NUREG/CR-5625 (ORNL-6698) - Oak Ridge National Laboratory

Procedure
It is recommended not to use the decay heat values of this
report if any of the following conditions occur:
if T c # 10 years and P e /P ave > 1.3,
if 10 years < T c # 15 years and P e /P ave > 1.7,
if T c # 10 years and P e-1 /P ave,e-1 > 1.6. (12)
Although it is safe to use the procedures herein, the heat
rate values for p may be excessively high when
final
T # 7 years and P /P < 0.6,
c e ave
T # 7 years and P /P < 0.4. (13)
c e-1 ave,e-1
5.2.3 The Excess Power Adjustment Factor f p
The maximum specific power, P max , used to generate the
data in Tables 5.1–5.3 and Tables 5.5–5.7 is 40 kW/kgU
for a PWR and 30 kW/kgU for a BWR. If P ave , the
average cumulative specific power, is more than 35%
higher than P max (i.e., 54 kW/kgU for PWR fuel and 40.5
kW/kgU for BWR fuel), then the guide should not be used.
When 1 < P ave /P max < 1.35, the guide can still be used, but
an excess power adjustment factor, f , must be applied.
p
The excess power adjustment factor is
For P # P , f = 1.
ave max p
5.2.4 The Enrichment Factor f e
f = P ave
/P max
.
(14)
p
The decay heat rates of Tables 5.1–5.3 and Tables 5.5–5.7
were calculated using initial enrichments of Tables 5.4
and 5.8. The enrichment factor f is used to adjust the
e
value p for the actual initial enrichment of the assembly
tab
E s . To calculate f e , the data in Tables 5.4 (BWR) or 5.8
(PWR) should be interpolated linearly to obtain the
enrichment value E that corresponds to the assembly
tab
burnup, B . If E /E < 0.6, it is recommended not to use
tot s tab
the guide. Otherwise, set the enrichment factor as follows:
f = 1 + 0.01[a + b(T - d)][1 - E /E ]
e c s tab
when E s /E tab # 1.5
f e = 1 – 0.005 [a + b(T c - d)]
when E s /E tab > 1.5 (15)
where the parameters a, b, and d vary with reactor type,
E s, E tab , and T c . These variables are defined in Tables 5.9
and 5.10.
5.2.5 Safety Factor S
Before obtaining the final heat rate p final , an appropriate
estimate of a percentage safety factor S must be
determined. Evaluations of uncertainties performed as
part of this project indicate the safety factor should vary
with burnup and cooling time.
For BWR assemblies:
S = 6.4 + 0.15 (B tot – 20) + 0.044 (T c – 1) . (16)
For PWR assemblies:
S = 6.2 + 0.06 (B tot – 25) + 0.050 (T c – 1) . (17)
The purpose of deriving spent fuel heat generation rates is
usually to apply the heat rates in the computation of the
temperatures for storage systems. A preferred engineering
practice may be to calculate the temperatures prior to
application of a final safety factor. This practice is
acceptable if S is accounted for in the more comprehensive
safety factors applied to the calculated temperatures.
5.2.6 Final Heat Generation Rate Evaluation
The equation for converting p tab , determined in Sect.
5.2.1, to the final heat generation rate of the assembly, is
p = (1 + 0.01S) f fN f f p , (18)
final 7 7 p e tab
where f , fN , f , f , and S are determined by the
7 7 p e
procedures given in Sects. 5.2.2 through 5.2.5.
5.3 Acceptability and Limits of the
Guide
Inherent difficulties arise in attempting to prepare a heat
rate guide that has appropriate safety factors, is not
excessively conservative, is easy to use, and applies to all
commercial reactor spent fuel assemblies. In the endeavor
to increase the value of the guide an effort was made to
ensure that safe but not overly conservative heat rates were
computed. The procedures and data recommended in the
guide should be appropriate for the mainstream of power
reactor operations with only minor limitations in the range
of applicability.
NUREG/CR-5625 26