Fuel element burnup measurements for the equilibrium LEU silicide RSG GAS (MPR-30) core under a new fuel management strategy

abstract
After the equilibrium LEU silicide core of RSG GAS was achieved, there was a strong need to validate the
new fuel management strategy by measuring burnup of fuel elements comprising the core. Since the reg-
ulatory body had a great concern on the safety limit of the silicide fuel element burnup, amongst the 35
burnt fuel elements we selected 22 fuel elements with high burnup classes i.e. from 20 to 53% loss of
U-235 (declared values) for the present measurements. The burnup measurement method was based
on a linear relationship between reactivity and burnup where the measurements were conducted under
subcritical conditions using two fission counters of the reactor startup channel. The measurement results
were compared with the declared burnup evaluated by an in-house in-core fuel management code,
BATAN-FUEL. A good agreement between the measured burnup values and the calculated ones was found
within 8% uncertainties. Possible major sources of differences were identified, i.e. large statistical errors
(i.e. low fission counters’ count rates), variation of initial U-235 loading per fuel element and accuracy of
control rod indicators. The measured burnup of the 22 fuel elements provided the confirmation of the
core burnup distribution planned for the equilibrium LEU silicide core under the new fuel management
strategy.