Thermal hydraulic performance of naturally aspirated control rod housing assemblies

The Savannah River Site uses heavy water reactors for the production of nuclear materials. Heavy water (process water) provides core cooling by downflow through the fuel assemblies, ultimately discharging into the reactor vessel. The stainless steel vessel contains fuel/target assemblies, control rods, and various internals for coolant mixing and diversion. Four hundred twenty seven (427) control rods grouped in sixty one (61) control rod sleeves (septifoils) modulate core power. The septifoils are nominally 20 feet long, 3-5/8 inch diameter aluminum tubes subdivided into 7 sections, six channels radially arranged around a center channel. During reactor charging operations, septifoils are positioned onto nipples extending into the tank from a ring header which injects process water into the septifoils to cool the control rods. The coolant exits the septifoils through holes and slots at the top. The rods produce heat through a radioactive decay process initiated by the capture of neutrons during reactor operations. Normally, approximately 90 gpm cools each septifoil, however, it is postulated that during charging operations, a septifoil may be inserted without seating on the nipple. Should this occur, forced cooling would not be provided. Coolant would be aspirated into the septifoil from below by the thermal-hydraulic action caused by the hot control rods. Coolant may also re-flood through the exit slots and holes at the top. Various computer codes are used to model thermal-hydraulic response in the reactor. These codes are typically semi-implicit finite difference based numerical models which solve the transient mass, momentum and energy equations associated with water and steam flow. To benchmark the models, experiments were conducted to determine the response of a septifoil to the unseated condition. Experiments included several mockups of the septifoil with simulated control rods. Uniformly-distributed power levels were supplied to the rods with no forced flow. Parameters measured included basic cooling phenomena, incidence of film boiling, coolant flow rate, pressure rise, and the ratio of heat transfer through the wall of the assembly versus heat transfer to axial water flow through the assembly. In parallel, the experimental assembly was analytically modelled using the computer codes and blind predictions were made of the test outcome. A power level of about 3.4 KW/ft of control rod constituted the target power for the analyses. Testing was conducted at several powers including 3.4 KW/ft, the power level which resulted in film boiling in the assembly. The data and observations from these tests suggested the cooling mechanisms associated with this unique geometry. For instance, at low and moderate power levels, the flow within the septifoil is primarily driven by classical buoyancy effects while at higher powers, chugging/reflooding become the dominant processes. Subsequent, sighted analyses were made to evaluate predictions of film boiling. Once data from the inputs was developed, an analysis was conducted to determine the degree of conformance. An independent review was completed to recommend enhancements to the codes to improve their predictive ability.