Procedure for Critical Experiment
This is Copy 53 of __________, Series A.
with best regards
of Raleigh Reactor
Went Critical at
PROCEDURE FOR CRITICAL EXPERIMENT
of Raleigh Reactor
1. The purposes of the critical reactor experiment are:
(a) To determine the critical mass volume and concentration of U235, at
room temperature conditions, without water cooling of the core.
(b) To find the effectiveness of the control rod in terms of flux changes
and equivalent fuel mass to determine the value of the shim plates.
(c) To note defects, if any, in mechanical or electrical equipment that should
be corrected before the approach to full power.
2. The experiment is to be performed in a manner to minimize the possibility of
exceeding the critical mass. Such excess would not be hazardous, in view of
the strong negative temperature coefficiant, but could result in the neutron
activation of components on which work is yet to be done. to avoid diffi-
culty, the following viewpoints are adopted.
(a) The experiment shall be understood fully by all participants. Each
person will know his role, including both the operation and the timing
of it in the sequence.
(b) All activities will be under the direction of one individual. No par-
ticipant is to act without specific instructions from the director.
The latter will perform no manual operations.
(c) All actions should be performed deliberately, carefully and reversibly.
For example if a portion of solution is added to the core, the operator
should be prepared to withdraw that same amount immediately.
(d) The experiment will be performed with a minimum of comment and conver-
sation and with complete attention to the operations.
(e) If a participant observes that some step in the procedure is not in
keeping with safe practice, he may request that the operating group
discuss the matter before going on.
3. The Operating Staff and assignment, for each are listed:
|Addition of Fuel|
|Assist in adding fuel; radiation monitoring|
|Control desk and instruments|
| ||Source manipulation; recording data|
| || Alternate Director; log book and recording data; |
Interpretation of results
After start of experiment,
4. The four phases of the experiment are
Phase 1 - Check-out of Equipment
The completion of each item below will be reported to the Director and checked
off on his list, with the name of the person performing the duty.
1. Assemble group of observers and operating personnel in auditorium to state
rules of conduct and outline plans for experiment. (
List names of observers and participants in log book. (
2. Assemble operating personnel in contrel room for briefing. Provide seating
facilities, writing space. (
3. Check that film badges and pocket dosimeters are on. (
4. Test control room and reactor room monitor alarms with source. (
5. Check filling system. Air pressure, vacuum available. Valves all operable.
6. Supply of distilled water, assorted graduates, containers on hand. Rubber
gloves for handling solutions. (
7. Check public address system to and from reactor room and to auditorium.
8. Source in place. (
9. Record books, graph paper, data on fuel available, clock or watches. (
Ink supply and replacement pens for recorders, (
10. Check that all exposure ports plugged and locked except source opening. (
Run with water out of cooling coils
11. See that cooling water valves are closed. Remove valve handles. (
12. Lock all building doors except one designated for exit. Post signs no
admittance. Brief department secretary on telephone calls if experiment
performed in daytime. (
13. Lock doors entering to reactor room. Check electrical controls of doors.
14. Test action of control rod and recorder indication of position. Repeat for
safety rod. (
15. Test rod drops. (
16. Test operation of shim control circuit. (
17. Test trips of PCP channels (
18. Collect prints of wiring diagrams for reference. Also have necessary tools,
fuses, check instruments. (
19. Survey meters on hand and in operating condition. (
20. Record specific list of operating instruments at time of experiment, which
ones connected to alarm, which to trips; location of every instrument.
List channels by name, location + detector locations
, type of trips.
21. Test stabilitv of instruments with repeated counts. B10 #1, B10 #2, Fission
chamber, BF3. Record observations. (Staff) also
+ Auto control reading + PCP
22. Note response of instruments to introducing source in reflector. Record
of source by
Phase 2 -- Addition of Material During Counting Period
In this phase, the bulk of the U235 is added to the core. Predictions of
the critical mass Mc are made after successive increments. The number of counts
taken over a fixed period increases as the multiplication of neutrons increases.
Thus 1/C approaches zero as the mass approaches Mc. Graphs of 1/C are repeatedly
extrapolated to obtain improved estimates of Mc,
The enriched uranyl sulfate is supplied in individual bottles with U235
contents as listed
The chemical concentration is approximately 600 grams per liter. The total
volume of solution as shipped is thus 1.67 liters. The final solution volume for
the critical experiment is to be close to 13 liters. If the critical mass turned
out to be 715 grams, this would mean that about 12 liters of water are needed.
At full power an excess of 75 grams over the critical mass will be present. The
ratio of hydrogen to uranium atoms then should be between 450 and 500. The
addition of water and sulfate solution should be made in such a way that the atom
ratio Nh/Nu is essentially fixed.
Assume that 430 is near but below the proper Nh/Nu ratio. To maintain this,
It is to be noted that there is noise in the switches
on console + thus spurious counts in
in A.C. lines.
the water addition per 100 grams must be 1.5 liters. In the event that the
critical mass is larger than 715 grams the water content should be kept low during
the initial stages.
(Predicted critical mass 715 ± 50 grams) 12.46 12.7
1. Initial conditions, Safety rod up, control rod down, shims up.
2. Take two background counts on instruments* with source far from core and no
water in reactor. Time, 5 minutes. As the counting rate increases, the time
may be reduced, being sure to keep statistical accuracy.
3. Insert source in normal position, centered under core.
4. Take two 5 minute background counts with source in position, Leave source
5. Study reproducibility of data.
6. Add exactly 6.0 liters of distilled water to core. Record at sampling station.
Note and report liquid level measurements. Note solution temperature. One
person is stationed on shield, one at sampling control board.
7. Take two 5 minute counts. Study consistency of data.
8. Add two 200 gram bottles of fuel solution slowly. (400 grams). Record at
9. Agitate solution by air pressure on lower filling tube for about one minute.
10. Take two counts with control rod in. Plot responses in form of 1/C vs. mass
on graph paper. Extrapolate to expected critical point.
11. Check for a rise in neutron level. Write appropriate notes on recorder chart.**
12. Retract control rod completely at full speed, watching recorders, Note on
chart. Take two counts with control rod out.
|B10 #2||B10 #2|
13. Lower control rod in place. Estimate criticel mass with rod out with a
separate plot of 1/C vs. mass. (Continuation of these curves down to the
horizontel axis yields an estimate of the rod value.)
14. Add 1.5 liters of water to core, slowly, watching recorders. because add H2O would be possible
15. Add 100 grams of fuel solution. Record operation at sampling station, note
solution level. Note solution temperature,
16. Agitate solution by air pressure on lower filling tube.
Repeat steps 10 - 13.
17. Compute appropriate amount of water and solutien to add in next step, using data
on manometer readings at shield end density-level graphs. Ultimate goal:
25.4 cm solution level at critical mass, Nh/N235 between 450 and 500, The
corrolation of manometer readings and level may be taken from previously-
Repeat steps 10 - - 17 as needed until within 5% of critical mass when the
control rod is out.
Phase 3 -- Fine Adjustment of Mass Using Source and Control Rod
1. With all rods and shims out, pull source rod until source is back within shield.
Note response on recorders. Exponential drop denotes sub-critical condition.
PUT SOURCE BACK IN PLACE
2. Add water and solution as directed, in small increments.
Note: If within 5% of critical, total that will be needed is no more than
40 grams of U235.
3. Retract central rod slowly after each addition, watching rise of flux as
measured by recorder. Stop frequently to see if flux levels out. It is
necessary to wait a sufficient time to tell whether the level is climbing to
establish equilibrum or whether the reactor is exactly critical. At the exact
critical point the counting rate climbs linearly. The critical mass is the
one for which this occurs with the rod completely out.
4. Pull source into shield after each addition with the control rod out, noting
response on recorders. (When flux stays constant without the source, reactor
is critical.) PUT SOURCE BACK IN PLACE.
5. Lower control rod in the solution.
6. After critical point is reached, add an excess of one gram of U235 in standard
manner. (Recorder traces should rise exponentially with time with the source
in place, and exponentially with a longer period with the source out. This is
the super critical condition, based on delayed neutrons. To detect the
difference between the linear rise and exponential rise, the increases in
neutron level in successive equal times are compared.) Put source back in
place. Lower control rod until flux level is constant with source out.
7. Lower one shim. Observe amount of flux decrease on recorders, compare with
level at a mass prior to the critical point.
8. Raise shim and lower other, to see if any differences exist, lower shim after
9. Lower control and safety rod, interchange roles of the two circuits, then remove
control rod to see if the value of the two rods are the same.
10. Check all ports + holes.
Phase 4 -- De-activation
1. Lower control rod and safety rod. De-energize controls and instruments.
2. Close valves of sampling system. Lock storage vault.
3. Close valves to off gas system. Put floor concrete block in place.
4. Remove source rod and store in wall.
5. Close and lock exposure port used for source pull.
6. Check that all other exposure ports are closed and locked.
7. Close valves in sampling lines, replace lid on sampling box and lock, being
sure locking bars are in place.
8. Transport all containers involving U235 to safe. Check work areas for spills
by survey instruments.
9. Annotate records of critical experiment.
Approval of Procedure by Participants:
|C. K. Beck|
|J. T. Lynn|
|A. P. Sanders|
|J. C. Lundholm, Jr.|
|H. A. Lamonds|
|A. C. Menius, Jr.|
|R. L. Murray|
#3 + #9 reserviced so that will reset.