More on Buffer Zones
Activities in Buffer Zone
Some industrial activities can be allowed in the buffer zone. Obvious candidates include electricity intensive processes such as CO2-free metals manufacture and hydrogen production. Releases at any one plant will be rare, roughly one in every four thousand reactor-years, based on past performance. But that does not mean nuclear power plant releases somewhere on the planet will be rare. On a ten thousand nuclear plant planet, we can be confident we will have at least one release every few years. Wind and solar also easily qualify for the buffer zone. The problem is, if nuclear is as cheap as it could be and as it needs to be, wind and solar will be economic almost nowhere.
Political Implications
I tend to dismiss selfish, cowardly, next election obsessed politicians as unnecessary evils; but wiser heads point out these creeps will always be with us. With this in mind, it's worth pointing out some political implications of buffer zones.
1) Buffer zones can replace Emergency Planning Zones (EPZ). EPZ's are areas within which evacuation drills must take place. Holding up those drills was an effective delaying tactic of the anti-nukes in the late 1970's.
2) In doing plume analysis, it would be reasonable to assume the buffer zone (and nothing else) is evacuated.
3) Buffer zones will make it much easier for weak politicians to not call for an evacuation. They can instead tell people to leave the buffer zone, go home, and shelter in place. They've done something.
4) Buffer zones make it easier to allow controlled venting. At Fukshima, at 12:20 AM on March 12, the site manager, Maseo Yoshida decided he had to vent Unit 1 and asked Tepco-Tokyo for permission.1 Tepco forwarded the request to Prime Minister Naoto Kan. Kan said not until a 3 km radius around the plant has been fully evacuated. Evacuation of this area was not confirmed complete until around 9:00 AM, and venting did not start until 10:00 AM. By then enough hydrogen had seeped into the outer building to cause the first explosion, which not only released a large amount of radioactive material, but also knocked out the mobile emergency diesel, which six minutes earlier had started sending power into Units 1 and 2, allowing high pressure water injection and core cooling. The debris also obstructed the attempt to get another mobile diesel generator to Unit 3. Without the delay in venting, Fukushima might have looked a lot like Three Mile Island.
With proper buffer zones, the plant management should be given discretion to vent, if they decide it is warranted.
Mini-Buffer Zones
The requirement for a buffer zone, as well as other economies, will tend to group multiple plants at the same site. There is nothing wrong with this, as long as the layout is such that a casualty at one unit, is astronomically unlikely to spread to other units. At Fukushima, neighboring plants shared the same stack. This resulted in hydrogen released by Unit 3 causing an explosion in Unit 4, which was not even on-line at the time.
At Chernobyl, four units not only shared the same building, but a pool of water in the basement. If the molten core had reached that water before it was heroically drained, it is quite possible the resulting steam explosion would have taken out the other units.
Each unit must be independent; separated by mini-buffer zones from the other units.
This has implications for modular reactors. ThorCon puts two 250 MW reactors in the same hull, feeding a single 500 MW turbogenerator. The design is such that it is quite unlikely that a problem on one reactor will spread to the other. But the two reactors are not truly independent. Each ThorCon must be treated as a 500 MW plant for buffer zone purposes. Similarly a Nuscale six pack is a 462 MW plant, not six 77 MW plants. It's the total source term (radioactive material) within each mini-buffer zone that counts.
If and only if the mini-buffer rule is complied with, the buffer zone extent will not need to be changed when more plants are added to the site.
Corrice, L., Fukushima: the first five days. Kindle. Amazon, 2014.
I had not heard of the politician's blunder at Fukushima. Not surprising in my experience. When a disaster strikes, technical decisions are made by lawyers and business types. The engineers are not even in the room.
I'm trying to imagine what might cause a molten-salt reactor to release radiation outside of its silo. Maybe an accident during loading or unloading? Do we need a buffer zone, if there is no possibility of a release? When a used reactor is pulled out of its silo, what is the radiation level 10 meters away?
In the sub-chapter on buffer zones in your book, you mention the buffer zone could be farmland.
In a cold climate I like the idea of putting greenhouses in the buffer zone where they could get very cheap heat from the cooling water that would otherwise just go into the lake river or sea, the water came out of before providing the cold sink for the heat engine.