ML20149F626

Source Term Re-evaluation & Emergency Response

ML20149F626
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Issue date:	02/04/1988
From:	Bernero R NRC
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FOIA-87-743 NUDOCS 8802170288
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SOURCE TERM REEVALUATION AND EMERGENCY RESPONSE Robert M. Bernero, Director Division of Risk Analysis and Operations U.S. Nuclear Regulatory Comission i

For some time now reactor accident releases or source tenns have been studied and one area which depends on understanding them, emergency response, has been significantly affected. It wasn't too long ago,1978, that emergency preparedness around nuclear reactors was based on TID-14844 release models and included general plans for action in the Low Population Zone, a radius of just a few miles. Then, in 1978, the NRC published a reevaluation of accident releases which should serve as the basis for emergency planning, NUREG-0396. This reevaluation took as its basis the risk 6nalysis t

of the Reactor Safety Study, WASH-1400. This figure, taken directly from NUREG-0396, shows the reactor risk portrayed by WASH-1400. Given a core melt accident, there is a chance of exceeding Protective Action Guideline doses out to some appreciable distance from the reactor. There is a lower chance and a shorter j range for exceeding the dose threshold where clinical effects could be observed, 50 rem. And there is still a lower chance that life threatening doses, above 200 rem whole body, would be exceeded.

The range for life threatening doses is even shorter reaching out no more than about 10 miles.

b With thtt technical basis the post TMI emergency preparedness '

. changes were implemented. The NRC regulations adopted planning t.

l radii of 10 miles for the imersion pathway and 50 miles for the food chain pathway, along with a body of planning standards and the requirement for periodic exercise of the emergency response capabi-l lity. With the Federal Emergency Management Agency FEMA). NRC published in 1980 more detailed guidance on emergency (planning in NUREG-0654. The essential message of this guidance was to prepare a spectrum of responses for a spectrum of possible accidents. This figure, taken from NUREG-0654, illustrates the typical response envisioned, protective measures taken in a short radius circle around the plant and a longer radius downwind segment. The protec-tive actions chosen and the action zone dimensions would be based on accident circumstances.

i The implementation of these requirements and guidance has been generally successful at almost all reactor sites. There has been some controversy and there are some problems. Many do not treat the 10-mile zone as a preplanning zone but as an evacuation zone.

As a result, there is the misconception of constant risk out to 10

,iles and more beyond. If we are ever to have widespread under-standing and use of preplanning for a spectrum of responses, it would be useful to further examine our portrayal of reactor risk with ymore attention on parameters of interest to the emergency 8002170200 B00204 ,

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planner. As we noted before, the technical basis for emergency planning is the risk portrayal of the Reactor Safety Study. That study of two reactors has since been ;ioined by a derivative study, the Methodology Applications Program (RSSMAP), which was published in four volumes of NUREG/CR-1659 in 1981 and 19U. This study of four more plants of different design expanded the Reactor Safety Study base to six plants, all using the WASH-1400 source tenns '

deriving from similar models of fission product transiert and containment performance. These six plant studies were used to synthesize a set of Siting Source Terms (SST) which describe a simple spectrum of accidents for typical light water reactors using the WASH-1400 models (see NUREG-0773, published in 1982). The three Siting Source Terms of greatest interest are SST-1, an unmitigated core melt with early containment failure; SST-2, a par-tially mitigated core melt with some function of containment sys-tems; and SST-3, a well mitigated core melt with the containment holding. Most current risk assessments suggest that the majority of core melt accidents would fall in the SST-3 or SST-2 categories l

with only a low likelihood of the SST-1 outcome. -

Let me show you some interesting Time-Dose-Distance relation- ,

ships based on these WASH-1400 Siting Source Terms. These figures show the exposure time after release which is required at some distance from the plant to accenulate a certain whole body dose.

The doses displayed range from PAG 1evels to life-threatening levels. This first figure is for the most severe release SST-1, from a large reactor under bad weather conditions, Pasquill F I stability and 5 m.p.h. windspeed. Note that life-threatening doses l

can be accumulated very quickly in the first 2 or 3 miles an in a period of about 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> at a distance of 10 miles. Planning for

. this event would call for very rapid action, starting before

? release to evacuate the first few miles and timely followup, Y possibly shelter and prompt relocation for people at greater distances.

Now let us look at the same release under slightly better weather conditions, Pasquill D and 10 m.p.h. windspeed. Here there is much more time before a given dose is accumulated except for those at very close range. Once again very rapid action is called i

for for the people in the first mile or two, and more deliberate l action for those at greater distance.

l Now if we look at the SST-2 release, about a hundred times I

lower than SST-1, under the bad weather conditions Pasquill F and 5 m.p.h. windspeed. Here the life-threatening dose range is only a mile or so, and clinical effect exposure range is only to 4 or 5 miles. PAG doses can still reach out quite far.

My last figure shows the SST-3 release under bad weather con-ditions. Here, even the PAG doses are limited to short range.

Looking at this set of figures and recognizing that other weather conditions would be more dispersive, giving even greater time, a reasonable strategy of response for all cases would be

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prompt action, even before release, for the first 2 miles and selective action beyond that radius based on accident circum-stances. We would be actively campaigning now with this infonna-tion for such a strategy at all plants. But we may change the basis for emergency planning once again--based on a new reeval-uation of source terms.

In 1981 the NRC published NUREG-0772 which presented the state of knowledge for severe accident behavior and suggested a number of factors which indicated that the WASH-1400 source terms are pessi-mistica11y high. In the last few years a major program has been underway to reevaluate the severe accident source terms based on all the newly available test data and analysis. The centerpiece of this work is the multivolume report of our contractor Battelle Columbus Laboratories BMI-2104, which is new being published. The working drafts of this reoort have been publicly available since last year as part of its extensive peer review process. Along with it, NRC and its contractors are publishing a major reestimation of containment performance this summer. THe extensive peer review of these elements of the reevaluation includes review by specialists, the Group. industry IDCOR group, and an knerican Physical Society Study The peer review should be complete by the end ef 1984.

And so today we face the question for emergency response planning of whether to use WASH-1400 source terms or wait for new ones. The WASH-1400 terms are better known, easy to work with, and would evidently be a conservative approach. New source terms might justify significant changes and it would certainly avoid the con-fusion of working with WASH-1400 source terms at the same time new i

ones are publicly available. The NRC is now on the second path--

p. with new estimates available and reviewed by the end of this year, we will hold new initiatives in emergency planning until then.

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SOURCE TERM REEVALUATION AND EMERGENCY RESPONSE ROBERT M. BERNER0 U.S. NUCLEAR REGULATORY COMMISSION MAY 24, 1984 D-CONFERENCE OF RADIATION CONTROL PROGRAM DIRECTORS DES MOINES, IOWA i

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Whole body dose calculated includes: external dose to the whole body due to the f

, passing cloud, exposure to radionuclides on ground, and the dose to the whole body from Inhaled radionucikles.

Dose calculations assumed no protective actions taken, and straight line plurne l tre)ctory.

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POST TMI EMERGENCY RESPONSE NRC RULES - 1980 o PLAN FOR IMMERSION PATHWAY OUT TO ABOUT 10-MILE RADIUS o FOLLOW PLANNING STANDARDS .

I o PERIODIC EXERCISE NRC/ FEMA GUIDANCE - 1980 o NUREG-0654 .

6 i o DETAILED GUIDANCE o SUGGESTED SPECTRUM 0F RESPONSE i

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IMPLEMENTATION SINCE 1980 o CONTROVERSY BUT SUCCESSFUL AT ALMOST ALL SITES o SOME PERCEIVED PROBLEMS ATTITUDE OF CONSTANT RISK TO 10 MILES AND NONE BEYOND ATTITUDE THAT EVACUATION BEFORE PLUME IS ONLY COURSE OF ACTION - EPZ l

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FOR EMERGENCY PREPAREDNESS o OVEST10N: USE WASH-1400 SOURCE TERMS OR NEW SOURCE TERMS o WASH-1400 SOURCE TERMS EASY TO WORK WITH A CONSERVATIVE APPROACH i o NEW SOURCE TERMS

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l 1 to 100 1000 i DISTANCE (MILES) l Figure 111. Conditional Probability of Exceeding Whole Body Dose Versus Distana.Probabilit*ms

are Conditional on a Core Mett Accident (5 x 10-5).

Whole body dose alculated includes: external dose to the whole body due to the

. possing cloud, exposure to radionuclides on youMI, and the dose to the Whole body from Inhaled radionuclides.

Does calculations assumed no protective actions taken, and straight Ene plume trajectory.

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