ACCEPTABLE
UNACCEPTABLE
Poison Basket
Flux Trap Basket .
0 10 20 30 40 50 60
0 1 2 3 4 5
Fresh Fuel Enrichm ent, w t%
Required Minimum Burnup, GWd/MTU
Yucca Mtn French Isotopics Actinide Only No Poison
Basket
FIG. 5. Effects of built-in neutron absorber relative to fission products.
The simplified approach with five fission product isotopes could result in more rapid approval by the regulators because there are fewer isotopes to defend. Given that the five-isotope approach yields nearly the same benefit as the 16-isotope approach, little is lost by the simplification. This suggests another possibility: that only one or two isotopes might yield nearly the total fission product benefit. Our calculations (not reported in this paper) show that this is unlikely to be the case. Rather, each of the five fission product isotopes contributes a part of the benefit, and the law of diminishing returns has not caused any of the five to be ineffective. Thus the five-isotope approach may be an optimum solution for a commercial burnup credit methodology.
A perspective on risk
This analysis examines the risks associated with using burnup credit. The quantitative estimate of risk associated with using burnup credit is shown to be negligibly small. Although not estimated quantitatively, the overall risk reduction, which is realized by the expectation of fewer shipments when burnup credit is used, is sizable.
The NRC transportation regulations require consideration of the effect of fresh-water in-leakage for criticality analysis of a single package for transport of fissile material [U.S.
Nuclear Regulatory Commission, Title 10 Code of Federal Regulations (CFR) Part 71]. The regulations do not preclude the use of burnup credit in demonstrating sub-criticality; they simply require that the package (i.e. cask and contents) be shown to be sub-critical.
Current regulatory practice, which has been applied to all currently approved spent fuel transport casks, does not account for the reduced reactivity of spent fuel in demonstrating sub-criticality under prescribed regulatory conditions. The rules and practices currently applied to NRC-Certified transport casks are listed below:
(1) Sub-criticality is assured, that is, keff < 1.
(2) Moderation by water occurs to the most reactive credible extent.
(3) Full reflection of the system on all side by water occurs.
(4) The system is in its most reactive credible configuration consistent with the chemical and physical form of the material.
Items (1) to (4) are part of the U.S. regulations.
(5) The allowed keff is then reduced from 1 to account for such things as modeling and calculational biases and uncertainties.
(6) Additionally, the allowable keff is further reduced by applying an arbitrary criticality safety margin of 5%, i.e. _keff = -0.05.
Item (5) is simply an acceptable approach to criticality safety analysis. Item (6) is an arbitrary safety factor applied to analysis.
• The fuel is assumed to be in its most reactive state, which is generally unburned.
Item (7), the “Fresh Fuel” assumption is the practice that is modified when “Burnup Credit”
(BUC) is used. It should be noted that the word "modified", not "eliminated”, is used. The modified item (7) might read:
(7a) "The fuel is assumed to be in its most reactive arrangement, after credit for the spent fuel's burnup is determined using conservative depletion analysis."
Burnup credit only seeks a change in the fresh fuel assumption.
The issue of “Burnup Credit” vs. the “Fresh Fuel” assumption for the evaluation of PWR spent fuel reactivity in a transportation package involves a trade-off. On one hand, consideration and credit for the reduced reactivity of the spent fuel allows for a better utilization of the package volume; this results in a greater number of assemblies per package, and, in turn, in a smaller number of shipments. On the other hand, the fresh fuel assumption provides additional margin for criticality considerations as it leads to the addition of engineered poisons within the package cavity.
Can any incremental reduction in criticality likelihood (and subsequently risks) be justified against the reduction in transportation risks deriving from using burnup credit?
NRC-sponsored work [NUREG/CR-4829, referred to as the “Modal Study”) discusses the likelihood of a rail cask accident with a greater than 2% strain coupled with a concurrent submersion [Modal Study, Subsection 9.3.2.4]. Rail shipping is particularly relevant because it is required for the dual-purpose systems that are or will be implemented at reactor sites.
Under the rail-shipping scenario of the Modal Study, “ this type of accident is estimated to occur once every ten million years.”
The estimated frequency of a criticality event is then obtained by multiplying the frequency of the accident referred to above [i.e. 10-7/year] by the likelihood that the specific package involved in the accident contain enough reactivity under the moderation and geometric conditions of the accident to result in a critical configuration [i.e. 10-x /accident].
Assuming that the package system is a BUC-designed system, such a likelihood, i.e.10-x/accident, would be acceptably low if:
• Non-conservative errors associated with the specific BUC methodology are smaller than the sum of (i) the administrative margin (ǻkeff = 0.05) and (ii) the systematic bias in keff
introduced in the methodology to account for enveloping conditions and uncertainties.
• The potential for human errors is small enough to protect against non-conservative fuel assembly insertion errors (misloadings)
Based on probability data for human error [Homes & Narver in NSS-8191.1, Transportation Accidents Risks in the Nuclear Industry], and given that:
• Misloadings can introduce less reactivity as well as more reactivity,
• Only misloadings in specific cask or canister locations have a marked effect; and
• Two checks are required for every fuel movement
It can then be estimated that the probability of a non-conservative misloading can be as high as 10-3 and as low as 10-5for a large package. In addition, past analyses have shown that more than one misloading is required to approach criticality conditions. This brings the likelihood of having to deal with a critical configuration, given a severe enough accident, to an estimated (10-3 to 10-5)n/accident, where “n” is the required number of non-conservative misloadings.
Using the conservative assumption that only two non-conservative misloadings are required, the likelihood is (10-6 to 10-10)/accident.
Given that the frequency of an accident severe enough to result in significant damage to the package (coupled with submersion) is already very low [10-7/year], the expected frequency of a critical configuration under the rail shipping scenario of the “Modal Study” is essentially zero (10-13 to 10-17/year is a meaningless number!) Estimates of the consequences of a criticality accident are inconsequential from a risk standpoint.
Therefore, the fresh fuel assumption results in a negligible numerical reduction in critical configuration likelihood. On the other hand, by using a BUC approach, the reduction in the number of shipments is real, and results in a measurable reduction in risk associated with lower exposures to workers and public, lower fatalities and injuries from non-radiological consequences of accidents, and lower property damage.
A risk-informed approach would seek an overall reduction of the risks associated with spent fuel shipments. In a few years, the Department of Energy will begin a thirty-year program of shipping approximately 63 000 metric tons of spent nuclear fuel from commercial nuclear reactors to a first deep geological repository. The use of burnup credit offers an opportunity to reduce the numbers of shipments required to move this material by ~30%. The benefits of reducing risks associated with transport are obvious. This analysis suggests that the increased risk of a criticality associated with properly using burnup credit is negligible. Comparing this negligible risk component with the reduction in overall risk associated with transport actually supports using burnup credit.