Welcome to the PWR 1000MWe SBO TD AFW Q&A

[matt222]

Hello everyone,

I would like to ask you question related to PWR 1000MWe, in the event of Station Black Out SBO there is a safety system called TD AFW to supply steam generator with enough water so that can remove the heat from the primary side indirect way as a heat sink. My question approximetly what is the pressure in the primary side is it going to be less than 10 MPa or slightly higher about 12 MPa.

My second question if we have two steam generator and we experience an SBO do we have to turn both TD AFD or for one steam generator will be enough

 

[Astronuc]

The pressure depends upon assumptions in the accident analysis. For example, does one assume all EDGs fail, or not.

Here is a recently published assessment from KAERI. They assume EDGs are not available.
COMPARATIVE ANALYSIS OF STATION BLACKOUT ACCIDENT PROGRESSION IN TYPICAL PWR, BWR, AND PHWR
http://article.nuclear.or.kr/jknsfile/v44/JK0440311.pdf
See Figure 1 for results of a 1000 MWe PWR of a current Gen3 design (The OPR 1000-like (ABB-CE type PWR)). The results show that one's plant better have at least one EDG available, and preferably more.

In LWRs, there are two or more safety trains. There is some mandatory redundancy that ideally do not fail due to a common mode.

If one has two independent systems, one each for SG, then ideally one uses all available cooling to cool the plant. Certainly one could analyze a scenario with cooling from two SGs and one SG and compare results.

 

[NUCENG]

The pressure depends upon assumptions in the accident analysis. For example, does one assume all EDGs fail, or not.

Here is a recently published assessment from KAERI. They assume EDGs are not available.
COMPARATIVE ANALYSIS OF STATION BLACKOUT ACCIDENT PROGRESSION IN TYPICAL PWR, BWR, AND PHWR
http://article.nuclear.or.kr/jknsfile/v44/JK0440311.pdf
See Figure 1 for results of a 1000 MWe PWR of a current Gen3 design (The OPR 1000-like (ABB-CE type PWR)). The results show that one's plant better have at least one EDG available, and preferably more.

In LWRs, there are two or more safety trains. There is some mandatory redundancy that ideally do not fail due to a common mode.

If one has two independent systems, one each for SG, then ideally one uses all available cooling to cool the plant. Certainly one could analyze a scenario with cooling from two SGs and one SG and compare results.

You beat me to that reference Astronuc!

The OP asked about SBO in which the diesels are not available (a la Fukushima) In Two-Loop PWRs I have worked at there is a single TD AFW pump supplied from steamfrom both steam generators. So there is a possibility of loss of makeup if that pump fails.

See
http://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6890/cr6890v2.pdf
for a 2005 assessment of that risk.

The SOARCA analysis of a long term SBO for the Surry (three-loop) PWR is available in
http://pbadupws.nrc.gov/docs/ML1202/ML120260681.pdf
That analysis assumes TD AFW operation until it empties the ECST.

The fact is that Fukushima Daiich Units 2, and 3 had two turbine driven systems (HPCI and RCIC). Unit 1 had a passive ECCS system (Isolation Condensers). I know the OP was asking for technical information on the TD AFW system, but it started me thinking if PWRs may need to look at this more.

 

[Astronuc]

As far as I know, last year the NRC required all stations to review their programs/procedures for addressing SBO and LOOP. The utilities with which I interface were proactive and immediately established review teams within a few days of the Fukushima event, and they asked themselves if it (combined natural events of extraordinary nature) could happen to them and how would they respond.
http://www.nrc.gov/reading-rm/doc-c...lides/2011/20110428/staff-slides-20110428.pdf

From 2003 - http://pbadupws.nrc.gov/docs/ML0324/ML032450542.pdf

I know that there has been an industry program on EDG maintenance and reliability.

Example SER for Watts Bar - http://pbadupws.nrc.gov/docs/ML0732/ML073200315.pdf
I believe they re-evaluated based on Fukushima.

 

[NUCENG]

As far as I know, last year the NRC required all stations to review their programs/procedures for addressing SBO and LOOP. The utilities with which I interface were proactive and immediately established review teams within a few days of the Fukushima event, and they asked themselves if it (combined natural events of extraordinary nature) could happen to them and how would they respond.
http://www.nrc.gov/reading-rm/doc-c...lides/2011/20110428/staff-slides-20110428.pdf

From 2003 - http://pbadupws.nrc.gov/docs/ML0324/ML032450542.pdf

I know that there has been an industry program on EDG maintenance and reliability.

Example SER for Watts Bar - http://pbadupws.nrc.gov/docs/ML0732/ML073200315.pdf
I believe they re-evaluated based on Fukushima.

Thanks. I hadn't considered the PWR TD AFW pump as a single point of failure before. I am certain that the PWR community hasn't been ignoring the Fukushima accidents, but I have been focused on BWR specifics for the last year and this just struck a chord today. It helps kickstart a broader perspective. In this case, it was a fairly light kick.