ML20129B341

Discusses Review to Estimate Production of Hydrogen by Radiolysis in Sf Storage Cask,Concluding That Radiolysis Not Significant Contributor to Hydrogen Gas Production at Point Beach & Not Expected to Be Source for Other Sf Storage Sys

ML20129B341
Person / Time
Site:	Point Beach
Issue date:	08/05/1996
From:	Howe A NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
To:	Sturz F NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
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ML20129A284	List: ML20129A278 ML20129A316 ML20129A489 ML20129A525 ML20129A557 ML20129B341
References
FOIA-96-322 NUDOCS 9610220404
Download: ML20129B341 (6)

Text

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4 MEMORANDUM TO: Fritz Sturz, Chief Technical Review Srction Spent Fuel Project Office FROM: Allen Howe, Nuclear Engineer, Technical Review Section Spent Fuel Project Office p i

SUBJECT:

Estimate of Hydrogen Production From Radiolysis in Spent Fuel Casks l

Following the unanticipated gas burn during VSC-24 cask loading at Point Beach on May 27, 1996, the Spent Fuel Project Office (SFPO) initiated a review to estimate the production of hydrogen by radiolysis in a spent fuel storage cask. The NMSS staff conducted a limited literature search on radiolysis and made some simplified calculations to estimate a hydrogen production rate. The details are attached.

From this review, the following pointo are clear:

• Radiolytic decomposition of water (and production of hydrogen gas) in a spent fuel derage cask is a complex process influenced by the types and intensities of radiation present, the boron concentration, and the presence of chemical impurities and dissolved gases.

• Experimental results show that in a mixed radiation field with a constant gamma source, a minimum threshold of neutron interaction with boron is required to produce a not yield of hydrogen gas. Other experiments have shown that gamma radiation is effective in the recombination of free hydrogen gas in a mixed radiation field.

• Gamma radiation dominates the radiation dose rates in the spent fuel storage cask.

• On the basis of this review, the not production of hydrogen gas from radiolysis in a spent fuel storage system is expected to be small (well below the quantities needed for combustion) but an absolute not hydrogen production rate cannot be quantified.

Based on the above, the staff believes that radiolysis was not a significant contributor to the hydrogen gas production at Point Beach and is not expected to be a source for other spent fuel storage systems. A more definitive quantification of the net hydrogen production rate could be obtained by significant additional research, data collection from systems in place, and possible experimentation. This further research is not recommended because hydrogen production is expected to be low and the anticipated response to NRC Bulletin 96-04 that requests licensees to demonstrate the safety of the cask loading and unloading operations.

Attachment:

As stated OPC SFPO [ NMSS/DWM NAME AHowe d DVinson om onem osigm .

C = COVER E = COVER & ENCLOSURE N = NO COPY OFFICIAL RECORD COPY 9610220404 961009 " ,/

PDR FOIA DUMS96-322 PDR n/- /

.. . . . - . . ~= . . - - . - - - . _ - - ... - - .- - -. . _ .

Attachment

Background

Factors affecting hydrogen production include the type of radiation (alpha, gamma, and neutron), the linear energy transfer (LET) rates from the radiation, the water chemistry and dissolved gases, and physical conditions such as temperature and pressure. Hydrogen gas production is a complex process involving decomposition of water molecules, recombination of decomposition products back to water (back reactior.;), chemical reactions of the decomposition products to form hydrogen peroxide and molecular hydrogen (H2 ), and the formation of various radicals such as OH, HO,, etc.

The literature states that mixed radiation fields will produce a competitive effect between water decomposition and recombination. In experiments, reducing the neutron flux in a mixed gamma-neutron field resulted in reduced rates of decomposition. Likewise, reducing the gamma flux increased rates of decomposition'. These results suggest that gamma radiation plays a role in recombination.

A series of experiments by Hart, McDonnell, and Gordon2 used varying concentrations of boric acid solutions in a constant gamma-neutron field to measure hydrogen production.

The key variable in this experiment was the a energy density from the B' (n,a)Li7 reaction.

The experiments found that a minimum boric acid concentration (proportional to the a energy density) was required before any appreciable Ha production was observed. This is further evidence of a Ha removal factor. Once H2production began, it was linear with increasing boric acid cuncontration and thus a energy density.

3 Calkins reported that hydrogen gas evolution from experiments with borated water was a linear function of the rad dosage calculated as En - Ey, Where En is the combined energy

- ebsorption from neutron moderation and the neutron-alpha reaction with boron and Ey was the y energy absorption.

The production of H2from decomposition in an air free boric acid solution can be ge..arally expressed as follows:

dH2 /dt = gross production - removal Much work has gone into quantifying H2 production from various types of radiation. The production factor is referred to as a "G" value and is usually expressed in terms of molecules produced (in this case, H2 ) per 100 eV of absorbed energy. The G(H 2) values used in this discussion are provided below:

Beta, gamma G(H,), = 0.45 neutrons G(H2 ),,1.12 (fast scattering)

For thermal neutrons, G(H2 ),is used due to the production of gammas (2.2MeV) from neutron capture in hydrogen B'D(n,a)Li7 G(H,), = 1.70

A-2 The gross hydrogen production can be calculated from the following expression:

gross production = G(H 2)a(E ) + G(H,),(E,) + G(H ),(E,,,) 2 where: E, = a energy absorption density (eV/cmSmin) (assume all a energy deposited in water)

E, = y energy absorption density (eV/cm*-min) 8 E,,, = thermal neutron capture energy absorption density (eV/cm -min)

The experiments by Hart, et al, saeasured a y energy absorption density of 11.9E20 eV/ liter-- l min and a thermal neutron flux of 8.34E13 n/cm2. min in the experiment. A fast neutron  ;

flux was not discussed and is assumed to be negligible. The H(n. y)D and the B"(n,a)Li 7 I reaction rates can be calculated to provide energy absorption densities from those reactions.

H, removal is assumed to be due only from the y interaction and can be expressed as:

removal = G,(E, + E,n). l l

Given the net H, experimental production rates and calculated gross production rates, experimental G, values can be derived. The following experimental G, values wei calculated for each boric acid concentration. ,

Boric Acid Concentration Net H2 production G, E, E,/E, (moles / liter) (pmole/ min) (H2 /100eV) (10"eV/

cm 8-min)

O.02 0 0.99 1.77 1.49 0.0313 21*2 1.07 2.76 2.32 0.05 53

• 2 1.24 4.415 3.71 0.0732 93 t 5 1.44 6.46 5.43 0.10 14712 1.59 8.83 7.42

. Hydrogen Removal vs. Energy Density

-to -

e a

4 2

1~-

0.99 1.07 1.24 1.44 1.59 H removal sate (per 100 eV)

Figure 1

I A-3 From the graph, the hydrogen removal factor (G,) varies nearly linearly with the a energy absorption density in the higher energy ranges and appearr to be approaching a minimum value for lower energy ranges. G, would be expected to approach G(H2), for a pure gamma field where no not H 2production is observed. The gamma / neutron fluxes are constant in  ;

this case. i VSC-24 Conditions ,

1 The conditior:s in the VSC-24 cask before draindown include the presence of spent fuel pool .;

water with a minimum concentration of boron at 2850 ppm. Soron is usually in the form of I dissolved boric acid assumed to be H3 BOs. The pool water contains some concentration of l dissolved gases from exposure to air (primarily nitrogen and oxygen) and the products of l radiolytic decomposition from the fuel in the pool. The cask is at roughly atmospheric I pressure depending on the tightness of the fit of the shield lid and the water temperatures I range from 80 to 120*F.

Data on the fuel assemblies loaded in the cask at Point Beach:

Westinghouse 14X14 bundles peak decay heat 0.403 kw/ assembly (design basis s 1kw/ assembly) peak gamma source 2.4d15 y/sec - assembly (design basis 6.8E15 y/sec - assembly) peak neutron source 9.28E7 n/sec - assembly (design basis 1.2E8 n/sec - assembly)

The design basis values except boron concentration were used for this evaluation since the decomposition rate is dependant upon the radiation dose rates and will provide a bounding condition. Boron concentration used was 3000 ppm.

Quahtative Radiolvsis Estimate The staff ran a SAS1 calculation for a cylindrical homogenous model with a fuel water mix I' representative of the fuel t:isket to estimate neutron and gamma fluxes and dose rates.

The staff also ran a dry model to see the difference in dose rates and therefore estimate j the fraction of radiation absorbed in the water. The results follow: l Wet radial wall neutron dose rate at axial centerline 0.556 rem /hr radial wall neutron flux at axial centerline 7.69E3 n/cm3 sec radial wall gamma dose rate at axial centerline 2.64E4 rem /hr radial wall gamma flux at axial centerline 1.92E10 y/cm' see Dry radial wall neutron dose rate at axial centerline 1.67 rem /h radial wall neutron flux at centerline 2.17E4 n/cm'sec radial wall gamma dose rate at axial centerline 2.98E4 rem /h radial wall gamma flux at axial centerline 2.13E10 y/cm8 sec An estimate of the absorbed energy deposition (related to dose) is based on the method given in reference 3 using the following input data.

N . . - _ . . _ _ _ _ . __ _ . - - . ._- _ ,

i A-4 The average neutron energy is 2.29MeV The neutron flux inside the " fueled zone" was assumed to be 10X the " dry" wall flux equal to 2.17E5 n/cm8 sec The average gamma energy was assumed at 0.362 MeV The gamma flux inside the " fueled zone" was assumed to be the " wet" wall flux equal to 1.92E10 y/cm'sec Boron concentration used was 3000 ppm. The calculated macroscopic cross-section was I. =0.0987/cm. The thermal neutron flux was assumed to be 10X the " wet" flux at energies of 10eV and below equal to 1.00E4 n/cm'sec.

Alpha energy absorption density E =2.3E9 eV/cm* sec (assumes all energy deposited in water)

Gamma energy absorption density E, = 1.92E14 eV/cm'sec Fast neutron energy absorption density E,= 2.20E11 eV/cm*-soc Thermal neutron energy absorption density E. - 3.08E8 eV/cm*-sec The gross H, production is G(H,),(E ) + G(H,),(E,) + G(H2 ),(E.) + G(H,),(E,)

= 8.665E11 molecules H,/cm*-sec Considering the case for design basis fuel where there is no removal of hydrogen, the gross production rate for the entire VSC 24 water volume (5.21E6 cm', p.11-47, SAR) is calculated at 7.6E-6 moles per second. This equates to about 0.65 moles of hydrogen produced in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

This calculated value is far less than the.approximately 4 moles found in the cask about 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> after the bum (16cc/kg dissolved and 5.44% in the free space). These values alone confirm that another mechanism for hydrogen production existed in the VSC 24.

Considering only the gross H, production rate is conservative since removal is ignored, if little hydrogen is assumed to remain in solution, this rate yields a sufficient quantity of H, for a flammable mix in the 30-gallon free space of the VSC-24. Absent additional research and/or experimental data, there is no direct way to calculate a not H, production rate for a spent fuel storage cask. However, a gross production without removal is not expected because gamma radiation dominates the radiation dose rates in the spent fuel storage cask. The E,/E, for the cask is 1.20E-5 as compared to the experimental threshold E,/E, value of 1.49. This comparison suggests very little H, production from the alpha interaction and significant removal potential by the gamma flux. Thus, little not H, production is expected.

If the not H, production is assumed to be zero, a G, can be calculated and compared with experimental values.

H, removal = G,(E, + E.) .

Therefore G, = (8.665E11 molecules H,/cm'-sec)/ (E, + E.) = 0.00451 molecules H,/eV

= 0.451 molecules H,/100 eV This value is comparable to G(H3 ), and well below the values shown in figure 1. Smaller removal factors would yield not H, production but do not correlate to the experimental observations.

Additionally, reference 3 states that the not H, production rates are a linear function of the absorbed

A-5 energy calculated as E,-E, where E, = E + E. + E, = 2.23E11 eV/cm*-sec. In this case, E, is greater and thus there is no not H, production rate.

Conclusion The staff believes that radiolysis was not a significant contributor to the H, generation at Point Beach. The above calculations are based on experimental results that qualitatively show that net H, generation rates should be small. Significant research and/or experimentation would be necessary to quantify the not H, production in a spent fuel cask. This research is not recommended because other causes of H, generation were identified at Point Beach and all cask users have been requested to demonstrate safety in response to NRC Bulletin 96-04.

REFERENCES

1. R. G. Sowden, "Radiolytic problems in Water Reactors", J. Nucl. Material.,B (1963) p. 81

2. E.J. Hart, W. R. McDonnell, and S. Gordon, "The Decomposition of Light and Heavy water Boric Acid Solutions by Nuclear Reactor Radiations," Proceedings of the First U.N.

International Conference on Peaceful Uses of Atomic Energy, Geneva,1955, Vol. 7, p. 593

3. H. Etherington, editor, Nuclear Enaineerina Handbook.1958, p.10-126, McGraw Hill Co.,

Contribution from V.P. Calkins, " Radiation Damage to Liquids and Organic Materials" j I

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