In depth: Safe nuclear power
A major argument against the use of nuclear power is the risk of releasing radioactive material into the environment. In the case of light water reactors, this may be caused by a steam explosion, a hydrogen explosion or a meltdown. What many people do not realise however, is that while meltdown and release of radioactive material are valid concerns, nuclear (bomb type) explosion is not possible in today’s light water reactors. The physical properties of the core exclude the possibility of a nuclear explosion. The same is true for a thermal molten salt reactor.
A steam explosion is what happened to the Chernobyl reactor in 1986. Although this lead to a serious disaster, it is much less destructive than an actual nuclear explosion (Wilkins, 2011). Due to the nature of molten salt reactors, the risk of either a steam explosion, a hydrogen explosion or a meltdown is effectively eliminated.
Light water reactors (LWR) are today’s most commonly used nuclear reactors. There are three types of LWR’s: BWR’s (boiling water), PWR’s (pressurized water) and SCWR’s (supercritical water). The majority of reactors worldwide are PWR’s (60%) so they will be further described here. PWR’s have a steel reactor vessel, in which pressurized water at 155 bar serves both as coolant and moderator for the fuel rods in which the nuclear reaction takes place. The water in the pressure vessel is pumped around through the reactor core and the primary cooling circuit, which delivers the heat to a secondary cooling circuit that drives the steam turbine that produces the electricity.
Containment structure of LWR’s needs to withstand steam explosion
The most volatile aspect of nearly all reactors in operation worldwide is their pressurized water. The water is kept at high pressure (155 bar) in order to raise its boiling temperature. This high temperature is needed to carry more thermal energy, and high pressure is the only way to achieve these high temperatures in liquid water (Hargraves & Moir, 2010, p. 310). If there is an accidental breach in the vessel and the pressure is lost, the boiling temperature suddenly drops, and a steam explosion occurs. To prevent this scenario, the high pressure needs to be safely contained, which is achieved by a system of highly engineered, highly expensive piping and pressure vessels called “the pressure boundary”. For cases in which the pressure boundary fails, reactors are equipped with a ‘containment structure’: a huge concrete dome constructed to contain a steam explosion. The ultimate goal of these structures is to prevent core materials from spreading in the environment. The Chernobyl reactor did not have such a containment structure (Hargraves & Moir, 2010, p. 310), (LeBlanc, 2009, p. 1645).
Meltdown prevented by injection of cooling water
The nuclear fuel in a light water reactor core needs to be cooled to keep temperatures within limits. This is achieved by pumping coolant water to the core. When the reactor has to be shut down in an emergency, the nature of the nuclear material in the core causes it to produce a diminishing but substantial amount of heat, during the first days after the power has been shut off. Pumping coolant water is then taken over by emergency backup generators. But if these fail, the heat in the core may boil off all the water surrounding it. This eventually exposes the nuclear fuel, which then can reach very high temperatures and melts, hence meltdown (which happened in Fukushima). This highly radioactive molten fuel then has the potential to burn through the protective layers of steel surrounding the core and causes release of volatile fission products into the outside environment, with potentially dire consequences (Matson, 2011), (Gamble, 2011).
No steam explosion possible in a LFTR
Contrary to traditional reactors, the thorium-MSR (liquid fluoride salt) operates at ambient pressure. The molten salt mixture operates around 500-700 °C and acts both as fuel and coolant. The boiling point of this mixture is over 1400 °C, at a comfortable distance from the operating temperature. Operation at ambient pressure eliminates the risk of a pressure explosion. Any break in the primary circuit would result in a leak, not in a steam explosion. Any leaking fluid salt would simply be captured in a catch basin where it can passively cool and harden (Hargraves & Moir, 2010, p. 310) (LeBlanc, 2009, p. 1644).
Meltdown risk is irrelevant with molten salt reactors
Because the molten salt mixture in the LFTR is already in a molten state, a ‘meltdown’ accident – worst case scenario of a light water reactor – is simply not an issue. In case of a hypothetical runaway reaction, temperatures could increase but this would only potentially damage the vessel containing the salt mixture. Even so, thorium-MSR’s have two passive mechanisms that prevent runaway reactions: a negative temperature coefficient and a freeze plug.
Negative temperature coefficient keeps reactivity in check
If the temperature in a molten salt rises, the nuclear reaction starts getting less efficient. This is called ‘a negative temperature coefficient of reactivity’. Engineers use this property to design the core structure and the composition of the fluid fuel. These are tuned in such a way that the reactor ‘follows the load’: if heat is drawn from the core, the liquid cools and the reaction intensifies. If no heat is drawn from the core, the liquid heats and the reaction diminishes. When the temperature in a reactor rises beyond a certain level (for instance in case of malfunction of the secondary coolant or the generator), the fuel will expand, which reduces the effective area for neutron absorption. This automatically decreases the rate of fission, thus acting as a self-limiting property of the fuel. This requires no human intervention (Hargraves & Moir, 2010, p. 310), (LeBlanc, 2009, p. 1645), (Juhasz, et al., 2009, p. 4).
Melting plug drains the salt when overheated
Another simple yet effective safety feature of the (thorium) MSR is the melting plug in the piping system beneath the reactor core, made from frozen salt. The plug is kept frozen by cooling it from the outside by an electric fan. When temperature rises too high or when the power to the reactor (and the fan) is lost, the plug will melt and the liquid fluoride fuel simply flows away into a safe containment basin (Hargraves & Moir, 2010, p. 310) (LeBlanc, 2009, p. 1644), (Juhasz, et al., 2009, p. 3).
Light water reactors require presence of water and external power to shut down the reactor safely. The mechanism of the freeze plug in molten salt reactors does not need external power. It was successfully demonstrated during the Molten Salt Reactor Experiment in Oak Ridge in the 1960s. It worked so well, that it became the standard shut-down procedure at the time. Of course, further design specifics will need to be developed that will ensure the reactor can be shut down under all circumstances, but the intrinsic properties of MSR technology grant engineers a head start.