NUREG/CR-3 139,. 0RNL/ENG/*-TMw25ý.; MA JV 77 JV MA MIALF7754111

Scenarios -and Analytical,

.Methods for:UFa6 Releases

at'NRC-Licensed Fuel

Cycle Fa cilities

M. Siman-Tov:

J. Dykstra

D. ýD. Holt,

W. P. Huxtable

R. -A.'Just:

W. R. Williams

Prepared for the,

-U.S. Nuclear Regulatory Commission

Office of Nuclear Regulatory Research

Division of Risk Analysis

Transportation and Materials Risk Branch

(S. Bernstein, Project Manager)

Washington, DC 20555.

under Interagency Agreement

DOE 40-550-75

OPERATED BY

MARTIN MARIETTA ENERGY SYSTEMS, INC.

FOR THE UNITED STATES

DEPARTMENT OF ENERGY
Printed in the'United States of America. Available from

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road, Springfield, Virginia 22161

Available from'

ýGPO Sales Program

Division of Technical Information and Documnent' Control

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555

..This report was prepared as an account of work sponsored by.an. agency of the.*

*United States Government. Neither the U nited States Government nor any agency

*thereof, nor any of their employees, makes any warranty,, express or implied, or

assumes any legal. liability or responsibility for the accuracy, completeness, or

usefulness of any information, apparatus, product, or process disclosed, or

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to any specif ic commercial product, process, or service by trade name, trademark,

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*any agency thereof. The views and opinions of authors expressed herein do not

necessarily state or reflect those of the United States Government or any agency

thereof..
NUREG/CR-3 139

ORNL/ENG/TM-25

RZ

Martin Marietta- Energy Systems,ý Inc., Engineering

SCENARIOS AND ANALYTICAL METHODS FOR UF REL EASESAT NRC-LICENSED, FUEL CYCLE FACILITOE

M. Siman-Tov, Project Manager

J. Dykstra

D. D. Holt

W. P. Huxtable

R. A. Just

W.R. Williams

Manuscript Completed-mJanuary. 1983

.,Date of Issue- June 1984

Prepared for the

U.S. Nuclear Regulatoryý Commission

Office of Nuclear Regulatory Research.

Division of Risk Analysis

Transportation and Materials Risk Branch

(S. Bernstein, Prject Manager)

Washington, DC 20555

under Interagency Agreement

DOE 40-550-75

FIN B0495

Oak Ridge National Laboratory

Oak Ridge, Tennessee 37831

operated by

MARTIN MARIETTA- ENERGY SYSTEMS, INC.

for the

U.S. DEPARTMENT OF ENERGY

under Contract No. DE-AC05-840R21400

CONTENTS

.1. INTRODUCTION ANDý SCOPE......................... ................ 1

,2. SUMMARY........................................ .......... ............. 5

3. LITERATURE.REVIEW..........................9

4.DESCRIPTION OF UF6 HANDLING SYSTEMS .... ................................ 21

:4.1. UFý6 PRODUCTION FACILITIES.. . . ............................. ............... 21:-

42ý FUEL FABRICATION FACILITIES..................... ................... 26

5.~~~~~~ CIETSEAIS.......................................... UF 2

5.1UF CYLINDER FAILURES ............................................. ..... 31

5.2 UF6 PROCESS SYSTEM FAILURES.................... I.......... .. 34

5.3' NUCLEAR CRITICALITY EVENTS....r..................................... .- 36

5.4, OPERATOR ERRORS.-....................................................3

6.- APPLICABILITY OF. AVAILABLE METHODS FOR ANALYZING, UF6 RELEASES ... 39:

6.1 APPLICABILITY OF AAH CODES..................... I............... ...... 3

6.2 OTHER AVAILABLE METHODS ................................ ... 44

7. PERSPECTIVES ON UF 6 ACCIDENT SCENARIOS AND ANALYSIS .........49

7.1 METHODS FOR CATEG;ORIZING SCENARIOS ............ 49

72BOUNDING CONSIDERATIONS FOR UF 6 . RELEASE. EVENTS ........ý.56

7.3 RELEASE PHENOMENA, AND MODELING CONSIDERATIONS ......... 60

7.4 PERSPECTIVES:SUMMARY.................. ....... ....... ......... 71

8. CALCULATIONAL METHODS NEEDED FOR ANALYZING UF 6 RELEASES ...... 75

8.1 METHODS FOR PREDICTING FAILURE MODES .... ........ :..75

.8.2 METHODS FOR PREDICTING SECONDARY FAILURES ............ 76

8.3 METHODS FOR PREDICTING RELEASE. RATES 80...........

8.4ý METHODS FOR PREDICTING BEHAVIOR AFTER RELEASE ... 80

8.5 METHODSý FOR PREDICTING BEHAVIOR IN A FIRE OR EXPLOSION ...... 82

8.6 METHODS NEEDED TO MODEL FLOW THROUGH.A VENTILATION. SYSTEM .83

8; CIICLTY METHOD ..... . .. ...... .... 83

9. CONCLUSIONS .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..8

REFERENCES . ........................................ ... 87

SELECTED BIBLIOGRAPHY .OF DOCUMENTS RELEVANT TO NRC UF 6 RELEASE

SOURCE TERM DEVELOPMENT ............... ... ......... 89

iii

SCENARIOS. AND ANALYTICAL METHODS FOR UF6 RELEASES.

AT NRC-LICENSED FUEL CYCLE FACILITIES

ABSTRACT.

.This report identifies and discusses potential scenarios for 'the accideni- tal release of UF 6 at NRC-licensed. UF6 production and fuel fabrication

facilities based on a'. literature review, site visits, and DOE enrichment

plant experience. :Analytical. tools needed for evaluating source'terms for

such releases are discussed, and the applicability of existing methods is

reviewed. Accident scenarios are Adiscussed under the broad headings of.

cylinder failures, UF6 process system failures, .nuclear criticality events, and operator errors and are categorized by location, release source, phase of UF 6 Prior to release, release flow ~characteristics, release causes,.initiating events, and UF6 inventory at risk.* At least three types of releases are

identified for further examination: (1) a release from a liquid-filled cyl- inder:'outdoors, (2) a release from a pigtail or cylinder in a steam chest, and.(3) an indoor release from either (a) a pigtail or liquid-filled cylinder

or (b) other indoor source depending on facility design and operating pro- cedures. Indoor release~ phenomena may be analyzed to determine input terms'-for a ventilation model by using a time-dependent homogeneous

compartment model or a more complex hydrodynamic model if timedependent, spatial variations in concentrations, temperature, and.. pressure

are important. Analytical tools for modeling directed jets and explosive releases are discussed as well* as some of the complex phenomena to be

considered in analyzing UF6 releases both indoors and outdoors.

v

1. VNTRODUCTION-AND SCOPE.-

The Nuclear Regulatory Commission (NRC), Office of Nuclear Regulatory Research, is -sponsoring theFuel Cycle Facility Safety Research Program for the purpose of -developing improved methods for determininig and 'characterizing accidental releases of radioactive: materials at NRC-licensed fuel cycle facilities., As

part ~of that program, the NRC Division of Risk Analyi ipraing the Fuel Cycle Facilit Accident

Analyss Handbook (AAH),to provide analytical techniques and; a data base for preparing realistic accident

assessments. The NRC Transportation and Materials Ris .k Bradch has requested that the Engineering Divi-,

Sion' of Union ;Carbide Corporation, Nuclear Division (UCC-ND Engineering) assist in developing that

handbook by considering UF6 handling systems. The objectives of this- project are (1) to identify and

define the major accident Scenarios in fuel cycle facilities that involve the release of UF6 , (2) -to determine

the importan t parameters and, data required for UF6 release accident assessments, (3) to evaluate available

methods for' determining Source -term s for such accident assessments, and,(4) to document the above infor-

*mation for inclusion, in Chapters. .2, 3,' and 4 of the AAH.

* The 'accidental r Ielease of UF6 results from the violation of containment of equipment, containing UF 6

either by equipment failure or operator error.. Once .a release occurs, UF6 and its hydrolysis products may

followoone or several of a number of pathways to the outdoor environment depending on the point of release

and the design and operating procedures, of the facility. Possible UF6 release pathways to the environment

-are illustrated in Figures 1 and 2.- -For example, consider a release from a cylinder that ruptures as a result

of being dropped from an overhead crane. The UF6 would be released into a room where some or all of the

UF6 could be hydrolyzed. If doors are open to the outside, then a release to the environment of at least

some of the UF6 ,products would.,occur, and UF6 and/or its hydrolysis products might also enter the ventila-,

ti~on system. If the ventilation system included filters, some products of a UF6 release, could be trapped,,

.while untrapped pr~oducts would pass on to the stack and out to the environment.

The scope of this study has been limited to. the' initial release of UF6 from containment into a room,

steam chest, or the outdoor environment, (pathways A, B, C, and D in Fig 'I), with some consideration for

the behavior *after release. Accident scenarios were to be developed from information in the literature

1
2

ORNL-DWG-83-16697

Fig. 1. Schematic diagram of some possible UF6 release pathways to the outdoor environment.

ORNL-DWG-83-1 6698

Fig. 2. 'Diagrammat~ic. representation of 'some of the UF6 : release, pathways shown in Fig. 1. Letters

on this-figure refer to pathways similarly lettered in, Fig. 1.
,(prima~rily documents in -NRC docket- files-. for. facilitie6', of itest)z, site. visits- toNRC-lcene faiiies,

and experience in DOE uranium enrichment facI6ities (to the extent that -op erIationfs and processes ar .e simnilar in 'nature to those in NRC-licensed UF6 productin and fuel fabrication faciliitis). These scenarios were;.

to address ýmajor accid~nii inivolving UF6 both inside, and outside 'theý buildingis but ithin facility bounida-

-ries, exclusv oacietinlving cyliniders secured for, transport to other fclte.The areas of 'interest

within facility boundaries include outdoor. cylinder storage, aeas transport to and, from sto'rage-areas, in the

:plan ,t, UF6 p, proce ssing ope rations, and sam ,pling and transfer operations,% The primary accident, analysis' concern in, this study was dev-elopment of methods for estimating the source term. itself *including the, amount o

material released, "As well as other source term information necessary. to' analyze a' release. :Quantitative.

analysis of release phenomena and 'co'nsequen'ces (such ais atmospheric' dispersion, dose commitments, structural damage, etc).was, beyond the scope of this investigation.

SAs a part of'th~e !Fuel Cyc ,le 'Facility Safety Research. PrIogram-, Los. Alamos Nat ional: Laboratory

.(LANL) and. BattelIle' Pacific Northwest ]Laboratories (PNL) are' deeoping- a. large ventilation. system'

model, a compartment model, a~nd a source term, model for accidents involving fire and explosion. These

areas are. excluded 'from this present -study except for a ýreview of the models for th.eir applicability to acci-

"'ental UF6 releases>.

Several. terms' have, been d~efined that are required, for.. this study and for subsequent activities.

important parameters are quantities that must be known or estimated to appropitl alyeancidt

and -its consequences. An event, 'controlling parameter' is any. important parameter that significatly afet

the event or the source terms characterizing the event.- ,A, small error in an' important ýparamekter that is not~i

an event. controllinig parameter will nt, signfcantly affect the consequences of the' a~ccident.. Source terms

are parameters that define And "c~haracterize the release into the. Affected surroundings*. Source terms do not

include ambient conditions.

The concept of source term is sometimes confusing because of its application. In t he casc'of a cylinder

rupture in a room, the mass, temperature, pressure, and phase of UF6 in the cylinder: at' the time of release

are source terms for evaluating the amount of UF6 vpr released from the cylinder, -which in turn is a

source term for evaluating the mass of UF6 hydrolysisý products, Anid %hence room air, concentrations. The.
A

mass: of UF6L hydrolysis products and air concentrations'are source terms for a ventilation system model that

calculates source'term's for an atmospheric dispersion model.

This report includes the results of the literature review, effort,, schematic descriptions of typical UF 6 handigsystems, a compilation. and discussion of credible UF 6 release .accident scenari~os, and a review, Of

methods for determining source terms for UF'6 rlease accbident assessment. The information-provided in

this report- will serve as a basis for preparing material for inclusion in the AAH4.
2. SUNMMARY.7

The objectives focused on in the *initial phases of this project. were (1) the identification and definition

of major accident scenarios in fuel cycle facilities that involve the release of UF6 and (2) the evaluation of

available methods for determining source terms for such accidents. A review of information obtained from

eight NRC dockets identified a few accident scenarios considered to be "bounding' cases in same docu-,

ments. The lack of specified scenarios from NRC sources led to the development of a list of potential.

major accident scenarios involving the release of UF6 based priarl on'the experience of and studies per-ý

formed by enrichment facility contractor personnel for the Department of Energy (DOE).. Available analytical methods, inicluding codes under development by LANL and PNL for the AAH, were reviewed for their

applicability to developing source terms and analyzing accident consequences for UF6 releases. To bring.

some perspective to the results of these activities, the scenarios were categoriied and discussed in several

different ways, which led to a: discussion of some bounding'considerations for UF6 releases.ý Modeling tools

needed for analy zing various ýUF 6 releases 'were also discussed.

Chapter 3. discusses information- from NRC dockets and licensees and presents the findings based on this

information. About 7000 pages of NRC documents for eight facilities were searched,, including -environmental impact assessments and radiological ,contingency -plans. These documents describe only two basic

scenarios -and' contain. little- useful informration for developing accident scenarios, involving the release of UF.

The principal goal of the NRC documents with respect to UF6 accidents was to consider aý bounding, event

that invariably involved the release -of :UF6 from a ýsingle. cylinder, containing liquid UF6. -Cylinder releases,,

may not, however, represent the, bounding event with respect to off-site consequencesý-. (see Section 7.2).

Furthermore,- it is interesting to noteý that several NRC-licensed, facilities predici..no significant off-site

consequences for a postulated multi-ton. UF6 release taking place only 'several hundred feet from public

facilities. Visits to several NRC-licensed facilities, while yielding a few new scenarios, did support the appli-

**cability of several UF6 release accident scenarios.

Descriptions of, UF6 handling systems in both UF6 production and fuel fabrication facilities are provided

in Chapter 4. These descriptions, which include block flow:daraso principal UF6 handling operations

5
6

and a list of equipment and operating'conditions, are, based primarily On material develope by PNL for

NRC's Fuel Cycle Risk Assessment -Program.

Major postulated accident scenarios involving the release of, UF6 at NRC-licensed facilities are introduced and discussed individually in Chapter 5 unde r general headings of cylinder failures, process"- system

failures, criticality events, and operator errors. Because detailed scenario data for accidents involving the

release of UF6 were not found in NRC documents,- a great Ier reliance on DOE experience 'and site visits to'-

NRC-licensed facilities was necessary than originally anticipated for, this project. The credibility, probability, and severity of consequences for each scenario are highly dependent on site-specific factors such as UF6

inventory, UF6 phase(s), UF6 enrichment, facility and site design, operating -procedures, emergency pro-.~.

cedures, and operator training. The distance to plant boundaries and the. off-site population density are

important in assessing off-site consequences.

Chapter 6 reviews the applicability_ of currently available methods for analyzing UF6 -releases. Section 6.1 presents an evaluIation of three codes. developed for the AAH (TORAC, EXPAC, and FIRAC)

with respect to their applicability to accidental UF6 releases including several DOE methods.," Three princi-.

pal conclusions resulted from this review:

1. At least five -areas were identified that must, be addressed in 'the tornado anlsscode (TORAC), the

explosion code (EXPAC), and the fire, analysis code (FIRAC) before they can simulate-UF6 ýtransport

without potentially large errors.

2. DOE has concentrated primarily. on the simulation of UF6 releases -outside buildings, whereas NRC has

concentrated on the .simnulation of hazardous material transport inside 'buildings; it is, .therefore,

believed that both agencies may benefit from an information exchange in these areas.

3. Several methods in 'the open literature exist that could be modified-for estimating the UF6 release rate

from ruptured cylinders.

Chapter 7 has been included to bring some perspective to the-scenarios introduced in Chapter. 5 And to:--

identify UF 6 release phenomnena andz, analytical tools needed for. analyzing UF6 releases., Uranium -hexafluoride accident scenarios aecategorized in Seict. 7,1 by location, source, initial pha'se(s) prior. to release,
flow characteristics, release causes, initiating events, And inventory :at rs. Telte ehdo aeoz

tion is 'used in Sect. 7.2 as' a biasis' for discussing some bounding consideration's for UF6 release 'events

because an inventory at risk can be defined for systems, of equipment separAted by batch operations. 'This

discussion leads to the selection of at least three types of UF6 releases that should be considered further:

1. release from a liquid-filled cylinder outdoors;,

2.release from a pigtail or cylinder inside a steam chiest; n

3. release indoors, from either, (a) a, pigtail or liquid-filled cylinder or (b) other 'indoor system, depending

on release rate and-duration.

It. is suggested that generic source terms for the release of UF6 from_ pigtails and cylinders be developed

because of their, applicability to the above releases.

Section 7.3. discusses two general approaches to modeling indoor UF6 releases as well- as an overview of

outdoor, release phenomena. The'two approaches are the use of either a' batch-mixed or time-depenidenthomogeneous mixture model for determining source terms- ~to .a ventilation model or A more: accurate hydrodynamic model. when time-dependent, spatial variations, in,ý temperature, pressure, and composition arc

important. Analytical tools needed to model directed and explosive releases of UF6 arelalso identified, anhd,

aspects of UF6-,hydrocarbon reactions in the presence of fireý or'inside a cylinder-are briefly discussed.,

Chapter 8 presents 28 calIculational methods required for tither i first-ordeir approximation or a more

accurate analysis,"fapsuae release of UF6. The applicability of thei methods to. the scenaro

presented. in, Chapter 54is, given as well as the current availability of those, methods.

Conclusions, many of which have been discussed above, are listed in Chapter 9.

References for this study are given in Tables I and 4, which ma Iy be found in Chapter 3, and in the.,reference lis .t following the text of this report. fi bibliography of other documents, relevant to UP6, source term

development is also provided. Uranium hexafluoride release studies, UF6 physical and thermodynamic data,

experimental studies, historical release studies, and other studies are cited in this bibliography.

3. LITERATURE REVIEW

'A lit:ratur reiw aS. codce ooti urntly available information on accident scenarios and

analyses as' well.,as to better understand the UF 6 handling operations at NRC-licensed facilities., This

review was supplemented by information on -DOE experience ini enrichment facilities and by visits to several

NRC-licensed facilities.

Documents pertaining to, eight NRC-selected facilities including two UF6 production plants and Six fuel

fabrication facilities were obtained -via several routes. An initial iearch of the DOE library facilities, in. Oak

Ridge yielded few, documents. After requesting* assistance from the NRC Project Manager, arra Ingementsý

were made to inspect NRC-licensing filesin Silver Spring, Maryland. This route yielded some useful documents, but time did not permit An in-depth review of all the available material. The NRC Public Document

Room was also contacted for information ,on facilities of interest. A computer printout listing references for'

all docket materials filed for the selected facilities 'since 1978 was obtained. along with a less exhaustive. ref-.

.erence list'of pre-1978 materials pertaining. to the eight selected facilities. After reviewing these lists, a

number of document Is :were obtained through the Public Document Room. The NRC Project, Ma nager also

supplied several requested documents.' In total, about 7000 pages of NRC docments were reviewed.

~Most, information on major accident ~events was contained in Environmental Impact Assessments (EIAs)

or in Radiological: Contingency Plans (RCPs). The event usually considered in these documents -was the

release of U176 from' a damaged cylinder containing liquid UF6 or from. associated auxiliary equipment.

Release quantities postulated' varied considerably (23 -96% of cylinder contents) from -one document to'

another. Although initiating events and detailed accident scenarios, were not given, postultd:oniin

included rupture on, heating from prior overfilling or contamination 'lwith foreign gas and failure of a cylin-ý

der valve or associated piping. Failure of a cold. trap in a UF6 production facility was also postulated, to

have consequences similar to a cylinder rupture. Table 1 lists dates of the most recent ElAs and RCPs for

facilities of interest as well a-s sections of those reports that discuss accidents involving the release of UF6.

The EJAs and RCPs reviewed did not contain details of the analyses performed, although'chemical. and

radiological exposures to individuals at several locations (e.g., nearest resident) were given in several reports.

9
10

Table 1. References to postulated UF6 release accidents and criticality events found

In environment impact assessments and radiological contingency plans

Environmental Impact Assessment Radiological Contingency Plan

Facility Docket Date Applicable. Date Applicable

and location number published section's published sections

UF6 production

1. Allied Chemical

Metropolis, IL

2. Kerr-McGee

Gore, OK

40-3392 8-77

4MO827 10-77

6.1.2, 6.1.3,

6.2.2

None

6-81* 3.1, 3.3

3-82 3.3

Fuel fabrication

3. Combustion Engineering

Hematite, MO

4. Babcock & Wilcox

Apollo, PA

5. Nuclear Fuel Services

Erwin, TN

6. General Electric

Wilmington, NC

7. Westinghouse

Columbua, SC

8. Exxon

Richland, WA

70-36

70- 135

70-143

9-81

10-78

1-78

5.2.2, 5.3.2

6.2.3

None

None

None

5.2.1

1-82

8-81

&S-I b

1-~82c

Appendix G

8-8 1d

8-8 1le11-819

3.1

None

None

3.3.3 &

None

None

70-1113 6-75

70-1151 4-77

70-1257 8-81

Other documents

9. Supplemental Environmental Information Related to Installation of Uranium Hexafluoride Conversion

Capability, B& W Commercial Nuclear Fuel Plant Expansion, Lynchburg, Virginia, BAW-141 2, Annex

(Draft), Battelle Pacific Northwest Laboratories, Richland, Washington, June 1976, Sects. 5.3 and 5.4.

10. Potential Radiological and Chemical Toxicity Consequences of an Accidental UF6 Gas Release In the

Exxon Nuclear U0 2 Plant, .XN-NF-562, Exxon Nuclear, Inc., April 198 1.

11. Radiation Control at NFS-ErWin and Generic Considerations for. Other Fuel Cycle and Materials Plants,

SECY-80-519, Nuclear Regulatory Commission, November 24, 1980.

12. Environmental Survey of the Nuclear Fuel Cycle, USAEC, November 1972, pp. E-32 t6 E-34.

13.- C. M. Vaughn (GE, 70-1113), letter to NRC Director (to attention of W. T. Crow), Attachment 1,

June 1, 198 1. (Letter Subject: *Modification 2 to Application Amendment N-2, Expansion of Plant

Conversion Capacity", 12-21-79).

*Updated 1-82.

bUpdated 3-82 and 4-82.

'Titled as Radiological Contingency and Emergency Plan.

dTitled as Emergency Plan.

"Titled as Emergency and Radiological Contingency Plan.

fRev. 10 of Part 1.

'Rev. 6 of Part' II.
11

'Several other 'docu menits, containing analyses of postulated UF' releases are also listed in. Table 1. Table 2

tabulates available information' on postulated releases,: including total UF6 released, release duration, and

other assumptions.

Two criticality events were postulated, by General Electric, (GE). In their Radiological Contingency and,

Emergency Plan (1-82, paragraph 3.3.3 and Appendix G), a 'criticality ent ispsulated to result frm

the introduction of UF6 into a vessel containing water when the uranium enrichment exceeds the design

-basis" enrichment. A letter from C. M. Vaughn (GE) ýto the NRC Director (June 1, 1981) included. a

complete criticality sa fety -ana lysis for UF 6 cylinders in steam autoclaves- based on 5% 2"

5U enrichment.-

Additional details of these postulated criticality events are givenmin TableA. while additional reference information is given in Table 1.

Reported historical releases at facilities of interest ranged. from much less than a pound. to over-100 lb

of UF6. Several documents reporting these historical events are listed in Table 4. These events. involve

pigtails, valve seal or other valve failures, and piiggse alrsoccurring in UF 6 fe vpratio

areas of fuel-fabrication fabrication facilities and distillation sections and cold trap systems of UF6 production facilities. Table .5 gives information on reported historical releases of UF6.

Most other NRC documents that were reviewed did not contain information on 'postulated or. historic

UF6 accidents;, however, several documents repeated information contained in the documents noted, Above.

These other. documrents included safety evaluation reports, radiological assessments of individual dose' from

rotine oprations, a health and safety manual, several emergency procedures predating the, RCPs, and various licensing applications and renewals for the fuel fabrication facilities. In general, little information useful for estimating UF6 'release source terms for inclusi'on .in the AAH was found in the eight -NRC dockets

searched.

Review of the NRC and NRC-licensee documents did not result in a comprehensive list Of credible UF6

release events. This was, generally the result of -the NRC and the licensees concentrating their efforts on

postulating what they considered to be bounding UF6 release aciet.Uniformly, the'rupture of a cylinder containing liquid UF6, either inside or outside a facility (but not both locations), was considered by

NRC and their licensees-to be a-bounding UF6 release event. Cylinder releases may not, however, represent
Table 2. Postulated UF6 release events documented for NRC-licensed facilities

Licensee UN6 Release .Av rate Phase of

Event and released duration of release UF6 Other assumptions Refs

docket no. (lb) (min) (lb/mmn) source Table I

Rupture of a

10-ton cylinder

Rupture of a

.10-ton cylinder

or cold trap

Rupture of a

2.5-ton cylinde

Allied

40-3392

9200 Vapor Temperature > 1490 F

UF6 reacts totally

with moisture in

ambient Air

Released outside bl~dg.

1, EIA

Kerr-McGee

GE'

r 40-113

4500

4800

50

long

112 Vapor 1.5-in,. hole in cylinder

100% .of release in bldg

leaves through roof vent

Cylinder cooled with water

spray to minimize release

Solid Bounding release outside

facility

2

6 -4

t.o

Release of highly

enriched uranium

from a 16-kg

cylinder

Valve or line

failure of a

cylinder being

unloaded

NFS

70-143

Combustion

ý70-'36

33 Particle Size = 5 pum

(3 gm AMAD)

,UF6 reacts with water

in atmosphe're

Highly enriched material,

22% of c ylinder contents

released as vapor

UF6 reacts totally with

moisture in ambient air

1200 15 80 Vapor/ 3, EIA

solid

UF 6 transfer

line leak

B&W 85-140 _ 15-25 5.7 Vapor Total UF6 release in

building gosto, the

.environment

Cylinder maintained at 212"F

durin'grelease

I -in,.. line releases UF 6

at a rate of 340 lb/h

9.
Table 2. (continued)

Licensee UF6 Release. Av rate Phase of

Event and released duration of -release UF6 Other assumptions Refs~s. '.'docket no. (Ib) (min) (lb/mmn) source Table 1

Vaporizer- line

break lingide

Exxoin

70-1257

11.00

(insid'e),

110

(outside)

3 37

10% of UF6 released

inside is released to

the environment*

Rapid release* to the

environment via exhaust

stack-or-out of building

at ground level

.5wt%9ý 23.

5u

HF concentration outside

at least ten times airborne

u .ranhium concentration,

UF. reacts totally. with.

water vapor in air to

produce U0 2F2 tpito

:release to. the environmentVapor 1% of cylin'der'cointents is

released to the environment.

Release via. pressure relief

valve and UF6 Scrubber, both

of which'function',0roprly

Evaporator temjerature = 212*F

10

Release fro'm

2.5-ton -

cylinder inside

an evapprator

.B&W 50 ý9
Table 2. (continued).

Licensee UF65 Release Av rate Phase of

Event and released duration of release UF6 Other assumptions Refs

docket no. (0b) (min) (lb/min) source Table I

Release from

2.5-ton

cylinder

inside an

evaporator

M~w

70-135

112965. .

(evaporator)

(258 lb

U0 2F2ý to

building)

50 118 lb/mmn

of U0 2F2

initially

to environment

10% of *UF6 released vents

to work area'

Fans without. HEPA filters

will not stop during

release to work area

50% of release will deposit

on building floor :

Rate of release to environment

decays with time (half-life

constant =3010s)

.AllUF6 released reacts with

water vapor-in building air

prior to release to the

environment

More than half of U0 2 172

aerosol will deposit

1% of released- uranium

escapes building

10% of HF formed escapes

building L

HEPA filter plugs resulting

in a slow release to the

rest of the building

4

Overf illed

2.5-ton

cylinder

Generic

fuel

fabrication

1540.

(inside)'

15

(outside)

35 44 12
Table 2. (continued)

Licensee UF6 Release Av rate Phase of

Event and released duration of release UF6 Other assumptions :I Refs

dokt no (lb), (min) (lb/mmn) source.Tbl

Fire in

cylinder,

storage area

(cold storage)

Tornado-induced

release

B&W

-B&W

555-.2775 2.5-12.5, 222

6.6

Vapor" Twelve 30A cylinders per:

1000. ft2 of concrete pad

Truck crashes into, cylinders

releasing, 100,gal of gasoline

Truck ruptures two 2.5-ton

cylinders

Gasoline distributed over- 'A

of pad and burns 2.5 to.,

12.5 miii

Radiative heat, transfer:

coefficient for a 20000 F fire

and an 800 F cylinder =32.4

Btu/h'ft2 'o 

F

.Heat flux at 10..ft from,

center 61f..20-ft diameter pool

of burning gasoline = 0.25

times the radiative. heat

transfer coefficienit

The cylinder-fire temperature

difference =17800 F, resulting

in,.6500 Btu/min of absorbed

heat per cylinder

Tornado shears line from U6

evaporator

All released UFj reacts with

water in atmosphere to form'

U0

2

F2 p aticulates of <1 0 gim

size 

.

.

-

9

C.11

4750 S720

9
16

Tabe 3 Potulted cti cality events involving UF6

Licensee UF6 Rlease

Event and released period Other assumption s Refs,

docket no. (lb) (h) Table: 1

Criticality due to GE .1018' fissions per event, 6

high enrichment UF 6 70-1113 -Vessel not safe for high

introduction into enrichment UF 6

vessel containing Volatile noble gases and

water iodines released

Steam autoclave GE Variable <40 5% 235

U -enrichment 13 a

criticality 70-1.1131 Water accumulates in annulus

(>92 gal) or inside cylinder

(>41 gal) following loss .of..

containment

'Information was taken from a-complete criticality safety analysis for. steam autoclaves in the GECO

System.

Table 4. Documents reporting historical accidents

Preliminary Notices of Event or Unusual Occurrences

Facility

1. GE2. GE

3. Westinghouse

4. Westinghouse

5. Westinghouse

6. Exxon

Notlice no.

PNO-II-80-92

PNO-11-8 1-76

PNO-II-80-54

PNO-11-80- 133

PNQ-II-80- 135

PNO-V-82-1 1

Date

5-22-80

9-16-81

4 -07-80

7-31-80

8-04-80

2-26-82

Other documnents

7., EIA for Allied Chemical, 8-77, Sect. 6.1.2.

8. A. L. Kaplan t6.J. T. Sutherland (NRC), January 19, 1979, letter and attachment.

9. Radiation Control at NFS-Erwin and Generic Considerations for Other Fuel Cycle and

Materials Plants, SECY-80-5 19, Nuclear Regulatory Commission, Nov. 24, 1980.
17

Table 5.. Summary of historical accidents involving UF6

at selected NRC-licensed facilitiesO

Licensee and Date of UF 6-released - as.Refs.

docket no. release (lb) Table 4

1 . Allied 12-6-68 :'95 Valve failure in UF6 7

,40-3392 distillation section

2. GE 12-3-78 not Main line, block -valve 8

70-11113 known opened after nitrogen

purge

:3. NFS 8-7-79 < 6.6 Accidental venting of 9

70-143 cylinder to exhaust

stack

4. '-GE 5-20-809 2.2 Pipe flange failure 1

70-1113

5. GE 91-1 < 163' Flange gasket leak 2

70-1113

6. Exxon 2-2-82 50Softening of Teflon seal 6

70- 1257 on conversion line #I

- vaporization chamber

7. Kerr-McGee 100" Overheated Teflon gasket c

40-80217 while 'melting UF6 in cold

trap drain system

" Several nuisance releases of a~ few grams involving pigtail connections or.

removal of, UF6 Plugs from pigtails are not. listed (Table 4, Refs._ 3, 4, 5).

bMOSt Of the UF6 released may have been collected by the' ventilation cleanup

-, system.

'It dentified during site visit.

t he bounding' event with respect to off-site consequences. Other types of accidents were not postulated,

'although several lesser. historical accidents have been documented. Apparently, only. a- few of the NRC

licensees have released documentation of detailed calculations of the analysis of their bounding UF6 release

accidents. Other "references useful to this study but not included in Tables 1 and 4 are provided in the bibliography for completeness.
18

Because detailed scenario: data for accidents involving the release of UF6 *ere not found in NRC .documents, a greater reliance on DOE experience and site visits to NRC-licensed facilities was necessary than

originally anticipated for their project. Project team members who have significant UF76-related experience

and who have participated in safety analyses of the DOE uranium enrichment facilities provided the

greate st amount of information on potential accidents involving UF6. Based on this experience, possible

UF6 accidents in UF6 production plants and fuel fabrication facilities have been postulated (see Chapter 5).

After reviewing the NRC dockets for the eight NRC-selected facilities and gathering- a list of potential

accident scenarios involving the release of UF6, it was decided that visits to several NRC-licensed facilities

would be made. The purpose of the visits was to gain a greater famil~ia .rity with these .facilities and to

expand and/or confirm the list of potential UF6 release scenarios for NRC-licensed facilities.

During the visits, it was -noted that there is a variety of site specific factors that can strongly affect the

potential for a UF6 release. For example, tornadoes and high, winds can be important safety considerations

at some sites. Also, site specific designs and operating procedures very strongly affect the potential for UF6

releases.

Info Irmal discussion s were: held with operatin g and manageme nt personnel to develop or est .ablish the

credibility of scenarios that could result in significant UF 6 releases,. but few new scenarios were identified.

It was apparent from discussions and observations during these visits that the NRC licensees have adopted

several engineering features to help prevent UF6 releases. For example, legs were added to UF6 cylinder

carriages to prevent cylinder dropping in the'event of ca.rriage axle failure. From discussions with NRC'

licensees, it was concluded that features such as this resulted from an informal sharing of UF 6 release experience among fuel cycle facilities-.

Discussions with licensees revealed no major'* UF6 rees;hwvr eea iorlases not found

through the literature review were mentioned. A release of about 100 lb of UF 6 resulted -from the

overheating of a Teflon gasket when an operator was attempting to melt UF6 that had solidified in a cold

trap drain system. In another incident,, a cylinder valve was, sheared off a cylinder containing solid UF6.

Since the cylinder contained solid UF6, the amount of UF6 released in the* latter incident was. very small.
19

At seVeral 'sites, 'cold traps are elevated., 110. to 15 ft above the plant-floor; therefore, a seismic release. scenario during a b eating cycle May be plausible. At one facility. where -there is .a poten Itial for hi Igh -wind or

tornado damage, a special plant shutdown policy has been instituted. to minimize th .e potential for a se Irious

UF6 release during periods when these conditions' are likely.

Pigtail reliability was a major concern at one facility where, eachi pigtail is replaced after using it to

empty, at most, 15 cylinders. At a facility handling,. highly enriched materi al, criticality was a concern

which led to the use of electrical heating -for UF6 feed, vaporization;ý however, other, facilities that handle

highly enriched :material use steam heat. Automatic UF6 ,le&ak-etection methods for -enclosures, where a

UF 6 release might not be irmmediatel y evident to operators were of in terest at .UF6 handl ing facilitie s.

Of major interest on one visit was a discussion about UF 6 release' management goals. 'One goal of UF6'

release management might be to contain any. UF6_ release inside a building. This goal is, however, in conýflict with another possible goal of minimizing worker exposure within a process building to UF6 and its

hydrolysis products. Depending on the goal desired, various UF6 handling facilities could develop quite dif-,

ferent safety system designs, ventilation requir~ements,' emergency procedures, etc.

4.` DESCRIPTHON OF UF6 HiANDLING SYSTEMIS.

.::Uranium hexafluoride is currently handled in three phases of the'comnmercial :nuiclear fuel cycle:. UF6

production, uranium enr~ichment. foperations,' and fuel fabrication.. In this section, we are concerned with

describing UF6 operations at UF6 ,.prod~uction and fuel fabrication "facilitie's licensed by NRC. Additional

UF6 handling process information can be found in a recent PNL report describing representative nonreactor

facilities.'

4.1 UF6 PRODUCTION FACILITIES

There are only two major UF6 Production facilities licensed -by. NRC. These facilities are located, nhear

-Metropolis, Illinois, and Gore, Oklahoma, and are operated by! Allied Chemical and Kerr-McGee, respectively. The handling of UF6 at these two facilities differs significantly; the, chief difference involves uranium

,purification. The Kerr-McGee. facility uses solvent extraction to purify the uranium before fluorination, and

the Allied facility uses distillation to purify the uranium as UF 6 after 'fluorination. These facilities served as'

generic models for the PNL report. A process flow diagram -for UF6 handling operations based on the

fluorination - fractional distillation process is presented in Fig. 3-an~d another, based on the. solvent extraction process, in Fig. 4. The various opeain shw-nFg.3 and 4 are discussed in subsequent para-ý

graphs.

A summary of important parameters including UF 6 temperatures, pressures, inventories, and phase as

well as numbers of vessels for UF6 production facilities and fuel fabrication facilities is presented in

Table 6. With the exception of cold product cylinders, UF 6 is handled at pressures ranging from slightly

above, atmospheric pressure to over 5 atm. These pressures help prevent* inleakage of moist air that would

react with UF6 to' ioduce a U0 2 1`2 deposit that could plug equipment and increase release potential.

4.1.1 Fluorination

Solid UF 4 is~ reacted with F2 gas to produce UF6 gas in this first UF6 handling step. Process temperat ures and pressures used in the. two processes are similar. The fluorination-fractional distillation process

21
22

ORNL.DWG-93-16699

.UF4

STACK

GAS

,TO ASH TREATMENT LIME/KOH/wATER

URANIUM

STORAGE*TRANSPORT- OF Liouib-FiLLED CYLINDERS

TO ENRICHMENT FACILITIES

Fig' 3. Flo diagram for UF6 handlig operations based on the fluorination-fatoa ditillato

process.-
.23

ORNL-D)WG4-8.16700

F2 FLUORINATION.RATO

(FLAME low L )

UP6 GAS. UF6 GAS

DOLPING/ TCAOLING/TRPIGSRUSG

PRDUALCUA

CLT INDEREIN

tFILLEING AHA

OF CYLINDERS

* TRANSPORT OF LIQUID-FILLED CYLINDERS

TO, ENRtICH4MENT

FACILITIES

Fig. 4. Flow diagram for UF6 kandling operations based on the solvent extraction process.
Table 6. Important UF6 process. parameters at NRC-licensed facilities'

Process LieseSaeApproximate Approximate UF phse Number of Max UF6 inventory

LieseSaepressure (atm abs) temp (*F) 6 pas units (lb/unit)

Fluorination Kerr-McGee Primary -1 750 t 110 VN 5 -3'

Compressor -INA V 2 .NA

Cleanup .. . (I 80V 2 NA

Allied, Primary -I795-815 V 2-+ 1 spare NA

Cold trapping Kerr-McGee Primary <1 36 V/s 4 21,0600

(collection). Secondary <1 -67 V/S 2 3,000

Allied Primary <1 -20 V/S 10 10,000

Secondary <1 NA V/S 6 2,000

Tertiary & sample <1 NA. V/S 5 1,000

Cold trapping All All <1-7 147-250 L NA NA

(liquefaction)

Distillation Allied Still feed -5.7 -200 V/L 3 20,000

Vaporizer -5.7 -200 V/L 1 10,000

Low boiling column .. 5.7 200 (avg) V/L 12,000

Low boiler reboiler -5.7 -ý200 V/L 110,000-

Low boiler condenser -5.7. -ý200: V/L 4 1,000

High boiling column 6.4 240 (avg) V/L. 1,000

.High boiler reboiler -6.4 -240 V/L 110,000

High boiler condenser -6.4 . -240 V/L tI 1,000

Cylinder filling Kerr-McGee Inside building <1.7 Ambient-250 V/L/S 2 214,000--27,600

and sampling Outside building/ <1-7 Ambient-250 V/L, 21,000-27,600

Steam chests

Allied Inside building <1-7 Ambient-250 V/L/S NA 28,000

Cylinder storage Kerr-McGee Cooldown outside <1-7 . Ambient-250 V/L/S <10 21,000--27,600

Cold storage outside <1 Ambient 5 >10 21,000-27,600

Allied ooldown outside <17Ambient-250 V/L/S <14 2,0

Cold storage outside <1 Amrbient 5 >14 28,000

Cylinder heating/ .All fuel fitb. <i-6 Ambient.2'50' V/L/S 2-6 1 10d-~4800

steam" chests

Hydrolysis All fuel fab. I Near ambient V 1-2 <50

UF6 scrubbing All. I Ambient: V 1-2 -0.

0NA =not, available in the public domain.

bV -vapor or gas; L = liquid; S = solid.

'The actual inventory at risk in an accident situation may involve the in~ventory of several units depending on site specific. configurations, operating procedures, and accident conditions.

dHighly enriched uranium.
25

uses ~a cýalcium fluoride (CaF2) fluidized'bed- to -carry -out the reaction. 6The;CaF2 bed, becomes contamiýnated by impurities in the UF`4 feed-and must periodically be replaced. The spent c-atalyst or "ash' from the

bed isosent to 'an ash treatment process to recover any uranium. The solvent- extraction process uses a-

'flame tower". Imcompletelyý reacted uranium and some impurities in, the process are collected at the bot-

,tom'of the flame tower in an "ash receiver" and are recycled to recover uranium.

The fluorination reaction is, strongly exothermic., The reaction temiperature is controlled by, the ýdiluting'

effect of the calcium fluoride -and'by an air-cooled jacket in the: fludiz ed -bed. The flame, towers are, cooled

by an external steam.,coil.

The 'fluo'rination product gases from the fluidized beds flow -through two. I O-Asm sintered nickel filters

.and then on to the primary cold traps. Product gases from the flame towers are cooled, passed through two

sintered metal filters and a bag filter, :and then compressed prior to piary cold trapping'.

4.1.2 UF6 Cailection-and. Gas Cleanup

In both processes,' product gases from the fluorination reactors are, passed through cold trapsý to! condense

UF6 as a solid. Gases that, pass through the cold trap, including unreacted F2, HF,-and other noncondensibles, as well as a-trace amount of UF6 are removed from the second primary cold trap by an air ejector or

by pressure difference, and,.subsequently pass to a gas cleanup system.

The gas. cleanup system for the fluorination-fractional -distillation process reacts cold trap off-gases with

potassium' hydroxide (KOH) in a two-step process and recycles the uranium, precipitates.. The gas cleanup

approach in the solvent extraction process is to react excess F2 in -the off-gases with fresh UF4 solid at elevated temperature in a 'cleanu .p reactor. After slightly cooling,-the cleanup reactor off-gases .in a. "screw

cooler," the gases pass through two sintered metal filters and a bag filter. These filtered. gases are then

Passed through a primary cold trap (360 F) and then a very cold secondary cold trap (-58o F) to condense

UF6. Off-gases from the secondary cold trap are mixed with air, burned, and fed to a burner/hydrofluoric

,acid scrubber before being released to the environment.

Once the.:cold traps are full,--the UF 6 is melted 'and drained by gra~vity to' the distillation feed vaporizer

the fluorination-fractional distillation' or* directly to product cylinders in th-e. solvent-extraction process.
26

ý4.1.3. UF6 Distillation. Purification

Saturated UF6 vapor from the distillation feed vaporizer is fed to, the low boiler stripping column that

removes high boiling contaminants from the raw UF6 as the overhead condenser product. Off-gases from'

the overhead condenser on the low boiler pass through a cold trap and are routed to the off-gas treatment

system for uranium recovery and scrubbing. Stripped UF6 liquids are pumped to the high boiler rectifying

column where high boiling contaminants are. removed as. column bottoms. Purified. U176 vapor from the

hig h boiler column overhead, is collected in cold traps The UF6 ste etdi heetasadfosb

gravity to a 10- or 14-ton product cylinder.

4.1.4 UF6 Product Cylinder Handling

Filled UF6 cylinders are either-sampled or sent directly to a cool-down area outside. At one facility,

filled cylinders are moved outside the building to be homogenized by heating in. a near-atmospheric-pressure

steam chest. The homogenized cylinders are then moved to a sampling station inside the facility, where a

sample of less than 5-lb is taken.. *After sampling, the cylinders are-moved to a. cool-down area outside the

main building where they are left several days before being moved to a cold storage area. ICold cylinders

are ready for shipment to enrichment facilities.

Product cylinders for UF6. production facilities have nominal capacities of 10 or 14 tons., Some 14-ton

cylinders have a wall thickness about half that currently acceptable for new cylinders. 2 Additio nal data on

cylinders can be found in, refs. 2 and 3.

4.2 FUEL FABRICATION FACILITIJES

Details of UF6 feed processes at NRC-licensed fuel fabrication facilities vary' significantly. depending on

cylinder size, uranium enrichment, U176 oxidation process, etc. Most fuel, fabrication facilities -in this study

handle UF6 as shown in Fig. 5. Cold cylinders are moved by forklift or other device to either a steam autoclave (usually just a steam ýchest) or an enclosure where the cylinders can be heated by electrical resistance

pads. Cylinder temperature and often pressure, are monitored during. cylinder heating. In the ammonium
.27-

ORNL7DWG-83.16701'

TO STACK

U% CYLINDER

STORAGE

SOFF-GAS

FILTRIN WASTE SOLIDS

SAPORIZATION VETt_ _ :UF 6

________SCRUBBING

[,HYDROLYSIS' VN

OWOXIDATION

URANIUM PRECIPITATES, TO HOLDING POND

URANIUM OXIDES

IN SOLUTION

Fig. 5. Flow diagram for JF,6 handling operations at a typical fuel fabrication facility.

diuranate process for producing U0 2, UF 6 vapors (typically at about 2300 F and 80 psia) are passed,

through heated piping to a. hydrolysis unit where the UF6 vapor reacts with water. to produce .U0 2 172. A

gas-phase hydrolysis process, (direct conversion. fluidized bed) is being investigated for future use by at least

one fuel fabrication facility, but in any case all uranium in this UF6 , vapior is converted into nonvolatile uranium compounds. !Effluent gases .from these UF6 conversion processes are. passed through venturi 'or,

packed-tow ,er scrubbers and then through roughing filters and a HEPA: filter before they are veit~ed to the

atmosphere. It is not.. known whether all' NRC-licensed facilities vent effluent ga~ses to: scrubbers and/or

through HEPA._filters.
i1⁄4t

w~'
S. VF6 ,ACCIET SCENARIOS

Based on. a review of available literatu .re, experience at DOE facilities, and results of DOE enrichment facilities safety, a .nalyses, a -.list, of generic scenarios fo~r NRC-licensed UP6 handling facilities (Table 7) has been developed. Many of these incidents are based on historical events..

Table, 7. -iF6 aleident; scengAios

1I. UF6 cylinder failures -

1.1 introduction of reactive hbydrocarbons 1.2 Impact into a cylinder of a liquid~filled~cylinder against an object or impact of an object on a;, cylinder

1.3 Valve or pigtail- failure due to movement of a connected cylinder containingý UF 6 ,

1A4 Hydraulic rupture of a cylinder exposed. 1.5 Hydraulic to fire rupture of in overheated 1.6 Hydraulic cylinder rupture of an overfilled 1.7 Heating cylinder or filling a defective cylinder 1.8 Heating a cylinder containing excessive volatile and/or gaseous contaminants 1.9 Dropping a liquid-filled cylinder

2. UF6 process system failures

2.2

2.3

2.5

.2.6

2.7

2.V

2.9

2.10

2.11

Excessive heating of process equipment containing solidified UF6 Fatigue failure of a process systemn Im*pact on a process Valve failure of 

system containing UF6 a cylinder or a system containing UF6 :Pigtail failure

Process- system loss of containment Heating caused; bnarlphnomna a cold trap, containing excessive volatile and/or gaseous contaminants Heating an overfilled cold trap Overheating a cold trap Cold. trap failure- caused by corrosion,. fatigue, or thermal Venting of UF shock 6 through a hydrolyzer

3. Nuclear criticality event

-1.1 Nuclear criticality in a UF6 vaporizer 3.2 Nuclear criticality resulting from, a safe spacing -violation 4. Operator error

4.1

4.2

4.3

Valving a cold trap to a vacant' position Bypassing, safety'controls Removing a valve from a cylinder containing UF6

29
30:

Thes e postulate :scenarios are. believed to becrdile ut :their* occurrence may be ifequn.Hsoi

cally, few significant UF 6 releases have been experienced at UF 6 handling facilities within the three. DOE

.uranium enrichment, plants and the NRC-licensed facilities, and these releases have not resulted in fatalities

to on-site, or off-site personnel. As each reportable' event occurred, its' cause and consequence were

evaluated, and in many cases, administrative or design fixes have been made'to reduce the likelihood and/or

the potential consequences of a future UF6 release. Compliance with the American National Standard

Institute (ANSI) standard, ANSI N 14.1 entitled Packaging of Uraniam Hexafluoride for. Transport can

prevent or, minimize, the consequences of future incidents involvn cyliders.2

No attempt has been made to suipply .information on consequences or probabilitie's for .the postulated

scenarios. Consequence- analysis requires such site specific details as cylinder size, isotopic 'enrichmnent, produc~tion capacity, building volumes, site boundaries, locations,- population density, meteorological conditions,

postulated natural phenomena, containment philosophy,' process layout, operating procedures, and process

parameters such as temperatures, pressures, and flow rates.: Experience in consequence analysis for similar

scenarios at uranium enrichment plants indicates' that conseune s wl eedsrnl nst specif ic

details and that -consequences can vary considerable between different operations within a facility. Similarly, personnel of specific.UF 6, handling facilities should be involv'ed directly in -assessing the probability of

-occurrence based on site specific operating procedures, design parameters, and historical experience

Although. some. scenarios may not be credible at all NRC-licensed UF6 handling facilities, (e.g.,, nuclear

criticality in a UF 6 feed production facility), at' least two events are believed- to be credible at all such

facilities: (1) release from a cylinder containing 'liqid, UF 6 and (2)ý failure of a pigtail. Following

approved operating procedures and using safety systems such as UF6 containment devices may prev .ent the

release of significant quantities, of UF.6 from a facility.

In summary, consequence analysis of postulated UF6 -release scenarios for NRC-licensed fuel cycle facilities will require de~tailed .site, specific information. The plausibility of each scenario listed in Table 7 and

described in the remainder of this section should be considered lor each. facility,. as appropriate.

In the following scenarioý descriptions, an understanding of UF phsical properties is assumed, as well as

a working understand ing of 'equipment and terminology generic toUF6 han dling facilities. These scen.arios
31

appeAr* capable of, resulting in signific~ant cons'equences under .certain ~conditionis;, scenarios resulting only in

small, nuisance-type r -ele'ases ar.eý not included..

5.1 UF6 CYLINDER FAILURES

Uraniumi hexafluoride' cylinders are used to transport solidified UF6 'at subatmospheric pressure. A

release may result if a cylinder' is damaged 7in transport and the :damnage -is not detected and acted upon

prior. to pressurizing the cylinder by heating.!

Cylinders containing. liquid UF6 Are susceptible to, rupture when dropped during handling operations or

when impacted. Cylinders that do not comply with ANSI N14.1 or thin-walled cylinders'used at some UF6

production plants are more. susceptible to such failure; these cylinders have been accepted under the current

version~ ofý the. standard as existing equii-pment. Cylinder fill limits based on- UF6 , Purity' specifications such

.that the'liquid UF6 occupies not more than. 95% of the cylinder volume at 2500 F are also specified in

ANSIý N1 14. 1. Violations* of these c-...nditions ýincrease the probability of the postulated release scenarios.

Atogh the mechanism of the chemical reaction between .-the UF6 an yrocarbons issomewhat

uncertain,. it is. known that the reaction of gram to kilogram quantities of hydrocarbon contaminants with

liquid UF6 is capaPble of producing sufficient energy, to explosively rupture equipment from the size of

pigtails to cylinders,, respectively. Because analytical techniques for detecting the presence of these contamn-,

inants are not practical for routine use, care must be exercised -to, prevent introduction of these contaminants

into any Uf 6,cylinder as specified in ANSI* N1 4ý 1

In the case of noncatastrophic cylinder failures, if effective corrective a ction is not or -cannot be taken

immediately when the failure occurs, most of the contents of a, cylinder containing -liquid. UF6 can be

released. The exact quantity of UF6 released will depend on the temperature and pressure of the liquid:UF6

prior to release as, well as on the characteristics of the cylinder failure. For example, if-a cylinder fails

above the liquid, level shortly after removal from an. autoclave or, electric heater system,, 'and if the UF6

within the cylinder is at 11 3*C ad5.61 atm prior to release, approximately 60% of the-liquid-UF 6 may be

released as UF 6 vapor. The remaining 40% will form solid UF6 particles. If a small hole or crack has

formed, most of the solid UF6 particles may be retained withinl the cylinder; however, a large crack may
32

release most of the particles along with the.'gas. If a large cylinder breach occurs below: the liquid level;

nearly all of the UF6 in the cylinder. may be released.

Nine cylinder failure scenarios have been developed., Compliance with, ANSI N 14.1 could, prevent or

minimize the consequences of these postulated scenarios:

1.1 Event: Introduction o~f reactive 'hydrocarbons into 'a cylinder.

*Description: The use of oil-lubricated vacuum-pumps to eva .cuate residual UF6 from cylinders could

*transfer oil to a cylinder containing UF6 either by operator error or inadvertent pump shutdown. -Sub-

*sequent refilling and then heating of the cylinder could result in an explosive re~action; 'thus releasing

the cylinder contents.

Comments: Although the' mechanism of the reaction 'between UF6 and hydrocarbons is somewhat

uncertain, several historical 'incidents have occurred during whichz cylinders were bulged or ruptured.

.1.21 Event: Impact of a liquid-filled cylinder against an object Or impact of an object on a cylinder.-

Description: Operator error or.equipment failure may subject a cylinder to an impact from' a moving

object or a cylinder may impact a stationary object while being transported.

Comments: Unprotected cylinder. valves are vulnerable,.to such' incidents. -Operating procedures at

.some facilities require the use-of protective valve covers when, liquid-filled cylinders are being' moved.

The movement of such a cylinder'. as well as the lift heights are also minimized.

1.3 Event: Valve .or pigtail failure ~due to movement of a connected cylinder' containing UF 6.

Description: Inadvertent movement of a cylinder connected to the process system can be 'caused by

operator error or failure of a cylinder support and could result in failure of the cylinder valve or connecting pigtail.

1.4 Event: Hydraulic rupture of a cylinder exposed to fire.

Description: Exposure of. a cylinder to an intense heat source, such as burning fuel, could result in

hydraulic, rupture:,of a cylinde r; however, slower release of the UF 6 due to solder failure in the valve

coupling threads .by melting is more 'probable. Fuel could come from sources such; as a fuel. storage

tank. or a fuel tank truck.'
33

1. vn:Hydraulic rupture of an overheated cylinder.

Description:,, An operator error, or the malfunction of temperature controls could result in cylinder

failure while heating a cylinder during sampling or vaporizing operations.

Comments: UF6 density changes from 318 lb/ft3 at 68*F to 1 90 lb/ft3 ait 300*F.- ANSI N14.lI

specifies a maximum, cylinder, temperature o 2500 F, where U46 density' is' 203 lb/ft3

. Ru Ipture is

more likely here than; in scenario, 1.4 because failure of the solder. in the valve coupling threads Ib .y

melting is not probable..

1.6 Event: :Hydraulic rupture of an overfilled cylinder.

Description: Normal he'ating of a cylinder during sampling or vaporizing operations could result in

hydraulic rupture if the cylinder fill limit has been exceeded.

Comments: Verification of cylinder weight and volume for comp~liance with ANSI N 14.1 fill limits

would preclude this event.

1.7 Event: Heating or filling a defective cylinder.

Description:- Cylinders 'or cylinder valves may be damaged in handling or transport incidents. If a

defect is not detected, a UF6 release could occur when the cylinder is pressurized during heating or,

filling.

Comments: Compliance with ANSI -N 14.1 would reduce the probability of this event.

1.8 Event: Heating a cylinder containing excessive volatile and/or gaseous contaminants.

Description: Cylinder fill limits are based on UF6 specifications defined in, the Federal Register.' The

presence of volatile impurities such. as HF .or, fluorocarbons anid/or gases such as air may-'cause

excessive pressures and subsequent cylinder failure when contaminated cylinders- a-re heated at normal

-.- temperatures.

1.9 Event: Dropping a liquid-filled cylinder.

Description: Causes of cylinder drops include operator error. in securing. lifting devices,, failure of cyl-

-inder support structures, failure of cylinder handling equipment hydraulic systems, and failure of,

crane components or lifting fixtures. The probability that a cylinder drop will result in a. UF6 release

is believed to increase as the lift height -(drop -height) increases.
1.34

5.2 UTF 6 PROCESS SYSTEM FAILURES

A significant UF 6 release may, res ult from a failure of any of the following process equipment handling

UF6. The extent of the release is dependent on the effectiveness of corrective' action.

Piping -Because, of the high coefficient of thermal expansion of solid UF6, piping and. instrument tubing containing solidified UF6 can rupture if heat is improperly.applied.- These systems are also subject to

fatigue resulting from vibrations and cyclic stresses,, and to impacts, from falling or moving objects.

Valves - Cylinder or process valves can fail from leakage through the valve seat, valve body failure

valve stem packing failure,- mechanical damage, or leakage through valve coupling threads as a result of Solder melting as in a fire..

Pigtails -Failure of a, Pigtail. connecting a -cylinder to the process system can result from defective,

damaged, or improperly designed pigtails, fatigue, overheating, or operator error.

Cold Traps -Cold traps may rupture if overfilled traps are heated to norm Ial temperatures'or- if properly, filled traps are overheated. Failure may result from -excessive pressure if ~A trap contains excessive vola-.

'tile impurities.' Fatigue or thermal shock may also lead to trap failures.

Hydrolyzers -Inadvertent release of UF6 from feed cylinders to. the atmosphere through hydrolyzers orý

U0 2F2 storage columns can result from. inadvertent system shutdown or operator error.

Natural phenomena, such as seismic, tornado, high wind, or flooding events, may also disrupt processing

equipment, cylinders, or! their supports.

.Eleven scenarios leading to releases from process equipment have been postulated:

2.1 Event: Excessive heating of process equipment containing solidified UF6.

Description: A pigtail,. a valve,: or process piping, containing condensed UF6 may rupture if heat is

improperly applied.. This event may result from' operator error or' it may result inadvertently when

system hea~t is restored after a system shutdown.

12.2: Event:, Fatigue failure of a process system.

Description: Process piping, systems, and 'instrument tubing are subject to vibrations or' cyclic

stresses and can fail, causing significant UF6 releases when systems contain UF6.
35

,2.3 Event: J1rnpact on a process system containing UF 6.

..Description: Inadvertent mnovement of maintenance equipment or process equipment being repairedý

in an operating area may impact on-stream systems, .causing failures that'result in loss'of UF6 Containment.

2.4 Event:. Valve failure, of 'a, cylinder or a systemr containing UF6.

Description: Valves .can fail from either seat leakage, valve body failure, or valve stem packing failuire. The valve mnay be inadvertently. removed from the cylinder by remote ýor automatic valve operators. Failure can. also be cause by operator error, scasoetrigwhlopenin orcoig, or,

by. improper assembly.

Comments:' Solidified UF6 or corrosion productions within the valve often make valve operations dif-!

ficult leading to the misloperation of overtorquing. Some valve components are vulnerable to stress

corrosion.

2.5;-. Event: Pigtail failure.

Description: -Failure of the- flexible connection (pigtail)ý between. a cylinder and the process' system

may be -caused -either by operator error involving improper fitting or by physical abuse, .Iinadequate

deisign, material fatigue, or overheating.

.2.6 Event' Process system loss of containment caused by natural phenomena.

Description: ýSeismic, tornado, *wind, 'or'flooding eventsý mfaiy disrupt 'processing equi Ipment,' cylinders,

.or their supports, resulting in. failure and, UF6 release.

2.7 Event:. Heating a cold trap containing excessive voiaýtileAnd/or gaseous conitamrinants.

Description: Operator failure to monitor cold trap pressure instrumentation during the -heating cycle

can permit excessive system:. pressures,: resulting in.vessel failure'and relea~se of UF6.

2.8 Event: Heating an overfilled cold trap.

Description: Operator error or failure of cold trap weight monitoring instruments, which .permits the

trap to be overfilled, may result in hydraulic rupture of the trap during, the heating cycle.

2.9 Event: Overheating a cold trap.

Descrition:Operator erro r or failur e of trap. temperature, controlsmycuehdalcrpueo

the vessel during the heating cycle.
86

2. 10 Event: Cold trap failure caused by corrosion, fatigue, or thermal shock..

Description: Exposure of cold traps to the corrosive UF6 atmosphere and repeated thermal shocks

may cause failures under normal operating conditions and result in UF6 releases external to the trap

or internal to the refrigerant system with ultimate release of UF6 to the atmosphere.

2.11 Event: Venting of UF6 through a hydrolyzer.

Description: Atmospheric venting of UF 6 from feed cylinders through hydrolyzers and U0 2F2 storage columns can be caused by the inadvertent shutdown of a recirculating water system, or by operator error which could either terminate the water flow or overload the system capacity by i ntrodu cing

excessive feed from the feed cylinders.

5.3 NUCLEAR CRITCALIT EVENT'S

Postulated scenarios include release of enriched assay UF6 into a steam-heated vaporizer containing an

excessive accumulation of condensate or violation of safe spacing of enriched assay containers. Criticality is

dependent on the enrichment of the urtanium involved, the presence of a neutron moderator, and the shape

and dimensions of the space in which the accident is postulated to occur. Therefore, analysis on a case-bycase basis is required to determine whether or not an accident can occur. The following postulated

accidents would not be expected in UF6 production facilities because natural uranium presents no plausible

criticality hazard. These accidents may occur, however, in fuel fabrication facilities where enriched uranium is present. These scenarios should be considered illustrative of the types of criticality events that

could occur.

3.1 Event: Nuclear criticality in a vaporizer.

Description: A UF6 release from a cylinder or pigtail into a steam-heated vaporizer of unsafe geometry containing an excessive accumulation of condensate could result in nuclear criticality. Causes of

the excessive condensate accumulation include operator error, condensate trap failure, or obstruction

of a condensate drain line in a vaporizer.
.37

3.2- Evenv:": Nuclear critcly reuting from a safe spacing violation.

Descripti on: Opera tor error o r mech an ical failure coulId cause a viol ation of: nuclear safe spacin.gý

of cylinders' containing enriched UF6 rslignnulearciiaiy

5.4 OPERATOR: ERRORS

A number Of the events described in Sects. 5. 1, 5.2, and 5.3 can be initiated or allow~ed to progress as a

result of operator error. Several other events are described in. thi~s'section that can be directly attributed to.

operator error. Tese events, which can result in significant releases of UF6 , include opening cold trap

drain valves to a position that is not connected to "A receiving cylinder, circumventing pr essure or temperature controls required for safety, or, inadvertent removal of the valve from a cylinder containing UJF 6.

4.1 Event: Valving a cold trap to a vacant position..

Description: Operator error -involving misvalving a cold trap to a position that is not connected to a

receiving cylinder could result in a significant release of UF 6.

.4.2 Event: Bypassing safety controls.

Description:. If controls required for safety, such as UF6 system pressure or temperature controls, are,

circumvented, subsequent failures could. result in significant releases of U176.

4.3 Event:. Removing'a valve from a cylinder containing UF 6.

Description: Operator "error while attempting to open a valve can- result, in valve removal" if condensed UF 6, or, corr .osio~n I-products cause a valve stem to freeze. Most cylinder valves are screwed'into

the cylinder head.. Opening the valve can result in removal of the valve if excessive torque is applied

by using a ,mechanical lever rod, for example.

6. APPLICABILITY OF AVAILABLE METHODS FOR-ANALYZING MF RELEASES

This chapter discusses available methods for analyzing UF 6 releases. The first section reviews the,

Applicability of several codes developed by LANL and PNL for the AAH for simulating UF6 release

phenomena. Methods that have been used or developed for analyzin UF6 releases are discussed- in. the

second section.

6.1 APPLICABILITY OF, AAH COD'ES

The -tornado analysis code (TOR'AC), the explosion analysis code (EXPAC), and the fire analysis,

code (FIRAC), 'as described in Appendices A, B, and C, re~spectively, of draft material for the AAH,

have been evaluated with respect-to (1) their applicability to' Accidental UF6 releases and, (2) 'their

ability to simulate such releases. In this respect, ,the review investigated the. basic assumptions and

physical -phenomena but not the details of the mathematical formulation or the adequacy'of the models

for purposes other than UF6 applications. Based on this review, at'least five areas were identified with

:respect to UJF 6 releases that should be addressed:

I1. phase changes and chemical reactivity of UF6 and UF6 .hydrolysis produýctsare neglected;

2. A uniform temperature and chemical composition withiný a compartment is Assumed;

3. an approximate. graviystln elto oe witi a compartment is used;

4. the ability to simulate changing particle size distribution. as a function of time is limited; and

5. chemical degradation effects of UF6 and its hydrolysis produicts.,on the Performance- 'of xplan't

equipment such as fans, filters, and ducts are not considered.

With respect to the first area listed above, consideration should be given to

1:. inclusion of water, UF6, UF6 hydrolysis products, and. their thermo-p'hysical properties in the

models;

2. simulation of chemical. reactions of UF6 and UF6 hydrolysis products with water, hydrocarbons,

ventilation duct walls, filters, fans, etc.;

39
40

3. in clusion of phase changes such- asý UF6 sublimation and HF condensation~ n

4. incorporation of the effects of energy released by these chemical reactions.

Because HF polymerization has only a small effect on the final specific volume, pressure, and temperature that results from a UF6 release inside a ,compartment, the" neglect of HF polymerization is

believed to be reasonable.

The models should include the simulation of the transport and chemical reaction of. water, UF 6, and

UF6 hydrolysis'products., Although FIRAC does allow the userf to model the transport of inert substances, the, user cannot model the transport of a chemically reacting species, such as UF6 . TORAC

and EXPAC can model the transport of only a single inert material. To obtain 'reasonably accurate

results, transport of a multicomponent mixture must be modeled during the simulation of a Iccidental

UF6 releases. Specifically, the models should consider, at. a minimum, the multispecies: transport and

*chemical reactions of air, H2 0, UF 6, U0 2F2 9, 'and HF'. The hydrolysis of UF6 and the vapor-liquid

equilibrum" of HF and H2 0 should. be simulated because they significantly affect temperature, pressure, s .pecific heats, molecular weights, deposition rates,- and particle size distributions of 'materials

inside a compartment. Also, phase changes such as UFA sublimation as well as HF-H120 vapor-liquid

equilibrium should be simulated.

Because accidental releases .of UF6 inside a building can produce -large changes in the building

atmospheric temperature and. pressure," it is important to include the heat of reaction, which is strongly

exothermic.

Several observations can be made about UF6 releases inside a building by considering a release, of,

UF6 into a compartment containing moist air at a specif ied initial temperature, pressure, and relative.

humidity. This cas .e can be analyzed by assuming that a UF6 release in side- an airtight'- comoa rtment

will result in 'a homogeneous mixture of air, UF6, and UF6 hydrolysis. products and by' not 'considering

heat transfer' to plant equipment and. to the. building itself. For example, if the air in the compartment

is initially at* 1 .atm and 800 F with a 60% relative humidity, the estimated resulting temperature and

-pressure in. the closed compartment following an instantaneous UF6 release are those "shown in -Figs. 6

ýand 7, respectively. As can be seen in Fig. 6, the temperature rise resulting from some releases can be
41

ORNL - DWG 83-16702 A .240. iltl

-LIQUID RELEASE, 148 OF

YIELDING 0.421 lb. VAPOR

220 AT 133.8 *F/lb LIQUID/

-- ` LIQUID RELEASE, 250 OF

YIELDING, 0.647.1lb VAPOR

* AT 133.8 OF/ Ib LIQUID

200 ~VAPOR. RELEASE, 148F

-- VAPOR RELEASE,250OF

W) 180

wk 160:"

120

100,

w 00. -.

120"

1O-;_411 0-3 '10-2 1O-1 100

lb UF6 RELEASED/f?3 0F:'COMPARTMENTVOLUME

.Fig. 6. Compartment temperature change due to an instantaneous UF6 release assuming perfect mixing of .releas . UF . in ia closed compartment containing, air at ,,atmosphe Iric presuead8 0 iharl atve huiiyo 0.U 6 temperatures were selected based on;7ANSI'N 14.1 concerning cylinder fill

limits (95% of ýcylinder volume- at. 250,*F and the triple point of UJF 6 (1-47.2 0 F).

large. The. -pressure rise, due to both the heat. and the additional moles of gas generated -from the

hydration of UF6 and its hydrolysis. product HF, may, under certain. circumstances, be enough to damage. the building and/or to increase leakage from ducts, rooms, etc. The "no leakage"- or. airtight compartment assumption. previously noted for this. model may not be realistic for many-existing facilities

handling UF6, but it does yield an upper bound estimate of the. pressure rise within a .compartment.

This idealized prediction of expected pressure rise does suggest, however, that total containment of al

UF 6 release into the building atmosphere may be an impractical, if not undesirable, goal.
:42

-ORNL;- DWG 83-16703 A

-LIQUID RELEASE,`148 OF

40ý YIELDING 0.421 lb VAPOR.

-. AT `133.8 OF/Ilb LIQUID

-LIQUID RELEASE, 250.PF

YIELDING, 0.647 lb VAPORt

AT 133.8 OF/Ilb LIQUID/

a. VAPOR RELEASE, 148OF I

CIL- VAPOR RELEASE,,250*F -30 WI, I

0-

-j

-4 ~ 10-3 10-2 1o0 10

lb UF' REESD/tO COMPARTMENT VOLUME

Fig. 7. Compartment pressure chang "due to an instantaneous UýFo release aissuming ~perfct mxn

of released UF6' in a closed comnpartent containing .air at atmospheric pressure and 80OF with a relative

humidity of 60%. UF6 temperatures were selected based on ANSI N 14.1 concerning cylinder fill limits

(95% of cylinder volume at 250*F) and the triple point of UF6 (147.290F). ,

The second area that -must be-addressed is the, assumption that the composition and temperature

are uniform throughout a; compartment. Although this should not be a problem with the'"multidimensional models. being developed by LANL, the problem will exist at least until those multidimensional

models are completed and are-available for use. In actual circumstances, concentration and thermal

gradients will, exist in a building after an accidental UF6-release and their' effects could. be significant.

For example, low concentration areas of UF6 co*uld totally react with incoming moisture and leave the
43

higercocetrtio aea o U 6 .ureacted. Thspoints out the need to. mode multidimensional spa-ý

tial variations ichmclratostepaueand- concentration, eseilyif time consideain

such asevacuation and response time are important.

The third area, to be, addressed, is that the deposition model uses only. a gravity settling correlation.

Other effects for example,, .condensation on soid surfaces of species,. such :as, HF, may have a significant -impacto,ýn the rates of'deposition -of UF6 'a~n-d UF6 hydrolysis products. The depo~sition -models,

used could affect which reactant is predicted to limit ,the reactions.

Theý fourth area of concern, s- the method of simulation of particile size distributions and -agglomera'-

tion -rates. For FIRAC, the deficiency is not the inability to deal with changin particle size distribu-,

tion's over time, which, FIRAIC can do crudely, but-the, requiremrent-th~at. the user specify the; timedependent distributions. For TO.RAC and. EXPAC, the user can only specify the average particle size

of the distribution; howeveri,size distributions of particles; are' expected to have a wide range of shapes

that are complex functions~ of the temperature, pressure, humidity of the'air, and the condition of the

UF6 prior to release.. Therefore, specifying only- an average particle size may lead to large errors when,

for example, removal mechanisms for certain particle sizes are significantly different from those for the

other sizes. The state of the art has not 'advanced enough to a ccurately: comipute,,changinig size distributions with -the physical ýdata available.

The. 'requirement that the user specify the particle. size distribution in FIRAC will probably cause

the user to take one of wo approaches. The first approach is, using iterative computer runs to estimate

the time-varying particle size distributions because changing conditions will change the size distributions over timne.- Changing size, distributions can alter the conrditions that determine subsequent size,-

distributions through such; mechanisms as filter plugging or heat generation rates. The second'

approach is assuming one distribution for all time. This latter approach will probably lead to significant errors;. for example, the removal of relatively small particles by a filter is much different from

removal after significant agglomeration has occurred.

The fifth area to be addressed is the inability to simulate corrosive effects of UF6 and its hydrolysis

products on. equipment performance in' a building. This would include possible degradation of filters,
44

fans, ducts; and other systems, as a result of che mical -re actions with UF6 7and 'its hydrolysis- products

during a release. -.Possible- adverse_ effects that corrosive UF6 and UF6 hydrol Iysis product's ma'y'have on

systems and equipment performance should be taken into account in the ventilation systems' modeling

and simulation effort.

Several areas, that should be addressed in the'models to properly simulate Accidental UF6 releases;

have been noted. Not addressing these areas may result, in poor simulation of stream, iom rpositions,

.pressures, and temperatures which, in turn, may lead to large errors in estimated concenitrations, flow

rates, and directions of flows. The overall effect of :temperature and pressure -changes on the flow dis-,

.tributions will be system and'accident dependent. Ut is known that the estimated pressure drops will be

incorrect, because a change in the composition will result in a change in the, density and viscosity of,

the mixture.

6.2 OTHER AVAILABLE METHODS

The methods discussed in the preceding section ha .ve been developed for inclusion in the AAH,. but

they have not been developed specifically for UF 6 releases. This section discusses methods used or specifically developed for analyzing UF 6 release-s, including information from N4RC and their licensees,

methods from the open literature, and preliminary information on available DOE source termimodels.

6.2.1 NRC Licensee Models

Most of the methods used by. NRC licensees for analyzing accidental UF6 releases have been concerned with modeling atmospheric dispersion rather than dispersion within a building. Because a Itmospheric dispersion is outside" the scope of this p~roject, no comments on the techniques used will be given.

No detailed source term models or methods were described in the NRC and licensee documentation

reviewed to date: however, assumptions made by NRC licensees in analyzing UF6 releases are summarized in Table 2 and briefly discussed in Chapter 3.
45

6.2.2 Methods from the Open Literature

A review of the open literature likewise revealed no methods for analyzing releases of UF.6 inside a

building, but two reports were found with information that is applicable to the NRC AAH.

The frust report by Okamoto and Kiyose5 descri~bed analytical methods and indicated the predicted

UF6 vapor release rate to the atmosphere as a function of time from a gas line connected to a heated

cylinder (type 30A). The results were summarized in a plot of cumulative UF6 vapor released versus

time for various initial cylinder temperatures. The expected effects (including failure) of UF6 hydrolysis products on HEPA filters were also. discussed assuming that all UF,6 had been hydrated.

The second report by Ericsson and Grundfelt' described analytical methods and results that provide

estimates of the mass flow rate of gaseous UF6 as a function of time from an unheated cylinder (type

30B) containing liquid UF6. It also discussed the volumetric flow rates of ambient air necessary to

totally react the UF6 released and the dispersion of the hydrated UF6 plume in the atmosphere assumning a neutrally buoyant, chemically inert plume. It is unclear whether the solid UF6 formation within

the cylinder is properly modeled. The results of this report should be used with caution.

6.2.3 DOE Source Term Methods

A number of methods are used by DOE for. analyzing accidental UF6 releases. Some of these

methods might be useful for developing source terms or for modifying the NRC codes. It should be

recognized that these methods are preliminary in nature; they have not been documented,,nor have

they been verified by field experience.

One method is a batch-mixing homnogeneous compartment model for predicting the final average

composition, pressure, and temperature following a postulated .accidental release of UF6 and/or HF

inside a single closed compartment with allowance for HF polymerization. The program requires as

input data the building's volume and its initial pressure, temperature, and relative humidity as well as

the mass of IJF6 and/or HF released, the temperature of the UF6 and/or HF released, the molecular

weight of uranium, and the UF6 phase (liquid or vapor but not a mixture of both). Output from the
'L.46

program includes the. final. average pressure (assuming the ideal gas -law-'and polymerized HF), the'

final average 'temperature, and the final mass and ave ,rage mass frac tio n o f each component in the

ýcompartment's atmosphere.

A second method is a transient homogeneous compartment model that computes as a function of

time the average composition, pressure, and temperature following a postulated Accidental release of

UF6 or HF inside a compartment with allowance for HF polymerization and deposition Of U0 2F2

and/or hydrate.. The program requires as input data the volume, initial pressure, initial temperature,

initial relative humidity, and'ventilation rate of the compartment and the ambient pressure, temperature, and relative humidity as well as the total mas's and temperature of the source, the mass flow rate

from the source as a step function of time, and the UF6 phase (i.e., the mass flow'rates of solid and

gas). The program's output includes the average pressure, temperature, and composition of the

compartment's atmosphere at user-specified times.

A third, method, which is based on the Hirst model for axisymmetric jets, has been. formulated;

however, there are currently no plans to develop and implement this method. The ~modification would

extend th .e' Hirst model to negatively buoyant 'flows such.'as pos Itulated accidental UF6 releases. The

modified Hirst model requires as inpu It data the ra dius .,'density ad , i horizontal and~ 'vert.ii.cal velocit Iies of

the exit jet and the ambient. density, and horizontal velocity. Such a method can predict. the entrainment velocity of moist air into the jet, and from that the mass of moist air entrained into-the jet can be

estimated. This model can bet developed by combining a homogeneous mixture moi del with an entrain-'

ment rate model (modified Hirst, model) and by solving horizontal and vertical momentum equations.

A fourth method can be u~sed. to calculate the gaseous UF6 release rate and the total mass of UF 6

released as a functioni of time from a UF6 cylinder. The calculations assume that UF6 i an da a

*that unde rgoe .s an isentropic expa In si Ion with -unchoked flow. The c alcula~tions all~ow for ,the cooling of

the UF6 and the cylinder .due 'to the release of the UF6 gas. The input data are the initial cylinder

tempe .rature and pressure, the'volu .me'of the cylinder,.and the cross.-sedtio'nal area of the opening in the

cylinder. The output data incdlud at' specif ied times the cylinder, pressure and temperature, the mass

flow rate of UF6 out of the cylinder, and the total amount of UF6 released.
47

Afifth method is a tool for eva-luating,:an arbitrary UF6 flow syte taisdfined b h sr

The method is applicable to a"'steady-state adiabatic release of nonideal UF6 gas in either choked or

unhkdflwtrug ievalves, orifics and/or a nozzle. The input data, include pressure and

temperature. of UF6 in a cylinder and the piping, valve, orifice, and/or nozzle arr .angement. Other'

data may be required as inpu o h eie outu dt.Fr example, the output may include noz-.'

zle dimensions, orifice dimensions, and either the mass flow rate or the exit velocity.

The above calculational methods require UF6 physical 4nd thermodynamic property data a's well as

U6 hydrolysis product characterization. -The bibliography includes some reference documents for suc'h

information.

7. .PERSPECTIVES OF UF6 ACCI]DENT SC.ENARIOS AND ANALYSIS'

A list of accidental UF6 release scenarios was presented -and discussed in Chapter 5. The individual scenarios 'may be grouped in a number of ways to permit greater:'understanding orisght that

could lea~d to an overall approach for considering scenario analysis and consequence assessment. ,Sec-ý

tion 7.1' discusses various methods for categorizing .the 'scenarios.. A particula .r method of categorization is considered furth .er in Sect. 7.2: that may aid 'in selecting specific events for analysis. UF,6

release phenomena and modeling: considerations are disclassed in Sect. 7.3. Section ;7.4 provides a

summary of this chapter..

7.1 METHODS FOR CATEGORIZING SCENARIOS

The scenarios presented in Chapter 5 were grouped for, convenience under four general headings

that are neither independent nor of the same general nature. In this section, no attempt will be made

to separate, scenarios into independent, groups; however, the various groupings presented may lead to

some ins~ights as to an overall approach -for considering scenario analysis and -consequence assessment.,

The various groupings, which,, include location, 'release source (equipment)','phase of release, flow

characteristics, release causes, initiating, events, and. inventory. groupings, are .summarized in Table 8

.and are discussed.below.

7.1.1 Location,

Accident scenarios can be divided into those occurring indoors, which may offer additional levels of

containment, *,And those, occurring outdor.Poeseupetadppn o F is generally

assumed to be located indoors. A further consideration is the possible (though perhaps marginal) containment afforded by steam chests used to homogenize UF6 in cylinders at U F6 production facilities

and to vaporize UF6 at fuel ;fabrication facilities. While most steam chests are located indoors, some

are located outdoors (at least at one UF6 production facility). Therefore,.: three location categories are

used: indoors, outdoors, and inside. steam chests.

49
50

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51

-7.1.2 Release, Source (Equipment)

Due to their frequent handling and use, it is believed that pigtails.* are the most likely source for'.

,,re~leasIe. of UF6_, followed -by cylinders and their associated ývalves and fittings. Fixed process equipment

and piping are. expected to be less vulnerable. Scenarios Are divided by equipment into several release.

source groups: .cylinders; fittings (inc'luding pigtails) for charging and discharging cylinders; cold traps;

vessel-type equipment. (tanks, distillation columns, etc.); and other process equipment (including fixedpiping systems with their pumps, valves, etc.).

7.1.3 Phase of Release

All accidents involving liquid UF6 released to atmospheric pressure Will ultimately release solid UF6

particles and UF6 vapor that will react with* moisture in the ambient environment to form U0 2F2 o r

U0 2F2 -COated UF 6 particles. Prior to release, UF6 will exist in eithe r the liquid or vapor phase 'or 'in

two-phase equilibrium of either liquid and vapor or vapor and solid.. UF6 in piping systems will probably be in a single phase, but; UF6 in.cylinders or vessel-type equipment will always exist in two phases.-

For practical purposes, however, a cold cylinder containing UF6 mayb assumed to contain only solid

UF 6 becaus e the UF6 vapor. available would produce only a niusance release.

7.1.4 Flow Characteristics

The flow characteristics of a UF6 release will be important,, particularly if- a very exact simulation

of 'a release is attempted. The' flow characteristics will set release rates' and the, rate of mixing of UF6

with the ambient atmosphere. 'There. are three categories of initial release characteristics that can be

conceptualized following loss of containment: an explosive release, a jet, and a slowly expanding cloud.

A release .that can be categorized as a slowly expanding cloud is expected to result only from an accident involving solid UF6 ; therefore,_ becaus e the release will not be significant, this release category is

not considered further.- However, an explosive release or a jet release may often-,be characterized by a

slowly expanding cloud after. the. initial momentum of the release has been transferred to entrained, air.
:52

A hydrodynamic, model (see Sect. 7.3) can then be used to analyze the masso transport of UF 6 in -a

compartment. Releases involving liquid UF6 will generate large amounts of vapor as the liquid flashes

to solid and vapor as it expands to I atm.s A jet may originate from either a regular opening (e.g., a

circle when a pipe breaks) or an irregular opening (e.g., a rupture in a cylinder wall). Releases from

irregular openings are normally approximated by jets resulting from well-defined openings, (e.g., circles,

rectangles,,slots, etc.). Releases from very large openings may often be approximated as instantaneous

releases if the release time is relatively short. If equipment has relatively weak. areas, possibly caused

.by corrosion or defective welds, overpressurization will be more likely to cause a rupture resulting in a

jet. If the equipment has no weak areas or if overpressurization occurs rapidly, however, the equipment may explode and form fragments.- Analysis methods for directed jets and explosive releases are,

discussed further in Sect. 7.3.

7.1.5' Release Causes

A release of UF6 may result from a sequence of events or one event.. A release cause is defined as

the end event; that is, the last event in a sequence that results in the release of UF6 (e.g., overpressure

for scenario 1.5 and operator 'error for scenario 4.1). Possible release causes include mechanical failure from overpressure, impact, or dropping, mechanical failure under normal process conditions, and

operator error. A comparison of primary release causes and release sources yields several generic

events that may be analyzed. in a similar manner:

I. loss of Cylinder, containme6nt by overpressure (1. 1, 1.5, 1.6, 1.7,J.8, 1.4);

2.loss of cylinder containment by impact/dropping (1.2,.1.9, 4.3);

3. loss of cold trap containment by overpressure (2.7, 2.8, 2.9, 2'.10); and

4. pigtail failure (13, 2.1, 2.3, 2.5).

Actual conditions for a generic event may varydpnigo the specific scenario under consideration

(1.7, could occur at, or below normal operating, conditions), and post release analysis could* be6 ciomplicated [e.g., conse quences of a UF6 release in a fire'(1.4) may- differ significantly from other' -cylinder*
.53

releases]. -Scen~arios 1.1: and, 1.4 may require specialalyilcosdrtnbeue of the posibilty

,of a rapid, pressure rise.

7.1.6 Initiating Events

Initiating events include operator err ,or, equipment failure -under normal conditions [e.g., failure of

a cylinder lifting device. may lead to rupture of a liquid-filled 'cylinder (scena'rio 1.9)], natural

phenomena, and fire. An initiating event may be the primary release cause, as 'in the case of scenario, 4. 1.

7.1.7 Inventory Groupings

The UF6 handling processes described in Chapter 4 may be grouped into several systems that are

-separated by batch operations and that have a definable maximum inventory of UF6 that is placed at

ýri~sk following a breach of containment. These systems are

1. System A: Fluorination and Cold Trapping,

2. System B: Distillation,

3. System C: (C.A) Cylinder Filling and (C.2) Indoor Handling of Liquid-Filled Cylinders,

4. System D:ý (D.A) Outdoor Handling of Liquid-Filled Cylinders and, (D.2) Cooldown Storage of

Liquid-Filled Cylinders, and

5. System E: (E. 1) Homogenization of Cylinder Contents and (E.2) Fuel Fabrication Feed.

All systems except System E.2 are applicable to UF6 production facilities. System E.2 (and possibly

Systems C.2, D. 1, and D.2) is applicable to fuel fabrication facilities. Each of these systems, consists

of process equipment and/or cylinders as well as associated piping; therefore, an accident involving any

portion of a system places the total i nventory of that system at risk. It should be noted that cold traps

operate in several modes, including cooling, heating, and standby. The systems defined above assume

that cold traps are being operated in the mode appropriate for that system. Interconnecting piping Systems between cold traps operating, in. different modes are assumed closed in the following discussion;
.54

however,. operator error in opening and/or closing -valves could combine several sys tem s" together. For"

clarity, such interconnecting piping is not shown in figures illustrating the systems.

Systems A, B, and C. 1 are illustrated in Fig. 8 for -a UF6 production facility based on the

fluorination-fractional distillation process. The inventory of UF6 in System A. (Fluorination and Cold

Trapping) includes the UF6 produced in the fluorination reactor as well as- the UF6 in the cold traps;

however, the UF6 in the. cold traps is primarily solid, so it. can probably be neglected in determining

the UF6 inventory. Therefore, the "inventory" at risk in System A' consists primarily of the UF6 production rate multiplied by the time required to shut down after a breach of the system. System B

(Distillation) contains. several ma'jor vessel, of whi'ch**the distillation columns are assumed to. operate

continuously. If the distillation feed tanks operate. in a batch mode, then this, system could, be divided

into, two systems: feed tank filling and distillation. The maximum UF6 inventory at risk in this system

is the sum of the UF6 initially in the head end cold trap (or traps, if several are drained simultaneously

into the feed tanks), the UF6 in the feed tanks, and the UF6 in the distillation columnsa and associated

ORNL-DWG.83.16704

SYSTEM A' FLUORINATION aCOLD TRAPPING

U AO CO-LD F TO OFF-GAS

TRA F HFP TREATMENT

CoF o

LIQUIDE

HEAIN SYTMB'ICLIDRFIL

COIL ISTILATIO

Fig. 8. Major UF6 handling components and flow paths in a ITF6 production, facility based on the

fluorination-fractional distillation process illustrating Systems A, 10, and C.2 (see Chapter 4 and Fig*.Z 3

for additional process details). Interconnecting piping between systems -is assumed- closed an .d is not

shown for clarity.
55.,

equipment. UF 6 in the tail end cold trap should be solidified and, therefore, would not bea major concern. The inventory of System C. 1 (Cylinder Filling), which could total. about twice the inventory of a'

full cylinder, is, the sum of the UF6 .initiailly in the cold trap and of the UF 6 heel initially in the cylinder. All of, these systems are assumed to be indoors.

For A facility based on the solvent extraction process, Systems A and C.A are illustrated in fig. 9.

Note that some details of Syst~emIi A differ. between the two facilities but that the. major contributori to

the UJF 6 "inventory". at both facilities. is the UF6 production rate of the fluorination reactor'., -Because

cold traps and cylinders usually used in a facility have appro -xima .tely the same nominal capacity,

Systems C.A and C.2 (Indoor Handling of Liquid-Filled Cylinders) are similarý and ,are therefore

grouped together. Systems' C. 2, D. 1 (Outdoor Handling of Liquid-Filled Cylinders), and D.2 -(Cooldown Storage of Liquid-Filled Cylinders), which are not illustrated, consist of a cylinder containing liquid UF6 alongý With a small amount of UF6 vapor. These systems differ only in location and whether

or not the cylinder is; in transit. The UF6 at risk in Systems C.2, D. 1, and D.2 is the inventory-of UF6

in the cylinder. -

System E.2 (Fuel Fabrication Feed), which is found in fuel fabrication facilities, s illustrated by.

Fig.' 10. System EA (Homogenization of Cylinder Contents), which is founid in UF6 productioin faciliORNL-DWG-93-16105.'

SYSTEM A:FLUORINATlON &

0 RECYCLE GASESZ

- - ZJ ~---RECYCLE- UF4 . . . . . . .

U RPTRAP TRAP

F2

IMPURITIES / JCYLINDER

SYSTEM C. I CYLINDER FILLING

Fig'. 9. Major UF6 handling components and nlow paths ýin a.UF(, production facility based on. the

solvent extraction process illustrating Systems A and C.1 (see Chapter 4 and Fig. 4 for additional proc-,

ess details). .Interconnecting piping between systems is, assumed closed and is not. shown for. clarity.
56

ORNL-DWG-83-16706

SYSTEM E.2: FUEL FABRICATION FEED

STEAM CHEST

IHYDROLYZER

CYLINDEOR

UF6 VAPOR OXI DI ZER

Fig. 10. Major UF6 handling components and flow paths in a typical fuel fabricationi facility illustrating System E.2 (s'ee Chapter 4 and Fig. 5 for additional process details).

ties, is similar to System E.2 except that -there is no UF6

' flow out of the cylinder. System E.2 is usually located indoors and will hand le cylinders having a smaller capacity than those found in

System E. 1, which may be located either indoors or outdoors. The maximum U .F6 inventory for

System E is the contents of a filled cylinder.

These systems as defined above are based on the assumption that a plant is operating in its normal

configuration. If interconnecting piping or manifolds are opened, possibly by -operator error, to more

,than on'e system, or. t parallel systems, different cons .equences would be expected because of combined

UF6 inventories.

7.2 BOUNDING CONSIDERATIONS FOR UF6 RELEASE EVENTS

A list of scenarios ssuch as that given in Chapter 5 (see Table 7 or 8) is desirable for identifying

potential problem areas and for taking steps to avoid them; however, a consequence analysis for each

scenario would be time consuming and probably unnecessary. On the other hand, selecting appropriate

release. events to hopefully bound the consequences of the various scenarios can be difficult due to the

Many factors that must be considered. While it may, at first appear re asonable to select appropriate

release events for consequence analysis; based on. the -total' amount of UF6 released, such a basis may
not yield theý mnost severe, consequences (health. effects,- etc.). For. example, a low-flow-rate, long-,

duration release may be more severe than A high-flow-rate, short-duration release. even though less

total UF6 is released in the former case than in te latt-er. Nevertheless, aafirst pass, use oftoa

UF6 released as 'a major factor for selecting: boundmig cases may be reasonable. (unless UF6: Of several

different assays is being handled). A review of the scenario list reveals a number that involve cylinders

either directly' or indirectly via pigtails. Each scenario places the same amount of UF - at, risk.- Such

reasoning, when. extended to other equipment, leads- to consideration, of UF6 rees vnts based-on

systems, rather than on specific pieces of equipment.' Representa~tive sy stems have been described in the

last art of Sect. 7.1 under.Inventory Groupings. it should be reiterated, that release consequences are

not nece .ss Iarily-proportional to the quantity of UF6 rlaebu that the rt n uaino ees

and the location within -a facility where the loss of containment occurs can also be important factors.

In looking at potentially bounding events,; Systems C, D, and E can be most easily ýconsidere~d. All

three systems involve liquid. UF6_ in-cylinders. The maximum inventory. for Systems C.2, D9 and E is

the cylinder capacity, while that of System Cý. 1 may be. greater than 'cylinder capacity, .(perhaps by

about a factor of 2) depending on 'cylinder filling- procedures (e.g., topping' off an almost full cylinder

from a full, cold trap). )Differences. between -the systems indicate that UF6 could be ,released indoors,

outdoo6rs, 'or into a steam chest that 'may be located indooris or outdoors depending on the facility. At a

U4, production 'facility, -the nominal capacities of the cylinders handled are 10 and 14 tons, while

2.5-ton and 55-lb cylinders are handled at fuel fabrication facilities, depending on the uranium enrichment of the UF6. The results of preliminary calculations at saturated conditions (see Fig. 11) indicate

that more than 40% of the liquid UF6 ý released from a cylinder will flash to vapor when released to, the

atmosphere. As the temperature of the liquid, UF6 increases,. so does the tamount of vapor produced.

The two curves shown in Fig.. 11 thermodynamically bound -the initial vapor mass fraction expect .ed. as

.a function of temperature.

The primary contributor to the inventory at risk In System A is. the fluorination rea ctor. Existing

facilities in the United States have nominal production rates of 55 and 77 lb/min.1 Based* on a rate of

77 Ib/min, it would take over 100. min?'to, produce 8000 lb of UF6 vapor (40% of. the capacity. of a,
58

ORNL-DWG 83-16707A

0.65

E

q-0.60

z

0-

0.5

LL

"0.50

0 0.45

ADIABATIC

ISENTHALPIC

'EXPANSION

0.557

0.537 , ý-IQPKITPAPIP 7~ I EXPANSION.

1210OF

I. I

0.40 ' I I I I I I 1J

10 160 180 200 220 240 260

SATURATED LIQUID UFro TEMPERATURE (OF)

Fig. 11. ntals vapor mass fraction reulting from a release of saturated UFg liquid to a pressure of

1 atmn. (Example: for saturated UF6 liquid at 210*F, the initial vapor mass fraction will be between

0.537 and 0.557 when a release to a pressure of I atm occurs.)

10-ton cylinder). If the fluroination system can be shut down within that time frame, analysis of an

indoor cylinder release may bound a release from the fluorination process.

The distillation system (System B) is the most complex system of the five considered, and its inven-.

tory will probably exceed that of a system containing a cylinder. In fact, this system may yield the

greatest indoor UF4 release potential at a UF,6 production facility utilizing distillation to remove impurities found in the natural uranium. If this system can be subdivided because of batch operation of the

feed tanks, then the inventory that could be released would be reduced for the two new systems that

are subsystems of System B. A further partial reduction in the inventory released might also occur,

depending on system configuration and where in the system a breach of containment occurs.
59

The primary factors in determining which system (A, B, C, or E) presents the greatest release

potential, indoors are the release rate and release duration-. For example, the rate of UF6 production'in

the~ fluorinatio reator may or may- not exceed the. rate, of UF6 'release from a ruptured cylinder.

,Thus, the flow rate of ýUF 6 out of a cylinder orcy'lmider containing system 'is needed as a function of.

time and cylinider size, and in both cases, an, analyst would also. nee~d'to .know the maximum duration of

the release (i.e., the maximum time required to: bring the release. under. 'control) to:determine and compar 'hrease% potential., Similarly,. UF6 flow rtsfo beksithdsilaion system, wihi

pressurized, would need to :be determined before An appr~opriate boun'ding indoor release co'ulId be cho-.,

sen.

'At least three' types of release events! appear reasonable to evaluate at UF6 handling facilities:

(1) a release fronm a liquid-filled. cylinder outdoor (See ) 2 ees fo iti rcln

der inside a se~am chest (System E), and (3 eease indoorsI'friomi either (a) a. pigtail or liquid-filled

cylinder (S Iystem C) :or (b) other indoor sytm(Systexs '-A and B)., It' should be ntdthat on eior

more of teeeetmanobeplicable to a specific facility Fo exaple, if liquid-filled cylinders

are not handled external to the steam' chest at a' fuel 'fabrication facility, then releases directly -to, the,,

indoor and. outdoor environment (as in cases, I1 and 3) need not be considered; however, the steam

chest should not be assumed to 'contain the UF6 unless designed to do so.

Selection, of: the more severe indoor- release 'will require the determination of release, rates and'

release potentials given the time required' to bring a' releaseý unider ýcontrol. Facility'design' 'may require

consideration of more than one indoor release event. A:'release, from..a p "Igti ora ylnei ea

steam. chest differs from the other releases because of the high ambient. mnoisture, And the secondary,

(although perhaps marginal) containment afforded by the ste'am' chest.

While facility, design and procedures may greatly affect the ultimrate, consequences of a release,

dvlpmeint of source terms for pigtail and cylinder releases is desirable because such source terms

could be used for- any facility. Such.. source terms should.:beý functions of process conditions and UF6

inventory (e.g., cylinder~or cylinder plus cold traps).
60

7Th discussion in this section has so far been limited. to the release of UF,6 from a single breach

that occurs during normal operation. In the event of a mome extensive accident (explosion, fire, earthquake, etc.), UF46 could be released from multiple source points [e~g., several cylinders, a cylinder

(fragmented) and other vessels (ruptured by the impact of cylinder fragments), etc.). Also, if operator

error has resulted in opening interconnecting piping or manifold systems, the inventory of several parallet or series systems could be released.

Fire, as a heat source, has the potential for causing vessels containing UF6 to fail from overpressurization or by weakening welds or walls of vessels or pipes. Once released, UFig may react with

unburned hydrocarbons. The amount of UF(5 in jeopardy will depend on facility construction and the

ability to extinguish the fire. Releases that occur within a building may or may not be closed off from

the environment. Outdoor fires could jeopardize cylinder storage areas.

Natural phenomena, such as earthquakes, high winds, tornadoes, and floods, may also jeopardize

UF6 operations. For example, earthquakes could lead to failure of elevated vessels and piping, tornadoes could generate missiles that could penetrate process equipment, and flooding could lead to criticality or equipment damage.

7.3 RELEASE PHENOMENA AND MODEUNG CONSIDERATIONS

Previous sections of this chapter have introduced a number of variables and approaches usefufl for

analyzing potential UF,6 accident scenarios. This section will address more specifically the phenomena,

that could be observed following a breach of containment and the types of models that would be

needed for simulating these phenomena. The status of some currently existing models applicable to

UF6 release analysis has been discussed in Chapter 6.

7.3.1 Initial Characteristics of UF6 Releases

Postulated UF6 release scenarios at NRC-licensed fuel cycle facilities include releases from process

equipment, piping, or cylinders containing UF6. The release form may be a multiphase mixturte of UF6
61

solid, liquid,: and vapor; A multiphase gas-solid-,mixture; o:r: a, singl-hsvao release. The'ý release

can be eiithe r a short release t hat may. be approx imately -modeled as an' instant aneouIs.. release. or a

longer -release that must be modeled as a finite duration release., For many engicneeing applications, a

release lasting less than a. few minutes may be approximated as an instantaneous re~lease.. The process

:and. ambienti conditiobns prior to the release, as well1 as the physica'l characteristics.of the',breach in the

cylinder or pressure equipment, must be considered in developing A source, term for A postulated UF6

release.

At NRC-licensed facilities ýduring nomloeation UF6 ma-xsta vapor, asoi-armx'

ture (e.g., in-a cold trap), a liquid'-vapor,4mixture (e.g., in. a cooling cylinder, a cold tra-p on its heating

cycle, or distillation column), or as a solid. Batch operations of cold. traps and cylinders will result

routinely ýn either liquid-vapor or vapor-solid mixtures being present, but three-phase mixtures would

not be expected. Thermodynamic conditions for the'existence of UF6- as a single-phase or as a' multiphase mixture can be seen in Fig. 12, a UF 6 temperature-entropy diagram.

Some characteristics of.-UF 6 behavior' following a breach -of containment can be illustrated by considering rupture under various: initial conditions. For practical purposes, a, cold cylinder contains- only,

UF6 solid, although a small amount of UF 6 vapor and trace amounts of noncondens able gases fill the

void space within a cylinder at subatmospheric pressure. Therefore, the rupture of a cold cylinider w~ill

result only in a small nuisance release limited by. the rate of, sublimation rather than in a significant

health hazard. If a defective cylinder is not inspected prior 'to subsequent heating, however, a. significant release may occuro9n heating because the undetected damage would result in. a weak spot susceptible to failure during heating.,.On the, other hand, the failure of a cylinder containing either pressurized liquid UF6 or a large amount of UF6 vapor (up to 'several hundred pounds in. a 10-ton cylinder

containing only saturated UF6 vapor) can result in a significant release. The UFf6 phbase composition

(i.e., solid-vapor'fraction) after the released material has expanded to 1 atm will-depend 'on the proicesis

conditions prior to the release and the release process.

Upper and lower limits of the UF 6 vapor fraction can be estimated. from the thermodynamic considerations. If the* expansion to 1 atm is assumed to be a reversible, constant entropy (isentropic)
62

1 110 ý

Sol id o -s =-Vao -1 0.- 04- .-

-0.02-

730 -20 -10 a 0 1 3

Entropy, a. a./oeaCi e

Fig. 12. An approximate temper Iature-entropy diagram for UF6 (adapted from ref. 4.4, p. .102).

The reference point for'both entropy and enthalpy is liquid UF6 at the triple point.

process, a lower bound on the vapor fraction can, be estimated. If an adiabatic, constant enthalpy

(isenthailpic) process is assumed, an upper, bound on the vapor fraction and an upper bound on the

change in entropy can be estimated. As shown in Fig. 11, the vapor fraction after an expansion of liquid to 1' atm, will increase with an increased change in entropy. Because the change in entropy is proportional to the exhaust rate of a high velocity release, the vapor. fraction for many release scenarios.

will increase with an increase in the exhaust rate.

7.3.2'Characteristics of Equipment Failure

There are two primary causes for equipment failure:. internal overpressure and external mechanical

forces (impact or dropping). Potential causes of overpressure. include overheating; heating when fill

limits or contaminant limits. have been exceeded, and. chemical reactions between UF 6- and a
hjdrocarbon(s).. Depending on the condition of the eqimntheý rate of overpressurization, andth

magnitude of the overpressure,: the equipment will either rupture (e.g., weld. failuire,' formation of a

crack, etc.) or explosively fragment.

If the equipment. has, relatively weak areas, possibly caused by corrosion or defective welds, or, in

the case of cylinders, by transportation damage',' an equipment rupture will be the more likely result of

overpressurization. However, if the equipment has no weak areas or if. the overpressure -results from a

rapid pressure rise, it may explode and form fragments. These fagments may cause secondary failures

,by-4impacting othe r equipment, and they, may represent a significant safety hazard to personnel. in the

vicinity of-the accident.

To determine the failure mode; the postulated accident scenario must be known. For example, if a

cylinder containing UF6 is impacted, it may rupture; however, if a cylinder containing UF6 fails from

an overpressure, it could either rupture or explode. A. stres analysis can be used toprdc teosi

ble failure mode(s) using information related. to the mechanical and thermal loads. on the process

equipment.

1.3.3 Introduction. to Release Analysis

*Consideration of: the- equ~ipmient failure modes discussed above leads to' the conclusion, that' at least

two generic 'failure modes are possible: a directed release through an equipment. ruptureý (including

pipe breaks and leaking valves) and* a multidirectional explosive release. Therefore, multiple analysis

tools may be required to develop source termnsfor postulated releases from equipment containing UF6.

There are two different approaches that the analyst can pursue in charactstizing a release into a coin-

-partmnent from equipment 'containing UF6 that will be discussed before looking more closely at analyzing the release from the equipment.

The simpler, less accurate approach is to neglect the detailed concentration, pressure, and temperature profiles that develop within the compartment. For certain instantaneous, or finite duration releases

inside, building compartments, a satisfactory source term ,(for a ventilation model) can be-developed by

assuming a homogeneous mixture of UF(,, Air, and UF 6 hydrolysis products that may vary uniformly
64

within a compartment as a function of time. This type of model will be most Accurate when the

release is rapid, the, ventilation rate from the compartment is'sufficiently low, and the size of the compartment is sufficiently small to allow the, UF6 hydrolysis products to become well mixed prior t

release from the compartment. If the ventilation rate is high or if the compartment is large, the homogeneous mixture'assumption may be significantly in error.

As an alternative to the homogeneous mixture model, the analyst may elect to,,use a more accurate

approach--a hydrodynamic model--to calculate -ventilation. flow patterns and temperature, pr Iessure, and

composition profiles inside the'compartment.- The use of a hydrodynamic model would yield more

accurate estimates and becomes necessary if time-dependent, spatial variations in concentrations of

UF6 and its hydrolysis products are important.. Its use, however, would be expensive, not only' -because

a single run would be more expensive,. but also because multiple runs may be required to filly characterize potential releases (e.g., a liquid-filled cylinder could be dropped at several different points in a

compartment, with each drop having different results). the source term for such a. hydrodynamic

.model would probably-be described by source characteristics at the release point such as the release

rate, direction, composition, and exhaust area, rather than a description of the process conditions prior

to the release and the failure ,mode of the UF6 containment. However, the error that may be associated with the attempt' to model the mixing of a chemically reacting substance such as UF6 within a:

compartment or a section of a compartment may be deemed sufficiently, large to justify-the use of the

less rigorous homogeneous mixture model.

7-3.4 Analysis of a Directed. Release

A directed release can be categorized according to the nature of- the flow (i.e., choked or unchoked)

and the level. of interaction, with solid surfaces. If the flow is choked, the release rate can be determined from `the process conditions and flow area at theý choked. flow location. However, if the flow is

not choked, a flow analysis must be used to determine the, release rate from the pressure differential.
.65

Across the flow,, passage and the geometry ,ofý the flow passage. As"'noted in Sect. 6.2 several 'techniques-are available for. predicting -UF6 flow rates through pips nozzles, valves.. and, other flow pas-,

sages. The calculated flow rate could. then,,be used as a source term either for the homogeneouscopartment model or the more. accurate hydraulic models.

.,After the UF6 release rate from a rupture has been determined, the next step is' to analyze the initial flow characteristics. of a directed release. As shown in 'Fig. 13, there are three basic configurations: a vertical release directed down, a vertical release directed up and a horizontal release.

OkNL-DWG-83-16708

Fig., 13. Three basic counfiurations for a directe release.
`66.

If. a release is directed dow n, usually thele'xhaust stream will impinge on the ground o floor of a

compartment), resulting in an "axisymmetric," expanding,. ground-hovering cloud of UF6. However,- for

low'velocity or elevated releases, the downward momentum may be completely transferred to the sur-'

rounding air through entrainment, resulting in a UF6 cloud below the release point.

If the release is directed up, the UF6 will usually be diluted with ambient air through entrainment

prior 'to significant interaction with. process -equipment or the walls and ceiling of the compartment.

However, -if the release has a large initial horizontal velocity component, the UF6 will often have significant interactions with process equipment and/or compartment enclosures.

The initial flow characteristics of a directed UF 6 release that does not impinge on process equipment or compartment enclosures may be analyzed using a jet method such as the one described in

Sect. 6.2 Such a method can be used to predict the initial trajectory of the directed UF6 release, the

dilution rate with the ambient air, the chemical reactions with the ambient moisture entrained into the

jet. etc. After the jet has expanded to 1 atm and after the initial momentum has been dissipated to

the entrained air, the jet characteristics can be used to develop the input data for a hydrodynamic

model of the compartment.

-If UF6 contacts solid surfaces, such as process equipment, the velocity, direction, and temperature

of the UF6 may be significantly altered. A multidimensional. hydrodynamic model. is required to predict the characteristics of .the mixture of air, UF6, and UF6: hydrolysis products that result from a

directed UF6 release. The model must be able to predict the flow of the mixture of UF' and. UF6-

hydrolysis products in the, vicinity of solid surfaces (e.g., around process equipment) and the exothermic chemical reactions associated with UF6 hydrolysis, if composition, temperature, and pressure

profiles are to be "accurately" predicted. The accuracy of such a model would be further improved if

heat transfer to so Ilid surfaces and depletion mechanisms are also simulated. It is believed that a significant effort, w ould be required to develop such a' model.

7.3.5 Analysis of an Explosive Fragmentation

As noted previously, if a piecer of. equipment fails e xplosively, the resulting fragments may form

projectiles that may endanger personnel inithe vicinityý, of the accident and that may cause significant.
damag'e, ,,anr~d :'additionail f.ailures- by impacting process equipment. The number of projectiles, their sizes,

and their velocities may be estimated, using standard techniques.' If bUF 6 hydrolysis is neglected -during

the initial expansion, the explosive release of UF6 Vapor can also. be characterized (temperature, pres-

, sure, velocity, etc.) by using available techniques for the analysis of a pressurized gas release.' If the,

effects of UF6 hydrolysis are included, a hydrodynamic model similar to the model described- above will.

be- required to accurately predict composition, pressure, and temperature profiles during the. initial

expansion of a UF6 vapor release. Alternatively, a homogeneous mixture model m~ay be used to

developa source term (temperature, pressure, composition, etc.) for a ventilation model.

The analysis of an. explosive liquid UF6 release is very,. complex. Th e liquid UF6 Will flash. to form

a gas-liquid mixture as the UF 6 Ieýxpands to the triple point at 1.54 atmn. As the UF6 mixture further,

expands to I 'atm, it becomes a vapor-solid miixturie. Therefore, an explosive release of UF6 liquid may

result in a three-phase "cloud" containing a mixture 'of, UF6 liqud sldand vapor. Because the standard techniques for analyzing.. an explosion assume the isentropic: expansion of an inert perfect gas,9-they

are inapplicable for the proper analysis of a' flashing chemically reacting substance such as. UF6.

If liquid UF6 is rapidly depressurized, a rarefaction wave will pass through the liquid UF6, forming

small UF6 vapor bubbles. *Although only trace amounts of UF6 bubbles will be present- initially, the

Ipresen Ice of these bubbles can reduce the sonic velocity in the liquid UF6 significantly (see Figs.'14 -and

.15 for some preliminary results). As the rarefaction Wave passes through the UF 6, theý outer layer of

liquid UF6 will for an'. expanding 'Cloud of UF6.. The rate of this expansion "will 'be limited by the

sonic -velocity of.:the, UFmtreadhtofi.Te a nayi of this expansi .on, should account for

shock waves either in the UF6 or in the ambient air, flashing of liquid UF6 in the cloud, and the UF6

hydrolysis reaction.,

7.3.6 Analysis of a UJF6 Release in the Presence of a Fire

The analysis of a fire inside a UF6 handling facility will include characterization of the fireiýphe'-

nome'non itself and Associated heat transfer to process equipment, the failure of process equipment due

to the effects of the, fire, and the interactions between released U176 and the fire. The fire transport
68'

ORNL-DWG-83:j 6709

8-

In.

0.n

Cn

LIOUO WIh ,ICE BUBBSLES

0 i

60 T0 1090 00 1:0 120 130 TEMPERATURE (*C)

Fig. 14. Pýredicted soni.c velocities as functions of temperature for UF6 at saturated conditions.

and energetics can be simulated, by' codes such as FIRAC prior to the release of UF6, and possible

failures of process, equipment may be predictable given the fire characteristics.. When UF6 is released

from containment, howvever, the chemical. reactions between UF 6 and hydrocarbons pr esent in the fire

will be much harder to characterize. Therefore, the more important UF6-hydrocarbon reactions, should

,be incorporated into FIRAC as it is further. refined and developed.

7.3.7 UF6-Hydrocarbou Reactions:

The reaction between UF6 and hydrocarbons is expected to only marginally increase the severity of

a fire from the standpoint :of total heat generated; however, the rate of combustion mna-y increase. If

Baedon Ref. 10 and personal communication with E. J. Barber, Oak Ridge Gaseous Diffusion Plant, Oak Ridge,,

Tennessee, February 24, 1983.
69

ORNL-DWG43-16710

VELOCITY OF

LIQUID

0

0

U,

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 09 1.0

VAPOR MASS FRACTION

Fig. 15. Predicted sonic velocity as a function of vapor umas fraction for saturated UF6 at 100*C

and 4.12 atm.

U176 reacts with water formed by hydrocarbon combustion~the fire could be slightly more severe than if

UF46 reacted only with hydrocarbons. The UF6-hydrocarbon reactions will form carbon-based fluorides

(such as C174), UF4, and HF.

UF6 liquid can also react with hydrocarbons inadvertently introduced into a cylinder and may lead

to failure of the cylinder by overpressure if sufficient hydrocarbons are present. The rate of reaction

between liquid UF6 and hydrocarbons is expected to increase with time as UF,6 dissolves into the oil
.70

phase. 'The energy -release -rate may reach, such a velocity that the' reacting: UF 6-oil: phase, actually

detonates.

.7.3.8 Special Concerns When Handling Highly Enrichied Uranium

Some fuel fabrication facilities handle highly enriched uranium (including fully enriched uranium

that is approximately 97% 211U). In additi on to'criticality, another concern beyond tihe scope' of this

study that should be kept in mind when assessing conse,.quences of postulated accidents"'involving UF6 Iis

the high alpha activity r,6sultinig from the presence of .11U in. the highly. enriched uranium. because the

concentration of 234U is also increased o~ver its na Itu Iral concentration by the enri .chment proc .ess. It is

p~ossible that radiological concerns may become primary in assessing the consequences of a postulated

release at those facilities- handling highly enriched uranium while chemical -toxicity effects will probably be of greater concern to facilities handling only low enriched uranium'

Methodologies 'for determining conditions that may result in criticality have not been considered

,during this study.:

7.3.9- Analysis of a UF6 Release-Outdoors

A release of UF6 from damnaged equipment' outdoors at ground level results, in a direct intrusion of

UF6 into -the environment., Although source term determination for an :outdoor release can follow

approaches similar to. those discussed for, indoor, releases, analysis of, the dispersion of UF6 and its

hydrolysis products after the release is very complex in both the near- and far-field zones. Near-field

analysis ma'y be greatly affected by buildings and other. structures and site topography. UF6 may be

trapped between buildings or.-on the downwind side of a building and only slowly be,,entrained into the

bulk' flow of air. The presence of v .enti .lation system air intakes near a release may d .raw UF6 into

buildings. Such near'-field concerns, if inadequately addressed, could. lead to nonconservative estimates

of on-site worker exposure. Other complicating factors to consider include the use of plugs (to stop

the release) and, knock-down procedures (to 'wash out," UF6;-from the air) used to minimize the release

of UF6.
71

Far-field analysis, will be complicated because of the unique aspects of UF6 plume: behavior'.,- The

plumne may. be pOsitively, negatively, or neutrally -buoyant,. depending on its comIposition and temperature, and it may be elevated or ground hovering (gravity spreading).,. Exothermic reactions associated

with the 'hydrolysis of UF6 and ýHF will occur. and: solid UF6 particles may sublimate while. HF,

hydrates. and condenses.. Plume density may decrease signiicantly as UF6 reacts with ambient moisture and the plume is diluted by entrained-air (the, density, of UF6 _ is about 0.93 lb/ft3 , while the'den-

-sity of air is about 0.0.76 lb/ft3 at,600 F). Figure 16 illustrates the possible behavior. of- a plume. fol-.

lowing a moderate-velocity, vertical release of UF6.

7.4 PERSPECTIVE SUNMMAY

UF6 accident scenarios can be categorized by location, source, initial phase(s), flow. characteristics,

release causes,and inventory at risk.' Release locations include indoors, outdoors, andi inside a steam

chest. Sources for UF6 releases include cylinders, cylinder fittings (including pigtails and valves), cold'

traps, And other proces's equipment. Immediately, prior to release UF6 myeitaa liqui ovapor or

as ,a multiphase mixture of liquid and vapor or vapor and solid. Flow characteristics can. be

represented by'a jet release or an explosive release. A number of primary causes for, release were identified to show that some'accident scenarios could. be consolidated for analysis of event consequences.

Some initialting events were also identified.

A, basis for discussing. some bounding. considerations for UF6 release e vents was: to define systems -of

equipment that are separated by batch operations and that have a definable maximum nvetr at

risk. -Subsequent discussion led to the selection of at least three types of UF 6.releases that should be

considered further:

-1., a release from a liquid-filled cylinder outdoors;

2. a release from a pigtail or cylinder inside a steam chest; and

3. a release indoors- from either (a) a pigtail or liquid-filled cylinder or (b) other indoor system.
72

ORNL-DWG-83-16,71 1

Re~i on

Reg ion 4 Region

1 I I 3

Region

5

I Region 4I

I

I

A

E

i

I;

I

I

Downwind Distanc

Region I: A negatively buoyant mixture of UF6, air, water, andUF6 hydrolysis..products rises untilithe Initial

vertical momentum Is ",transferred" to the entrained

air and water.

Region 2: The negatively buoyant Plume will fall to the ground .-u nless the decrease in density resuiting.from chemical

reactions and entrained air Is sufficient to form a

.buoyant Plume Prior to Plume touchdown.

Region 3: A ground-hovering (gravity-spreading) plume IS

transported downwind, while entraining air and water

ati a reduced rate, until the Plume beomsbuoyant.

Region 14: The buoyant Plume lifts off from the ground, rises

until further entrainment and .UF6 hydrolysis Produces

aninert, neutrally buoyant plume.

Region 5iý The plume may be-adeciuately modeled as a Gaussian'

Plume.

Fig. 16. Example o f a. possible plume trajectory from a moderate-velocity, vertical release of UF6

vapor.,

. 11
'<73

Selecti-on of an. indoo'r release, would be based on-an .,evaluation of release rates from the various Systems containiing UF6 .

'it is suggested that generic source" terms for the release of UJF 6 from pigtails and cylinders be

deve'loped because of their applicability to the above releases. These source terms should be functions

of process conditions: and UF6 inventory and would be' time dependent.

There are two. general, approaches to modeling indoor UF6 releases. One approach is, to analyze a

worst case scenario using approximate models to obtain a conservative ,estimate. This approach would

use a homogeneous mixture model for determining source terms for a ventilation model. A more accurate-. (and.; also more expensive). approach is to characterize release 'behavior in "the momentumdominated zone of a compartment by using a directed or explosive release model and -in the rest of the

compartment by using, a hydrodynamic model to -det~ermine the time-dependent, spatial variation in

temperature,' pressure, and composition. The relative benefit of using :one model over the other, would

need to be considered with respect to, the relationshipl between release rate, ventilation rate, and compartment size; -the accuracy of the ventilation system and atmospheric dispersion models to be used;

'And the type of information desired (e.g., the: time available for evacuating the room with respect to

operator location)'."

Analysis of directed UF6 releases will require analytical tools to predict

I1. the perturbations in process conditions caused by the release;

2. the flow characteristics through process equipment to determine release rate, compositioni and

temperature;

3. the behavior. of, a jet'in the region where the jet's momentum: and, buoyancy dominate the jet tra-.

jectory;

4.. interactions between a jet and'solid, surfaces (e.g., change in -flow direction, deposition,: And heat

transfer);

5. the behavior of the resulting cloud 'of UF6 and UF6 hydrolysis products when diffus~ion and convective processes are dominant (including hovering plumes); and
'74

6. the time-dependent composition, temperature, pressure, etc., of the com .partment atmosphere during and following the release (homogeneous or hydrodynamic model).

Analysis of explosive-type UF6 release will require analytical tools t o predict

1. possible failure mode(s) of equipment (e.g., weld failure, expl osive fragment ation);

2. the number of fragments, as well as their siz~es and velocities;

.3. the behavior of flashing liquid UF 6;

4. the flow characteristics of an expanding cloud of UF6;. and

.5. the time-dependent composition, temperature, pressure, etc.; of the compartment atmosphere during and following the releaseý (homogeneous or hydrodynamic model).

A few additional problem- areas of particular interest when analyzing UFj accident scenarios were also

,discussed briefly. These areas- included fire-related releases of UF6, UF6-hydrocarbo6n ýreactio~ns, the

release-of higb~rassay UF6, and UF6 .plume behavior outdoors.
S. CALCULATIONAL METHODS NEEDED FOR ANALYZING UF6 RELEASES

Based on the scenarios. presented in Chapter 5 and the modelling considerations discussed in Chapter 7,

a list of 28 calculational methods required for either a first-order approximation or a more accurate analysis

of a postulated UF6 release was prepared. The applicability of the 28 methods to the 25 accident scenarios

is given in Table 9 based on whether the scenario can result in a directed or an explosive release of UF6.

Table 10 shows the availability of the various methods and the. level of need (first-order approximation or

more accurate analysis). Methods 8 through 13 and 21 are considered as necessary for a frust-order

approximation. Some requirements for these methods are briefly discussed below in the same order as they

appear in Tables 9 and 10.

8.1 METHODS FOR PREDICTING FAILURE MODES

The frust six methods appearing in Tables 9 and 10 are useful for predicting the failure mode of equipment containing UF6. Methods 1 and 2 are concerned with predicting internal forces acting to breach the

containment, while methods 3 through 5 deal with external forces. Method 6 applies applicable forces to

the containment to determine. potential failure modes and characteristics' of the breach.

1. liquid UF6 - Hydrocarbon Chemical Reactions

An estimate of.- the heat liberated from these reactions, the rates of reaction, and the change in

chemical composition is required to predict the pressure and temperature inside a containment vessel

(e.g., a cylinder).

2. Physical and Thermodynamic Conditions Immediately Prior to Failure of Containment

The temperature and pressure of the process stream immediately prior to failure must be, known to

determine the possible failure mode(s). The characteristics of the process stream (e.g., phase, composition, etc.) must be known to evaluate the initial release rate. Temperature-pressure time histories may

be required for some stress analyses.

75
76

3. Effects of Natural Phenomena (Earthquakes, Tornadoes,. High Winds, etc.)

The effects of natural phenomena, such as pressuredo impulse loading, must be known to predict possible failure modes. For example, the pressure force resulting from a high wind. might cause a crane-to

fall with aý resultant loss of containment through an impact on a containment vessel.

4. Characteristics of an Impact on Process Equipment.

The characteristics of, an impact including the mass, inertia, and shape of projectile(s) and their

poinit(s) of impact are required to predict the failure mode; of an impacted piece of equipment. For

example, a highly localized impact force could puncture the containment vepssel, while a, distributed

force could result in a weld failure.

5. Characteristics of a Drop of Process Equipment

As for an impact on process equipment, the prediction of the failure mode of a dropped piece of

process equipment requires the evaluation of the impact forces. Analysis "will use the mass, inertia,

shape,, and orientation of containment at the. point of: impact and the characteristics of the impact -sur-

-faces.

6. Failure Mode(s) of Containment

To predict the release rate after a loss in UF6 containment, a failure mode must be predicted.. If.a

directed release is predicted (e.g., from a weld failure, valve failure, etc.), the geometric characteristics

of the flow channel must be known to predict the release rate..

8.2 METHODS FOR PREDICTING SECONDARY FAILURES

The seventh. method appearing in Tables 9 and 10 is useful for evaluating secondary effects following

loss of containment from the primary equipment of interest.

7. Number of Fragments, Their Sizes and Velocities

If an ex'plosive'-type release is postulated, the resultant projectiles 'may cause secondary failuresi by

impacting other equipment.
ORNL MGM 054 56

Table 9. Applicability of caculational methods

,to analysis of postulated UF5 release scenarios

I

Ooetto,. to .1n alto tott . otibote - to t n iotlte~ io, ntegone, 1, 2, and 3.

KEVY

-ohod w~.po w.b tit both. a tente ,nd an 400100w teliowe to, tine p0.~itoI~~ 0

D. ninhod applcshle to.a Itinoted teleoc fo, the potoatedw Sinanato

E * tninhod .nnlicible to in c00100w tel.. tot the pitolatumtd b-6,tt

fFI
78

OffNL 0mG aNg65?

I *-.-.. .1

P--odpplbiitttmnttton Docurnasrtation nrust be

Sarsd lendi of .naId eoailabtf mvinad or prepared P

Table 10. Status of calculatlonal methods for.

.analysis of postulated UF6 release sceanrios

* ..

'P~ ~ r J, 

dP o' SF' A.

ce ~ 06' 

. ?/ .~# 6 *f~ 01?

x'6' ~ 51 0 0 * 4 6

r.fr.,taai,,,.aI osatfoofs -

1. Liquid UP0 - hydroarbon cfhenmical reactions (e.g.. inside a cylinder). C C : .V v

2. Physical and thermodynamic conditions immediately prior to failure of C C. containment Itiree history may be required for some stress analyses). -

'3. Effects of natural phnnomena learthtquakes. tornadoes, high rý,indls. etc.). C V v

4. Characterist ics of an impact on prooess equipment (includes the mass, C

-inertia, and shape of proiectilels) and their pointls) of impact).

5- Characteristics otaýdrop of.peo-esequipment (includes the mass. inertia,- C

shape, and orientation of containment at the point of impact and the - V V . .. V

characteristics of the impact surface. ,.

6. Failure models) of containment (including geometric descriptiont of failure). C C 7 V . V

7. N Iumber of fragments, their sizes and Weocisies . . C .V

S. Characteristics of flow through equipment and piping. S v b b b

9. Characteristics of flow through a rupture in containment. S V b It

0:Time-dependent, physical and thermodynamic conditions during release. S -. V V

1f. Characteristics of flashing liquid UP6

. S V VV

12. Choke flow criteria for multiphase U~F systems Isonic oelocityl. S C v

13. Chemical reactions between UP6

. and moist air (including energy balancns. S C v

density changes, etcI.).

1d. UP6-steam interactions inside a steam chest. - C v V .V V,

15., Release ot.UP6 and UP6 hydrofysis products from a steam chest. C -V -- V, -v

16.: Deposition rates of UP6 and UP6 hydrolysis products. C.,. . e -

17. Agglomeration rates of UP6 and UP6 hydrolysis products. Cf a e

18. PFree jet cftaracteristics (including effects of a chemically reactingC

mixture of UP6 and UP6 hydrolysis products;-- Note: Jets resulting from irregular openings ore often approximated using jets from regular opening'), ____

1g- Plomocharacteristicsof a chemically reacting jet impinging on a surface. C -VV

20. Characteristics of an expanding cloud of UP6 and UP6 hydrolysis C

products in a compartment (including effects of a chemically-reacting . -. - tVV

m insure of UP6 and UP6 hydrolysis products),-

21. Anerge physical and thermodynamic prnperties within a compartment.- S-- -

(homogeneous compartment model; - can be used to predict a steady -

-state or transient source term fot a ventilation model yiu- the'release. :

rate, duration. aod compositi~l

22. Characteristics of an expanding cloud of UP6 and UP6 hydrolysis products S

resulting from an outdoor release near the release point.

23. UP6 - hydrocarbon reactions in the presence of fire. - - - C

,24. Mast transport within a fire. .C C`i k.-

26: -Mass transport that results from an explosion. - - -I

26. Filtration efficiency. -- : - S; S v 

27. Tramport of UP6 and UP6 hydrolysis products throughoventilation sysems.- -ý S

26. Criticality model.+ v I-#

KEYS needed for first-order approximation as well astfor mote accurate analysis !order of unalysis may affect formulation of method)

C =useful for more naccorate analysis
79

Footnotes. to Table' 10: "Status of. Caiculational 'Methods for Analysis of Postulated UF 6 Releases

Scenarios

~A xlsv-ype reese isconsidered much less- likely than a directed release. Although several

postulated scenarios could 'possibly culminate in an explosive-type release, -only scenario, 1.1 (introductinof reactive hydrocarbon into a cylinder, which results in a UF6-hydrocarbon reaction) is considered

reasonably likely to~occur.

bThe fourth and fifth methods discussed-in Sect. 6.2 under DOE Source Term Methods as. well as

the two reports discussed earlier in the same section' under Methods from the Open Literature., include

-some information applicable to these Calculational Methods (8ý and 9). These. reported methods, -do not.

'cover all release possibilities. -The DOE methods need to be documented.

Information is geeal viable for estimating sonic velocities.

dThis method is needed if particle -deposition rates are'required. This method. can::also be uXsedr to

imp rove the' accura .cy of- a ,nalytical results.

- Additional data are needed to* *apply~ these methods.,

~fhis'Pmethod is -needed if particle size distributions are required. This method can also be used to

improve the accuracy of analytical results.

.

8Two levels of methods for- "frees jet characteristics need to be developed. First, -a "free" jet method

needs to be developed. for a- chemically, reacting 'jet, then this method needs to be expanded to handle

flashing in a mnultiphase jet.--- -

'A "free" jet. method -is not believed -to 'be. required to obtain a reasonable first-order approximation

of a source term for an indoor release; however-,.a "free" jet method may be required to obtain a reasonable approximation outdoors.

'The third -method discussed in, Secti. 6.2 DOE Source Term. Methods has. not been- implemented or

documented.

r The first and second methods discussed in Sect. 6.2 DOE Source Term' Methods are documented

in internal memoranda. These documents need .to be revised to, reflect changes in the computer,'programs they describe.

k FIRAC*-may 'be applicable with modifications to handle IUF6 (see Sect. 6.1).'

'EXPAC may be applicable wiith modifications to handle UF6 (see Sect. 6.1).

"'FIRAC,* TORAC, and/or EXPAC may be applicable with' modifications to handle UF6 (see

Sect. 6. 1) --
80

8.3 MIETHODS FOR PREDICTING RELEASE RATES

Methods 8. through 12 listed in Tables 9 and 10 are needed for determining release rates from breached

equipment. Four of the 'methods - 8, 9, 11, and 12 - deal with flow phenomena, while method 10 dea.ls

with the behavior of UF6 within the equipment following loss of containment.

8. Characteristics of Flow Through Equipment and Piping

The pressure drop of a compressible,. flashing mixture needs to be calculated for flow through pipes,

valves, etc. Some correlations exist that reduce to equations for single-phase:, incompressible flow under'ý

appropriate conditions.

9. Characteristics of Flow Through a Rupture in Containment

This methodology estimates the flow rate through an irregular opening. The methodology would

probably assume a rough pipe, developing flow, and an equivalent diameter approximation.

10. Time-Dependent, Physical and Thermodynamic Conditions During Release

A methodology will be needed for predicting the temperature, pressure, and UF6 phase(s) inside a

UF6 cylinder during a postulated release.

11. Characteristics of Flashing Liquid UF6

A model to predict the solid/vapor split of- liquid UF6 after either an isentropic or an isenthalpic

expansion to a given pressure will be needed.

.12. Choke Flow Criteria for Multiphase UF6 Systems

Choke flow, criteria are needed to bound the release rate of UF 6. This method may involve predicting the sonic velocity of a multiphase UF6 mixture and the sonic velocities -of UF6 liquid and, vapor.

8.4 METHODS FOR PREDICTING THE BEHAVIOR AFTER RELEASE

Methods 13 through 22 given in Tables 9 and 10 are useful for evaluating the physical consequences of

a release of UF6. Method 13 deals with chemical reactions and phase equilibria associated with the hydrol

ysis of UF6. Method 21 is the simplest -and most uncertain - method for analyzing the behavior of
UF6 in a compartment following a: release. ,Methods 14 and 1 5 incorporate basic information. used in.

methods 13 and 21 along with chara cteristic' information related, to. steam, chests. The other methods are

much more complex models neededifo~r advanced levels of analysis req uiring spatial resolu Ition of the 'char .acteristics of a release.,,

13. Chemical Reactions Between UF6 and Moist Air,

The primary reaction associated with UFhyrlssmt be icroated inoamethodology for

estimating the resulting, composition and temperature after hydrolysis.

14. UF6 -Steam Interactions Inside A Steam Chest

This method would be similar to Method 13- excep that., the ýpressur and temperature inside the

steam chest must be predicted.

15. Release of UF6 and UF6 ydrolysis Products from a Steam Chest

The methodology forpeitn threas rate from, a steam ches It considers the scnario inwhich

arelease from a steam chest is through a short. piece of pip fo0wn h fiue"o utr ik

16. Deposition Rates of, UF6 and UF6 Hydrolysis Products

Deposition, rate data and a deposition rate model are needed to determine the,, fraction. of, the

released material that is d~epo~sited on surfaces, within a building or ventilation system. If reentrainment

can-be- neglected, the deposited material will not be released to theý ambient environment.

1,7.. Agglomeration Rates of:UF 6 -and, UF6 Hydrolysis Products

An agglomeration rate model is -required to predict -particl e size distributio ns as functions of time.

Deposition rates and filter-efficiencies will usually ,be strongly depenident, on the average particle size.

18,.-,"Free" Jet Characteristics

A free jet model is required if either: spatial variations of jet characteristics in the momentumdominated flow regime are to be pred icted or if a source term is needed to simulate an expanding cloud

(see Method 20). Important jet characteristics, include the jet size, trajectory, composition, temperature, and concentration profile. The model should include the effect of a'chemically reacting mixture

of UF6 -and UF 6 hydrolysis products. Jets resulting from irregular openings would be approximated

using jets from regular openings.
82

19. Flow Characteristics of a Chemically Reacting Jet Impinging on a Surface

An estimate of the flow field associated with a chemically reacting jet impinging on a surface may be,

required if spatial variations are important. Heat transfer, deposition processes, and ,chemical reactions

with solid surfaces may be important.

20. Characteristics of an Expanding Cloud of UF6 and UF6 Hydrolysis Products in a Compartment

This model is also required if. spatial variations are important. Because this model requires .the.

results of method '18 as input and because' it will provide informati~on simnilar to the jet model,

methods 18 and 20 could be combined, in a single model. The effects'ofa chemically reacting mixture

of UF,6 and UF6 hydrolysis products should be included.

2.1. Average Physical and The rmnodynamic Properties within a Compartment (Homogeneous Compartment

Model)

This model can be used to predict a batch-mixed or transient Source term for a ventilation model

given the, release rate, duration, and phase, composition of a postulated UF6 release. Output from this:

model would include the time-dependent, spatially averaged composition and the temperature within a

compartment. This model can be used, as an alternative to melthods 18 through 20 if spatial gradients

are not important.

22. Characteristics of an Expanding Cloud of UF6 and UF6 Hydrolysis. Products Resulting from an Outdoor Release Near the Release Point

This model would be similar to Model 20 except that ambient characteristics such as wind and pre-m

cipitation may be important.

8.5 M[ETHODS FOR PREDICTING BEHAVIOR IN A FIRE OR EXPLOSION

Methods 23, 24, and 25 (see Tables 9 and 10)- deal with phenomena associated with fires and explo-'

sions.
23. ,,UF6-Hydrocarbon Reactions in the Presence of Fire

The methodology simulates UF6-hydrocarbon reactions in a fire including, changes in composition,.

heats of reaction,' etc.

.24. Mass Transport Within. A Fire

A transport model is required to sfimulate the movement of UF6 and ;UF 6 "combustion products",

.within a fire.

25. Mass Tranisport that Results from an Explosion

A transport model is required to, simulate. the mass transport. that results from :an. explosion.

Phenomena such as shock waves and flashing of liquid'UF 6 may be important.

:8.6 METHODS NEEDED TO MODEL FLOW THROUGH A VENTILATION SYSTEM

Methods 26 and 27 (see Tables 9 and 10) are useful for evaluating the effects of a ventilation system

on the UF6 release products vented to the atmosphere.

26. Filtration Efficiency

A m.iodel to prdict the fraction of..UF6,,hydrolysis products. retained by: a. filter is. required -if flow

through a filter is to be simulated.

*2.Transport of UF6 and UF6 Hydrolysis Products through Ventilation Systems

Aventilation transport model, which considers the effects oUF6 yrlss srqie fasgii

cant quantity of UF6-is transported through a ventilation-.system containing moisture.

8.7 CRITCALITY METHOD

The last method appearing in Tables 9 and 10 is needed because of the radioactive nature of uranium

compounds.

28. Criticality Method

Existing criticality methods can be used for evaluating the plausibility of nuclear criticality resulting

from an accident.

9. CONCLUSIONS

This study and this report have focused on determining the various possible accident scenarios in NRClicensed fuel cycle facilities that involve the release of UF(5 to identify and evaluate the analytiWa methods

needed and available for determining source terms for such accidents. The study is part of a program that

will lead to documentation of the necessary analytical techniques and data bases for realistic' accident

assessments in a Fuel Cycle Facility Accident Analysis Handbook (AAH).

Conclusions derived from this study are as follows:

1. Review of available NRC and NRC-licensee documents did not reveal a comprehensive list of credible

UF6 accident scenarios or any specific analytical methods for assessing the consequences of such

accidents.

2. Heavy reliance on site visits to NRC-licensed facilities and on operating experience from uranium

enrichment facilities was necessary to establish a list of credible UlF6 accident scenarios (Table 7),

many of which are based on historical events.

3. No attempt has been made to assess the probabilities and/or consequences of the listed scenarios. Such

an assessment requires detailed site-specific information and is best done on a case by case basis. Criticality events and radiological concerns related to the release of high assay Mr, have been touched on

briefly but need to be addressed further.

4. UF6 thermodynamic, chemical, and physical characteristics and behavior are unique and are considerably different than those for most other compounds. Therefore, modifications to most available analytical methods, including those currently being developed for the NRC AAH, are necessary for them to

be applicable for assessing situations involving U`F4 releases.

5. In addition to classical methods for determining UF,6 flow rates through process equipment (e.g., pipes,

valves, orifices, etc.), DOE/UCC-NI) has several other analytical methods that are within the scope of

the AAH that are in various stages of development but that are not formally documented.

6. The procedures and criteria for selecting (a) bounding release events to be analyzed, and (b) the minimum generic analytical tools to be developed for and/or documented in the AAH are very complex.

85
This report, and -especially Chapters 7 and 8 on perspectives and calculational methods, respectively,

gives the basic material for arriving at such selections.

7. Consideration of the inventories, relative probabilities, and generic nature involved,.leads to the identification of at least three types of releases that are definitely worth investigating:

a. release from ,q liquiid-filled cylinder outdoors;

b. release from a pigtail or cylinder inside a steam chest; and

c.. -release indoors from either( (1) a pigtail or liquid-filled cylinder or (2) otherý indoorý system.
REFERENCE&:

[1 :J: Schneider,' Nuclear Fuel Cycle Risk. Assessment: Descriptions of Representative NonReactor Facilities, ýNUREG/CR-2873 (PNL-4306), Vo.1, Pacific Northwest ýLaboratory, Richland, Washington, Septemrber:1982, Sects. 11I and 13.

[2], Packaging of Uraniu Healurd fo rnsport, ANSI N14. 1-1982, American National Standards Institute, Inc., New York, July 6, 1981.

[31 Uranium Hexafluoride: -Handling Procedure and Container Criteria, R-51 e.4 Oak Ridge.

- perations Office, U.S. "ERDA, Oak Ridge, Tennessee, April 1977;ý

*4 Uranium Hexafluoride," Fed. Regist., 32 (230), pp. .16209-162941i November'29, 1967.

5]T. Okamoto and. R.' Kiyose, "Evaluation of. U.F6 Vapor Release in Postulated Accident,"

J.* NucL.: Sci. Technol,' 15(6), 455L460, June, 1978.

_[6] 'A. Ericsson and B. Gru~ndfeld, "Atmospheric Dispersion and Consequences of a UF6 Release Caused

by; Valve Rupture on. a Hot 30B Cylinder," Proceedings of the Specialists' Meeting on the Safety

Problems 'Associated. with the Handling -and Storage of 'UF6, Boekelo, Netherlands,

June 27-29, 19.78, Organization for Economic Corporation: and Development, Paris, France (contains several errors).

[71 Departments of the Army, the Navy, and the Air Force, Structures to Resist the -Effects. of Accidental Explosions, TM 5-1300, NAYFAC P-397, AEM 88-22, June 1969.-

[81 C. Grelecki, Fundamentals of Fire and Explosion Hazards Evaluation, AIChE Today Series, American Institute of.Chemical Engineers, New. York, 1976.

[9] G. F. Kinney, Explosive Shocks, in Air, The Macmillan Company, New York, 1962.

[10], D. F. Norsworthy and C. Howarth, "Safety Consideration Involved in the Choice of Containers for

Storage of UF6 TaIls", Proceedings of the Specialists' Meeting' 'on: the Safety Problems Associated

With the Handling-and Storage. of UF6, Boekelo, Netherlands, June 27-29, 1978, Organization for

Economic Corporation and Development, Paris, France.

87

SELECTED BIBLIOGRAPHY OF DOCUMENTS

RELEVANT TO NRC UF6 RELEASE SOURCE .TERM[ DEVELOPMENT,,

Standards,, ProceqssandfSystem D~escripto, S.9

and Cylinder Descriptions

See Refs. 1,2, :3a, and 4.

Postulated and Actual Scenarios

Ivolving the Release of (IF6

A.- Price, "Safety Aspects of Uranium :Hexafluoride Manufactur n eovrina Sprnfed

Work", rocedigs f 'the Specialists Meeting on th'eSafety Problems Associated with the Handling and

Storage, of UF6, Boekelo, 'Netherlands, June' ý27-29,' 1978,' Organization- for Economic-' Co-operation and

Development, Paris, France..

Methods for Analyzing (IF6 Releases:

See Ref. 5, 6, and 10O..t

H. Bouzigues, M. Mezin,'and E. Mestre, "Fundamentals of UF6 Accidental Release;" Proceedings Of the Specialists'. Meeting on the, Safety. Problems Associated with the Handling. and -Storage 'of (IF6,

Boekelo, Netherlands,. June 27-29,-,1978, Organization for IEconomic Corporation and Development, Paris,`

.France.

E. A. Hirst, Analysis of Round, 'Turbulent, Buoyant Jets Discharged to Flowing Stratified.. Ambients,

U.S. AEC Report No. QRNL-4685, June 1971.

P. Maitre, T. Meslin, and P. Pages, Evaluation of Safety in the Transportation of Natural Uranium

Hexafluoride, Report No. 4, nr d'Etudi sur l'Evaluation del rtcindn eDmie Nucaie

Fontenay-aux-Roses, 92 (France),' May 1977, (also, BNWL-tr-269,' Battelle Northwest -Laboratory, Richland, Washington,: September 1977).

A. Raggenbass, M.'1 Berlin, and J. _C. Malet, "Expeiental Approach of Mi aaeesfrSft

Evaluation of Uranium' Hexafluoride Industrial Utilizations", Proceedings ofteSpecialists' Meeting on the

Safety Problems Associated w-ith'Handling ,and Storage of UF6 9 Boekelo, .Netherla nds,~ Junte:27-29, 1978,

Organization for Economic Corporation and Development, Paris,, France.

W. Thomas, "Environme ntal Impact of a Potential UF6 Release Resulting from an Accident-in a U0 2

Fuel Fabrication 'Facility", Proceedings of the Specialists'. Meeting on the Safety Problems _Assýo'ciaited with

Handling and Storage of (IF6, Boekelo, Netherlands, June 27-29, 1978, Organization for Economic' Corporation,-andD Dvelopment,. Paris, France.

Physical Characterization Data for (IF6

and (IF6 Hydrolysis Products

Allied Chemical Corporation, Hydrofluoric Acid,,Morristown, New:Jersey,. 1978.

S. P. Anderson, A Siud)y of the Hydrolysis- of Uranium Hexafluoride. by -Fourier Transfor m Infraried

Spectroscopy, master's thesis, The University of Tennessee, Knoxville, Tennessee (in preparation).

89
90

S. P. Anderson, Prolog to Studies on the -Hydrolysis of Uranium Hexafluorie akruddt o

ma'ster's thesis, Chemnistry Department, The University of Tennessee,. Knoxile Tennessee, Aug ust 1982

(in preparation).

R. DeWitt, Uranium Hexaftuoride:- A Survey of the Physioco-Chemical Properties, GAT-280, Goodyear Atomic Corporation, Portsmouth, Ohio, January 29, 1960..

J. Ishida, H. Ishigro, and J. Kato, "Investigation of the UF6 Aerosol Behavior in Air (II) -'Experimental Results", Semiannual Progress Report of Power Reactor and Nuclear. Fuel Development Corporation, Tokai Works, Tokai, Ibaraki (Japan), January-June 1978, -PNCT-83t-78-O1,, January 1979,

pp. '109-113.

J. Ishida, et. al., "Investigation of, the UF6 :Aerosol Behavior in Air (II) - Experimental Results",

Semiannual Progress Report of Power Reactor and Nuclear Fuel Development Corporation, Tokai Works,

Tokai, Ibaraki (Japan), January-June, 1978, PNCT-831-78-01, January, 1979, pp. 151-158.

J.. Ishida, et al., "Investigation. of the UF6 Aerosol Beha'vior 'in Air (IV) A-The Characteristic of the .

Monitors Detection UF6 Release," Semiannual Progress Report of Power Reactor and Nuclear Fuel

Development -Corporation, Tokai Works, Tokai, Ibaraki (Japan), July-Decembe~r 1979, PNCT-83 1-79-02,

July 1980, pp. 203-207.

H. Ishigro, J. Ishida,. and J. Kato, "Investigation of the UF6 Aerosol Behavior in Air (I) Summary

of the Instrument", Semiannual Progress Report of Power Reactor and Nuclear Fuel Development Corporation, Tokai Works, Tokai, Ibaraki (Japan);,Q'January-4unie 1978,-,PNCT-831-78-0l, January 1979,

pp. 109-110.

G. K. Johnson, "The Enthalpy of Formation of Uranium Hexafluorid~e", J.. -Chem. Thermodynam., 11,

483-490, 1979.

J. J. Katz and. E.' Rabinowitch, The Chemistry of Uranium, 'Part .1, 1 st ed., McGraw-Hill Book Company, New York, 1951.

1. E. Knudsen, et al., A Fluidized-Bed Process for the Direct Conversion of Uranium. Hexafluoride to

Uranium Dioxide,' U.S.. AEC ReportNo. ANL-6606, February 1963.

W.' H. McCulla, Determination of the Rate of HF Hydration and the Effects of HF on Moisture Condensation", K/PS-l 55, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tennessee, April 30, 1982.

P. A. G. O'Hare and J. G. Malmn, 'The.rmoche~mistry of Uranium Compounds XIII.,. Standard Enthalpies of Formation at 298.1 5*K of Uranium Oxide. Tetrafluoride (UOF4) and Uranyl Fluoride (U0 2F2)

Thermodynamics of (Uranium + Oxygen + Fluorine)', J. Chem.. Thermodynam., 14, 331-336, 1982.

B. IL. .Parks and D. W. Burton, Thermodynamic Properties of Gaseous Uranium Hexafluoride, K1458, Oak Ridge Gaseous Diffusion Plant,.Oak Ridge, Tennessee, July 1.973.

P. W. Pickrell, Characterization of the 'Solid Product(s) Formed when UF6 is' Released into Ambient.

Air in a Contained Volumhe (U.), K/ET-503, Oak Ridge Gaseous Diffusion Plant, 'Oak Ridge, Tennessee,,

May 7, 1980.

P. W. Pickrell, C~har~acterization of the Solid, Airborne Materials Created by. the Interaction of (IF6'

with Atmospheric .Moisture in a, Contained Volume, K/PS- 144, Oak Ridge Gaseous Diffusion Plant, Oak

Ridge, Tennessee, April 30, 1982.

Historical Releases

D. I. Bodeau, et al1; Data Base-fior Radiation"Events in the Commercial-Nuoclear Fuel Cycle 1950-1978,

ANL/ES- 123 and NUREG/CR-24-29',, Argonne National Laboratory, IChicago,, Illinois, December' 198 1.
91

A. 3. Ducouret, 'An Experience of Accidental Release of UF,6," Proceedings of the Specialists' Meeting

on the Safety Problems Associated with the Handling and Storage of UF6, Boekelo, Netherlands,

June 27-29, 1978, Organization for Economic Co-operation and Development, Paris, France.

G. T. Hull, UF6 Release Experience and Prevention In United States Gaseous Diffuson Plants, KY690, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tennessee, June 27, 1978.

A. 3. Legeay, "Handling of UF46 in U.S. Gaseous Diffusion Plants', Proceedings of the Specialists'

Meeting on the Safety Problems associated with the Handling and Storage of UF6, Boekelo, Netherlands,

June 27-29 1978,' Organization for Economic Corporation and Development, Paris, France (also,

K/P-6452, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tennessee, 1978).

A. I. Mallett, ORGDP Container Test and Development Program Fire Tests of UF6-Filled Cylinders,

K-D-1894, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tennessee, January 12, 1966.

Methods for Analyzing Explosions

See Refs. 7, 8, and 9.

93

NUREG/CR-3 139

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NCFORM 335 1.REPORT NUMeE R Asslpned bIYoDC) U.S. NUCLEAR REGULATORY COM4MISSIONNRG/R33

- BIBLIOGRAPHIC DATA SHEET (RN/EG/M-5

.4. TITLE AND SUBTITLE tA dc Volume No., '11 a-opropriere) ( Leave blankJ

Scenarios and'Analyti'cal Methods for UF6 ýReleases at'

NRC-Licensed Fuel Cycle Facilities 3. RECIPIENTS ACCESSION NO.

7. AUTHOR(S) e,5. DATE REPORT COMPLETED

M. Siman-Tov,- J.* Dykstra, D. D. Holt, W. P. Huxtable., MONTH' Y YEA.9

.R. A. Just, W. R.' Williams January 1983

9. PERFORMING ORGAN IZATI ON'.NAME A .ND MAILING ADDRESS (Include Zip. Code) DATE REPORT ISSUED

-MONTH YEAR Oak Ridge Nationa Laboratory June 19ý84

PostOffce X6. Bx .(Leave blank) Oak Ride TN 3831,

8. ('Leave blank)

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Division'% of Risk Analys.is . 10PRECTAKIR(

office of Nuclear Regulatory ýResearch '1.FINO

U.S. Nuclear Regulatory Commission

Washington, DC 20555 B049 ,5

13: TYPE OF REPORT PE RIOD COVE REO.(6ýlfliusivesOtes)

Topical,

15. SUPPLEMENTARY NOTES_1;(eveoak

V6. ABSTRACT (200 words or less) 'This report identifies and discusses..potentiali-scenarios for the

accidental release of UF6 .at NRC;--licensed UF' production and fuel fabrication facilities

based on a literature review, site, visits, and,-DOE enrichment plant expcerience. Calcu-_,ý

lational~metho~ds *needed fo~r analyzing ý.such releases are' also, reviewed. Acc~ident scenariosate presented under 'the headings of cylinder, failures, 'process system failuires,.criticality events, 'and operator errors and are categorized by locatibn; -release source, UF6 phaside prior to relea~se, release flow characteristics,- release causes, initiating events,, and UFý6 inventory at risk.. Releases identified for further,'examiniation-include: (1) a rele'ase

from a cylinder outdoors, '(2) a release from a7"pigt~ail or cylinder ,in a steam chest,

and.(3) an indoor release from either (a) a,.'pigtail or cylinder :or (b)' other i1ndoor .s~ource

depending on, 'facility design and operating procedures.. Indoor rele"ase phenomena may be

analyzed :using a time-dependent homogeneous compar .tm ,ent model or a more ýcomplex hydro-,

dynamic model 'if' time-dependent, ''spatial' variations in concentrations,' te-mperarture,:and.

pressure are important'. Analytical tools for.modeling directed jets and'exoplosive,

.releases are.Kdiscussed 'as well as' some of 'the complex phenonfena to be conside'red 'in.

,analyzing U F6 releases bo th indoors, and outdoors.

17. KEY WORDS AND DOCUMENT ANALYSIS 1 7

a. DESCRIPTORS

uranium hexafluoride (UF) cylinder:. initiating events

accient *. cold trap ' modeling'

scenarios ' pigtail

releases .. steam chest

analysis fuel cycle

17ti bE INTI FIE RS.'OPE NE NDCED TERMS

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