1

 Table 1: Radionuclides of Concern

Radionuclide Quantity of

Concern1

 (TBq)

Quantity of

Concern2 (Ci )

Am-241 0.6 16 

Am-241/Be 0.6 16 

Cf-252 0.2 5.4 

Cm-244 0.5 14

Co-60 0.3 8.1

Cs-137 1 27

Gd-153 10 270

Ir-192 0.8 22

Pm-147 400 11,000

Pu-238 0.6 16 

Pu-239/Be 0.6 16 

Se-75 2 54 

Sr-90 (Y-90) 10 270 

Tm-170 200 5,400 

Yb-169 3 81

Combinations of radioactive

materials listed above3

See Footnote

Below4

1

 The aggregate activity of multiple, collocated sources of the same radionuclide should be included when the total activity

equals or exceeds the quantity of concern.

2

 The primary values used for compliance with this Order are TBq. The curie (Ci) values are rounded to two significant

figures for informational purposes only.

3 Radioactive materials are to be considered aggregated or collocated if breaching a common physical security barrier (e.g.,

a locked door at the entrance to a storage room) would allow access to the radioactive material or devices containing the

radioactive material.

4

 If several radionuclides are aggregated, the sum of the ratios of the activity of each source, i of radionuclide, n, A(i,n), to the

quantity of concern for radionuclide n, Q(n)

, listed for that radionuclide equals or exceeds one. [(aggregated source activity

for radionuclide A) ÷ (quantity of concern for radionuclide A)] + [(aggregated source activity for radionuclide B) ÷ (quantity of

concern for radionuclide B)] + etc........ >1
2

Use the following method to determine which sources of radioactive material require increased

controls (ICs):

! Include any single source equal to or greater than the quantity of concern in

Table 1

! Include multiple collocated sources of the same radionuclide when the combined

quantity equals or exceeds the quantity of concern

! For combinations of radionuclides, include multiple collocated sources of

different radionuclides when the aggregate quantities satisfy the following unity

rule: [(amount of radionuclide A) ÷ (quantity of concern of radionuclide A)] +

[(amount of radionuclide B) ÷ (quantity of concern of radionuclide B)] + etc.....> 1
3

Guidance for Aggregation of Sources

NRC supports the use of the IAEA’s source categorization methodology as defined in

TECDOC-1344, “Categorization of Radioactive Sources,” (July 2003) (see http://wwwpub.iaea.org/MTCD/publications/PDF/te_1344_web.pdf) and as endorsed by the agency’s

Code of Conduct for the Safety and Security of Radioactive Sources, January 2004 (see

http://www-pub.iaea.org/MTCD/publications/PDF/Code-2004_web.pdf ). The Code defines a

three-tiered source categorization scheme. Category 1 corresponds to the largest source

strength (equal to or greater than 100 times the quantity of concern values listed in Table 1.)

and Category 3, the smallest (equal or exceeding one-tenth the quantity of concern values

listed in Table 1.). Increased controls apply to sources that are equal to or greater than the

quantity of concern values listed in Table 1, plus aggregations of smaller sources that are equal

to or greater than the quantities in Table 1. Aggregation only applies to sources that are

collocated.

Licensees who possess individual sources in total quantities that equal or exceed the Table 1

quantities are required to implement increased controls. Where there are many small (less

than the quantity of concern values) collocated sources whose total aggregate activity equals or

exceeds the Table 1 values, licensees are to implement increased controls.

Some source handling or storage activities may cover several buildings, or several locations

within specific buildings. The question then becomes: When are sources considered 

collocated for purposes of aggregation? For purposes of the additional controls, sources are

considered collocated if breaching a single barrier (e.g., a locked door at the entrance to a

storage room) would allow access to the sources. Sources behind an outer barrier should be

aggregated separately from those behind an inner barrier (e.g., a locked source safe inside the

locked storage room). However, if both barriers are simultaneously open, then all sources

within these two barriers are considered to be collocated. This logic should be continued for

other barriers within or behind the inner barrier. 

The following example illustrates the point: A lockable room has sources stored in it. Inside the

lockable room, there are two shielded safes with additional sources in them. Inventories are as

follows:

The room has the following sources outside the safes: Cf-252, 0.12 TBq (3.2 Ci); Co60, 0.18 TBq (4.9 Ci), and Pu-238, 0.3 TBq (8.1 Ci). Application of the unity rule yields:

(0.12 ÷ 0.2) + (0.18 ÷ 0.3) + (0.3 ÷ 0.6) = 0.6 + 0.6 + 0.5 = 1.7. Therefore, the sources

would require increased controls. 

Shielded safe #1 has a 1.9 TBq (51 Ci) Cs-137 source and a 0.8 TBq (22 Ci) Am-241

source. In this case, the sources would require increased controls, regardless of

location, because they each exceed the quantities in Table 1. 

Shielded safe #2 has two Ir-192 sources, each having an activity of 0.3 TBq (8.1 Ci). In

this case, the sources would not require increased controls while locked in the safe. 

The combined activity does not exceed the threshold quantity 0.8 TBq (22 Ci).

Because certain barriers may cease to exist during source handling operations (e.g., a storage

location may be unlocked during periods of active source usage), licensees should, to the

extent practicable, consider two modes of source usage — “operations” (active source usage)
4

and “shutdown” (source storage mode). Whichever mode results in the greatest inventory

(considering barrier status) would require increased controls for each location.