| CLOSING THE SWU BALANCE
ON A RANDOM BASIS Marco
Marzo, Hugo Vicens
Definition of the Problem
The safeguards’ objectives for enrichment facilities encompass the
detection of the diversion of declared nuclear material and facility misuse.
Diversion scenarios on declared nuclear material include inflating the MUF
and or concealment of diverted material introducing gross, partial or bias
defect into uranium or isotopic content in some items, complemented with
data falsification.
Relevant scenarios of facility misuse include the production of high-enriched
uranium (HEU) or low-enriched uranium with enrichment higher than 5%; and
undeclared production of low enriched uranium with enrichment lower than
5%.
While the installed separative work capacity and the throughput are small,
misuse scenarios are the dominant concern from the safeguards point of view,
particularly those associated with feeding the plant with undeclared LEU.
One on the main misuse scenario at centrifuge enrichment facilities is
the SWU deviation. The operator declares that has used X SWU/y, but effectively
he used Y<X for production of declared enriched uranium and uses the
difference X-Y to enrich undeclared uranium. That means the operator can
divert SWUs. Swipe environmental sampling is presently the most powerful
tool to confirm the absence of unrecorded production of direct use material.
The verification of the SWU balance, associated with U and U-235 mass balance,
is another relevant measure to detect SWU diversion. The traditional approach
for enrichment facilities foresees the closing of the SWU balance once per
year, at the end of the Material Balance Period, at the Physical Inventory
Verification. This has two disadvantages: the uncertainty associated with
the SWU closing is relatively high because it is performed only once per
year and this scheme does not introduce any deterrence into the verification.
The introduction of enrichment and flow monitors on line in the headers
would be a measure to confirm the SWUs that are really used in the enrichment
process. Such monitors however are under development and not yet available
for safeguards purposes. As an alternative measure to the enrichment and
flow monitor, a scheme of closing periodically the SWU balance at randomly
selected opportunities is proposed.
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