose rates can be offset by shorter exposure times. Another area that significantly affects the calculated number of early fatalities is the assumed medical treat-
444
H. Specter, P. Bieniarz / Is Mark 1 shell failure really important? - Part 1
ment that people might receive after exposure to radiation. People given supportive medical treatment have an improved chance of survival compared to those receiving minimal medical treatment and heroic treatment should be even more effective [6]. Extensive medical capabilities exist throughout the country to treat radiation exposure. Many consequence analyses assume minimal medical treatment. Many of the above comments address reasons why the offsite early fatality consequences are likely to be at or near zero, if calculated more realistically. Such low consequence values virtually assure that the early fatality safety goal will be met. For example, consider two highly populated sites, Indian Point and Shoreham, postulate a severe release, and assume that a Graded Response was used. All the conservatisms of large source terms, no buoyancy, " p u f f release", minimal medical treatment, no thermal islands, etc. are to be assumed. Using EPRI and N R C analyses and further assuming that all the early fatalities appeared in the inner two miles, one can construct the following chart: site
Indian Point Shoreham
0 - 2 mile population
mean number of fatalities
fatalities /population
15000 10500
0.22 0.0012
146 x 10 -7 1.14 x 10 -.7
In order to exceed the individual early fatality safety goal of 5 × 1 0 - 7 / R y the frequency of severe releases would have to be: Indian Point: 3.4 x 1 0 - 2 / R y Shoreham : 4 . 4 / R Y The over 1200 reactor years of U.S. operating experience without a severe release to the environment suggests that the above release frequencies are far in excess of actual experience. PRA results also indicate that severe release frequencies would be well below such values. Lastly, it is useful to compare the calculated early fatality risk values to the early fatality safety goal where PRA techniques have been used. Although a complete comparison awaits the results from the IPE process, insights can be gained from the N R C ' s recent assessment of five U.S. nuclear power plants. All five plants showed wide margins below the early fatality goals [8, Vol. 1, Fig. 12.6]. Of particular interest was the Zion analyses, since this is a highly populated site. In this analysis the source terms, containment failure frequency,
and plume buoyancy were treated more realistically. However, the Zion analysis assumed a five mile radius inner evacuation zone with a resultant slow evacuation speed of 2.5 mph (slower than typical walking speeds). Evacuation time estimates of actual high population density sites show, not surprisingly, that as the size of the evacuation radius increases, the average evacuation speed decreases. It is anticipated that if the Zion analy~ sis utilized a smaller inner evacuation zone, higher evacuation speed combination - i.e. more like the Graded Response - calculated risks would be even lower. Nonetheless, even though a less than optimal emergency response was assumed for this highly populated site, results were well below the early fatality safety goal. In summary, advances in our understanding of phenomena that affect plume concentrations (e.g. source terms, buoyancy, plume release duration) serve to lower calculated dose rates while more efficient emergency responses can lower the time of exposure. Both effects lower the calculated early fatality risk, which has repeatedly been shown to be below the N RC's early fatality safety goal when such effects have been accounted for. All plants and sites are expected to meet the NRC's individual latent fatality safety goal, again assuming severe releases of radioactive material. The latent fatality goal, as presently formatted,is very conservative. Comparisons of nuclear plant latent risks are made against non-nuclear latent risks within 10 miles of the plant. Had the former N R C comparison basis of 50 miles been used, margins would be wider by about another order of magnitude. Even comparisons based on a 50 mile radius may be too short for many sites. For example, assuming a very severe release and a uniform population distribution, it takes about 150 miles to "capture" about half of the number of projected latent fatalities. One can argue that if comparisons are to be made between nuclear and non-nuclear health effects, these comparisons should be made over a sufficiency large area wherein the bulk of these effects would be manifested. The present 10 mile radius only represents an area in which a few percent on the latents would be projected to occur. Since the ratio of nuclear to non-. nuclear latent fatalities decreases as the area over which this comparison is made increases, the present format is quite conservative. In spite of the conservative way in which the latent safety goal is formatted, all published comparisons that we are aware of show wide margins between the calculated median latent fatality risk and this safety goal. For example, fig. 2 [11] shows N R C results of 16 PRA's;
H. Specter, P. Bieniarz / Is Mark I shell failure really important? - Part l
This figure, which was published several years ago, does not reflect reductions in the calculated latent cancer fatality risk that would result from accounting for numerous improvements made since the TMI-2 accident or from the use of more realistic source terms. More recently, the five plants analyzed by the N R C had very large margins below the latent fatality goal [8, Vol. 1, Fig. 12.6]. All median values were below 1 0 - S / R e a c tor Year for internal initiators. As shown by the TMI accident and also shown by analyses and experiments, not all core damage accidents lead to reactor vessel failure. Additionally, during the time period since WASH-1400 publication, analyses and experiments have shown that containment pressure retention capabilities far exceed the original design basis accident pressure. Failure pressures are in the range of 2.5 to 4 times larger than design basis accident pressures. This means that many reactor vessel failure accidents would be incapable of producing containment overpressure conditions or would only do so after long periods of time. For example, earlier studies of plants with large, dry containments showed that only about one core melt/reactor vessel failure accident in ten might eventually lead to containment overpressure. Latent fatality risks decrease as the containment failure frequency decreases. Even those accidents which might lead to a slow containment overpressure may not be risk significant; they would likely have very small source terms. This last observation is also especially relevant to the early fatality risk. Extreme external events may
retard offsite responses while also causi,~ core damage. Should such events predominantly lead to slow overpressurization sequences, significant time would be available to take offsite preventative actions and the source terms would likely be below the - 0 . 1 release fraction early fatality threshold. One can also make some general observations about latent fatality risks without relying on reduced source terms, and with minimum use of PRA techniques. Testimony provided by the N R C in connection with the Indian Point hearing showed that even at the nation's most populated site, the latent fatality safety goal would be met with WASH-1400 type source terms, even if there were no containment [12]. One would need a severe containment release frequency greater than about 1 0 - 3 / R y tO possibly exceed0tis goal. Based on operating experience, there is considerable confidence that the U.S. nuclear national average severe release frequency is well below 1 x 1 0 - 3 / R y . For example, Minarick [13] calculates core damage frequencies in the range of 3 to 6.7 x 1 0 - 5 / R y for nuclear plants which have more than one year of commercial operation. These core damage figures are based upon reviews and analyses of actual operating experiences. Use of precursor data to estimate core melt frequencies l:nlnirnizes the PRA 1301"tion of such analyses. Since only a fraction of the core damage events might lead to releases from the containment, the national average severe containment release frequency would be below Minarick's core damage values and clearly well below 1 × 1 0 - 3 / R y .
I ! I I
I%'RC 8 A F E T Y G O A L 4--- [10 NILE RADIUS]
x~ It
v !
K
~t I x
i I !
I
W W
i I i
Z, g
I I i
',
10-8/Ry
,
445
I
10-6/Ry
"
I
10-4/Ry
r
BELOW X O - 8 / R Y
Fig. 2, Latent cancer fatality risks ( x = median individual latent cancer fatality risk per reactor year).
446
1t. Specter, P. Bieniarz / ls Mark I shell failure really important? - Part 1
The consequences of actual large releases of radioactive material are also of interest. The most serious release of radioactive material from a nuclear power plant ever came from the accident at Chernobyl. Yet D O E [14] analyses indicate that over the next 70 years the increment in latent fatal cancers in the northern hemisphere due to Chernobyl would be 0.00004. Stated differently, where there might be 100000 latent cancer fatalities from non-nuclear causes, Chernobyl might raise it to 100004. More recent data indicate less exposure actually occurred so that this small calculated increment should be revised in a downward direction. There is considerable uncertainty in predicting the long term effects of low levels of exposure. However, as this D O E report points out, the possibility of zero health effects at very low doses and dose rates cannot be ruled out. Areas closer to Chernobyl are projected to experience higher incremental latent fatality rates than the northern hemisphere value. In European U S S R the increment over naturally occurring cancer rates is projected to be 0.0012, and for the 116000 or so evacuees, 0.024. It should be recognized that there is an upper limit on the incremental latent cancer rate that even the most exposed individual could obtain. This is because as radiation exposure increases over 200 rads, the individual's fatality risks quickly shift from latent to early unless more extensive medical assistance is provided. Using risk coefficients in the range of 1.4 to 2.3 x 10 -4 fatal cancers per rad, an individual exposed to 200 rads would have a cancer risk heightened by 2.8 to 4.6%. This increment would be added to normal cancer risks which are in the 12 to 18% range. Actions taken to minimize the early fatality risk, such as using a Graded Response, would most likely prevent any individual from incurring a 200 rad exposure. A m o n g the Chernobyl evacuees the average exposure was about 12 rads will some subgroups in the 35 to 54 rad range, Therefore our starting point is the recognition that both health consequences and risks from all U.S. L W R nuclear power plants are quite small and that the risks are expected to be well within N R C safety goals. This includes all Mark I plants, regardless of drywell shell failure concerns. Since overall risks are small, the risks from drywell shell failure are also small. Even though overall health risks from nuclear power plants have repeatedly been shown to be very small, there may be practical ways of further reducing the
residual risk. Of particular interest is the use of present plant equipment with little or no modification. The safety philosophy employed here for severe accidents is similar to the A L A R A (As Low As Reasonably Achievable) principle. It is desirable to reduce accident frequencies a n d / o r mitigate releases of radioactive material, even though risks are low, provided that such improvements are reasonably achievable. Implementing this requires a specific analysis for each nuclear power plant, e.g. an IPE, to identify risk outliers and possible "success paths" to reduce their importance. In Part 2 of this paper, information is provided on practical ways which could help reduce risks associated with drywell shell failure.
References [1] USNRC, Reactor Safety Study, PWR-1,2 and BWR-1,2 source terms, Table 5-1, WASH-1400 (1975). [2] NUMARC, Graded response: the preferred emergency strategy for nuclear power plants, Nuclear Management and Resource Council, NUMARC/NESP-005 (February, 1989). [3] Martin, J.A. A perspective on emergency planning, risk, and the source term issue, USNRC, April 13, 1983 (internal memo). [4] USDOE, Issues, information needs and programs for improved emergency preparedness, DOE Working Group on Emergency Preparedness (November 1984, draft version). [5] EPRI, Risk based evaluation of emergency response planning, Nuclear Safety Analysis Center, Electric Power Research Institute, NSAC 115 (November, 1988). [6] USNRC, Reactor Safety Study, Fig. VI 9-1, WASH-1400 (1975). [7] USNRC, Technical guidance for siting criteria development, NUREG/CR-2239 (December 1982). [8] USNRC, Severe accident risks: an assessment for five U.S. nuclear power plants, NUREG-1150 (June, 1989). [9] Kaiser, G.D. Implications of reduced source terms for ex-plant consequences modeling and emergency planning, Nuclear Safety 27, No. 3 (July-September, 1986). [10] Martin, J.A. Personal Communication, USNRC. [11] USNRC, Safety Goal Evaluation Report, Vol. 2, Fig. 4.3. USNRC (April 1985) (Adaptation). [12] NRC Staff presentation before the USNRC Commissioners, September 5, 1984. [13] Minarick, J.W. The U.S. NRC Accident Sequence Precursor Program: Present Methods and Findings" (to appear). [14] USDOE, Health and environmental consequences of the Chernobyl nuclear power plant accident, DOE/ER-0332 (June, 1987).
441
IS MARK I SHELL FAILURE REALLY IMPORTANT?. - PART O N E
Herschel S P E C T E R New York Power Authority, 123 Main Street, White Plains, New York 10601, USA
and
Peter B I E N I A R Z Risk Management Associates, 2309 Dietz Farm Road NW, Albuquerque, New Mexico 87107, USA Received 15 May 1989
This two part paper discusses Mark I shell failure from two perspective& In Part One a general overview of nuclear health consequem~= and risks is provided. Comparis~ between calculated risks and the NRC's safety goals are offered. Use is made of ps,emat PRA rmults to calculate risks as well as shnple~ techniques that do not rely on more real~tic calculations of source terms. These ~n'mlar teehniqucs for estimating nuclear risks also minimiT~the use of PRA methodoloS~. In Part Two the results of specific Mark I accident sequences is analyzed. Source term& azcid~t ~ containment pressure transisat plots, and a discussion of the relevance of this information to the Mark I shell failure isane is provided. Of particular interest is the mitigation potential of extended use of the automatic deprmsur~tion s y ~ m during station blackout conditions.
1. h m m l m i m
2. Overview of nadear lmwer [Ihmt risks
A principal concern for nuclear power plants with Mark I containments is the possibility that in a severe accident molten core material will penetrate the reactor vessel, spread across the drywell floor, and then attack the drywell shell. This high temperature attack might lead to shell failure and releases of radioactive material into the environment. It may be some time before additional experiments and further development of our analytical capabilities can describe the drywell shell failure process with much greater certainty. What safety actions should be taken in the meanwhile? That depends on how well we can answer the question: "Is M a r k I shell failure really important?" The above q u ~ t i o n will be addressed at bQ~ gener~ and specific levels in this two part paper. In the first part, a discussion is provided which gives an overview of nuclear power plant risks. In part two a set of Mark I analyses that bear on this specific issue is given.
In one fundamental respect we already have the answer to the above question. Even if Mark I shell failure is assumed to occur each time there is a core melt, nuclear health risks are expected to be quite small. Furthermore, this can be shown without reliance on smaller, but more realistic, source terms and with minimum use of PRA. Assuming a large, WASH-1400 type [1], release of radioactive material into the envirommmt, virtually no early fatalities would be expected, provided an efficient emergency response was taken. A highly efficient emergency response, described in a recent N U M A R C report [2], is the "Graded Response". This response would use a combination of prompt ~ a z u a t i o a of ~ two mile O,r ~ i n n ~ z o ~ ~ ~ p!aut and sheltering, as
appropriate, ~
in the ~
Phan~. Z 0 ~
(EPZ). Some downwind sheltered people might be relocated later if radiation m o n i t o r i ~ showed that they were in areas of u n a c c ~ t a b l y high dose rates (fig. 1).
0 0 2 9 - 5 4 9 3 / 9 0 / $ 0 3 . 5 0 © 1990 - Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d )
442
H. Specter, P. Bieniarz / Is Mark I shell failure really important? - Part 1
Table 1 EPRI analysis of Indian Point Impacts of different emergency responses PWR-2 release/six hours of exposure
EVACUATE[m
I
\
i0 MILES
Emergency response
Mean number of calculated early fatalities
Mean number of calculated early injuries
1. Normal activities
466.00
1550.00
22.50
465.00
0.22
126.00
2. All sheltering (0-10 Miles) 3. Graded response (10 mph evacuation of inner two mile zone beginning at time of release, sheltering downwind beyond two miles)
PHASE #1:
NOTIFYPUBLIC TO EVACUATEAN INNER ZONEOF ABOUT A Z-NILE RADIUSAND TO SHELTERIN THE REST OF THE IO-MELE EPZ, AS APPROPRIATE.
PHASE #2:
MONITORHADIATTON LEVELS.
PHASE #3:
NOTIFYSHELTEREDPEOPLEIN PLUMEPATHWAYTO RELOCATE IF REASUREDRADIATION LEVELSARE TOO HTEH.
Fig. 1. Graded response strategy.
Analyses by the N R C [3], D O E [4] and E P R I [5] show that if the Graded Response were fully utilized, essentially zero early fatalities would be expected, even at our most populated sites. The E P R I effort analyzed various emergency responses at the nation's most populated site, Indian Point, using a very large source term. Referring to table 1, a total lack of response (normal activities) for six hours after a severe release of radioactive material might be considered an extraordinary failure in the emergency response process. Even in this extreme situation, less than 0.2% of the approximately 270000 people within this EPZ might become early fatalities. This limited impact is largely due to the inherent risk reduction associated with plume dilution as the radioactive plume moves away from its point of release a n d the fact that only a portion of the EPZ would be downwind from the plant. N o t e that taking shelter provides a 20 fold reduction in the number of calculated early fatalities relative to normal activities. Still further improvements are obtained when the G r a d e d Response is used. Assuming a total, prompt evacuation at 10 mph of the inner 2 miles and 6 hours of sheltering in the two-to-ten mile outer
zone, another 100-fold reduction in the early fatality risk is observed. In summary, finite plume size and natural dilution effects limit the number of early fatalities to about 0;2% of the EPZ population. Graded Response can further reduce this percentage approximately 2000 fold, that is, to an effectively zero early fatality risk. Thus the N R C ' s early fatality safety goal * can be met at any site and with any L W R design, even assuming severe releases, by taking simple protective actions, i.e., prompt evacuation in the immediate area accompanied by sheltering downwind of the plant. This conclusion is reached independent of core melt or containment failure frequency, and without reliance on P R A ' s or more realistic source terms. At many sites there are very few people within two miles of the plant. Such sites are, in effect, " p r e evacuated" and have largely accomplished the first phase of a Graded Response. The graded response assumes total evacuation of the inner zone. Recent evaluations of actual evacuations indicate a 99.5% participation level. Much of the residual early fatality risk is associated with that small percentage of the population which does not take protective actions. By first concentrating the emergency resources on the inner zone in a G r a d e d Response, it is probable that the percentage of the public that does not participate would be further reduced. * Both the early and latent fatality safety goals are set at limiting nuclear power plant risks to one part in a thousand of corresponding background non,nuclear risks.
H. Specter, P. Bieniarz / Is Mark I shell failure really important? - Part I
A t this time the Graded Response has not been gene~lly implemented at U . S . n u c l e a r sites, although some emeagency plans utilize similar approaches. The Graded Response has been favorably reviewed by the ACRS and the N R C staff. In addition to the use of an efficient emergency response, there are a number of other ways in which early health effects can be minimized. Small reductions in the amount of radiation a person is exposed to would bring about large reductions in the mortality rate. Earlier studies show that, assuming minimal medical treatment, a 400 tad exposure results in an 80~$ chance of a mortality within 60 days of exposure. If the exposure is one half of this value, 200 rads, there is about a 0.5~ chance of causing an early fatality [6]. A two fold reduction in exposure results in about an 160 fold reduction in mortality rates. More recent studies do not display quite as strong a relationship between dose and mortality rate. Two principal ways in which radiation exposure can be minimized is by reducing the dose rate and by reducing the time of exposure. Dose rate itself is largely driven by the concentration of the radionuclides in the vicinity of the receiver. If the receiver is at some distance from the point of release, then the diffusion of the radioactive plume through the atmosphere will rapidly decrease radioactive concentrations. The normal diffusion process, coupled with haman sensitivity to small reductions in exposure, is the fundamental reason why the early fatality risk is geographically very short ranged. If the radioactive plume is buoyant, concentrations within a few miles of the plant will also be reduced, as will early health effects. Many well known consequence analyses, e.g., N U R E G / C R - 2 2 3 9 [7] (generally referred to as the Sandia Sitting Report) assumed zero plume internal energy upon release from the containment, thereby overstating nearby concentrations and the calculated early health consequences. More recent analyses [8] account for plume buoyancy. Accounting for buoyancy is especially important in early containment failure accident sequences. Such sequences minimize the time available for source term reduction and offsite protective actions. However, a characteristic of many early release scenarios is that they are very energetic, e.g., they contain enough energy to overpressurize the containment. Therefore they are quite likely to produce buoyant plumes. Chernobyl was an extreme example of plume buoyancy. No offsite radiation induced early fatalities or injuries were reported at Ch~nobyi. A third mechanism that affects the concentration of radionuclides in the plume is source term reduction.
443
Many natural chemical and physical removal processes rapidly reduce the amount of ~ radioactive material in the containment. Studies conducted by Kaiser [9] show that once the average core release fraction (of iodine, cesium and tellurium) falls below - 0.1, the conditional mean number of early fatalities is very small or zero. Based on earlier source terms studies in large, dry PWR containments, very low concentrations of airborne radionuclides occur if containment integrity can be maintained for about 8 to 10 hours after core. melt. It is expected that all containments types will limit releases to below the - 0.1 early fatality threshold, should post core melt containment integrity be maintained for about 10 hours. Other mechanisms exist that also can lower early health effects. If the duration of the plume release from the containment is long, then wind shifts are likely to occur and average downwind exposures will be lower. Even though a larger area would be affected, lower total early fatality consequences are usually expected. Various past consequence analyses have assumed rapid or "puff type" releases, rather than prolonged releases. "Puff" releases would be more justified with early containment failure sequences, but these are likely to be buoyant. If a radioactive plume traverses over a populated area, it may experience a "thermal island" effect. Such plume heating from the populated area itself can cause reductions in the plume concentration. The heat that rises from city areas may result in shifting plume stability one or more categories [10]. For example, consider a highly concentrated plume identified as a Pasquill category F. A shift from Pasquill category F to category D at a point 3 km downwind from the plant would increase the plume cross section area by about a factor of 13, with a corresponding reduction in concentration. If a narrow, highly concentrated plume should strike a tall building, its concentration would be lowered. The calculated full width of a Pasquill category F plume is only 3 m, even 3 km from the release point. The 3 m width is narrower than the sides of most buildin~. The effects of thermal islands and tall buildings are not accounted for in present consequence analyses. Concentrations can also increase. The ground concentration of radioactive material will be higher if the plume passes through an area where there is rain. This can result in local hot spots. These can be readily identified by radiation surveys, and people can be relocated if necessary. Thus higher dose rates can be offset by shorter exposure times. Another area that significantly affects the calculated number of early fatalities is the assumed medical treat-
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H. Specter, P. Bieniarz / Is Mark 1 shell failure really important? - Part 1
ment that people might receive after exposure to radiation. People given supportive medical treatment have an improved chance of survival compared to those receiving minimal medical treatment and heroic treatment should be even more effective [6]. Extensive medical capabilities exist throughout the country to treat radiation exposure. Many consequence analyses assume minimal medical treatment. Many of the above comments address reasons why the offsite early fatality consequences are likely to be at or near zero, if calculated more realistically. Such low consequence values virtually assure that the early fatality safety goal will be met. For example, consider two highly populated sites, Indian Point and Shoreham, postulate a severe release, and assume that a Graded Response was used. All the conservatisms of large source terms, no buoyancy, " p u f f release", minimal medical treatment, no thermal islands, etc. are to be assumed. Using EPRI and N R C analyses and further assuming that all the early fatalities appeared in the inner two miles, one can construct the following chart: site
Indian Point Shoreham
0 - 2 mile population
mean number of fatalities
fatalities /population
15000 10500
0.22 0.0012
146 x 10 -7 1.14 x 10 -.7
In order to exceed the individual early fatality safety goal of 5 × 1 0 - 7 / R y the frequency of severe releases would have to be: Indian Point: 3.4 x 1 0 - 2 / R y Shoreham : 4 . 4 / R Y The over 1200 reactor years of U.S. operating experience without a severe release to the environment suggests that the above release frequencies are far in excess of actual experience. PRA results also indicate that severe release frequencies would be well below such values. Lastly, it is useful to compare the calculated early fatality risk values to the early fatality safety goal where PRA techniques have been used. Although a complete comparison awaits the results from the IPE process, insights can be gained from the N R C ' s recent assessment of five U.S. nuclear power plants. All five plants showed wide margins below the early fatality goals [8, Vol. 1, Fig. 12.6]. Of particular interest was the Zion analyses, since this is a highly populated site. In this analysis the source terms, containment failure frequency,
and plume buoyancy were treated more realistically. However, the Zion analysis assumed a five mile radius inner evacuation zone with a resultant slow evacuation speed of 2.5 mph (slower than typical walking speeds). Evacuation time estimates of actual high population density sites show, not surprisingly, that as the size of the evacuation radius increases, the average evacuation speed decreases. It is anticipated that if the Zion analy~ sis utilized a smaller inner evacuation zone, higher evacuation speed combination - i.e. more like the Graded Response - calculated risks would be even lower. Nonetheless, even though a less than optimal emergency response was assumed for this highly populated site, results were well below the early fatality safety goal. In summary, advances in our understanding of phenomena that affect plume concentrations (e.g. source terms, buoyancy, plume release duration) serve to lower calculated dose rates while more efficient emergency responses can lower the time of exposure. Both effects lower the calculated early fatality risk, which has repeatedly been shown to be below the N RC's early fatality safety goal when such effects have been accounted for. All plants and sites are expected to meet the NRC's individual latent fatality safety goal, again assuming severe releases of radioactive material. The latent fatality goal, as presently formatted,is very conservative. Comparisons of nuclear plant latent risks are made against non-nuclear latent risks within 10 miles of the plant. Had the former N R C comparison basis of 50 miles been used, margins would be wider by about another order of magnitude. Even comparisons based on a 50 mile radius may be too short for many sites. For example, assuming a very severe release and a uniform population distribution, it takes about 150 miles to "capture" about half of the number of projected latent fatalities. One can argue that if comparisons are to be made between nuclear and non-nuclear health effects, these comparisons should be made over a sufficiency large area wherein the bulk of these effects would be manifested. The present 10 mile radius only represents an area in which a few percent on the latents would be projected to occur. Since the ratio of nuclear to non-. nuclear latent fatalities decreases as the area over which this comparison is made increases, the present format is quite conservative. In spite of the conservative way in which the latent safety goal is formatted, all published comparisons that we are aware of show wide margins between the calculated median latent fatality risk and this safety goal. For example, fig. 2 [11] shows N R C results of 16 PRA's;
H. Specter, P. Bieniarz / Is Mark I shell failure really important? - Part l
This figure, which was published several years ago, does not reflect reductions in the calculated latent cancer fatality risk that would result from accounting for numerous improvements made since the TMI-2 accident or from the use of more realistic source terms. More recently, the five plants analyzed by the N R C had very large margins below the latent fatality goal [8, Vol. 1, Fig. 12.6]. All median values were below 1 0 - S / R e a c tor Year for internal initiators. As shown by the TMI accident and also shown by analyses and experiments, not all core damage accidents lead to reactor vessel failure. Additionally, during the time period since WASH-1400 publication, analyses and experiments have shown that containment pressure retention capabilities far exceed the original design basis accident pressure. Failure pressures are in the range of 2.5 to 4 times larger than design basis accident pressures. This means that many reactor vessel failure accidents would be incapable of producing containment overpressure conditions or would only do so after long periods of time. For example, earlier studies of plants with large, dry containments showed that only about one core melt/reactor vessel failure accident in ten might eventually lead to containment overpressure. Latent fatality risks decrease as the containment failure frequency decreases. Even those accidents which might lead to a slow containment overpressure may not be risk significant; they would likely have very small source terms. This last observation is also especially relevant to the early fatality risk. Extreme external events may
retard offsite responses while also causi,~ core damage. Should such events predominantly lead to slow overpressurization sequences, significant time would be available to take offsite preventative actions and the source terms would likely be below the - 0 . 1 release fraction early fatality threshold. One can also make some general observations about latent fatality risks without relying on reduced source terms, and with minimum use of PRA techniques. Testimony provided by the N R C in connection with the Indian Point hearing showed that even at the nation's most populated site, the latent fatality safety goal would be met with WASH-1400 type source terms, even if there were no containment [12]. One would need a severe containment release frequency greater than about 1 0 - 3 / R y tO possibly exceed0tis goal. Based on operating experience, there is considerable confidence that the U.S. nuclear national average severe release frequency is well below 1 x 1 0 - 3 / R y . For example, Minarick [13] calculates core damage frequencies in the range of 3 to 6.7 x 1 0 - 5 / R y for nuclear plants which have more than one year of commercial operation. These core damage figures are based upon reviews and analyses of actual operating experiences. Use of precursor data to estimate core melt frequencies l:nlnirnizes the PRA 1301"tion of such analyses. Since only a fraction of the core damage events might lead to releases from the containment, the national average severe containment release frequency would be below Minarick's core damage values and clearly well below 1 × 1 0 - 3 / R y .
I ! I I
I%'RC 8 A F E T Y G O A L 4--- [10 NILE RADIUS]
x~ It
v !
K
~t I x
i I !
I
W W
i I i
Z, g
I I i
',
10-8/Ry
,
445
I
10-6/Ry
"
I
10-4/Ry
r
BELOW X O - 8 / R Y
Fig. 2, Latent cancer fatality risks ( x = median individual latent cancer fatality risk per reactor year).
446
1t. Specter, P. Bieniarz / ls Mark I shell failure really important? - Part 1
The consequences of actual large releases of radioactive material are also of interest. The most serious release of radioactive material from a nuclear power plant ever came from the accident at Chernobyl. Yet D O E [14] analyses indicate that over the next 70 years the increment in latent fatal cancers in the northern hemisphere due to Chernobyl would be 0.00004. Stated differently, where there might be 100000 latent cancer fatalities from non-nuclear causes, Chernobyl might raise it to 100004. More recent data indicate less exposure actually occurred so that this small calculated increment should be revised in a downward direction. There is considerable uncertainty in predicting the long term effects of low levels of exposure. However, as this D O E report points out, the possibility of zero health effects at very low doses and dose rates cannot be ruled out. Areas closer to Chernobyl are projected to experience higher incremental latent fatality rates than the northern hemisphere value. In European U S S R the increment over naturally occurring cancer rates is projected to be 0.0012, and for the 116000 or so evacuees, 0.024. It should be recognized that there is an upper limit on the incremental latent cancer rate that even the most exposed individual could obtain. This is because as radiation exposure increases over 200 rads, the individual's fatality risks quickly shift from latent to early unless more extensive medical assistance is provided. Using risk coefficients in the range of 1.4 to 2.3 x 10 -4 fatal cancers per rad, an individual exposed to 200 rads would have a cancer risk heightened by 2.8 to 4.6%. This increment would be added to normal cancer risks which are in the 12 to 18% range. Actions taken to minimize the early fatality risk, such as using a Graded Response, would most likely prevent any individual from incurring a 200 rad exposure. A m o n g the Chernobyl evacuees the average exposure was about 12 rads will some subgroups in the 35 to 54 rad range, Therefore our starting point is the recognition that both health consequences and risks from all U.S. L W R nuclear power plants are quite small and that the risks are expected to be well within N R C safety goals. This includes all Mark I plants, regardless of drywell shell failure concerns. Since overall risks are small, the risks from drywell shell failure are also small. Even though overall health risks from nuclear power plants have repeatedly been shown to be very small, there may be practical ways of further reducing the
residual risk. Of particular interest is the use of present plant equipment with little or no modification. The safety philosophy employed here for severe accidents is similar to the A L A R A (As Low As Reasonably Achievable) principle. It is desirable to reduce accident frequencies a n d / o r mitigate releases of radioactive material, even though risks are low, provided that such improvements are reasonably achievable. Implementing this requires a specific analysis for each nuclear power plant, e.g. an IPE, to identify risk outliers and possible "success paths" to reduce their importance. In Part 2 of this paper, information is provided on practical ways which could help reduce risks associated with drywell shell failure.
References [1] USNRC, Reactor Safety Study, PWR-1,2 and BWR-1,2 source terms, Table 5-1, WASH-1400 (1975). [2] NUMARC, Graded response: the preferred emergency strategy for nuclear power plants, Nuclear Management and Resource Council, NUMARC/NESP-005 (February, 1989). [3] Martin, J.A. A perspective on emergency planning, risk, and the source term issue, USNRC, April 13, 1983 (internal memo). [4] USDOE, Issues, information needs and programs for improved emergency preparedness, DOE Working Group on Emergency Preparedness (November 1984, draft version). [5] EPRI, Risk based evaluation of emergency response planning, Nuclear Safety Analysis Center, Electric Power Research Institute, NSAC 115 (November, 1988). [6] USNRC, Reactor Safety Study, Fig. VI 9-1, WASH-1400 (1975). [7] USNRC, Technical guidance for siting criteria development, NUREG/CR-2239 (December 1982). [8] USNRC, Severe accident risks: an assessment for five U.S. nuclear power plants, NUREG-1150 (June, 1989). [9] Kaiser, G.D. Implications of reduced source terms for ex-plant consequences modeling and emergency planning, Nuclear Safety 27, No. 3 (July-September, 1986). [10] Martin, J.A. Personal Communication, USNRC. [11] USNRC, Safety Goal Evaluation Report, Vol. 2, Fig. 4.3. USNRC (April 1985) (Adaptation). [12] NRC Staff presentation before the USNRC Commissioners, September 5, 1984. [13] Minarick, J.W. The U.S. NRC Accident Sequence Precursor Program: Present Methods and Findings" (to appear). [14] USDOE, Health and environmental consequences of the Chernobyl nuclear power plant accident, DOE/ER-0332 (June, 1987).