May-June 2018 NPJ

Radiation Source Term Assessments By Jen Jarvis and Al Jarvis, Structural Integrity Associates, Inc. Jen Jarvis At Structural Integrity Associates, Dr. Jarvis has worked on a variety of projects primarily dealing with chemistry in BWRs, including radiolysis, IGSCC mitigation, and chemistry control for radiation field management. Some of her past projects have included chemistry control for advanced BWRs, chemistry impacts of flexible operations in BWRs, and an independent review of methanol injection data at a BWR. Dr. Jarvis also performs crack growth estimates for chemistry transients in BWRs using EPRI BWRVIP correlations. She has a Doctor of Philosophy - Nuclear Science and Engineering from Massachusetts Institute of Technology, a Bachelor of Science - Nuclear Engineering, from Pennsylvania State University, a Bachelor of Science - Mechanical Engineering, from Pennsylvania State University, and a Bachelor of Science - French and Francophone Studies, from Pennsylvania State University. An article from the 2018 Volume 44 of Structural Integrity Associates, Inc.’s News and Views. Nuclear plant workers accrue most of their radiation exposure during refu- eling outages, when many plant systems are opened for corrective and preventive maintenance. The total refueling outage radiation exposure can be 100-200 per- son-Rem at a typical Boiling Water Reac- tor (BWR), and 30-100 person-Rem at a typical PressurizedWater Reactor (PWR). Accrued refueling outage radiation expo- sure values can be significantly greater than these values depending upon radia- tion fields, outage work scope, and emer- gent work. Outage radiation exposure is one metric used by a plant to determine outage success and by industry regula- tors in assessing the overall performance of a plant. Plants with high personnel radia- tion exposure tend to be those plants with more equipment problems and more unscheduled shut- downs; consequently, they may be subjected to increased regula- tory oversight. Radiation source term assessments are performed to understand the causes of high collective radiation exposure and to help plants evaluate their strategies for source term reduction. This involves understanding how a plant’s material choices and chemistry and operational history influence the radiation fields that develop in the plant systems. Consequently, a source term evaluation is very plant- specific, but can help a plant identify which strategies may be most effective for their specific situation. As part of Structural Integrity’s Single Point of Contact (SPOC) program implemented during the spring 2017 outage season to assist plants with emergent outage issues, SI noted in the daily outage reports of two BWRs that accrued outage radiation exposure significantly exceeded the outage goal. The SI Nuclear Chemistry and Materials Group followed up with station Chemistry and Radiation Protection Department personnel offering our assistance in determining if increased radiation source terms resulted in higher radiation fields, leading to radiation exposure goal exceedance. We were subsequently contracted by one of the plants to participate in the event root cause evaluation and by the other to perform a formal radiation source term assessment project. From Corrosion to Radiation Field Formation The radiation fields formed in BWRs and PWRs are primarily caused by the deposition of activated corrosion products inside plant piping and equipment. Fission products and water activation products have less of an effect on radiation fields. The content of alloying elements in the materials of construction ultimately determine the corrosion products released into the coolant. Most of the surface areas are austenitic stainless steels, however nickel-based alloys and low alloy steels are used in some of the of the reactor internals. Stellite™, a cobalt based alloy, is used in many valves. In BWRs, carbon steels are used in the condensate, feedwater, and steam systems. In PWRs, steam generator tubing, typically Alloy 600 or 690, makes up a large portion of the exposed surface area in the primary circuit. Corrosion products are released from component and piping surfaces by dissolution and wear. These products can be soluble or particulate, and once released into the coolant, they are transported by the reactor water around the primary circuit. Some of the corrosion products may be removed by the coolant purification systems (the Reactor Water Cleanup System in BWRs and the Chemistry and Volume Control System in PWRs). The corrosion products will deposit around the circuit, including on the fuel. The corrosion products deposited on the fuel become activated by the high levels of neutron flux in the core. The activated corrosion products can be released from the fuel by dissolution, erosion, or spallation. Once released into the coolant, the activated corrosion products can deposit on reactor coolant system piping surfaces, inside reactor coolant valves, and in other systems that are connected to the reactor coolant system, leading to radiation field 34 NuclearPlantJournal.com Nuclear Plant Journal, May-June 2018