By Ed Pheil
I mentioned at the Thorium Energy Conference 2013 that accelerator driven systems (ADS) were likely to hurt the nuclear industry more than help it with the following discussion.
The ADS proponents are justifying their participation, including with thorium, on the basis of two concepts that cater to the anti-nuclear crowd.
A) ADS supporters imply that nuclear reactors can blow up like a bomb. They imply this by saying we should not allow reactors to go critical. They think ADS systems are needed because they allow the reactor to stay subcritical, and inject the extra needed neutrons to create power. In actuality, commercial nuclear reactors have NEVER had a problem with criticality problems coming any where near a risk to the public. Also, to minimize the power of the accelerator needed you need to keep the reactor very close to critical, so it still needs control rods. That means you can STILL have the same criticality accident risk with rods, plus a steam line rupture equivalent accident that could take you critical. If the keff is too high and you turn on the accelerator at too high a neutron generation rate, you could also take the reactor super-prompt critical; being able to shut down the accelerator as claimed by the ADS community would not help you.
B) ADS advocates also say that transuranics (TRU’s), also known as minor actinides (MA) in Europe, ARE a major problem, again catering to the anti-nuclear community. All of the MA’s in the world would not fill the room we were in at the conference center, barring criticality control and cooling systems. That is, only the MA’s after reprocessing by removing the air, cladding, useful fuel, and fission products (FPs). Spent fuel with all those things un-reprocessed would only cover 1 football field to a depth of 7 meters. So, there is no possible way that either spent fuel and, especially MA’s/TRUs, are a real waste problem because the volume of waste is SO VERY TINY.
The only real issue is that countries like the US and many others do NOT REPROCESS fuel because they fear proliferation. LWR fuel has too much 240Pu, so is NOT weapons grade, and can’t easily be used for weapons. Further, I said that MAs/TRUs may have a half-life of 10,000 years, but, with such a small volume, why does it matter — from a purely technical point of view. The toxic waste from coal, solar, wind, etc. could bury Geneva in toxic waste that has an INFINITE half-life; it lasts forever. And those wastes are not nearly as well controlled as nuclear waste.
At the very least we must weigh the risks of the different wastes on a fair basis and not treat nuclear waste as if it should be burned at the stake for being a witch (it was Halloween). They challenged me, saying that I did not care about getting rid of nuclear waste. I said that the normal environmental method is to reduce the production of waste. That can be done with liquid fueled reactors and fast breeders. It is not done by building more LWR’s and producing more waste so that the ADS reactors have enough Pu for fuel with the Th reactors. ADS advocates have it backwards on reducing waste; they say we need to produce more and then use the ADS to get rid of it.
Then I went over what I thought were the top three most important things to work on for future reactor designs to improve them
1) We must ensure that decay heat can be removed for an infinite grace period with NO power available. The ADS system does not address that explicitly. It makes it worse because it adds a huge high power accelerator that can overheat, cause huge capital losses, injuring the utility financially if repairs are required. The reactor that goes with the ADS system would have to address the decay heat removal, but that is no different from a reactor of the same type without the ADS, so there is no advantage.
2) We should reduce the potential of fission product dispersal from all reactor designs.
a. Water has high pressure steam blow down, steam-zircalloy exothermic reactions producing hydrogen that can explode, damaging the both the reactor vessel containment, and the large containment, i.e. the two other barriers to FP dispersal. I assume that FP’s have already been released to the coolant if the steam – zirc reaction is occurring, which is the first FP barrier of the three. These are all instigated by failing to successfully address criteria 1 above. Water-cooled reactor designers have to take these risks into account and provide mitigations like hydrogen recombiners and high pressure capable containment vessels or filtered vents.
b. Sodium cooled reactors have little or no pressure, but have air and water exothermic reactions that can break containment and/or disperse FP’s. Designers have to provide mitigations like large pool coolant systems and double tube heat exchangers.
c. Gas reactors have high pressure blowdown dispersive mechanisms, and the solid fuel could still overheat, depending on the design. Ensuring fuel integrity under accident conditions is a significant design challenge.
d. Pb and Pb/Bi have no pressure but tend to have steam cycles in the primary heat exchangers due to the need to limit temperatures to control corrosion. The corrosive nature of Pb coolants in the past has led to high corrosion in the heat exchangers, and occasional leaks of the higher pressure water into the Pb coolant. That can cause a steam explosion, which provides a dispersive mechanism of toxic and radioactive 210Po contamination, as well as any fission products that pass the fuel cladding due to corrosion in Pb/PbBi.
e. Liquid fueled (LF) Fluoride (F) or chloride (Cl) MSR reactors have low pressure, do not react aggressively/exothermically with air or water, would have an intermediate loop so the power loop would not be adjacent to the primary loop. Even if a volatile power cycle fluid like water is used, any steam explosion is not on primary coolant/FP’s, and air or nitrogen could be used in the high temperature power cycle. LF reactors can be designed to constantly remove gasses from the reactor, and require systems designed to protect them, so any reactor accident would have very little FP gasses to release.
The boiling point is 600C above the operating point and the reactor shuts down due to fuel expansion. Under certain accident conditions it dumps the coolant to a dump tank which must be designed to provide sufficient decay heat removal to prevent it from heating to the boiling point, but this is easier with a coolant that can take high temperature because heat transfer rate is proportional to the delta T, and the surface area required to remove goes down with increasing temperature.
F and Cl are very chemically active and want to bond with everything, so most other FP’s and MAs/TRUs will be tightly bound in liquid or solid form, so are less likely to volatilize out of the fuel/coolant. The core operating temperature of LF – MSRs is just above freezing, so when the fuel gets dumped into the passive cooling tank or similar passive cooling system kicks in, it can freeze locking in most other fission products, unlike in water or gas reactors.(although this may be an advantage in Pb, Na, Li reactors as well).
The LF-MSR, would need to maintain the 3 fission product barrier rule, but since it eliminated the fuel cladding to get the liquid fuel advantages, another barrier would need to be added, such as a second double reactor vessel. All of the reactor vessels are low pressure, as is the containment vessel since there are no pressurizing mechanisms. This keeps adding the extra barrier outside of the primary vessel from being expensive.
As for ADS, if a liquid fluoride molten salt reactor (LF-MSR) is used they have these same advantages. BUT, due to the accelerator the design needs a reactor vessel (RV) window to get the beam in. Any dry well, or target at the center of the core would receive very high proton and neutron (1 GeV) material damage at the center of the core, potentially causing failure of the internal RV containment. If the target is water-cooled that is a potential water ingress (criticality) concern and steam explosion concern at the center of the reactor. Plus if the super-conducting accelerator cavities heat up, they can explode, very quickly risking the containment.
3) Today’s reactor capital and operating costs must be reduced. Adding an accelerator to fix non-existing reactor problems (1 and 2 above) violates this need for the nuclear community. When I said that an ADS system might double the cost of the reactor system, I was corrected. That was too low an estimate. Increasing the capital cost of nuclear reactor systems is THE PRIMARY method used by anti-nuclear activists for fighting the expansion of nuclear power.
At less than 20% efficiency in the accelerator, the accelerator will eat much of the electricity reducing the overall efficiency and thus profit. The low reliability of accelerators (32 trips/day, with a goal of 1 trip per day, equivalent to solar power “trip/day”) will reduce plant capacity factor of reactors from the current very high state of 95% in the US, to something lower, further reducing profit where as the LF-MSR will increase system thermal efficiency to 40-50% and may be able to increase the capacity factor beyond 95%, thus increasing electricity and profit/reduced cost to consumers.
Other than the challenge that I did not care about nuclear waste, which was solidly answered, the ADS community did not respond. They did say that Europe has a law to require getting rid of MA’s. I said that that is NOT a real/technical problem, but a problem of perspective and a political problem. At the end of the summaries they decided NOT to open the floor to the general community for comment as originally planned, but directed one ADS community person to make final statements, thus preventing further discussion on the subject. So I suspect the ADS community either didn’t want to address my comments, and/or didn’t want to hear others support my comments.
In summary, the ADS community is catering to the anti-nuclear community by saying:
- critical reactors are unsafe (they are NOT),
- MA’s/TRUs are a problem (they are a political problem, but not a real problem when weighing risks properly)
- they dramatically increase the capital and operating cost of reactors
Feedback/Corrections/Additions/Opposition/Discussion is not only welcome, but encouraged!
I got a LOT more attention at the conference after that soliloquy.
About the author: Ed Pheil is an advisory nuclear engineer who has been working as a nuclear energy professional for thirty years. He is current focusing on the potential for liquid fuel molten salt reactors with a variety of actinide fuel cycles.