Indian Point: Its Core Components Highly Irradiated Differential Of Expansion, Stupid

***“Unfortunately the internals of most Western PWRS and WWERs were constructed with two of the most the swelling–prone steels that are commercially available.”

between 304SS plates and 316SS bolts

There are at least three things going on here.

1) The startling lack of knowledge with the attributes of core components being highly irradiated and under stress: high temperature and bolt stress (tensile). Under high prolonged radiation, core components change their dimensional measurement such as in swell, creep, void formation and ductility, amount others.


2) The baffle plates swell and creates a beyond design tensile stress on the bolt shank and head. This is the area where the IASCC cracking occurs. The swelling of the baffle tries to pull out the bolt leading to dangerous tensile stress. It is much like the damage of frozen pipe (the tremendous forces).

3) The differential expansion…swelling…between the between 304SS plates and 316SS bolts. This can de-torque the bolt and later create beyond design tensile stress on the bolt.

Selectively replacing bolts can create a dangerous condition within deferential expansion between the new bolt and old baffle beyond the bolt design tensile stress at plant end of life. In other words, bolt tensile stress could be much more severe for an “old baffle plate and a replacement new bolt” than an “old bolt and old baffle plate” both at end of plant life.  

The reason the industry chose 316ss for bolt is the high tensile stress this alloy can withstand. The reason why 304ss was used for the baffle plate is because this was much more inexpensive. We wouldn’t have the cracks in the bolts if the baffle plate was made from the same alloy as the bolt.

This is why Entergy wants to keep information about the bolt problem close to the vest before startup. They are hoping to restart the plant before the inspection comes out. This way they won’t have to replace the baffle plates.

I wouldn’t at all allow the plant to restart without all new baffle plates and all new bolts. I’d have identical stainless alloy for both the bolt and the baffle plate.

The NRC and Entergy should release all inspections reports on this event and Entergy should release all their investigative reports concerning the baffle-reformer before allowed to start-up.

***“Unfortunately the internals of most Western PWRS and WWERs were constructed with two of the most the swelling–prone steels that are commercially available.”

 between 304SS plates and 316SS bolts

Response of PWR Baffle-Former Bolt Loading to Swelling,Irradiation Creep and Bolt Replacement as Revealed Using Finite Element Modeling

Conference: 12th International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, At Snowbird UT

Abstract 

Baffle-former bolts in pressurized water reactors (PWRs) tend to degrade with aging, partially due to radiation-induced hardening and also due to the often complex stress history of the bolt in response to time-dependent and spatial gradients in temperature and neutron flux-spectra that can alter the stress distribution of the bolts. The time-integrated stresses must play some role in bolt cracking, however, and therefore it is of interest to study the time dependence of bolt stresses even for idealized cases. These stresses have been quantified in the present analysis using newly developed material constitutive equations for swelling and creep at light-water reactor (LWR)-relevant temperatures and dose rates. ABAQUS finite element calculations demonstrate that irradiation creep in the absence of void swelling tends to relax bolt tension before 10 dpa. Subsequent differential swelling leads to an increase in bolt tension, but only to stresses below the yield strength and usually below the initial bolt loading. Various assumed bolt replacement scenarios are considered. 

Conclusions

An FEA and constitutive equation approach quantified bolt stresses in a PWR baffle-former assembly. Spatial variation in temperature and dose affect the calculated stresses. Maximum stress occurs at the corner of the bolt head and bolt shank and is at the same location for occurrence of IASCC. Irradiation creep relaxes bolt shank stresses, whereas irradiation-induced swelling promotes bolt loading. Differential swelling rates between 304SS plates and 316 SS bolts cause increase in bolt tension. The bolt remains in tension throughout life and reaches a minimum at 10 y of irradiation. Calculations of bolt replacement options indicate that end-of-life stress is greater for bolt replacement when compared to no bolt replacement.

Response of PWR Baffle-Former Bolt Loading to Swelling, Irradiation Creep and Bolt Replacement as Revealed Using Finite Element Modeling (PDF Download Available). Available from:

https://www.researchgate.net/publication/258883539_Response_of_PWR_Baffle-Former_Bolt_Loading_to_Swelling_Irradiation_Creep_and_Bolt_Replacement_as_Revealed_Using_Finite_Element_Modeling [accessed May 1, 2016].

***“Unfortunately the internals of most Western PWRS and WWERs were constructed with two of the most the swelling–prone steels that are commercially available.”