Thermal Hydraulic Features of the Accident - ACS Symposium Series

2 Thermal Hydraulic Features of the Accident B. Tolman, C. Allison, S. Behling, S. Polkinghorne, D. Taylor, andJ.Broughton

Downloaded by AUBURN UNIV on December 2, 2016 | http://pubs.acs.org Publication Date: December 23, 1986 | doi: 10.1021/bk-1986-0293.ch002

Idaho National Engineering Laboratory, EG&G Idaho, Inc., Idaho Falls, ID 83415

The TMI-2 accident resulted in extensive core damage as confirmed by recent video data showing extensive prior molten core debris in the bottom of the reactor vessel. A hypothesized TMI accident scenario is presented that consistently explains the TMI data and is also consistent with research findings from independent severe fuel damage experiments. The TMI data will prove useful in confirming our understanding of severe core damage accidents under realistic reactor systems conditions. This understanding will aid in addressing safety and regulatory issues related to severe core damage accidents in light water reactors. TMI-2 c h a r a c t e r i z a t i o n d a t a over the past 2 y e a r s c o n f i r m t h a t e x t e n s i v e damage t o the r e a c t o r core r e s u l t e d from t h e TMI a c c i d e n t . To u n d e r s t a n d t h e mechanisms r e s u l t i n g i n such e x t e n s i v e c o r e damage, i t i s n e c e s s a r y t o r e c o n s t r u c t the important thermal h y d r a u l i c events w h i c h c o n t r o l l e d t h e a c c i d e n t p r o g r e s s i o n . The p r e v i o u s paper (I) summarizes the sequence o f p l a n t events t h a t l e d t o e v e n t u a l l o s s of r e a c t o r v e s s e l c o o l a n t , core heatup and e v e n t u a l f a i l u r e , and t h e subsequent e f f o r t s t o r e - e s t a b l i s h l o n g term f o r c e d c o o l i n g t o the damaged c o r e . T h i s paper d i s c u s s e s t h e thermal h y d r a u l i c f e a t u r e s of the a c c i d e n t t h a t d i r e c t l y i n f l u e n c e d the c o r e damage p r o g r e s s i o n d u r i n g t h e p e r i o d from 100 t o 300 minutes ( a l l times u n l e s s noted a r e r e l a t i v e t o the b e g i n n i n g o f the a c c i d e n t ) . A h y p o t h e s i z e d a c c i dent p r o g r e s s i o n s c e n a r i o i n which the c o r e f i r s t a t t a i n e d a nonc o o l a b l e geometry and s u b s e q u e n t l y p r o g r e s s e d t o a f i n a l c o o l a b l e s t a t e i s d i s c u s s e d . Our u n d e r s t a n d i n g o f the a c c i d e n t p r o g r e s s i o n s c e n a r i o and a s s o c i a t e d thermal h y d r a u l i c s w i l l be p r e s e n t e d based on a p p l i c a b l e TMI d a t a , t h e r e s u l t s o f independent severe f u e l damage e x p e r i m e n t s , and f i r s t o r d e r thermal h y d r a u l i c p r i n c i p l e s . Summary o f the End-State Core and R e a c t o r V e s s e l C o n d i t i o n s P r i o r t o d i s c u s s i n g the thermal h y d r a u l i c a s p e c t s o f t h e a c c i d e n t , i t i s n e c e s s a r y t o summarize the damage t o the r e a c t o r v e s s e l 0097 6156/ 86/0293-0026506.00/0 © 1986 American Chemical Society

Toth et al.; The Three Mile Island Accident ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


2.

T O L M A N ET A L .

27

Thermal Hydraulic Features

e n v i r o n s and t h e known e n d - s t a t e c o n d i t i o n s o f t h e c o r e m a t e r i a l s * T h e t h e r m a l h y d r a u l i c s o f t h e c o r e damage p r o g r e s s i o n m u s t t h e n b e consistent with this data. The known s t a t e o f t h e r e a c t o r v e s s e l i s s u m m a r i z e d i n F i g u r e 1. Approximately o n e - t h i r d of the o r i g i n a l f u e l i n t h e u p p e r c o r e r e g i o n i s no l o n g e r t h e r e , l e a v i n g a v o i d r e g i o n extending to the outermost f u e l assemblies. Measurements from the c o n t r o l - r o d d r i v e s t r u c t u r e s i n d i c a t e t h a t upper plenum s t r u c t u r e t e m p e r a t u r e s ( a b o v e t h e c o r e ) r a n g e d f r o m 700 t o 1350 K , s e v e r a l hundred degrees below the s t a i n l e s s s t e e l m e l t i n g tempera t u r e s (2). A d e b r i s bed r a n g i n g from 2 t o 3 f e e t deep i s r e s t i n g on top o f the e x i s t i n g c o r e . C h a r a c t e r i z a t i o n of p a r t i c l e s from t h e d e b r i s i n d i c a t e t h a t t h e z i r c a l o y i s h i g h l y o x i d i z e d , and l o c a l i z e d t e m p e r a t u r e s e x c e e d i n g 2900 Κ were r e a c h e d , s u f f i c i e n t t o m e l t t h e UO2 f u e l ( 3 ) · E f f o r t s t o p r o b e down t h r o u g h t h e r e m a i n i n g c o r e m a t e r i a l s i n d i c a t e t h a t an i m p e n e t r a b l e l a y e r o f m a t e r i a l e x i s t s a t the 5- t o 6 - f o o t e l e v a t i o n (above the bottom o f the o r i ginal core). Recent v i d e o scans of the lower regions of the r e a c t o r v e s s e l i n d i c a t e t h a t 5 t o 20% o f the c o r e f u e l and/or s t r u c t u r a l m a t e r i a l s now r e s i d e o n t h e v e s s e l b o t t o m , n e a r l y 7 t o 8 f e e t b e l o w the bottom of the o r i g i n a l c o r e . The e x t e n t o f damage t o t h e l o w e r c o r e s u p p o r t r e g i o n i s n o t p r e s e n t l y known s i n c e t h e v i d e o s c a n s were unable t o c h a r a c t e r i z e the c e n t e r r e g i o n s o f the lower plenum. Based on the v i d e o i n f o r m a t i o n , which i s l i m i t e d t o the p e r i p h e r a l r e g i o n s o f the r e a c t o r v e s s e l , the r e a c t o r v e s s e l s t r u c t u r e does n o t a p p e a r t o b e s i g n i f i c a n t l y d a m a g e d , e v e n t h o u g h a s much a s 20 t o n s o f c o r e m a t e r i a l may r e s i d e o n t h e b o t t o m o f t h e v e s s e l . Overview

of the Core Degradation

Period

(100

t o 300

Minutes)

E s t a b l i s h i n g a n a c c e p t a b l e a c c i d e n t s c e n a r i o b a s e d o n l y o n TMI d a t a would be d i f f i c u l t s i n c e t h e a v a i l a b l e c h a r a c t e r i z a t i o n d a t a r e l a t e p r i m a r i l y t o t h e e n d - s t a t e c o n d i t i o n o f t h e c o r e and r e a c t o r s y s t e m . L i m i t e d p l a n t d a t a were t a k e n d u r i n g the a c c i d e n t t h a t r e l a t e d i r e c t l y t o t h e c o r e damage p r o g r e s s i o n . Independent severe f u e l damage e x p e r i m e n t s , h o w e v e r , p r o v i d e a n i m p o r t a n t k e y f o r i n t e r p r e t i n g t h e e n d - s t a t e TMI d a t a . A l s o , s e v e r e c o r e damage c o m p u t e r m o d e l s a r e h e l p f u l t o i n t e r p r e t and i n t e g r a t e b o t h t h e e n d - s t a t e c h a r a c t e r i z a t i o n d a t a and t h e a v a i l a b l e t r a n s i e n t response d a t a recorded during the a c c i d e n t . Using these resources, together w i t h t h e TMI d a t a , a m o r e c o m p l e t e u n d e r s t a n d i n g o f t h e c o r e damage p r o g r e s s i o n i s emerging. The t i m e d u r a t i o n o v e r w h i c h t h e m a j o r c o r e d e g r a d a t i o n o c c u r r e d c a n be s u b d i v i d e d i n t o t h r e e i n t e r v a l s : 1.

I n i t i a l c o r e h e a t u p and f u e l / c l a d d i n g f r o m 100 t o 174 m i n u t e s .

melting/liquefaction

2.

A t t a i n m e n t o f a n o n - c o o l a b l e c o r e geometry and subsequent h e a t u p t o a b o v e 2 4 0 0 Κ f r o m 174 t o 227 m i n u t e s .

3.

Melt progression i n t o the lower regions of the r e a c t o r v e s s e l and s u b s e q u e n t a t t a i n m e n t o f a c o o l a b l e g e o m e t r y f r o m 227 t o 300 m i n u t e s .


core

28

T H E T H R E E MILE I S L A N D ACCIDENT


Control lead screws (intact) temperature range 700-1255K Localized regions of oxidized and molten stainless steel Core void cross section —30% of total core volume Upper debris • Prior moltel (-3100K) • Fully oxidized zircaloy Hard layer (63-69 in above bottom of core' State unknown Bolts appear undamaged Estimated 10-20% of original fuel in lower plenum

Thermocouple junction locations near vessel inner surface Figure

1·

Known s t a t e

o f t h e core and r e a c t o r v e s s e l

components.


2.

TOLMAN ET A L .

29


The s i g n i f i c a n t t h e r m a l h y d r a u l i c periods are discussed below.

features

of

these

three


I n i t i a l Core Heatup and F u e l / C l a d d i n g M e l t i n g / L i q u e f a c t i o n 174 M i n u t e s )

time

(100

to

When t h e l a s t p r i m a r y c o o l a n t pump was s h u t o f f a t 1 0 0 m i n u t e s , f o r c e d c o o l a n t c o n v e c t i o n t o t h e r e a c t o r c o r e was l o s t , a n d t h e l i q u i d r e m a i n i n g i n the r e a c t o r v e s s e l and v e s s e l o u t l e t p i p i n g (hot l e g s ) s e t t l e d r a p i d l y i n t o the r e a c t o r v e s s e l . The r e s u l t i n g r e a c t o r v e s s e l l i q u i d l e v e l i s n o t known e x a c t l y , a l t h o u g h i t has been e s t i m a t e d t o be n e a r o r s l i g h t l y above the t o p o f t h e c o r e region (4). In a d d i t i o n , considerable u n c e r t a i n t y e x i s t s i n the r e a c t o r s y s t e m makeup f l o w s d u r i n g t h i s p e r i o d . The c o o l a n t i n t h e r e a c t o r v e s s e l s u b s e q u e n t l y b o i l e d o f f d u r i n g t h i s p e r i o d as a r e s u l t o f t h e c o r e d e c a y p o w e r s l e v e l s ( 1 0 0 t o 2 0 0 MW). The i n i t i a l r e a c t o r v e s s e l l i q u i d l e v e l and t h e s y s t e m makeup f l o w s u n d e r t h e s e c o n d i t i o n s c o n t r o l t h e a b i l i t y t o c o o l t h e c o r e as w i l l be seen s h o r t l y . D u r i n g t h i s p e r i o d , measurements o f s u p e r h e a t e d t e m p e r a t u r e s i n t h e r e a c t o r v e s s e l o u t l e t p i p i n g ( h o t l e g s ) and r a d i a t i o n r e l e a s e f r o m t h e p r i m a r y c o o l i n g s y s t e m g i v e c r i t i c a l t i m i n g i n f o r m a t i o n on t h e r e a c t o r v e s s e l b o i l d o w n and c o r e h e a t u p r a t e s . Additional i n f o r m a t i o n r e l a t i n g t o t h e c o r e t e m p e r a t u r e r e s p o n s e and t h e r e a c t o r v e s s e l l i q u i d l e v e l s may a l s o b e i n f e r r e d f r o m t h e i n c o r e t h e r m o c o u p l e s , i n c o r e n e u t r o n d e t e c t o r s , and t h e e x c o r e s o u r c e range m o n i tors. Of p a r t i c u l a r i m p o r t a n c e a r e the d a t a t r e n d s from the i n c o r e instrumentation. T h e s e i n s t r u m e n t s i n c l u d e d 364 s e l f - p o w e r e d n e u t r o n d e t e c t o r s ( S P N D s ) a n d 52 t h e r m o c o u p l e s . These i n s t r u m e n t s were p l a c e d i n i n s t r u m e n t a s s e m b l i e s , e a c h h a v i n g s e v e n SPNDs a n d o n e t h e r m o c o u p l e and were l o c a t e d a t d i f f e r e n t a x i a l e l e v a t i o n s . The instrument assemblies were i n s e r t e d i n the core r e g i o n from the bottom of the r e a c t o r v e s s e l . The c o n f i g u r a t i o n o f t h e i n s t r u m e n t a s s e m b l i e s w i t h i n t h e r e a c t o r c o r e a r e shown i n F i g u r e 2 . Extensive a n a l y s i s and i n t e r p r e t i n g o f t h e s e i n s t r u m e n t s a r e documented i n R e f e r e n c e 5. A general overview of the core thermal response during t h i s time i n t e r v a l based on the a v a i l a b l e i n s t r u m e n t a t i o n d a t a i s summarized below; 1.

I n i t i a l h o t l e g s u p e r h e a t was d e t e c t e d a t 113 m i n u t e s ( 4 - 7 ) . The s t e a m s u p e r h e a t i m p l i e s t h a t i n i t i a l c o r e u n c o v e r y and h e a t u p o c c u r r e d p r i o r t o 113 m i n u t e s .

2.

Large increases i n containment r a d i a t i o n l e v e l s occurred at 142 m i n u t e s . The i n c r e a s e d r a d i a t i o n r e s u l t e d f r o m t h e i n i t i a l f u e l rod f a i l u r e s (cladding b u r s t ) . The c o r e - t o - c o n t a i n m e n t t r a n s p o r t t i m e has b e e n e s t i m a t e d t o be f r o m one t o s e v e r a l minutes ( 5 ) . Thus, i n i t i a l f u e l f a i l u r e occurred s e v e r a l m i n u t e s p r i o r t o 142 m i n u t e s .

3.

A t 135 m i n u t e s , t h e u p p e r c o r e t e m p e r a t u r e s a r e e s t i m a t e d t o i n t h e r a n g e o f 810 t o 860 Κ b a s e d on d a t a f r o m t h e i n c o r e neutron detectors (6).


be

30

T H E T H R E E M I L E I S L A N D ACCIDENT

«:-.

2

rrpjf


Background sensor Thermocouple

Zircaloy Al 0 Inconel 3

H0 2

Inconel sheath

Instrument tube Instrument tube sleeve Spacer tube

Instrument lead ^to instrument panel

In-core Instrumentation Configuration

A Β

Ο

C

D

Ο

Ο

F

ο ο

ο

ο

Ο

Κ

ο

ο ο

Ο

Ο

ο

Ο

Ο

Ο

ο ο

ο ο

L

ο

ο

ο

ο

M

ο

ο

Ν

ο

Ο

ο

Ο

ο ο ο

0

Ο

Ο

ο ο

Ο ο

G

Ο

ο

Ο

Ε

H

Ο

Ο

ο

ο

ο

ρ

ο

R 1

2

3

4

5

6

7

ο 8 9 10 11 12 13 14 15

Τ Μ Ι In-Core Instrument String Locations Figure 2 . Incore instrumentation c o n f i g u r a t i o n and s p a t i a l d i s t r i b u t i o n o f instrument assemblies w i t h i n the core.



2.



31

4.

A t 150 m i n u t e s , the c o r e temperatures i n the upper o n e - t h i r d o f the c o r e a r e i n f e r r e d t o be i n the range o f 1700 t o 1800 Κ based on d a t a from the i n c o r e n e u t r o n d e t e c t o r s (6).

5.

D u r i n g the i n t e r v a l o f 150 t o 174 m i n u t e s , the e x - v e s s e l s o u r c e range m o n i t o r ( n e u t r o n d e t e c t o r ) s u g g e s t s s e v e r e damage t o t h e f u e l ( 6 ^ . I n a d d i t i o n , s e v e r e damage i s suggested by the i n c o r e n e u t r o n d e t e c t o r s l o c a t e d i n the upper t w o - t h i r d s o f the c o r e region.

To a i d i n i n t e r p r e t i n g t h i s s e t o f o b s e r v a t i o n s , the Severe Core Damage A c c i d e n t Package (SCDAP) computer code has been used t o e s t i m a t e the c o r e heatup and d e g r a d a t i o n ( 8 ) . The code was developed t o model the p h y s i c a l p r o c e s s e s c o n t r o l l i n g s e v e r e c o r e damage b e h a v i o r and has been v e r i f i e d a g a i n s t the a v a i l a b l e f u e l d e g r a d a t i o n e x p e r i m e n t a l d a t a base ( 9 ) . T a b l e I summarizes t h e model f e a t u r e s o f t h e SCDAP code. To e s t i m a t e bounding l i m i t s on the c o r e heatup, c a l c u l a t i o n s were performed f o r t h r e e assumed r e a c t o r v e s s e l c o o l a n t l e v e l h i s t o r i e s . The bounding c a s e s a r e : 1·

Nominal Core Uncovery Case. The r e a c t o r c o o l a n t l e v e l v s time was t a k e n from R e f e r e n c e 4. The nominal boundary c o n d i t i o n s i n d i c a t e the r e a c t o r v e s s e l c o o l a n t l e v e l t o be n e a r the t o p o f the c o r e a t a p p r o x i m a t e l y 110 m i n u t e s , 10 m i n u t e s a f t e r t h e p r i m a r y c o o l a n t pumps were shut o f f .

2.

E a r l y Core Uncovery Case. These boundary c o n d i t i o n s assumed t h e same r e a c t o r v e s s e l b o i l d o w n r a t e s as t h e n o m i n a l c a s e , e x c e p t the r e a c t o r v e s s e l l i q u i d l e v e l vs time i s s h i f t e d 13 minutes t o r e s u l t i n an e a r l i e r c o r e heatup. The e a r l y c o r e heatup r e s u l t s i n p r e d i c t e d c o r e t e m p e r a t u r e s more c o n s i s t e n t w i t h measured s u p e r h e a t i n the r e a c t o r v e s s e l o u t l e t p i p i n g by 113 m i n u t e s .

3.

E a r l y Core Uncovery W i t h R a p i d B o i l d o w n . T h i s case i s t h e same as the e a r l y uncovery case but w i t h no makeup c o o l a n t i n j e c t i o n i n t o t h e p r i m a r y c o o l i n g system d u r i n g the a c c i d e n t . T h i s c a s e r e p r e s e n t s the most r a p i d c o r e b o i l d o w n r a t e thought t o be possible.

F i g u r e 3 compares t h e e s t i m a t e d c o r e l i q u i d l e v e l v s time f o r each o f t h e s e t h r e e c a s e s . The p r e d i c t e d peak c o r e temperatures a r e compared t o the TMI d a t a i n F i g u r e 4. Three i m p o r t a n t t r e n d s from the SCDAP c a l c u l a t i o n s a r e n o t e d f r o m t h e r e s u l t s shown i n F i g u r e 4. 1.

The c o r e temperature h i s t o r y i s v e r y dependent on the assumed reactor vessel hydraulic conditions.

2.

The peak f u e l r o d temperature i n c r e a s e s r a p i d l y once c l a d d i n g temperatures o f 1500 t o 1700 Κ a r e r e a c h e d . T h i s i s a r e s u l t o f the e x o t h e r m i c c h e m i c a l r e a c t i o n between the steam and ζ i r c a l o y f u e l r o d c l a d d i n g a t these t e m p e r a t u r e s .


32

T H E T H R E E MILE ISLAND ACCIDENT Table

I.

Summary o f

SCDAP P h y s i c a l

Models

G e n e r a l code c a p a b i l i t i e s * Hydrodynamics TRAC-BD1/CHAN, q u a s i - e q u i l i b r i u m (1-D

only)

Radiation


TRAC-BD1/CHAN, g e n e r a l i z e d L o s s o f bundle

geometry

Liquefaction Ballooning Fragmentation F u e l rod behavior O x i d a t i o n ( P a w e l , U r b a n i c k i n e t i c s , Chung H2 ) N u c l e a r heat g e n e r a t i o n D e f o r m a t i o n ( a x i s y m m e t r i c and non-axisymmetric) F i s s i o n p r o d u c t release-PARAGRASS ( f u e l ) Gap r e l e a s e ( L o r e n z Model) L i q u e f a c t i o n and r e l o c a t i o n — Z r C ^ m e l t i n g , UO2 d i s s o l u t i o n , ZrÛ2 breach F r a g m e n t a t i o n — K a s s n e r and Chung e m b r i t t l e m e n t Control rod—Zr/SS/Ag-In-Cd Oxidation—Zr/SS N u c l e a r heat g e n e r a t i o n Heat c o n d u c t i o n L i q u e f a c t i o n and r e l o c a t i o n Shroud—Zr/SS/Other Oxidation—Zr N u c l e a r heat g e n e r a t i o n Heat c o n d u c t i o n L i q u e f a c t i o n and

relocation—Zr

Debris Oxidation N u c l e a r heat g e n e r a t i o n Heat c o n d u c t i o n F i s s i o n p r o d u c t r e l e a s e (PARAGRASS) Liquefaction



2.

100

Figure 3. uncovery»

4.

120

140

Estimated

core

40001

\

1

« 120

I 140

01 100 Figure

33



160 180 Time (min)

liquid levels

1

200

f o r three cases o f core

\

I I 160 180 Time (mln)

Γ

I 200

SCDAP p r e d i c t e d p e a k c o r e t e m p e r a t u r e s

core uncovery

I 220 f o r the three

cases.


34


3.

T H E T H R E E MILE ISLAND ACCIDENT

For a l l t h r e e c a s e s , the z i r c a l o y m e l t i n g temperatures a r e exceeded. Thus, i t i s e x p e c t e d t h a t s i g n i f i c a n t c l a d d i n g r e l o c a t i o n (downward f l o w ) would o c c u r d u r i n g t h i s p e r i o d . T h i s downward f l o w o f m o l t e n c l a d d i n g would be e x p e c t e d t o i n t e r a c t w i t h the c o o l a n t i n the lower r e g i o n s o f the c o r e c a u s i n g the z i r c a l o y t o f r e e z e thus b l o c k i n g the c o o l a n t f l o w p a t h s t h r o u g h the c o r e .

I n a d d i t i o n t o the c l a d m e l t i n g , the m o l t e n z i r c a l o y tends t o r e a c t w i t h the UO2, through o x i d a t i o n , p r i m a r i l y a l o n g the g r a i n boundaries. T h i s i n t e r a c t i o n d i s s o l v e s the UO2 i n the l i q u i d z i r c a l o y and a l l o w s g r a i n s o f UO2 t o be c a r r i e d away by the m o l t e n ζ i r c a l o y . T h i s p r o c e s s i s r e f e r r e d t o as f u e l l i q u e f a c t i o n . The e x t e n t o f f u e l l i q u e f a c t i o n i s c o n t r o l l e d p r i m a r i l y by the w e t t i n g b e h a v i o r o f the UC^/Zr m a t e r i a l s which i n t u r n i s a f f e c t e d by r o d t e m p e r a t u r e , d u r a t i o n o f the f u e l / z i r c a l o y c o n t a c t , and the geome t r y o f the deformed c l a d d i n g t u b e s . R e f e r e n c e 10 d i s c u s s e s the 1 i q u e f a c t i o n p r o c e s s and d a t a d e s c r i b i n g t h i s p r o c e s s i n more detail. Separate e f f e c t e x p e r i m e n t s p r o v i d e d a t a f o r u n d e r s t a n d i n g the importance o f the z i r c a l o y m e l t i n g and f u e l l i q u e f a c t i o n p r o c e s s e s . F i g u r e 5 shows the e n d - s t a t e c o n f i g u r a t i o n o f a n i n e rod c l u s t e r o f UO2 f u e l rod segments from an experiment w i t h a heatup t r a n s i e n t s i m i l a r t o the SCDAP p r e d i c t e d e a r l y uncovery case (JJ.). E x t e n s i v e r e l o c a t i o n o f the z i r c a l o y i s e v i d e n t which r e s u l t s i n n e a r l y com p l e t e b l o c k a g e o f the c o o l a n t f l o w c h a n n e l s at the bottom o f the f u e l assembly. A l s o , r e c e n t e x p e r i m e n t s conducted i n the Power B u r s t F a c i l i t y under s i m u l a t e d TMI c o r e heatup c o n d i t i o n s show e x t e n s i v e c l a d d i n g m e l t i n g and f u e l l i q u e f a c t i o n (12 ). Extensive f l o w b l o c k a g e i n the lower r e g i o n s o f the PBF f u e l assembly r e s u l t e d from the r e l o c a t e d m o l t e n m a t e r i a l . The SCDAP code models the f u e l l i q u e f a c t i o n , c l a d d i n g m e l t i n g , and r e l o c a t i o n p r o c e s s e s . The p r e d i c t e d zones o f h i g h l y o x i d i z e d but i n t a c t f u e l rods and the zones i n which m o l t e n and/or l i q u e f i e d f u e l a r e p r e d i c t e d are compared f o r the "nominal" and " e a r l y " u n c o v e r y cases i n F i g u r e 6. The upper o n e - t h i r d o f the core f o r both cases i s h i g h l y o x i d i z e d . M o l t e n z i r c a l o y and 1 i q u e f i e d f u e l a r e p r e d i c t e d i n the mid-core r e g i o n s and would have r e l o c a t e d ( f l o w e d ) downward, f r e e z i n g near the c o o l a n t i n t e r f a c e between the 1- t o 2 - f o o t l e v e l i n the c o r e . I t i s c l e a r from F i g u r e 6 t h a t the e x t e n t o f l i q u e f i e d m a t e r i a l i n the lower core r e g i o n s i s dependent on the r e a c t o r v e s s e l h y d r a u l i c s c e n a r i o ( b o i l d o w n r a t e ) . The d a t a t r e n d s o f the i n c o r e n e u t r o n d e t e c t o r s d u r i n g the 150 t o 174 minutes p e r i o d c o n f i r m t h a t the m e l t i n g / l i q u e f a c t i o n and downward r e l o c a t i o n o f the m o l t e n m a t e r i a l d i d o c c u r . The c e n t r a l l y l o c a t e d i n c o r e n e u t r o n d e t e c t o r s i n the upper t h i r d o f the c o r e showed v e r y h i g h anomalous o u t p u t c u r r e n t s a t about 150 m i n u t e s . B e s t - e s t i m a t e e n g i n e e r i n g i n t e r p r e t a t i o n o f the n e u t r o n d e t e c t o r d a t a suggests the p r o b a b l e e x p l a n a t i o n o f the anomalous, l a r g e p o s i t i v e c u r r e n t s i s m e l t i n g o f the i n s t r u m e n t s h e a t h . By 165 m i n u t e s , n e u t r o n d e t e c t o r s i n the mid-core r e g i o n a l s o showed the anomalous o u t p u t , s u g g e s t i n g severe core damage i n these regions. I f t h i s i n t e r p r e t a t i o n i s c o r r e c t , by 170 minutes c o r e t e m p e r a t u r e s above 1700 Κ were reached i n the c e n t r a l r e g i o n s o f the c o r e as low as 30 i n c h e s from the core bottom ( L e v e l 2 i n s t r u m e n t s ) .




35


2.

F i g u r e 5· E n d - s t a t e c o n d i t i o n o f f u e l r o d s f r o m r e c e n t G e r m a i r o d m e l t d o w n e x p e r i m e n t , ESBU-1. R e p r o d u c e d f r o m Ref. 5.


fuel

T H E T H R E E MILE I S L A N D ACCIDENT


Early Uncovery Case

100

80

SO

40

20 0 20 Core radius (%)

40

60

80 100

Nominal Uncovery Case

i

Cladding embrittlement boundary • Area where liquefied fuel and cladding solidified

F i g u r e 6 . SCDAP p r e d i c t e d z o n e s o f h i g h l y o x i d i z e d and l i q u e f i e d c l a d d i n g f o r T M I .

(embrittled)



2.



37

The measured r e a c t o r system p r e s s u r e i n c r e a s e from 8 0 0 t o 1000 p s i d u r i n g t h e p e r i o d o f 140 t o 174 minutes (see F i g u r e 7 ) . T h i s i n c r e a s i n g p r e s s u r e l e v e l i n d i c a t e s s e v e r e c o r e damage w i t h an accompanying l a r g e hydrogen p r o d u c t i o n from t h e z i r c a l o y o x i d a t i o n p r o c e s s and/or molten c o r e m a t e r i a l s r e l o c a t i n g downward and i n t e r a c t i n g w i t h t h e c o o l a n t i n t h e bottom o f t h e r e a c t o r c o r e . Without s e v e r e c o r e damage, t h e system p r e s s u r e would n o t be e x p e c t e d t o i n c r e a s e b u t r a t h e r would decrease as t h e r e a c t o r v e s s e l c o o l a n t l e v e l recedes i n t o t h e lower c o r e r e g i o n s , where t h e decay heat i s much l e s s . The a v a i l a b l e TMI o b s e r v a t i o n s , t h e b e s t - e s t i m a t e c o r e damage p r e d i c t i o n s , and s e p a r a t e e f f e c t s core d e g r a d a t i o n experiments a l l i n d i c a t e that the core experienced s i g n i f i c a n t m e l t i n g / l i q u e f a c t i o n and downward r e l o c a t i o n o f t h e molten core m a t e r i a l s p r i o r t o 174 m i n u t e s . As a r e s u l t , a molten zone o f ζircaloy and l i q u e f i e d f u e l i s e s t i m a t e d t o e x i s t a c r o s s n e a r l y 60 t o 80% o f the core r a d i u s t o a h e i g h t o f 0.5 t o 1.0 meters. The bottom o f t h e l i q u e f i e d zone was supported by a c r u s t o f f r o z e n , p r i o r - m o l t e n c o r e m a t e r i a l formed a t t h e 1 i q u i d i n t e r f a c e i n t h e bot torn o n e - f o u r t h o f t h e r e a c t o r c o r e . The upper r e g i o n s o f t h e c o r e s t i l l m a i n t a i n e d t h e o r i g i n a l r o d - l i k e geometry; however, t h e c e n t e r and p a r t o f the upper o n e - t h i r d o f the c l a d d i n g had m e l t e d and r e l o c a t e d downward l e a v i n g columns o f f u e l p e l l e t s . The c o n d i t i o n o f t h e fue1 rods w i t h i n the upper and mid-core r e g i o n s a r e expected t o be s i m i l a r t o those shown i n F i g u r e 5. Any i n t a c t z i r c a l o y c l a d d i n g i n t h e upper r e g i o n s o f t h e c o r e would be h i g h l y o x i d i z e d and v e r y b r i t t l e . F i g u r e 8 summarizes t h e h y p o t h e s i z e d c o r e c o n d i t i o n s j u s t p r i o r t o t h e Β pump' t r a n s i e n t which o c c u r r e d a t 174 m i n u t e s . 1

'B' Pump T r a n s i e n t and Subsequent Core Heatup (174 t o 227 M i n u t e s ) The p r o g r e s s i o n o f t h e a c c i d e n t was s i g n i f i c a n t l y a l t e r e d a t 174 m i n u t e s , when one o f t h e 'Β' l o o p p r i m a r y c o o l i n g pumps was s t a r t e d and remained r u n n i n g f o r a p p r o x i m a t e l y 15 m i n u t e s . The c o o l a n t d e l i v e r y t o t h e r e a c t o r v e s s e l and c o r e r e g i o n as a r e s u l t o f t h e pump t r a n s i e n t i s n o t known. O r i g i n a l e s t i m a t e s ( 4 ) i n d i c a t e as much as 1000 c u b i c f e e t may have been pumped i n t o t h e r e a c t o r v e s s e l . However, t h i s e s t i m a t e assumed an i n t a c t c o r e geometry and no f l o w b l o c k a g e . F o r h i g h l y degraded c o r e c o n d i t i o n s w i t h s i g n i f i c a n t c o r e f l o w b l o c k a g e , c o o l a n t f l o w through t h e c o r e would have been l i m i t e d and would n o t have been s u f f i c i e n t t o a r r e s t t h e heatup o f t h e c e n t r a l , m o l t e n c o r e r e g i o n . F i g u r e 7 shows t h e r e a c t o r system p r e s s u r e t o i n c r e a s e by s e v e r a l hundred p s i i n t h e f i r s t minute o r so a f t e r t h e pump was t u r n e d on. T h i s l i m i t e d p r e s s u r i z a t i o n suggests r e s t r i c t e d c o r e f l o w and heat t r a n s f e r which i s c o n s i s t e n t w i t h e x t e n s i v e c o r e f l o w b l o c k a g e and t h e i n a b i l i t y t o c o o l t h e h o t c o r e w i t h t h e l i m i t e d c o o l a n t f l o w . The c o o l a n t f l o w t o t h e steam g e n e r a t o r would a l s o have decreased t h e system p r e s sures . The r e l a t i v e importance o f these i n t e r a c t i o n s i s n o t p r e s e n t l y known and c a n o n l y be e s t i m a t e d by r e a c t o r systems c a l c u l a t i o n s t h a t model t h e complex t h e r m a l h y d r a u l i c i n t e r a c t i o n s between t h e degraded c o r e and t h e r e a c t o r system h y d r a u l i c s . The m e c h a n i c a l f o r c e s and f u e l r o d t h e r m a l s t r e s s e s g e n e r a t e d as a r e s u l t o f t h e r a p i d steam g e n e r a t i o n and r o d c o o l i n g would



T H E THREE MILE ISLAND ACCIDENT

i g u r e 8. H y p o t h e s i z e d TMI c o r e c o n f i g u r a t i o n j u s t p r i o r t o t h e B' pump t r a n s i e n t a t 174 m i n u t e s .


2.

TOLMAN ET A L .

39



have s h a t t e r e d the o x i d i z e d , e m b r i t t l e d f u e l rods i n the upper c o r e region. T h i s c o n c l u s i o n i s s u b s t a n t i a t e d by d a t a f r o m a r e c e n t 3 2 - r o d , s e v e r e - c o r e damage e x p e r i m e n t c o n d u c t e d i n t h e P o w e r B u r s t Facility. The d a t a i n d i c a t e t h a t n e a r l y c o m p l e t e s h a t t e r i n g and r e l o c a t i o n of the upper e m b r i t t l e d p o r t i o n s of the f u e l rods r e s u l t e d from r a p i d r e f l o o d i n g of the experiment (14). The h y p o t h e s i z e d c o r e c o n f i g u r a t i o n s h o r t l y a f t e r t h e ' B ' pump t r a n s i e n t i s s h o w n c o n c e p t u a l l y i n F i g u r e 9. The m o l t e n / l i q u e f i e d z o n e w o u l d s t i l l be i n n e a r l y t h e same l o c a t i o n a s b e f o r e t h e pump was i n i t i a t e d , s i n c e f l o w upward t h r o u g h t h e m o l t e n m a t e r i a l w o u l d be l i m i t e d b e c a u s e o f e x t e n s i v e c o r e f l o w b l o c k a g e p r i o r t o 174 m i n utes. The s h a t t e r e d f u e l f r o m t h e u p p e r c o r e r e g i o n formed a d e b r i s bed a p p r o x i m a t e l y a m e t e r deep on o r n e a r t h e t o p o f t h e m o l t e n / l i q u e f i e d zone. T h i s upper d e b r i s bed would tend t o i n s u l a t e the center region. Heat c o n d u c t i o n c a l c u l a t i o n s have been performed t o i n v e s t i g a t e t h e c o o l a b i l i t y o f a m o l t e n / l i q u e f i e d zone o f c o r e m a t e r i a l as h y p o t h e s i z e d i n F i g u r e 9. A s i m p l e s l a b g e o m e t r y m o d e l was u t i l i z e d a n d i n i t i a l s e n s i t i v i t y c a l c u l a t i o n s v a r y i n g t h e assumed s l a b c o m p o s i t i o n , t h i c k n e s s o f t h e l i q u e f i e d / m o l t e n z o n e , and s u r f a c e h e a t t r a n s f e r c o n f i r m t h a t t h e e s t i m a t e d TMI m e l t z o n e ( c e n t e r r e g i o n t o 80% r a d i u s and f r o m 2 0 - t o 3 0 - i n c h e s h i g h ) w o u l d n o t be c o o l a b l e , i . e . t h e s u r f a c e h e a t t r a n s f e r w o u l d n o t be s u f f i c i e n t t o m a i n t a i n the center region below the m e l t i n g temperatures. For t h i s nonc o o l a b l e c o n f i g u r a t i o n , the m o l t e n zone would c o n t i n u e t o heat and e v e n t u a l l y m e l t t h r o u g h t h e b o t t o m c r u s t and f l o w i n t o t h e l o w e r plenum r e g i o n s o f the r e a c t o r v e s s e l . T h e TMI i n c o r e n e u t r o n d e t e c t o r s s h o w e d n o f u r t h e r a n o m a l o u s b e h a v i o r d u r i n g t h e 174 t o 27Λ m i n u t e p e r i o d s u g g e s t i n g t h a t n o f u r ther melt progression occurred during this period. The i n c o r e t h e r m o c o u p l e s show a g e n e r a l c o o l i n g t r e n d i m m e d i a t e l y a f t e r t h e pump t r a n s i e n t f o l l o w e d b y a s l o w h e a t u p t r e n d s t a r t i n g a t a b o u t 190 m i n u t e s . D u r i n g t h i s p e r i o d , i n t e r m i t t e n t h i g h p r e s s u r e emergency c o r e c o o l i n g i n j e c t i o n was i n i t i a t e d , a n d t h e r e was a b r i e f p e r i o d w h e n t h e b l o c k v a l v e was o p e n e d . As a r e s u l t o f these a c t i o n s , e s t i mates o f the r e a c t o r v e s s e l l i q u i d l e v e l vs t i m e and t h e c o r e t e m p e r a t u r e r e s p o n s e i s n o t known and c a n o n l y be e s t i m a t e d w i t h m e c h a n i s t i c computer code c a l c u l a t i o n s as i n d i c a t e d e a r l i e r . M e l t P r o g r e s s i o n I n t o t h e Lower R e g i o n s o f t h e R e a c t o r V e s s e l and S u b s e q u e n t A t t a i n m e n t o f a C o o l a b l e G e o m e t r y (22 7 t o 3 0 0 M i n u t e s J A t 22 7 m i n u t e s , 53 m i n u t e s a f t e r t h e ' Β pump t r a n s i e n t , a g l o b a l change i n the c o r e c o n d i t i o n o c c u r r e d as i n d i c a t e d by the i n c o r e i n s t r u m e n t a t i o n , r e a c t o r s y s t e m p r e s s u r e and t e m p e r a t u r e s , s o u r c e r a n g e d e t e c t o r , and e x - v e s s e l r a d i a t i o n m o n i t o r s . Of p a r t i c u l a r i n t e r e s t are the incore neutron d e t e c t o r s i n the c e n t r a l lower core r e g i o n s which i n d i c a t e d anomalous b e h a v i o r ( i n f e r r e d core m e l t i n g ) f o r t h e f i r s t t i m e d u r i n g t h e a c c i d e n t a t 227 m i n u t e s . F o r many o f the instrument s t r i n g s ( p a r t i c u l a r l y near the core c e n t e r ) t h i s a n o m a l o u s b e h a v i o r was r e c o r d e d a t a l l a x i a l e l e v a t i o n s . In a d d i t i o n , the lowest incore neutron d e t e c t o r s i n d i c a t e d f o r the f i r s t time d u r i n g the a c c i d e n t l a r g e changes i n o u t p u t . A summary 1



40


o f t h e i n c o r e n e u t r o n d e t e c t o r s w h i c h i n d i c a t e d a s i g n i f i c a n t change i n o u t p u t i s shown i n F i g u r e 10· These d a t a suggest a s i g n i f i c a n t change i n c o r e c o n f i g u r a t i o n a n d damage s t a t e o c c u r r e d a t 22 7 m i n u t e s . It is hypothesized that a t t h i s t i m e t h e m e l t z o n e , as shown i n F i g u r e s 8 and 9, i s h e a t e d s u f f i c i e n t l y to melt and/or f a i l the bottom c r u s t or core support structure. The m o l t e n c o r e m a t e r i a l t h e n e i t h e r slumped o r f l o w e d i n t o t h e l o w e r c o r e and l o w e r plenum r e g i o n s . The d e t a i l s o f t h e m e l t p r o g r e s s i o n and c o o l a b i l i t y o f t h e m o l t e n c o r e m a t e r i a l a f t e r f a i l u r e o f the lower support s t r u c t u r e i s not known. The f l o w o f m o l t e n c o r e m a t e r i a l i n t o t h e l o w e r p l e n u m r e g i o n may h a v e b e e n i m p e d e d somewhat b y t h e l o w e r c o r e s u p p o r t structures and e l l i p t i c a l f l o w d i s t r i b u t o r p l a t e i n the lower plenum r e g i o n . However, i t can a l s o be h y p o t h e s i z e d t h a t t h e l a r g e s u p e r h e a t o f t h e m o l t e n m a t e r i a l would have a l l o w e d the m o l t e n m a t e r i a l t o f l o w r e l a t i v e l y unimpeded i n t o the l o w e r plenum r e g i o n s , s e t t l i n g r a p i d l y onto the reactor v e s s e l . Scoping c a l c u l a t i o n s i n d i c a t e that i f the c o r e m a t e r i a l w e r e t o r a p i d l y f l o w downward o n t o t h e r e a c t o r v e s s e l , melt-through o f the v e s s e l w a l l would occur w i t h i n s e v e r a l minutes (15). T h u s , t h e i n t a c t TMI r e a c t o r v e s s e l s u g g e s t s t h a t ( 1 ) t h e progression of the core melt occurred over a longer period of time; (2) the f u e l / c o o l a n t i n t e r a c t i o n i s i m p o r t a n t i n l i m i t i n g the down w a r d r e l o c a t i o n o f t h e m o l t e n c o r e m a t e r i a l ; and (3) t h e o u t e r s u r faces o f the r e a c t o r v e s s e l were c o o l e d s u f f i c i e n t l y t o prevent f a i l u r e of the v e s s e l . The l o w e r plenum v i d e o i n f o r m a t i o n i n d i c a t e s t h a t instantaneous r e l o c a t i o n of the molten m a t e r i a l onto the r e a c t o r v e s s e l d i d not o c c u r as s u g g e s t e d by: 1.

F r o z e n , p r i o r - m o l t e n m a t e r i a l on the upper s i d e o f the f l o w d i s t r i b u t o r p l a t e , i n d i c a t i n g t h a t the lower plenum s t r u c t u r e s were e f f e c t i v e i n slowing the melt p r o g r e s s i o n .

2.

A v a r i e t y of frozen m a t e r i a l structures ( s i z e s , shapes, m a t e r i a l ^ and t e x t u r e ) suggest t h a t d i f f e r e n t m a t e r i a l s t r a t e d the plenum r e g i o n s p o s s i b l y at d i f f e r e n t t i m e s .

pene

The t r e n d s from t h e i n c o r e thermocouples g i v e a d d i t i o n a l i n s i g h t i n t o t h e c o o l a b i l i t y o f the m o l t e n c o r e m a t e r i a l as i t p e n e t r a t e d the lower plenum r e g i o n s . The t h e r m o c o u p l e s w e r e m e l t e d d u r i n g t h e c o u r s e o f t h e a c c i d e n t a n d f o r m e d new j u n c t i o n s i n t h e lower, cooler regions of the reactor v e s s e l . Thermocouple measure m e n t s f r o m t h e r e l o c a t e d j u n c t i o n s made b e t w e e n 2 4 0 t o 330 m i n u t e s i n d i c a t e t e m p e r a t u r e s above 800 Κ (>1000 F ) i n t h e c e n t e r r e g i o n s o f t h e r e a c t o r v e s s e l as shown i n F i g u r e 1 1 . Resistance measure m e n t s made o n t h e t h e r m o c o u p l e l e a d s a f t e r t h e a c c i d e n t i n d i c a t e t h a t f o r most o f t h e c e n t r a l l y l o c a t e d t h e r m o c o u p l e s , t h e r e l o c a t e d j u n c t i o n s are located near the bottom of the r e a c t o r v e s s e l . These m e a s u r e m e n t s a r e c o n s i s t e n t w i t h t h e k n o w n s e v e r e c o r e damage a n d i n d i c a t e t h e l o w e r p l e n u m f u e l d e b r i s was a b o v e 8 0 0 Κ f o r t i m e s g r e a t e r t h a n a p p r o x i m a t e l y 2 h o u r s a f t e r t h e "22 7 m i n u t e " c o r e r e l o c a t i o n event.



2.

T O L M A N E T AL.

41


1

2

4

3

6

5

7

8

10 11 12 13

9

14 15

A 31 30

Β C

34

Ε F

•• I

36

G 37

9)

Κ L M Ν

51

33

D

H

52

29 28

32

38 39

•

2

14

41

15 47

43

M

/18 /V

:13

0

21 20

12

40

22

48

1

Ρ

Γ

R

•

45

46

Multiple level failures on same instrument string Level 1 and 2 only failures on same string Other alarms - (Levels 3-7)

F i g u r e 10· Summary o f i n c o r e n e u t r o n d e t e c t o r l o c a t i o n s i n d i c a t e d a c h a n g e i n c o r e c o n d i t i o n s a t 22 7 m i n u t e s .


which

42



1

2

3

4

5

6

7

8

9

F i g u r e 11. Temperature measurements r e c o r d e d a t 2 4 0 t o 330 m i n u t e s .

10

11

12

13

from incore

14

15

thermocouples


2.

TOLMAN ET

AL.


43

C o n c l u s i o n s and Summary


A h y p o t h e s i z e d TMI a c c i d e n t p r o g r e s s i o n s c e n a r i o has been d e s c r i b e d t h a t i n t e r p r e t s t h e TMI d a t a and i s c o n s i s t e n t w i t h o u r under s t a n d i n g o f s e v e r e c o r e damage phenomena g a i n e d from s e p a r a t e e f f e c t s e x p e r i m e n t s . A l t h o u g h many q u e s t i o n s remain, a more com p l e t e u n d e r s t a n d i n g o f t h e a c c i d e n t i s emerging. The proposed a c c i d e n t s c e n a r i o s e r v e s as a b a s i s f o r i n t e r p r e t i n g t h e r e s u l t s and c o n c l u s i o n s p r e s e n t e d i n papers t o f o l l o w i n t h e Symposium. The major t h e r m a l h y d r a u l i c f e a t u r e s t h a t c o n t r o l l e d t h e accident are: 1.

Severe c o r e m e l t i n g and/or f u e l l i q u e f a c t i o n o c c u r r e d p r i o r t o the ' Β ' pump t r a n s i e n t . The i n i t i a l molten m a t e r i a l ( z i r c a l o y c l a d d i n g and some f u e l ) formed a c r u s t i n t h e lower r e g i o n s o f the core o f s u f f i c i e n t t h i c k n e s s t o support t h e c o r e m a t e r i a l s as t h e c o r e m e l t p r o g r e s s e d . The m o l t e n c o r e r e g i o n s c o u l d have extended over 60 t o 80% o f t h e c o r e r a d i u s t o a h e i g h t o f 2 0 t o 30 cm. T h i s m a t e r i a l would n o t have been c o o l a b l e based on o u r i n i t i a l s c o p i n g s t u d i e s .

2.

The Β pump t r a n s i e n t caused e x t e n s i v e s h a t t e r i n g o f t h e b r i t t l e f u e l i n the upper core r e g i o n and r e s u l t e d i n a d e b r i s bed 50 t o 70 cm h i g h which f u r t h e r tended t o i n s u l a t e t h e c e n t e r m o l t e n r e g i o n . As a r e s u l t , t h e m o l t e n / l i q u e f i e d zone c o n t i n u e d t o heatup and e v e n t u a l l y m e l t e d through the lower s u p p o r t i n g c r u s t a t 227 minutes.

3.

P r o p a g a t i o n o f t h e molten c o r e m a t e r i a l c o n t i n u e d i n t o t h e lower plenum a f t e r 22 7 m i n u t e s . The i n t e r a c t i o n o f the m o l t e n m a t e r i a l w i t h t h e c o o l a n t i n t h e lower plenum r e g i o n s i s n o t c e r t a i n ; however, the m e l t p r o g r e s s i o n appears t o have been impeded by t h e lower plenum s t r u c t u r e s . F i n a l c o o l i n g times may have r e q u i r e d s e v e r a l h o u r s .

1

1

The TMI d a t a form an i m p o r t a n t b a s e l i n e f o r c o n f i r m i n g o u r under s t a n d i n g o f severe core damage phenomena, p a r t i c u l a r l y i n l i g h t o f m o l t e n m a t e r i a l c h a l l e n g e t o t h e r e a c t o r v e s s e l . The d a t a p r o v i d e the n e c e s s a r y benchmark f o r a s s e s s i n g t h e c a p a b i l i t y o f t h e a n a l y t i c a l models t o p r e d i c t t h e c o n t r o l l i n g c o r e d e g r a d a t i o n phenomena d u r i n g a severe a c c i d e n t i n a l a r g e r e a c t o r system environment. The TMI d a t a w i l l a l s o p r o v i d e the d a t a f o r e v a l u a t i n g t h e t y p i c a l i t y o f s m a l l e r s c a l e , severe a c c i d e n t e x p e r i m e n t s . A d d i t i o n a l work i s n e c e s s a r y t o c h a r a c t e r i z e t h e lower plenum, t h e c o r e r e g i o n s , and t h e r e a c t o r system f l o w paths t o p r o v i d e the needed d a t a f o r u n d e r s t a n d i n g t h e m e l t p r o g r e s s i o n i n t o t h e lower r e g i o n s o f t h e r e a c t o r v e s s e l and the f i s s i o n product b e h a v i o r d u r i n g t h e a c c i d e n t . T h i s d a t a w i l l p r o v i d e a r e a l i s t i c b a s i s f o r j u d g i n g o u r under s t a n d i n g o f s e v e r e a c c i d e n t s and t h e i r r e l a t i o n s h i p t o r e a c t o r safety.


44

T H E T H R E E MILE ISLAND ACCIDENT

Acknowledgments T h i s work was supported by t h e U.S. Department o f Energy A s s i s t a n t S e c r e t a r y f o r N u c l e a r Energy, O f f i c e o f Remedial A c t i o n and T e r m i n a l Waste D i s p o s a l , under DOE C o n t r a c t No. DE-AC07-76ID01570.

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3. 4. 5. 6. 7. 8. 9. 10.

Thomas, G., "Description of the TMI-2 Accident," These proceedings, May 1985. Vinjamuri, K. et al., Examination of H8 and B8 Leadscrews from Three Mile Island, Unit 2, EG&G Idaho, Inc. Report EGG-TMI-6685, October 1984. Akers, D., et al., Draft Report: TMI-2 Core Debris Grab Samples--Examination and Analysis, EGG-TMI-6853 (Part 1), July 1985. Ardron, Κ., Cain, D., "TMI Accident Core Heatup Analysis," NSAC-24, January 1981. NSAC-28, "Interpretation of TMI-2 Instrument Data," May 1982. Taylor, D., EG&G Idaho, Inc., personal communication. NSAC-80-1, "Analysis of Three Mile Island Accident," March 1980. Allison, C. et al., "SCDAP/MOD1 Analysis of the Progression of Core Damage During the TMI-2 Accident," EG&G Idaho, Inc. Report SE-CMD-84-006, July 1984. Allison, C. et al., "Draft Preliminary Report for Comment: SCDAP/MOD1/Theory and Models," EG&G Idaho, Inc. Report FIN A6360, January 1985. Hofmann, P., Kerwin-Peck, D., and Nikolopoulos, P., "Physical and Chemical Phenomena Associated with the Dissolution of Solid UO by Molten Zircaloy-4," Zirconium in the Nuclear Industry: Sixth International Symposium, ASTM STP 824, D. G. Franklin and R. B. Adamson, (eds.), American Society for Testing and Materials, 1984, pp. 810-834. Hagen, S., Peck, S., "Out-of-Pile Bundle Temperature Escala tion Under Severe Fuel Damage Conditions," KfK-3568, August 1983. McCardell, R. et al., "Severe Fuel Damage Test Series: Severe Fuel Damage Scoping Test Quick Look Report," EG&G Idaho, Inc., January 1984. Rogovin, Μ., "Three Mile Island--A Report to the Commis sioners and to the Public," Nuclear Regulatory Commission Special Inquiry Group Report. McCardell, R. et al., "Severe Fuel Damage Test 1-1 Quick Look Report," EG&G Idaho, Inc., October 1983. IDCOR Technical Summary Report, "Nuclear Power Plant Response to Severe Accident, November 1984. 2

11. 12. 13. 14. 15.

RECEIVED

October 1, 1985


Thermal Hydraulic Features of the Accident - ACS Symposium Series

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