Fully Moderated and Reflected Solids

DOE-HDBK-3010-94

6.0 Inadvertent Nuclear Criticality; Total Fission Yield

The 100 liters is a general value assumed for release fraction purposes, and is not assigned

any specific technical meaning for real systems.

6.2.3.2

F u lly M od erated an d R eflected S olid s

McLaughlin (1991) discusses the basis for the total fission yield reported by Woodcock

(3E+20 fissions) that is based upon an autocatalytic phenomena ("... dry powder becomes

flooded, goes prompt critical as an equivalent very rich solution, and then the mixing and

dilution which accompany the excursion introduces additional reactivity since one is sliding

down the critical mass versus concentration curve.") Competing negative feedback

mechanisms (e.g., thermal expansion and microbubble formation) are ignored. Woodcock

admits "this estimate is rather a shot in the dark." The possibility of positive feedback

during dilution of rich solutions is considered by Stratton but "it is difficult to imagine an

explosive reaction". A prompt total fission yield in a few hundred liters of liquid would be a

significant explosion. McLaughlin (1991) cites the Woodriver Junction criticality as most

closely approximating this situation with a total fission yield of 1E+17 fissions.

For water-moderated metal pieces, McLaughlin (1991) notes that Woodcock does not provide

a discussion on the basis for prompt yield of 3E+18 and total yield of 1E+19. Instead of the

values referring to a "system of small pieces of fissile material" as indicated in the footnotes

to the Table 6-5, the values are for "1 or a small number of pieces".

The three historical excursion listed in Olsen et al. (July 1974) are bounded by a value of

3E+16 total fissions. Given the types of situation encountered in DOE facilities where it is

difficult to accumulate the quantity of materials required, contain the material and moderator,

and assume any shape that would be unfavorable, a reference value of 1E+18 fission in a

single burst is assessed to be the bounding reference value and is believed to be very

conservative.

6.2.3.3

D ry P ow d er an d M etal

"Criticality accidents with solid metal systems (including alloys) should be readily controlled

at a likelihood of occurrence that is vanishingly small. ... Only rarely are there operational

requirements which necessitate working with more than the water reflected spherical critical

mass (233U 6.0 kg, 235U 20.1 kg, 239Pu 5.0 kg) ... " (McLaughlin, 1991). Eleven excursions

are listed for Metal Systems in Olsen et al. (July 1974). All the historical excursions have

taken place in experimental systems. Five resulted from mechanical failures that increased

the reflection in the system. Three resulted from incorrect operations, and three resulted

from computational errors. None are especially representative of anticipated accidental

conditions, and are likely to be conservative representations of expected conditions.

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