Temperature Control – Core Inlet Temperature

Steam generator - counterflow heat exchanger

Temperature gradients in typical PWR steam generator.

As was written, the interfacing variable is in pressurized water reactors the core inlet temperature, which is fully determined by steam pressure inside steam generators. Note that, the core inlet temperature and the steam pressure are interconnected, since heat (or power) transferred across a steam generator is:

q = h . ΔT

where:

  • q is amount of heat transferred (heat flux), W/m2 i.e., thermal power per unit area
  • h is heat transfer coefficient, W/(m2.K)
  • ΔT is the difference in temperature across the steam generator (in this case, the difference between the average temperature of the reactor coolant – Tavg and the saturation temperature determined by system pressure.

For all practical purposes, the heat transfer coefficient (h) is constant, since the heat transfer coefficient is a function of the materials used in the construction of the steam generator and the U-tubes are completely covered with water.

Reactor Control - Turbine Control

Sliding Tavg and Sliding psteam Regime. The automatic system follows Tavg, set, but Tavg, set is a function of turbine load and it is programmed for best performance of the entire system.

When a reactor is in the automatic control, it follows the core inlet temperature – Tin(or the core average temperature – Tavg). Note that Tavg = (Tout + Tin) / 2. When there is a difference between actual Tin, actual and the temperature Tin, set, which is set in the system, the reactor control system initiate control rods movement. For example, when Tin, actual > Tin, set, the reactor control system automatically inserts control rods in order to decrease Tin, actual. The reactor thermal power remains constant, because the rods movement only offsets the reactivity from the difference (Tin, actual – Tin, set) x MTC = moderator defect.

On the other hand, if the energy demand in the external system increases, more energy is removed from reactor system causing the temperature of the reactor coolant (Tin) to decrease. As the reactor coolant temperature decreases, positive reactivity is added and a corresponding increase in reactor power level results. This reactor power increase occurs without any change in control rods position and without any change in boron concentration. The same inherent stability can be observed as the energy demand on the system is decreased.

Core inlet temperature is directly given by system parameters in steam generators. When steam generators are operated at approximately 6.0MPa, it means the saturation temperature is equal to 275.6 °C. Since there must be always ΔT (~15°C) between the primary circuit and the secondary circuit, the reactor coolant (in the cold leg) have about 290.6°C (at HFP) at the inlet of the core. As the system pressure increases, the core inlet temperature must also increase. This increase causes slight increase in fuel temperature.

See also: Power Plant Control

The most significant effect of a variation in temperature upon reactor operation is the addition of positive or negative reactivity. As previously discussed, reactors are generally designed with negative temperature coefficients of reactivity (moderator and fuel temperature coefficients) as a self-limiting safety feature. A rise in reactor temperature results in the addition of negative reactivity. If the rise in temperature is caused by an increase in reactor power, the negative reactivity addition slows, and eventually turns the increase in reactor power. This is a highly desirable effect because it provides a negative feedback in the event of an undesired power excursion.

Operating with a negative MTC is favorable operational characteristics also during power changes. At normal operation there is an exact energy balance between the primary circuit and secondary circuit. Therefore when the operator decreases the load on the turbine (e.g. due to a grid requirement), the steam demand decreases (see the initial electrical output decrease at the picture). At this moment, the reactor will produce more heat than the steam turbine can consume. This disbalance causes the steam pressure, the saturation temperature in the steam generators to increase (see II. pressure at the picture). As a result of increasing saturation temperature in the steam generators the moderator temperature will simply increase (see inlet temperature). Increasing the temperature of the moderator adds negative reactivity, which reduces reactor power (without any operator intervention). As can be seen, to a certain extent the reactor is self-regulating and the reactor power may be controlled  via the steam turbine and via grid requirements. This feature is limited, because also the range of allowable inlet temperatures is limited. It is power plant specific, but in general, power changes of the order of units of % are common.

See also: Reactor Stability

self-regulating reactor-min

Nuclear and Reactor Physics:

  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

  1. K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
  2. K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
  3. D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2. 
  4. E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See above: