THE BORON ISSUE AND CALCULATIONS FOR OECD/NEA/CSNI PKL PROJECT DIPARTIMENTO DI INGEGNERIA MECCANICA, NUCLEARE E DELLA PRODUZIONE UNIVERSITA' DI PISA 56100 PISA -ITALY THE BORON ISSUE AND CALCULATIONS FOR PWR AND VVER-1000 F. D’Auria, G.M. Galassi, W. Giannotti, D. Araneo, M. Cherubini OECD/NEA/CSNI PKL PROJECT PKL Analytical Workshop – University of Pisa, Pisa (I), Oct 11-12, 2005

CONTENT FRAMEWORK & OBJECTIVES NOMENCLATURE & BACKGROUND 2/36 FRAMEWORK & OBJECTIVES NOMENCLATURE & BACKGROUND THE ADOPTED INPUT DECKS AND THE CODE RUNS THE NC SCENARIO THE SBLOCA SCENARIO CONCLUSIONS 02/65

FRAMEWORK & OBJECTIVES 2/36 FRAMEWORK • The PKL SETH 2004/2005 • The investigation of boron dilution in PWR-NPP OBJECTIVES ° To identify & characterize NPP (state) conditions causing diluted boron in loop seal ° To investigate the mechanisms of transport of de-borated plugs into the core ° To evaluate the reactivity effect due to de-borated water in the core Definitely, ° To constitute a database of results to support the design of PKL experiments 04/65

NOMENCLATURE & BACKGROUND 2/36 BORON DILUTION The process where a local decrease in boron concentration in PS occurs with (nearly) constant average boron concentration DE-BORATION The process where a net loss of boron from PS occurs that implies a decrease in average boron concentration Consequently, the following definitions can be introduced: BORATION (the opposite of de-boration) BORON CONCENTRATION RECOVERY (the opposite of boron dilution) DE-BORATION RATE (the rate of de-boration) DILUTION RATE (the rate of boron dilution) 05/65

NOMENCLATURE & BACKGROUND 2/36 BORON TRANSPORT The movement of diluted boron plugs in the primary loop (this occurs at a system level and is affected by ‘integral’ phenomena like NC, SG-heat transfer: a system code is needed for the investigation) BORON MIXING The process when volumes of fluid with different boron concentration, temperature and void fraction come into contact (this occurs at a local level: a CFD code is needed for the investigation) Boron Dilution, De-boration, Mixing & Transport occur simultaneously in case of selected Boron Relevant Accidents (BRA) 06/65

NOMENCLATURE & BACKGROUND 2/36 THE STUDY HAS BEEN CONDUCTED BY A QUALIFIED SYS-TH CODE The phenomena BORON DILUTION, DE-BORATION AND BORON TRANSPORT are calculated, together with the effect of DILUTED VOLUME OF WATER upon NK reactivity. The main assumptions are FULL MIXING in the LP of the RPV and use of POINT NK in the core (including the ‘boron-feedback’). 07/65

NOMENCLATURE & BACKGROUND 2/36 THE QUALIFICATION LEVEL THE ADOPTED CODE IS QUALIFIED FOR THE SELECTED TH TRANSIENTS. THE ADOPTED NPP NODALISATIONS (INCLUDING BIC) ARE QUALIFIED (LICENSING & SAFETY APPLICATIONS, ETC.). THE ADOPTED CODE-NODALISATIONS ARE QUALIFIED FOR BORON DILUTION, DE-BORATION AND BORON TRANSPORT (COMPARISON WITH AVAILABLE PKL TESTS). THE CODE-USERS ARE (PRESUMED TO BE) QUALIFIED. THE ‘FULL MIXING’ IN LP AND CORE (& PARTIAL MIXING IN DC) CONSTITUTES AN ‘UNQUALIFIED’ SIMPLIFYING ASSUMPTION. THE 0-D NK FEEDBACK IS AFFECTED BY THE SELECTED REACTIVITY FEEDBACK. THEREFORE, “WEAK POINTS” FOR THE ANALYSIS ARE THE RPV MIXING AND THE NK FEEDBACK. 08/65

THE PWR INPUT DECK 2/36 09/65

THE PWR INPUT DECK 2/36 10/65

THE PWR INPUT DECK 2/36 11/65

THE VVER INPUT DECK 2/36 11/65

THE CODE RUNS SELECTED ASSUMPTIONS NC 2/36 SELECTED ASSUMPTIONS NC Starting from NPP nominal operation, MCP trip and scram are imposed that bring the PS to NC conditions at decay power level value with controlled SG level and SG pressure (by SRV opening). ‘Steady-state’ NC steps at different PS mass inventories are calculated, to find the relationship between boron dilution rate and PS mass inventory: step-wise mass draining of PS. Each draining steps lasts 100 s, afterwards 500-1000 s NC flow-rate stabilisation period is calculated. No actuation of ECCS. Calculation end (and stop of mass draining) when dry-out is calculated. SBLOCA The SG cooling is available assuming: % FW flow at the same value as % core power, constant (at the nominal conditions) FW temperature, availability of SRV, or alternatively, of the 100 K/hr procedure. DBA or BDBA assumptions for ECCS (identified in the table of code runs). Calculation end following re-criticality occurrence. 17/65

‘Initial’ &‘Fine-UT’ nodalisations: PS & SG pressure THE PWR NC SCENARIO 2/36 ‘Initial’ &‘Fine-UT’ nodalisations: PS & SG pressure 23/65

‘Initial’ &‘Fine-UT’ nodalisations: PS mass inventory THE PWR NC SCENARIO 2/36 ‘Initial’ &‘Fine-UT’ nodalisations: PS mass inventory 24/65

‘Initial’ &‘Fine-UT’ nodalisations: Core inlet flow-rate THE PWR NC SCENARIO 2/36 ‘Initial’ &‘Fine-UT’ nodalisations: Core inlet flow-rate 25/65

‘Initial’ &‘Fine-UT’ nodalisations: HL liquid velocity THE PWR NC SCENARIO 2/36 ‘Initial’ &‘Fine-UT’ nodalisations: HL liquid velocity Reflux Condensation start 27/65

‘Initial’ &‘Fine-UT’ nodalisations: HL steam velocity THE PWR NC SCENARIO 2/36 ‘Initial’ &‘Fine-UT’ nodalisations: HL steam velocity 28/65

‘Initial’ &‘Fine-UT’ nodalisations: Boron in LS THE PWR NC SCENARIO 2/36 ‘Initial’ &‘Fine-UT’ nodalisations: Boron in LS LS ALWAYS FULL OF LIQUID 29/65

‘Initial’ &‘Fine-UT’ nodalisations: Boron dilution rate THE PWR NC SCENARIO 2/36 ‘Initial’ &‘Fine-UT’ nodalisations: Boron dilution rate Quantity affected by time span at each PS MI Boron dilution rate (R5 units) Boron dilution rate > 0 at high PS MI values PS Mass Inventory (-) 31/65

THE NC SCENARIO ‘Fine-UT’ nodalisation: The sensitivity study – run061 2/36 ‘Fine-UT’ nodalisation: The sensitivity study – run061 Step 3. Stop of PS mass draining 33/65

THE NC SCENARIO ‘Fine-UT’ nodalisation: The sensitivity study – run061 2/36 ‘Fine-UT’ nodalisation: The sensitivity study – run061 Stop of PS mass draining Step 3. NC restart at constant PS MI (removal of diluted boron plug) 34/65

‘Fine-UT’ nodalisation: The sensitivity study – runs 064-066 THE NC SCENARIO 2/36 ‘Fine-UT’ nodalisation: The sensitivity study – runs 064-066 SG SS pressure Step 3. P 37/65

‘Fine-UT’ nodalisation: The sensitivity study – runs 064-066 THE NC SCENARIO 2/36 ‘Fine-UT’ nodalisation: The sensitivity study – runs 064-066 SG SS pressure Step 3. 38/65

‘Fine-UT’ nodalisation: The sensitivity study – runs 064-066 THE NC SCENARIO 2/36 ‘Fine-UT’ nodalisation: The sensitivity study – runs 064-066 SG SS pressure Step 3. 39/65

THE PKL III NC EXPERIMENT 2/36 THE BORON DILUTION AND TRANSPORT THE BORON DILUTION RATE VS PS MI MEASURED IN PKL EXPERIMENT AND CALCULATED (BY RELAP5) IN NPP CONDITIONS PKL DATA 2005 NPP CALCULATION 2004 24/32

THE SBLOCA BORON SCENARIO 2/36 FOUR LOOP PWR: SEARCH FOR THE WORST RPV-LP BREAK RANGE FOR INVESTIGATION Mode-1: instantaneous time value 20-40 CM2 BREAK IN LP WITHIN THE CRITICAL RANGE 46/32

THE SBLOCA BORON SCENARIO 2/36 FOUR LOOP PWR: SEARCH FOR THE WORST RPV-LP & CL BREAKS CL break RANGES FOR INVESTIGATION Mode-2: average time value LP break LP break 20-40 CM2 BREAK IN LP WITHIN THE CRITICAL RANGE, HOWEVER…. CL break 46/32

THE SBLOCA BORON SCENARIO 2/36 FOUR LOOP PWR: SEARCH FOR THE WORST RPV-LP & CL BREAKS WIDER RANGE FOR BREAK AREAS CL break ADDITIONAL RANGES FOR INVESTIGATION Mode-2: average time value LP break LP break 20-40 CM2 BREAK IN LP WITHIN THE CRITICAL RANGE, HOWEVER…. CL break 46/32

‘Three Fine-UT’ nodalisation: Relevant BIC THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Relevant BIC The results documented in Appendices 10 to 18. Relevant BIC are: Break opening at time 0 s, (break area and position* varied in sensitivity studies) HPIS available (No. of HPIS trains varied in sensitivity studies) ACC available 100 K/hr Boron concentration in ECC storage tanks equal to 1000 ppm (varied in sensitivity studies between 1000 and 4000) CR worth following scram: 8 $ (conservatively low) ** NK feedback values (moderator, Doppler and boron): BE values  * LP break not possible in Framatome-ANP [formerly Siemens-KWU] NPP is considered in W DBA analyses ** 20 $ scram rod worth prevents re-criticality in code run 3mnb8 (see below) Step 5. 44/65

‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13 THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13 The results documented in Appendices 10 to 18. 30 cm2 break in LP and 2 HPIS Main remarks HPIS is adequate in keeping cooled the core. LS boron dilution (almost “complete”) occurs in LS. Power excursions are due to break deboration, boron diluted liquid from LS and low boron content of ECCS.   Step 5. 45/65

‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13 THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13 Step 5. PS 46/65

‘Three Fine-UT’ nodalisation: Code run 3mnb8 THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Code run 3mnb8 Run 3mna8 (same as 3mnb8 & W/0 HPIS) CHF due to lack of cooling 47/65

‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13 THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13 Step 5. Loops w HPIS Loops w/o HPIS 48/65

‘Three Fine-UT’ nodalisation: Code run 3mnb8 THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Code run 3mnb8 Nominal power Boron diluted plug in core NC restart CHF 49/65

‘Three Fine-UT’ nodalisation: Code run 3mnb8 THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Code run 3mnb8 NC restart 50/65

‘Three Fine-UT’ nodalisation: Code run 3mnb8 THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Code run 3mnb8 51/65

‘Three Fine-UT’ nodalisation: Code run 3mnb8 THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Code run 3mnb8 52/65

THE SBLOCA SCENARIO ‘Three Fine-UT’ nodalisation: Sensitivity studies 2/36 ‘Three Fine-UT’ nodalisation: Sensitivity studies Influence of break position upon “break de-boration integral” Step 5. Break in LP Break in CL 56/65

‘Three Fine-UT’ nodalisation: Sensitivity studies THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Sensitivity studies Influence of the presence of HPIS upon “LS boron dilution” (loop w/o HPIS) Step 5. 57/65

‘Three Fine-UT’ nodalisation: Sensitivity studies THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Sensitivity studies Influence of the presence of HPIS upon “LS boron dilution” (loop with HPIS) Step 5. 58/65

‘Three Fine-UT’ nodalisation: Sensitivity studies THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Sensitivity studies Influence of the boron concentration in the ECCS tanks upon the power excursion caused by boron dilution Step 5. 59/65

‘Three Fine-UT’ nodalisation: Sensitivity studies THE SBLOCA SCENARIO 2/36 ‘Three Fine-UT’ nodalisation: Sensitivity studies Core power excursion caused by boron dilution calculated by three different codes: Relap5/mod3.2-gamma, Relap5/mod3.3-beta and Relap5/3D © 2.2.4. Step 5. 60/65

THE SBLOCA SCENARIO ‘Three Fine-UT’ nodalisation: Sensitivity studies 2/36 ‘Three Fine-UT’ nodalisation: Sensitivity studies 50 cm2 break in CL – 3 HPIS, 8 ACC, full LPIS 1 deborated ACC Step 5. Effect of LS boron dilution Effect of ACC deborated liquid 61/65

Main findings from the NC study CONCLUSIONS p. 1 of 4 2/36 Main findings from the NC study The boron dilution process in LS starts at PS MI around 70% Reflux condensation enhances the boron dilution. The (complete) boron dilution of the LS liquid, about 3 m3 per each LS, occurs in a time span <several 100 s> - <1 hr> depending on the flow regimes into the hot leg (PS MI and pressure have a role). Restart of NC may occur at constant PS MI (i.e. minor changes of SG power and liquid accumulation in CL may trigger the event). However, ECC flow-rate is the main reason for NC restart (more effectively when ECCS is injected in CL compared with HL). Pressure influence is negligible upon the TH scenario. However, boron transport is affected (to a larger extent than the boron dilution process in LS). The influence of nodalisation (three input decks used) is negligible (locally relevant). However, asymmetries in the boron dilution process are calculated when the fine nodalisation (3-UT) is used. The influence of time step and even typing frequency has been noted. The results must be seen considering un-identified code limitations. 62/65

Main findings from the SBLOCA study CONCLUSIONS p. 2 of 4 2/36 Main findings from the SBLOCA study    Break area and position are relevant in the SBLOCA-BRA. The phenomenon ‘break-de-boration’ has been characterized Boron concentration in the ECCS tanks and line affects the scenario. However, following the creation of boron diluted slugs, re-criticality occurs with boron concentration in ECCS lines and tanks up to 4000 (break in LP). A boron diluted plug in the ECC lines or in one ACC may cause re-criticality (or, worse, unrecoverable core power excursion – part of residual risk). 63/65

CONCLUSIONS p. 3 of 4 OPEN ISSUES INPUT DATA AND CODE QUALIFICATION 2/36 OPEN ISSUES INPUT DATA AND CODE QUALIFICATION Sensitive BIC are: CR worth, initial boron content of ECCS tanks, modelling of ECCS lines (including possible boron diluted plugs), NK parameters. Code-nodalisation qualification proofs available but not fully documented NEED FOR COUPLED 3D NK –TH CALCULATIONS. Boron XSEC sets must be derived and qualified USE OF CFD CODES (MOSTLY FOR BORON MIXING IN RPV) The full mixing in LP does not avoid the re-criticality occurrence. Realistic mixing in CL, DC, LP and core is required. 64/65

HOW TO ADDRESS THE BORON ISSUE CONCLUSIONS p. 4 of 4 2/36 HOW TO ADDRESS THE BORON ISSUE ECCS lines connected with the LS (boron dilution timing and ECCS reliability are relevant). Discharge of boron diluted LS liquid. Valves installed in the bottom of LS could be opened/closed. Actuation of the Boron Injection System (JDH). MCP restart. Proper AMP might be designed (negative implications expected). Avoiding loop seal. New plants (e.g. AP-1000) are designed accordingly (well known solution). Increasing the scram rod worth (well known solution and negative implications expected). Using burnable poisons. New plants (e.g. AP-1000) are designed accordingly (well known solution). 65/65