Operations with fissile materials may be performed safely by complying with any one of the subcritical limits given in Sec. 8.2 provided the conditions under which it applies are maintained. A limit shall be applied only when the effects of neutron reflectors and of other nearby fissionable materials are no greater than reflection by an unlimited thickness of water, t The limits shall not be applied to mixtures of 235U, and 233U, and 239Pu.

Process specifications shall incorporate margins to protect against uncertainties in process variables and against a limit being accidentally exceeded

Uniform aqueous solutions. Any one of the limits of Table 10.25 is applicable provided a uniform aqueous solution is maintained and provided, for 239Pu, at least four nitrate ions are present for each plutonium ion. The 239 Pu limits apply to mixtures of plutonium isotopes provided the concentration of 240Pu exceeds that of 241 Pu and provided 241 Pu is considered to be M9Pu in computing mass or concentration.

Uniform slurries. The limits of Table 10.25 may be used for macroscopically uniform slurries, provided:

1. There are at least four nitrate ions intimately associated with each plutonium atom, and

2. For the dimensional and volume limits, the ratio of hydrogen-to-fissionable material does not exceed that in an aqueous solution having the same concentration of fissionable material.

^Section 8.2 is taken verbatim from Sec. 5 of [A4] except for footnotes and changes in references to tables, sections, and literature citations. Extracted from American National Standard N-16.1-1975 (ANS-8.1), with permission of the publisher, the American Nuclear Society.

*The limits do not apply to reflection by graphite, beryllium, or heavy water.

The limit on the 1.00 wt % enrichment of uranium is valid only for the slurries in which the ratio of surface-to-volume of the particles is at least 80 cm"1.

Nonuniform slurries. The limits on cylinder diameter and slab thickness in Table 10.25 may be used for nonuniform slurries provided:

1. Four nitrate ions are intimately associated with each plutonium atom,

2. The restriction on the ratio of hydrogen-to-fissionable atoms, specified in Condition 2 for uniform slurries is met everywhere throughout the system,

3. For cylinders, the concentration gradient is only along the length, and

4. For slabs, the concentration gradient is only parallel to the faces.

For 239Pu in the absence of nitrate ions, but with the proviso that no localized regions of density greater than 0.25 g of 239Pu/cm3 are permitted, limits of 15.1 and 5.4 cm on cylinder diameter and slab thickness, respectively, are applicable under Conditions 2, 3, and 4 above.

The areal densities given in Table 10.25 are valid for nonuniform slurries provided these densities are uniform.

The subcritical mass limits for 235U, 233U, and 239Pu in nonuniform slurries are 0.70, 0.52, and 0.45 kg, respectively. Nitrate ions need not be present.

Metallic units. The enrichment limit for uranium and the mass limits given in Table 10.26 apply to a single piece having no concave surfaces. They may be extended to an assembly of smaller units provided there is no inter-unit moderation.

The 235 U and 233 U limits apply to mixutres of either isotope with 234 U, 236 U, or 238 U provided all isotopes except 238 U are considered to be 235 U or 233 U, respectively, in computing mass. The 239Pu limits apply to isotopic mixtures of plutonium provided the concentration of ^Pu exceeds that of 241 Pu, all plutonium isotopes are considered to be 239 Pu in computing mass, and no more than 1% 238Pu is present.

550 NUCLEAR CHEMICAL ENGINEERING Table 10.26 Single-parameter limits for metal units