ML20198H076

Responds to Item 7.3.2 Re Corrections to Shielding of Castor V/21 Tsar,Per L Rouse .Mod to Gamma Shielding Proposed,Based on Dose Rate Measurements Obtained During Demonstration Test at Inel.Figures Encl

ML20198H076
Person / Time
Site:	Surry, Turkey Point, 07100510
Issue date:	01/02/1986
From:	Kingsley K GENERAL NUCLEAR SYSTEMS, INC.
To:	Roberts J NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
References
REF-PROJ-M-37 26252, C8512-14, NUDOCS 8601300063
Download: ML20198H076 (6)

Text

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Subj ect: NRC Project M-37  % ~

CASTOR V/21 TSAR Corrections

Reference:

(A) HRC letter dated September 30, 1985; Project M-37; L. C. Rouse to V. J. Barnhart

Dear Mr. Roberts:

This transmittal provides you with our response to iten 7.3.2 Shielding of the TSAR corrections listing provided in Reference (A). Item 7.3.2 stated that the calculated cask doserates reported in the TSAR should be increased to account for the effects of neutron subcritical nultiplication. The following discussion orovides our assessment of this effect. In addition, a modification to the CASTOR V/21 gamma shielding is proposed based on doserate measurements obtained during the demonstration test at INEL.

Subcritical Heutron Multiplication The effect of neutron subcritical multiplication is not considered in our shielding calculations. This effect is significant only if fresh undepleted fuel is loaded into the cask together with spent fuel. For storage of typical 15 x 15 PWR fuel assemblies, the keff for fresh fuel at 3.7 % U-?35 in a helium environment will be about 0.d. In spent fuel the U-235 content is depleted to less than 1.0%. The keff will also be lower (in the range of 0.1), and the overall effect of the subcritical neutron nultiplication in the case of spent fuel (about 10%) is covered by the conservatisns of the calculation nethod used in our TSAR. This conclusion is further supported by the doserate measurements at INEL. Prior to perfomance of the test, GHS/GHSI conducted a doserate study that accounted for the source strength and characteristics (including power history and cooling time) of the actual fuel selected for the test. The shielding analysis was performed using the same methods described in Section 7 of the TSAP,. The enclosed three figures demonstrate the good agreenent between measured and calculated results.

Therefore, it is our conclusion that the doserates presently reported in the TSAR are acceptable.

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Gamma Shieldino Design The doserate measurements at INEL demonstrated that the CASTOR V/21 performed well within the design criteria of 200 mrem /hr at the surface and 10 mren/hr at 2 meters. Figure one for the sidewall indicates that a gamma peak was observed at the elevations corresponding to the fuel upper and lower end fittings. Even though the design requirements were satisfied the following modification is proposed to reduce this local peak,

l. The depth of the moderator bore holes will be reduced by 250 mm.

2. Steel plugs, 600 run long, will be inserted at the cask bottom end of the borings instead of the polyethylene material.

These changes will result in a reduction by a factor of eight of the gamma peak in those regions. Since the neutron dose in these regions is very low, the effectiveness of the neutron shielding is not impaired. This shielding improvement affects only the local gamma peak and does not change average doserate values presently reported in the TSAR (Table 7.3 a).

This change will be appropriately documented in the forthcoming revision of the TSAR.

4 Very truly yours, GENEPAL NUCLEAR SYSTEMS, INC.

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