Large Storage Arrays - h3010v10248

DOE-HDBK-3010-94

6.0 Inadvertent Nuclear Criticality; Total Fission Yield

The range of the eleven excursions is from 3E+15 to 4E+17 total fissions. All the

excursions are bounded by 4 yields at or around 1E+17 (1E+17 to 4E+17 fissions) and

involved large masses of metal (47 to 96 kg U) well beyond the water-moderated spherical

critical mass. The highest yield is from a mechanical failure. The remaining 3 yields are

1E+17, 1.2E+17 and 1.5E+17 fissions. It appears adequately conservative to assess

1E+17 fissions as the bounding reference total fission yield for dry solid systems.

6.2.3.4

L arge S torage A rrays

As McLaughlin (1991) states "... common sense storage practices and case-specific analysis

should lead to the conclusion that either the critical state cannot be credibly reached or, if the

upset condition is so severe that criticality cannot be precluded, then the consequences of the

criticality accident are minor compared to the total accident consequences. Under no

circumstances can an accident scenario be envisioned which would incorporate the

simultaneity, speed, and neutron source requirements which would lead to anything

approaching the '3 X 1022 fissions' and 'serious explosion' Woodcock proposes...The

fundamental storage practice for unmoderated fissile material should be a maximum effective

density, i.e. the fissile mass divided by the outer container volume, which does not exceed

about 1 kg/liter."

Excursions in reactor and reactor experiments listed in Table 6-2 (Nyer, Bright and

McWhorter, 1965) and Tables 6-3 & 6-4 (Table I - Inhomogeneous Water-Moderated and

Miscellaneous Systems, Stratton, Sept 1967) are considered partially representative of

potential excursions in an array. The total fission yields range from 3E+16 to 1.2E+20 for

the nine inhomogeneous water-moderated excursions listed in Table 6-3 (Stratton, September

1967) and 3.3E+17 to 4.7E+18 for the reactor excursion experiments covered in Table 6-2

(Nyer, Bright and McWhorter, 1965). A value of 1E+20 (rounding the coefficient to a

single digit) bounds the reported values and is assessed to be the bounding reference yield.

This value is generally considered only applicable to reactor fuel arrays, typically with

moderation. For analytical purposes, it is not considered representative of uranium or

plutonium vault storage conditions, even under severe stress conditions such as earthquakes.

6.3

M A T E R I AL R E L E A S E I N C R I TI CA L I TY E X C U R S I ON S

The radionuclides generated by an excursion are a function of the fissionable material

undergoing the reaction. The quantity of each fission product or actinide formed is a

function of the total fission yield. Reference values for the various systems were designated

in the previous section. 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 regulatory guides do

Page 6-16