Patent Number: 055531091
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring to FIGS. 2(a)) and 2(b), there is illustrated a closed pressure vessel 10 having a coolant, for example, water, within the vessel 10. Heating elements according to the present invention are disposed within vessel 10 in contact with the coolant. Such heating elements may comprise a single-ended heater rod 12, as in FIG. 2(a)), or a double-ended heater rod 14, as illustrated in FIG. 2(b). A single-ended heater rod exits the pressure vessel 10 only at one end, while the double-ended heater rod exits the pressure end at both the top and bottom ends of vessel 10. One or more of the heater rods embodying the two heater elements as described below may be disposed in the vessel 10. Referring now to FIG. 3, there is illustrated an example of a single-ended heater rod comprised of a double helix heating element. Heater rod 16 includes internal double helix heating elements 18 and 20 separated from an outer tubular metallic cladding 22 by suitable electrical insulation, typically boron nitride. The heating element can be fabricated from a uniform wall thickness tube using a numerically controlled machine tool. Thus, two continuous helices are generated with a width versus length variation which represents the desired power versus length relationship for each of the two length terms f.sub.1 (x) and f.sub.2 (x), as set forth in Equation (2). One end of each of the two helices is connected to ground and the other ends are coupled to two independently variable power supplies. Thus, the heater rod 16 may be connected to the independent power supply 24 having a controller 26 for varying the power output from supply 24 as a function of time. Similarly, power for the heater rod 16 is supplied independently from a separate power supply 28 having a separate controller 30 for varying the power output from supply 28 to the heater rod 16 as a different function of time than the controller 26. Referring now to FIG. 4, there is illustrated in cross-section a pair of coaxially and radially spaced heating elements forming a heater rod. The coaxial heater elements 32 and 34 are separated one from the other by electrically insulating material 36. It will be appreciated that more than two coaxially arranged heating elements can be provided to more accurately represent Equation (1) for a nuclear fuel rod. Also, the cladding may serve as the outer element 34 for the fuel rod simulator. These elements can be either solid or of a helical configuration. If the elements are solid, the axial power profile can be realized by either using tapered wall tubes of one material or uniform wall thickness tubes of more than one material with different coefficients of electrical resistivity. The ends of the two heating elements have a common ground and the other two ends are connected to electrically separate power supplies, similarly as in the embodiment of FIG. 3. With respect to double-ended heater rods as illustrated in FIG. 2, those rods exit the pressure vessel at both the top and bottom ends. Double-ended heater rods may be of the helix or coaxial type, or a combination of both types. With reference to FIG. 1(a)), there is graphically depicted the output of the two heater element FRS versus length, i.e., the length along the simulated fuel rod from its lower end to its upper end, at three different times. As will be appreciated, the power supplied to each heater rod element can be independently and continuously changed over time. Thus, by continuously changing the power supplied to the heating elements, with one element being weighted toward an initial steady state condition and the other weighted toward a transient condition, the total power developed is additive of the two heater elements at each location along the simulated nuclear rod and the change in the flux shape as a function of time of a nuclear transient condition can be simulated. Thus, while previously only the total bundle power output as a function of time was available, with the present invention, both the total bundle power output and the axial change in the flux shape of the simulated nuclear fuel bundle versus time can be approximated. A simplified example is given in FIGS. 1(a)) and 1(b). In FIG. 1(a)), the power output of two heater elements at time t=o is given s wherein heater element (1) is at 100% power and heater element (2) is at 0% power, giving an average relative power of about 1.75 for the four given axial locations (nodes). At time t=t.sub.1, the heater elements are both at 50% power thereby giving an average relative power of about 1.312 for the four given axial nodes. At time t=t.sub.2, the heater element (1) is at 0% power and heater element (2) is at 100% power, giving an average relative power of about 0.875 for the four axial nodes. In FIG. 1(b), the graph gives the total relative power variation for both heater elements with axial position and time. These values for the simplified representative example of FIGS. 1(a)) and 1(b)) may thus be tabulated as follows: ______________________________________ Tabular Values for FIG. 1 Relative Powers t = o t = t.sub.1 t = t.sub.2 Axial (Element #) (Element #) (Element #) Node (1) (2) (1) (2) (1 + 2) (1) (2) ______________________________________ 1 1 0 0.5 0.25 0.75 0 0.5 2 3 0 1.5 0.5 2.0 0 1.0 3 2 0 1.0 0.75 1.75 0 1.5 4 1 0 0.5 0.25 0.75 0 0.5 Average 1.75 0 0.875 0.4375 1.3125 0 0.875 Relative Power ______________________________________ Consequently, it will be seen that the independently controlled heater elements may approximate or simulate in power output and axial flux shape a nuclear fuel bundle as a function of time. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.