Criticality Safety Evaluation of Mixtures Containing Americium and Curium (0)

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

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

DOE Contract No. DE-ACO9-89SR18035 This paper was prepared in connection with work done under the above contract number with the U. S. Department of Energy. By acceptance of this paper, the publisher and/or recipient acknowledges the U. S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper, along with the right to reproduce and to authorize others to reproduce all or part of the copyrighted paper. I

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Available to the public from the National Technical Information Service, U. S. Dcpanmcnt of Commerce, 5285 Port Royal Rd.. Springfield. VA 22161 DISCLAIMER Portions of this document may be inegible in electronic image produch~ mes are produced from the best available original document. 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 &om HRXN module, resulting in the use of the ANSVANS-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 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 1 calculated using the B1 method. The condition is precipitation of the met GLASS STAND- version, 84 group, the form of a water reflected sphere cross section library was used, and containing either the metal and Cm Am collapsed to ximate the 16 mixture or the Cm and Pu mixture, Hmsen-Roac ture. The macroscopic whichever mi is most reactive. A cross sections are input to ANISN which water reflector of 30 cm thickness is used, was to determine the subcritical radius because a metal reflector is not considered used for a sphere of fissionable material. The credible for this spherical model. Other subcritical mas limits were based on a kff key assumptions are 1) that americium will of 0.90, rather than 0.95, to account for the not separate 'Om under any lack of critical experiments for code credible tank conditions, which has been validation with the americium and curium justified, and 2) the ratio of cross sections. ANISN is a one isotopes remains fixed, excep dimensional discrete ordinates neutron account for Of transport code. 5-16 quadramre and frst 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 de ation of neutron below. multiplication const inf and lgR, and , RESULTS - subcritical of various Am, Cm, and Pu homogeneousmass water mixtures. Savannah Selected results of GLASS-ANISN 3t River Site main frame computers, IBM calculations of neutron multiplication 3090 and IBM 3081, were used. The .constants, kinfand kR,and subcritical mass HRXN module, containing the 16 group of various Am, Cm, and Pu homogeneous water mixtures are summarized. i

concentrations of 12 and 30 grams Cm- 1) Use of 0.90 k,, as the safety factor, 24Yliter are summarized below. 2) Metal densities are assumed for the Mixtures kinf ' kinf precipitated product, at 12 g/1 at 30 g/l 3) Precipitated product was assumed to Cm 2.1 ' 2.3 accumulate with the geometry of a sphere, Cm-Am 0.37 0.63 Cm-Am-Pu 0.35 0.72 and Fiss/Am243 0.46 0.52 4) No credit was taken for neutron absorption by uranium, nitrogen, iron, Subcritical Mass for Dry Am/Cm/Pu: lanthanides and other non fissionable elements in Tank 17.1. 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 subcritical mass is 148.2 Containing Americium and Curium (U)", grams Cm-245, and is less than the NMP-NCS-94-0090 (5/23/94). calculated subcritical mass for a dry Am- 2. H. K. Clark, "User's Manual - Cm metal mixture of 174.6 Cm- grams JOSHUA Nuclear Criticality Safety 245). Not, all of the calculations for Modules", DPSTIk-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,,ANSI/ANS-8.15-1981. subcritical mass than Pu-240, according to 4. H. K. Clark, "Subcritical Limits for ANSVANS-8.15. Special . Fissile Aclinides", NUCLEAR CONCLUSION TECHNOLOGY, VO~48, pp 164-170 (1980). The calculations demonstrate that the contents of the tank remain subcritical under all credible storage conditions. The tank contains less than 67 percent of the calculated maximum subcritical mass (148 - grams Cm-245 or 23.3 kilograms Am, Cm e and Pu metal). Significant conservatism includes following:

Figure. 1

Mimimum Critical Mass versus Cm-245 Concentration

GlasslANiSN Calculation for homogeneous Cm-245 metal-water mixtures P

41.2 gram reference // rn I. I. 1.1 I I. 411 ' I ' I' 1'1 1 1 6 a 10 12 . 14 16 18 20

Concentration, grams of Cm-245Iliter