Nuclear plant workers accrue most of their radiation exposure during refueling outages when many plant systems are opened for corrective and preventive maintenance. The total refueling outage radiation exposure can be 100-200 person-Rem at a typical Boiling Water Reactor (BWR), and 30-100 person-Rem at a typical Pressurized Water Reactor (PWR). Accrued refueling outage radiation exposure values can be significantly greater than these values depending upon radiation fields, outage work scope, and emergent work. Outage radiation exposure is one metric used by a plant to determine outage success and by industry regulators in assessing the overall performance of a plant. Plants with high personnel radiation exposure tend to be those plants with more equipment problems and more unscheduled shutdowns and consequently, they may be subjected to increased regulatory 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 which 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 term 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.
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, make 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 formation. Figure 1 above shows the primary system of a BWR. Figure 2 illustrates the steps in Co-60 activation and transport, ultimately leading to the formation of radiation fields.
The gamma-emitting isotopes are responsible for the radiation fields. However, there are differences between the isotopes which are most prolific in the coolant and those which are the major contributors to the out of core radiation fields. How much an isotope contributes to the radiation fields depends on:
1. The quantity of the isotope.
2. The half-life: isotopes with short half-lives (a few hours or less) will decay away quickly, and not have much of an effect on shutdown dose rates. For example, Manganese-56 has a half-life of 2.58 hours, so it will decay quickly.
3. The yield and energy of gamma rays: isotopes with small yields and that emit low energy gammas will have less of an effect on radiation fields. For example, a Chromium-51 decay will emit, on average, 0.1 gamma rays of low energy, so it tends to not be a large contributor to radiation fields.
Cobalt-60 (Co-60) is typically the isotope of most concern in BWRs, while in PWRs, Cobalt-58 (Co-58) and Cobalt-60 are of most concern. Table 1 compares Co-58 and Co-60. Co-60 is formed from the neutron adsorption of non-radioactive Co-59, while Co-58 is formed from the neutron adsorption and decay of non-radioactive Nickel-58 (Ni-58). The primary source of non-radioactive Co-59 is the corrosion and wear of materials with Stellite™ hardfacing, as the concentration of Co-59 in Stellite™ is typically between 50 and 65% by weight. Trace Co-59 is also present in any material that contains nickel. The primary source of Ni-58 is the corrosion of various stainless steels and nickel alloys used in the plant. The Ni-58 content of typical Type 304 stainless steels is between 8 and 12% by weight. In BWRs, there are many more components with Stellite™ hardfacing that interface with the primary coolant than at PWRs. In PWRs, the large surface area of nickel alloy steam generator tubes results in a substantially greater nickel source than in most BWRs. Accrued outage radiation exposure is typically higher in BWRs than in PWRs because BWRs have higher Co-60 source term.
Other isotopes which also contribute to radiation fields include Manganese-54 (Mn-54, which originates from Iron-54), Chromium-51 (Cr-51), and Iron-59 (Fe-59). In some instances, Silver-110m (Ag-110m) and Antimony-124 (Sb-124), and Tungsten-187 (W-187) have also been identified as contributing sources. Table 2 lists some of the commonly found activation corrosion products.
For many years, SI’s Nuclear Chemistry and Materials Group have performed radiation source term assessments for BWRs. These projects have either been contracted directly through a utility or for a utility through the EPRI Radiation Management and Source Term Technical Strategy Group (RM&ST TSG). SI’s BWR expertise in this area led to EPRI RM&TSG contracts for performing source term assessments at two PWRs in 2016-2017.
A typical work scope for each source term assessment includes reviews of operational and shutdown chemistry data, radiological data, water treatment equipment performance, outage schedule and water management plans, and 5-year As Low as Reasonably Achievable (ALARA) and cobalt reduction plans. In the assessment process, data reviews are completed and documented in a draft report followed by a site visit to discuss initial findings from the data reviews and to fill in data gaps. A preliminary list of findings are typically reviewed with senior management at the end of the site visit. The final report is issued after the site visit.
The findings from a radiation source term assessment will help plants identify specific actions to reduce the radiological source term and collective radiation exposure. This can include evaluating the effectiveness of chemical injection programs, such as BWR and PWR zinc injection, and BWR hydrogen water chemistry programs. Elemental cobalt source term reduction programs can be reviewed for effectiveness. Methods to optimize the operational and shutdown water treatment and chemistry control can be identified.