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By Charles Rhodes, P.Eng., Ph.D.

As set out in the document titled: Mitigation of Global Warming the only dependable and economic source of non-fossil energy with the capacity to sustainably displace fossil fuels is urban sited fuel breeding Fast Neutron Reactors (FNRs). These reactors should feature autonomous operation, passive temperature control, low sodium coolant pressure and no need for a surrounding off-site public safety exclusion zone.

The US nuclear electricity industry has become trapped in water moderated thermal reactor dogma. In a water moderated reactor loss of coolant causes the reactivity to decrease which stops the nuclear fission reaction. Existing US nuclear power reactor safety regulations rely on this phenomena.

In a liquid sodium cooled FNR sodium voids (sodium vapor bubbles) have the potential to increase the reactivity which would both increase heat production and reduce rate of heat removal. This is a positive feedback process that can potentially cause thermal runaway.

In the FNR literature the term sodium void instability is a euphemism for this potential fuel melting/sodium boiling problem.

. The sodium boiling point temperature is somewhat dependent on the sodium pressure. For a sodium cooled FNR the sodium coolant pressure is typically about 0.2 MPa and the sodium pressure inside the fuel tube can range from 0.1 MPa when the fuel is new up to 5 MPa_____, when the fuel is old and the fuel tube plenum pressure is high. Hence for a fuel tube rated for a working pressure of ____ MPa the sodium boiling point inside the fuel tube can range from ____ degrees C up to ____ degrees C.

At the normal FNR operating temperature, which is several hundred deg C less than the sodium boiling point temperature, with the movable fuel bundles at normal insertion the reactivity should be zero.

Provided that the core fuel centerline temperature is always below 600 degrees C, then sodium void instability should not occur. However, for safety certainty there should be a mechanism that rapidly reduces the reactivity with increasing core fuel temperature. For safety certainty the decrease in reactivity between the reactor operating temperature and the sodium boiling point should be larger than the increase in reactivity due to boiling of sodium.

Liquid sodium cooled FNRs with metallic fuel have a sodium void instability issue that, if not addressed, in adverse circumstances could cause major equipment damage. There is typically only a small temperature difference between the design peak FNR fuel operating temperature and the fuel centerline melting point. During adjustment of the FNR temperature setpoint it is desireable to avoid fuel centerline melting.

Fissile fuel center melting concentrates zirconium along the core fuel rod center line and concentrates uranium around the outside of the central melt region.

A sudden unplanned change in the FNR nuclear fuel geometry, possibly caused by a military missile attack or a control system failure, might cause the FNR temperature setpoint to rise far above the normal operating temperature of the liquid sodium coolant. Should that happen we need to ensure that sodium void instability does not damage the reactor.

Sodium boiling forms sodium vapor bubbles (voids) which will increase the FNR reactivity further increasing the FNR temperature setpoint and hence further increasing the FNR fuel temperature and the rate of coolant boiling. This is a positive feedback process that can potentially cause thermal runaway.

In a pool type FNR sodium coolant boiling must be sustained for a long time before the increased sodium vapor pressure will structurally threaten the reactor enclosure. The main concern is fuel melting and fuel geometry distortion.

To avoid sodium void instability it is necessary to:
1) Reduce the reactivity with increasing temperature sufficiently quickly to over ride sodium void instability with respect to internal sodium at low pressure
2)Blow the fixed fuel bundle core fuel toward the fuel tube plenum on a core fuel over heat condition caused by prompt neutrron criticality

Unless suitale protective measures are taken rapid fissile fuel melting can cause equipment damage before the mechanical reactor shutdown system has time to act.

Once a FNR's temperature setpoint reaches the sodium boiling point, absent a mechanism for FNR Axial Disassembly, the FNR temperature setpoint will spontaneously rise out of control due to the change in FNR reactivity caused by sodium void formation.

The thermal output power of a fission type nuclear reactor is controlled by controlling the reactor parameter known as reactivity. Reactivity is a mathematical parameter which indicates how fast the reactor's free neutron population will grow or decay and hence how fast the reactor's thermal output power will grow or decay.

The FNR reactivity at a particular fissile fuel temperature can be changed by changing the shape, composition or atomic concentration of the reactor's fuel assembly. In a FNR the change in reactivity with temperature is due to thermal expansion of the fuel assembly components, including the fuel, fuel tubes, liquid coolant and the fuel bundle structural steel. Thermal expansion changes the atomic concentrations.

The operating temperature of a FNR that has a pancake shaped core zone is a strong function of the FNR's core zone thickness. Changing this thickness by even a few mm results in a large change in FNR temperature setpoint. The FNR core zone thickness is carefully mechanically adjusted so that the FNR reactivity is zero at the desired FNR operating temperature.

The following graph shows a family of Reactivity versus Temperature curves.

The lowest curve is for a reactor that is turned off by full withdrawal of the movable fuel bundles. Note that it does not allow a positive reactivity at any temperature between - 100 C and + 900 degrees C.
The middle curve corresponds to a setpoint of 500 degrees C achieved by partial insertion of the movable fuel bundles. At the setpoint the reactivity is zero.
The top curve corresponds to thermal runaway caused by over insertion of the movable fuel bundles. For this curve there is no temperature at which the reactivity goes negative.

Consider an operating reactor:
If the FNR temperature falls below its setpoint the reactivity becomes positive causing the FNR thermal power output to exponentially increase with time until the thermal power is sufficient to restore the FNR temperature to its setpoint.

If the FNR temperature rises above its setpoint the reactivity becomes negative causing the FNR thermal power output to exponentially decrease with time until the drop in thermal power is sufficient to restore the FNR temperature to its setpoint.

In normal FNR operation the reactivity is close to zero and the thermal power output is nearly constant.

Provided that thermal expansion gives a FNR the necessary negative slope reactivity versus temperature characteristic the FNR temperature will maintain at its setpoint independent of the thermal load.

However, at the coolant boiling point coolant void formation causes the FNR reactivity to suddenly swing positive, causing thermal runaway. Similarly, there is a potential reactivity discontinuity at the fuel melting point.

In normal operation a sodium cooled FNR operates at about 500 degrees C at which temperature its reactivity is close to zero. At higher temperatures thermal expansion of the fuel assembly causes its atoms to move further apart resulting in negative reactivity. At lower temperatures thermal contraction of the fuel assembly causes its atoms to move closer together resulting in positive reactivity. Thus a FNR will normally maintain its setpoint temperature.

An important issue in sodium cooled FNRs is the ratio of the fuel assembly components. Some components such as Pu-239 and U-235 contribute positive reactivity. Other components such as Fe, Cr, U-238, Zr, and Na contribute negative reactivity.

The worst case is when the fuel is new so that no sodium is embedded in the core fuel. As a result the negative slope of the rectivity versus temperature curve is minimal, so tha at the sodium boiling point there might not be enough of a reactivity reduction to compensate for ths sodium void instability.

Remember that the net reactivity must be zero at the operating temperature and the slope of the net reactivity versus temperature curve must always be negative. At the coolant boiling point this slope may swing positive. Ideally at the sodium boiling point the reactivity should not swing positive. If it does there is potential for damage due to sodium void instability.

If there is sodium void instability there is serious potential concern about fuel melting potentially changing the fuel geometry. In this respect, during fissile fuel rod casting, Zr tends to concentrate along the core fuel rod centerline. That effect will lower the Zr concentration at the core fuel rod surface.

If a FNR has a pancake shaped core zone any axial change in the core zone thickness will affect the FNR temperature setpoint. If the temperature setpoint is suddenly increased due to a sudden decrease in core zone thickness the fissile fuel will almost instantly adopt the new setpoint temperature and the surrounding sodium coolant will adopt that temperature as soon as the required heat can thermally flow from the fissile fuel to the sodium. If the new FNR temperature setpoint is above the boiling point of the sodium coolant, then the FNR will attempt to evaporate the sodium coolant.

The contemplated FNR design has an atmospheric pressure liquid sodium coolant pool which provides abundant coolant thermal mass and has sufficient space for sodium vapor above the sodium pool so the main concern is prevention of damage due to fuel melting. However, if the reactor does not shut down, the sodium pool temperature will rise and the sodium vapor pressure may eventually become high enough to rupture the argon containment bladders.

It is essential to have reactor safety shutdown systems that recognize fuel overheating and quickly reduce force a FNR shutdown by reactivity reduction.

To minimize the consequence of an external overhead impact FNR's are mechanically designed such that any overhead impact causes the reactivity to decrease.

In the FNR literature the term sodium void instability is a euphemism for this potential fuel melting/sodium boiling problem.

To address this problem there should be two independent reactor shutdown systems and there should be a means for minimizing fuel melting while the shutdown is occurring.

Inherent Safety Aspects of Metal Fuelled FBR
Carbide as a Candidate Fuel for SFR
Measurement of Thermal Neutron Capture Cross Section of Cs
Na-23 Neutron Cross Sections

This web page last updated June 19, 2023.

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