Inadvertent Nuclear Criticality

DOE-HDBK-3010-94

6 .0 IN A D V E R T E N T N U C L E A R C R IT IC A L IT Y

6.1

S U M M A R Y O F B OU N D I NG R E L E A S E E S T I MA T E S

Under appropriate accident conditions, fissile and fissionable radionuclides may undergo a

self-sustaining nuclear reaction (chain reaction) called an inadvertent nuclear criticality. The

initial airborne release from a nuclear criticality is estimated by use of relevant factors of the

five-component linear equation used to estimate the airborne release from other events

covered in the previous chapters. However, because the evaluation of nuclear excursions is a

complex process, some additional topics used in the equation are discussed below.

For nuclear criticalities, the MAR is determined by the fraction of fission products generated

by the criticality and the fraction of the fissile/fissionable material that may be suspended by

the event generated conditions (primarily heat) [Note: since fissile materials [233U, 235U,

239

Pu] are also fissionable, both will be referred to as fissionable]. The amount of fission

products and actinides produced by the excursion is a function of the total fissions from the

criticality and the specific fissionable radionuclide involved. The fraction that is at-risk of

airborne suspension depends upon the physical form of the fissionable materials involved.

Estimation of fission product quantities can be done with computer codes, such as

ORIGEN2, or by simple ratios (total fissions for scenario/1E+19 fission) to values in NRC

regulatory guides prepared for fuel cycle facilities.

The airborne release from nuclear excursions in various physical systems can be estimated

for general purposes using the equations which follow. Unless otherwise noted, all

respirable fractions are equal to 1.0 and are not specifically mentioned. The physical

systems considered are: (1) solutions; (2) fully moderated/reflected solids; (3) bare, dry

solids; and (4) large storage arrays. Conservative fission yields are assigned to generate

maximum quantities of fission products, but physical effects to the fissionable material have

not been evaluated to the same degree of conservatism to avoid implying the need for

analytical calculations of severity exceeding a considerable historical record and contrary to

generally accepted consensus. Meaningful enhancements in criticality safety are more

appropriately accomplished through required criticality safety evaluations in accordance with

guidelines such as DOE-STD-3007-93, "Guidelines for Preparing Criticality Safety

Evaluations at Department of Energy Nonreactor Nuclear Facilities," than by speculative

consequence modelling.

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