Letter from Ignalina NPP regarding Positive Run Up of Reactivity

On the 2nd of February 1984, operators at Ignalina NPP issued a note in which they described a sudden increase of reactivity caused by movement of the control rods during the physical launch of the RBMK-1500 reactor. They presented their findings in a letter addressed to the Soviet authorities in Moscow, Kurchatov Institute, as well as several other nuclear power plants operating in the Soviet Union, including Chernobyl NPP.

Huge thanks thanks to Alex from chernobyllab.com, who was kind enough not only to translate the letter, but also provide detailed explanation of technical terminology and the mysterious address codes. Those are post box numbers used within the Soviet government organisations, used to keep the recipient’s address secret. Alex is also known as H. S. and his notes are in square brackets.

Translator’s notes:

1. Proper understanding of this document requires a certain knowledge, so seek appropriate technical comments by graded nuclear engineers. This translation is for historical purposes only.
2. p/b — «post box» — common practice in USSR to hide an identity of a particular organization, typically, military or nuclear-related by using a post box number only when addressing or mentioning it. Created slang terms «post box» for any such facility, or «workers of a post box» for their staff.
3. Translator’s comments placed in [… — H. S.], additions in […]

ORGANIZATION
post box
A-7291 [NIKIET, Scientific-Design Institute of Energy Technics, chief designer of the RBMK reactor — H. S.]

02.02.84 №050-0\1124
on №_________from_________
regarding the question of positive run up of reactivity [reactivity is value that describes the behavior of the chain reaction inside the active zone — H. S.]

To: Head of organization p/b V-2250 [unidentified — H. S.]
com. Kulikov E. V.
101000, city of Moscow

During the physical launch of a reactor of Ignalina NPP and calibration of rods a positive reactivity run up [effect] has been observed when individual rods moved from their uppermost position.

A positive reactivity run up has been observed within 1.5 m of movement for RR rods [manual control — H. S.] and 2.5 m for AR rods [automatic control — H. S.]. Maximum value of the observed run up was 0.01β for RR and 0.03β for 4 AR rods [β is an effective amount of delayed neutrons — H. S.]. While lowering 24 AZ rods [emergency protection — H. S.] run up value during the initial stage of movement was 0.03β.

Run up [effects] of a slightly smaller value were observed before during calibration of the individual rods on units 3 and 4 of Chernobyl NPP. In case of throwing [direct translation of term — lowering all rods in the same time — H. S.] of all the rods, meaning also the rods that were in interim position, no positive run up of reactivity was observed. It was not present also at unit 1 of Ignalina NPP.

Appearance of the positive run up of reactivity during the initial stage of rod’s movement from the uppermost trailer is related to design features of the SUZ RBMK [SUZ — system of control of the protection] and asymmetry of the neutron field by its height. While height of an active zone is 7 m, a length of the absorber rod is 6.2 m and displacer — 4.5 m. In [maximally] lowered position an absorber rod is placed inside the active zone symmetrically by height. In upper position, absorber rod is out of the active zone, and to displace water in the core, a displacer is placed symmetrically in height. During this, above and below remain columns of water of 1.2 m each. Due to special design of the connection joint of the AR rods, displacer is position asymmetrically, therefore has a column of water only under. As displacer is the same, the height of the column of water is 2.5 m.

During the rod movement from uppermost position, a rod is being inserted into upper part of the active zone, along with it moves the upper column of water, and the lower one is being ejected. This movement provides an injection from of negative reactivity for movement down and positive — for upward movement. But change of reactivity will be exactly as described only in case of designed field of energy emission. In case of significant deviation of the neutron field by height with distortions down may change the sign of the rods’ differential efficiency, what was described in experiments mentioned above.

[We] will underscore once more, a positive run up of reactivity will be observed only during the movement of rods only from the uppermost position and only while having a neutron field distorted down.

When reactor is being put into a critical state and during operation on various power levels a part of rods (up to few dozens) are in interim position. Their lowering cover the positive run up, therefore during throwing of the rods positive run up was never ever observed, despite neutron filed could be moved down.

Therefore, we deal with a known phenomenon. Decisions on it already have been made. There are some suggestions, core of which is as below:

Do not use the rods with short connection joint in automatic regulators. On all operating units displacers on such rods were cut away.

Limit the number of rods that being taken out from the active zone completely (up to upper trailers) with 150 for RBMK-1000 and 130 for RBMK-1500. Other partly immersed rods have be inserted into active zone not less than 0.5 m.

Study and gather an experience of the best operators on control of energy emission fields and prepare an addition to the instruction on reactor operation regarding preferred order of ejection of rods from an active zone and height [neutron] field control.

For RBMK-1500 reactor, to additionally study a stability and controllability of the distribution by height using built-in means of control and management in stationary and transitional modes. If needed, to develop additional means, for which it is proposed to provide an installation of additional USP [shortened absorber rod — H. S.] rods (instead or RR and AR-3 rods) in the quantity 8±12. For Ignalina NPP, a set of 30 additional absorbers, that has absorbing rings only in the lower half has to be prepared for installation inside the reactor.

To conduct a design research regarding increase of the length of the retractable joint between displacer and rod in order to remove a water column under the displacer from an active zone, that will prevent appearance of positive run ups of reactivity in case of any distortion of the field.

Use of SUZ rods without displacer with film-based cooling of the channel pipe removes a possibility of appearance of the positive reactivity run up in any states of reactor and in any configurations of height field of energy emission. This variant of SUZ rods and channels was included into the technical project, but due to insufficient experimental testing was rejected. As for today, a significant experience in film-based cooling is accumulated, so there are reasons to return to this decision.

A radical solution is to reduce the active zone height to 6 meters. This suggestion, along with removal of the positive reactivity run up having existing design of the SUZ rods, has other advantages: increases the stability of energy distribution by height, increases the profitability of the power plant due to reduction of loads of uranium. Tasks, described in the letter, are included into the plan XXXXXXXXX [removed in original text — H. S.], and works are being conducted on them.

Deputy head
Y. M. Cherkashov

MAILING LIST

[written notice — SEND TO ALL MAILING LIST Y. M. Cherkashov — H. S.]

Head of organization
p/b V-2250 [unidentified — H. S.]
com. Kulikov E. V.
101000, city of Moscow

Head of organization
p/b A-1616 [unidentified — H. S.]
com. Efimov E. F.
101000, city of Moscow

Head of the department of organization
p/b A-1758 [Kurchatov Nuclear Institute — H. S.]
com. Sydorenko V. A.
128093, city of Moscow

Head of organization
p/b A-7238 [unidentified precisely, but this is some nuclear center — H. S.]
com. Vperin A. P.
188510, city of Lomonosov, Leningrad region

Head of Kursk NPP
com. Gorelikhin V. K.
307239, city of Kurchatov, Kursk region

Head of Chernobyl NPP
com. Brukhanov V. P.
255614, city of Pripyat, Kyiv region

Head of Smolensk NPP
com. Saraev Y. P.
216535, city of Desnogorsk, Roslavl district, Smolensk region

Head of Ignalina NPP
com. Lukonin L. F.
234740, city of Ignalina, Lithuanian SSR