Renew process implementation for reactor bottom head

The present invention relates to mitigating crack initiation or growth of small cracks in a nuclear reactor vessel and, more particularly, to an implementation/scheduling process for mechanical surface conditioning of metals to mitigate crack initiation or growth. Delivery of other processes to the component surfaces is also contemplated.

The welds in the bottom head region of a reactor vessel in a boiling water nuclear reactor are susceptible to cracking. Repair of such cracking requires complex and expensive processes. A known ‘ReNew™ process’ has been used to mitigate susceptibility to crack initiation or growth of small cracks due to tensile surface stresses, stress corrosion initiation and fatigue crack initiation using a mechanical surface conditioning of metals. These stresses can be found in the welds and heat affected zones of the reactor bottom head. The quantity of welds in the vessel head region, however, is very large, and it is typically very time consuming to mitigate all welds.

To the inventors' knowledge, mitigation of all bottom head welds in a reactor vessel has never been done. Prior methods have performed mitigation using water jet peening or laser shock peening.

For those reactors going through a shroud replacement process, bottom head weld mitigation is economically viable if mitigation can be performed in about thirty days or less. It would thus be desirable to provide a renew process implementation/scheduling process to mitigate all bottom head welds in a reactor vessel in an economically viable period of time.

In an exemplary embodiment of the invention, a surface conditioning scheduling process is used to mitigate susceptibility to crack initiation or crack growth due to surface stresses in a boiling water nuclear reactor using a plurality of working crews. The process includes the steps of (a) defining a plurality of working zones in an annulus region of a reactor vessel flange; (b) positioning one of the working crews in each of the annulus region working zones; (c) defining a plurality working zones in a core region of the reactor vessel; (d) positioning one of the working crews in each of the core region working zones; and (e) the working crews performing surface conditioning in areas of the reactor vessel accessible from their respective working zones such that the surface conditioning process can be completed in no more than thirty days.

In another exemplary embodiment of the invention, the process includes the steps of assembling a circular platform on a reactor vessel flange; assembling an auxiliary bridge; positioning a first plurality of working crews on the circular platform adjacent a corresponding first plurality of working zones in an annulus region of a reactor vessel flange; positioning a second plurality of working crews on a reactor main bridge adjacent a second plurality of working zones in a core region of the reactor vessel; positioning a third plurality of working crews on the auxiliary bridge adjacent a third plurality of working zones in the core region of the reactor vessel; and the working crews performing surface conditioning in areas of the reactor vessel accessible from at least one of the circular platform, the main bridge, and the auxiliary bridge.

In yet another exemplary embodiment of the invention, the component surface treatment scheduling process is utilized for delivering other treatment processes to the reactor component surfaces.

FIG. 1 is a sectional view of a nuclear reactor vessel showing generally locations of an annulus region and a core region;

FIG. 2 illustrates preferred work areas of each of the working crews;

FIG. 3 shows the various working zones in each of the annulus region and the core region;

FIG. 4 shows the working crew positions for the annulus region;

FIG. 5 shows the working crew positions for the core region;

FIG. 6 is a critical path schedule for the working crews in the annulus region; and

FIG. 7 is a critical path schedule for the working crews in the core region.

A ReNew™ process is a process for mechanical conditioning of metals to mitigate susceptibility to crack initiation or growth of small cracks due to tensile surface stresses, stress corrosion initiation cracking and fatigue crack initiation. These stresses can be found in the welds and heat affected zones of a reactor bottom head. The scheduling process described herein allows the ReNew™ process to be implemented in the welds at the bottom head in no more than thirty days. In a preferred implementation, the scheduling process described herein relates to reactors that are having the reactor internals replaced, where all reactor internal components located between the top of the vessel flange and the top of a control rod drive housing have been removed. Additionally, the shroud above the shroud support has been removed as well as all jet pumps and jet pump components.

With reference to FIGS. 1 and 3 , a bottom head 12 of a reactor vessel 10 can be separated into an annulus region 14 and a core region 16 . Each region 14 , 16 can be further divided into reactor zones or welding zones including a first area zone (designated as H 8 ), a second area zone (designated as H 9 ), a first leg zone (designated as H 10 ), a second leg zone (designated as H 11 ), a vert zone (designated as V 8 ), and a combination zone (designated as H 8 / 9 ). The core region 16 includes reactor zones as stub tube zones, CRDH zones, ICMH zones, and dP/SLC zones.

In a preferred arrangement, the scheduling process utilizes eight working crews. Each region 14 , 16 includes a plurality of working zones, one each for each working crew. With reference to FIGS. 2 , 4 and 5 , prior to positioning the working crews, a circular platform 18 is assembled on the reactor vessel flange. As shown, crews 1 - 4 are positioned in the annulus region working zones on the circular platform 18 .

The reactor vessel 10 is typically provided with a main bridge 20 for various refueling and maintenance activities. In the scheduling process described herein, an auxiliary bridge 22 is assembled that extends between the sides of the vessel cavity. As shown in FIG. 2 , crews 5 and 6 are positioned on the auxiliary bridge, and crews 7 and 8 are positioned on the main bridge 20 .

The appropriately positioned working crews perform surface conditioning in areas of the reactor vessel accessible from their respective working zones. FIGS. 6 and 7 illustrate an exemplary critical path schedule for each of the crews including the reactor zone for which each crew is responsible and a time goal for completing the conditioning process in each assigned reactor zone. From the crew assignments in the exemplary critical path schedule shown in FIGS. 6 and 7 , it is clear that each crew has access to reactor zones in both the annulus region 14 and the core region 16 from their respective working zones. With the defined reactor zones and noted working crew responsibilities, mitigation of all bottom head welds can be performed in less than thirty days.

Although the methods described herein have been directed to an application of surface conditioning for mitigating crack initiation or growth of small cracks. The disclosed application tooling, service platforms/bridges, and multiple crew arrangements are also suitable for delivery of other processes to the component surfaces (e.g. GE's ReNew™ Surface Improvement). These other processes may include welding, coating, inspection, thermal treatment, excavation, etc. The invention is thus not necessarily meant to be limited to the disclosed application.

With the surface conditioning scheduling process described herein, a ReNew™ process for mitigating susceptibility to crack initiation or growth of small cracks (and other processes) can be performed for all bottom head welds in a reactor vessel in an economically viable time period.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.