What Are the Primary Radiation Concerns for Workers in Nuclear Plants?

[The Prince]

Hi Experts,

-I want to List and rank the radiation concerns based on threat level in Nuclear plants
- What are the radiation sources and locations of primary concern for workers in nuclear Plants?

Please tell me if you know a good link for these questions

Thanks in advance.

 

[Astronuc]

Here's a link. http://deqtech.com/Resources/PDF/Sources_at_NPP.pdf

Bascially the fuel is the greatest source of radioactive species at the NPP, especially once it has operated in the core. The fission products and transmuted fuel nuclei are supposed to be retained by the fuel - firstly in the ceramic pellets - then by the cladding surrounding the pellets. Problems arise when the cladding is breached (fuel failure). Fuel particles containing isotopes of U, Np, Pu, Am and Cm, and fission products get into the coolant, or in a BWR, may find their way to the turbine, and ex-core cooling systems. Spent fuel is quite radioactive.

Fission gases (Xe, Kr) and volatiles like I, Cs are also problematic because they can be transported out of the core. In BWRs with degraded failed fuel, Xe, Kr, I and Cs are problems for folks working on turbines and BOP. Sometimes workers must wait on sight to degass, release the Xe, Kr they have inhaled.

Next corrosion products (know as crud) which deposit on the fuel during operation will become activitated, and structural materials in the core also become activated by neutron absorption. These corrosion products may find their way out of the core. This is a problem when a reactor shutsdown for refueling and the top is removed. Prior to removal, a cleanup system runs in order to wash out the loose crud, which is collected on a filter. More tenacious crud will be carried on the fuel to the spent fuel pool.

In BWRs using Hydrogen water chemistry, the can be carry over of N-16 (produced by (n,p) reaction with O-16) to the turbine. Some BWR plants have had a problem of gamma shine on workers or control room personnel because the steam lines passed near the control room.

There are other radiation sources, e.g., primary and secondary neutron sources, as well as calibration sources.

There was one or two journals from Brookhaven National Lab that had a lot of articles on radiation control of nuclear power plants. They don't seem readily available on-line. One is entitled "Nuclear Safety".

 

[The Prince]

You are always helpful.
Thank you so much.

 

[Nspyred]

Is the BWR the older model or something? All the problems outlined above seem to come from BWRs.

 

[Astronuc]

Is the BWR the older model or something? All the problems outlined above seem to come from BWRs.

PWR and BWR were two main concepts that were commercialized from several different ideas. Liquid metal and gas cooled reactors were developed. In the UK, Magnox and AGR reactors using CO₂ for cooling were developed for power generation.

GE favored BWRs while Westinghouse, Babcock & Wilcox and Combustion Engineering favored PWRs. The concepts basically grew out of the naval nuclear power program.

BWRs are a direct cycle plant where steam is generated in the primary system, actually in the core, and is separated from liquid phase above the core before being transported throught main steam lines to the high pressure turbine. The liquid fall back into the annulus on the outside of the core and returns via the recirculation system. The recirculation system also collects the feedwater returning from the power system (turbines, reheaters and condenser). Although BWRs may be slightly more efficient than PWRs of similar thermal rating, the major disadvantage of BWRs is that any fuel particles or fission products from leaking (failed) fuel can find their way to the turbines, and this presents an source of radiation exposure to plant personnel. BWRs typically operate at saturation temperature of 285-286 C (at ~7.2 MPa)

PWRs in contrast to BWRs have two circulation systems. The primary (core cooling system) circuit operates under high pressure ~15.5-15.7 MPa (more than twice that of a BWR) and the secondary side operates under more moderate pressure of ~6.2 MPa.

Here's a reasonably good overview of PWR technology - http://www.ansn-jp.org/jneslibrary/npp1.pdf ,
and BWR technology - http://www.ansn-jp.org/jneslibrary/npp2.pdf

 

[Xnn]

Although BWRs may be slightly more efficient than PWRs of similar thermal rating, the major disadvantage of BWRs is that any fuel particles or fission products from leaking (failed) fuel can find their way to the turbines, and this presents an source of radiation exposure to plant personnel.

Sorry, but this a statement from somebody with limited working knowledge of radiation sources at a BWR.

Fuel integrity at BWR's has historically been sufficiently high that there are no detectable fission product particulates in the main turbine. The majority of BWRs' have completely intact cladding and no leakage at all. Cladding problems that do occur are generally quickly located and suppressed. This practice helps keep defects tight enough that the majority of fission products that come from any leaking fuel are noble gases (Krypton and Xenon). Iodine is rare while transurancis are seldom detected in the coolant when leaking fuel is present. Particulate carry over to the turbine is minimal since moisture carry over is slight and the steam lines are long. While radiation levels at the main turbine is significant during power operation, it's mostly from gaseous activation products (Nitrogen-16 & Fluoride-18). During reactor shutdown, radiation levels at the main turbine are minimal, even for BWRs with fuel failures.

The majority of radiation exposure occurs during refueling outages in the Reactor Building. Cobalt-60, an activation product, is responsible for the majority of worker exposure.

 

[Astronuc]

The matter of N-16 was mentioned in my initial post. I should also mention that a motivation to use noble metal chemistry addition (used in order to produce a more negative electrochemical potential (ECP in the upper core) is to reduce the H required for HWC, which reduces the carryover of N-16 to the steam lines.

Cobalt activity is a problem with any LWR - Co-60 and Co-58. Reducing the Co-content of Ni-alloys and steels was one factor, replacing Co-bearing alloys like Stellite was another, as was reducing quantity of Inconel in the core. Beyond that, controlling crud transfer and Ni dissolution from heat transfer or structural surfaces (e.g. steam generators in PWRs) has been a major factor in controlling dose. That was the reason Zn-injection was adopted first in BWRs and subsequently in PWRs. Some plants seem to have more effective dose reduction/crud control programs. Depleted Zinc (depleted in ⁶⁴Zn) was introduced to mitigate effects of Zn-activation.

I've been involved with most plants (Ps and Bs) with degraded fuel. Fuel reliability has improved greatly over the last two decades, but the early 90s had some problematic cases with severely degraded fuel with off-gas rates in the 20k-100k uCi/sec at the SJAE. I know of one plant where those working in containment had to degas before leaving - they had to exhale the Kr and Xe they absorbed while in containment. BWR operators now generally suppress failures with one or more control blades and do mid-cycle outages to remove failed fuel.

The previous company where I worked had extensive records on most NPPs operating in the 60's through 80's. In the 1970's there were some plants that huge numbers of failures due to CILC, and possibly PCI, debris and primary hydriding. Those plants had off-gas activities of several 100k uCi/sec. That was before people started getting serious about preventing fuel failures.

In the early 90s, one PWR was hours from shutdown because the DEI was approaching 0.5 uCi/ml in their coolant. Plant personnel were restricted in some areas because radiation fields. That was from one failed rod, which just happened to be close to one of the peak rods in the core.

More than two decades ago, one plant had problems with fuel particles (fleas) getting on personnel.

I got involved in one case during the early 90s where a plant was draining the reactor cavity when the rad alarms went off. They reflooded the cavity and took a look. There were fragments of fuel rod by the fuel handling machine by the RV. They eventually found 4 pieces including one about 5-6 feet long that was split down one side. It had contained about 1 kg of fuel, but all of that had washed out into the primary circuit. I'd indicated months earlier from the coolant activity, particularly the Np-239, that they had a severally degraded failure. For some reason, they just assumed they had some crud burst that released some tramp, and they didn't inspect for failed fuel. The fuel rod that had fractured actually broke outside of the core when they were retrieving the assembly from the fuel handling machined. There had been a guillotine break near one of the upper grids. The broken rod leaned out of the assembly far enough to catch on the machine and it was pulled out through the grids, and broke in two other places. They had reloaded the fuel assembly with the broken grids and minus the broken rod.

This is old but historical - Characteristics of fuel crud and its impact on storage, handling, and shipment of spent fuel.
http://www.osti.gov/bridge/purl.cover.jsp?purl=/6164184-i2i6ob/
or - http://www.osti.gov/bridge/product.biblio.jsp?osti_id=6164184


Here is a good set of references on radiation protection and monitoring:
http://hps.ne.uiuc.edu/natcisoe/brookhaven.htm

Brookhaven used to publish an journal called ALARA. I don't know if they still do. There was also a journal entitled Nuclear Safety that examined a lot of the same issues.