Criticality Control Factors

DOE-STD -1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

and alarm systems. Paragraphs 4.1.2, 4.2.1 and 4.2.2 shall be followed as modified in Section

4.3.3.c and e of DOE 420.1. ANSI/ANS- 8.7 - 1975, R87, Guide for Nuclear Criticality Safety in

the Storage of Fissile Materials and ANSI/ANS- 8.15 - 1981, R87, Criticality Safety Control of

Special Actinide Elements provide additional guidance.

c. ANSI/ANS-8.19, Administrative Practices for Nuclear Criticality Safety (ANSI 1984). This

standard provides the elements of an acceptable nuclear criticality safety program for operations

outside reactors.

d. ANSI/ANS-8.20, Nuclear Criticality Safety Training (ANSI 1991). This standard provides the

criteria for the administration of a nuclear criticality safety training program for personnel who

manage, work in, or work near facilities, or work outside of reactors, where the potential exists

for nuclear criticality accidents. This standard does not meet the training needs of nuclear

criticality safety personnel.

e. ANSI/ANS-10.3, Guidelines for the Documentation of Digital Computer Programs (ANSI

1986b). This standard presents guidelines for documenting computer codes (i.e., user

documentation) for engineering and scientific applications.

ANSI/ANS-10.4, Guidelines for the Verification and Validation of Scientific and Engineering

Computer Programs for the Nuclear Industry (ANSI 1987c). The objective of this standard is to

identify processes that will enhance the reliability of computer codes used in the nuclear industry

and reduce the risk of incorrect application.

7.2 CRITICALITY CONTROL FACTORS

As noted in ANSI/ANS-8.1, the critical mass is a function of the radionuclides in the material as well

as its density, chemical and physical form, shape, and surroundings (i.e., moderators, reflectors, neutron

absorbers). Nuclear criticality safety is achieved by controlling the quantity and distribution of fissionable

materials and other materials capable of sustaining a chain reaction and the quantities, distributions, and

nuclear properties of all other materials with which fissionable materials are associated. For new facilities,

DOE requires that design considerations for establishing the controls shall include mass, density, geometry,

moderation, reflection, interaction, material types, and nuclear poisons (neutron absorbers). Passive

engineered controls such as geometry control is the preferred method. The use of administrative controls is

to be minimized.

Nuclear criticality control factors can be classified as engineered (e.g., geometry controls and

volume controls) or administrative (e.g., mass limits and operating procedures).

7.2.1 Controllable Factors

Some of the criticality safety controls used to prevent a nuclear criticality accident are described

below.

7.2.1.1 235U Enrichment

Enriched uranium is normally required to provide sufficient fissionable material to sustain a critical or

sustained nuclear reaction in a small enough mass to meet the needs of the system. Handling of natural

(0.7% 235U) or depleted (<0.2% 235U) uranium is generally safe at DOE uranium-processing

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