Primary coolant

Safety. A large inventory of radioactive fission products is present in any reactor fuel where the reactor has been operated for times on the order of months. In steady state, radioactive decay heat amounts to about 5% of fission heat, and continues after a reactor is shut down. If cooling is not provided, decay heat can melt fuel rods, causing release of the contents. Protection against a loss-of-coolant accident (LOCA), eg, a primary coolant pipe break, is required. Power reactors have an emergency core cooling system (ECCS) that comes into play upon initiation of a LOCA. 

The primary water specifications for a PWR are given in Table 1 (4). Rigid controls are appHed to the primary water makeup to minimise contaminant ingress into the system. In addition, a bypass stream of reactor coolant is processed continuously through a purification system to maintain primary coolant chemistry specifications. This system provides for removal of impurities plus fission and activated products from the primary coolant by a combination of filtration (qv) and ion exchange (qv). The bypass stream also is used both to reduce the primary coolant boron as fuel consumption progresses, and to control the Li concentrations. 

Most nuclear reactors use a heat exchanger to transfer heat from a primary coolant loop through the reactor core to a secondary loop that suppHes steam (qv) to a turbine (see HeaT-EXCHANGETECHNOLOGy). The pressurized water reactor is the most common example. The boiling water reactor, however, generates steam in the core. 

For large motors, which use water as the secondary coolant in a closed circuit, the temperature of the cooling air, i.e. of the primary coolant, varies with the temperature of the cooling water inlet temperature and its rate of flow. For the performance of the motor output, this primary coolant, temperature has the same significance as the ambient temperature for an air-cooled motor. The motor output is unaffected by the ambient temperature. For such motors the output graph is shown in Figure 1.13 at different coolant temperatures and altitudes. The rating at 25°C inlet water temperature for water-cooled machines is the same as for air-cooled machines at an ambient temperature of 40°C. 

Frame surface cooled (using the surrounding medium) The primary coolant is circulated in a closed circuit and dissipates heat to the secondary ccxilant. which is the surrounding medium in contact with the outside surface of the machine. The surface may be smooth or ribbed, to improve on heat transfer efficiency (as, in a TEFC or tube venulated motor (Figures 1.19 and 1.20) 4 ... 

The four primary coolant pumps are connected to the secondary shield wall by three-link snubbers designed to be flexible under static applied loads (thus, allowing thermal expansion) but become stiff under dynamic loads that might occur during an earthquake. Accordingly, the system is coupled to the wall under seismic loading. 

This system is backed-up by a Standby Liquid Control System (SLCS) that injects sodium pentaborate into the moderator using a positive displacement pump (shown as a piston pump). Steam that originates in the core of a BWR, unlike the primary coolant in a PWR, exits the containment. The closing of the MSIVs isolates the radioactivity from the environment but when this is done, normal heat removal is not possible. The Residual Heat Removal (RHR) system... 

Loet of primary coolant flow LOB8 of feed water ricw Loae of steam Flow Turbine trip... 

Pressurizer The pressurizer maintains and controls both the temperature and the pressure in the primary coolant system. It contains a cold water spray and electric heater bank as a means of providing the necessary control. 

To slow down and control the rate of reaction, a moderator is also required. Typically, the moderator is boric acid, graphite, or heavy water (D20) and is present in the high-purity water, which also serves as a primary coolant for the fuel and the reactor vessel. The tremendous heat generated by nuclear fission is transferred to this closed-loop coolant, which is contained within a reactor primary-coolant circulation system. The high-purity water coolant also contains a suitable pH buffer such as lithium hydroxide, which has the additional effect of limiting the corrosion of fuel-cladding and other components. 

A pressurizer operating within the range of approximately 2,450 to 2,750 psig maintains pressure within the closed-loop circuit. The pressurizer contains the cold spray water system, which is typically supplied at around 550 °F (288 °C), and the electric heater-bank system. These systems maintain and control primary-coolant water pressure and temperature. The high-temperature water produced in the primary... 

Loss of expensive light- or heavy-water primary coolant... 

Boric acid [B(OH)3] is employed in primary coolant systems as a soluble, core reactivity controlling agent (moderator). It has a high capture cross-section for neutrons and is typically present to the extent of perhaps 300 to 1,000 ppm (down from perhaps 500 to 2,500 ppm 25 years ago), depending on nuclear reactor plant design and the equilibrium concentration reached with lithium hydroxide. However, boric acid may be present to a maximum extent of 1,200 ppm product in hot power nuclear operations. 

Today, all-membrane processes are also employed to ensure the integrity of high-purity primary coolant water and the removal of chlorides and fluorides. Crud (iron/steel corrosion debris) is removed by filtration. 

Section 10. II. I Chemical Program for PWR Primary Coolant p477... 

Liang, C.H. Huang, T. Chen. Boron-Lithium Chemistry in PWR Primary Coolant. Ultrapure Journal. Tall Oaks Publishing, Inc., USA, November 1992. 

Data are available for metallic impurities in the primary coolant of in-pile test loops at MOL which uses I.C.l. Na, and from Degussa Na which is used as the... 

Amey, M. D. H. and Bridle, D. H., Application and development of ion chromatography for the analysis of transition metal cations in the primary coolants of light water reactors, /. Chromatogr., 640, 323, 1993. 

The size and complexity of the N-reactor plant and the limited amount of computing equipment that was available necessitated a judicious use of simplifying assumptions. For instance, primary coolant temperature transport lags were lumped into two groups, one each for the hot and cold loop legs thermodynamic effects in the secondary system condensate headers and surge... 

Primary coolant hydraulic model (one primary pump). 

Primary coolant thermal model with lumped transport and mixing lags (the transport lag was varied as a function of the flow rate). 

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