Title of Presentation: Criticality Measurements for SNM Accountability

Authors: Joetta Bohman, E. Ray Martin, Ken Butterfield, Richard Paternoster

Institution: Los Alamos National Laboratory

FAX: 505-665-3657

Phone: 505-665-0449 (Ray Martin)

E-mai I : raymartinalanl. gov

Abstract:

Based on extensive operating experience with the Godiva IV fast metal burst

assembly at Los Alamos National Laboratory, we were able to create data plots for

reactivity worths of standard configurations at various temperatures and room return

locations. These plots show that the material uncertainties in criticality measurements are

within f 20 grams out of the 65.4 kilogram HEU Godiva core. This is superior to active

neutron well coincidence counter (AWCC) measurements. The criticality measurements

have the additional advantage of not requiring disassembly of the reactor. No

disassembly means the measurement takes less time -- it can be done during each

operation -- and there is less dose to measurement personnel.

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty. txprcss or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or USC- fulness of any information, apparatus, product, or process disclosed, or represents that its w would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark manufac- turer, or otherwise d m not necessarily constitute or imply its endorsement, rtcom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expmsed herein do not neassarily state or reflect those of the United States Government or.any agency thereof.

Criticality Measurements for SNM Accountability

Fast metal critical assembly machines contain large quantities of highly attractive

SNM which must be accounted for as with other SNM. However, the presence of SNM in an assembly creates unique challenges. There is a need to detect diversion of the

material in a timely manner, without disassembling the machine, and while avoiding high

doses to personnel. Criticality measurements at a standard configuration for each

machine can meet these challenges. These measurements are often called the PHYS

method (for “PHYSICS” in the U.S. and “Physical Weighing Method” in Russia).

This method is illustrated with the Godiva-IV fast metal assembly at Los Alamos

National Lab, although the technique is applicable to a wide variety of critical assemblies

and pulse reactors. Godiva is fueled with 65.4 kg of 1.5-wt% molybdenum-

uranium(93.2% 235U). Figures 1 and 2 show Godiva data over the past two years. The

weighted least squares line fit ( )corresponds to the full 65.4 kg of Godiva

fuel. The first set of dashed lines (-------- ) represents a * 10 g uncertainty. This

corresponds approximately to the 99% confidence interval ( ........................................ ). The next set

of dashed lines (------------) corresponds to an uncertainty o f f 20 g. The majority (>

95%) of points lie within 20 g of the expected mass.

Figures 1 and 2 show that ten grams of material correspond to 2.2 cents of reactivity because four inches of control rod have a measured worth of approximately

$1.60 (25 mills/cent average over length of travel), control rods are 0.86 inches in

diameter, and the nominal density of the uranium is 19.1 g/cm3. Therefore, one gram of

Godiva fuel has a reactivity worth of 0.22 cents.

When a point falls outside of the anticipated region, the operator would shut down

the assembly, do a visual inspection, and perform another measurement. After a series of

measurements (-10) over the course of a week, it should be readily apparent whether the

point was a statistical outlier or the result of tampering with the machine.

The Godiva machine is operated at various physical locations within the room in

order to accommodate various experimental setups. Figure 1 shows data for Godiva in

position 2, near the center of the room. Figure 2 shows data for Godiva in position 6, near a shielding wall. Position, and the corresponding room return, make a considerable

difference in reactivity. For example at 25"C, Godiva has an excess reactivity of 34 cents

in position 2 but 40 cents in position 6. Temperature also has a predictable and

reproducible effect on criticality measurements. In position 2 the temperature coefficient

of reactivity is -0.43$/"C; in position 6 it is -.32$/"C.

Comparable methods to criticality measurements such as active neutron well

coincidence counting (AWCC) give measurements with a resolution of, at best, f 20 g

per piece. Since a critical assembly consists of multiple pieces-Godiva, for example,

has six rings, three rods, and a safety block-the combined uncertainty is much greater. The advantages of using criticality measurements extend far beyond the accuracy

obtained. The measurements can be made quickly and therefore frequently. Because this

method relies heavily on a known standard configuration of he1 and reflector, a visual

inspection is performed before each operation. The assembly is then placed in a standard

configuration and data is taken at the start of each Godiva operation, typically one per

week. The measurement takes less than half an hour. Any measurement that required

disassembly would take several days to a week and could only be done a few times a year

at most without seriously limiting the ability to perform experiments on a regular basis.

After each measurement where the assembly had been taken apart, a time

consuming series of calibration measurements would need to be taken to reestablish

operating parameters because the reproducible geometry may have been affected.

There is no additional equipment required to make the criticality measurements.

Verification is based on known reactivity curves. AWCC requires the purchase or use of a rather large well counter to accommodate the size of the largest assembly pieces.

Using criticality measurements also limits the total dose to workers since the

machine is operated remotely. If the machine were disassembled, there would be a large

dose to the measurement personnel due to fission products and the activation of the

machine. There would also be contamination of the measurement area from uranium

oxide flaking off as the components were moved. Criticality measurements for SNM accountability have many advantages over

other methods including increased accuracy, higher frequency of measurements, time

savings, and dose savings. The method is still meant to work in conjunction with other

MPC&A measures such as physical security, access control, and routine inventory.

,

15

Reactivity vs. Temperature for Godiva in Position 2

I I I I I I I I

45

40

35

25

20

14 16 18 20 22 24 26 28 30 32

Temperature ("C)

+ 20 grams

+ 10 grams

65.4 kilograms

- 10 grams

- 20 grams

Figure 1 : Reactivity vs. Temperature for Godiva in Position 2

.

Reactivity vs. Temperature for Godiva in Position 6

50

45

40

35

30

b.

10 15 20 25 30 35 40

Tem pe ra t u re ("C)

+ 20 grams

+ 10 grams

65.4 kilograms

- 10 grams

- 20 grams

Figure 2: Reactivity vs. Temperature for Godiva in Position 6

.

Reactivity vs Temperature for Godiva

50

45

40

35

30

25

20

15 10

\ 0

15 20 25 30

Temperature (C)

-.32 $PC

-.43 $PC

35 40

Figure 3: Reactivity vs. Temperature for Godiva

50

45

40

35

30

25

20

Reactivity vs Temperature for Godiva

- Y

0 Position 6 1 Position 2

0

0

10 15 20 25 30

Temperature (C)

-.32 $PC

-.43 $PC

35 40

Figure 3: Reactivity vs. Temperature for Godiva

Report

DOE