D. Des t s o f Re d

All non-Soviet reactors are designed to have a negative temperature coefficient This makes the reactor self-regulating and stable against many perturbations. The lack of such a coefficient was one factor in the Chernobyl accident Another factor was that the reactor was deliberately operated in such a way as to render the temperature coefficient positive. Hence, a loss of liquid water would cause a power excursion.

What causes a negative temperature cmfficient? Neutrons are created from fission at high energy. In order to sustain a chain reaction, they must be slowed down. This is achieved by moderating them — they collide with light water ( the hydrogen ) and lose energy with each collision. This use of a moderator leads to two design options.

( a ) Under-Moderation — There is not enough water to thermalizc or moderate every neutron. So, we have some inefficiency in that some neutrons are wasted. But, there is also a huge advantage. If the water heats up, as it would at the outset of a power excursion, it becomes less dense. Hence, there will be fewer thermalizing collisions. This has two consequences. First, the average energy of a neutron increases. This is called spectral shift. Second, more neutrons leave the reactor core without interacting. This is called increased leakage. Both effects result in fewer neutrons to continue the chain reaction. So, the power level decreases and the excumion stops.

( b ) Over-Moderation — There is more than enough water to theimalizc every neutron. So, there is no loss of efficiency. We can build the smallest possible core. But, them is also a huge disadvantage. If the water heats up an the result of an incipient power excursion, nothing stops ie The neutrons remain fully moderated despite the loss of some water. So, the reactor overheats.

The following figure illustrates ( in a simple way ) the difference between over- and undermoderated. On the left is the amount of moderation versus the amount of water. On the left is the neutron energy versus the amount of water. A high neutron energy implies an undennoderated core.

- 2-

Undeminferaied Ovemioderated

% Moderation

0 M

Amount H 2 O

M

Amount H 2 O

If the amount of water in the core is initially less than M, the core is undermoderated. If the coolant density goes up, the amount of water decreases, and the reactor moves along the sloping pan of the curve into a state of increasingly less moderation. Fewer neutrons slow down, the neutron energy spectrum "hardens" ( i.e., neutrons do not thermalize ) , and the power level drops. If the amount of water in the core is initially greater than M, the core is overmoderated. If the coolant density goes up, the amount of water decreases but nothing happens to the neutron energy spectrum because there is still enough water present to therrnalize every neutron.

What happened at Chernobyl? The Chernobyl accident was the result of both design and

a ) — Chernobyl was a gmphite-moderated boiling warn reactor. It was designed to axiderate its neutrons completely. That is, the core was over- nxiderated. Hence, if the coolant heated up and less dens«, a<e would be no effect on the neutron specmim In contrast, all non-8oviet mrs are designed to be under-moderated and h«n>, u a« «›olant heats up, there is a negative feedback effecL Nai ly, fewer neurons are nvxleratcd and this slows the rate of

b ) Onerational — Light water both scatters and absorbs neutrons. Because the Chernobyl core was ovo-nx›derated, the dominant effect of its light-water coolant was neutron absorption. This in itseF would have been ax›eptab1e because the coolant was normally maintained at a certain quality ( mixture of liquid and vapor ) and control blades were used to absorb the neutrons and kwp the reactor exactly critical. But, prior to die accident the operands had been trying to do an experiment to evaluate a new energy storage mechanism. As a result, the of s sequence of power maneuvers was unusual and the opcrnors ended up with ( 1 ) the

- 3-

control blades fully oub and ( 2 ) die coolant alnx›st 1IXi& saturated liquid. So, the coolant was absorbing the excess neutrons from the neutron chain reaction. The operators then began to raise the reactor power. The coolant flashed to steam and hence its density decreased rapidly. This eliminated the neutron absorption and created a positive feedback effect — the pows rose faster and faster and die reactor was destroyed.

1000

100

10

0 0. 00t 0. 0t 0. 1 l.O 10 100 t000 \ 0’ f 0 5

NEUTRON ENERGY ( eV )

to‘ i o 7 i o 8

F ission C ro ss-Sec t i o n of U ranium-2 3 5 ( BW -32 5 )

Courtesy of Brookhaven National Laboratory.

, IiBfITRO R X0D8RA’£E0 X

- Neutrons aloo down or lose energy mist rap 1d1 y If they col tlde with nuc1l.des of emit ar aase • Material.s that slow down neutrons are theref ore those with lori ato&e nuabera. These Incl ude llght water, po1yethyl.ene, berylllua, heavy water, and graphl t e•

- I'he mtec1a1 of cho1ce 1n the \ In1ted States, Japan, and Burope 1s 11ght-vater• ’Z’h1s 1a ceCerred to aa the neutron aiodecator oc zioderatoc• ( Xote : Canada usea heavy xater Cor a aoderator• )

- Light water, like all other metsrials, can both scatter and absorb neutrona• U.S., Japanese, and European reactors are designed so that the xcatter£ng effect ia the dominant mode of interactions Loss of the light water therefore causes the chain reaction to stop and the reactor to shut down•

Conclusion:

- The tendency for a reactor to shut down on loss of its moderator is referred to as a ’negative power coefficient”. This is an Ln- herent ( i•e., paaa&ve ) safety feature that is required of all UPS ouslzar reac£orx.

NEGATIV E P M COEFFICIENT S

- Negative power coefficients make reactors self-regulating. Con- sider the sequence of events that occurs during a power increase:

( a ) A licensed operator causes the neut roti chain reactlon to In- crease.

( b ) Additional fissions occur This releases additional energy.

( c ) The moderator ( ice., light-water ) heats up. As it does, its density decreases. Then, there is more apace between the water molecules.

( d )

The fact that there is more space between the water mole- cules means that fewer neutrons collide with the water So, there is less neutron thermalization. Pewer neutrons slow down.

( e ) As fewer neutrons slow down, there are fewer fissions and the reactor power sCops rising and, In most cases, starts Co decrease.

Figure removed for copyright reasons.

Schematic diagram of the RBMK-1000, a heterogeneous water-graphite channel-type reactor ( source: Soviet report to IAEA )

CHRRhOBY L DN S H R P'KATBRB S

”''	Seveca1 dee1ga	features of the Chernoby1 Reactor atand	out	as be1ng
	d1st1nct €roo des1gn• These	reactors o€ U‹S‹, Japanese, Canad1an are:	or	European

Cor e S1z e - The huge s1ze oC the core oakea 1t d1C€1 cult to control. Each subregion of thc core can b* a critical reactor functioning independcotly of the rest of the core.

Pos1t1v e P o u' e c CoeCC1c1en t - The gcaph1te €u11y aoderates the neutrons produced Croa f1ss1on• Aa a result, the net eC€ect oC the 11ght••vatev coo1ant 1s neutcoo absozpt1on• EC the dens1ty o€ the coo1ant decreases, Cewec neutrons are absorbed and ttie C1ss1on rate x1ses‹ S o, these 1s poa1t1ve Ceedback• An 1ncrease 1n power causes a €ucthec, autoaiat1c 1ncrease 1n power• ( Note : 7ft1s efCect 1s present on1y at pooez 1evelc oC 20a oz 1esa• Ie waa the Cundaaenta1 cause oC the Cheznoby1 acc1dent• )

Stiutdow u System - Et tabes 20 seconds to shut down a Chernoby1- type reactor uodec eoecgency cond1t1ons• Contrast th1s v1th a

I CBCt fozg OF B 8eCOzt@ TOY’ • • ¥'GBCtO£8•

Contalnmen t S tex - The Chernobyl reactor had a containment around its primary piping and staam-producing components. The core itself was NOT within a cootainment•

Gcaph1t e leoperatuz e - The gcaph1te temperature 1s oa1nta1ned several hundred degrees ‘C higher than recomended by ides tern author1t1es •

D e s i g n F e a t ur e s

CAUSE S O F TH E CRfiRNOBY L AC C IDBTI T

1 . Po s i t ive power c oe f f xc i ent .

2. Lack of cont alnuien t .

3. Little or no use of automatic 9ystems to prevent incorrect oper-

ato r ac t ions such as wi thd racing the rod s too far .

4. Slow response of shutdown system.

5. Po s s ib iI i ty of excess ive or thd raoal of the cont ro 1 b'l ade s .

Procedura l Error s

) . Conduc t ing a research test on a commerc ia 1 reac tor .

2. Lack of a procedure for the tes t .

3. Reac tor was kept oper at Ing uhi le the tes I was performed .

Operato r Trainin g

l. Emphasis on keeping reactor in operation rather than on safety. The Chernobyl crew was rated Al in the Soy éet Union ba sed on the hours that they kept the ir reacto r at power .

2. Lack o£ underst and Ing of the phys lcs of reac tor oper at ion .

Op e ra t o r Error s

1. Bypassing any and all systems that might have impacted on their ability to conduct the test. Ten major errors of this type.