ML20138A624

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Submits Description of Method Used to Estimate Release of Respirable Radioactive Matl & Brief Comparison of Release from LWR Fuel to That Expected from HTGR Fuel
ML20138A624
Person / Time
Site: Fort Saint Vrain Xcel Energy icon.png
Issue date: 06/19/1984
From: Fentiman A
Battelle Memorial Institute, COLUMBUS LABORATORIES
To: Turel S
NRC
Shared Package
ML20136E458 List: ... further results
References
CON-NRC-04-84-113, CON-NRC-4-84-113, FOIA-84-682 NUDOCS 8512120068
Download: ML20138A624 (5)
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OBattelle Columbus Laboratories SOS King Avenue Columbus. ONo 41201 lelephone tbl4i 424 6424 Teles 24-5454 June 19, 1984 Mr. Stanley P. Turel U.S. Nuclear Regulatory Comission 5650 Nicholson Lane '

Rockville, Maryland 20852

Dear Mr. Turel:

Under Contract No. NRC-04-84-ll3, we are required to estimate the mass of respirable radioactive material expected to be released during our experi-ments with surrogate HTGR fuel. In those tests, a model cask loaded with surrogate spent HTGR fuel will be struck by a small shaped charge jet. This letter contains the release estimate, a description of the method used to make the estimate, and a brief comparison of the release from LWR fuel to that expected from HTGR fuel.

Before continuing, I must caution the reader that the release estimates presented in this letter are only estimates. The estimated releases from surrogate HTGR fuel are calculated using correlations developed from experi-ments performed on spent LWR fuel. There are major differences between the two types of fuel, and the extent to which these differences affect the re-lease of radioactive material will not be known until the HTGR experiments are done.

Six tests (three experiments done in duplicate) on model casks loaded with surrogate HTGR fuel will be conducted in July. The tests will simulate, as closely as p,ossible, an attack on a shipment of spelt fuel from the Ft.

St. Vrain comercial power plant. The surrogate fuel will be rods identical to those used in the Ft. St. Vrain reactor except that they are ccmposed of microspheres containing ThC2 only. No U"2 1s included. In each test, two rows of fuel rods will be loaded into holes drilled in a graphite block.

Coolant holes will also be drilled in the graphite block. Spacing of both the fuel rods and the coolant holes will be identical to that found in a full-scale Ft. St. Vrain fuel element. (A description of a Ft. St. Vrain fuel element is attached to this letter.)

- Three pairs of experiments will differ only in the thickness of the graphite between the inside cask wall and the first row of fuel rods. Cut away dia-grams of the model cask / graphite / fuel assemblies for the three different types of tests are shown below.

l 8512120068 851112 PDR FOIA PDR MILLAR84-682 l

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, Mr. Stanley P. Turel 2 June 19, 1984 graphite / fuel i graphite /fue1 cas w 11s 7 cas w 11s

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jet _

lO4 Jet  : O O IO path ' Q path l

O Ol i

Tests 1 & 2 Tests 3 & 4 graphite / fuel cas lls ru  %

Jet .

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O O O Ojo path 00 O O Ol Tests 5 & 6 e fuel o coolant hole Cut Away Diagram of Model Cask / Graphite / Fuel l

Assemblies for Experiments l

l Three pairs of tests are being done because we need to learn how the shaped charge jet's effect on a fuel rod changes as the jet's energy decreases. If the jet were to follow the most damaging path through a full-sized HTGR fuel element, it would strike 22 rows of rods and penetrate approximately 14 inches of graphite. As the jet moves through solid material, it loses some of its i energy, and therefore, does slightly less damage to each successive row of l fuel rods. In order to properly scale up the results of our experiments, we i

need to know how much respirable radioactive material is released from fuel rods at various locations in the fuel element.

To estimate the respirable release from the experiments, we are assuming that the shaped charge jet produces a hole of uniform size in each graphite / fuel rod assembly. Previous tests with LWR fuel rods indicate that this hole will be approximately one inch in diameter. If the jet strikes the model fuel

6 Mr. Stanley P. Turel 3 June 19, 1984 element at the point where it can do the most damage, it will disrupt one inch of each of two fuel rods (one rod in each of the two rows). In that case, approximately 5.5 cu em of fuel rod, or 12.2 grams, will be disrupted.

Most of the disrupted fuel will remain in the cask. Some non-respirable particles will leave the cask but fall to the ground very quickly. The dis-rupted fuel of interest to us is that which leaves the cask in respirable form (aerodynamic diameter 13.5 pm). Results of the LWR tests indicate about 1 x 10-3 of the disrupted fuel is released as respirable particles.

Therefore, we estimate that 0.0122 g of the fuel will be released from the model cask in a respirable form.

This estimated release is difficult to interpret unless it is scaled up and compared to the release resulting from an attack on a full-scale cask con-taining an LWR fuel assembly. Unfortunately, the scaling law for HTGR fuel elements cannot be developed until the laboratory scale experiments are com-pleted. The only scaling law currently available is the one developed for the LWR fuel. It is inappropriate to apply that law to the HTGR fuel, and such an application is expected to yield results which significantly under-estimate the release. Very preliminary considerations of scaling appro-priate for HTGR fuel suggest that the scaling factor should be the ratio of the areas of the holes produced by the laboratory-scale and full-scale shaped charge jets. That ratio is not currently available but is expected to be on the order of 20.

Recall that in the experiments the release from two rows of pins was estimated to be 0.0122 g of fuel. In a full-sized HTGR fuel element, the jet could strike 22 rows--11 times as many as it will in the experiment. The preliminary HTGR scaling estimate indicates that the reference (full-scale) shaped charge will disrupt 20 times as much material per row as the precision shaped charge used in the experiments. Thus, the estimated release from a full-scale HTGR ele-ment is:

0.0122 x 11 x 20 = 2.7 grams The release reported for a single LWR fuel assembly was 8.97 grams.II)

This completes our discussion of the estimate of the release from the experi-ments using surrogate HTGR fuel. However, we can provide further insight into the probable consequences of an attack on a shipment of HTGR spent fuel by briefly discussing the differences in the compositions of LWR and HTGR fuel.

II) E. W. Schmidt, M. A. Walters, B. D. Trott, J. A. Gieseke, " Final Report on Shipping Cask Sabotage Source Term Investigation", NUREG/CR-2472, (October,1982).

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Mr. Stanicy P. Turel 4 June 19, 1984 4

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\ l j General Atomic (2) has reported the nuclide content for. spent U-235 fuel  !

(once-through) and U-233 fuel (recycle) .as calculated by the GARG0YLE ,

code. We have chosen to base our analysis on'the spent once-through fuel '

- since that is the type currently used. However, the consequinces of dis-persal of recycle fuel can be obtained by using the ratio of the specific activities of the dominant nuclides. (The concentration of transuranic elements in recycle fuel is approximately 8 times that found in once-

through fuel.)

GARG0YLE data show that the specific activity (Ci/cc) of the ' fission pro-ducts in the HTGR fuel is an order of magnitude less than that found in LWR

, fuel, as can be expected from ithe lower specific power in the HTGR. The specific activity of most the transuranics is 40 to 700 times less than that found in LWR fuel snici was expected because of the low U-238 content of the highly-enriched HTGR once-through fuel . However, the specific activity of Pu-238 in the HTGR fuel is 1/4 of that of LWR fuel. The high Pu-238 concen-tration i_s due to the high U-235 content and the favorable production route

by neutron capture and e decay from U-235 to Np to Pu-238.

The' Pu-238 concentration is very significant to our study. In the evalu-ation of the consequences of the malevolent dispersal of LWR fuel, Pu-238 1

was found to account for 40 percent of the latent cancer fatalities. Since i- its concentration is ^relatively higher than all of the other significant nuclides in~HTGR fuel, Pu-238 can be expected to dominate the consequences in a release' of HTGR fuel. However, its concentration is less than that in LWR fuel so we do not expect the consequences of a release of HTGR fuel to be as great as those of a release of an equal amount of LWR fuel.

Sincerely, -

/&d&d<sm )

i IAudeen Walters Fentiman Program Manager l

Nuclear Systems Section i

AWF:erc Enc.

i l

I2) Hamilton, C. J., et al. , "HTGR Spent Fuel Element Composition 'and Fuel Element Block Flow", GA-A-13886 (Vol 1); 7/76.

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DESCRIPTION __0F FT. ST. VRAIN FUEL ELEMENT A Ft. St. Vrain HTGR fuel element is a graphite block 31.2*-in high with a hexagonal cross section 14.2-in* across the flats. Axial holes for coolant and fuel are located in a triangular array on a 0.74-inch pitch.

In each set of three holes in the triangular array, one is a coolant hole

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4 and two are fuel holes. The 0.625-inch diameter coolant holes completely penetrate the block and, when in the reactor, are aligned with coolant holes in blocks above or below. The 0.50-inch diameter fuel holes ar.e closed at each end and hold 2-inch long graphite rods which contain (Th-U)

C 2 r ThC2 particles coated with pyrolytic carbon. At the axial center on the top of the block is a blind hole no..inally 1-inch diameter to accommodate a lifting device for handling the block. Surrounding the lifting hole are six 0.5-inch diameter coolant holes. A 0.5-inch diameter hole at each corner of the block holds burnable poison. There are 210 fuel holes,102 large coolant holes, 6 small coolant holes, and 6 burnable poison holes.

Rounded to 3 significant figures.

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