Teachin g N o tes
Operation a l Reacto r Saf et y Course
Lecture : 4 – Fue l Depletio n an d Relate d Effects
Objectiv e : Review the concept of burn up, transm utation, conversion and breeding and the role that sam a rium and xenon play in operations.
Ke y Point s t o Brin g Out:
Slid e num ber Point s
3 Present the depletion equation a nd provide a clear understanding of the definition of burnup, and what it physically m eans. This is a commonly used term that is frequently m i sunderstood.
4 Transm utation should be described as a general process where it’s the ability to produce a particular type of nuclide which can be related to a neutron abs o rption a nd decay process. Thus where th e chart of the nuclides com e s in. W e can m a ke design choices to m a ke cores that are conv erters na m e ly users of the uranium , and also breeders to produce plutoni um and other fuels. The key param eter in term s of being able to successfully being able to do that is the eta(n), which is a num ber of neutrons produced per fission. And the concept of convers ion ratio should be explained.
5 Shows the various etas as a func tion of energy. The key point here is to show that plutonium 239, 241 and uranium 235 all have essentially the sam e etas at about 0.01ev. However there is an energy range where plutonium has cer ta in advan t ages in te r m s of num b er of neutrons per fission (about 100ev).
6 A broader range of etas as a f unction of energy indicating that at high energies, the num ber of neutrons produced is greater which would suggest the breeder reactors need to be high neutron energy m achines rather than con v erters to take advantag e of these ex tra neutrons for breeding fis sile Pu239 from fertile U - 238.
7 Exam ples of conversion ratios fo r various types of reactors with the liquid metal fast breeder re actor having breeding ratios, not conversion ratios.
8 Presents the build up of pl utonium during operation and the depletion of uranium 235 such that by the end core life, plutonium 239 and 241 produce a substantial amount of energy which com p ensates to a degree the cons um ption of U235. Shown also on the slide is a website for the chart of nuclides which is very useful in identifying typical cross sect ions of various isotopes which you could spend som e ti m e discussing.
9 Discusses th e concept of reactivit y penalties for different types of transm utation products. The buil dup of neptunium and a m ericum as a function of num b er or recy cles n e gativ ely affects the ability to m a intain criticality becau se these iso t opes are n e u t ron abso rbers.
10 Begins the discussion of the need to keep track o f som e of these fission products in term s of their creation and ho w they affect the ability to h a ve a cr itical reactor. The fission frag m ent balance equation is p r esented tha t ultim ately will be us ed to discus s th e buildup of sam arium and xenon, wh ich are fission products that can affect significantly the opera tion of reactors. Show students absorption cross-sections from the KAERI table of nuclides.
11 Shows the buildup of sam a rium as a function of the decay of the fission product neodym ium . The ch art shows how the decay chain functions in the buildup of Sm 149.
12 Shown is the effect of sam a ri um buildup during the start up, shut down and restart. The keys point is to write the fission fragm e nt equations to see how the buildup of sam a rium oc curs and it can only be destroyed by neutron ab sorption. The production occurs by the decay of fission prod ucts – a final balan ce is reached where the build up rate is m a tched by the bur nup site which is a function of flux level.
13 This char t is sim ilar to th e Sam arium chart except it has another fission prod uct decay ch ain source and Xe 135 also decays. W r ite the equa tion s. Point out that if the rea ctor is not re star ted with in a certain pe rio d of tim e, it is very d i f f i cult to res t ar t, to overr ide the xenon poiso ning the reactor. If not restarted one will have to wait until the peak xenon 14 has decayed.
14 This is an important s l ide since it re late s the f l u x level of th e core or power lev el to the ability to re s t art in a ce rtai n period of tim e .
For exam ple, if the neutron flux is 5x10 14 as shown in curve 4, there would be significant penalt y due to xenon build up that m a y prevent the restart of the reactor since there would not be sufficient control rod worth in the core override the xenon to restart.
15 Summ arizes som e of the operational im pacts asso ciated with fission products. Xenon oscillat ions m a y occur during flux redistribution in the core (m ove m e nt of control rods). Length of the fuel cycle is an im portan t elem ent in core design. Fuel m a nage m e nt strategies need to be developed to assure that with the reactor can be refueled in ways the can have the core operated for the next fuel cycle of a fixed period. During the core design of the refueled reactor, peaking factors have to be lim ited for the cycle to avoid derates due to exceeding safety lim its for transients.
16 Summ arizes the process used to do reacto r physics calculatio ns. This is an important slide for the students to understand in term s of summarizing all the basic concepts to date. Knief has a good summary.
MIT OpenCourseWare http://ocw.mit.edu
22.091 / 22.903 Nuclear Reactor Safety
Spring 200 8
For informati on about citing these mate rials or our Terms of Use, visit: http://ocw.mit.edu/term s .