atw - International Journal for Nuclear Power | 2.2024

Environment and Safety
61
Fig. 2.
Ex-vessel neutron dosimetry system(EVND) and Surveillance capsule in nozzle region
capsule and Ex-vessel neutron dosimetry(EVND)
measured in the core region to the calculated result
from the neutron transport calculation. (Figure 1)
shows Ex-vessel neutron dosimetry system in beltline
region. If the neutron dosimeter is attached to reactor
pressure vessel nozzle, accurate results can be obtained,
but it is impossible due to spatial limitation. Therefore,
as shown in (Figure 2), accurate result can be obtained
from the measured value of upper part of surveillance
capsule neutron monitor and Ex-vessel neutron
dosimetry(EVND) close to the reactor pressure vessel
nozzle region.
2.3 Determination of Adjusted Reference
Temperature for Reactor Vessel Inlet and
Outlet Nozzle
The adjusted reference temperatures (ARTs) for the
nozzle materials are calculated according to the
Regulatory Guide 1.99 Rev. 2 [3] . The ARTs are given by
the following expression:
ART = Initial RT NDT + ∆RT NDT + Margin (2)
Initial RT NDT of equation (2) is reference temperature
of unirradiated nozzle material and is determined based
on the certified material test reports (CMTRs) for
reactor vessel inlet and outlet nozzles. The reference
temperature shift by neutron irradiation, ∆RT NDT , is
determined by the following equation:
∆RT NDT = CF × f (0.28 – 0.1·logf) (3)
CF is the chemistry factor (CF) derived from the Copper
and Nickel weight percent (wt. %) values. f is neutron
fluence value (E+19 n/cm2, E > 1 MeV) at 48EFPY and
was calculated at the lowest extent of weld location
between nozzle and intermediate shell. The lowest
extent of weld location was chosen for conservatism.
2.4 Allowable Pressure Calculation
According to the ASME Code Section XI Appendix G
2013 edition [4] , the stress which have to be considered
in nozzle P-T limit evaluation are both internal pressure
loading and thermal loading. For level A&B service condition,
the following requirement shall be satisfied.
2K Ip + K It < K Ia (4)
K Ip is stress intensity factor due to internal pressure
loading, K It is stress intensity factor due to thermal
transient loading. In this study, K Ia fracture toughness
is considered in the generation of the nozzles corner
P-T limits. Thus, for the nozzle P-T limit curves, the K Ia
fracture toughness is calculated based on the following
equation [4] .
K Ia = 26.78 + 1.223 * e (0.0145(T–RTndt + 160)) ksi√in(5)
Allowable pressure can be obtained by defining the K Ip
of equation (4) as a function of internal pressure.
The applicable pressure and the thermal transient
stress are used to calculate pressure and the thermal
stress intensity factor. K Ip and K It , at the nozzle corner
cut are determined based on a finite element analysis
because the maximum stress occurs in the nozzle
corner. Only the cool-down transient stresses are considered
since the inside surface of the nozzle corner
would be in a tensile stress state during the cool-down
transient. The stress intensity factor calculation for the
nozzle corner regions is based on a 1/4t circular corner
flaw, as per ASME code Section XI Appendix G 2013 [4]
edition postulated flaw guidelines. (Figure 3) shows
postulated nozzle corner crack.
The stress intensity factor calculation method includes
postulating an inside surface 1/4t nozzle corner flaw
and calculating through-wall nozzle corner stresses for
Vol. 69 (2024)