United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #: 9503010082

U.S.  Nuclear Regulatory Commission
Brain K.  Grimes, Director
Division of Project Support
Office of Nuclear Reactor Regulation          February 17, 1995

SUBJECT:   IMPROPER JET EXPANSION MODEL IN BECHTEL TOPICAL 
           REPORT, BN-TOP-2, REV.2, DESIGN FOR PIPE BREAK EFFECTS,
           ISSUED: MAY, 1974

Dear Mr. Brain K. Grimes

      This letter is to inform NRC about an improper presentation of jet
expansion model (i.e., jet expansion profile) in the subject Bechtel
Topical Report.  Since the subject Topical Report has been previously
approved by NRC, NRC may need to consider evaluating the consequences of
potential misapplication of the concerned jet expansion model and taking
necessary corrective actions.

Improper Presentation of Jet Model

Topical Report Section 2.3 and Figure 2-3 describes and illustrates the jet
expansion model in the situation of steam or water-steam blowdown.  Figure
2-3 shows, in Region 1 (i.e., Moody Expansion), the jet axial travel
distance to the asymptotic area is approximately 5 times of the pipe I.D.

By reviewing the original Moody Expansion Model (Ref.: ASME Paper 69-HT-
31, "Prediction of Blowdown Thrust and Jet Forces", F.J.  Moody, August 6,
1969), it is found the jet axial travel distance to the asymptotic area is
4.5 times of the pipe I.D. for saturated water blowdown, or 2.0 times of
the pipe I.D. for saturated steam blowdown.

In conclusion, Figure 2-3 of Bechtel Topical Report can only be used for
saturated water blowdown.  To apply Figure 2-3 for saturated steam
blowdown, the axial travel distance to the asymptotic area shall be
corrected to 2.0 times of the pipe I.D.

Consequences of Potential Misapplications

PG&E Diablo Canyon Project Jet Impingement Reverification Program used the
jet model in Figure 2-3 of Bechtel Topical Report for identifying the jet
impingement targets and calculating the jet impingement forces in the
situation of both saturated steam and saturated water blowdown.  During a
recent jet impingement walkdown of 28" main steam lines, it was found a
Solid State Protection System (SSPS) electrical junction box would become
a jet target and being destroyed if the correct jet model for saturated
steam (i.e., axial distance to asymptotic area equal to 2.0 times of pipe
I.D.) is used.  However, this SSPS Junction box would not become a jet
target, i.e., outside of "Zone of Influence" of the jet, if the jet model
in Figure 2-3 of Bechtel Topical Report is used.


                          Sincerely Yours

                                         David Yang

                                         Peak Technical Services
                                         1020 Wickham Dr.
                                         Moraga, CA 94556
                                         (510) 631-1368

cc:   Gregory M. Rueger (PG&E) 
      Mike Angus (PG&E)
      Tien Lee (PG&E)


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