Thanks Bob, You have offered an educated description of some of the more intricate inner behavior of a light water fission reactor. I had been previously introduced to some of the processes at work but your input is much more of the type that engineers understand. It is always refreshing to be exposed to the real life secondary considerations that result in modifications to the original less sophisticated designs.
I find your information concerning the cooling factors quite interesting and demonstrates that where a problem exists a solution can be found. Perhaps a pile of Axil pellets would not work due to the very same issues that you discuss as applying to nuclear reactors, whereas a well engineered geometry should lead to a successful design. Dave -----Original Message----- From: Bob Cook <frobertc...@hotmail.com> To: vortex-l <vortex-l@eskimo.com> Sent: Fri, Jan 16, 2015 4:40 am Subject: Re: EXTERNAL: [Vo]:TRISO LENR pellet Dave--To answer your question about reactor control I offer the following: Light water fission reactors using U-235, U-233, and Pu-239 fissionable isotopes depend on thermal or relatively slow neutrons to react with those isotopes. The slower the neutron the more likely it will be absorbed by one of these isotopes and cause it to fission. Each fission produces more neutrons at high energies that are slowed down by collisions with water and other material in the reactor until they are thermalized--at an average energy determined by the temperature of the reactor. At criticality the population of neutrons is steady with as many being produced as are leaking out of the reactor (not to enter the fuel region again) or being absorbed by materials such as control rods. More power is produced as the temperature is decreased because the average energy of the population of neutrons is reduced and the interaction rate with the fissile isotopes in the reactor is increased. If the power generated is not extracted from the circulating coolant the temperature goes up and the reaction rate (fission rate) goes down on average because the energy spectrum of the neutrons is higher. This is a negative feed back called a negative temperature coeff. and is an inherent control feature of the power in the reactor. However if the water is cooled again the power increases and holds the reactor at a selected average operating temperature. Heat extracted from the primary coolant of the reactor by a steam generator is such a cooling mechanism for the primary coolant. The fissile isotopes also react with faster neutrons at various energies, however at lower probability than they do with the thermal neutrons. Thus, they there are many fewer fissions caused by fast neutrons before they are thermalized in the reactor during normal reactor critical operation. However, with a rapid addition of neutron population, power can drastically increase at a high rate and cause a large increase of fast neutron compared to the thermal neutron population. If this happens a condition of "prompt criticality" can occur and the reactor can explode because of a high energy production rate. Reactors are designed to add a poison--a control rod--to absorb neutrons if the rate of production--the rate of population increase--is too high. Such control rod action avoids prompt criticality. An accident called a cold water accident can occur in reactors which adds a slug of cold water to the reactor and causes prompt criticality before the control rod system has a chance to add poison. This must be avoided to keep the reactor in tact. The various assemblies in a core produce differing amounts of power with the colder regions near the entering coolant producing more power than the hotter regions. Thus at higher powers the differential temperature across the core is greater given a constant coolant flow rate. To keep the temperatures in a core closer to an average temperature the flow is increased as more power is generated. Fuel assemblies are loaded with differing amounts of fissile material depending upon the location of the fuel assembly in the core with higher loading in radial positions that may have a lower neutron population on average. The fuel design objective is generally to create a system with even power generation throughout the core. Such a condition can only be approached in practice and changes as fuel is depleted with operation. Most modern reactors include burnable poisons--for example boron--that are depleted as the same time the fuel is depleted. This reduction of the poison in the fuel allows an increasing thermal neutron population inside the fuel element and thus maintains an more constant fission rate with time as the local fissile isotopes decrease. Bob ----- Original Message ----- From: David Roberson To: vortex-l@eskimo.com Sent: Thursday, January 15, 2015 2:41 PM Subject: Re: EXTERNAL: [Vo]:TRISO LENR pellet You make a good point Robin. My concept is to make one pebble first that has the characteristic that you wish and then to work on the complete system of them to end up with a good overall plan. For instance, if a coolant is flowing through a large number of them, it will extract heat from the group. I suspect that the geometry of the complete system can be played with so that all of them contribute to the net heat being extracted. This may require that coolant be injected along the container sides or other structures so that none of the pellets is over stressed. I would not think that a big random pile of these devices would work properly due to problems with heat generation and extraction, but a good engineering plan should be able to solve the problems. I would assume that a nuclear reactor would face similar issues with their multiple fuel rod assemblies yet they seem to be able to operate properly. Perhaps one of our reactor experts can help with this issue. I am discussing a design of this type in response to the pebble concept mentioned by Axil. I am not biased either for or against that idea. Dave -----Original Message----- From: mixent <mix...@bigpond.com> To: vortex-l <vortex-l@eskimo.com> Sent: Thu, Jan 15, 2015 4:15 pm Subject: Re: EXTERNAL: [Vo]:TRISO LENR pellet In reply to David Roberson's message of Thu, 15 Jan 2015 10:18:27 -0500: Hi, [snip] >The amount of positive feedback can be adjusted by establishing the proper ratio of sphere surface area to volume. As the pellet becomes larger the surface area varies as the square of the radius. At the same time the volume varies with the cube of the radius. In an ideal case that suggests that the feedback ratio would vary directly with radius. If some form of insulating material is coated upon the outer surface the fuel volume can be reduced considerably. > Take into account that this is only valid for a single separate sphere. Once they are all bundled together, heat leaving one will enter another, so in the limit, the surface area is reduced to the external surface area of the conglomeration. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
