Evaluation of heat-removal intensification in research reactor cores

Abstract

Research reactors are technological proving grounds for solving applied problems and validating new technologies as well as a base for performing fundamental research in various fields [1][2][3][4]. The highest thermal-neutron flux required for such purposes Φ T max ≥ 10 15 sec -1 ·cm -2 in currently operating vessel or pool reactors is reached inside the reflector or in-core traps. This flux is proportional to the specific energy-release density q V = Q/V in core volume V at reactor power Q. As a result of high fuel concentration, ensuring a short diffusion length for thermal neutrons in the core, together with an external moderator, a burst of neutrons is produced at the core boundaries and a substantial coefficient of nonuniformity of energy release obtains -K V > 2.5. This produces a proportional increase of the heat flux from the surface of the peripheral fuel elements, reaching 10-15 MW/m 2 in vessel reactors. The presence of such a heat flux creates large differences between the coolant temperature T ƒ * and the surface temperature T W max of the fuel elements at the hot points:where S is the heat-emitting surface of the core; ζ ≡ S/V is the compactness of the core; q S max is the maximum heat flux; and α is the heat-emission coefficient of the surface of a fuel element.As follows from Eq. (1), depending on the design of and the operating regulations of the reactor as well as the safety requirements, the increase in the power of the reactor is limited by the admissible coefficient of excess K q = q S crit /q S max > 1 or K T = T W adm /T W max > 1, where q S crit is the critical heat flux, and T W adm is the maximum surface temperature admissible for a fuel element under concrete operating conditions. When the wall temperature T W max exceeds the coolant saturation temperature t S in a given region of the core, wall boiling arises on the surface of the fuel elements, and as a result the coolant density changes. Within certain limits, this does not violate any safety criteria and does not result in burnout of the fuel-element cladding. However, it does result in the appearance of statistical fluctuations of the neutron fluxes (neutron noise [5]) which can make an additional, and sometimes even determining, contribution to the error in high-precision scientific experiments. In this case, in a reactor where the pointkinetics model works in a wide range of low-frequency composition fluctuations or positions of the elements of the core in any region of the core, the source of the fluctuations gives rise to coherent noise in the interior detectors as well as in exiting neutron beams [6]. For this reason, it is best to use for the criterion limiting the increase of the total power Q 0 by a factor of K Q , where K Q = Q/Q 0 , the condition whereby the admissible surface temperature of a fuel element at a hot point in the plane Z max does not exceed the saturation temperature in this plane t s (Z max ):As follows from Eq. (1), different methods can be used to increase the reactor power while maintaining the composi...