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Fresh student of nuclear engineering

by Physics71
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Physics71
#1
Sep11-05, 11:27 AM
P: 4
Hi I am fresh student of nuclear engineering. I want to know why it is so important to make such cumbersome models for corrosion product and fission product activity in primary coolants of LWRs. What is importance of measuring corrosion and fission product activity. Hoping someone will answer my question. Thanx
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Astronuc
#2
Sep11-05, 04:52 PM
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Quote Quote by Physics71
Hi I am fresh student of nuclear engineering. I want to know why it is so important to make such cumbersome models for corrosion product and fission product activity in primary coolants of LWRs. What is importance of measuring corrosion and fission product activity. Hoping someone will answer my question. Thanx
At some point the reactor must be shutdown for refueling and maintenance, and utility personnel must refuel the core from a bridge over the reactor cavity (reactor is under ~30+ ft (10 m) of water). Utilities are required by law (10CFR50) to minimize dose (radiation exposure) to personnel.

Corrosion is an inherent problem in power systems in which water is used as the working fluid. In PWRs, highly pure water is used at the coolant under high pressure ~2200-2250 psia (15.2-15.5 MPa) and temperatures of Tin = 545-556F (285-291C) and Tout = 600-626F (315-330C). In addition to the coolant thermal-hydraulic conditions, the coolant chemistry includes boric acid (H3BO3, which is a soluble burnable absorber or chemical shim) and LiOH as a buffering agent (pH's vary from about 6.9-7.4, with some plants moving to higher pH at EOC). Plants at the upper range of temperatures have greater corrosion rates and greater transfer of corrosion products to the core.

Corrosion products come from the oxidation of metal surfaces of the pressure vessel, pressurizer, heat exchangers (steam generators), primary piping (hot and cold legs), and various other systems connected to the pimary system. Most metal is stainless steel (e.g. 304) or Ni-based superalloy (Inconel 600, 690, or Incoloy 800, which are primarily in the steam generator tubing). Control rods (RCCA, CRAs or CEAs) have been SS304 or Alloy 625 (IIRC), but more recently SS316 (with nitriding surface treatment for wear resistance) has been used as a structural material.

The Ni is relatively volatile (more readily soluble) in stainless steel and Ni-alloys than say Fe or Cr, and Ni along with mostly Fe finds itself to the fuel rod cladding and tends to precipitate at the hottest locations (highest heat flux), particular if nucleate where boiling occurs. The precipitate, known as crud, may form a Fe,Ni-spinel (with some Cr and other elements). The deposited crud is then activated. More recently, it has been determined that some crud redissolves from reloaded (reinserted) fuel during a second/third cycle and re-deposits on the fresh fuel.

During the cycle, the boric acid and LiOH concentration are gradually reduced and power shifts so that crud solubility changes. Invariably the activated crud may develop a looser structure and be transported elsewhere in the primary system. As shutdown, crud may actually come off the fuel and contaminate the primary coolant and that which is not cleaned up (filtered) may contaminate the water above the reactor when the pressure vessel head is removed - this represents a dose problem. Also the crud may be transported to the spent fuel pool, where it also becomes a dose issue.

Years ago, the some valves had Co-alloy or Co-bearing alloys. The cobalt found its way to the core and precipitated with the Ni. Co-59 becomes Co-60, and Ni-58 activiated to Co-58 by virtue of the (n,p) reaction. For this reason, alternative alloys without cobalt or very low cobalt (< 300 ppm) have been introduced. As another mitigating factor, Zn is now added to the cooling water to help reduce corrosion of the Ni-alloy steam generator tubes.

A major problem with current generation plants was the use of Inconel-600 which corroded much faster than originally predicted (here lack of experience in the industry). Rather than last 40 years, many steam generator tubes began failing after 15-20 years, less than the half-life of the plant. Many plants have replaced the original steam-generators, and the new ones use Inconel-690, with some using Incoloy-800. Siemens high temperature plants have used Incoloy from the beginning, and AFAIK, the steam generators are still performing well, with less of a crud problem than those of Inc-600.

There have also been some power distribution anomalies associated with the deposition of B in the crud. This has since been resolved, but it indicates the importance of undertanding the corrosion of materials and transport of corrosoin products in a nuclear reactor. It also applies to BWRs as well, but they a slightly different beast.
Physics71
#3
Sep25-05, 04:37 PM
P: 4
Thanks Mr. Astronuc for your detailed answer. It helped me too much.


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