ML20148H890
| ML20148H890 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 09/13/1963 |
| From: | Bryan R US ATOMIC ENERGY COMMISSION (AEC) |
| To: | |
| Shared Package | |
| ML20148H881 | List: |
| References | |
| NUDOCS 8011200127 | |
| Download: ML20148H890 (6) | |
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UNITED STATES ATOMIC ENERGY C05NISSION lll IIAZARDS ANA' LYSIS BY Tile RESE ARCll AND PONER REACTOR S AFETY BRANCil
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- . I DIVISION OF LICENSING AND REGULATION :
IN die MATTER OF Y ANKEE ATOMIC ELECTRIC COMPANY q PROPOSED CllANGE NO. 36 DOCKET NO. 50-29 tack ground .
Yankee Atomic Elcetric Company presently plans to begin their second refueling operation in September 1963, at which time Core III will be loaded into the reactor. Prior to establishing the enrichment for the Core III fuel -
assemblies, Yankee Atomic Electric Company previously requested guidance and advice from the Commission concerning certain aspects of their proposed opera-tion with this new core. In Yankee's letter requesting this assistance, dated .
January 3,1962, they described use of fuel assemblies with an increased enrichment of 4.1% as a part of the Core III loading. The core was to con- a tain 36 assemblies of the 4.1** enrichment in the peripheral region and 40 of ~ ~
the Icast. irradiated fuel assemblies from Core II (initial enrichment 3.4%)
in the central region of the core. The excess reactivity o f this configuration was to be approximately 2.5% higher than that of both Cores I and II. In order to provide the additional reactivity control which would be required, boric acid having a worth of approximately 2.5%, the same worth as equilibrium xenon, was to be used in the main coolant system for approximately the first 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> of power operation until xenon equilibrium could be established.
Yankee's equest for advice as to whether the proposed fuel element loading and use of boric acid with Core III would be acceptable from a safety standpoint was considered by the ACRS at its February 1962 meeting. In its .
1ctter to the Commission concerning this matter, the ACRS stated, "...recog-nizing that there are possible alternate steps which can be taken if serious difficulties should arise, the Committee can see no reason at this time to advise against specifying the higher enrichment with a view to using boric acid in the primary coolant at power with Core III." The Committee also stated that the results of the tests which had thus far been carried out by Yankee using boric acid in the primary system at full power were inconclusive and that several questions must be answered before a definite approval could j be given for the proposed operation of the reactor with Core III. Thess ques-tions related to such items as shutdown margin, integrity of the boron injec- _
tion system and control rod worth, J
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Discussion In Proposed Change No. 36, dated June 7,1963, and in supplements thereto, dated July 1 and 25, 1963, Yankee requested Commission authorization of their @
proposed loading configuration -for Core III and the use of boric acid during power operation with this core. 'The loading for Core III would be changed from the present single to a multi-region pattern. This would be accomplished by '
loading 36 Core III-type unirradiated fuel assemblics into the peripheral. '
. region of the core, four Core II-type unirradiated assemblies into the central region of the core and 36 irradiated assemblics from-Core II into the inter-mediate region. The 36 irradiated assemblies will have undergone' an average -
burnup of approximately 5500 Mh'D/T at the time of refueling.
Yankoo has also requested that the technical specifications be revised to allow them to operate the reactor with up to approximately 400 ppm of boron ,
in the main coclant system at any tine during core life. This request, of _
course, differs from Yankee's original concept of retaining boron in the system when operating with Core III only until equilibrium xenon is reached. ;
Evaluation $
Based upon our consideration of Yankee's request, we have concluded that I there are two principal safety questions in regard to the proposed loading and manner of operation with Core III. These are whether Yankee should be auth- ,
i:j ori:cd to operate the reactor at power with boric acid in the primary system '
throughout . core life and whether an adequate hot shutdown margin has been p rovided, These two questions are, of course, interrelated, as the shutdown
. margin depends on the amount of boric acid which will be present in the system. .
A. Ilse of Boric Acid During Core III Power Operation ,
The Yankee reactor was operated for seven days with Core I and for '
eighteen days with Core II with approximately 400 ppm of boron in the main coolant system. A loss of reactivity of approximately .5% was noted during ,
the first test, and a gain in reactivity of approximately .6% was found during the second test. Similar losses and gains of reactivity, all occurring over -
time intervals on .the order of a day or more, have been noted when the pH of the system has been changed at times when no boric acid was present in the system.
The principal means of detecting build-up of boron on fuel element sur-faces will be through loss of core reactivity. For this reason, we believe that it is important that as accurate an account as practicable be maintained of the excess reactivity of the core in order to prevent the undetected plate-out of significant quantities of boron on core surfaces. It is not possible to calculate precisely how large an amount of boren could be present on core ,
It appears that, at most, this quantity of boron l
surfaces due to plate-out.
would be worth 2 to 3% in reactivity.
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The Yankee core is approximately 7.5 feet high and the coolant velocity E soproxinately 15 feet per second. Assuming an. instantaneous crud burst, the reactivity increase which might result from boron removal from core surfaces could occur within a half a second; and the resulting nuclear excursion could reach a high power icvel before the control rods become effective in shutting do: n the reactor. Due to the negative Doppler coefficient, however, a sudden increase in reactivity in excess of 1% would be necessary to maintain the reactor prompt critical. It. is our opinion that an increase in reactivity ?si of that magnitude through loss of boron from core surfaces is not possible, d especially in so short a time that the control rods would not be able to shut the reactor down sufficiently fast to prevent core damage. Accordingly, it '
is our opinion that Yankee may be authorized to operate their reactor with up to 400 ppm of boron in the primary system (and, if required, as discussed below, for a short period of time with up to 500 ppm of boron in the system) throughout Core III life.
In view of the small possibility that boron plate-out may occur, we believe that Yankee should immediately notify the Commission should an unexplained reactivity change of greater than .008 occur after the burnout curve is computed -
during the first week of full power operation. (A period of approximately one week of full power operation will be required for equilibrium xenon to be fff established and for the predicted shape of the curve of reactivity loss due to burn-up to be computed to a reasonable degree of accuracy.) A similar _.
provision was included in previous authorizations for boron tests at power at :i this facility.
B. Ilot Shutdown Margin +
A concern regarding a reactor such as Yankee with a large negative temperature coefficient is that the primary system could be suddenly cooled to a degree that the control rods lose their ability to shut the reactor down.
Suppleranting the control rods in the Yankee reactor is the boric acid shut.
dcwn systen, This system can be used to introduce horic acid into the crimary ecolant water when it is in a hot, pressurized condition. (If the primary "
system piessure should fall below 800 psig, the safety injection system will automatically begin to operate and introduce borated water into the prinary coolant.) However, the boric acid shutdown system requires a significant amount of time to become effective in reducing core reactivity, in that approxi-mately five minutes will clapse before reactivity reduction starts through use of this system. Because of the rapidity with which a primary coolant system H temperature reduction might be sustained, the injection of boric acid into the system may not be effective in decreasing core reactivity before an uncon-trolled power increase can result.
With the segmented loading which will be used in Core III, a substantial fission product inventory will be present at the beginning of core life whea the shutdown capability of the control rods will be at minimum. Because of this and the above considerations, Yankee was requested to analyze an accident
4 involving rupture of the main steam line coincident with the occurrence of a 0 j
stuck control _ rod At this time in core life. During such an accident, the ruptured secondary system will rapidly cool the primary system and the stuck rod will result in n' substantial reduction in the amount of reactivity immedi- j ately availabic from the control rods to shut the reactor down. Yankee's analysis, which was supplied to the Commission in a letter dated July 1, 1963, 4 indicates that, providing the hot reactor meets a one stuck rod plus 2% shut-down margin, the reactor will not return to critical during such an accident M and no damage to the core will result. Yankee is presently required by the terms of their license to have a hot shutdown margin of 3%. It is our opinion that this should be. increased to 4%, and a "one stuck rod plus 2%" shutdown margin requirement should be added to the Technical Specifications attached as Appendix A to the license. Yankee has indicated that, because of uncertain-ties in their calculations, in order to guarantee a one stuck rod plus 2% ,
shutdown margin, the use of boric acid in the primary system at a concentra-tion slightly in excess of 400 ppm may be required. They have, accordingly, requested that they be allowed to use somewhat higher boron concentrations in the primary system during the period of minimum shutdown margin, if this is found to be necessary. l We belicyc it is important that Yankee adhere to the above shutdown requi rement s , It is, therefore, our opinion that these minimum shutdown margins should he -incorporated into the Technical Specifications, and Yankee should be authorized to use up to 500 ppm of boron in the primary system for a short period of time should this be necessary to maintain these shutdown -
margins.
C. Other Considerations ,
Yankee's license requires that they maintain their reactor suberitical by a reactivity worth of 5% when the reactor is in the cold condition. An additional safety matter reg-* ding the new core is that no critical experiment eill have been conducted prior to loading the fue1~into the reactor. The reactor vessel water will be borated to a concentration of 1600 ppm during refueling, which Yankee calculates will provide a shutdown margin of at 1 cast 7%. We believe that the calculated 7% shutdown margin plus periodic withdrawal of a control rod during loading will provide adequate assurance against inad-vertent criticality during the refueling operation. _
The only mechanical difference between the core III fuel assemblics and the Core I and Core 11 assemblics is that each Core III assembly contains two fue l rods with an increased cladding thickness and a decreased fuel pellet :)
l diameter, These changes will reduce flux peaking around the control rod followers. The average burn-up of the fuel from Core II which will be a g
part of Core III will be approximately 12,000 !MT/T at the end of Core III life, and the maximum burn-up will he approximately 18,000 ?MD/T. These values are less than those which have been authorized for similar fuel in other power reactors and less than the burn-up which will be attained by the "T-' -, _, ,
- 5'- A. .
two Core I. fuel assemblics left in Core II. . The Core I fuel assemblies remain-ing in ' Core . II have now undergone approximately 18,000 K1D/T. irradiation, and, y cs far at can be' determined, these assemblics have not developed any signifi- 4 cant cladding defects to date.
.The reactivity coefficients, except for the moderator temperature coeffi-cient, are the same as for Core II. The minimum value specified for the mod-
. crator. temperature coefficient will be changed from -1.5 x 10-4/ *F to -1 x 10*d/ 'F to provide for operation with approximately 400 ppm boren. Yank o has ]p stated, however, that the expected value of this parameter is -1.5 x 10 g/*F.
1hc hot channel factors for Core III have been calculated by Yankee to be less /
than -those- for Cores I and II, and the . steady state DNB ratios correspondingly 2 higher. This is due primarily to the . flux flattening resulting from the seg. O mented_ fuel loading. As a result of the lower hot channel factors, the cal-culated consequences of accidents are generally less severe than with Cores I and II.
The total fission product inventory will be somewhat greater at the end of Coro III life than at the end of Core II life. Because the maximum operat-
'ing power Icycis are the same, however, the potential downwind iodine doses (which are controlling) will be essentially unchanged; and the other potential radiation doses will not increase significantly.
D. ACRS Review g Proposed Change No. 36 was reviewed by the Advisory Committee on Reactor i Safeguards at its July 11-13, 1963, mootin g. In its letter to the Commission regarding this matter, the Committee stated that, on the basis of the informa-tion presented, they believed the proposed Core III loading and operation up to full power with boric acid in the primary coolant may be carried out without unduc hazard to the health and safety of the public.
Conclusion In view of the above, we believe that Proposed Change No. 36 may bc .f" authorized and the Technical Specifications modified as proposed, except that section D.2.a.(8) of the Technical Specifications should read:
"The Commission shall be immediately notified should an unexplained
. reactivity change greater than 0.85 4 K/K take place at any time subsequent to the first week of full power operation. This report.
ing requirement shall be in effect only when the boron concentration in the' primary system exceeds 80 ppm and within one week after a reduction to a boron concentration of less than 80 ppm."
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This reporting requirement would terminate one week after a reduction in boren concentration to less than 80 ppm, instead of at the time of reduction as pro-9 l
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6-posed by Yankee, With the Technical Specifications modified as stated above, we
!elieve that the health and safety of the public will not be endangered as a result of authorization of the Proposed Change.
C.'
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Robert 11. Bryan, Chief '
Research 6 Power i eactor Safety Branch Division of Licensin3 Regulation Date: ..
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