ML19241B329
| ML19241B329 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 04/09/1979 |
| From: | Desiree Davis Office of Nuclear Reactor Regulation |
| To: | Stello V Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 7907160102 | |
| Download: ML19241B329 (5) | |
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%, % p April 09,1979 MEMORANDUM FOR: Victor Stello, Jr., TMI Operation FROM:
Don Davis, Duty Officer Shift B
SUBJECT:
PARTIAL RESPONSE TO REQUEST FOR HEADQUARTER ADVICE ON C00LD0WN Attached is a Headquarter Advisory Report on boration and criticality aspects of cooldown from points A to C.
Other aspects of cooldown review will follow.
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Don Davis, Duty Officer Shift B cc:
R. Mattson G
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Advisory Report - TMI Cooldown HDQ Comments on Boron Precipitation and Criticality ction 1 Solubility of Boric Acid Assuming the initial concentration of boron in the primary coolant system of 3000 pp$ precipitation of boric acid will not occur until the temperature of the primary coolant is reduced to below 32*F.
Since there is no boil-off of coolant in the primary system the concentration of boric acid remains unchanged.
It would be necessary to increase the concentration of boric acid by a factor of 24.5 at operating point "B" (220 F and 1000 psia) and by a factor of 8.5 at operating point "C" (140 F and 1000 psia) before any precipitation of boric acid could occur.
It is concluded tiat no precipitation of boron in the primary system would occur when the operating conditions of the plant is changed from "A" to "B" and subsequently to "C".
In the letdown system the primary coolant is cooled from its initial operating temperature to about 120 F (FSAR value).
Then it is depressurized to 14.7 psia.
During the depressurization the liquid remains in a sub cooled condition and no boiling takes place. The concentration of boric acid in the letdown sytem stays the same as in the primary system and no precipitation is expected.
Lion 2 Criticality The reactor coolant is presently believe to contain~2200 ppm boron.
The reactor core geometry if fu ly intact, at 150*F, 2200 ppm boron, all rods in and burnable poison intact, would have a K# 0.85.
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Rods are worth m10%dk; the burnable poison 4%4k. Thus if rods are out, k goes to N95 and if the burnable poison is also out the k goes to N.99 (at 2200 ppm).
Ifthecladisremovedandboratedwater(2200 ppm) substituted (i.e. Zr to oxide and washed away) there would be little, if any, change in reactivity, since at this boron level and temperature the moderator reactivity coefficient is near zero (probably slightly positive).
Similarly the reactivity state is not sensitive to the temperature in this range (150'F to 280 F).
0 Thus, if fuel has not redistributed, a baron level of 2200 ppm will keep the system subcritical even if everything but fuel is removed (small effect fru... thimbles and grids).
If fuel is redistributed (in addition to the above removals) the system could - under optimum conditions of moderation - go critical at 2200 ppm (note: moderation is required since solid if02 sphere require enrichments over 5% to be critical). For the worst case - all fuel in a cylinder, or sphere - at optimum moderation - about 3000 ppm would be required to stay subcritical. The calculations are by the B&W Naval criticality group, using Monte Carlo (KEN 0) and tested cross sections. The calculation used pellet nuclear parameters (because these mcximize reactivity) and fuel-water (boron) ratios which had been optimized by sensitivity studies.
The configurations which require boron levels above 2200 appear to be ocu very probable (unless it is probable that the rods are not there), but B&W believes (strongly) that there should be protection from redistribution criticality by going to 3000 ppm boron.
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There are no problems with 3000 ppm from a physics viewpoint. The problems which have been expressed appear to be only the potential for systems blockage from boron at this level - a concern which appears to have no theoretical basis (see Section 1).
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Detectability If the reactor were to get to a k of about 0.95 subsequent changes of the order of 1%Ak should be reasonably detedible (25% increase in count rate for.95 to.96)on an excore startup detector ~if conditions are reasonably normal.
If may, in these circumstances, be partially masked by (relati.ely) weak sources, disturbed geometries, r.onnuclear changes (e.g., downcomer density changes).
At 2200 ppm the worst configuration is estimated to be several percent supercritic3: (i.e.,knel.03).
It is very difficult (if not, impossible) to estimate the power level such a system would maintain to compensate,
for this rr ctivity.
However, going to a water der;sity of about 0.5 should (at least) do it.
It is of course, not generally possible to predict the (hypothetical) rate of reactivity addition in the case of redistribution (whendetechbleabove.95)andthusprovidetheneeded boron insertion rate.
Conclusions and Recommendations At this time we do not have sufficient concern either to require boration or to prevent it. A primary coolant sample would likely provide information that would allow a more definitive recommendation (e.g., the presence or non-presence of control rod material and a better value on existing Baron Concentration).
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There is nothing directly about the cooldown process which would in itself affect the reactivity.
If boron blockage is not a problem, however, the boron level should be taken to 3000 ppsi to cover all (remote) possibilities.
If decision to borate is made a 100 ppm j
stepwise addition with monitoring of potential letdown blockage should be considered.
If blocEage is sufficiently strongly suspected to cause problems so that this is not carried out it would be highly advisable to be sure the startup range instrumentation is likely to be in good order (would be wise in any case) and it would also be advisable to be sure that baration at some " reasonable" rate is available.
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