End of simulation: 1300 sec.
No flow reversal was observed.
It keeps nominal direction.
It keeps nominal direction.
Neither stagnation nor flow reversal were observed.
It keeps nominal direction.
Idem Group 2.
No flow reversal was observed.
It keeps nominal direction.
Idem Group 2.
In all the channels the prompt effect is a great temperature decrease.
The final value is always less than 140°C.
ON
TABLE II: EFFECTS OF EMERGENCY COOLANT INJECTION
Tinio injection: 1007 me.
« Coolant flow At injection time:
•) It stagnant in Groups 1, ?., 5 find in the non-critical pass.
b) Is reverted in Groups 3 and -1.
In ovory case:
») Emergency coolant injection onuses a rnuitltorf flow oscfllfttfon.
b) Prior to the injicrion sheath temperatures begin to increase became of tho low flows.
c) The prompt effect of the injection is a great sheath temperature decrease.
Variable
Emergency coolant to all the headers
End of simulation: 187.0 nee.
A mninontnry flow nrrenn) wa« ohiorved during (Its oscillation period. Ttton it wm seen a lonjf «t»tn«r)on, but finally How in reestablished in nominal direction.
Similnr behaviour as Group 1 win observed.
Motnentnry t(«gn«(lom were observed during the oscilation period, aflor which nnmlnitl direction flow it reeatahltalied. However, it decreases itnd finally nrvrrsei definitively.
Similnr behaviour us Group 1 was observed.
A very tecentixted onclllntion WAD observed with successive stagnations and flow irveritli, After which nominal direction flow is rcmtablijhed.
Emorf^nc^' coolnnt mjection reestAblishes nominal dlrM^ion flow.
The low flow cauies an important tmnsitory increaio. The finnl vahio is lens thnn 1WC.
Similnr behaviour ns Group 1 . The fintl vnlue ii ltj« thnn 150°C.
A trnnsitory inr.n:nnc was observed here nlso, but not so important. The final value ii leai than 130°C.
Similnr behaviour tit Group 3, although the peak is more pronounced. The final value is less thnn 130°C.
The better circulntion condhionB prevent the temperature penk. The finnl v»hio it less than 130°C.
A sliglit trnnsitory incrense was obncrved. Tho final valiw is less than 150°C.
Emergency coolant to Inlet headers
End of simulation: 1820 soc,
Neither rcvonnl nor stapiation were observed during the oicfflition period. Kondatl direction Dow is rec;tabli«hod.
Neithor reversnl nor stapiation were observed during the otcfflttion period Th«tt there is a tntnsitory flnn'rcvm*!, but finaHy nomraal direction flow is reestablished.
After the oscillation period nnmlnnl direction flow is reestablished.
Similar behaviour as Group 1.
Simitar behaviour as Group 1.
Similar behaviour as Group 1.
The pronounced peak wa.i not observed. The final value Is loll thin 130°C.
A peak wns observed, although not as pronounced as when the emergency cootafl enten to all the headers. The final value is less than 130*C.
Non important incrsas: wns observed. The final vahie Is lost than 120°C., Idem Group 3.
Idem Group 3.
Idem Group 3.
4. CONCLUSIONS
The refill performance of a high -pressure accumulator tank emergency coolant injection system under natural circulation conditions was evaluated. The scenario consisted of a 55 kg/s reactor inlet header break coincident with a loss of electrical power supply.
The emergency core cooling system action was initiated manually, supplying injection to all the headers and to inlet headers only.
This was performed at 630 seconds and at 1000 seconds after the header break.
The results showed a better performance of the mentioned system whenever the injection is directed to inlet headers only.
The code accounted for some experimental tests carried out at RD-14M loop:
• core cooling may be satisfactory even with reversed flows.
• emergency coolant injected to inlet headers reestablishes nominal direction flows.
Although the results for the later injection showed a fuel temperature decrease, it is recommended not to delay the initiation of the emergency coolant injection beyond 600 seconds because it was seen that void would reach the critical pass inlet header, denoting an insufficient heat removal by the boilers
REFERENCES
[1] BEDROSSIAN, G. - GERSBERG, S. - Simulacidn de una rotura pequena en colector de entrada del sistema primario de transporte de color de la Central Nuclear de Embalse con perdida de summistro electrico normal - Comisi6n Nacional de Energia At6mica,
1025 (1993).
[2] ATOMIC ENERGY OF CANADA LIMITED - Small loss-of-cookmt accident and emergency core cooling operation - 59 SDM-7 Second Edition (1982).
[3] ATOMIC ENERGY OF CANADA LIMITED - 600 MWe CANDU-PHW, Central Nuclear en Embalse, C6rdoba, for Comisi6n Nacional de Energia At6mica - Safety Report (1985)
[4] CALABRESE, R. - GERSBERG, S. - Sistema de calculo de accidentes por perdida de refrigerante con acoplamiento neutrdnico-termohidrdulico para reactores tipo CANDU.
Informe preliminar - Comisibn Nacional de Energia Atdmica 1046 (1992) [5] LIN, M R. et al. - FIREBIRD III Program Description - AECL 7542 (1984)
A RISK-BASED MARGINS APPROACH FOR PASSIVE SYSTEM PERFORMANCE RELIABILITY ANALYSIS N.S. SALTOS, A. EL-BASSIONI
US Nuclear Regulatory Commission, Washington
C.P. TZANOS
Argonne National Laboratory USA
Abstract
Passive safety systems (e.g., those used in Westinghouse's AP600 and General Electric's SBWR designs) rely on natural forces, such as gravity, to perform their functions. Such forces are relatively small compared to pumped system driving forces, their magnitude varies from one scenario to another and they are subject to large uncertainties. These uncertainties affect the passive system thermal-hydraulic (T-H) performance reliability (for short, T-H reliability). For this reason, the T-H unreliability of the passive systems must be assessed
and accounted for in the Probabilistic Risk Assessment (PRA). The quantification of T-H
unreliability involves a prohibitively large number of computations. This paper presents a conservative risk-based "margins" approach which eliminates the need to quantify T-H unreliability for most, if not all, accident sequences. With this approach the issue of T-H unreliability is addressed by linking it to the passive safety system success criteria and, thus, to core damage frequency (CDF). This is achieved by assuring that the adopted success criteria for safety systems and operator actions are conservative enough so that the contribution to a sequence CDF from T-H uncertainties is significantly smaller than thecontribution from hardware failures and human errors.
INTRODUCTION
Current advanced "passive" reactor designs, such as Westinghouse's AP600 and General Electric's SBWR designs, have a number of unique features that distinguish them from both operating and advanced evolutionary light water reactor designs. Although, they use both
active and "passive" systems for accident prevention and mitigation, only the "passive"
systems are classified as safety related by the vendor. The word "passive" is in quotation
marks because these systems are not purely passive. Active components, such as motor operated valves and check valves, are used for system actuation. The mission of the active systems is to provide a first line of defense and reduce the challenge rate for the safety-related "passive" systems.The passive safety systems in these designs rely on natural forces, such as gravity and stored energy, to perform their accident prevention and mitigation functions once actuated and started. These driving forces are not generated by external power sources (e.g., pumped systems), as is the case in operating and evolutionary reactor designs. Because the magnitude of the natural forces which drive the operation of passive systems is relatively small, counter-forces (e.g., friction) can be of comparable magnitude and cannot be ignored as is usually done with pumped systems. This requires more accuracy in estimating "net"*«
79
driving forces. Moreover, there are considerable "knowledge-based" uncertainties associated with factors on which the magnitude of these forces and counter-forces depends (e.g., values of heat transfer coefficients and pressure losses). In addition, the magnitude of such
"natural" driving forces depends on the specific plant conditions and configurations which could be existing at the time a system is called upon to perform its safety function. For these reasons, the reliability of the thermal-hydraulic (T-H) performance of the "passive"