Original scientific paper
RELAP5/MOD3.3 Analyses of Core Heatup Prevention Strategy during Extended Station Blackout in PWR
Andrej Prošek
orcid.org/0000-0003-0641-3474
; Jožef Stefan Institute Jamova cesta 39, SI-1000 Ljubljana, Slovenia
Abstract
The accident at the Fukushima Dai-ichi nuclear power plant demonstrated the vulnerability of
the plants on the loss of electrical power for several days, so called extended station blackout
(SBO). A set of measures have been proposed and implemented in response of the accident at the
Fukushima Dai-ichi nuclear power plant. The purpose of the study was to investigate the
application of the deterministic safety analysis for core heatup prevention strategy of the extended
SBO in pressurized water reactor, lasting 72 h. The prevention strategy selected was water injection
into steam generators using turbine driven auxiliary feedwater pump (TD-AFW) or portable water
injection pump.
Method for assessment of the necessary pump injection flowrate is developed and presented.
The necessary injection flowrate to the steam generators is determined from the calculated
cumulative water mass injected by the turbine driven auxiliary feedwater pump in the analysed
scenarios, when desired normal level is maintained automatically. The developed method allows
assessment of the necessary injection flowrates of pump, TD-AFW or portable, for different plant
configurations and number of flowrate changes.
The RELAP5/MOD3.3 Patch04 computer code and input model of a two-loop pressurized
water reactor is used for analyses, assuming different injection start times, flowrates and reactor
coolant system losses. Three different reactor coolant system (RCS) coolant loss pathways, with
corresponding leakage rate, can be expected in the pressurized water reactor (PWR) during the
extended SBO: normal system leakage, reactor coolant pump seal leakage, and RCS coolant loss
through letdown relief valve unless automatically isolated or until isolation is procedurally directed.
Depressurization of RCS was also considered. In total, six types of RCS coolant loss scenarios were
considered. Two cases were defined regarding the operation of the emergency diesel generators.
Different delays of the pump injection start following the station blackout were assumed and
analysed. For each scenario, two kinds of SBO calculations for two-loop PWR were performed,
base and verification. Base calculations were needed to determine necessary minimum flowrate for
steam generators feeding in such a way that they are not overfilled or emptied. Namely, it was
assumed that instrumentation is not available during extended SBO. The verification calculations
have been then performed to verify if the determined minimum flowrates are sufficient to prevent
the core heatup.
The calculated results show effectiveness of the proposed extended SBO prevention strategy
provided that the water injection is available in the first two hours after SBO occurring at full
power. If diesel generator is running after loss of offsite power for some time, e.g. one hour, the
available time for steam generator water injection is significantly longer. The obtained results
demonstrate the need for assessment of the pump injection flowrates before the utilization of the
pump for mitigation of the event. The applicability of the developed method for assessment of the
required pump injection flowrate has been validated on PWR.
Keywords
Hrčak ID:
199642
URI
Publication date:
1.9.2017.
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