This paper reviews validation issues associated with implementation of burnup credit in transport, dry storage,
and disposal. The issues discussed are ones that have been identified by one or more constituents of the
United States technical community (national laboratories, licensees, and regulators) that have been exploring the
use of burnup credit. There is not necessarily agreement on the importance of the various issues, which
sometimes is what creates the issue. The broad issues relate to the paucity of available experimental data

From 1998 to 2004, a series of critical experiments referred to as the fission product (FP) experimental program was performed at the Commissariat à l'Energie Atomique Valduc research facility. The experiments were designed by Institut de Radioprotection et de Sûreté Nucléaire (IRSN) and funded by AREVA NC and IRSN within the French program supporting development of a technical basis for burnup credit validation.

This paper provides insights into the neutronic similarities between a representative high-capacity rail-transport cask containing typical pressurized water reactor (PWR) spent nuclear fuel assemblies and critical reactor state-points, referred to as commercial reactor critical (CRC) state-points. Forty CRC state-points from five PWRs were analyzed, and the characteristics of CRC state-points that may be applicable for validation of burnup-credit criticality safety calculations for spent fuel transport/storage/disposal systems were identified.

A conservative methodology is presented that would allow taking credit for burnup in the criticality safety analysis of spent nuclear fuel (SNF) packages. The method is based on the assumption that the isotopic concentration in the SNF and cross sections of each isotope for which credit is taken must be supported by validation experiments. The method allows credit for the changes in the 234U, 235U, 236U, 238U, 238Pu, 239Pu, 240Pu, 241Pu, 242Pu, and 241Am concentration with burnup. No credit for fission product neutron absorbers is taken. The methodology consists of five major steps:

A flat, uniform axial burnup assumption, preferred for its computational simplicity, does not always conservatively estimate the pressurized water reactor spent-fuel-cask multiplication factors. Rather, the reactivity effect of the significantly underburned fuel ends, usually referred to as the "end effect," can be properly treated by explicit modeling of the axial burnup distribution based on limiting axial burnup profiles.

The calculation of isotopic concentrations in spent nuclear fuel (SNF) assemblies and the subcritical multiplication factor of SNF packages are two of the essential requirements of the actinide-only burnup credit methodology. To justify the accuracy of the computed values, the code systems used to perform the calculations must be validated. Here, the techniques used for actinide-only burnup credit isotopic and criticality validation are presented and demonstrated.

Until now, the Nuclear Regulatory Commission's (NRC) approval of criticality safety evaluations for spent fuel in transport and storage casks has been based on analyzing the fuel as though it were fresh and without burnable poisons. The well-known nuclide composition of fresh fuel has provided a straightforward and bounding approach for showing that spent fuel systems will remain subcritical under normal and accident conditions. Burnup credit refers to the approval of criticality safety evaluations that consider the decrease in fuel reactivity caused by. irradiation in the reactor.

One of the significant issues yet to be resolved for using
burnup credit ~BUC! for spent nuclear fuel ~SNF! is establishing
a set of depletion parameters that produce an adequately conservative
representation of the fuel’s isotopic inventory. Depletion
parameters ~such as local power, fuel temperature, moderator temperature,
burnable poison rod history, and soluble boron concentration!
affect the isotopic inventory of fuel that is depleted in a
pressurized water reactor ~PWR!. However, obtaining the detailed

Criticality safety analysis devoted to spent-fuel storage and transportation has to be conservative in order to be sure no accident will ever happen. In the spent-fuel storage field, the assumption of freshness has been used to achieve the conservative aspect of criticality safety procedures. Nevertheless, after being irradiated in a reactor core, the fuel elements have obviously lost part of their original reactivity. The concept of taking into account this reactivity loss in criticality safety analysis is known as burnup credit.

The effect of fixed absorbers on the reactivity of pressurized water reactor (PWR) spent nuclear fuel (SNF) in support of burnup-credit criticality safety analyses is examined. A fuel assembly burned in conjunction with fixed absorbers may have a higher reactivity for a given burnup than an assembly that has not used fixed absorbers. As a result, guidance on burnup credit, issued by the U.S. Nuclear Regulatory Commission's Spent Fuel Project Office, recommends restricting the use of burnup credit to assemblies that have not used burnable absorbers.