1. 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

2. CONTENT FRAMEWORK & OBJECTIVES NOMENCLATURE & BACKGROUND
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FRAMEWORK & OBJECTIVES
NOMENCLATURE & BACKGROUND
THE ADOPTED INPUT DECKS AND THE CODE RUNS
THE NC SCENARIO
THE SBLOCA SCENARIO
CONCLUSIONS
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3. FRAMEWORK & OBJECTIVES
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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
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4. NOMENCLATURE & BACKGROUND
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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)
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5. NOMENCLATURE & BACKGROUND
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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)
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6. NOMENCLATURE & BACKGROUND
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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’).
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7. NOMENCLATURE & BACKGROUND
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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.
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8. THE PWR INPUT DECK
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9. THE PWR INPUT DECK
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10. THE PWR INPUT DECK
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11. THE VVER INPUT DECK
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12. THE CODE RUNS SELECTED ASSUMPTIONS NC
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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 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.
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13. ‘Initial’ &‘Fine-UT’ nodalisations: PS & SG pressure
THE PWR NC SCENARIO
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‘Initial’ &‘Fine-UT’ nodalisations: PS & SG pressure
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14. ‘Initial’ &‘Fine-UT’ nodalisations: PS mass inventory
THE PWR NC SCENARIO
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‘Initial’ &‘Fine-UT’ nodalisations: PS mass inventory
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15. ‘Initial’ &‘Fine-UT’ nodalisations: Core inlet flow-rate
THE PWR NC SCENARIO
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‘Initial’ &‘Fine-UT’ nodalisations: Core inlet flow-rate
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16. ‘Initial’ &‘Fine-UT’ nodalisations: HL liquid velocity
THE PWR NC SCENARIO
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‘Initial’ &‘Fine-UT’ nodalisations: HL liquid velocity
Reflux
Condensation
start
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17. ‘Initial’ &‘Fine-UT’ nodalisations: HL steam velocity
THE PWR NC SCENARIO
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‘Initial’ &‘Fine-UT’ nodalisations: HL steam velocity
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18. ‘Initial’ &‘Fine-UT’ nodalisations: Boron in LS
THE PWR NC SCENARIO
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‘Initial’ &‘Fine-UT’ nodalisations: Boron in LS
LS ALWAYS
FULL OF LIQUID
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19. ‘Initial’ &‘Fine-UT’ nodalisations: Boron dilution rate
THE PWR NC SCENARIO
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‘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 (-)
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20. THE NC SCENARIO ‘Fine-UT’ nodalisation: The sensitivity study – run061
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‘Fine-UT’ nodalisation: The sensitivity study – run061
Step 3.
Stop of PS mass draining
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21. THE NC SCENARIO ‘Fine-UT’ nodalisation: The sensitivity study – run061
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‘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)
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22. ‘Fine-UT’ nodalisation: The sensitivity study – runs 064-066
THE NC SCENARIO
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‘Fine-UT’ nodalisation: The sensitivity study – runs
SG SS pressure
Step 3. P
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23. ‘Fine-UT’ nodalisation: The sensitivity study – runs 064-066
THE NC SCENARIO
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‘Fine-UT’ nodalisation: The sensitivity study – runs
SG SS pressure
Step 3.
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24. ‘Fine-UT’ nodalisation: The sensitivity study – runs 064-066
THE NC SCENARIO
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‘Fine-UT’ nodalisation: The sensitivity study – runs
SG SS pressure
Step 3.
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25. THE PKL III NC EXPERIMENT
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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
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26. THE SBLOCA BORON SCENARIO
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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
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27. THE SBLOCA BORON SCENARIO
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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
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28. THE SBLOCA BORON SCENARIO
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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
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29. ‘Three Fine-UT’ nodalisation: Relevant BIC
THE SBLOCA SCENARIO
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‘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.
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30. ‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13
THE SBLOCA SCENARIO
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‘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.
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31. ‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13
THE SBLOCA SCENARIO
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‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13
Step 5. PS
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32. ‘Three Fine-UT’ nodalisation: Code run 3mnb8
THE SBLOCA SCENARIO
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‘Three Fine-UT’ nodalisation: Code run 3mnb8
Run 3mna8
(same as 3mnb8 &
W/0 HPIS)
CHF due
to lack
of cooling
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33. ‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13
THE SBLOCA SCENARIO
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‘Three Fine-UT’ nodalisation: Code run 3mnb8 – App. 13
Step 5.
Loops w
HPIS
Loops w/o
HPIS
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34. ‘Three Fine-UT’ nodalisation: Code run 3mnb8
THE SBLOCA SCENARIO
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‘Three Fine-UT’ nodalisation: Code run 3mnb8
Nominal power
Boron
diluted plug
in core NC
restart
CHF
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35. ‘Three Fine-UT’ nodalisation: Code run 3mnb8
THE SBLOCA SCENARIO
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‘Three Fine-UT’ nodalisation: Code run 3mnb8 NC
restart
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36. ‘Three Fine-UT’ nodalisation: Code run 3mnb8
THE SBLOCA SCENARIO
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‘Three Fine-UT’ nodalisation: Code run 3mnb8
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37. ‘Three Fine-UT’ nodalisation: Code run 3mnb8
THE SBLOCA SCENARIO
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‘Three Fine-UT’ nodalisation: Code run 3mnb8
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38. THE SBLOCA SCENARIO ‘Three Fine-UT’ nodalisation: Sensitivity studies
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‘Three Fine-UT’ nodalisation: Sensitivity studies
Influence of break position upon “break de-boration integral”
Step 5.
Break in LP
Break in CL
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39. ‘Three Fine-UT’ nodalisation: Sensitivity studies
THE SBLOCA SCENARIO
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‘Three Fine-UT’ nodalisation: Sensitivity studies
Influence of the presence of HPIS upon “LS boron dilution” (loop w/o HPIS)
Step 5.
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40. ‘Three Fine-UT’ nodalisation: Sensitivity studies
THE SBLOCA SCENARIO
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‘Three Fine-UT’ nodalisation: Sensitivity studies
Influence of the presence of HPIS upon “LS boron dilution” (loop with HPIS)
Step 5.
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41. ‘Three Fine-UT’ nodalisation: Sensitivity studies
THE SBLOCA SCENARIO
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‘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.
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42. ‘Three Fine-UT’ nodalisation: Sensitivity studies
THE SBLOCA SCENARIO
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‘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 ©
Step 5.
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43. THE SBLOCA SCENARIO ‘Three Fine-UT’ nodalisation: Sensitivity studies
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‘Three Fine-UT’ nodalisation: Sensitivity studies
50 cm2 break in CL – 3 HPIS, 8 ACC, full LPIS deborated ACC
Step 5.
Effect of
LS boron dilution
Effect of ACC
deborated liquid
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44. Main findings from the NC study
CONCLUSIONS p. 1 of 4
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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.
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45. Main findings from the SBLOCA study
CONCLUSIONS p. 2 of 4
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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).
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46. CONCLUSIONS p. 3 of 4 OPEN ISSUES INPUT DATA AND CODE QUALIFICATION
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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.
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47. HOW TO ADDRESS THE BORON ISSUE
CONCLUSIONS p. 4 of 4
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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).
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