SOURCE TERM ON NPP SAFETY ANALYSES Marino Mazzini Professore Ordinario nel s.s.d. Impianti Nucleari Università di Pisa Facoltà di Ingegneria Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione

DOCUMENTAZIONE PER LA LEZIONE SUL SOURCE TERM 1.Slides (M. Mazzini) 2.Note sul problema del Source Term (F. Fineschi) 3.G. Petrangeli: Nuclear Safety Elsevier Pub. 2006, reperibile anche su CD NUCLEAR ENERGY E-Book Collection

SUMMARY Miles-Stones in Source Term EvaluationMiles-Stones in Source Term Evaluation Inventory of Fission Products in the CoreInventory of Fission Products in the Core Calculation of the FP Release to GapCalculation of the FP Release to Gap Calculation of the FP Release in Case of Severe AccidentsCalculation of the FP Release in Case of Severe Accidents Comparison of Source Term Evaluation in the Rasmussen Report and more Recent PSA AchievementsComparison of Source Term Evaluation in the Rasmussen Report and more Recent PSA Achievements ConclusionsConclusions

History of source term assessment and relationship to regulatory process

Nominal Source Term Assumed in the Years 60.ies and 70.ies for the Regulatory Process Release of: 100 % of Noble Gases 50 % of Iodine, of which 1/2 deposites on Containment Internal Structures. The Remaining 25 % is 91% as I 2, 4% as Organic Forms (CH 3 I and Others), 5% Particulate Matter 10 % of Cs and Other Volatile Metals 1% of Other Fission Products Assumptions of Regulatory Guides 1.3 and 1.4 for Evaluating Accomplishment of 10CFR Part 100 Dose Limits

BLOCK SCHEME OF FP SOURCE TERM AND DOSE EVALUATION

FISSION PRODUCT INVENTORY Parameters determining FP inventory: Fission Power (1 W = fissions/s) and Irradiation time or Burn-up Fission yield Possible formation due to precursor decay Decay constant Neutron capture

FISSION PRODUCT INVENTORY Beattie Relationship: a = 8.4 y P(1-e -t/ ) (1) Where: a = Activity of the considered radionuclide (kCi) y = Fission yield (%) P = Thermal power (MW t ) t= Irradiation time (day) = Time constant of the radionuclide (day) When t>> relationship (1) becomes: a = 8.4 y P while, if t<< : a=8.4 y P t/ = 8.4 y Bu/ Bu= Burn-up (MW.day)

U 235 Fission Yield versus Mass Number

U 233 and Pu 239 Fission Yield versus Mass Number

Nuclidi, tempo di dimezzamento (giorni) e attività (MCi) per reattore da 3000 MW t (1/2)

Nuclidi, tempo di dimezzamento (giorni) e attività (MCi) per reattore da 3000 MW t (2/2)

FISSION PRODUCT RELEASES More dangerous isotopes (due to inventory, volatility and radio-tossicity): Alogena (in particular 131 I) Noble gases (isothopes of Kr and Xe) Alkali metals and similes ( 137 Cs, 134 Cs, 132 Te,…) Alkali-earth metals ( 89 Sr, 90 Sr, 140 Ba) Metals with volatile oxides ( 103 Ru, 106 Ru, 99 Tc,….)

FP RELEASE TO GAP The Main Mechanisms are: FP Nuclei Recoil (T<1000 °C) Diffusion (1000< T <1600 °C) Grain Equi-axial Growing (1600< T <1800 °C) Grain Columnar Growing (1800< T <2850 °C) Diffusion in Liquid Metal (T>2850 °C)

FP RELEASE TO GAP The Total Release is the Sum of the Releases from the Various Zones (Volumes) at Different Temperature Interval, Determined by the Conductivity Integral: Where W = Linear Power T S = Fuel Surface T T c = Central Fuel T

FP RELEASE TO GAP

Where: F = Total Release Fraction Lewis Method T r =1000°C, T d =1600°C, T ex =1800°C, f r = 0,001, f d = 0,1, f ex = 0,3, f col = 0,95

Comparison of methods for calculation of fission-gas release from UO 2

Rilascio dei gas di fissione nel gap e plenum degli elementi di combustibile (ORNL TM 2347)

FP RELEASE FROM FUEL IN SA: Simple Model

FP RELEASE FROM FUEL IN SA: Smoothed Curves

FISSION PRODUCT RELEASE FROM FUEL Rate Constants

Keys to PWR Accident Sequence Symbol (1/2)

Keys to PWR Accident Sequence Symbol (2/2)

Core Behaviour Calculated by MARCH for the Sequence AB

Calculation of Fission Product Release from Fuel for the Sequence AB

Radioactivity Releases Calculated for the Sequence AB

Core Behaviour Calculated by MARCH for the Sequence S 2 C

Calculation of Fission Product Release from Fuel for the Sequence S 2 C

Radioactivity Releases Calculated for the Sequence S 2 C

Comparison between Calculated Release Fractions This Calculation WASH 1400 RSS

CONCLUSIONS OF NUREG-0772 (1981) (1/3) 1.Fission Product Release from Fuel –Chemical Form of Iodine in Gap for T>950°CsI 2.Chemistry of Cs and I –Main Forms I I 2, CsI, HI at HighTemperature Cs CsOH, CsI, Cs Reducing Atmosphere at Low Temperature Oxidant Atmosphere Reducing Atmosphere

CONCLUSIONS OF NUREG-0772 (1981) (2/3) 3.Fission Product Transport through the Primary System: –Ritention = 0 =0-50% –Ritention Depending from Accident Sequence High for Sequences TMLB or TC 20% for Sequence AD Gas and Volatile Metals If Core Melt is not complete

CONCLUSIONS OF NUREG-0772 (1981) (3/3) 4.Fission Product Transport through the Containment: for 50% Core Melt and –Delay in ECCS Intervention –Containment Integrity – E.S.F. in Operation the Dose Attenuation Factor of Containment is > for All F. P.

Sequenze con rilasci esterni più significativi in PSA recenti (1/3)

Sequenze con rilasci esterni più significativi in PSA recenti (2/3) SEQUENZA TE SE V

Sequenze con rilasci esterni più significativi in PSA recenti (3/3) immediati SEQUENZA TE SE V

Accident Source Terms for LWR NPP US-NRC NUREG 1465 (1992) Intercapedine Rilasci rapidi nel "vessel" Rilasci fuori dal "vessel" Rilasci tardivi nel "vessel" Durata Ore0,51,3210 Gas Nobili0,050,950,00 Iodio0,050,350,290,07 Cesio0,050,250,390,06 Tellurio0,000,150,290,025 Stronzio0,000,030,120,00 Bario0,000,040,100,00 Rutenio0,000,0080,0040,00 Lantanio0,000,0020,0150,00

Accident Source Terms for LWR NPP US-NRC NUREG 1465 (1992) Per un BWR i rilasci sono lievemente diversi. La nuova proposta deriva dalla considerazione delle sequenze studiante e vuol rappresentare una media dei casi significativi. I rilasci dovuti ad interazione del nocciolo fuso con il calcestruzzo (rilasci tardivi fuori dal vessel) sono quelli derivanti dallipotesi di assenza di acqua al di sopra dello strato fuso: se interessa il caso in cui sia presente uno strato di acqua, allora il rilascio sarà minore a causa delleffetto di abbattimento dellacqua.

FP Releases at Various Accidents Involving Fuel Damage Chernobyl Unit 4Water5 x 10 7 > 5 x 10 9 > 5 % Inventory Reactivity 26/4/1986 excursion

History of source term assessment and relationship to regulatory process

BMI-2104 suite of codes as used in the source term reassessment

Radionuclides Contributions to Risks

Conclusioni sul tema del Source Term Quasi tutti gli impianti nucleari di potenza attualmente in esercizio sono stati licenziati sulla base di un Source Term nominale, relativamente semplice. Dopo il WASH 1400 è stato possibile valutare il Source Term in modo più realistico, dimostrando un livello di sicurezza migliore dei LWR. Oggi codici di calcolo come MELCOR ed ASTEC consentono valutazioni integrate e realistiche del Source Term, e sono validati a fronte di dati sperimentali.

Visitate il sito YouNuclear