THORIUM AS AN ENERGY SOURCE - Opportunities for Norway ...

Nuclear Reactors for Thorium
result in a slow coolant heat-up. This should normally lead to a manual shutdown. Fast and
rather large reactivity insertions (which are of very low probability in a system without control
rods) lead only to a limited power increase. This is due to the subcriticality of these systems. To
stop such a limited overpower condition, a beam shut-off or the insertion of safety rods is
necessary. The switching off of an accelerator is principally simpler than the insertion of safety
rods.
5.4.4.6 Advantages of the ADS
Due to its special design, an ADS has some advantages when compared to more standard
reactors.
1. The main advantage of an ADS is the subcriticality of the core. This gives an improved margin
to prompt criticality leading to a runaway reactor like the Chernobyl reactor. However, this
kind of reactivity-induced accident is highly improbable in modern nuclear power reactors,
since they are constructed with negative power reactivity coefficients. This means that the
reactivity decreases when the power increases, thus limiting power excursions. The Chernobyl
reactor, on the other hand, had positive power reactivity coefficients, which lead to a rapidly
accelerating power increase.
2. The largest benefit of the subcriticality is that it facilitates a more effective way of
transmuting the minor actinides. The control of traditional reactors depends to a large extent
on a small fraction of the neutrons emitted in the fission process that are delayed, varying
from a few tenths of a second up to around 1 minute. For U-235 the delayed fraction of
neutrons is 0.65 %, and this prevents rapid power increases for reactivity insertions less than
0.65 %. However, many of the minor actinides (Np, Am, Cm ...) as well as U-233 have lower
delayed neutron fractions than U-235. If a substantial amount of minor actinides or U-233 is
added to the reactor fuel, it will inevitably lead to a significantly reduced margin to prompt
criticality. This problem does not exist for an ADS because the reactivity of the core can be
reduced with an amount corresponding to the reduced margin to prompt criticality, at the cost
of increasing the accelerator power to keep the power constant.
3. The production of long-lived actinides (Np, Pu, Am, Cm ...) is much smaller than for a
conventional reactor fuelled with low-enriched uranium. The storage time for the waste may
therefore be shortened from several tens of thousands of years to less than a thousand years.
Here it should be mentioned that the storage time of waste from the fourth generation of
reactors also will be significantly reduced compared with today’s reactors, since actinide
separation and recycling will be an integrated part of the fuel cycle.
4. The absence of control rods may simplify reactor control, since the power is proportional to the
accelerator power. In any pressurized reactor system the penetrations of the control rods
through the reactor vessel are possible leakage paths from the primary system. There is also a
small possibility of control rod ejection, which would lead to a rapid reactivity increase.
5. Due to the high boiling point of the lead-bismuth eutectic (1670 °C), the pressure above the
core can be kept at, or close to, atmospheric pressure. Low pressure reduces the risk of
ruptures in the Cover Gas System (CGS). Of course, in the lower part of the reactor tank the
pressure will be higher (≈ 28 bars) due to the 25 meters of lead. This is still much less than the
160 bar pressure used in the present generation of pressurized water reactors.
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