ML19220B606
| ML19220B606 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 04/13/1979 |
| From: | Johnston W NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Ross D Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19220B586 | List: |
| References | |
| NUDOCS 7904270095 | |
| Download: ML19220B606 (4) | |
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APR 131979 l
MEMORANDUM FOR:
D. Ross, Deputy Director Division of Project Management Office of Nuclear Reactor Regulation FROM:
W. V. Johnston. Chief Fuel Behavior Research Branch Civision of Reactor Safety Research Of fice of Nuclear Regulatory Research
SUBJECT:
FUEL EXPERTS MEETING ON CONDITIOM OF THE TMI CORE A meeting of nuclear fuel experts was held on April 12 to update the estimates of the damage to the TMI core and to consider its effect on the desirability of moving to natural convection cooling of the core.
The Experts Group consisted of the following persons:
J. S. Tulenko, B&W; R. DeMars, B&W; T. Kassner, ANL; R. A. Proebstle, GE; K. A. Jordan, W; R. Duncan, CE; T. Fernandez, EPRI; T. Buhl, NRC; R. Meyer, NRC; W. Johnston, NRC, Chairman. Additional attendees included L. Rubenstein, NRC; C. Berlinger, NRC; M. Tokar, NRC; R. Majors, ACRS Staff and T. Mott, TEC.
Summary The group concluded thst although the core is badly damaged, essentially all of the fuel has remained in the core and that the overall packing density of the settled portion is not expected to exceed 70%.
Therefore, shutting off the RC pumps should not seriously threaten further damage to the reactor.
It was further concluded that the thermocouples (Tc's) located in the upper end fittings are the most important indicator of core condition during transi on to natural convection cooling.
If feasible, the addition of a spectrometer to monitor the activity of the loop coolant for new fis ion products released the transition to natural convection cooling will provide an indepen ent alert to possible difficulties.
A Two questions were considered:
Is a pump trip likely to lead to an unsafe condition? and What signals 91?1 indicate undesirable conditions in the core?
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APR 131979 i
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_Sumary of Core Status Summaries of amount of damage to the co're based upon measurements or calculations of fission gas release, hydrogen produced by zircaloy oxidation, coolant analysis, coolant boil-off rates, incore and excore instrumentation were presented by J. Tulenko and R. Meyer. A relatively large pressure drop across the core is inferred by TH calculations.
If the pressure drop is real, blockage must also exist in the peripheral assemblies (perhaps by ballooning).
The shift in location and magnitude of the high reading core Tc's following the pump trip on April 6 was believed to indicate either a change in the core flow path through more heavily damaged sections of the core to a redistribution of debris surrounding some of the thermocouple beads. An alternate explanation for the change in Tc temperature distribution patterns was presented by T. Mott of TEC.
He suggested that the Tc temperature difference may be due to non uniform flow distributions caused by operation of a single pump rather than non uniformity within the damaged region of the core.. Due to this non uniform flow distribution portions o; the core may already be experiencing similar cooling to that expected during natural convection.
Mott estimates smaller core pressure drop and suggests the B&W estimates may include substantial external pressure drops.
The group visualizes the core as consisting of a heavily damaged region resembling an inverted bell extending across nearly the full width of the top of the core and reaching down about five - six feet into the core at the center and a less damaged remainder of the core. In the heavily damaged region,100% oxidation of the zircaloy and less of a regular geometry is expected.
The guide tubes and poison rods are damaged similarly to the cladding. Spacer grids should be located at or near their original locations.
The important coolability conclusions are that although some settling may have taken place, the overall packing density of the settled portion is not expected to be greater than 70%
and that 85% to 98% of the fuel and cladding from this region is believed to have remained in the " core" region including the upper end fitting.
The remainder of the core is less damaged although considerably oxidized.
The original flow geometry is probably retained although the rods may be twisted or warped and broken in a few places and the spacer grids may have collected some loose debris.
The above conditions should not preclude satisfactory achievement of natural convection ficws.
What should be monitored to determine undesirable changes in the core during the transition to natural convection cooling? The temperature distribution of the core exit thermocouples are the most important.
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s D. Ross APR 131979 condition monitoring signals.
The group believes that all exit Tc's should be continuously tracked and recorded.
B&W suggested the f111owing criteria for remaining in natural convection cooling:
No more than 2 Tc's above 800'F and at least 10 Tc's below Tsat.
There were some reservations among the group about allowing so many Tc's to read abeve Tsat (as many as 39) and radiolysis was an expressed concern.
There was a lot of discussion but no concensus on hcw nany interior Tc's should be permitted to exceed Tsat. Tc's in peripheral assemblies should not exceed Tsat.
The following table sumarizes the available instrumentation and its possible application to monitoring core condition.
Detector Event-Core Overheating Criteria Basis
- 1. Exit Tc's Limit no. in film boiling Not to exceed previous Limit no. above 800 F core damage reverse procedure.
- 2. RTD Maintain positive AT across No flow reversal Hot leg core.
Cold leg permitted.
- 3. Ion chambers Void formation Ambiguous signal since 6 and N If +, record for future some local superheat interpretation, watch Tc's.
may be permitted.
- 4. Noise detection If + indicates bubbles in Same as above.
core or loop, check Tc's, SG.
- 5. System Pressure If increasing system effects Not direct indication Brancb should review this.
of core condition, but for gas bubble formation detection.
- 6. Pressurizer Level Same as above.
Same as above.
Additional Detection - Feasibility needs to be established.
spectroscopy Increasing activity of Xe, I Overhe tad core alert of coolant via 2
sampling line for major cror in procedure.
H 0 analysis on Boron, 0 ' H2 Core criticality and l$nemonitor 2
chemistry control radiolysis and H content control.2 W. V. Johnston), ChiefW hv1A 80 260 Fuel Behavior Research Branch Division of RAu tnr h #a+"
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April 13, 1979 ATTACHAEur F.
NRC ADVISORY -- THERMOCOUPLE READINGS We recommend that all incore thermocouples be read in a continuous manner with provisions for rapid retrieval and permanent storage.
It is clear that a continuous reading of thermocouples will be needed during the transition to natural circulation.
It is also clear that temperature t-ends, which were not recorded during the pump changeover on April 6, would have given additional clues to core behavior in natural circulation.
Since future flow transients cannot be ruled out and since the transition to natural circulation could occur involuntarily, we reccmmend that the continuous recording of all thermocouple readings be initiated as soon as possible.
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\\ <ws; Ralph 0. Meyer, Leader Reactor Fuels Section
- Core Performance Branch 8
80-261
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