EnBG .
Energiemanagement für KMU
Risk of Uncontrollable Re-Criticality
due to a Steam Generator Tube Rupture
NURIS 2015, 1st International Conference On Nuclear Risk, Viena 17. April 2015
Dipl.-Ing. Helmut J. L. Mayer, EnBG Energiebüro Gorxheimertal
Photo: Südhessen Morgen
17. April 2015, NURIS
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Content
EnBG .
Energiemanagement für KMU
Purpose:
Publish the risk of non-borated water inside the reactor core
1. Steam generator tube rupture (SGTR)
according to the emergency operation procedures
 probability of occurrence
 procedure
2. SGTR and shutdown of main cooling pumps (MCP)
 probability of occurrence
 procedure
3. Scientific investigations and main results
(BMUB, GRS, RSK, AST, ISR, ATHLET-analyses, ROCOM-tests)
4. Summary and conclusions
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Probability of SGTR
EnBG .
Energiemanagement für KMU
Probability of SGTR
 3,000 to 16,000 tubes in one steam generator (SG),
2 to 4 SGs in one pressurized water reactor (PWR)
 End of 2012: worldwide 366 PWRs in operation
(Reference: ATW Kernenergie-Weltreport-2012)
 Very high stresses and strains,
high pressure, temperature, transients,
bending load and so on …
 Significant tube thinning
(Reference: RSK)
 Stress corrosion cracking
Wikipedia
(Reference: RSK)
 Multiple ruptures have to be assumed
(Reference: RSK)
 Highest probability of all loss of coolant accidents
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SGTR according to emergency operating procedures (example)
EnBG .
Energiemanagement für KMU
Damaged SG
Intact
SGs
Quelle: GRS, Internet
Procedure
1.
2.
3.
4.
Emergency plant shut down
Secondary pressure rises to 72 bar
Primary pressure is sprayed down to 80 bar
The damaged SG will be locked by the valves
at the secondary side
7.
8.
9.
With the intact SGs the plant is cooled down by
the steam bypass station
Long-time heat discharge by the heat discharge
system
The damaged tube may be repaired in the
depressurized primary system
17. April 2015, NURIS
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SGTR according to emergency procedures (example)
EnBG .
Energiemanagement für KMU
Damaged SG
Intact
SGs
Quelle: GRS, Internet
Thermodynamic conditions after spray process (example)

Secondary side of all SG: 72 bar, correlating temperature of saturated steam at 288 ° C

Primary side: 80 bar (sat. steam-temp. 295 ° C), average coolant temperature at 290 ° C

Supercooling of the MCP in normal operation: 99 bar (155 bar – 56 bar correlating to 271°C)

Supercooling of the MCP during SGTR: merely 7 bar
(80 bar – 73 bar correlating to 289°C)
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Probability of shutdown of main coolant pumps
EnBG .
Energiemanagement für KMU
Loss of external power sources:
Regular emergency case, specified in emergency procedures
Possible causes in the primary side:

Undershoot of the spray process

Different possible measuring tolerances (e. g. foaming of the SG-water secondary side)

Delayed closing or stuck opening of spray valve(s) (Three Miles Island Accident)

Leakage of spray valve (corrosion products, dissolving during the very rare spray process)

Stiffness of seldom used spray valves
Possible causes in the secondary side:

Overshoot of the non-safety-designed steam bypass station

Inaccurate measurements, possibly in opposite directions between the primary and secondary
side
Possible causes due to human errors:

Several imprecise manual operations,
e. g. when tracking the primary pressure on the secondary pressure
 There are a lot of reasons in this extraordinary situation
(conclusion from many years of operating experience)
Failure of only one device,
no multiple failures and no unusual human errors assumed
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SGTR followed by a shutdown of main coolant pumps
EnBG .
Energiemanagement für KMU
Damaged SG
Intact
SGs
Non-borated Water
Quelle: GRS, Internet
Non-borated relatively cold water,
may flow through the defective tube and
may enter the primary loop as well as the reactor core
PWR:
For sufficient subcriticality it is absolutely essential to have borated coolant in the core!
What happens to the criticality of the reactor core?
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History and scientific investigations and tests until today
EnBG .
Energiemanagement für KMU
Nov. 2012 Recognition of the seriousness by my novel “Der Störfall”
Feb. 2013 Information provided to the „Reaktor-Sicherheitskommission“(RSK)
May 2013 Forwarded to the „Bundesministerium für Umwelt, Naturschutz, Bau und
Reaktorsicherheit“(BMUB)
Apr. 2013 BMUB commissioned the Gesellschaft für Reaktorsicherheit (GRS)
Apr. 2013 GRS is investigating the incident including ATHLET-Analysis and
ROCOM-Tests des Helmholtz-Zentrum Dresden-Rossendorf (HZDR);
( support by the author/initiator was refused by BMU and GRS!)
Jan. 2014 Supported by the “Büro für Atomsicherheit” (Prof. Renneberg) and
the “Institute for Safety and Risk Research” (ISR, Report of 23. Jan. 2014)
May 2014 Presentation and discussion with the „RSK-Ausschuss Anlagen- und
Systemtechnik“ (RSK-AST, minutes of the 97. and the 103. meeting)
May 2014 RSK-Discussion (minutes of the 446. and the 471. meeting)
Feb. 2015 The author/initiator was informed by BMUB with a letter.
Main results:
There are still different views.
RSK-AST advises a comprehensive clarification.
BMUB issued an order to the RSK.
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Main contradictions
EnBG .
Energiemanagement für KMU
Number of damaged tubes
RSK assumes that ruptures of
more than one tube will occur
(RSK-Report 447. Meeting)
 only the case of one rupture has been investigated
Critical boron concentration
(Control rods inserted! At hot conditions!)
 RSK assumes that a “reactor core will go critical again at
600 ppm” (RSK-report 446. meeting, during hot conditions!)
 850 ppm are required by BMU (RSK report 446. meeting)
 GRS calculated a plug of 8.000 kg with less than
100 ppm (GRS-report of 16. Sept. 2013)
What happens to the criticality of the reactor core at 100 ppm?
17. April 2015, NURIS
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Individual assessment
EnBG .
Energiemanagement für KMU
My personal assessment:
 Contradictions between RSK requirements and the GRS analysis
 Predominant theoretical and formalistic approach
 A lot of different sequences are possible, but
only 1 case considered,
only 1 nuclear power plant,
only 1 plant technology,
only 1 emergency operation procedure,
only 1 course of actions
………
There is too much inconsistency to my practical experience.
There is no comprehensive solution.
17. April 2015, NURIS
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Summary and conclusions
EnBG .
Energiemanagement für KMU
I´m afraid, but it´s not just fantasy, it could come to reality
 High probability x severe impact = very high risk
 More than two years extensive research and discussions by
GRS, RSK, AST, ISR, BMUB, …..
 Identified and initiated by the author
 but cooperation with the author/initiator was not accepted,
he did not get a chance to be involved
There is an urgent need for external specialists
to work in the field of nuclear safety!
And it is extremely important
that independent experts get involved
and organize events such as NURIS!
Thanks a lot for NURIS - and thanks for your attention!
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