ML20078K993
| ML20078K993 | |
| Person / Time | |
|---|---|
| Site: | Farley  |
| Issue date: | 09/06/1991 |
| From: | Lutz R WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML20078K997 | List: |
| References | |
| NUDOCS 9411280291 | |
| Download: ML20078K993 (7) | |
Text
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.O CORE DAMAGE DEFINITION / JUSTIFICATION FOR IPE ANALYSES September 6,1991 Pmpared by:
R. J. Lutz, Jr.
e Westinghouse Electric g.
Strategic Operations and Risk Assesenent O
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CORE DAMAGE DEFINTTION / JUSTIFICATION INTRODUCTION
. For the IPE project the definition of core damage for the FWRs has been Mnad based on the peak car temperature during the transient caum! by the accident progression. De fmemal dermition for successful core cooling (i.e., no core dunage) is: " Core cooling is rmdu! if the core temperatures do
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not exceed 1200' Fahrenheit for s prolonged period of time", as predicted by realistic computer codes.
This document provides 3 dets.ned tecbnical basis for this definitico.
De definition of successful core cooling is required in order to bin accident seq-es from the IPE program into those which do not contribute to risk and those which could have a catnbution to risk.
Thus, the definition of core damage becomes a key criterion in binning accident sequences for an IPE program; the definition of core damage must provide a realistic assearment d the potential of an accident sequence to contribute to overall risk from an Eddaa' in IPE accident sequenca. evaluations, for those acquences which do not result in cae damage (according to the Mntrian chosen), the possible fission product releases from the plant under any conceivable subsequent plant condition must be small enough that they will not contribute to the overall accident risk profile of the plant. In accident management considerations, the possible accident management alternatives may be different for accident sequences whh and without core damage since the fission product and hydrogen inventaries in various plant locaticas may be considerably different.
Previous Probabilistic Risk Analyses have conservatively used core uncovery as a basis for Mala; core damage. This definition assures that there will be no releases of fission products ime the fuel rods to the reactor coolant system as a result of the accident, since there is always adequate cooling of the core by intimate contact with water. The rmimum possible release of fission products frosn the plant for any accident would therefore be limited to the inventory of fission products in the reactor coolant system prior to the accident; additional fission psoduct release to the reactor coolant systesn as a result of "iodme spiking" is also included in these considerations). De Reactor Safety Stwty (WASH.1400) concluded that fission product releases from the plant for sequences in which there was no core damage would not contribute to the severs accident risk profile of the plant. That is, those accidents which can result in the release of the entire fission product inventories normally fcnmd in the reactor coolant during power operation will have a negligible contribution to risk. Subsequent studies have shown that ateam cooling of the upper portions of the cme (after core uncovery) can be effective in delaying or preventmg cae damage. Thus, core uncovery is not equrvalent to core damage; cae uncovery is a very conservative approximation of core damage.
1[the IPE program, an emphasis has been placed on derming a realistic, best estimate accident O
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ne following paragraphs document the a=chale=1 basis for the definfrion of core cooling.
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EMERGENCY OPERATING PROCEDURE BASIS De Emergency Operating Procedures (EOPs) for Westinghouse PWRs de8ine Inadequate Core Cooling as a core exit thermocouple (T/C) readmg of greater than 1200' Fahrenheit. If the core sait h~=>ple indication (s) exceed 1200* Fahrenheir, the EOPs transfer the guidance to a set of Pimetion Restoration Procedures, FR-C.1: Response to Inadequate Core Cooling". At the tkne when the T/O indacate 120Cr Fahrenheit, the actual peak fuel rod cladding temperatures may be as high as 1500' Fahrenhch, daaaadiag on the design of the T/Cs and their location relative to the fuel rods. His difference can be attributed to the difference in steam and fuel rod cladding tesoperatures. De core exit TSs measure local fluid temperatures at the top of the core; there can be as much as a 300'F between the core exit steam tempera:ure and the cladding temperatures durmg core uncovery.
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FUEL ROD BEHAVIOR BASIS nere are five temperature ranges for fuel rods that may be dermed when looking at core coolability and core integrity, as described below.
Q A)
At a core temperatures up to 1400' Fahrenheit, analyses and evaluations of the behrvior of the core indicate that there are no changes to the.b
.! integrity (metallurgical or marhantemi) of the fuel rods in the core. Dus, the ability to cool the core by providing water would not be compromised and there would be no additional releases of fission products from the fuel rods to the reactor coolant system.
B)
At core temperatures between about 1400 and 1600' Fahrenbah, the zirconhan fuel rod ewia!
begins to lose some of its structural capability. Experiments 6ndicaw that the cladding becanes very plastic in this range (described as the consistency of chewing gum). At these tesnperatures, ballooning of the fuel rod cladding and bowing of the fuel veds may occur. De arperiments and subsequent analytical models indicate that the capability to cool the core under these conditions (clad ballooning and rod bowing) is not compromised for any accident sequences. Dese same analyses indicate that some local breaches (bunting) of the cladding integrity may occur in this temperature range due to pressure differences across the fuel rod claddag. De breach of the cladding integrity would permit the release of those fission products which are accumulated in the imemal fuel rod gu space (between the fuel rod pellet stack and the fuel sod cladding) to be released to the reactor coolant system. His release component, which is sennad " gap release",
represents less than 1% of the cut inventory of noble gases, nodines and calums in the fuel rod; no other fission products would be released from the fuel rods in significant quantities.
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C)
At core temperatures between about 1600 and 200Cr Fahrenheit, the zirconian fuel continues to lose its structural capability and additional local breaches in the fuel rod O
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'rier over about 160(r Fahrenheit, ddfusion of volatile fission products Ace te ceramic fuel pelle matrix to the cladding gap would become significant. *nds tesnperature sahanced diffusion phenomena would increase the release of those fission products &ce te $mst rods to'the reactor coolant system, throgh any breaches in the fuel rod cladding which exist (or are created increased temperatures). De contribution frosa semperature =hanemt diffusion depends on time at which the fuel rod temperature is held above 160lY Fahrenheit; a remnanahle appr=I==
indicates that the compcnent may represent apprmi==*1y 1 to 3% of the core inventory of no e
gases, and volatile fission products, such as lodine and ceshan. De non volatile fission products are not impacted by this release mWalem D)
At core temperatures between about 2000 and 400Cr Fahrenheit, the zirconium fbel rod cladd:
undergoes an exothermic reaction with any steam in the reactar vessel (Le, zirc-water reactions De zirc-water reactions are very temperanne depsamt and incsesse %-==Many with temperature. Extensive zire water reactions will cause the fuel rod cladding be transformed to zircon]um dioxide. Experimental evidence shows that the zirconium dioxide et=Mia; willlose its structural capability via cracking, resulting in wide spread breaches in the fuel rod *1=Mi' The initiation of zirc-water reactions wiU begin to compromise the capability to cool the core, O
rrs==riir a= io ia aaitiae 6 i whica==>i 6 4-A i o. i== in ciia 5-exothermic, the reaction heat will cause lower temperature pertions of the core to beat at a rate greater than that which can be attributed to decay heating. His resulu in a very rapid temperature rise which limits the time frame over which recovery of core cooling can be successful Analyses and evaluation indicate that core cooling is still possible during this time frame. In this temperature range, significant gaseous and volatile fission products would be released from the fuel rods to the reactor coolant rystem as a resuk of temperature =haned diffusion and the additional loss of cladding integrity. A reasonable approximation indicates that this paaaat may represent up to 50% of the core inventory of noble gases, and volatDe fission products, such as iodine and cesium. As described above, the non volatile fissico products are not impacted by this release mechanism, even in this temperature range.
E)
At core temperatures e=A 4000* Fabienheit, the zirconium fuel rod cladding and the uranium dioxide fuel pellets form a eutectic, melt and begin to relocate downward in the core (freezing en cooler surfaces toward the bottom of the core). As relocation occurs, cooling chanaela are blocked and the capability to cool the core becomes more difficult However, analyscs and expsiase indicate that core relocation does not prevent core cooling in this time period; the time required for complete care cooling increases significantly and further relocation of core mataial is still possible after core cooling is re-established (e.g. TM12 experience). De primary ~ reason for this O~
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phenomena is that convective beat removal from the core may no longer be the Amninant heat
" Q transfer Mant conductive beat transfer through substantial masses of relocated core material may dominate. At these core temperatures, all of the gaseous and volatste fission products would I
be released from the fuel rods to the reacta coolant systeun as a result of cess uselting (the fission
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products held in the ceramic fuel matrix would be free to leave the ibel peDets at melting). Dus, 100% of the core inventory of gaseous and volatile fission products would be released to the reactor coolant system at this time. Additlocal analyses and evaluations indicate that the non-volatile fission products are not impacted by this rel:ase new=. even in this tesnperature range.
BASIC CONSIDERATIONS Based on :he behavior of the fuel rods during temperature transients resulting frosn accident conditions, it is reasonable to choose a value below 1400' Fahrenheit to represent the peak fuel rod tesnperature (as predicted by realistic, best estimate analyses) for the dermition of core damage. A choice of 1200'is justified, based on the following considerations:
1)
At a fuel rod temperature below 1200' Fahrenheit there is no mechanIn for addnional fission product releases from the fuel rods to the reacter coolant system. Dus, the maximum release from the plant for any accident sequence is limited to the pro accident Q
reactor coolant system inventory of fission products (plus "iodme spiking").
2)
At a fuel rod temperature below 1200' Fahrenheit, there is no mehani-fcr changing the coolability of the fuel rods in the core. Dus, the ccse can always be Erwed by simply providing sufficient water to remove the core decay beat.
3)
Dere is no " ruff" at a point just beyond the fuel rod temperature of 1200' Fahrenheit where the accident situation changes drastically.
4)
At a fuel rod temperature below 12M Fahrenheit, the plant operators are in the " normal" accident procedures and are not taking extraordmary steps to re establish core cooling (see consideration #6 for additional inforinatinn).
5)
There is some margin (approximately 200' F) between 1200* F and the temperature at which the onset of any core damage (in terms of fuel rod Mine integrity and fission product releases) begins. His margin provides assurance that the success criteris is not dependent on the accident sequence modelhng (IA, cv-code models and assumptions for ccre heat transfer).
6)
Dere is some numerical consistency between the Minition of core damage and the EOPs,
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although the temperature is referenced to differnt locations, ne 1200'F value used for defining core damage refen to the peak fuel rod predicted by realistic, best estimate analyses; the 1200FF value used in the EOPs for danaiag " Inadequate Core Cooling" refen to steam temperatures at the exh of the core, as measured by the core exh thermocouples. Dere can be a 200 to 300' F difference in these temperatures for a given accident scenario.
j ADDITIONAL CONSIDERATIONS If core temperatures (as predicted by realistic analyses) exceed 1200* Fahre 6eh for a short period of time (but do not exceed 2000' Fahrenheit), the possible fission product seleast. to the astor coolant system are limited to a very small fraction of the total core inventory. MAAP 5.0B analyses indicate that if the j
maximum core temperature r==Im less than 2000* Fahrenheit and does acd exceed 1600' Fahrenheit for longer than 30 minutes, less than 1% of the total cae fuel rods will experience a temperature in excess of 1200' Fahrenheit. Assuming that the additional fission product release to the reactor coc.lant systaan due to failure of the fuel rod cladding occurs in all fuel rods over 1400' Fahrenheit, the release from the plant from an accident can be found. Table 1 shows the releases from the plant for various types of accident sequences. De results displayed in Table 1 indicate that the p-eta 1 fission poduct releases from an accident sequence in which the fuel rod temperature exceeds 1400' Fahrenheit are two orders d magnitude less than a similar accident scenario in which the entire core is severely damaged (T/C > 4000*
Fahrenheit). Previous PRA studies (Lt., WASH 1400) have shown that accident sequences which result O
in fission product releases on the order of 1/1,000,000 of the core inventory are not contributors to severe accident risks, snespective of their frequency of occurrence. Dus, a core temperature greater than 1400' Fahrenheit for a short period of time (i.e. less than 30 minutes), but not erdag 2000' Fahrenheit, would result in a negligible impact on the plant risk profile. However,if the core temp.s e were sustained at these levels for periods longer than about 3G minutes, the fuel rod e1=Miag failures and subsequent releases of fission products to the RCS would increase; this increase may be =Igatara* in terms ofimpact i
on the plant severe accident risk profile, for some accident sequences (e.g., n=talammat bypass sequences and impaired containment sequences).
nerefore, an acceptable situation in tenns of successful core cooling can am nmmiate fuel rod temperatures in excess of 1200' Fahrenbeh for a shortperiod of time, which can be defined as about 30 minutes.
OVERALL CONCLUSIONS ne success criteria for core cooling can be stated as: " core fuel rod maxhnum temperatures which do not exeged 1200" Fahrenheit for a prolonged period of time, where a prokeged period of time is defined as greater than 30 minutes".
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FISSION PRODUCT RET.F ASES FOR VARIOUS ACCIDENTS ACCIDENT DESCRIFTION FISSION FRODUCT REIXACE j
(5 erCareinumeery)
N.G.
I/ Cs Other TS < 1200' F; 100% RCS P ata==ad to 1B4 154
< 1 B.9 Atmospbere; No lodine Spike TS < 1200' F; 100% RCS Released to 2 54 1B6
< 1 E-9" i
Atmosphere;1odine Spike TS < 2000' F; 100% RCS #*1*==ad to; 1B4 1B4
<1B4 Containment C-nla=*ar Intaa '
TS < 2000' F; 100% RCS Released to; 1E4 1 54 1B4 Cootainment: Containment Bypassed
- TS > 4000* F; 100% RCS Released to 1B4 1B4 1B4 Containment: No Comamment Intact '
Notes: 1.
A - 15 st3 ease of Ip. to RCS fran 15 of deel rods; 99% :eeemian in RCS; 995 O-retection in n=tainm***
2.
Assunes 1% release of Ip. to RCS imm 1% of dee! sods; 995 sesentico la RCS 3.
Assumes 100% ac!rese of Lp. to RCS imm 100% of fuel rods; :=* in and comamment as modeBad by MAAP 3.0B O
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