Pressurized Water Reactors 1

[JEFF1]

I do not work in the nuclear industry. However, I work with a district heating system that uses pressurized hot water. I would like to know if there is a similar experience base in the two industries. Specifically, in a typical district heating system, one might be dealing with 380 F pressurized water at an operating pressure of about 265 psi. In the nuclear industry, are pressures and temperatures much higher?

 

[Radiation]

The General Ellectric symplified boiling water reactor (sbwr) built in Taiwan runs at an average pressure of about 7 MPa (1015 psi) and an average temperature of about 550K (530 F).Pressurized reactor will run at much higher pressures than boiling water reactors do.

 

[JEFF1]

I know that for highest thermal efficiencies, nuclear reactors will run at high temperatures. But I thought that there might be subsystems that operate at lower temperatures and pressures.<br>
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The district heating system where I work handles thermal expansion and contraction with an expansion tank that is pressurized by nitrogen. Level control in the tank is accomplished by sensing both liquid level and pressure. I have heard that in at least some nuclear plants, liquid expansion is handled by pressurization of an expansion vessel with steam, and that water sprays are used to modulate steam conditions, thus accomodating the expansion. <br>
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I thought that subsystems in a nuclear power plant might use the same type of expansion control that we use, and that it might be worthwhile to exchange operating experience. Our system was designed by Black & Veatch in Kansas City, and I feel that the overall design is excellent.

 

[ScottFree]

Our Nuclear Power plant operates at 2250psi 550F on the primary side with a water spray controled pressurizer. It heats a secondary side steam generator at 900 psi 530F.

 

[EHD]

Jeff - In response to you 9/3/99 question about your expansion tank. Your expansion tank sounds like it is used for pressure control as well as an expansion tank and is similar in principle to the pressurizer in a nuc plant. The nitrogen in you plant is probably being used to maintain contant sys pressure and poss as an inert blanket to keep other gases, such as O2 ,H2, out of the system or from collecting in the tank. <br>
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Most likely, the reason the control sys takes input from both the press and level trans is because as level increases or decreases, you don't want your process filled with N2 and you don't want water backflowing into your nitrogen system.<br>
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In a PWR nuc, the pressurizer is also uesd for pressure control and as an expansion tank but uses water and heaters to control press as level changes. Also due to the temp and rad flux in a nuc plant, N2 would not be a good thing to put in your water. Besides chem reactions with the reg water chem you probably would form nitric acid or some other chem just from the heat and rad field which would not be good for plant equip life. <p> <br><a href=mailto: eugene.decker@srs.gov> eugene.decker@srs.gov</a><br><a href= > </a><br>

 

[nuke]

Some additional PWR chemistry info....As stated in the other responses, the chemistry for PWRs is very rigorous because of the potential materials(40 year design life) impact combined with the extreme pressures and temperatures. The chemistry specs for the various reactor modes vary with the temp & pressure as well as the reactor power which is determined by the neutron flux. All of these effect what material conditions exist. Corrosion controls require minimum dissolved O2(DO)concentrations (ppb range) depending on whether the reactor is subcritical or critical & at power (0-100%). The reactor system DO is controlled using a H2 overpressure on a "volume control tank" in a letdown purification system which is recycled back into the system continuously. Operating at 100% power the H2 concentration is maintained in the range of 40-50 cc/kg. Additionally, the PWRs use boron dissolved in the coolant to act as a chemical neutron poison to control the reactivity and power.The boric acid adds its own unique chemistry issues. Another issue is the control of corrosion products that do plate out on the fuel due to surface nucleate boiling. The chemistry is managed to prevent this radioactive material from being released into the system during cool down and then it is solubilized to allow removal in the clean-up system prior to openning the system. This is to minimize radiation exposure to the workers. When the reactor is shut down for refueling and the core/coolant systems are opened, the H2 is removed and replaced with N2 during the depressurization shutdown process so that explosive mixtures of H2 and O2 cannot occur. Removal of the N2 and O2 is a critical aspect of startup and the residual N2 that can hideout creates ammonia that can collect in the steam space of the pressurizer which does negatively effect reactor chemistry.<br>
Coal fired plants that can also operate at very high tempeartures & pressures may have conditions more similar to the water plant described in the inquiry. Might want to inquire among fossil plant engineers & chemists for info.

 

[CMEng]

One other factor re: N2. Nitrogen passing through the core, is readily activated to N16 which decays producing a high energy gamma. This would result in high dose to workers in the plant.

 

[TSSD]

Nitrogen is used as a Tertiary shutdown method in Advanced Gas Reactors because of its high Neutron Absorption properties.