### Final Report



### History and Basis of NESHAPs and Subpart W

Prepar ed by S. Cohen & Associates 1608 Spring Hill Road , Suite 400 \Ttenna , VA 22182
Under Contract Number EP-0-05-002 Work Assignment No. 4-11 , Task 4

Prep ared for U.S. Environmenta l Protection Agency

Office of Radiation and Indoor A ir

1200 Penn sy lvania Avenue , N. 

### W. Washington , DC 20460
Reid J. Resnic k Work Assignment Manager
September 25, 2008In accordance with the 

### Quality Assurance Project Plan: Technical and Regulatory Support to Develop a Rulemaking to Modify the NESHAP Subpart W Standard for Radon Emi ss ions from Operating U ranium Mills (40 CFR 61.25), thi s document has been reviewed and approved by the following individual s :
/ I 

### _) Work A ss ignment Tas k Manager: , '-- jft_J,;J 

## J--tZUi: j:**~** Date: 0 9/2 5/2008 I 

### Harry Pettengill
Project Manager:

# (~~~ Date: 09/25 /2008 

### Abe ZeitoufiJ
Corporate Quality A s surance Mgr:

# G~~tr- Date: 09/25 /2008

Work Assignme nt QA Manager:

# ~:l~ Date: 0 9/25/2008 

### St

ep en Os trow

WA 4-11 , Task 4

- NESHAP s History SC&A

- September 25, 2008

### TABLE OF CONTENTS
1 .0 Intr o duction .................................................. .... ............................................ . ...................... I 2.0 Back g r

o und on th e Develo pme nt of R adion ucl ide NE SHA PS
......... .......... . ....... ............... I 2.1 The 1977 Amen dment s to the C l ean Air Ac t .......................... .................. ........ . ..... 2 2.2 Regu la tory Activ ities between 1979 and 1987
.... .... . .............................................. 2 2.3 Regulatory Activ iti es between 1987 and 1989 ....................... ................. ............... 4 2.4 19 89 Radio nucl ide NESHA Ps Rec o n s ide ration Rulemaking ................................. 4 3.0 Basis for the S ubpart W Risk Assess ment ............ ........ ... ......................... ... . .................. .... 5 3.1 Existin g Impo u nd ments ..... ........... .......... ................ .. ............. .. . .. ............................ 5 3.2 New Imp ou ndme nts .... .................. ......... ............ ................ .......................... ........ ... 6 4.0 Compari son of the NES HAP s Risk Assess ment with C urre nt R isk Assessme nt A pproache s .............................. ................... . ............. ........................................... .......... ...... 7 5.0 Conc lu sions ................. ............... ............. ... .............................. .......... ............................... I 0 6.0 References
... . ........ .................................... .. . .......... .................. .... ....................... ..... .......... I 0

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- September 25 , 2008

### 1.0 INTRODUCTION On December 15, 1989 , the Environmental Protection Agency's (EPA) Office of Radiation and Indoor Air (ORIA) promulgated a National Emission Standard for Hazardous Air Pollutant (NESHAP) for radon emissions from operating uranium mill tailings (Subpart W). Section 112(q) oftlhe Clean Air Act (CAA), as amended, requires the EPA to review, and if appropriate, revise or up date the Subpart W standard on a timely ba s is (1 

### 0-year interval). To date, EPA has not revisited this standard since its promulgation and now desires to do so . The purpose of this work assignment is to provide essential technical and regulatory support necessary for EPA to review the techn ical basis on which the standard was based and the decision-making process that was used to determine th e requirements of the standard. Furthermore, this effort will assist the EPA in determining the appropriateness of the standard for this type of pollutant , and to decide if modification of Subpart W is warranted. The facilities covered by Subpart Ware uranium mills licensed and regulated by the U.S. Nuc lear Regulatory Commission (NRC) or its Agreement States. There are two separate standards outlined in Subpart W
. The first states that ex isting sources must e n sure emiss ion s from tailings impoundments no t exceed 20 pCi/m 2



### - sec of radon-222 (Rn-222). The second prescribes a work practice specifying one of two types of impoundment structures that new sources must construct to handle tailings during operations. The work practice also applies to operations at existing sources once their existing impoundments can no longer accept additional tailings. As part of these two standards, Subpart W also requires that existing sources fi le an annual report of t he facility's e missions. The standard as promulgated was a health-based standard. It is important to review how the initial standard was developed and to ascertain if it is sti ll valid. Furthermore, it needs to be determined ifthe standard requires revision to satisfy the requirements in CAA Section 112(d), w hi ch r equires a technology-based standard. It is also important to determine if this type of standard is appropriate for this type of pollutant. The reports collected for this review include the original risk assessment material used by EPA to develop the Subpart W standard, including the final rulemaking package for Subpart W (FR 1989), the background information document (BID) for the Subpart W rulemaking (EPA 1989 , Vo lume s I and 2), and the economic impact analysis for the Subpart W rulemaking (EPA 1989, Volume 3). This regulatory informat i on was evaluated and compared and contrasted to current risk assessment modeling methodologies. The purpose of this report is to illuminate th e differences, if any, and detail what impact this information would have on the original radon risk standard as provided i n Subpart W. 

### 2.0 BACKGROUND ON THE DEVELOPMENT OF RADIONUCLIDE NESHAPS In the following subsections, we present a brief history of the development of environmental radiation protection standards by the EPA, with particular e mphasis on the development of radionuclide NE SHAPs
.

WA 4

- 11, Task 4

- NESHAPs History SC&A- September 25 , 2008

### 2.1 The 1977 Amendme n ts to the Clean Ai r Act On January 13, 1977, the EPA established e n vi ronm enta l protec ti o n s tandards for nuc lear power operations pursuant to its authority under t he A tomic E ne rgy Act (AEA) (FR 1977). T he standard s, Ti tle 40, Part 190, of th e 

### Code of Fede ral Regulations (40 CFR 190) , wh i c h covered a ll licensed facil it ie s that are part of the uranium fue l cycle , esta bl is he d an ann ual limit of exposure and the requ irement t hat lic ensees keep all exposures "as low as reasonab ly achievable" (ALARA). Because of the unc ertai nti es associate d w ith the risk of inhaled radon , t he standards exemp ted Rn-222 from the annual limit. After the promulgatio n of 40 CFR 190 , the 1977 amendments to the Clean Air Act (CAA) w er e passed. These a me nd ments included the requirement that the Administrator of the EP A d etermi ne whether or not radionuclides should be regulated und e r t he Act
. In December of 1979 , the Agency pub li shed its determination in the 

### Fede ral Register (FR 1979) that radionuclide s co nst itu te a hazardous ai r pollutant within the meaning of Section 112(a)(l). As was stated in the 

### Federal R egi ster, rad ionucli des are known to cause cancer and ge netic defects , and contribute to air pollution that may be anticipated to resu lt in an increase in mortality o r an inc rea se in serious irrev ers ib le o r in capacitating reversible illness. The Agency further determined th at the ri s ks posed by e missi ons of radionuclides into the ambient air warranted r egu lati on, and listed radionuclides as a hazardous air pollutant und e r Section 112. Sec tion 1 12(b)( l 

### ) (B) requires the A dmin is trator to es tablish N E SHA Ps as a " l eve l w hi ch (in the judgment o f the Administrator) provides an ample margin of safety to protect the publ i c health" or find that they are not hazardou s and de list them
. 2.2 

### Regu l atory Activities between 1979 and 1987 To s upp ort th e deve l opment of radi onuclide NESHAPs, the Age nc y de ve loped a B ID to c haracteri ze "source categories " of facilities that emit radionuclides into ambient air (EPA 1979)
. For each source category, information needed to characterize the expos ure o f the publi c was developed. T hi s included charac teriz ation ofthe facil it ies included in the sou rce category (nu mbers, l ocations, proximity of nearby indi vi duals) ; radionuclide source terms [c uri e/year (C i/y)] release rates by rad ionuclide, solubility class , and particle size); release point data (stack height, vo lumet ric flow, area size); and effluent controls (ty pe , efficiency). Doses to nearby individuals and regional popul atio n s caused by releases from e ither actual of mode l fac il ities were esti mate d using the comp uter codes AIRDOS E

- EPA and DARTAB. In 1983, th e E PA p ropo sed radi onuc li de NESHAPs fo r four source categories based on the resu lts r eporte d in a new BID (E PA 198 3)
. T he four source categorie s for which N E S H APs were proposed were D e p artment of Energy (DOE) and Non

-NRC-Lic e n se d Fe d e ral Facilities , NRC

L ice n sed Facilities, E lemental Phosphorus Plants, and Und erground Uranium M ine s. For all of t he ot her sou rc e catego r ies that i t had considere d in it s BID (i.e
. , coal-fired bo il ers, the phosp hate indu st r y, other extraction industries, uran ium fu e l

-cycle facilit ies , uranium mill tailings , highlevel waste disposal, and low-energy acce lerators), the Agency found t hat N ESHA P s were not nec essary . In reachin g this conc lusi o n , the EPA found that (1) the leve ls of rad io nucl ide emissions d id not caus e a significant dose to nearby ind ivi dual s or t he regional populations,

WA 4-11, Task 4- NE SHAPs History 2 SC&A

- Septembe r 25 , 2008(2) the costs of additional effluent controls were not cost effective , or (3) existing reg u lations under o th er au th or iti es we re su fficient to keep emissions at a n acceptab le level. During the public co mme nt period on the proposed NES HAP s , the Agency co mpl e ted it s rul e mak in g efforts und e r the Uranium Mill Tailings Remedial Cont rol Act (UMTR CA) to establish standards (40 CFR 192) for the disp osa l of uranium mill tailings. With re spect to the emission of Rn- 222, the UM TRCA standard s established a d es i g n standard calling for a Rn-222 flu x rat e of no more than 20 pCi/ m2

- sec . In February of 1984, the Sierra Club sued the EPA in th e U.S
. Di strict Court for N orthern California (Sierra C lub v. Ruckel shaus, N o. 84-0656) ( E PA 1989), demanding that the Agency promulgate final NESHAPs or de li st radionuclides as a hazardo u s a ir pollutant. The court sided with the S i e rra C l ub and ordered the EPA to promu lgate final re g ulati ons. In October of 1984 , the E PA withdrew the proposed NE SHAPs for Elemental Ph os phoru s Plants, DOE-Facilities, and NR C- Li ce n se d Facilit i es, finding that existing control practice s protec ted the public health with an ample marg in o f s afety (FR 1984). The EPA al so wi thdrew the NES HAP for Underground Uranium Mines , but stated its intention to promulgate a diff e re nt s tandard , and publishe d an Adva nc e Notice of Proposed Rulemaking (ANPR) to solicit additional inf o rmation on control meth ods. It a lso publ ishe d an ANPR for licen se d uranium mills. Finally, the 

### Federal Register n o tice affirmed the decision not to regulate the ot her source catego ri es identified in the proposed rule , with the excepti o n that it was doing further studies of phosphogypsu m stacks to see if a standard was needed. In December of 1984 , the District Court f or Nort hern Cal if o rni a fo und the EPA's action withdrawing th e NESHAPs to be in contempt of its order
. Given the ruling , the EPA is sued the final BID (E PA 1984) and promulgated final s tandard s fo r E leme ntal Ph os ph o ru s Plants, DOEFacilities, a nd NRC-Licensed Faci lities in Februar y of 1985 (FR 1985a) , and a work practice s tand ard f o r Un de rg r o und Uranium Mine s in April ( FR 1985b). T he En v iro nmental D e fen se Fund ( EDF), the Natural Reso urc es Defen s e Counci l (NRDC) , and the S i erra C lub ( SC ) til ed court peti t ion s seeking rev iew of the October 1
. 984 final dec ision not to reg ulate the so urce categories identified above , the Fe bruary 1985 NESHAPs, and the Apri l 1985 NES HAP. T h e American Mining Congr ess (AMC) al so filed a petition seeking judicial rev iew of the N ES HAP for underground uranium mine s. On Se pte mb e r 24 , 1986, the Agency issued a final NESHAP for Operating Uranium Mill Ta ilin gs , establish in g an emission standard of20 p Ci/ m2 -sec for Rn-222 and a wo rk prac tice standard requ irin g that new tailings be di sp osed of in s m a ll impoundments o r b y continuous disposal. The jus tifi cati o n s for the wo rk practices inc luded the f ac t that wh ile lar ge impoundments did not pose an unacceptable risk durin g active operatio n s , the cyclica l nature of the uranium milling industry could lead to prolonged p eriods of plant stan d-by and the risk that the ta il ing imp ound ments cou ld experience s ignifi ca nt drying with the res ultin g inc rease in Rn- 222 emiss ions. Furthermore , the two acc e ptab le wo rk practices we re believed to actually save the industry the sig nificant costs of constructing and cl os in g large imp o undments before they were co mpl etel y filled
. With the promulgation of th e N ES H AP for Operating Ura nium Mill

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### Tailings, releases of radionuclides to air during operations and tailings disposal at uranium mills were covered by three EPA regulation s, 40 CFR 190, 40 CFR 192, and 40 CFR 61, Subpart W. In November of 1986, the A MC and the EDF filed petitions challenging th e NES H AP for operating uranium mill tailings
. 

### 2.3 Regulatory Activities between 1987 and 1989 While the petitions filed by the EDF, NRDC, SC, and AMC were still before the courts, a decision by the U.S. District Court for the District of Columbia in NRDC v . EPA (FR 1989) , found that the Administrator had impermissibly considered costs and technological feasibility in promulgating the NESHAP for Vinyl Chloride. The court outlined a two-step decision process that it would find acceptable, first establishing a standard based so lely on an acceptable lev e l of risk, and then consider ing additional factors, such as costs to establish the "ample margin of safety." Given the Court's decision, the Agency reviewed how it had conducted all i ts NESHAP rulemakings and requested that the Court grant it a vo luntary remand for its radionuclide NESHAP s. As part o f an agreement with the Court and the NRDC, the Agency agreed to reconsider all iss ues that were currently being litigated, and it agreed that it would explic itly consider the need for a NESHAP for two additional source categories ; rad on from phosphogypsu m stacks and radon from DOE facilities. The subsequent reconsideration became know as the radionuclide NESHA Ps reconsideration rulemaking. 

### 2.4 1989 Radionuclide NESHAPs Reconsideration Rulemaking In the radionuclide N ESHA Ps rec onsideration rulemaking, the Administrator relied on a "bright line" approach for de te rmining whether or not a source category required a NESHAP. Namely, no NESHAP was required if all individuals exposed to the radionuclid e emi ss ion s from the facilities in the source category were at a life-time cancer risk of less than 1 in 1 

### , 000 ,000, and less than 1 

### fatal cancer per year was estimated to be incurred in the population. For so urce categories that d id not meet th is "bright line" exclusion, a two-step multi-factor approach to setting the emission s tand ards was adopted. The first step estab l ished a presumptively acceptable emissions level corresponding to a maximum individual risk (MIR) of about l 

### in 10,000 life

- time cancer risk, with the vast majority of exposed individuals at a lifetime risk lower than 1 in 1,000,000, and with the total f atal cancers per year in the exposed population of less than 1
. If the baseline emissions from a source category met these criteria, they were presumptive ly adequate ly safe
. If they did not meet these criteria , then the Administrator was compelled by his non - discretionary duty to provide adequate safety to determi ne an emission l imit that would correspond to risks that were adequately safe. After baseline emiss ions were determined to be adequately safe, or an adequately safe alternative li mi t defined , the analysis moved to the second step, where reduced risks for alternative emiss ion limits were evaluated, along with the technological feasibility and costs estimated to be associated with reaching lower levels. In the two-step approach, the Administrator retaine d the discretion to decide whether or not the NESHAP shou ld be set at these lower limi ts.

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- N E SHAPs History 4 SC&A

- September 25, 2008As with previous radionuclide NESHAPs, exposures were estima ted using a combination of actual site data and model or representative faci lities and computer models. For all radionuclides other than Rn

- 222, the CAP88 computer code, which included updates to AIRDOSE-EPA and DARTAB, and the addition of RADRlSK, was used to calculate total effective dose equivalent and li fetime fata l cancer risk , and t he distribution of risks in the exposed populations. For Rn-222, the CAP88 computer codes were used to estab li shed ambient concentrations (pCi/m3 ) in each of the sectors in a 0-80 

### km radius of the source . The concentration within each sector was then converted to working level months (WLMs), based on a 0.70 equilibrium fraction between Rn-222 and its decay products, and a respiration rate appropriate to members of the general public. Using risk factors derived from human epidemiological studies, the WLM exposure data were converted to risks. Ofthe N ES HAPs promulgated in 1989, those for Phosphogypsum Stacks (Subpart R), Radon from DOE Facilit i es (Subpart Q), and Operating Uranium Mills Tailings Disposal (Subpart T) are the most relevant for comparison with Subpart W, in that they all address the emiss ion of Rn-222 from relatively large area sources. Furthermore, the basic post-disposal Rn

-222 emission limit is consistent at 20 pCi/m2 -s. Regarding the operational phase, no standard was deemed to be necessary for phosphogypsum stacks, since the large ponds that cover most to the top of the stacks and are part of the plants normal operation to control Rn-222 emissions to an acceptable leve l. For DOE facilities, there is no operational phase, as all these site are simply storage sites for radon-emitting wastes that were generated in the past (most as part of the Manhattan Project). The NESHAP for disposal of uranium mill tailings (Subpart T) was determined to be ne cessary by the Administrator to assure that the tailings did not remain unreclaimed for lon g periods of time, as was possible under the UMTRCA standard (40 CFR 192). Additionally, the design standard of the UMTRCA regulations were enhanced by the requirement that the 20 pCi /m2 -sec Rn-222 design standard be demonstrated to have be en achieved by post-d isposal radon monitoring. 

### 3.0 BASIS FOR THE SUBPART W RISK ASSESSMENT 3. 1 Existing Impoundments As noted previous ly, the NE SHAP for operating uranium mi ll tailings addressed both existing and future tailings impoundments. For the existing impoundments , the radon emissions and estimated risks were developed using site

- specific data for each ofthe 12 mills that we re operating or operable at the time the assessment was made. These data included the average radium-226 (Ra-226) content of the tailings; the overall dimensions and areas of th e impoundments (developed from licensing data and areal photographs); areas of dry (unsaturated) tailings; the existing populations within 5 

### km of the cent ers of t he impoundments (identified by

· field enumeration); 5

- 80 

### km populations derived from U.S. Census tract data; meteorological data Uoint frequency distributions) from nearby weather stations; mixing heights; and annual precipitation rates. The CAP88 codes were used to estimate airborne concentrations based on the calculated Rn-222 source term for e ac h facility. Radon-222 source terms were estimated on the assumption that a Rn-222 flux of l pCi/m2

- sec resu lts for each I pCi/g Ra

- 226 in the tailings and the areas of dried

WA 4-1 1, Task 4- NESHAPs History 5 SC&A

- September 25, 2008tailings at each site. The radon flux rate of l pCi/ m2-sec per pCilg R a-226 was derived ba sed on both theoretica l radon diffusion equations and the ava ilable radon emissions measurements. For each sec to r in the 0

- 80 km grid around eac h f ac ili t y, the est imat ed Rn-222 airborne
. concentration was c o nverted to c umu lative WLMs assuming a 0 .7 0 equili brium fraction bet wee n radon and its decay products, an average respiration rate appropriate for members o f the general public , and the a ssumptio n of continuous exposure over a 70

-year lif et ime. Us in g a risk coeffic ient of760 fatalitie s! I 0 6 WLM, lif etime risk, fatal cancers per yea r, and the r isk di stribution were calculated for the exposed population
. The ba selin e r isk assessme nt for exis tin g urani u m tailings showed MIR of 3 x I 

# o · 5
, with 0.004 3 co mmitted cancers in the 2 million persons living w ith in 80 km of the mill s. The d istribution of the cancer risk s howed th a t 240 persons w e re at ri s ks between 1 x 1 0 ' 5 and 1 x 1 0' 4
, and 60,000 we re at risks be twee n I x 1 

# o· 6 and 1 x l 

# o· 5

. The remainder of the popu lati on of ab out 2 milli on we re at a risk of less than I x 10 ·6

. Bas ed on these findings , the EPA concluded that baseline ri sks were acce ptab le. The dec ision o n a n ample mar gin of safety co n s idered all of the risk data presented above plus costs, scient ifi c u ncertainty, and the technical feasib i lity of control techn o l ogy necessary to lower e mi ss ions from operating uranium mill tailin gs piles. As the ri sks from existing em iss io n s were very low, the EPA determined that an em i ssio n standard of20 pCi/m2

- sec, which represented current emissions , was all that was necessary to pro vide the ample marg in of safet y. T he necess it y for the s tandard was expla in e d by the need to assu re that mill s co ntinu ed the current co ntrol practice of keep in g tailings wet and /o r covered. Finally, in order to ass ure that groundwater was not adversely affected by continued ope ration of existing piles that we re not sy nthetically lined or clay lined , the NES H AP ended the exemption to th e requi rements of 40 CFR I 92.32 ( a) , which protects water s upplie s from contamination. 

### 3.2 New Impoundments The risk assessment f o r new mill tailings imp ou ndment s was ba sed o n a set o f mo del mills , defined so that t he imp act of alternative disposal st rateg ies could be eva luated. Fo r the purpos e of estimating the r is ks, the model mill s were c h arac terized to reflect ope rat ing mills, and the d ispe rsion modeling and population exposures were based on the arid con dit ions arid sparse population dens ity that characterize existing impoundments in the southwestern states. Th e results o f the mo de l ing exercise indi cated an MIR of l. 6 x I 0-4, a fatal cancer incidence of 0.014 per year, a nd only 200 persons at a risk g reater than l x l 

# o · 4

. Given the numerous uncertainties in estab li s hin g the param eters for the ri sk assess ment and in modeling actu a l emissions and exp osu r es, th e A dmini stra tor fo und that the baseline em issions for new tailings imp ou ndmen ts met the c riteria for presumptiv e l y safe
. The decisi on on an ample margin of safety for new tailings con s idered two a lternatives to the baseline of one large impound ment; phased disposal using a series of sma ll impoundment s and continu ous d isposa l. The evaluation of these alternatives s howed a modest reduction in the M IR and the number o f fatal cancers per year, but a s ignificant increase in the numbe r o f individuals at a lifet ime risk of less than I x I 

# o · 6

. The costs estimated for the two alternatives s howed that

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### phased disposal would lead to an incremental cost of $6.3 million, while continuous disposal was believed to actually result in a modest cost saving of$1 million. Given the large uncertainties associated with the risk and economic assessments performed for the new tailing impoundments, and considering the boom and bust cycles that the uranium industry ha s experien ced, the Adm inistrator determ ined that a work practice standard was necessary to prevent the risks from increas ing if an impoundment were allowed to become dry. Finally, although continuous disposal showed slightly lower over-all risks and costs than phased disposal, the Administrator recognized that it was no t a proven technology for di sposal of uranium mills tai l ings. Therefore, he determined that the work prac t ic e standard should allow for either phas ed disposal (limited to 40 -acre impoundments, with a maximum of2 impoundments o pen at any one time) or continuous disposal. 

### 4 . 0 COMPARISON OF THE NESHAPS RISK ASSESSMENT WITH CURRENT RISK ASSESSMENT APPROACHES Since i t is neither feasible nor practical to directly measure the exposure of indi viduals in the 0

- 80-km area surrounding an emission so urce, risk assessments rely on mod e lin g exposures. The essential elements in assessing the risk from a source emitting Rn-222 to the ambient air are as follows:
• The so urc e term (Ci/y)
• The disp ers ion of the Rn-222 from the source to the receptor
• The ingrowth and depleti on of the short-lived Rn-222 decay products from the source to the receptor
• The locat ion of the receptors within the assessment area
• The duration of the receptors' exposures
• The fatal cancer risk/unit of exposure (risk/cumulative WLM) The following paragraphs discuss how each of these elements were derived for the risk assessments performed in support of the Subpart W rulemaking and suggest where different values might be used if the assessments were performed now. The source terms for existing impoundments were based on radon flux rates (pCi/ m2), 

### Ra-226 concentrations in the tailings, and the areas of unsaturated tail ings exposed at eac h s ite. The radon flux rate that was used, l pCi/m 2 

### Rn-222/pCi/g Ra-226, was selected based on theoretical diffusion rates from thick sources. As diffusion rates depend on the porosity of the matrix and its moisture content, the specific rate that was selected was chosen to reflect site conditions that pertain in the southwestern part o f the country where t he industry is concentrated. The concentrations of Ra-226 in the tailings reflect measurement data, while the areas of unsaturated tai lings were estimated based on both photographs of the impoundments and informat ion

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### supplied by the industry during the public comment period. Given that the NESHAP imposed an annual requirement for the facilities to measure and report their Rn

- 222 emissions , it should now be possible to develop the source term for each mill based on measurement data. If thi s approach were to be taken, it would be necessary to know the configurat ion of the impoundment during each measurement period. The dispersion of the Rn-222 from the tailings impoundments to the receptors in the assessment area were estimated using the AIRDOSE-EPA model in the CAP88 assessment code. AIRDOSE-EPA uses a Gaussian plume dispersion model to calculate air concentration s at the locations of the receptors. The model uses meteorological data (in the form of joint frequency distributions of w ind s peed and direction by stability class) , annual precipitation rate, and lid height supplied by the user, and it accounts for removal from the plume by dry deposition and scavenging. The model allows for the source to be characterized as either a point source or an area source, and allows the user to input the effective re lease height. For the risk assessments, impoundment s were characterized as area sources with an effective release height of 1 meter. As on - site meteorologica l data were not available, joint frequency data representing long-term averages were obtained from the nearest weather station. Annual precipitation rates and lid heights were site-specific
. While the EPA has developed a number of dispersion models, including some that are more sophisticated than the AI ROOSE-EPA model, the AIRDOSE-EPA model is preferre d for assessments involving radionuclides. When its predicted concentrations have been be nc hmarked against measured values, it has demonstrated good agreement; in fact, it has often s urpassed the performance of more "sophisticated" models. The ingrowth and decay of the Rn-222 decay products is very important to estimating the risks from Rn-222 exposure . When the Rn

- 222 emanate s from the tailings, the fraction of its short

lived decay products is zero, as they are retained in the tailings matrix
. However, their ingrowth begin s immediately, and theoretically could reach 100% (total equilibrium) at some distance. As a practical matter, 1 00% ingrowth is unlikely to ever be attained, due to dry deposi t ion and scavenging during plume transport. For the Subpart W assessments, the equilibrium fraction of decay products was set at 0
. 70
. The equil ibrium fract i on of0.70 is appropriate for distances beyond approximately 15,000 meters from the impoundments (where the majority of the exposed populations are located) and assumes that an individua l spends 75% of their ti me indoors. For individuals nearer t o the impoundments than 15,000 meters, the assumption of a 0.70 equilibrium fractions will over- state their exposure and resulting risk
. Since the NESHAP Subpart W was promulgated, there has been a great deal of wo rk done to better characterize the appropriate equil ibrium fraction for assessing exposure to Rn-222. In general, these studies suggest a somewhat lower fraction than was used in the Subpart W assessments, which would have the effect of lowering the estimated risks (UNSCEAR 2000)
. The locations of the receptors in the 80-km assessment area around each site were determined in one of two ways. For the 0-5 

### km radius, actual site visits were conducted, and the populations in each of 16 directions were determined for distances of0.5, 1.0 , 1.5, 2.0, 2.5, and 5.0 km. 

### For distances between 5 and 80 km, 

### increments of5 km 

### and 10 km were used, and U.S. Census tract data were used to estimate the population in each sector.
WA4-ll, Task 4- NESHAPs History 8 SC&A

- September 25, 2008Locating the nearb y recept o rs by actual e numeration is se ldom possibl e when conduct ing risk assessments. Genera ll y , the best that can be done is to identify nearby sec tors where person s are presumed to re s ide
. Us in g the U .S. Cen s u s tract data to popula te the 5

- 80 km assessment grid , while not perfec t, as it places all of the tract's p o pulati o n at the centroid of th e tract, is as accurate a means of calculating co llecti ve exposure as w e have
. For the NES HAPs r isk assess m ent , indi viduals in the population are pre sumed to exp eri e nce a constant leve l of exposure over a 70

-year period ( i.e. , lif eti me)
. Whi l e the Agency has oft e n been c riticized for making t hi s assumption , it has r e p eated ly f o und that it is appropriate for evaluating radiati o n risks w here age at exposure is an important facto r in t he over-a ll risk. The final step in the assessmen t of the ri sks fro m Rn-222 released from o perating uranium mill tailings impoundments is to correlate the exposures with risk. For almo st all rad ionuc lide r isk assessments , the ri s k is based o n do simetry mode ls. H owever, fo r Rn-2 22 , it is based o n epidemiology. Specifically, based largely o n work presented by the Nation Co uncil on Radiati o n Protectio n (NCRP 1984) and the International Co mmi ssio n on Radiological Protect ion (TCRP 1977), the N ES HAP used a ri sk of 760 fata l cancers/ I 06 cumulative WLM (re fl ecting a risk coefficient of7.6 x I 0 -4/ WLM). As with the equilibrium fraction, considerabl e additi o nal studies have bee n conduc ted to determine the ap pro priate ri s k coeffic ient for Rn- 222
. EPA's current recommended ri s k coefficient is 5.38 x 

### 10 " 4 per WLM
. Thi s value is based primarily on the National Acad emy of Sciences Report B E J R VI (NAS 1999). The EPA reco mmended risk coefficient is rep orted in Sectio n VI of"EPA Assess ment of Ri sks from Radon in Homes ," EPA 402-R-03-003, June 2003. Table I 0 of that report , reproduced below, provides a more detailed breakdown of the risks. 

### Table 10: E stimates of risk per WLM by smoki n g category and gender for a s tationary population in which 53% of males and 41% of females are ES (Authors note: In this table, ES refers to ev er smoked an d NS refers to never smoked). Gend er Smoking Category Risk per WLM• (10-4) E xpected Life Span (yea rs) Male ES 10.6 71.5 NS 1. 74 72 .8 ES and 

## s 6
.40 72.1 Female ES 8
. 81 78.0 NS 1.61 79.4 ES and N S 4.39 78.8 Male & Female ES 9
. 68 74.2 NS 1.67 76.4 E S and NS 5.38 75.4 • Based on 1990 adult (ages g reater than or equal to I 9 y) ever smoke d prevalenc e d ata (58.7 males and 42.3

females are ES) and assumptio n that 37% males and 36% females or children (ages < 18 y) wi ll become ES)
. If the ri s k asse ssme nt s for Subpart W were d o ne today, the over-all ri sks that would b e estimated wo uld like ly be somewhat l owe r than those estimated in 1989
.

WA 4-11, Task 4- NES HAPs History 9 SC&A- September 25, 2008

### 5.0 CONCLUSIONS The risk assessments that were performed for the NESHAP for Operating Uranium Mill Tailings were based on the best data and science that were available at the time. While it is unlikely that the over-all risk profile would change significantly if the assessments were redone today, additional data and new scientific findings have become available. Most significantly, measured emissions data shoul d be available, and the current scientific consensus on both equ il ibr ium fractions and th e radon risk coefficient could be used in the assessment. 

### 6.0 REFERENCES 40 CFR 61, Title 40 , Part 61: National Emission Standards for Hazardous Air Pollutants, Subpart W National Emissions Standards for Radon Emissions from Operating Mill Tailings. 40 CFR 190 , Title 40 , Part 190: Environmental Radiation Protection Standards for Nuclear Power Operations. 40 CFR 192 , Title 40, Part 192: Health and Environmental Protections Standards for Uranium and Thorium Mill Tailings. N AS (National Academy of Sciences) 1999. Biological Effects of Ionizing Radiation (BEIR) VI: The Health Effects of Exposure to Indoor Radon. National Academy of Sciences, National Research Counci l. 1999. EPA (Environmental P rotection Agency) 1979
. Radiological Impacts Caused by Emission of Radionuclides into Air in the United States, EPA 520/7-79-006, Office of Radiation Programs, Environmental Protection Agency, Washington DC. August 1979. EPA (Environmental Protection Agency) 1983. Draft Report, Background Information Document Proposed Standards for Radio nuclides, EPA 520/1-83-001, Office of Radiat ion Programs, Environmental Protection Agency, Washington DC. March 1983. EPA (Environmental Protection Agency) 1984. Final Background Information Document Proposed Standards for Radionuclides, EPA 520/1-84-022-1, Office of Radiation Programs, Environmental Protection Agency, Washington DC. October 1984. EPA (Environmental P rotection Agency) 1986. Final Rule for Radon-222 Emissions From Licensed Uranium Mill Tailings Background Information Document, E PA 520/ 1-86-009, Office of Radiation Programs, Environmental Protection Agency, Washington DC. August 1986. EPA (Environmental P rotection Agency) 1989
. RiskAssessment Methodology Environmental Impact Statement NESHAPS for Radionuclides Background Information Document, Volumes 1 - 

### 3 , EPA 520/1-89-005, Office of Radiation Programs, Environmental Protection Agenc y, Washington DC. September 1989.

WA 4-11 , Task 4

- NESHAPs History 10 SC&A

- September 25, 2008

### EPA (Environment a l Protection Agency) 2003
. EPA Assessment of Risks from Radon in Homes, EPA 402-R-03

- 003, June 2003. FR (Federa l Register) 1977. EPA estab l is hed environmental protection standards for nuclear power operations pursuant to its authority under the Atomic Energy Act (AEA). Volume 42, page 2858. J anuary 13, 1977
. FR (Federal Register) 1 979. EPA determ i nation that radionuclides constitute a hazardous air pollutant within the meaning of Section 112(a)(1). Volume 44 , page 78738. December 27, 1979. FR (Federal Register) 1984. EPA w ithdrew the proposed NES HAPs for E l emental Phosphorus Plants, DOE

- Facilities, and NRC

- Licensed Facilities
. Vo l ume 49 , page 43906. Octob e r 23, 1984
. FR (Federal Regis ter) 1985a. EPA promulgated final standards for E lemental Phosphorus Plants, DOE-Fac ilities , and N RC

- Licensed Facilities. Volume 50, page 7280. February 8 , 1985. FR (Federal Regi ster) 1985b. EPA established a work prac t ice standard for Underground Uranium Mines. Volume 50, page 15385
. Aprill7, 1985. FR (Federal Register) 1989
. F inal ru l emaking package for Subpart W. Vo lume 54 , page 51679. December 15, 1989. ICRP ( International Commission on Radiological Protection) 1977. Recommendat ions of the International Commission on Radiological Protection, ICRP Publ icat i on 26, Ann. ICRP, 1, (1 ), 

### Pergamon Press, 1 977
. NC RP (National Council on Radiation Protection and Measurements) 1984. Evaluation and Occupational and Environmental Exposures to Radon and Recommendations, NCRP Report No
. 78 , Washington, DC. UNSCEAR (Un i ted N at i ons Scientific Committee on the Effects ofAtomic Radiation) 2000. U

nitedNations Scientific Committee on the Effects of Atomic Radiation Report to the General Assembly, with Scientific Annexes
. United Nations
. New York 2000.

WA 4

- 11, Task 4

- NESHAPs History 11 SC&A

- September 25 , 2008