Reactivity void coefficient

Positive void reactivity coefficient of the reactor, made tolerable by the high thermal inertia of the sodium coolant in the amounts generally used and by the consequent difficulty for the reactor to reach boiling conditions. 

The coolant void reactivity coefficient was first analyzed for a core with a 10% coolant fraction, a 10% enrichment, and fuel fractions ranging from 10% to 50%. The results (Fig. 3.2) show that for fuel fractions less than 30%, complete voiding of the Flibe coolant from the core could result in a positive reactivity addition. As the fuel concentration is increased to provide more realistic excess reactivity values and longer core bumup times, the relative importance of the absorption and moderation in the Flibe is reversed, and the overall void coefficient is then negative as the uranium-to-carbon atom ratio exceeds approximately 0.05. For an NaZrFs salt, the void coefficient is positive for fuel fractions less than 60%. 

Design and neutronic characteristics of the reactor core ensure negative power, temperature, and void reactivity coefficients. As a result, self-limitation of the reactor power at the reactivity accidents and transients without scram takes place. Reactor self-control properties enable to change reactor power in the range of 20 - 100% of rated power (No) at a rate up to 0.5%No/s without control rods displacement and just for automatic control of feed water flow rate. 

The core has self-regulating and self-stabilizing features due to the negative temperature, power and void reactivity coefficients. Also, the use of burnable poison reduces the excess reactivity margin to be compensated by the mechanical reactivity control system. 

Detailed neutronics calculations were performed in spherical geometry using the transport code DTF-IV in the P1-S4 approximation with 27 broad energy groups. A space-independent perturbation cross section was also computed at the center of the system for comparison with measured results. The first-order transport perturbation theory program GAPER was used in evaluating central material worths as well as Doppler and sodium-void reactivity coefficients. 

The Cirene reactor was a 40-MWe prototype power plant constructed at Latina, 80 km south of Rome. Construction start in 1976 and completion was scheduled for 1984. Commissioning stopped in 1988, before work to reduce the positive void reactivity coefficient was complete, by the general moratorium on nuclear reactor operation imposed by the Italian Government following the Chernobyl accident. 

Negative temperature reactivity coefficients and negative void reactivity coefficients ... 

For loss of coolant accident, it has been assumed that coolant is unavailable in the upper plenum, core and lower plenum of the reactor. Due to the absence of a heat removal medium, temperatures of the core will start increasing, leading to heating of all core components. The negative void reactivity coefficient will limit the power and thus, the temperature of the core components. The neutronically limited power would reach 200 kW(th). For this case, a system of 12 variable-conductance heat pipes, made of a carbon-carbon composite with a metallic liner, has been provided. These heat pipes penetrate the core. The condenser end of these heat pipes extends beyond the upper plenum and the interface vessels of heat-utilizing systems to the atmosphere. At the condenser end, these heat pipes have radiator fins to dissipate heat to the atmosphere. In case of a postulated accident due to loss of load or loss of coolant, core temperature will start increasing. As long as the temperature of the core is within... 

Void reactivity coefficient 0.07 %dK/%void (initial state) Voiding by boiling will not occur in fuel salt, because the boiling temperature is 1800 K and higher than the maximum fuel temperatures reached even in accident conditions... 

Explain the effect the selection of the proper moderator/fuei ratio has on moderator temperature and void reactivity coefficients. 

Validation of negative void reactivity coefficient (achieved by the use of a scatterer cum... 

The fuel cycle concept of the RMWR is basically a closed cycle and is the same as for FBRs (sodium cooled fast breeder reactors). It has been confirmed that the high conversion ratio, more than 1.0 and the negative void reactivity coefficient can be achieved in the RMWR core under the multiple recycling of Pu including advanced fuel reprocessing schemes. 

Transients Not important due to increased design margins and intrinsic self-protection by negative void reactivity coefficient. 

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. 

The PEACER safety concept draws upon the experience with lead-bismuth coolants and metallic fuel for fast reactors. Specifically, a Pb-Bi coolant is not only chemically inert but has a high natural circulation capability that could lead to certain inherent safety features. For example, core melting can be prevented by natural circulation of the Pb-Bi during a loss-of-flow accident. Since the Pb-Bi coolant has a much higher boiling point (1670°C) than sodium, the role of a positive void reactivity coefficient becomes essentially diminished. The safe nature of the metallic fuel in a fast reactor was demonstrated in the EBR-II programme in the 1980s, and these results are also applicable to the PEACER reactor concept. 

These had to be chosen to achieve an acceptable economic performance and to result in a steam void reactivity coefficient compatible with the limitations set by control system design and fault transient analysis. 

This performance would have a number of beneficial effects. Fuel life would be longer. The reactor stability both overall and spatial, would improve, due to the absence of the sub-cooled portion of the power channel, which leads to a substantial reduction of the power-density feed-back. Obviously the smaller water content in the core would improve the safety aspects connected with the voiding reactivity coefficient. 

For the boiling channels this programme Included, for Instance, very extensive burn-out and instability tests which culminated with tests of full scale 36-rod assemblies in a 6 to 8 MW rig where natural circulation powers for an excess of the hot channel power to the Marviken reactor was achieved. Fig. 5< In-plle tests were also performed in the Agesta and Halden reactors, whilst the physics -including the vital void reactivity coefficients - were established by extensive tests In the critical RO and hot exponential facilities at Studsvlk,... 

In contrast to most other MSRs previously studied, the MSFR does not include any solid moderator (usually graphite) in the core. This design choice is motivated by the study of parameters such as feedback coefficient, breeding ratio, graphite lifespan, and initial inventory. MSFR exhibit large negative temperature and void reactivity coefficients, a unique safety characteristic not found in solid fuel fast reactors. 

Coolant void reactivity coefficient High Low Maximize overpower transient. Delay HTS high pressure trip. 

Positive watCT density reactivity coefficient (negative void reactivity coefficient)... 

The positive reactivity coefficient or negative coolant void reactivity coefficient is necessary for the inherent negative feedback of the Super LWR and Super FR at the loss of coolant accident. The reactor power should decrease automatically at the loss of coolant accident. 

---