The operating margin of reactivity (ORM) is not measured by directly any sensors and not registered by any showing or recording devices. ORM value is calculated by the routine for physics calculation "[PRIZMA]" on the computer of Centralized Monitoring System (SCM) "[SKALA]". This calculation takes certain time and its result it is possible to see on listing which the shift engineer on computer facilities should make and bring specially on CRU.
What we see here?
1) ORM it is not any operational parameter, operatively supervising, but a certain quantitative characteristic of condition of the reactor which did not leave yet from under science guardianship.
2) This characteristic is connected with stability of fields of power allocation in the reactor and possibility them to operate. Here there is no speech about any weakening of emergency protection system, and furthermore about possibility of an explosive situation.
3) Restrictions on ORM size concern a steady state mode, when this parameters varies very slowly and there is time for its definition and comprehension.
4) Restriction of ORM in 15 rods presented extremely lightly. It is not described, what exactly means the term immediate shutdown (if it is SCRAM then so should be written), and does not explained why such requirement caused. The term "immediate shutdown" does appear in no one point of this chapter of regulations any more.
Very strange looks either tough reaction to a malfunction of such harmless by sight parameter, and a context in which it is written down.
If, ORM is not operative parameter, but only certain characteristic of physics which is not impact the reactor nuclear safety, and the chief designer knew nothing about danger of small ORM then the context is explainable, but such tough reaction is not clear.
If ORM is really a safety-significant parameter, and the chief designer knew about it (though has told anything to no one) then the tough reaction is clear. But then such record of this restriction in regulations smells slightly already of a criminal act.
Other record of ORM restriction in size 15 rods is in chapter 6 Lifting of the reactor power and start-up of the unit after a short-term stop and partial unloading, in paragraph 6.6 Order of power lifting . Here this record. (*2)
As we see, any promptness and haste in the reactor shutdown is not present here.
First, to see that the reactivity margin has reached limiting value and continues to fall it is necessary some time (some minutes). All this time ORM will be less than 15 rods and will be becomes ever less.
Secondly, the anxiety in this point as it will be visible from further, goes about an idle time as a result of getting into "poison out" time (characteristic time – hours) that does not make fill harry too. There is no a word about any disaster from small ÎRM.
Thirdly, the same remark as to chapter 9 – an illegibility of formulations. For example, what does it mean – reactivity margin in the course of power lifting, how it to define in this fast enough transient? To its beginning – it is clear, after its finish too, but in the process how to determine it?
And main thing is, what for such point exist in regulations in general if in it there is point 6.2 (see below) which accurately orders at what ORM it is possible to put the reactor to power state after a short-term stop (and how this reactivity margin to determine). How safety is secured on putting the reactor to a critical condition and the further lifting of power, is described in detail in many paragraphs of regulations without any refer to ORM. Point 6.6.4 in essence does not add anything to it.
The third mention of ORM contains the point 6.2.(*3)
Even in this accurately enough formulated point there is not done without incomprehension. What in your opinion according to this point should be minimum of initial ORM if the reactor before shutdown worked on power of less than 50 % – 30, 26 or 15 rods? Or may be it is need to understand so, that if the reactor before stopping worked on power of less than 50 %, then it is forbidden in general to raise the reactor power again without passage the ‘poison out’ time? Then why it so directly is not written?
There is a question.
How it is possible to explain so indistinct representation in technical regulations (TR) of such important parameter for the reactor safety, as an operative reactivity margin (ÎRM)?
If not to suspect authors of regulations in something bad the explanation can be only one, they did not consider ÎRM as safety-significant parameter. And restrictions on its size, written down in regulations, are connected not with nuclear safety at all, but with something another. What with?
Let's look what the chief designer of the reactor wrote about ÎRM before Chernobyl accident. In his book [E1] is written about ORM the following (on p. 34-35) (*4)
And that is all, nothing more presented in this book about ORM. And it is absolutely clear from this that is said in the given text, that the admissible operative reactivity margin in reactor RBMK is chosen not at all by reasons of nuclear safety, but by reasons of efficiency, good manoeuvrability when work together with electric power transmission system and efficiency of operating in transitive modes. Values ÎÇÐ from 1 % to 2 % (it approximately also there are 15 and 30 rods, see the curve 1 on fig. 2.5) are considered as the optimum. If ÎRM less than this range there will be too great idle times because of a fall into ‘poison out’ time holes and also there will be a low operational control, but if ÎRM more than this range then too expensive will manage fuel, and there will be low efficiency of its use.
It is hard to read restrictions on ORM in regulations otherwise, as made by these reasons.