WSRC-MS-95-0006

Criticality Safety Evaluation of Mixtures Containing Arnericium and Curium (U)

by R. R. Rahn Westinghouse Savannah River Company Savannah River Site Aiken, South Carolina 29808

A document prepared for INTERNATIONAL CONFERENCE ON NUCLEAR CRIT1CAUTY SAFETY/ANS at Albuquerque from 09/17/95-09/22/95. •••-.-'

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Criticality Safety Evaluation of Mixtures Containing Americium and Curium

Robert R. Rahn (Westinghouse Savannah River Company) (Savannah River Site, Aiken, SC) Present Address: Martin Marietta Energy Systems P.O. Box 2009 Y-12 Plant, Building 9110, MS 8238 Oak Ridge, IN 37830-8238 615-574.3261 615-241.2772- FAX [email protected] (For attribution at ICNC-95, September 1995)

/ tank, containing dilute solutions of metal r*'-t.^OTe..s This paper provides a summary of an ""*akrfctes v The mass for some of the NCSE (nuclear criticality safety evaluation) fissionable isotopes contained in this F [1] which was performed to establish Caayon tank is provided in Table I. The conditions for which the contents of an F amounts of curium isotopes, Cm-242 and Canyon tank would remain subcritical. The Cm-243, are very low and are assumed to tank contains fissionable isotopes of be zero. Uranium (primarily U-238), iron, americium, curium, plutonium and: the lanthanides, and some other elements uranium. The evaluation addressed storage identified in the tank were assumed to have of the 'tank contents and provided a zero concentration for the criticaiity information relative for processing to safety calculations. Briefly, the isotopes1' recover or to dispose of the americium and , considered in the analysis are: curium isotopes. Calculations were 10.2 kilograms Am, performed using the Savannah River Site JOSHUA 1-70 version modules of 2.7 kilograms Cm, and GtASS-ANISN |2] for the determination 2.5 kilograms Pit, of neutron multiplication constants, fc^ and Am-243 accounts for more than half of k^jv and subcritical mass of various Am, the fissionable, isotopes in the tank. Cm, and Pu homogeneous water mixtures. However* Cm-245 is the dominant fissile INTRODUCTION isotope, with the tank containing slightly more than three times its safe mass. But F Canyon tank number 17.1 is a large Cm-245 accounts for only 98 grams (0^4 10 foot diameter by 11 foot high process weight percent) of the total 15.4 kilograms Hansen-Roach cross sections, is usually of fissionable metal isotopes. used for criticality calculations. However, The small amount of material needed to nuclear cross section data for the americium form a critical mass is an interesting aspect and curium isotopes are not included in the of this work. Specifically, mass limits from HRXN module, resulting in the use of the ANSI/ANS-8.15 [3] are 13 grams for Am- GLASS cross sections which were 242m and 30 grams for Cm-245, which are developed mainly for use in analysis of considerably smaller than the 450 gram SRS reactors, limit for the more common fissile isotope GLASS provides the nuclear cross Pu-239. section data for a given mixture MODEL composition. Atom densities are calculated,

macroscopic cross sections and kinf are The model assumes the limiting calculated using the B1 method. The condition is precipitation of the metals into GLASS STANDARD version, 84 group, the form of a water reflected sphere cross section library was used, and containing either the metal Am and Cm collapsed to approximate the 16 group mixture or the Am, Cm and Pu mixture, Hansen-Roach structure.. The macroscopic whichever mixture is most reactive. A cross sections are input to ANISN which water reflector of 30 cm thickness is used, was used to determine the subcritieal radius because a metal reflector is not considered for a sphere of fissionable material. The credible for this spherical model. Other subcritical mass limits were based on a k.ff key assumptions are 1) that americium will of 0.90, rather than 0.95, to account for the not separate from curium under any laGk of critical experiments for code credible tank conditions, which has been validation with the americium and curium justified, and 2) the ratio of fissionable cross sections. ANISN is a one isotopes remains fixed, except that it is dimensional discrete ordinates neutron acceptable to account for alpha decay of transport code. S-16 quadrature and first Cm-244 to Pu-240. order scattering coefficients were used for Calculations were performed using the all calculations except that S-4 quadrature JOSHUA J-70 version modules of GLASS- was used for a validation check described ANISN [2] for the determination of neutron below. multiplication constants, k and ^ , and inf r RESULTS subcritical mass of various Am, Cm, andPu homogeneous water mixtures. Savannah Selected results of GLASS-ANISN River Site main frame computers, IBM calculations of neutron multiplication constants, k and 1^, and subcritical mass 3090 and IBM 3081, were used. The inf HRXN module, containing the 16 group of various Am, Cm, and Pu homogeneous water mixtures are summarized.; SEP-07-I39S- «te--i«i Sltod •

Illustrations are included as Figures 1, 2, on this curve are at 12 grams Cm-243/iiter and 3, and are discussed further below. and at 482 grams Cm-245/Uter (a dry metal Validation Check/ Cm-245Solutions: at 13500 grams Cm/liter). The respective subcritical masses (k^ « 0.90) are 44.8 The dominant fissile isotope in the grams Cm-245 and 114.8 grams Cm*245. evaluation is Cm-243, Standard These calculations for Cm water mixtures ANSI/ANS-8.I5 [3], table 2, indicates 30 are only important to show the trend of grams as a subcritical mass limit for this subcritical mass with concentration. They, isotope* and Appendix B indicates 41 are not used to directly determine any mass grams as minimum computed critical mass limits because of the key assumption that at a concentration of about 12 g/L Clark [4] americium will always remain with the reports a calculated 41.2 grams for the curium. minimum critical mass of Cm-245 using similar Q1AS3 cross section^. / As a check on me GLASS-ANISN calculations used in this evaluation'.,'Cm--/ 245 minimum critical mass was calculated for concentrations from 8 ,g/l to 18 g/L Results are plotted in Figure 1. The mass at kisf for Mixtures of Cm, Am, and Pu:

12 g/I is approximately 1,3 grams higher kw (the neutron multiplication constant than the 41.2 grams reported by Clark* for an infinite system) is shown in Figure 3 , This difference is because Clark's for Cm, Am, and Pu mixtures, Cm and Am 'calculation used S-4 quadrature. This mixtures, and for Cm mixtures, Cm-245 is calculation was repeated also using the S-4 in all of the mixtures. Therefore, the Cm- quadrature, resulting in 41.2 grams as me 245 concefltraioh provides a common x minimum critical mass. This agrees very axis for all mixtures. The compositions well with that reported by Clark* Note that used in the calculations have an Am:Cm:Pu there h no difference between critical ratio consistent with that of me actual masses which are modeled as a metal solution composition in Tank 17.1. Note versus those modeled as an oxide for a that the americium poisons the fissile Cm- solution at 12 g/1, as is expected (both are., 245 in the 12 gram/liter concentration 42,5 grams using S-16 quadrature). range, k^ is also shown for a fourth mixture (Fiss/Am243) which consists only Subcriticai Mass for Aqueous Mixtures of fissile isotopes and Am-243, and is of Cm: provided to demonstrate that tank materials G£ASS-ANISN calculations were used are safe with alpha decay of Cm-244. to develop me plots ofsubcritical mass for . Calculated values for k at Cm water mixtures in Figure 2. Key points w

TOTAL P.04 concentrations of 12 and 30 grams Cm- 1) Use of 0.90 kefT as the safety factor, 245/liter are summarized below. 2) Metal densities are assumed for the precipitated product, Mixtures •^inf *Hnf at 12 g/1 at 30 g/1 3) Precipitated product was assumed to Cm 2,1 ••' 2.3 accumulate with the geometry of a sphere, Cm-Am 0.37 0.63 and Cm-Am-Pu 0.35 0.72 Fiss/Am243 0.46 0.52 4) No credit was taken for neutron absorption by uranium, nitrogen, iron, Subcriticai Mass for Dry Am/Cm/Pu: lanthanides and other non fissionable elements in Tank.1.7.1. The limiting condition was determined to be precipitation in the form of a water- REFERENCES reflected sphere containing a dry metal 1. R. R. Rahn, "Nuclear Criticality mixture of americium, curium, and Safety Evaluation for F Canyon Tank 17.1 plutonium. This subcriticai mass is 148.2 Containing Americium and Curium (IT)", grams Cm-245, and is less than the NMP-NCS-94-0090 (5/23/94). calculated subcriticai mass for a dry Am- 2. H. K. Clark, "User's Manual - Cm metal mixture of 174.6 grams Cm- JOSHUA Nuclear Criticality Safety 245). Not, all of the calculations for Modules", DPSTM-86-700-3 (March justifying this conclusion are provided in 1987). this paper, but it makes sense that the addition of Pu (primarily Pu-240) would 3. American National Standard for increase fast fission reactivity because it Nuclear Criticality Control Special Actinide displaces Am-243 which has a larger Elements, ANSl/ANS-8.15-1981. subcriticai mass than Pu-240, according to 4. H. K. Clark, "Subcriticai Limits for ANSI/ANS-8.15. Special Fissile Actimdes", NUCLEAR CONCLUSION TECHNOLOGY, vol 48, pp 164-170 (1980). The calculations demonstrate that the contents of the tank remain subcriticai under all credible storage conditions. The tank contains less than 67 percent of the calculated maximum subcriticai mass (148 grams Cm-245 or 23.3 kilograms Am, Cm and Pu metal). Significant conservatism includes the following: Table I Tank 17.1 Isotopes and Half Lives

Mass, grams * Half Life * Decay Mode Am-241 458 458 yr. alpha Am-242m 2 152 yr. IT(99.52%),alpha Am-242 0 16 hour Beta-(82%),EC Am-243 9701 7,370 yr. alpha (total) 10161

Cm-242 0 163 days alpha Cm-243 0 32 yr. alpha(99.7%),EC Cm-244 2557 17.6 yr. alpha,SF Cm-245 98 9,300 yr. alpha Cm-246 85 5,500 yr. alpha(79%),SF Cm-247 1 16Myr. alpha (total) 2741

Pu-238 145 , 86yr. alpha,SF Pu-239 62 : 24,400 yr. alpha,SF Pu-240 2165 6580 yr.^ alpha,SF Pu-241 1 13.2 yr beta-(99+%),alpha Pu-242 124 379,000 yr. alpha,SF (total) 2497

* From a Handbook of Chemistry and Physics Figure. 1

Mimimum Critical Mass versus Cm-245 Concentration

46 Glass/ANISN Calculation for homogeneous Cm-245 metal-water mixtures

45

o 44 E O o to f= 43

42 41.2 gram reference

41 -*—-i 1- -t- 'I ' I ••-. I •• 6 8 10 12 . 14 16 18 20

Concentration, grams of Cm-245/liter Figure 2

Subcriticai Mass versus Cm-245 Concentration

Keff is calculated to be 0.90 for a water reflected sphere containing the homogeneous Cm-water mixture.

The subcriticai mass is that mass of Cm-245 for a mixture containing Cm isotopes in the same ratios as are in Tank 17.1.

4- 100 200 300 400 500 Concentration, grams of Cm-245/liter Figure3

Kinf versus Cm-245 Concentration

o.o T ' 1—' I ' 1—* 10 20 30 40 50 60 70 Concentration, grams of Cm-245/liter