Boiling water reactors stability

In the operation of BWRs, especially when operating near the threshold of instability, the stability margin of the stable system and the amplitude of the limit cycle under unstable condition become of importance. A number of nonlinear dynamic studies of BWRs have been reported, notably in an International Workshop on Boiling Water Reactor Stability (1990). The following references are mentioned for further study. 

Boiling water reactor stability in the time and frequency domains... 

AMBROSINI, W., ANEGAWA, T., BLOMSTRAND, J., DE BETOU, J., LANGENBUCH, S., LEFVERT, T., VALTONEN, K., State of the Art Report on Boiling Water Reactor Stability (SOAR ON BWRS) (D AURIA, F., ed.), OECD-CSNI Rep. OECD/GD (97) 13, Paris, 1997. 

Kozlov, B. K., 1954, Forms of Flow of Gas-Liquid Mixtures and Their Stability Limits in Vertical Tubes, Zh Tekh. Fiz. 24( 12) 2285—2288. Transl. RJ-418, Assn. Tech. Service, East Orange, NJ. (3) Kramer, A. M., 1958, Boiling Water Reactors, Addison-Wesley Pub. Co., New York, (4)... 

Hu, R., 2010. Stability Analysis of the Boiling Water Reactor Methods and Advanced Designs (Ph.D. dissertation). Massachusetts Institute of Technology. 

The fuel was a mixture of oxides of thorium and uranium-235 (THUD fuel), a material expected to have high physical and chemical stability in a reactor atmosphere, and to offer possibilities of breeding in a boiling water power system. Small cylinders of this mixture were pressed, sintered and stacked inside aluminum to form a fuel pin 60 inches long and -q inch ta diameter. Two versions of this fuel were available in quantity, the first having a thorium to uranium-235 atomic ratio of 25 1 the second an atomic ratio of 50 1. ... 

This made it possible to utilize directly the technology on materials and fuel elements and most of the technology on components built up by the vast US light water reactor (LWR) programme. Pressurized water (D2O) was preferred to boiling water at this early stage primarily because of the better known water chemistry and stability. 

Peng, S.J., Podowski, M.Z., Lahey Jr., R.T., Becker, M., 1984. NUEREQ-NP, a computer code for the stability analysis of boiling water nuclear reactors. Nuclear Science and Engineering 88, 404-411. 

Flow instabilities are undesirable in boiling, condensing, and other two-phase flow processes for several reasons. Sustained flow oscillations may cause forced mechanical vibration of components or system control problems. Flow oscillations affect the local heat transfer characteristics and may induce boiling crisis (see Sec. 5.4.8). Flow stability becomes of particular importance in water-cooled and watermoderated nuclear reactors and steam generators. It can disturb control systems, or cause mechanical damage. Thus, the designer of such equipment must be able to predict the threshold of flow instability in order to design around it or compensate for it. 

An exothermal reaction is to be performed in the semi-batch mode at 80 °C in a 16 m3 water cooled stainless steel reactor with heat transfer coefficient U = 300 Wm"2 K . The reaction is known to be a bimolecular reaction of second order and follows the scheme A + B —> P. The industrial process intends to initially charge 15 000 kg of A into the reactor, which is heated to 80 °C. Then 3000 kg of B are fed at constant rate during 2 hours. This represents a stoichiometric excess of 10%.The reaction was performed under these conditions in a reaction calorimeter. The maximum heat release rate of 30Wkg 1 was reached after 45 minutes, then the measured power depleted to reach asymptotically zero after 8 hours. The reaction is exothermal with an energy of 250 kj kg-1 of final reaction mass. The specific heat capacity is 1.7kJ kg 1 K 1. After 1.8 hours the conversion is 62% and 65% at end of the feed time. The thermal stability of the final reaction mass imposes a maximum allowed temperature of 125 °C The boiling point of the reaction mass (MTT) is 180 °C, its freezing point is 50 °C. 

The stabilizing reactivity feedback caused by negative reactivity temperature coefficients for the fuel and coolant as well as the void reactivity coefficient mean that heating up the core structural components, including fuel, or water boiling in the core would eventually result in a spontaneous reduction or self-limitation of the reactor power irrespective of the positions of control rods, including scram rods. 

Fuel pin and fuel element stability tests have been conducted both In alr/water and freon loops, in the latter case under nucleate boiling conditions. These tests have all shown the fuel pins to be inherently very stable in the reference design, but have revealed some cluster vibration at about 7 c/s which is initiated by flow around the neutron scatter plug. An early type of twisted strip scatter plug was rejected because it set up torsional oscillations in the fuel the current torpedo shaped plug can be stabilized by a device already developed, but this will not be applied to reactor fuel unless the instability proves to be damaging to fuel or pressure tubes. 

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