ML19276G194
| ML19276G194 | |
| Person / Time | |
|---|---|
| Site: | 07109120 |
| Issue date: | 04/06/1979 |
| From: | Goldman K TRANSNUCLEAR, INC. |
| To: | Macdonald C NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| E-1598, NUDOCS 7905150788 | |
| Download: ML19276G194 (3) | |
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47 E-1598
_y April 6, 1979 Mr. Charles E. MacDonald, Chief Transportation Branch Division of Fuel Cycle and Material Safety Nuclear Regulatory Commission Washington, D.C.
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SUBJECT:
Doc k e t No. 71-9 ' "
Fracture Toughness for TN-12 Steels
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REFERENCE:
Transnuclear Letter, Kurt Goldmann to
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Charles E. MacDonald, NRC, E-1515, date d January 31, 1979
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Dear Mr. MacDonald:
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4 Based on further discussions, we wish to withdraw the approach contained in the referenced letter.
In its stead, we propose to demonstrate the suitability of TM-12 containment vessel steels from a fracture toughness point of view as follows:
A.
DESIGN 1.
Material Acceptance Criteria The containment vessel of the TN-12 is basically a cylinder nade of 12 inch thick ferritic steel forgings. The material should have sufficient fracture toughness to preclude brittle failure at low temperature.
During normal conditions of transport at low ambient temperatures, stresses in the containment vessel are low and brittle failure is not likely.
The hypothetical accident condition which is considered to be the most adverse for brittle failure is a free drop or puncture of an empty cask at -20F, Material acceptance criteria must be selected to assure that under these conditions the material has sufficient fracture toughness to prevent brittle failure.
In particular, during the impact a crack should not propagate through the thickness of the containment vessel and thereby violate the containment function of the vessel.
In the absence of specific regulations or industry standards on fracture toughness criteria for spent fuel shipping casks, Transnuclear, Inc.
Proposes the following acceptance criteria for fracture toughness of the material for the TN-12 containment vessel:
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Charpy V-notch test: Cyil 20 ft-lb at -50F.
(ASME Section III, Class MC) b.
Charpy V-notch test: Cy 2: 35 ft-lb, at the lowest service temperature of -20F.
c.
Drop weight test: NDT 6 -40F (ASTM E 208) d.
Ultrasonic examination of shell, bottom and lid forgings.
(ASME Section III, Class 1, NB-2540).
Flaw size a 5: 0.5 in.
within 15 in. of trunnion centerline, a != 1 in. all other locations.
Criterion (a) is taken from ASME Code,Section III, Class MC.
The rules for Class MC components have been developed for vessels whose function is to contain radioactive materials in case of an accident and which normally operate at a pressure which is less than atmospheric pressure.
The TN-12 serves a similar function and its cavity is also under negative pressure during normal transport.
In the absence of high pressure, there is no large strain energy stored in the vessel material or in the internal fluid to assist the propagation of a crack through the vessel wall.
For the TN-12 there will also be no extensive residual stresses which could act as crack drivers since the vessel will be stress relieved.
In the further absence of thermal stresses under the postulated accident conditions, the propagation of a flaw would be driven only by the transient forces generated by the impact. These forces are applied only during a period of approximately 30 msec.
Criterion (b) has been included to assure reasonable fracture toughness at the lowest service temperature of -20F.
The rules of Class MC do not require drop weight NDT determinations if Charpy V-Notch test requirements are met.
Nevertheless, criterion (c) is proposed to assure that the material drop weight NDT is lower than the lowest service temperature (LST) of -20F.
To give further assurance against brittle failure at low temperature, it is proposed to perform ultrasonic inspections of all forgings in accord with Class 1 rules. Criterion (d) shall be the basis for acceptance.
2.
Drop Tests of TN-12 Model Drop tests with a 1/3 scale model have heer. used to demonstrate that the TN-12 will meet the requirements for free drop and puncture hypothetical accident conditions of 10 CFR 71.
A combined total of nine tests have been performed at normal ambient temperature.
Without refurbishing, this same model was recently subjected to another free drop from a 30 ft. height and two puncture tests from 40 in. heights
. while the ecdel was at a temperature of approximately -40F.
No evidence of brittle f ailure was noted from visual observations and dye penetrant and ultrasonic examinations after the tests.
As further proof that the materials criteria of 0) above are acceptable, it is proposed to perform additional drop tests with the same 1/3 scale model at a temperature of -40F + 10F as follows:
a)
Free drop through a distance of 30 ft. on an essentially unyielding surface with the model impacting in a horizontal position on two trunnions. The trunnions will be refurbished prior to this test.
b) A free drop through a distance of 40 in, onto a 2 in, diameter mild steel bar mount.d on an essentially unyielding surface, with the model in a horizontal positica and an area adjacent to the front trunnion located above the center of the bar.
Cask orientation and points of impact have been selected as being mcst severe for brittle failure.
Surface flaws shall be machined into the model at the most critical locations prior to the test.
One flaw shall be machined into the thinner section of the shell above the front trunnion in the plane of the thickness discontinuity. Another flaw shall be located on the outside surface at the center of the shell in a plane perpendicular to the axis of the model.
Both flaws shall be 1 in. in depth and 6 in. long, and shall be centered in the impact plane in the lower part of the shell.
The radius at the notch tip of both flaws shall be less than 0.003 in.
Visual observations, photographs, dye penetrant and ultrasonic or equivalent examinations prior to and after the tests shall be utilized to confirm the absence of brittle failure.
In particular, it shall be demonstrated that cracks from the machined flaws or new cracks do not propagate through the thickness to the opposite surface of the shell.
Available Charpy V-notch tests confirm and NDT drop weight tests shall be performed in the near future to confirm the expectation that the low temperature fracture toughness properties of the model material are no better than the criteria established in la, b,
and c above.
Please contact us, if you have any questions. We shall await your concurrence with our proposed approach before proceeding with the test of the 1/3 scale model.
Very truly yours,
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Kurt Goldmann Chief 2ngineer
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